Textbook conceptions of light-matter interactions have been redefined by two recent material advances -metamaterials and parity-time (PT) -symmetric materials. On the one hand, metamaterials have enabled control over both the electric and magnetic fields of light. This control allows the permittivity and permeability to be precisely tuned throughout
positive, negative, and near-zero values. Metamaterials have enabled negative refraction, optical lensing below the diffraction limit of light and invisibility cloaking. On the other hand, PT-symmetric potentials have emerged as a new technique to control field distributions in loss and gain media, so that light propagation can be asymmetric and even unidirectional. In wavelength-scale optical devices, PT-symmetric potentials have enabled loss-induced optical transparency, lossless Talbot revivals and unidirectional invisibility. Combined with non-linear media, they have also been suggested for optical diodes, insulators, circulators, and perfect cavity absorber-lasers.
In this presentation, we introduce a new class of optical media - PT symmetric plasmonic metamaterials. These materials combine concepts from both metamaterials and PT-symmetric materials to enable nanoscale optical devices capable of both lossless and asymmetric light propagation. As a case study, we consider a prototype metamaterial composed of a periodically-stacked planar plasmonic waveguide. This metamaterial is designed to behave as an isotropic, three-dimensional negative refractive index material at optical frequencies. By subjecting the plasmonic modes to PT-symmetric optical potentials, we demonstrate the broad tunability of the band curvatures, band gaps and effective refractive indices of the material. Small but non-zero PT-symmetric potentials increase the transmission of the isotropic negative index metamaterial to unity. Larger potentials morph the material from isotropic to anisotropic and directional. In particular, double negative refraction, Bloch power oscillations, unidirectional invisibility, and reflection and transmission coefficients that are simultaneously equal to or greater than unity can all be achieved with increasing PT-symmetric potentials. We discuss how the highly unusual optical properties of these PT-symmetric metamaterials can enable lossless and far-field sub-diffraction-limited Veselago lenses and new nanoscale perfect laser-absorbers.

High-performance integrated optics, solar cells, and sensors require the ability to concentrate, redirect and manipulate light at the surface of the device, in order to optimize the coupling and trapping of light. Metasurfaces composed of (complex) patterns of individual optical scatterers are key to achieving this function. Here, we present metasurfaces composed of high-index dielectric nanoparticles that support (Mie-like) geometrical resonances in the visible spectral range. Unlike plasmon resonances, these geometrical resonances are driven by displacement currents rather than actual current flows. The resonances therefore have low absorption losses and correspondingly high quality factors that are purely determined by radiative loss.
Dielectric Mie scattering is described by both electric and magnetic multipole resonances, the latter driven by displacement current loops. Due to the different scattering patterns of electric and magnetic resonances and the ability to tune their spectral overlap, the light scattering from engineered dielectric metasurfaces can be controlled in a unique way.
Here, we use optical experiments and numerical simulations to study the electric and magnetic resonances of silicon nanoparticles that are coupled to a planar substrate. While Mie scattering from isolated nanoparticles is well-understood, the strong coupling to the substrate in metasurfaces results in very complex modal interplay of the scattering and incoupling of light. We investigate the influence of particle shape, particle dimensions (d = 75-250 nm, h = 50-250 nm), substrate index, particle index, excitation mechanism (in-plane and out-of-plane polarization) and presence of low index spacer layers on the particle eigenmodes.
We show that careful engineering of the geometry and photonic environment gives accurate control over the spectral position (lambda; = 400-800 nm), resonance width and angular radiation pattern, and allows the selective enhancement or suppression of the magnetic or electric eigenmodes. We find that the excitation of the displacement current loops is determined by retardation of the incident light across the particle and can thus be controlled by the thickness.
Next, we investigate metasurfaces composed of coupled Mie resonators and find that near-field coupling of resonant Mie scatterers leads to strong mode splitting. We use electron-beam lithography to fabricate arrays of coupled Mie scatterers with a controlled spacing in the range 0-100 nm. By controlling the coupling, the angular radiation patterns can be precisely controlled.
We use substrate-conformal soft-imprint lithography in combination with reactive ion etching and thermal evaporation to fabricate metasurfaces over a full 4” silicon wafer. Scattering nanoparticles are composed of either TiO2 or Si. Optical scattering and cathodoluminescence spectroscopy are used to identify the electric and magnetic modes, which are identified by their distinct angular radiation profiles.

Refractive lenses are some of the most ubiquitous optical components and are widely used in applications that range from imaging to concentrating light. However, the focusing capabilities of these dielectric based lenses deteriorate as their physical dimensions are reduced to optical wavelength scale. They also carry uncured problems of chromatic and spherical aberration. Even though diffraction based microlens (micro-Fresnel lenses) show lower chromatic aberration, the micro circuitry required in its fabrication process requires precise alignment among multiple lithography steps.
Surface plasmons, collective charge oscillations on a metal surface, provide opportunities to reduce the size of optical elements by strong confinement of electromagnetic radiation. Here we report, a novel planar plasmonic lens which concentrates wavelengths of light across the entire visible spectrum to a single spot along its axis. Numerical simulations obtained using finite difference time domain methods suggesting chromatic aberration correction along with concentration of incident electromagnetic waves using novel metallic devices will be presented.

Microlenses are key elements in many optoelectronic devices, such as image displays, communications instruments, and advanced solar cells. Microlenses, however, require complex 3D surface profiles for optimal focusing, which are challenging to fabricate. Due to these limitations, the combination of one or more lenses to create multicomponent optical systems has not been achieved at the micrometer scale. This presentation describes how a lattice lens composed of different arrangements of sub-wavelength holes in a metallic film can focus light to a desired focal distance with optimal focal spot size. Since the design for lattice lens considers contributions from each nanohole individually, lattice lens could direct light into multiple, spatially separated focal points, which is not possible for traditional refractive lenses. We show that we could shift the focal point of a single lattice lens up to 10-µm away from the optical axis, allowing for the combination of two or more independent laterally shifted optical elements. Furthermore, we have created lenses with polarization-dependent optical properties by fabricating elliptical nanoholes. We anticipate that the unprecedented properties associated with these lattice lenses will allow for many new applications in biological imaging research and related applications.

Optoplasmonic materials that amalgamate the synergistic material properties of metallic and dielectric components provide new opportunities for enhancing photoemission, long range photon transfer, and optical switching. We investigate in this contribution the tunable electromagnetic properties of optoplasmonic molecules that comprise plasmonic antennas and photonic resonators in well defined geometries obtained through template guided self-assembly strategies. In particular, we provide an experimental demonstration of efficient photon transfer from quantum emitters in discrete optoplasmonic molecules comprising gold nanoparticle (NP) dimer antennas located in the evanescent field of a 2 mu;m diameter optical microcavity (OM). Optoplasmonic molecules are versatile electromagnetic materials. Provided appropriate fabrication technologies are available, the electromagnetic properties of optoplasmonic molecules can be adjusted to specific requirements through choice of the size and composition of their components as well as their geometric arrangement. Design approaches for enhancing both electric and magnetic field components in optoplasmonic molecules and the underlying electromagnetic interactions of the metallic and dielectric components are discussed.

We show that far-field thermal radiation can be enhanced through the use of thermal extraction scheme. We also discuss the implications of control of thermal radiation towards radiative cooling in both daytime and nighttime.

Radiative heat transfer across micro- and nano-scale gaps between hot and cold surfaces is exploited in a variety of applications ranging from thermophotovoltaics to nanoscale heat management and heat-assisted magnetic recording. The radiative heat flux can be enhanced by orders of magnitude by using materials that support surface plasmon polariton (SPP) waves.
Recently, new plasmonic materials including not only noble metals but also transitional metals, oxides and nitrides has been explored for ‘traditional&’ applications such as sensing and optical information processing. Typically, the dissipative losses in such materials have to be minimized to improve the performance of plasmonic devices. We use the fluctuation-dissipation theorem and the dyadic Greens function approach to perform a comprehensive analysis of the role of dissipative losses on the efficiency and spectral characteristics of the SPP-mediated nanoscale radiative heat transfer.
To bridge previous estimates of the transfer limits for doped semiconductors and for traditional metals such as Ag, Au, Al, etc., we compare the losses needed to maximize the heat flux between materials with the plasma frequency spanning from the ultra-violet to the mid-infrared band. We also compare the heat transfer efficiency through s- and p-polarized channels, within different frequency ranges, and for both high and low temperatures. First, we use a generic free-electron Drude model to characterize material properties of plasmonic materials in order to paint a broad picture of the effects of the SPP mode spectral position (relative to the thermal emission spectrum) and SPP peak bandwidth on the heat transfer efficiency. We then expand our analysis to investigate a range of recently-discovered plasmonic materials as well as metal-dielectric metamaterials, and to reveal the effect of the dissipative losses higher than those predicted by the Drude model.
Our results show that the high dissipative losses are not detrimental to the performance of the nanoscale thermal devices; to the contrary, high losses can be used to enhance the radiative heat flux. We identify the best material candidates for maximizing the high-temperature and high-frequency heat transfer, which might be useful for emerging applications in the near-field thermophotovoltaics.
This work is supported by DOE BES grant no. DE-FG02-02ER45977 and AFOSR MURI via grant no. UIUC FA9550-08-1-0407.

In the emerging field of thermoplasmonics, Joule heating associated with optically resonant plasmonic structures is exploited to generate nanoscale thermal hotspots. The ability to control and probe thermal processes at the nanoscale has opened the door for several promising applications in medicine, chemical catalysis, and data storage. In the present study, new methods for designing and thermally probing thermoplasmonic structures are reported. A general design rationale, based on Babinet&’s principle, is developed for understanding how the complementary version of ideal electromagnetic antennas can yield efficient nanoscale heat sources with maximized current density. Using this methodology, we show that diabolo antennas are much more suitable for heat generation compared with their more well-known complimentary structure, the bow-tie antenna. We also develop a new thermal microscopy method based on the temperature dependent emission lifetime of thin-film thermographic phosphors to experimentally characterize the thermal response of various antenna designs. Data from FDTD simulations and the experimental temperature measurements are used to confirm the validity of the design rationale. It is also shown that the thermal microscopy technique, with its robust sensing method, could overcome some of the drawbacks of current micro/nanoscale temperature measurement schemes.

The large scattering and absorption cross section of localized surface plasmon resonance (LSPR) can overcome the partial spectral coverage and weak absorption in semiconductor devices. The increased scattering enhances absorption at energies above the band edge, while the energy stored in the plasmon can be transferred to the semiconductor at energies below the band edge, increasing spectral utilization. The energy transfer can occur through transfer of hot plasmonic carriers, or as our recent work has shown, through the energy stored in the local field by a non-radiative dipole-dipole coupling. The local electromagnetic field mediated mechanism is known as plasmon-induced resonant energy transfer (PIRET). Although the presence of PIRET has been confirmed, the details of the enhancement mechanism need further exploration.
Using various plasmonic metal/SiO2/Cu2O semiconductor shell nanoparticles, we have investigated the PIRET mechanism in depth. Varying the shell thickness and metal core allow the dipole-dipole coupling to be varied through distance and spectral overlap. The enhancement in charge separation below the band edge is analyzed theoretically and confirmed by action spectrum photocatalysis measurements. PIRET allows enhanced spectral utilization in semiconductor devices where restraints on band alignment or direct interfacial contact prevent hot carrier transfer.

Using a fully quantum mechanical model for surface plasmon decay to hot carriers, we observe a strong correlation between surface plasmon polarization and the hot electron momentum distribution, and discuss the implications for current transport in hot carrier devices.
Typically theoretical descriptions of the optical response of plasmonic systems have successfully used classical electromagnetic models to account for absorption, though quantum calculations have been applied to certain plasmonic phenomena such as coupling between nanoparticle dimers. For resonantly excited nanometer-scale metallic particles or antennas at metal-semiconductor interfaces, semiclassical Fowler theory predicts much lower photocurrents than are observed experimentally in ‘hot carrier&’ devices. While these processes occur on much larger length scales than systems that have been addressed with a quantum approach so far, to capture the detailed physics of surface plasmon decays into hot carriers a fully quantum mechanical (QM) model is necessary.
Our model treats the electromagnetic field as fully quantized and models electronic states using the orthogonalized plane wave method. In addition to the standard plane-wave modes of the vacuum, there are bound modes in the vicinity of the structure of interest (antennas and nanoparticles). These bound modes, supported by the dielectric response of the material, provide a quantized approach to plasmons. Combined with an analytic and computational methodology for electron coupling to the quantized plasmon and EM fields, we compute the momentum and polarization dependences of hot carrier production specifically for optical frequencies. The energy distribution of hot carriers after excitation, role of multi-photon excitation processes and correlation with plasmon polarization will be discussed using the model we have developed. The Fowler-model and internal photoemission predictions of isotropic momentum distribution on the Fermi sphere for generated hot carriers are violated in the QM approach, which will be shown. We also compare our theoretical predictions to experimental polarization dependent absorption spectra photocurrent measurements at different wavelengths (300nm-1800nm).

Resonant dielectric nanostructures can serve as optical antennas, light directors, impedance match-ing and light trapping structures in a way very similar to plasmonic architectures, yet without the intrinsic Ohmic losses suffered in metallic nanostructures. We demonstrate the use of dielectric Mie scatterers made of SiO2, TiO2, ZnO and Si in a variety of dielectric light management architectures for ultra-high efficiency photovoltaics. We demonstrate black silicon, low-reflective nano-glass, and enhanced light trapping in ultra-thin silicon solar cells. The resonant excitation of orthogonal dielectric and magnetic modes in dielectric nanostructures enables many different ways to control light that cannot be achieved with plasmonic structures, and enables novel designs for spectral splitters, light directors and much more

Nano- and microwire photovoltaic devices have received great attention in recent years due to their promising light collection capabilities. Less material use in these devices results in cost reductions, and radial junctions are often used to allow for easy carrier collection, which opens the door to materials with short carrier diffusion lengths. As these wire-based devices begin to approach the power conversion efficiencies of planar structures, it is natural to ask if the fundamental efficiency limits for these nano/micro-structures is the same as their planar counterparts.
Here we take an approach similar to the one of Shockley and Queisser to determine the thermodynamic power conversion efficiency limit for nanowire solar cells based on the method of detailed balance. This method describes the balance between absorption and emission of photons. We find that the nanowires act as effective concentrators of light and yield maximum efficiencies that are equivalent to those of planar cells with macroscopic concentrating optics. While the light collection effects lead to higher current densities, these current density enhancements alone do not cause an increase in the efficiency. The high current densities can, however, lead to improved voltage characteristics, as is common for concentrating solar photovoltaics, and it is this effect that leads to improved power conversion efficiencies. For a material like InP, we find that the ultimate efficiency of a nanowire device is 43%.

Photoinduced generation of electric currents with a very high current density was observed under conditions of surface plasmon polariton resonance in metal films and nanostructures. We have studied this effect in systems of different type and geometry as a function of the light wavelength and angle of incidence. We demonstrated a possibility to engineer the magnitude and polarity of the iplasmon-related electro motive force with nanoscale geometry. The effect of dc electric currents on plasmon propagation was explored as well. The results can find applications in integration of plasmonic and electronic circuits.

Metallic and dielectric nanostructures have highly tunable resonances that have been used to increase light absorption in a variety of photovoltaic materials and device structures. Metal nanowires have also emerged as a promising candidate for high-performance transparent electrodes for local contacts. In this paper we propose combining these electrical and optical functions. As a first step, we use rigorous solutions to Maxwell&’s equations to demonstrate theoretically extreme absorption in semiconductor thin-films wrapped around metal nanowires. We show that there are two key principles underlying this extraordinary light trapping effect: (1) maximizing the absorption of each individual resonance by ensuring it is critically coupled and (2) increasing the total number of degenerate resonances. Inserting a metal core into a semiconductor nanowire creates such a degeneracy: polarization-dependent Mie resonances are transformed into polarization-independent Fabry-Pérot-like resonances. We demonstrate that by reducing the polarization sensitivity and increasing the number of critically coupled resonances, this hybrid coaxial nanowire geometry substantially outperforms solid semiconducting nanowires, even though the semiconductor volume is significantly reduced. Second, we will give an update on experimental verification of resonant photon trapping in such core-shell nanowire structures. Overall, these results suggest that metal nanowires with semiconductor shells might be ideal building blocks for photovoltaic and solar fuel applications.
Reference:
S.A. Mann, E.C. Garnett, Extreme light absorption in thin semiconductor layers wrapped around metal nanowires. Nano Letters ASAP 2013, DOI: 10.1021/nl401179h

The efficiency of thin film solar cells can be enhanced by nanoscale structuring the absorber, notably in the form of periodic Nanowire (NW) & Nanohole (NH) arrays. Even though both are 2D nanophotonic diffraction gratings and are each other&’s complements, the isolated inclusions of high refractive index NWs have their own modes which makes them strikingly distinct from the connected high index background in NH arrays. This produces strong differences in the absorption properties of NW & NH arrays.
Electromagnetic simulations of NW and NH arrays are numerically involved as they require accurate solutions to Maxwell's equations. Calculating the absorption across the solar emission spectrum is therefore computationally expensive.
Here we apply fundamental physical arguments and semi-analytic calculations to find optimal design parameters for both NW and NH arrays in seconds; this would otherwise require intensive simulations of a large parameter space. Our approach also provides insights that ensure that a global maximum of photovoltaic efficiency is found which only depends on the complex refractive index of the materials used. As an established test case we apply our method to Silicon (Si) NW and NH arrays on a silica substrate with an air superstrate.
Since the bandwidth of the nanostructure is limited we first select the wavelength range for which the absorption is to be maximised so as to optimise the ultimate efficiency. Wavelengths less than 450 nm are below the peak of the solar photon flux, and are already strongly absorbed due to the large Im(n) of silicon. For wavelengths over 900 nm, Im(n) of silicon and the solar flux are both low and we therefore focus on maximising the absorption in the 450-900 nm range.
It is key in our approach to minimise the number of loss channels in reflection and transmission, whilst maximising the number of highly absorptive modes in the absorber. For NWs the dominant loss mechanism is transmission which is limited by restricting the period, d<620 nm, such that no more than two propagating diffraction orders exist in the substrate. The strongly absorbing modes in NW arrays, Key Modes (KMs), can be accurately described by a dipole model [Sturmberg et. al. 2011]. Here we use this model to calculate the number of KMs that exist across the solar peak wavelengths. To maximise this number, the NW radii (r) should be as large as possible. Although reflection tends to be low for SiNW arrays, the Fresnel reflection grows as the silicon fill fraction (ff) increases, which limits the ff. By applying these constraints on the interrelated parameters of d, r, ff, we find a small region of parameter space where all conditions are simultaneously met. The procedure for NH arrays is similar, except that the reflected, rather than the transmitted diffraction orders are important. The optimal parameters found for NWs and NHs are in excellent agreement with full numerical calculations & with results from the literature.

We demonstrate general principles for the rational design of subwavelength semiconductor structures with capability of fully absorbing any arbitrary incidence. To illustrate this notion, we use arrays of silicon nanostructures as an example. We design two Si nanostructure arrays following the proposed principles, and experimentally demonstrates fully absorptions (> 97%) for incident wavelengths of 532 nm and 800 nm.

Finding new methods for light detection has been of great concern for decades. The most traditional method has relied on the use of semiconductor materials and electron hole pair creation upon photon absorption. Recently, another method that has attracted significant attention is based on photocurrent generation through hot electron excitations in metallic nanostructures. We present work on metal-insulator-metal (MIM) junctions that are fashioned into plasmonic antennas. For this study, Au was used because of its plasmonic characteristics and a nanometer-thin aluminum oxide film grown by atomic layer deposition was used as the insulating spacer between the metals. By tailoring the size and shape of this type of antenna, the light absorption in the metal can be enhanced and redistributed in a favorable way to increase the photocurrent over planar MIM junctions.
We describe a detailed model that quantitatively predicts the spectral and voltage dependencies of the experimentally observed photocurrent from illuminated MIM junctions. This model involves the photon absorption in the metal, followed by charge transport of the hot electrons to the metal-insulator interface and across insulator. Based on our model, we can estimate the limits to the power conversion efficiency of MIM junctions and discuss how they might complement current photodetection technologies.

We demonstrate all-semiconductor thin-film plasmonic absorbers, where the absorption is linked to the excitation of highly-confined negative-index surface plasmon polaritons. Strong (>98%) absorption is observed experimentally. Highly doped semiconducting systems have recently attracted significant attention due to their compatibility with existing semiconductor fabrication technology and unique optical response that provides a flexible platform to tune their plasma frequency. In particular, these systems provide the opportunity to scale visible-frequency plasmonics to the mid-IR frequency range with a range of health- and security-related applications. Here we present all-semiconductor perfect absorber (PA) structures where absorption can be efficiently controlled by geometry of the patterned doped-undoped-doped InAs sandwich. To provide quantitative analysis of light interaction with patterned all-semiconductor systems, Maxwell&’s equations were numerically solved with rigorous coupled wave analysis (RCWA). Results of the simulations largely agree with the observed experimental data. The analysis of intensity distributions in the vicinity to PA resonance suggests that the PA behavior is related to excitation of guided modes supported by the system. The dispersion of these guided modes was calculated using the T-matrix technique, which revealed a family of highly-confined plasmon modes. In contrast to the majority of optical guided modes, the waves supported by the doped-semiconductor structures have a negative refractive index, in the sense that their in-plane component of the wave vector points opposite to their in-plane component of the Pointing flux. Strong optical confinement explains the fact that the spectral position of our absorption resonance is shown to be effectively independent of device geometry, while the absorption strength to be highly dependent on the etch depth of the top patterned layer. The strong absorption of light in the structures&’ subwavelength volumes holds promise for integration with semiconductor optoelectronic devices, such as detectors or lasers.

High efficiency thin film solar cells exhibit unusual scattering and radiative emission properties. Recent measurements indicate that the external radiative efficiency for record GaAs solar cells has reached 49% of the Shockley Queisser detailed balance limit. We discuss the role of radiative emission from cells and radiative coupling between subcells on efficiency of solar cells with high external radiative efficiencies. We show from simple analytic considerations that limiting the emission angle of a high-quality GaAs solar cell is a possible route to achieving power conversion efficiencies above 38% with a single absorber layer, and present the first experimental validation of increases in open circuit voltage for GaAs single junction solar cells coated with angle-restricting dielectric silicon dioxide/niobium oxide multilayer optical filters. The highest efficiencies are predicted for a thin, light trapping cells with an ideal back reflector, though the scheme is robust to a non-ideal back reflector. Comparison with a conventional planar cell geometry illustrates that limiting emission angle in a light-randomizng absorber layer not only allows for much thinner cells, but also for significantly higher overall efficiencies combined with an excellent Lambertian scattering rear reflector.
Multijunction solar cells can enable also varying degrees of radiative coupling between subcells and new technologies for synthesis of optically-in-series or optically-in-parallel multijunction solar cells have further opened this design space for very high efficiency photovoltaics employing interesting optical environments. We show here that multijunction cell efficiencies are significantly influenced by radiative coupling with, e.g., an efficiency increase of 1.4% (absolute) for full radiative coupling among 20 subcells. We provide a general framework to understand the effects of radiative coupling on multijunction solar cell performance. We find for example that for designs with less than 100 subcells, higher efficiencies are achieved via radiative coupling between subcells than for the “idealized” situation of restricting the reemission for each subcell.

Colloidal quantum dot solar cells have seen rapid progress in recent years, much of it attributable to improved transport and recombination properties within the CQD solid. Here we review instead the role that photonic and plasmonic enhancements have had, and can potentially have, in further improving the performance of a device made using the best available CQD solids.

Controlled coupling between localized surface plasmon resonances (LSPRs) and a quantum emitter (QE) is key to many exciting applications in photonics including single photon sources, super-bright light emitting diodes, photovoltaics, photodetectors and sensors. The exponentially decaying electric field of the LSPR modifies the local density of states of a nearby quantum emitter, simultaneously enhancing radiative and non-radiative rates. Studies in solution, on surfaces and at the single particle level indicated that the separation for maximum enhancement depends on the shape, size, composition and spectral overlap of the units. Relative to lithographic approaches, chemical colloidal routes provide emitters and plamonic nanoparticles that are single crystalline. Compatible methods for oxide spacers however lack comparable quality, with a large dispersion in thickness and lack sub-nm control with high yield. As a result observations are scattered with optimal distances from 1-10 nm, and maximum values of photoluminescence enhancement from 5 to 30. Hence, well-controlled morphology of these heterostuctures is key to large scale consistent fluorescence enhancement and thereby practical device applications. Herein we will demonstrate sub-nm tunability of dielectic gaps over large scale using atomic layer deposition of Titanium dioxide on self-assembled sub-monolayer gold nanoparticles (NPs). Appling this method a systematic study of fluorescence enhancement/quenching and lifetimes of Cadmium Selenide quantum dots over-coated with a Zinc Sulfide layer (CdSe/ZnS) on self-assembled gold NPs of different sizes using fluorescence lifetime imaging microscopy (FLIM) will be presented. A systematic study performed with energy filtered transmission electron microscopy (EFTEM), X-ray reflection and atomic force microscopy revealed the morphology of the complex heterostructures which is correlated with the optical response.

Degenerately doped metal oxide semiconductors exhibit plasmonic resonance at wavelengths tunable by varying their composition. We have recently developed robust chemical strategies for incorporating high levels of dopants during colloidal nanocrystal synthesis and thereby fabricated a wide range of dispersible, processible nanocrystals with infrared plasmon absorption features. These nanocrystals have been incorporated into conducting networks and embedded in solid matrix materials to derive a new generation of electrochromic switching materials that operate on plasmonic principles. In this talk, we will describe new opportunities for manipulating optical properties of such electrochromic nanocomposites and strategies for integrating them into practical prototype devices such as energy-saving smart windows.

The field of optical quantum technology has been boosted by the integration of single semiconductor quantum dots (QDs) in microcavity structures. For maximum light-matter coupling strength between a target QD and the optical mode of the microcavity, both spatial and spectral mode matching need to be controlled precisely. To achieve spatial alignment, site-controlled growth of QDs has received great attention. However, the resulting QDs still suffer from a reduced optical quality as compared to self-organized QDs (e.g. via the Stranski-Krastanow growth mode). Moreover, for a fully deterministic device technology it will be vital to ensure also spectral alignment which has been achieved up till now only by optical in-situ lithography.
In this work, we report on a novel, fully-deterministic nanophotonic device technology using high-resolution in-situ electron-beam lithography. Our approach combines low-temperature cathodoluminescence (CL) spectroscopy with electron-beam lithography and is demonstrated on samples containing self-organized QDs in the active layer. The lithography process starts by spin-coating the sample with the standard electron-beam resist PMMA.
Then we map the local origin of the QD luminescence by CL at low electron-beam dose to avoid an over-exposure of the resist. The luminescence map allows us to select and determine the position of single QDs with suitable spectral features. Next, we write nano-structures into the resist aligned to the selected the chosen QDs. For writing structures, a higher electron dose is applied locally to invert the resist. This way the exposured PMMA acts as an etch mask with feature sizes of standard high-resolution electron-beam lithography.
To demonstrate this approach, a sample with a low density of self-organized QDs is mapped and sub-µm mesa structures are spatially aligned to single QDs. After etching the sample, another CL-mapping is applied to verify the positioning process. It nicely demonstrates that all mesa-structures are at the pre-selected positions and 90% of them show luminescence from the chosen QDs. To further investigate the optical properties we performed micro-photoluminescence measurements at low temperatures. We observe resolution-limited emission linewidths of 9 µeV, proofing the good optical quality of our nanophotonic structures. Single-photon emission is verified by second-order correlation measurements on single emission lines, resulting in g(2)(0) = 0.04 [1].
Our deterministic lithography technique combines the pre-selection of quantum emitters by means of their lateral and spectral positions with the ability of patterning low-dimensional structures with nm-precision and thus will pave the way for a new generation of fully deterministic quantum devices with unprecedented complexity and quality.
[1] M. Gschrey et al., Appl. Phys. Lett., accepted for publication (2013)

Phosphorescent molecules offer the highest quantum efficiencies (> 20%) in organic light emitting diodes thanks to their emissive triplet states. Alternatively, colloidal quantum dots (QDs) stand out as promising light emitters owing to their high photoluminescence quantum yields as well as tunable and narrow emission spectra. Therefore, hybrid nanocomposites synergistically integrating QDs with phosphorescent molecules, which potentially combine the benefits of both, are scientifically and technologically important. Moreover, understanding the excitonic phenomena including Förster resonance energy transfer (FRET) is crucial for such hybrids of QDs and phosphorescent molecules. In this work, exciton transfer dynamics in the constructs of core/graded-shell CdSe/CdS/ZnS QDs interfaced with the blue-emitting phosphorescent small molecules (known as FIrpic) is studied using time resolved and steady state measurements. Also, a physical model that takes exciton diffusion into account is employed to analyze and understand the experimental results.
Here, the experimental measurements indicate that an efficient FRET from the blue-emitting FIrpic into the red-emitting core/graded-shell QDs is viable. In terms of the donor phosphorescent molecules, FRET efficiencies exceeding 90% have been achieved with a concomitant enhancement in the emission of the acceptor QDs by up to 3.2-folds. As the thickness of the phosphorescent layer is systematically varied from 5 to 20 nm via a 5 nm step size, the enhancement in the fluorescence of the QDs is observed to change correspondingly, with the best case being the thickest phosphorescent layer. These results are supported by the model relating the fluorescence quenching in the donor to the exciton diffusion and transfer. The thickness dependence of the exciton transfer is attributed to the strong assistance of the exciton diffusion to the exciton transfer, which substantially helps the excitons in the phosphorescent layer reach the FRET-zone easily. These findings are essential to designing hybrid organic-inorganic nanocomposites towards highly efficient optoelectronics.

Multiexcitons represent the fundamental limit of light and carrier generation in colloidal quantum dots. While these states are not yet thoroughly understood, they are involved in a variety of important phenomena such as carrier multiplication and photoluminescence blinking. Recent work has shown that the extent of antibunching in emission from single dots is related to the quantum yields of the biexciton and exciton. We extend this work with a photon correlation method which identifies the photons originating from biexciton and exciton emission as the first and second photons emitted from a single dot after a single pulse of excitation, respectively. We use this technique to study dynamic emission intensity, lifetime, and relative quantum yields of the biexcton and exciton in single CdSe quantum dots. We find that the fluctuations of emission can be described by dynamic quenching of the exciton, while the biexciton is minimally affected.

Interest in graphene as a two-dimensional quantum-well material for energy applications and nanoelectronics has increased exponentially in the last few years. The recent advances in large-area single-sheet fabrication of pristine graphene have opened unexplored avenues for expanding from nano- to meso-scale applications. The relatively low level of absorptivity and the short lifetimes of excitons of single-sheet graphene suggest that it needs to be coupled with light sensitizers in order to explore its feasibility for photonic applications, such as solar-energy conversion. We employ red-emitting CdSe quantum dots for photosensitizing single-sheet graphene with areas of several square centimeters. Pyridine coating of the quantum dots not only enhances their adhesion to the graphene surface, but also provides good electronic coupling between the CdSe and the two-dimensional carbon allotrope. Illumination of the quantum dots led to injection of n-carrier in the graphene phase. Time-resolved spectroscopy reveals three modes of photoinduced electron transfer between the quantum dots and the graphene occurring in the femtosecond and picosecond time-domains. Transient absorption spectra provide evidence for photoinduced hole-shift from the CdSe to the pyridine ligands, thereby polarizing the surface of the quantum dots. That is, photoinduced electrical polarization, which favors the simultaneous electron transfer from the CdSe to the graphene phase. These mechanistic insights into the photoinduced interfacial charge transfer have a promising potential to serve as guidelines for the design and development of composites of graphene and inorganic nanomaterials for solar-energy conversion applications.

Quantum dots (QDs) hold tremendous potential for low-cost processing and fabrication of optoelectronic devices. In addition to their size- tunable electronic structure and high photo-stability, their ability to support multiple optical excitations is being explored as a strategy for developing more efficient solar cells and brighter LEDs. Metal nanoparticles are able to collect and focus light over very small length scales by forming surface plasmon oscillations of electron density. When assembled with resonant QDs they have been shown to enhance photoemission and absorption rates.
Using a newly developed multi-pulse time-resolved fluorescence technique we are able to separate the recombination dynamics of multiply excited QDs from the dynamics of singly excited QDs in an ensemble solution. This allows us to selectively determine the effect of plasmonic structures on the emission and absorption rates of multi-excitons and other multiply excited species. In this talk we will demonstrate the technique and show how bi- and tri-exciton species can be generated in reasonably high yields in CdSe QDs in the presence of nanostructured gold, but not at all when no gold is present. Some of the factors that control the magnitude of this plasmonic coupling will be illustrated and the implications for optoelectronic devices will be discussed.

Plasmonic-based structures exhibit promising features mostly due to the highly concentrated electromagnetic fields at the metal-dielectric interface, and have been recently exploited to enhance second-order nonlinear interactions to assemble efficient subwavelength and coherent light sources. However, surface plasmon resonances (SPR) excited in these structures mostly localizes light in a small region near the metal-dielectric interface, not involving any significant interaction and modal overlap with nonlinear materials with high second order susceptibility to produce efficient second harmonic generation (SHG). Here, we report giant enhancement of SHG from single CdS nanowires integrated with plasmonic nanocavities in which the leaky-mode resonances instead of SPR are engineered to concentrate light mostly into the nonlinear material (CdS). These devices exhibits efficient, stable and directional SHG output and consumes low excitation powers. Detailed spectroscopic and numerical analyses will be presented, which are important to understand the mechanism of enhanced SHG from CdS nanowires integrated with plasmonic nanocavities. The potential of these devices for fabricating tunable subwavelength coherent light sources with applications in bio-sensing and imaging, and for integrated nanophotonics will be discussed.

With their size tunable electronic structure and optical properties, colloidal quantum dots (CQDs), also known as semiconductor nanocrystals, are highly attractive materials for lasing applications. When compared to the most commonly used semiconductor disc lasers, CQDs offer in principle great advantages including lower lasing threshold and capability of wide-range color tunability [1]. However, there are various problems and challenges associated with CQD-based lasers. CQDs typically suffer from the enhanced Auger recombination (AR) processes and decreased photostability. To address this issue, CQDs having suppressed AR and exhibiting higher stability are strongly required. For this purpose, in this study, we propose and demonstrate low-threshold, highly stable optical gain performance using new-generation zinc blende CdSe/CdS core/shell CQDs with two-photon optical pumping to decrease the probability of photodamage. In our previous study, we have demonstrated a low optical gain threshold value (~6 mJ/cm2) with two-photon optical pumping from wurtzite CdSe/CdS core/shell CQDs with almost near-unity quantum yield, outperforming CQDs having similar sizes previously reported in the literature [2]. However, the possibility of further decreasing their optical gain threshold is limited for these CQDs due to the low two-photon absorption cross-section resulting from their small size. To overcome this problem, we synthesized core-seeded CdSe/CdS nanorods having giant two-photon absorption cross-section and achieved a lower optical gain threshold than the near-unity CdSe/CdS CQDs [3]. However, making high-quality film of these nanorods was comparatively more difficult.
In this study, to achieve a reduced level of optical gain threshold, we synthesized new-generation CdSe/CdS core/shell CQDs having both a near-unity quantum yield and a higher two-photon absorption cross-section as a result of the thicker CdS shell formation [4]. These CQDs exhibit zinc blende crystal structure with a highly monodisperse size distribution, which results in narrower emission bandwidths (<30 nm). For optical gain study, we prepared highly close-packed films with spin casting of concentrated solution of CQDs on bare quartz substrate. We pumped the samples with 200 fs laser pulses having a wavelength of 800 nm and achieve a very low optical gain threshold of < 3 mJ/cm2 without any loss compensation mechanism. This is thus far the lowest level reported for CQD-based amplified spontaneous emission to the best of our knowledge. The experimental results indicate that these CQDs are very promising for use as optical gain media in lasers.
REFERENCES:
[1] V. I. Klimov et al., Science, 290, 5490, 314-317, 2000.
[2] Y. Kelestemur et al., CLEO: Science and Innovations, Optical Society of America, 2013.
[3] A.F. Cihan et al., in review (2013) and also CLEO: QELS_Fundamental Science, Optical Society of America, 2013.
[4] O. Chen et al., Nature Materials 12, 445-451, 2013.

Electromagnetically induced transparency (EIT) is an optical non-linear phenomenon based on quantum interference that renders a medium transparent over a narrow spectral window. The extreme dispersion within this window has numerous technological implications including low-power high-contrast optical switching, high figure of merit sensing, optical information processing (pulse delay and storage) and lasers without inversion. Unlike atomic systems requiring ultralow temperature and stable laser excitation, plasmonic nanomaterials offer an on-chip physically-realizable platform for a plasmonic analogue of EIT, plasmon induced transparency (PIT), in the visible to near-infrared (VIS-NIR) range if fabrication limits can be overcome. The broadened resonances arising from limited resolution and surface roughness (large units >100 nm in length and 10s of nm in gaps fabricated by top-down lithography) restrict the sharpness and modulation depth of the PIT. To overcome this challenge, we experimentally demonstrate VIS-NIR PIT in self-assembled heterodimers of gold nanorods of two different aspect ratios arranged side-by-side with a few nm separation. Such a small gap (0.5-3 nm) mediates strong dipole-multipole interactions difficult to observe in larger counterparts whereas the crystalline smooth surfaces reduce the radiative scattering maximizing the sharpness and extent of PIT. The strong dipolar-multipolar couplings and broken symmetry create a transparency window due to destructive interference between dark and bright plasmonic modes. These self-assembled plasmonic heterodimers demonstrate a route to physically realizable PIT in VIS-NIR on chip-scale and will be of significant interest for future design of high figure of merit sensors, ultrafast switches and slow light devices for optical information processing.

Recently, inorganic and hybrid light absorbers such as quantum dots and organometal halide perovskites have been studied and applied in fabricating thin-film photovoltaic devices because of their low-cost and potential for high efficiency1-6. Further boosting the performance of solution processed thin-film solar cells to become close to crystalline silicon, without detrimentally increasing the complexity of the device architecture is critically important in their path of commercialization. Part of optimising the device performance is controlling the propagation of light through the solar cell by photonic design. Here I will firstly present the photophysical properties of perovskite based hybrid solar cells employing CH3NH3PbI3-xClx as the absorber layer. I will then present recent results demonstrating a plasmonic effect in perovskite based hybrid solar cells. We blend Au@SiO2 core-shell nanopartciles within a mesoporous Al2O3 scaffold layer, or a mesoporous TiO2 layer prior to coating with the perovskite absorber. We observe, through many hundreds of devices fabricated and tested, that the introduction of plasmonic materials significantly enhances the spectral response and power conversion efficiency of the devices. As a comparison, no enhancement is observed when SiO2 NPs with similar size are added. Peculiarly, the enhanced photocurrent in the devices can-not be explained by enhanced light absorption due to plasmonic light coupling. Through further spectroscopic studies, we attribute the origin of enhanced photocurrent to a previously unobserved mechanism of reduced exciton binding energy (from 100 to 35 meV) and enhanced free carrier generation efficiency with the incorporation of the metal nanoparticles, rather than enhanced light absorption. Our findings represent a new aspect and lever for plasmonics and photovoltaics, and could lead to facile tuning of exciton binding energies in solution processed semiconductors.

In this work we present experimental and theoretical evidence of the coupling between the optical features of porous one dimensional photonic crystals (1DPC) and dye sensitized cells to enhance their efficiency. Porous 1DPCs act like a light harvesting enhancers, approximately as good as the widely employed diffuse scattering layers, provided their thickness is on the same order of magnitude, but maintain one of the primary added values of the original cell: its transparency. [1] Our approach is based on the alternative stacking of two different types of nanoparticles combined with polymeric porogenes to deposit a multilayer on the standard nc-TiO2 electrode. Large overall porosity and wide pore size distribution result in improved electrolyte diffusion through thick multilayers with minimum impact on fill factor and photovoltage. These multilayers can be designed to reflect a wide spectral region in the visible region of the electromagnetic spectra by using multiple periodicities (panchromatic reflection).[2] We also studied the angular response of these structures.[3] In addition, we demonstrate the generation of resonant photocurrent in these types of multilayers sensitized structures.[4]
1. S. Colodrero, A. Forneli, C. Loacute;pez-Loacute;pez, L. Pellejagrave;, H. Míguez, E. Palomares Adv. Funct. Mater. 2012, 22, 1303-1310
2. C. Loacute;pez-Loacute;pez, S. Colodrero, H. Míguez Submitted
3. C. Loacute;pez-Loacute;pez, S. Colodrero, M. E. Calvo, H. Míguez Energy Environ. Sci., 2013, 6, 1260-1266
4. M. Anaya, M. E. Calvo, J. Luque-Raigoacute;n, H. Míguez J. Am. Chem. Soc. 2013, 135, 7803-7806

In photovoltaic devices, light harvesting (LH) and carrier collection have opposite relations with the thickness of the photoactive layer, which imposes a fundamental compromise for the power conversion efficiency (PCE). Unbalanced LH at different wavelengths further reduces the achievable PCE. Localized surface plasmons (LSPs) are the elementary excitation states in noble metal nanoparticles (NPs), and can improve LH of photo-absorbers. Here, we report a novel approach to broadband balanced LH and panchromatic solar energy conversion using multiple-core-shell structured oxide-metal-oxide plasmonic nanoparticles (core-shell-shell titania-gold-titania nanoparticles). These nanoparticles feature tunable localized surface plasmon resonance frequencies (visible to infrared) and the required thermal stability during device fabrication. By simply blending the plasmonic nanoparticles with available photoactive materials, the broadband LH of practical photovoltaic devices can be significantly enhanced. We demonstrate a panchromatic dye-sensitized solar cell with an increased PCE from 8.3% to 10.8%, mainly through plasmon-enhanced photo-absorption in the otherwise less harvested region of solar spectrum. This general and simple strategy also highlights easy fabrication, and may benefit solar cells using other photo-absorbers or other types of solar-harvesting devices.
The general and simple strategy for broadband LH enhancement and panchromatic DSSCs is realized by matching LSP resonance (LSPR) wavelength (lambda;_LSPR) of plasmonic NPs with lambda;_Lo (wavelengths with low extinction coefficient) of existing photo-absorbers. Demonstrated by both finite-difference time-domain simulations and experiments, matching lambda;_LSPR with lambda;_Hi (wavelengths with high extinction coefficient) or lambda;_Lo affects LH differently. Although matching lambda;_LSPR with lambda;_Hi readily improves LH and PCE for optically-thin photo-absorbing layers, matching lambda;_LSPR with lambda;_Lo maximally increases LH and PCE for practical photovoltaic devices with optically-thick layers, through enhancing photo-absorption in the weakly-absorbing region. The multiple-core-shell oxide-metal-oxide plasmonic NPs developed and utilized here are advantageous over other geometries, by featuring: adjustable lambda;_LSPR of 400-1,000 nm, substantially enhanced near-field electromagnetic (EM) intensity, and preservable plasmonic properties during device fabrication. By matching lambda;_LSPR with lambda;_Lo of a common dye (N719), a panchromatic DSSC with broadband balanced LH and a PCE of 10.8% is achieved (30% increase comparing to DSSCs without plasmonic NPs).

Photonic or optoelectronic management via tailor-designed photonic and plasmonic nanostructures is a crucial factor in enhancing the performance of photovoltaic devices and optical biosensors. Among the strategies to improve the solar cell efficiency, periodic photonic nanostructures can be utilized to increase the light harvesting capability. For instance, photonic elements such as 1-dimmentional Bragg stacks and 3-dimensional inverse opal structures are introduced into photoelectrode to absorb the non-absorbed photons by light active materials. We introduce soft-template-derived photonic bandgap films into conventional dye-sensitized solar cells (DSSCs) to improve the light harvesting of dye molecules. Periodic organic-inorganic multilayer films are constructed by stepwise buildup of block copolymer (BCP) inverse micelles. The multilayered BCP films exhibit strong reflective color and well-defined photonic stop bands. The layered films are integrated into the non-conducting side of counter electrodes and thereby remarkably enhanced overall power conversion efficiency is achieved, which can be attributed to the increased light harvesting by sensitized dye molecules.
Another promising method to improve the light trapping effect can be coupling of surface plasmon resonance (SPR) effects, which will lead to further enhancement of the photocurrent. Localized SPR (LSPR) phenomenon is induced by excitation of hot-electrons that can lead to the optical field enhancement at the resonance wavelength. In this work, plasmonic DSSCs containing two types of Au-TiO2 core-shell nanostructures with mutually antagonistic configuration are demonstrated. Enhancement in cell efficiency can be attributed to not only the increase of the dye excitation by LSPR effects associated with near-filed enhancement and scattering but also generation of additional photocurrent induced by direct hot-electron transfer from plasmonic core-shell structures to TiO2 conduction bands.
Next, we show that plasmonic coupling phenomenon can be a versatile platform to enhance the sensitivity of SPR-based biosensing. Au nanoparticles (AuNPs) tagged with dipicolylamine and bisindole moieties are utilized as highly selective sensors for metal anions/cations and thiol-containing proteins. We designed a plasmonic-coupling-based sensing device consisting of AuNPs separated from the Au substrate in SPR spectrometer through a stimuli-responsive polymer linker layer. We also demonstrate that bimetallic nanostructures containing another type of metal NP at the stimuli-responsive polymer periphery exhibit a controlled optical sensing property based on LSPR coupling phenomenon. The simple yet novel approaches presented in this work can be utilized as generalized protocols and applied to increase the function of various photonic and optoelectronic devices.

The use of clean, renewable energy has become increasingly crucial owing to the shortage of fossil fuels. Hydrogen is a good candidate for a clean energy source because the only product is water after it reacts with oxygen. In past decades, water splitting via the photoelectrochemical cell (PEC) and photocatalysts became two of the most potential courses to sustainable hydrogen production. The reaction limit by the light absorption, charge separation, charge transfer and charge recombination. Numerous methods try to overcome the limitation via the band engineering, sensitization, heterojunction and heterostructure. Increasing the light absorption is the straightforward method to enhance the PEC and photocatalysts conversion efficiency. A number of organic and inorganic material demonstrations show a significant enhancement of the solar energy conversion efficiency in the ultraviolet and visible regions of solar irradiation. In the present study, we demonstrate how a nanocomposite using ZnO and CdTe quantum dots and the upconversion nanoparticles (UCNs) to drive the photoelectrochemical reaction via the conversion of NIR light into high-energy photons. We also discuss how the amount of gold (Au) modified on the surface of UCNs enhanced the upconversion emission intensity via localized surface plasma resonance. Thus, we demonstrate how a nanocomposite using ZnO, CdTe quantum dots and the upconversion nanoparticles (UCNs) to drive the photoelectrochemical reaction and gas evolution via the up-converted the NIR light into high-energy photons. The photocurrent enhancement by the Au nanoparticles modified the UCNs significantly by a factor of about 40, which demonstrates the potential applications of the material in solar energy conversion systems.

Organic thin-film devices have achieved significant progress in the last years, with OLED displays achieving already widespread commercial use, while OLED lighting and solar cells have so far not achieved a broad market penetration. One of the key steps for further improvement is improved outcoupling and incoupling in OLED and solar cells, respectively. In this talk, I will review our recent work on improving the photon management in such devices. In particular, I will cover periodically structured substrates and metal nanoparticle approaches.

Organic solar cells present some unique properties that make them very attractive as a renewable source for energy production. Particularly interesting are semi-transparent organic solar cells that have the potential to be integrated in urban areas and largely reduce building emissions. However, to make an organic solar cell transparent one has to thin down the top metal electrode to just a few nanometers, drastically reducing its reflectivity and consequently the device capacity for photon trapping and sun light harvesting. Here, we propose and implement a path for light harvesting. Our reference was an opaque direct cell fabricated using PTB7:PC71BM blend as active material, which exhibited a power conversion efficiency of 7.7%. We reduced the thickness of the Ag back electrode to 10 nm and implemented a photonic crystal on top of the rest of the architecture. The fabricated semi-transparent cells exhibited a 30% visible light transmission and a power conversion efficiency of 5.6%. The architecture for the entire device was numerically calculated using inverse problem solving methods to design a non-periodic photonic crystal to trap near IR and UVA photons, not visible to the human eye. The corresponding light harvesting recovery lead to a short circuit current for the semi-transparent cell equivalent to 77% the one from its opaque counterpart. In addition, using the same type of inverse integration to modify the layer structure of the photonic crystal we could broadly tune the color of the device without any significant alteration on its overall performance.

Optical effects of the plasmonic structures and materials effects of the metal nanomaterials have recently been individually studied for enhancing performances of organic solar cells (OSCs). In this work, differently, the effects of plasmonically induced carrier generation and enhanced carrier extraction of the carrier transport layer (i.e. plasmonic-electrical effects) in OSCs are investigated. We propose and demonstrate enhanced charge extraction in TiO2 as a highly efficient electron transport layer by the incorporation of metal nanoparticles (NPs). While OSCs using pristine TiO2 can only operate by UV activation (< 400 nm, otherwise poor S-shape J-V characteristics are exhibited), efficient device performance is demonstrated by using Au NPs incorporated TiO2, at a plasmonic wavelength (560-600 nm) far longer than the originally necessary UV light. By optimizing the amount of Au NPs doped into TiO2, the performances of OSCs with various polymer active layers are enhanced and efficiency of 8.74% is reached [1]. In order to understand the fundamental physics, an integrated optical and electrical model (i.e. a multiphysics model), which takes into account hot carrier tunneling probability and extraction barrier between TiO2 and active layers, is introduced here. From experimental and theoretical studies, we attribute the enhanced charge extraction under plasmonic wavelength illumination to the strong charge injection of plasmonically excited electrons from NPs into TiO2. The mechanism favors better energy alignment at the TiO2 interface which facilitates carrier transport in OSCs. The work can contribute to new approaches and knowledge to utilize plasmonically electrical nanostructures in organic optoelectronic devices for enhancing device performances.
[1] D. Zhang, W.C.H. Choy, F. Xie, W.E.I. Sha, X. Li, B. Ding, K. Zhang, F. Huang, and Y. Cao, “Plasmonic-electrically Functionalized TiO2 for High Performance Organic Solar Cells”, Adv. Funct. Mat., DOI: 10.1002/adfm.201203776.

We report on a novel light management approach based on laminating nanostrucutured electrodes onto the active layer. This unusual fabrication method allows the gluing of the top electrical contact onto the active layer to form the hole-collection contact in OPV. In addition and most importantly, it offers a simple way to develop pre-patterned photonic electrodes that is usually incompatible with direct deposition onto a polymer-fullerene active layer. As such, we have carefully modeled, designed and fabricated photonic elements on the top metal contact specifically for trapping red and near-infra red photons in an OPV device. At this spectrum region, photoexcitation of the polymer in a typical polymer-fullerene bulk-heterojunction based on poly(3-hexylthiophene) (P3HT) leads to generation of free charge carriers via the population of an interfacial Charge Transfer Exciton (CTX) in the middle of the band gap. The direct photoexcitation of these mid band gap states lead to the generation of free charge carriers. Thus, the top contact is not only an electrical contact, but also a photonic element of our device for harvesting the additional charges in the mid band gap.
The synergy between light management optimization through modeling of the photonic nanostructures is discussed, as well the integration of the optimal photonic nanostructures onto devices. We will compare results for devices using flat electrodes and will show that our fabrication methodology is a promising approach to integrate light management schemes and easily scalable and applicable to a wide range of devices.

During the last years, the field of plasmonics has moved from a domain in which fundamental insights were developed into a discipline that becomes relevant to applications. In the specific case of light emission, the ability of plasmonic resonances to enhance the fluorescence of nearby emitters depends strongly on the efficiency of the emitter and on its location with respect to the metal dielectric interface. For high quantum efficiency (QE) emitters, losses in the metal lead to a reduction of this efficiency, limiting the emission increase. Therefore, it is generally believed that plasmonic structures only provide benefits for light emission when used with low QE emitters and there are not yet cutting edge applications of plasmonic structures able to compete with state of the art solid-state lighting (SSL) technologies.
Herein, we challenge this believe by demonstrating a very large directional enhancement of the emission (up to 60-fold for unpolarized emission) using emitters developed for SSL applications with an intrinsic QE close to 1 [1]. The emitters are in the proximity of a periodic array of aluminium nanoparticles. This directional enhancement is the result of the plasmonic enhanced excitation of the dye and its decay into collective plasmonic resonances supported by the array of nanoparticles and known as surface lattice resonances (SLRs). These resonances arise from the coupling of localized surface plasmon polaritons to diffracted orders in the array [2,3]. SLRs have a large spatial extension [4] and can couple very efficiently to free space radiation due to their hybrid photonic-plasmonic character. These features lead to a highly directional emission in defined directions [5], which can be controlled by the shape and dimensions of the particles and by the lattice structure and constants.
References
[1] G. Lozano et al., Light: Science&Applications 2, e66 (2013).
[2] G. Vecchi et al., Phys. Rev. Lett. 102, 146807 (2009).
[3] V. Giannini et al., Phys. Rev. Lett. 105, 266801 (2010).
[4] G. Vecchi et al., Phys. Rev. B 80, 201401 (2009).
[5] S. R. K. Rodriguez et al., Appl. Phys. Lett. 100, 111103 (2012).

OLED efficiency has experienced substantial progress over the last years. While the internal quantum efficiency (IQE) reaches almost 100%, external quantum efficiencies (EQE) are still relatively low caused by poor outcoupling of the generated light. The main loss mechanisms are lateral wave-guiding in different layers of the OLED (including the substrate) and plasmon excitation at the top and/or bottom contacts.
Light propagation and outcoupling of fabricated red OLED have been simulated (electromagnetic and ray optics simulations) for various wavelengths. It turns out that wave-guiding in the TCO bottom contact is one of the dominant loss mechanisms, caused by the relatively thick ITO layer normally employed as anode contact material and its high refractive index. Accordingly, different ITO thicknesses have been evaluated in simulations. Further simulations show that substitution of ITO by a thin titanium/gold layer leads to higher extraction efficiencies despite the significantly larger absorption of gold in the visible range.
Based on the simulation results, red OLED devices have been produced by the OVPD method on glass substrates using transparent metal anodes and ITO for comparison. First, thin gold layers were directly deposited on glass substrates. However, rough morphology and grain size limit transparency and sheet conductivity. The addition of a 1 nm Ti adhesion layer improved the morphology of an 8 nm thick Au layer by reducing grain boundaries and roughness. As a result, the transmission of longer wavelengths (> 580 nm) increased by about 15 % and lower sheet resistance could be measured (20 #8486;/sq.) (from 70 #8486;/sq. without Ti). Thus, the high electrical conductivity of gold and its excellent attributes as hole-injecting material allow concepts of OLED devices with Ti-primed gold anodes.
Subsequently, the OLED stack was deposited by OVPD using the phosphorescent material IrMDQ(acac) as red emitter. The resulting EQE of these devices has been improved by 20 % as compared to the reference OLED with ITO anode. This result is in very good agreement with our simulations.
The implementation of non-wave-guiding anodes is a novel concept to increase OLED performance. Further improvements could be achieved by the integration of micro-lenses on the substrate. An additional advantage of gold as anode material is the possibility to realize large-area devices (> 100 cm^2) for lighting applications. Furthermore, Ti/Au is well suited for the realization of flexible OLED, which has already been demonstrated on a 3 x 3 cm^2 device.

We propose an on-chip nanofocusing structure capable of confining electromagnetic energy within a volume of 3.17 × 10^-5 lambda;³ at an enhancement level of 2.2 × 10³ and a nanofocusing efficiency of 80%. The proposed structure can accomplish a Purcell enhancement of 3×10³, and is theoretically predicted to enable the operation of sub-wavelength-scale nano-LEDs and nano-detectors for digital communication over 1 THz (an improvement of about a two orders of magnitude over today's best light-emitting devices). The proposed structure uses two previously demonstrated devices: (1) the recently demonstrated three-dimensional nanoplasmonic photon compressor (3D NPC), which consists of a rectangular metal-insulator-metal (MIM) plasmonic nanofocusing waveguide that tapers three-dimensionally into a sub-wavelength-scale tip; and (2) the plasmonic crystal (PLC), which is made of an MIM waveguide with a periodic array of holes oriented perpendicular to the metal layers, similar to photonic crystals, and is used to manipulate the propagation of surface-plasmon polaritons. Because both devices share the same MIM configuration, a PLC can be seamlessly connected to the input (larger) end of a 3D NPC to form a single unit. And, joining two of these units at the sub-wavelength-scale tips forms the proposed structure. We investigated a silver-silicon-silver (Ag-Si-Ag) MIM configuration on a fused silica substrate for the 1.55mu;m telecommunication wavelength. The PLC allows us to tune the coupling rate between the sub-20nm tip and 200nm body of the 3D NPC and to minimize modal reflections. This greatly improves the efficient nanofocusing capability of the 3D NPC. We first investigate the band structure of the PLC and confirm the presence of the band gap around 1.55mu;m. By carefully adjusting design parameters such as the number and radius of the PLC holes and the distance between the taper and the PLC holes, we accomplish correct phase compensations at the designed resonance wavelength and optimize the coupling efficiency. Thus, we achieve a modal reflection of less than 1% at resonance (lambda; = 1.55mu;m), and the corresponding reflectance extinction ratio between on- and off-resonance frequencies is larger than 100. The effective nanocavity formed in the structure has a Q ~ 50. The difference in the field intensity enhancement between on- and off-resonance wavelengths is a factor of about 100. In the absence of PLC, we observe broadband transmission in the proposed structure, which agrees with our previous findings for the 3D NPC. In our presentation, we will discuss detailed simulation results of the proposed structure; simulation techniques; applications of the proposed structure to various nanophotonics implementations including nano-LEDs and other nanophotonic devices; and expected performance improvements.

Organic light-emitting diode (OLED) performance is significantly limited by poor light outcoupling efficiency (20-33.7%)[1,2] manifesting from: (1) non-radiative exciton decay within the organic semiconductor as heat to one of the electrodes (5.3% loss)[2]; (2) guided photonic modes (9.3% - 35.2% loss)[1]; and, (3) significantly, to tightly bound surface plasmon polariton (SPP) modes at the metal electrodes (up to 46.8% loss).[1,2] Major approaches to minimize optical energy loss to SPP modes in OLEDs include: (i) introduction of Bragg gratings or other wavelength-scale corrugated microstructures to fulfill the energy and momentum conservation of trapped modes and light propagating in air[1], (ii) anisotropic orientation of the dipole emitters to an in-plane configuration to avoid coupling to bound SPP modes[1], and (iii) index coupling to match the refractive index of the emitter layer to air.[1] These strategies have potential to reduce SPP losses but may introduce new leakage or optical dissipation channels, require additional and potentially complex fabrication, and significant changes to the device architecture.[2] Compelling evidence suggests that if the SPP mode character and leakage into the semiconductor can be controlled rather than suppressed entirely, it may boost performance in a host of visible-light-based optoelectronic devices including organic photovoltaics, organic lasers, and OLEDs.[3] Here, we have taken a theoretical approach towards mitigating optical loss to tightly bound SPPs in light-emitting organic optoelectronics by solving a set of three dispersion relations for a semiconductor-metal-insulator (SMI) asymmetric planar waveguide, comprised of a silver film sandwiched between a silica substrate, and a cover layer of the organic semiconducting polymer F8BT (Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] ). This structure serves as a direct means of studying the plasmonic coupling occurring at metal electrode/organic semiconductor interfaces. Through calculations of SPP mode effective index, propagation length, penetration depth and magnetic field confinement, over a range of metal film thickness and polymer emitter regimes (steady state and amplified spontaneous emission) we demonstrate that by simply tuning metal film thickness and emission regime, a significant fraction of a SPP mode can be radiated directly into the organic semiconductor emitter layer. This increased leakage decreases the amount of optical energy lost to parasitic coupling to bound SPP modes. At visible wavelengths, we demonstrate that the magnetic field leakage increases by factors of up to 3.8 and 88 by tuning metal film thickness and when the polymer emitter undergoes amplified spontaneous emission, respectively.
[1] B. J. Scholz, et al., Opt. Exp., 20 [S2], A206 (2012).
[2] B. Koo, et al., J. Mater. Chem. C., 1, 243 (2013).
[3] S. A. Maier, Plasmonics: Fundamentals and applications (Springer, New York, 2007).

By directing light, optical antennas can enhance light-matter interaction and improve the efficiency of nanophotonic devices. Here we exploit the interference between the different multipolar moments of a split-ring resonator to experimentally realize a compact directional optical nanoantenna which outperforms larger multi-element nanoantennas in both bandwidth and directivity while overcoming the need to position local emitters in its near field with nanometric accuracy.
Our multipolar interference antenna is in essence a U-shaped SRR with a narrow gap, optimized to enhance the coupling of electric dipole sources to the antenna modes. Thus, if a local photoluminescence source is coupled to this antenna then it will simultaneously drive interfering multipolar moments whose inverted field symmetries produce a directed light emission.
We experimentally demonstrate this concept using a microscope objective to collect the emission of quantum dots coupled to the nanoantennas. By imaging at the back focal plane, momentum-space images of single antennas are recorded and their directional behavior can be characterized. The collected luminescence around lambda; = 800 nm is observed to be highly directed over the entire quantum dot emission spectrum, in contrast to the relatively narrowband Yagi-Uda antenna based on an array of dephased electric dipoles.
Through FDTD simulations we find that the radiated power enhancement and unidirectional emission are both maintained over a broad bandwidth even when the emitter position is displaced within the gap, resulting in the robust performance observed experimentally when the emitters are attached to an antenna at random positions. Finally we fit the experimental back focal plane images to that of a point source multipole radiating near a substrate and estimate the directivity.
With a more compact, more broadband and more robust single-element design, our multipolar antennas direct light emission better than previously realized directional optical antennas. The antenna volume is only lambda;³/200, the directional bandwidth is at least 150 nm and there is no need to accurately position the emitters. These results are promising for the integration of directional nanoantennas in light-emitting and harvesting devices.

Nano-interfaces between metallic and dielectric materials bring unique optical properties which can be tuned by nano-structures. Especially, surface plasmons (SPs) generated on metal surface modify emissions, absorptions or scatterings, and produce the complicated optical properties combined with other optical effects. On the basis of this, here we propose a new concept of the nano-interface photonics, which should be applicable to enhanced optoelectronic devices.
In 2004, we have reported that the nano-interface is useful to increase the emission efficiencies of light emitting materials. Huge enhancement of the photoluminescence (PL) from InGaN/GaN quantum wells (QWs) was obtained when nano-structured Ag layers were deposited 10 nm above the QWs [1]. Excitons in the QW should couple to SPs at the Ag/GaN interface and this coupling should increase the spontaneous emission rate and the internal quantum efficiency. The increased emission rates were observed by the time-resolved PL measurement and found that the enhancement factor of emission rate is proportional to the density states of the SPs [2]. This method should be useful to develop the higher efficiency light-emitting diodes (LEDs) and also high-efficiency solar cells, because the SP-exciton and SP-photon coupling processes are reversible.
In order to develop the device applications of plasmonic LEDs and solar cells, further optimization of the metal nanostructure and tuning of the SP coupling process are required. For example, we could obtain very efficient light extractions from the propagation mode of the SPs by using the metal nano-grain structures. The PL enhancement ratios depend on the grain sizes because the grains act as the nano-gratings [3]. By using this, we can excite the propagating SP polariton mode without the periodic nanograting structures fabricated by top-down processes.
Metal nanoparticle two dimensional sheets are also very promising structures for various applications. We succeeded in more flexible and remarkable tuning of the resonance spectra by making the multilayered Ag nanosheet structures on metal substrates [4]. This effect should originate not only from the near-field SP resonance but also from other far-field optical effects at the multilayered nanosheet. These nanostructures should be useful to develop both high efficiency LEDs and solar cells.
We believe that our approach of device applications based on Nano-Interface Photonics which includes Plasmonics would lead to new class of several photonic and electronic technologies.
[1] K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, A. Scherer: Nat. Mater. 3, 601 (2004).
[2] K. Okamoto, A. Scherer, and Y. Kawakami: Phys. Stat. Sol. C, 5, 2822 (2008) .
[3] X.-Y. Xu, M. Funato, Y. Kawakami, K. Okamoto, and K. Tamada: Opt. Exp., 21, 3145 (2013).
[4] K. Okamoto, K. Okamoto, B. Lin, K. Imazu, A. Yoshida, K. Toma, M. Toma, and K. Tamada: Plasmonics, 8, 581 (2013).

To mitigate the ever worsening bandwidth crisis of current data communication architectures, here we present nanoscale optoelectronics devices including multiplexed plasmonic nanolasers and high speed graphene optical modulators for future photonic integrations with high density. Nanoscale light sources are known for their strong divergence of emission, preventing the practical integration. We demonstrate nanolasers that direct more than 70% of the radiation into the embedded semiconductor waveguides. An array of lasers with different emission wavelengths are multiplexed on a single semiconductor waveguide, illustrating the potential for large scale photonic integration. On the other hand, integrated optical modulator with high modulation speed and small footprint has been poised to be the enabling device for on-chip optical interconnects. We experimentally demonstrated a broadband, high-speed, electroabsorption modulator based on graphene, a single layer of carbon atoms. By electrostatically tuning the Fermi level, we demonstrated over 1 GHz modulation of the guided light with a broad operation spectrum ranging from 1.35 - 1.6 µm under ambient conditions. The high modulation efficiency of graphene, ~0.1 dB/µm, results in an active device area of merely 25µm2.

The ability of surface plasmon polaritons (SPPs) to confine light to sub-wavelength volumes has garnered significant attention for the realization of miniaturized photonic components. This confinement, however, comes at the expense of a greatly reduced propagation length. In this work, we show that the propagation losses of confined SPPs can be completely compensated and demonstrate SPP amplification with gains as high as 0.093 dB/mu;m using an organic solid-state gain medium [3].
Convincing demonstrations of SPP amplification have recently been realized for long-range SPPs, a low-loss mode specific to symmetric insulator-metal-insulator structures [1,2]. This low loss, however, come at the expense of poor confinement due to high field delocalization within the dielectric media resulting in typical mode areas Am >10lambda;^2. In this work, amplification will be demonstrated for a confined mode with area Am = lambda;^2/2.
Power dependence and gain measurements were performed on a variety of waveguide lengths and for different fabricated structures. We will show that at 200 mu;J/cm2 a mean net gain of 0.002 dB/mu;m was obtained, while at the highest pump fluence investigated, gains up to 0.096 dB/mm were recorded. These results will be shown to be in good agreement with the calculated gain.
[1] S. Kéna-Cohen et al., Nano Lett. 13, p. 1323 (2013)
[2] P. Berini and I. De Leon, Nat. Photon. 6, 16-24, (2012)
[3] M.C. Gather, K. Meerholz, N. Danz and K. Leosson, Nat. Photon. 4, 457-461, (2010)

Periodic dielectric structures are typically integrated with a planar waveguide to create photonic band-edge modes for feedback in 1D distributed feedback (DFB) lasers and 2D photonic crystal lasers. Although photonic band-edge lasers are widely used in optics and biological applications, drawbacks include low modulation speeds and diffraction-limited mode confinement. In contrast, plasmonic nanolasers can support ultrafast dynamics and ultrasmall mode volumes; however, because of the large momentum mismatch between their nanolocalized lasing fields and free-space light, they suffer from large radiative losses and lack beam directionality. Here we report lasing action from band-edge lattice plasmons in arrays of plasmonic nanocavities in a homogeneous dielectric environment. We found that optically pumped, 2D arrays of plasmonic (Au, Ag) nanoparticles surrounded by an organic gain medium showed directional beam emission (divergence angle < 1.5° and linewidth < 1.3 nm) characteristic of lasing action in the far-field and behaved as arrays of nanoscale light sources in the near-field. Using a semi-quantum electromagnetic approach to simulate the active optical responses, we discovered that lasing was achieved because of stimulated energy transfer from the gain to the band-edge lattice plasmons in the deep subwavelength vicinity of the individual nanoparticles. Femtosecond-transient absorption spectroscopy verified that lattice plasmons in plasmonic nanoparticle arrays could enable a 200-fold enhancement of the spontaneous emission rate of the dye because of their large local density of optical states (LDOS).

The last decade has seen photovoltaics emerging as a promising key technology for a sustainable energy future. To meet the economic requirements for a global terawatt-scale deployment of photovoltaics, further efficiency improvements and cost reductions are mandatory. In the first part I will focus on recent developments in improving the efficiency of a cheap absorber material, silicon. I will show that progress has been achieved through the introduction of novel optoelectronic materials, such as substoichiometric silicon oxide window layers and high mobility hydrogenated indium oxide electrodes, as well as innovative processes, such as nanoimprinting and nanomoulding. In the second part I will present a recently developed method for reducing the cost of an efficient solar cell absorber, indium phosphide. Using a vapor-liquid-solid growth technique we are able to grow polycrystalline indium phosphide thin films with excellent optoelectronic properties on cheap metal substrates presenting a promising route towards high efficiency at low cost.

Photonics Crystals (PhC) have a great potential to enhance the thin film photovoltaic cells absorption for both solar and indoor devices thanks to their so-called slow Bloch modes. If those nano-structures enable to reach high absorption in limited spectral ranges, achieving broad band absorption is challenging. It has been reported an even enhanced absorption in not strictly periodic structures, having a controlled amount of disorder.
In this communication, we review the mechanisms responsible for this improvement through the simple case of multiple pattern supercell PhC. We focus on a 200 nm ultra-thin film absorber made of hydrogenated amorphous silicon deposited on a glass substrate.
Several symmetric, non symmetric and disordered supercell PhC with different magnitudes of perturbation and number of patterns are simulated using the Finite Difference Time Domain method, and compared to a single pattern PhC. It is thus shown that:
* band folding mechanisms implied by the perturbation enable to foreseen the additional modes in the absorption spectra
* the magnitude of perturbation controls the coupling of the incoming light, and thus its absorption.
The enhanced absorption in multiple pattern supercell is thus the result of a better coupling of the incident light in a larger number of modes in the spectral range of interest. As a result, this full understanding of the enhancement enables us to demonstrate that the absorption of an optimized single pattern PhC can be easily overcome by introducing a perturbation.

Development of a highly efficient light trapping system is critical in solar cells to achieve improved photovoltaic performances by increasing the absorption of light. While the texture-mediated light trapping plays a dominant role in thin-film Si solar cells, the use of plasmonic effects derived from nanoscale metal structures have also been explored as a potential tool for light trapping. The ability to introduce controlled metal nanostructure inside solar cells is thus essential to design efficient devices, and with this respect, we recently proposed a simple transfer-printing approach to integrate Ag-based periodic nanostructures (nanodisks) with thin film Si solar cells. Herein, we report on the plasmonic light trapping effect using such Ag nanodisks applied in superstrate-type hydrogenated amorphous Si (a-Si:H) solar cells.
The a-Si:H cells with the transfer-printed Ag nanodisks were fabricated as follows: a-Si:H p, i, and n layers and a ZnO:Ga (first) layer were deposited on flat or textured transparent condictive oxide (glass/SnO2:F) superstrates using plasma enhanced chemical vapor deposition and magnetron sputtering. Ag nanodisks with various diameters and heights were then transfer-printed on the ZnO:Ga first layer using nanopatterned poly(dimethylsiloxane) stamps. ZnO:Ga second and Ag layers were overcoated using magnetron sputtering again to complete the back reflectors. A-Si:H cells without the Ag nanodisks were also fabricated as reference cells.
External quatum efficiency (EQE) was measured with a series of thus-fabricated cells to compare the effect of adding the Ag nanodisks. It was confirmed in the case of flat suparstrates that the degree of the light trapping depends on the size (diameter and height) and position of the Ag nanodisks. For example, a 20% EQE enhancement was observed in the wavelength range of 600 to 800 nm when the Ag nanodisks with the diameter of 200 nm, height of 60 nm, and disk-to-disk distance of 460 nm were introduced at 10 nm away from the a-Si:H n layer (i.e., the thicknesses of the first and second ZnO:Ga layers were 10 and 90 nm, respectively). Since the open-circuit voltage and fill factor were unchanged by introducing the Ag nanodisk, the overall photoconversion efficiency (eta;) was also enhanced (7.5%) compared to a reference cell (7.2%). In contrast, no similar enhancement was observed in the case of textured superstrates, suggesting that the simple collaboration of texture-mediated and plasmonic light trappig does not lead to further enhancement of EQE, thereby photocurrent generation. The scope and limitations of the transfer printing-based plasmonic light trapping using transfer-printed Ag nanodisks will be discussed in the presentation.

Light-trapping is essential for high performance thin-film silicon solar cells applying optically thin amorphous (a-Si:H) or microcrystalline silicon (mu;c-Si:H) absorber layers. Conventional devices apply randomly textured transparent conductive oxide substrates serving as light-scattering front contacts as well as reflective light-scattering metal back contacts. In recent years, several novel periodic light-trapping schemes, including plasmonic gratings and photonic crystals, have not only been suggested but also tested and prototyped. One of the promising light-trapping schemes, called plasmonic reflection grating back contact, applies periodically arranged plasmonic Ag nanostructures at the rear side of thin-film silicon solar cells. For an optimized period of this plasmonic reflection grating, even an enhanced light-trapping effect compared to the conventional random texture in thin-film silicon solar cells was demonstrated by the authors.
In this contribution, the light coupling at the plasmonic reflection gratings to leaky waveguide modes in the thin-film silicon solar cells is investigated. Both, a-Si:H solar cells with absorber thickness of around 350 nm as well as mu;c-Si:H solar cells of absorber layer thickness above 1 µm are studied. The presented results include numerical studies based on three-dimensional electromagnetic simulations as well as experimental optimizations of the plasmonic reflection grating back contacts in prototype thin-film silicon solar cells. The shape, geometry and unit cell of the plasmonic reflection grating back contacts were varied. The impact of these variations on the light-coupling to leaky waveguide modes in the solar cells and, in turn, the light-trapping effect is evaluated. Additionally, the measured light-trapping effect is compared and explained with the simulated light-trapping effect of the solar cell layer stack. For the thin a-Si:H solar cells much less leaky waveguide modes are apparent than for the thicker mu;c-Si:H solar cells. Of particular interest is the disorder which was introduced into the periodic order of the plasmonic reflection grating back contact. Surprisingly, already for a moderate deviation from the perfect periodic order of the nanostructures, the leaky waveguide modes in the spectral response of the solar cells disappear but the spectrally averaged light-trapping effect in the solar cells maintains at a high level.
In conclusion, the light-trapping effect induced by plasmonic refelction grating back contacts of thin-film silicon solar cells is studied under variation of the geometrical parameters of the plasmonic grating. In particular, the role of disorder in the periodic grating structure with regard to the light trapping is measured in prototypes, analysed and explained on the basis of optical simulations.

Periodic and random arrays of dielectric nanostructures placed on top of thin-film solar cells can significantly enhance the cell efficiency in two different ways. First, the preferential forward scattering of light through leaky Mie resonances in the individual nanoparticles improves light coupling in the solar cell over a broad spectral range [1]. Second, collective light scattering from nanoparticle arrays causes enhanced light trapping in ultrathin (1-5 mu;m) Si slabs due to coupling to waveguide modes.
In this work we study the absorption of light into thin (1-100 mu;m) crystalline Si solar cells patterned with Si nanocylinder arrays on top of the cell. Numerical simulations show that for all thicknesses a broadband absorption enhancement is observed due to the increased light coupling. Furthermore, for ultrathin Si slabs, distinct absorption peaks are observed in the 800-1100 nm spectral range, due to coupling to waveguide modes of the slab. We use an analytical model to calculate, based on the simulated optical absorption spectra, the photocurrent, open circuit voltage and efficiency of crystalline Si solar cells with a Mie scattering surface coating and a metallic back-contact. Using realistic values for bulk recombination rate (1 ms) and surface recombination velocity (10 cm/s) we find that efficiencies as high as 21.5% can be achieved for 20-mu;m-thick cells with a Si nanocylinder Mie coating. This is a significant improvement compared to the efficiency that we find for planar crystalline Si cells of the same thickness, coated with a standard Si3N4 anti-reflection coating (17.5%). For an ultrathin 1-mu;m-thick Si cell the short-circuit current is enhanced by as much as 75% with respect to the planar reference, due to strong light trapping. We compare periodic and random nanoparticle arrays and find the latter lead to best light trapping as they cover a broader range of spatial frequencies required for matching to the waveguide modes.
To experimentally test the Mie scattering light trapping designs we fabricate thin poly-crystalline Si layers with thicknesses of 1, 2 and 4 mu;m on top of glass substrates using hot-wire chemical vapor deposition. We then use substrate-conformal soft-imprint lithography and reactive ion etching to fully cover the front surface of the Si slab with a periodic array of Si nanocylinders. Optical spectroscopy shows that the AM1.5-solar-spectrum-averaged reflectivity of a 4-mu;m-thick Si slab is reduced from 21.4% to 6.5% by using a Si nanocylinder Mie coating. At the same time the average transmission reduces from 37.5% to 9.1%. From a comparison of transmission and reflection spectra measured on flat and patterned Si slabs under different angles of incidence the coupling to waveguide modes in the ultrathin Si solar cells is directly derived.
[1] P. Spinelli, M.A. Verschuuren, and A. Polman, Nature Comm. 3, 692 (2012)

The recent development of nanophotonics has triggered the emergence of novel concepts for light management in photovoltaic solar cells. This includes incident light trapping and strategies to control light absorption in thin film solar cells. Such novel approaches are based on various promising Nanophotonic structures, including planar photonic crystals, and their abilities to control light propagation and photon capture.
The use of such nanophotonic concepts is expected to greatly increase the efficiency of moderate cost thin film solar cells. When combined with up- or down-conversion processes, these concepts could also be at the basis of ultra-high efficiency 3rd generation solar cells.
This communication will include an overview of these Nanophotonic concepts. They will be illustrated by examples, with a particular focus on the emergence of photonic crystals-assisted thin film silicon solar cells.

Random texturing of a semiconductor slab cladded with a lossless back reflector can theoretically enhance light absorption by a factor of 4n² (also known as Yablonovitch limit, YL). Recent theoretical studies and optical modeling have predicted that YL can be surpassed by deploying nanophotonic light trapping structures, such as periodic gratings and plasmonic structures. However, a definitive experimental study to demonstrate this in a real solar cell is still lacking. Thus understanding the limiting factors to approach or even surpass YL is necessary to achieve a high short-circuit current (Jsc) and thus high efficiency in nanophotonic solar cells. In this contribution, we present a guideline on how to achieve high Jsc in realistic thin-film silicon (TF-Si) solar cells using nanophotonic structures, combining both experimental studies and optical modeling. We explore two-dimensional periodic hexagonal grating for light trapping in p-i-n type a-Si:H solar cells. Investigated solar cell has a structure of glass/grating (P1=1µm)/ZnO:Al/p-SiC:H/i-a-Si:H 300nm/n-a-Si:H/n-SiOx:H/Ag and results in Jsc=16.5 mA/cm². Such solar cell exhibits no degradation of Voc or FF and 0.4 mA/cm² higher current density in the wavelength range 500-850nm, with respect to the Asahi-U TCO-glass. However, the Jsc is still lower than Jsc implied by YL (JY) in the entire range of interest 300-850nm (69.9% of JY=23.6 mA/cm²). Finite element method-based optical modeling, using optical parameters of realistic materials, is performed on aforementioned device structure under perpendicular light incidence to evaluate light absorption of each layer in a realistic solar cell. The optical model is validated by matching the measured EQE and simulated absorptance of i-layer. Performance of nanophotonic light trapping is then improved by (i) replacing p-a-SiC:H layer with a more transparent p-SiOx:H layer; (ii) deploying highly transparent In2O3:H in place of ZnO:Al; (iii) using low absorption n-SiOx and non-doped ZnO before Ag back reflector; (iv) optimizing grating periods; and (v) considering a perfect anti-reflective layer on glass. These modifications lead to an absorptance of i-layer above YL in weak absorption regime lambda;=650-850nm, and increase the Jsc to 22.9 mA/cm² (97% of JY). By assuming no parasitic absorption losses, Jsc above 24 mA/cm² beyond YL can be achieved over broad incident angles. Our results indicate that absorption losses in the front TCO contact and doped layers are the main limiting factors to approach YL in current TF-Si solar cells. Given that all optical properties used in our modeling are referred to materials currently available in laboratories, Jsc above 20 mA/cm² are practical for realistic a-Si:H solar cells with nanophotonic light trapping. To further reduce the gap with YL or even surpass YL, efforts on developing highly transparent front contacts and doped layers are necessary.

Cost reduction of silicon solar cells by using thin absorber layers requires the application of advanced light-trapping schemes to prevent a reduction of the conversion efficiency. An important element of light trapping in solar cells with thin absorbers is the back reflector (BR). In this contribution we focus on the development and application of (i) plasmonic BR for thin-film silicon solar cells and (ii) one-dimensional (1-D) photonic crystal-like-structures in the role of a Distributed Bragg Reflector (DBR) as a part of the textured metal-free BR reflector in ultra-thin crystalline silicon (c-Si) solar cells.
Plasmonic metal nanoparticles have attracted extensive interest for highly efficient light trapping in solar cells, as they have the potential to exceed the 4_n2 absorption enhancement limit of randomly-textured absorbers with refractive index n. We present the design of a plasmonic BR with self-assembled Ag nanoparticles (NPs) for thin-film silicon solar cells to achieve strong scattering and low parasitic absorption over a broad spectral range between 400 to 1100 nm. In addition to the size and shape of the Ag NPs, the thickness of the spacer layer between the Ag NPs and a planar reflector strongly influences the scattering efficiency of the BR by manipulating the intensity of the electric field which drives the excitation of the plasmons. Following the design principles, we developed an optimized plasmonic BR and integrated it into both thin-film amorphous and nano-crystalline silicon (a-Si and nc-Si) solar cells. We demonstrate that they are competitive with the state-of-the-art randomly textured BR for light trapping. Further we present a plasmonic flattened light-scattering substrate (FliSS) that has excellent light trapping properties without sacrificing the electrical performance. The plasmonic FLiSS has an average haze value of more than 0.6 between 650 and 1200 nm and scatters light into a broad angular distribution close to the Lambertian distribution. The nc-Si:H silicon solar cells deposited on the plasmonic FLiSS demonstrate improved V_oc and FF in comparison to the non-polished plasmonic BR or textured Ag/ZnO BR.
The DBR structure basically consists of periodic repetition of two alternating layers with different optical properties. Design and fabrication of different DBRs with wide-band reflectance close to 100% using materials such as a-Si, a-SiN, a-SiO, and ZnO will be presented. The reflectance spectra of the DBR based on a-Si/a-SiN layers as the back reflector in ultra-thin c-Si solar cells was optimized using optical modelling. We deposited the designed DBR on the random pyramids at the back side of silicon wafers with different thicknesses (80, 100, 140 and 190 mu;m), while black silicon was used as the front-side texture. Measured absorptances of the textured wafers with DBR demonstrated up to 98% of 4_n2 absorption enhancement limit in the long wavelength spectral range up to 1200 nm.

Introducing disorder in a controlled way into photonic crystals allows studying the disorder-related change of the underlying transport mechanisms. Recently, controlled disorder has been introduced by mixing different ratios of spheres of different materials and assembling them into fcc opaline photonic crystals [1]. This leads to a random distribution of the defective sites. Here, we introduce deterministic aperiodic disorder in woodpile and cubic photonic crystals. Deterministic aperiodic structures offer the possibility to reproducibly create specific potential landscapes whose Fourier components are determined by the underlying aperiodic sequence. In accordance with Lebesgue&’s spectral theorem the Fibonacci, Thue-Morse and Rudin-Shapiro sequences are examples of the three basic spectral measures, namely pure-point, singularly-continuous and absolutely-continuous, respectively. Especially, the Rudin-Shapiro series is said to be indiscernible from randomly disordered samples concerning their diffraction patterns/properties [2]. Varying the structural parameters, i.e., filling fraction and lattice spacing, according to the aperiodic sequences allows us to introduce deterministic aperiodic disorder into the photonic crystals. Samples are fabricated via direct laser writing in negative-tone photoresist. Using a dip-in lithography approach, samples of considerable height (> 70 unit cells) are prepared with constant high quality throughout the volume. Employing additionally a galvanometric scanning unit, large volumes can be filled, reducing writing times by about two orders of magnitude. After development the samples are characterized with reflectance and time-resolved transport measurements as well as with Laue diffraction. The different types of aperiodic disorder can clearly be discerned in the Laue diagrams. Results from the samples with deterministic aperiodic disorder are compared to such from randomly disordered samples. The different series show characteristic localization behavior, which might be tunable by tailoring the underlying series. Hence, 3D deterministic aperiodic structures could present themselves as interesting materials for electro-optic applications. [1] P.D. Garcia et al., Phys. Rev. A 84, 023813 (2011) [2] M. Baake and U. Grimm, J. Phys. Conf. Ser. 226, 012023 (2010)

The energetic efficiency of current photovoltaic devices suffers from incomplete light trapping. A promising method for achieving enhanced light trapping is the incorporation of 3-dimensional metallo-dielectric photonic crystals into PV devices. We herein present the fabrication methodology and characterization of 3D silver hollow nano-lattices whose hierarchical dimensions span from tens of nanometers in individual strut diameter, to several microns in unit cell size, to hundreds of microns in overall structure extent. The tailored design of hollow Ag nanolattices allows these structures to not only act as 3D waveguides which direct optical mode propagation, but also as surface plasmon coupling structures. Enhancement in light trapping is thus achieved by creating interference between different modes propagating throughout the periodic array, as well as through the excitation of surface plasmon resonances in the metal-dielectric interfaces through absorption of frequencies in the solar spectrum. The novel 3D plasmonic waveguides, or nanotruss lattices, were fabricated utilizing state of the art two photon lithography, followed by plasma sputtering of the plasmonic material, resulting in conformally coated complex silver nanostructures. The core polymer within the nanotruss structures was then selectively etched out, resulting in the creation of silver-air metallo-dielectric photonic crystals.

Amongst the photonic crystals architectures, the 3D design remains the most challenging to fabricate. This originates from the stringent requirements on the constituent materials and the quality of processing. Structuring dielectric materials at the 100-nm scale and with 3D periodicity is not straightforward and several methods have been proposed to address this bottleneck. In this talk, we will discuss a large area 3D structuring strategy for advanced photonic materials by adding the third dimension to 2D etch masks. Surface structuring by nanosphere lithography (NSL) is merged with a novel silicon etching method to fabricate ordered 3D architectures. The SPRIE method, Sequential Passivation Reactive Ion Etching, is a one-step processing protocol relying on sequential passivation and reactive ion etching reactions using C4F8 and SF6 plasma chemistries. Instead of generating smooth and straight etch profiles we have adapted the procedure in such a way that it produces regular size variations in the etch profile. The diffusion of fresh reactants and etch product species inside the etched channels is found to play an important role affecting the structural uniformity of the designed structures and the etch rate drift is corrected by adjusting the reaction times. High quality photonic crystals are thus obtained by adding the third dimension to the 2D colloidal crystal assemblies through SPRIE. Careful adjustments of both mask design and lateral etch extent balance allow the implementation of even more complex functionalities including photonic crystal slabs and precise defect engineering. We demonstrate 3D photonic crystal lattices exhibiting optical stop-bands in the infrared spectral region proving the potential of SPRIE for fast, simple and large-scale fabrication of photonic structures. Numerical modeling based on a structural characterization of the fabricated structures correctly predicts the optical response of the obtained photonic structures [1]. The SPRIE protocol is presently investigated for the realization of tapered structures with axial diameter modulation designed to enhance the light absorption in silicon solar cells or in conductive polymer Schottky junction solar cells [2].
[1] A. Vlad et al., Advanced Functional Materials 2013, 23, 1164;
[2] A. Vlad et al., in preparation.

Increasing solar cell efficiency while keeping low cost process is a key issue for the Si solar industry in the next years. One of the solutions consists in developing frequency conversion layer compatible with the Si industry, to absorb and convert one UV photon in two Infra-Red ones. To achieve such a goal, we based our work on the development of a matrix which will be compatible with this PV technology but which should also contain sensitizers to efficiently excite the rare earth ions. The use of sensitizers allows to overcome the low absorption cross-section of the lanthanide ions when they are embedded in glasses. Thus, a SiNx matrix doped with the couple of rare earth ions, Tb-Yb, has been fabricated by reactive magnetron co-sputtering and studied. The first step of this work consists in achieving an intense emission of Tb3+ ions by optimizing the deposition parameters to enhance the coupling between sensitizers and Tb3+. This has been evidenced by deep time resolved- and excitation- photoluminescence experiments as a function of the Tb3+ concentration. The second step has consisted in the incorporation of an increasing content of Yb3+ ions to the optimized Tb-doped matrix with the aim to obtain an intense emission at 980 nm through a non resonant excitation laser line. By a careful analysis of the optical properties of this system, the excitation mechanisms of Tb3+ and Yb3+ ions will be discussed. An internal quantum efficiency as high as ~190% has been achieved. With the aim of always improving the efficiency of such a conversion frequency layer, a plasmonic structure has been added. It consists in depositing a thin silver layer using the same fabrication technique. After a suitable post annealing process, the effect of this metallic layer on the Tb3+, and Yb3+ emissions under laser excitation has been analyzed and the most relevant results obtained such as enhancement of the emission, interaction Plasmon-Emitters, a.s.o. will be presented. Finally, different Si solar prototypes have been fabricated and tested to demonstrate the gain expected in the efficiency of the cell using these two different optimized structures.

Single-layer graphene has been shown to have intriguing prospects as a plasmonic material, as modes having plasmon wavelengths sim; 20 times smaller than free space (lambda;p sim; lambda;omicron; /20) have been observed in the 2minus; 6 THz range, and active graphene plasmonic devices operating in that regime have been explored. However there is great interest in understanding the properties of graphene plasmons across the infrared spectrum, especially at energies exceeding the graphene optical phonon energy. We have used infrared microscopy to observe the modes of tunable plasmonic graphene nanoresonator arrays as small as 15 nm. We map the wavevector-dependent dispersion relations for graphene plasmons at mid-infrared energies from measurements of resonant frequency changes with nanoresonator width. By tuning resonator width and charge density, we probe graphene plasmons with lambda;p le; lambda;omicron; /100 and plasmon resonances as high as 310 meV (2500 cm^-1) for 15 nm nanoresonators. In this talk, we also demonstrate how the plasmonic absorption of a graphene sheet is enhanced and perturbed in controllable ways by controlling the thickness and permittivity of the supporting substrate. We will show the results of recent experiments where 20% absorption is achieved in the plasmonic modes of a graphene sheet by carefully selecting the properties of underlying boron nitride and silicon nitride substrates. We also demonstrate how additional absorption pathways can be created by modifying the surrounding dielectric environment to have optical resonances that can couple to the graphene plasmons. These new composite resonances are a versatile tool in expanding the working range of graphene plasmonic devices, as well as exploring the fundamental physics of plasmons in two-dimensional materials. A theoretical model that explains our results has been developed and is used to predict the performance of optically active graphene structures.

The self-assembly of mono-disperse inorganic nanoparticles into highly ordered arrays represents an exciting route to advanced materials with novel functions. It allows programming their properties by changing the size, shape, composition and packing order of the assemblies. While substantial progress has been achieved in the fabrication of superlattices with isotropic building blocks, limited advances have been made with anisotropic building blocks.
In this article, we report the controlled growth large-scale, monolayered Au and Ag nanoprism superlattice by polymer ligands in an entropy-driven interfacial self-assembly process. Furthermore, a 3-dimensional finite element method was used to investigate the plasmon resonance properties for individual nanoprisms in different dielectrics and plasmon coupling in nanoprism aggregates. Scattering enhancement of more than 10x has been observed experimentally with these nanoprism nanosheets.
Our rapid and simple strategy may be extended to other anisotropic plasmonic building blocks, offering a robust and inexpensive avenue to plasmonic nanosheets for various applications such as nanophotonic devices, biochemical sensing, optical processing and solar photovoltaics.

We demonstrate that it is possible to fabricate optical antennae that match the theoretically predicted resonance quality factor for a flawless antenna structures using helium ion lithography (HIL).
The concept of optical antennae has revolutionized the field of nano optics and allows in principle to manipulate and control light with a single digit nano meter resolution. This development has been made possible due to recent accomplishments in nanofabrication that has permitted structuring materials such as Au, Ag or Al with single digit nm resolution to enable plasmonic coupling between nano antennae. Hence, much of the work has been dedicated to reproducibly fabricate optical resonators with a nanometer precision using state of the art fabrication techniques such as Focused Ion Beam (FIB) milling, Electron Beam (e-beam) Lithography, and Induced Deposition Mask Lithography. However, due to the fabrication resolution limitations of > 10 nm, the optical properties of plasmonic antenna fall short of their theoretical predicated resonator quality factor and enhancement factors. Geometrical deviations from the ideal geometry - such as tapering, sidewall roughness, and corner rounding on length scales comparable to the SPP skin depth are the reasons for the shortcomings of actual optical antennae compared to their theoretical model structure.
In this work, we fabricated coaxial optical antennae by using Helium Ion Lithography (HIL) that allows the fabrication of coaxial optical antennae with reproducible gap sizes of 8 nm, perfectly parallel sidewalls and edges with radii of curvatures of less than 4 nm. Wavelength dependent transmission measurements are used to determine the resonance peak and the resonance quality factor then compared to the theoretical response of an ideal geometry, determined by a numerical Finite Difference Time Domain (FDTD) simulation. We found a very good match between the experimental values and the simulated resonances of perfect structure.
For comparison, coaxial antennae with 30 nm critical dimensions were fabricated using both HIL and the more common Ga Focus Ion Beam lithography (Ga-FIB). The quality factor of the Ga-FIB resonators was 60% of the ideal HIL results for the same design geometry due to limitations in the Ga-FIB fabrication process. In summary, we demonstrate that it is possible to fabricate coaxial optical antennae that match the theoretically predicted resonance quality factor for flawless structures using helium ion lithography (HIL).

Nanogap hybrid antennas, consisting of two metal nanostructures joined by a gap material serving as an impedance load, can utilize optical-frequency resonances for enhanced collection and localization of optical fields. These structures have been identified as potential building blocks for optical analogs to RF circuit elements with applications in sensing, photovoltaic energy conversion, single photon sources, and optical communication.
Using electrochemical deposition into alumina templates, we have fabricated and characterized nanogap antennas comprising two Au nanorods (with individual rods ranging from 105 to 130 nm in length) joined by a CdS-filled ~40-nm gap, with a uniform diameter of ~65 nm. Antennas were dispersed on ITO and individually characterized with polarization-dependent, wavevector-resolved dark-field microspectroscopy and imaging, which revealed three distinct optical resonances: a longitudinal mode that resonates with a peak response ~950 nm, and two nearly degenerate transverse modes that are split by substrate interactions. A strong, out-of-plane, transverse mode peaks near 570 nm which is red-shifted compared to a weaker in-plane resonance near 540 nm.

The nonlinear response of the antennas was probed by measuring the second harmonic generation (SHG) of a ~100-fs pulsed pump-beam tuned to the longitudinal resonance. Polarization studies found that the SHG intensity strongly peaked when the incident polarization was aligned with the nanoantenna long axis. These experimental results are consistent with finite-difference time-domain (FDTD) simulations which show the magnitude of the electric field in the gap material is ~an order of magnitude larger when the system is excited under longitudinal polarization as compared to transverse excitation. Simulations also accurately predict the measured longitudinal resonant-wavelength, which is found to be that of a single arm with a slight gap-dependent redshift. For nanogap antennas with asymmetric arms, the overall resonance is localized on the longer of the two arms with the gap fields shifted towards the longer arm.

We report on a new candidate for near-infrared (near-IR) to mid-infrared (mid-IR) optoelectronics and transparent contacts: InGaBiAs films grown by molecular beam epitaxy (MBE). Near-IR and mid-IR optoelectronic devices cover a broad range of functionality including free space optical communication, environmental gas monitoring and thermal imaging, and many of these devices are based on an InP platform. Dilute bismuthides (i.e. III-V semiconductors containing small amounts of bismuth in the alloy) are a relatively new class of materials which possess many interesting properties. Our group have systematically studied the growth conditions and the potential applications of InGaBiAs on InP substrate1-3. Incorporating bismuth will cause valence band anticrossing (VBAC) and effectively reduce the bandgap. Tuning the composition of InGaBiAs can make it lattice-matched to the InP substrate. Upon further investigation, we found that degenerately doped InGaBiAs:Si are ideal candidates for near-IR and mid-IR transparent contact materials. They are highly transmissive and conductive.
We have experimentally and theoretically demonstrated that the bandgap of undoped InGaBiAs is effectively reduced with the increase of the Bi concentration at a rate around 56meV/Bi%. The longest wavelength we have achieved so far is 2.7 mu;m. We have also demonstrated that degenerately doped InGaBiAs:Si are highly transparent in the infrared range with the exact transparency windows determined by the material's carrier concentration. Degenerately doped InGaBiAs:Si has been demonstrated to be a highly conductive semiconductor (up to 4850 S/cm) due to its high carrier concentration and good mobility.
In conclusion, we have investigated the effective bandgap of undoped InGaBiAs and demonstrated the VBAC both experimentally and theoretically. We also studied the transmittance and conductivity of degenerately doped InGaBiAs:Si films, and demonstrated that they are highly transparent and conductive in the near-IR to mid-IR range. Therefore, InGaBiAs is potentially an ideal candidate for near-IR to mid-IR optoelectronic materials.
1 J. P. Petropoulos, Y. Zhong, and J. M. O. Zide, Applied Physics Letters 99, 031110 (2011).
2 Y. Zhong, P.B. Dongmo, J.P. Petropoulos, and J.M.O. Zide, Applied Physics Letters 100, 112110 (2012).
3 P. Dongmo, Y. Zhong, P. Attia, C. Bomberger, R. Cheaito, J.F. Ihlefeld, P.E. Hopkins, and J. Zide, Journal of Applied Physics 112, 093710 (2012).

Graphene is emerging as an optical material which can be dynamically tuned by electrostatic doping. However, integration of graphene into optical and optoelectronic devices is limited due to its small thickness and the resultant weak interaction with light. By combining metal and graphene in a hybrid plasmonic structure, it is possible to enhance graphene-light interaction and thus achieve in situ control of the optical response.
Finite-difference time-domain (FDTD) simulation was used to design the hybrid plasmonic structure and understand the coupling between the plasmonic modes of the metallic antennas and the graphene. In our experiment, a CVD (chemical vapor deposition) single layer graphene was first transferred onto 30 nm silicon oxide layer on doped silicon substrate. Gold plasmonic structures were patterned onto the graphene sheet using electron beam lithography. Reflection spectra were taken at various gate voltages using a Fourier transform infrared (FTIR) spectrometer equipped with a mid-infrared microscope. A maximum wavelength tuning range of ~1100 nm (18% of the resonance frequency) is achieved experimentally from the hybrid metal-graphene plasmonic structure at mid-infrared (MIR) wavelengths (6-7 µm). The time response of the device was characterized by measuring the modulation depth of the optical reflection (The incident light is from a continuous wave quantum cascade laser.) for different gate voltage modulation frequencies. Our device exhibits a response time of < 25 ns, which can be further decreased to the picosecond scale by minimizing the contact pad sizes and increasing the insulator thickness underneath the contact pad. Further experiment will be carried out to improve the tuning effect and decrease the response time.
This study confirms that hybrid metal-graphene structures are promising elements for high speed electrically controllable optical and optoelectronic devices.

Because of its desirable optoelectronic properties, compatibility with Si electronics and lattice matching with III-V semiconductors, Ge has found important applications in advanced optoelectronics such as Si-based electronic-photonic integration and high-efficiency tandem solar cells. Growing high crystallinity Ge on amorphous materials at low temperatures <500 ^oC can potentially enable 3D electronic-photonic integration on Si as well as low-cost tandem cells on Ge/glass virtual substrates, yet it still remains a significant technical challenge. Here we demonstrate that, by incorporating Sn into Ge to enhance crystallization through the Ge-Sn eutectic system and to modify the band structure towards a direct gap semiconductor, we manage to achieve high crystallinity GeSn thin films on amorphous SiO₂ layers at low crystallization temperatures of 340~464 ^oC, and significantly improved optoelectronic properties simultaneously. The crystallization temperature decreases significantly with the increase of Sn composition in the as-deposited GeSn thin films. A highly (111) textured Ge_0.92Sn_0.08 thin film with large grain sizes up to tens of microns was achieved from a 9.5% as-deposited Sn composition, as revealed by both X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). EBSD also shows that the average crystal grain size is >5 microns, and that the grain boundaries are mostly twin boundaries and low-angle boundaries, which greatly reduces defect recombination centers. The large grains and the dominance of twin/small angle grain boundaries are due to an extraordinarily large spacing of 0.1~1 mm between nucleation centers from which GeSn grains grow radially, orders of magnitude larger than that in regular solid phase crystallization in semiconductor thin films. This striking observation indicates an exceedingly high lateral growth rate vs. a low nucleation rate in the eutectic-mediated crystallization process of GeSn. Furthermore, by fabricating nano-tipped tapered structures, nucleation is controlled to take place at the tip from which the single crystal grain grows subsequently, demonstrating a promising potential for GeSn single crystal growth on amorphous layers. Another remarkable feature is 8% Sn is incorporated into the Ge lattice, well above the equilibrium solubility limit of ~1%. Correspondingly, the direct band gap optical absorption edge is extended to lambda;=2500 nm (~0.5 eV), right on the verge of indirect-to-direct gap transition. For photonic integration, this transition greatly enhances photonic device performance. For tandem solar cells, this extension in optical response leads to more energy conversion from the infrared spectrum. This Sn-assisted crystallization process is free from common issues related to metal induced crystallization (MIC) such as deep-level defect centers or undesirable heavy doping. It not only offers higher throughput, but also strongly enhances optoelectronic properties due to GeSn alloying.

We demonstrate a computationally efficient and intuitive one-dimensional (1D) analytical model that significantly expedites the design and analysis of three-dimensional (3D) metal-clad nanocavities (MCNs) for various nanophotonics applications. The proposed approach is two orders of magnitude faster than a full 3D finite-difference-time-domain computation. Our 1D approach accurately predicts core cavity parameters including resonant frequencies and quality (Q) factors; quickly reveals tradeoffs between different design choices; and serves as a powerful tool with which to optimize the design of 3D MCNs. The configuration of the examined rectangular 3D MCNs includes a metal-coated, infinitely long, low index dielectric layer (or cladding) with an embedded nanocavity consisting of high index dielectric rectangular cavity core centered on the major axis and two slender posts protruding out of the core along the same axis and in the opposite directions. In our approach, we break down the 3D analysis into 2D and 1D tasks. Using the numerically calculated modal properties of the cavity&’s 2D cross-section, we develop a 1D analytical solution along the major axis (this is analogous to the well-known 1D finite potential well problem). Because the MCNs&’ 2D cross-sections exhibit four-fold symmetry, we investigate the antisymmetric-antisymmetric transverse-electric-like mode that yields a maximum photonic band gap. The high index nanocavity core supports the targeted mode, while the two post regions along the major axis outside the cavity core exhibit a near-zero refractive index and provide confinement. Consuming less than 1/100^th of the full 3D computation time, the 1D model predicts the resonant frequencies within a 4% deviation for wavelengths from 1.1 to 1.6 mu;m and the Q factors within 10% by considering the metallic loss in the nanocavity. When the length of the nanocavity core along the major axis decreases to the dimensions of its 2D cross-section, our 1D model overestimates the Q factors. A closer examination of the metallic loss at this condition reveals that multimode effects caused by the presence of additional discontinuous interfaces in the 3D geometry become significant and generate additional loss uniformly distributed along the major axis; these further reduce Q-factors below the values predicted by our 1D model. We have also examined the case in which the post regions have finite lengths and one of the regions ends with a metal contact. We discover that tuning the lengths allows us to control the magnitude of the radiation loss, and to keep the influence of the metal contact negligible on the cavity properties. Furthermore our study elucidates the underlying physics that relates the mode inside the cavity, its far-field patterns, and its coupling efficiencies with integrated waveguides. We will present the capabilities and limitations of our 1D approach for the design and analysis of 3D MCNs in nanophotonic applications.

A two dimensional metallic photonic crystal pattern with the addition of a SiNx ARC have been previously shown through simulations to increase the optical intensity within an active PV diode region; increase the photon/exciton interaction time; and decrease the number of recombination events. The simulated PhC implied a TPV cell with a greater over all conversion efficiency, as well as, a larger external quantum efficiency and internal quantum efficiency. This work experimentally demonstrates the enhancing effect of the metallic PhC on the diode stage of a Gallium Antimonide (GaSb) TPV device
Thermophotovoltaic generators have traditionally been composed of three parts (emitter, filter, diode) which convert infrared photons into an electrical current.[1] Present research in TPV devices demonstrate maximum conversion efficiencies of only around 20%.[2] However, theory predictions that total system efficiencies of 30-40% can be reached. [2] . In order to reach this limit, our research demonstrates that 2D PhC patterns can be successfully implemented as a TPV filter stage offering many benefits over current filter stage technologies. This is a new application of PhC in the field of TPV as present reseach only utilizes 2D PhC patterns for the Emitter stage. The focus of this research to increase the efficiency of the Gallium Antimonide (GaSb) TPV cell, through the use of a two dimensional metallic photonic crystal filter. Metallic 2D PhC have yet to be implemented into TPV filter stages and may provide many benefits over traditional 1D PhC filters. These benefits include narrower spectral responses and the ability to act as both a filter and a front side contact increasing the TPV diode surface area and thereby power out.
A photonic crystal was first designed and optimized in Lumerical FDTD to create a PhC standing wave corresponding to the bandgap of GaSb. It was determined that the addition of a PhC pattern increased photon absorption by The PhC cavity by 66% and the addition of SiNx as an ARC increased absorption by 80%. The PhC was fabricated from ohmic contact materials resulting in a decreased path length for excitons and therefore a decreased number of potential recombination events. Optical characterization of the metallic PhC patterns demonstrates ARC properties as well as an enhancement in generated electrical current (17%) which may allow the metallic PhC to replace standard TPV contacts as well as traditional ARC coatings.
[1] Nelson, Robert E., “A brief history of thermophotovoltaic development”, Semicond. Sci. Technol. 18 S141, 2003.
[2] Mauk, Michael G. “Survey of Thermophotovoltaic (TPV) Devices,” Mid-infrared Semiconductor Optoelectronics, Springer Berlin / Heidelberg, 2006 (pg 674).

Different optical designs of Dye Solar Cell (DSC) have been successfully employed to trap light in the active layer with the aim of improving power conversion efficiency.[1,2] Here, 1D and 2D relief gratings are patterned onto the surface of nanocrystalline titania electrodes using a combined soft-lithography and microcontact printing approach.[3] This method gives rise to an increase in the photocurrent of the cell as a result of the angularly back diffracted of selected wavelengths. The integration of such surface patterned electrodes shows promising photovoltaic performance enhancement of the cells.
References
1. F.E.Gálvez, E.Kemppainen, H.Míguez, J.Halme, J.Phys.Chem., 2012, 116, 11426.
2. S. Colodrero, A. Forneli, C. Loacute;pez-Loacute;pez, L. Pellejagrave;, H. Míguez, E. Palomares, Adv. Funct. Mater. 2012, 22, 1303.
3. D. Quin, Y.Xia, G.M. Whitesides, Nature protocols, 2010, 5, 491.

In this talk, the fabrication and characterization of 3D polymer photonic crystals using high resolution and high-energy electron beam lithography is presented. A novel type of resist and dose-modulated electron beam lithography (RDM-3D-EBL), extensively exploiting the intrinsic properties of resist-electron beam interaction is detailed. In the RDM-3D-EBL method, orthorhombic 3D polymer photonic crystals are obtained by a single exposure of a multilayer PMMA/MAA resist stack to a high-energy electron beam. Depending on the local exposure dose, either one or both of the resists are removed during a subsequent development step. 3D photonic structures can thus be produced in a single exposure step. During this step the periodicity and feature size in the z-direction are controlled by the resist layer thickness. The feature size and periodicity in the x-y-plane can be determined by the patterning step and the development conditions. The correct parameters for the EBL processing and the development step were found to be strongly dependent on the exposure energy, substrate type and pattern density and were calibrated accordingly. The theoretical prediction of electron trajectories in the resist multilayer realized using a Monte Carlo simulation code (CASINO) will be discussed [1]. While the realized structures have orthorhombic crystal symmetry, we will also discuss the implementation of other crystal symmetries. Extensive structural characterization by FIB cross-section analysis is combined with numerical modeling to correlate the measured optical response with theoretical predictions. The fabricated polymer photonic crystals display tailored photonic stop bands in the visible spectrum range. The stop band position is adjusted by morphology correct choice of the photonic crystal geometry. Nearly the entire visible spectrum range is thus accessed by simply adjusting the multilayer resist stack exposure dose. In conclusion, we show how photonic crystals with precise size and positioning as well as predefined optical stop-bands can be realized simply using a single-step, direct write electron-beam lithography protocol. An on-chip graded band-stop filter has been realized by direct writing of a chirped photonic crystal structure through dosed irradiation of the multilayer stack. This can lead to ultimate miniaturization of on-chip integrated photonic crystal based spectrophotometers [2].
[1] A. Vlad et al. Nano Letters 2009, 9, 2838; P. Jedrasik et al. J. Nanosci. Nanotechnol 2011, 11, 8924.
[2] A. Froelich et al. in preparation.

Metal nanoparticles have been very attractive materials owing to their particular character: Localized surface plasmon resonance and scattering for improving light harvesting ability in photovoltaic cells. In this regard, many researchers have reported that metal NPs are introduced to increase the performance of bulk-heterojunction (BHJ) solar cells as an additive without changing the thickness of active layer. However, it is necessary to modify the surface of metal nanoparticles for preventing aggregation of each particles and photo-induced charge trap. Very recently, introduction of silica layer and surface modification on the metal nanoparticles was tried. However, the effect of the silica layer and the morphology of active layer (BHJ) with nanoparticles were not sufficiently understood yet.
We prepared the silver nanoparticles having silica coating layer modified with oligo-thiophene (OT) (Ag@SiO2@OT NPs), which were well dispersible in organic solvent and showed good miscibility with poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) (P3HT:PCBM) composite films. The performance of the photovoltaic devices based on the active composition of P3HT:PCBM was improved in terms of the light harvesting ability and photo conversion efficiency value when Ag@SiO2@OT NPs were incorporated, comparing to the cases of pristine P3HT:PCBM device as well as nanocomposite devices with bare Ag nanoparticles. Serious problem of Voc drop, when the concentration of bare Ag NPs was increased, was not observed in the case of nanocomposite devices with Ag@SiO2@OT NPs. Furthermore, the location of Ag@SiO2@OT NPs in the nanocomposite devices could be confirmed as mainly distributed within P3HT domain, by TEM experiments after selective removing of PCBM region. Details including synthetic method, characterization of morphology of active layer, and the result of surface modification will be discussed during the presentation.

Anti-reflection (AR) coatings for flexible, organic substrates require organic-based materials with well-matched coefficients of thermal expansion that will not crack or delaminate, as do traditional AR materials designed for inorganic substrates. While quarter- and half-wave plates can reduce reflection at specific wavelengths, graded refractive index (GRIN) coatings are desirable because they offer AR properties across a broad spectral band and for a wide range of incident angles. One approach to achieve a GRIN coating is to continuously vary the porosity of an AR polymer film such that the refractive index is reduced from that of the substrate to that of air over a given thickness [1]. In order to fabricate such a coating, it is critical to co-deposit a polymer blend (AR polymer and porogen polymer) with isotropic nanoscale phase domains (< 40 nm). In this way, the AR coating behaves as an effective medium without light scattering across the visible spectrum. Yet, solution-based co-deposition of polymers often leads to micron-sized domains that are too large to prevent light scattering in the visible spectrum. Furthermore, layering of solution-cast films is difficult because of the inevitable dissolution of existing layers by repeated deposition of polymers with similar solubility.
A novel deposition technique known as resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE) has demonstrated nanoscale pore sizes suitable for the visible spectrum [2]. RIR-MAPLE, which is a gentle offshoot of pulsed laser deposition, has been developed to deposit one or more organic materials as thin films regardless of solubility. Importantly, RIR-MAPLE can actively control the polymer blend ratio during deposition to create the graded structure required for the GRIN coating. In this work, we will present the application of RIR-MAPLE to depositing styrene (AR polymer)/methyl methacrylate (porogen polymer) blends for the fabrication of organic GRIN AR coatings that are appropriate for polycarbonate substrates, and eventually, organic optoelectronics. In particular, RIR-MAPLE target chemistry, substrate temperature, and post-deposition surface treatment will be investigated. Scanning electron microscopy will be used to characterize phase domains in the porous structures. Optical characterization of the films, via integrating sphere reflection/transmission/absorption measurements, will demonstrate the reduction in reflectance achieved by such coatings across the visible spectrum. While porous polymer films with a graded material structure are nearly impossible to create with solution-casting methods, this work will demonstrate that RIR-MAPLE can enable these structures for application to GRIN AR coatings.
[1] X. Li, J. Gao, et al., Advanced Functional Materials, vol. 20, pp. 259-265, 2010.
[2] R. D. McCormick, E. D. Cline, et al., Proc. SPIE 8258, Organic Photonic Materials and Devices XIV, p. 825806, 2012.

Among many π-conjugated organic semiconducting building blocks, diketopyrrolopyrrole (DPP) has attracted significant attention due to their structural compactness, driven by strong πminus;π stacking in the solid state, their quasi-planar backbone structures, and the facile control over the MOs, which cooperatively yield outstanding charge carrier transport efficiencies in organic photovoltaics (OPVs) and organic field-effect transistors (OFETs). In particular, DPP-based small molecules are attracting keen interest due to their inherent advantages over polymeric analogues, such as perfectly defined chemical structure, easy purification and reproducibility without batch to batch variation.
In this work, we have synthesized a new series of small molecules comprising DPP backbone for organic electronic device applications. The frontier molecular orbitals in these series are elaborately tuned by introducing a strong electron-accepting functionality (dicyanovinyl) and various thiophene containing electron-donating units. Among these DPP-based small molecules, thiophene and dicyanovinyl-substituted DPP-based organic semiconductor, DPP-T-DCV, exhibited an outstanding electron mobility of up to 0.96 cm² V^-1 s^-1 in solution-processed single-crystal OFETs (SC-OFETs). Furthermore, by replacing the thiophene moiety to more strong electron-donating unit, we could effectively control the frontier energy levels to demonstrate ambipolar transistors. With such precise energy level tailoring, together with the solubility control and molecular structure engineering, we could realize highly efficient small molecule OPVs as well.

Improving light-harvesting in ultra-thin conjugated polymer active layers is an ongoing topic of interest in organic photovoltaics community. In recent work, we have shown that metal-polymer nanoheterostructures consisting of Au plasmonic nanoantennas coupled to a sub-50-nm poly(3-hexylthiophene) (P3HT) layers on a metallic ground plane are a promising solution to this issue [1]. The plasmonic nanoantennas are capable of concentrating and enhancing optical fields within deeply-subwavelength regions of the P3HT active layer due to strong coupling between localized surface plasmon resonances supported by the nanoantennas and free electron oscillations in the adjacent metallic plane. On resonance, the incoming light field is greatly amplified in the nanometric P3HT “gap” between the nanoantenna and the metal plane, an effect that is predominantly attributed to plasmonic gap modes resonances (i.e., hybridization between localized plasmon modes of the nanoparticle (NP) and the propagating plasmon modes of the metal substrate). Such significantly enhanced electric fields are of vital importance to optoelectronic devices for improving light absorption in the active layer..
Recent theoretical work has highlighted the strong dependence of gap modes resonances upon conditions such as the shape and size of nanostructures, dielectric properties of the environment as well as the substrate and the distance between NP and the substrate [2]. Here, we fabricated a metal NP/poly(3-hexylthiophene)/metal thin film system to experimentally study the absorption enhancement due to single-nanoparticle gap modes in ultra-thin conjugated polymer films. The conjugated polymer film is less than 50 nm thick and is inserted in the gap region between NP and metallic plane to take advantage of the highly intensified near field. We aim at seeking experimental validation and identification of how the gap mode wavelength, intensity and degree of light localization can be optimized by changing factors such as: gap thickness, nanoparticle height, material type, incident field polarization. Single-particle dark-field spectra obtained from this geometry reveal the shift of the scattering peak originating from highly confined gap modes at the interface due to variation of the geometric parameters of the system. This experimental work seeks to systematically elucidate the effects of gap modes resonances on the optical properties of conjugated polymer thin films which will be constructive in understanding and utilization of gap modes resonances in optoelectronic device fabrication.
1. Yu, B.; Goodman, S.; Abdelaziz, A.; O&’Carroll, D.M.; Light-management in ultra-thin polythiophene films using plasmonic monopole nanoantennas, Appl. Phys. Lett., 101, 151106 (2012)
2. Hutter, T.; Elliott S.; Mahajan S., Interaction of metallic nanoparticles with dielectric substrates: effect of optical constants, Nanotechnology 24, 035201 (2013).

A hybrid photonic crystal structure is designed to modify thermal emission spectrum tailored to suit applications in biosensors, detectors and thermophotovoltaic devices [1]. The hybrid photonic crystal structure uses the effect of surface morphology on the optical properties to introduce the selectivity in frequencies for the emission. The thermal emission in a certain set of frequencies can be suppressed while allowing the crystal to emit radiation limited in a narrow band of frequencies.
The emission spectrum tunability has been confirmed by using experimental setup as well as theoretical calculations. A metal-dielectric composite with the top metal surface etched into sub wavelength nanostructures is made the base case for the emitter design. From thereon, various periodic arrays of rectangular or circular holes as well as gratings were introduced in the base case [2-3]. While simulating the emission spectrum different metals including Silver, Gold, Tungsten, Nickel and Platinum were considered. Also, changing the dielectrics with varying indexes was also seen to impact the shape of the emission spectrum. Effect of etch depth in such periodic structures was also taken into account. Variations in period of the array and the dimensions of each hole were also noted to affect the narrowness of spectrum. Such frequency selectivity can be attributed to phenomena including Surface Plasmon Polariton (SPP) coupling as well as diffraction by the grating structure [4].
References:
1. M. U. Pralle, N. Moelders, M. P. McNeal, I. Puscasu, a. C. Greenwald, J. T. Daly, E. a. Johnson, et al. Applied Physics Letters 2002, 81 (25), 4685.
2. A. Heinzel, , V Boerner, A Gombert, B Bläsi, V Wittwer, and J Luther, 2000.” Journal of Modern Optics 2000, 47 (13) 2399-2419.
3. H. Sai and Y. Kanamori, Microscale Thermophysical Engineering 7 2003, 101-115.
4. H. Sai, Y.i Kanamori, and H.Yugami. 2005. “Journal of Micromechanics and Microengineering 3003, 15 (9) S243-S249.

Despite the ability of plasmonic metals to confine and manipulate optical energy at the nanoscale, plasmonic studies in the visible are limited by significant ohmic losses at room temperature. By lowering the temperature, we observed moderate to large reductions in ohmic losses on optically thick silver films. We report dielectric constants in the visible obtained from ellipsometry measurements at various temperatures down to liquid helium temperatures, and tabulate the relevant Drude parameters. Furthermore, the temperature dependence of the dielectric constants is strongly influenced by the roughness and crystallinity of the silver surface. Ultrasmooth, single-crystalline silver films showed the largest temperature-dependent improvements whereas rough, polycrystalline films showed little enhancement. Below 50K, the surface plasmon polariton propagation lengths on single-crystalline films were enhanced by 30% at 500 nm to 55% at 750 nm compared to those at room temperature. In rough, polycrystalline films, losses are dominated by the temperature-independent scattering from the roughness and grain boundaries, so very small improvements were observed.

Electrode-embedded few-mode single-crystal electro-optic (EO) LiNbO3fiber (FMELiOF) with silica glass cladding and In2O3-SnO (ITO) electrode embedded between core and cladding with one-step process by CO2 laser heated pedestal growth (CO2-LHPG) technique is demonstrated.
A half-wave voltage (Vπ) of 4.5 V, effective electro-optic coefficient of 29.0 pm/V and transmission loss of 0.92 dB/cm are achieved in the FMLiEOF with the core diameter of 5 mu;m, electrode distance 20 mu;m, ITO thickness of 300 nm, and overall diameter of 125 mu;m.
A technical comparison of this new type of LiNbO3 EO fiber waveguide against conventional LiNbO3 planar waveguide and its new potential applications will be presented.

The research of highly luminescent, chemically stable and photostable emitters is one of the workhorses of hybrid plasmonics. The hybrid architecture based on a metallic core surrounded by a luminescent shell is one of the most popular. From the fundamental point of view, a deeper understanding of the energy transfer at the nanoscale is needed to increase the properties of such hybrid nanoobject. Particularly, the distance between the metal core and the active media is one of the key parameters.
In this work, we present a systematic study of the behavior of core-shell (Ag@polyelectrolytes) systems with a layer doped by rhodamine 6G (Rh6G) when the Rh6G is really close to the metallic shell (typically less than 15nm).
The core-shells are based on metal nanoparticles (NPs) synthetized by citrate reduction [1]. Then the shell is made by layer-by-layer deposition of polyelectrolytes [2, 3] used as spacer. When the desired distance metal-fluorophores is reached, a Rh6G doped layer is deposited.
The study consists in photoluminescence measurements on these structures in order to characterize the effect of the metal on the photoluminescence. We compare these systems with a reference, which is a system with a close geometry except a polystyrene core. The compared photoluminescence signal shows us 3 zones: close and far to the core the PL is quenched, but at a distance around 7 nm the silver core significantly enhance the Rh6G emission. A similar behavior is observed in the total decay rate through lifetime measurements. This demonstrates that core-shell can be used as model systems to analyze the coupling between an emitter and a metallic NP at sub-10 nm distances.
References:
1. Jana K. R. et al, Chem. Mater. 13 2001, 2313-2322
2. Schneider G and Decher G., Nano Letters vol.4, n°10 2004, 1833-1839
3. Schneider G and Decher G., Nano Letters vol.6, n°3 2006, 530-536

We fabricated a series of high performance white organic light-emitting devices (WOLEDs) based on a novel phosphorescent material, bis[2-(4-tert-butylphenyl)benzothiazolato-N,C2&’]iridium (acetylacetonate) [(t-bt)2Ir(acac)], used as yellow emitting layer (EML). The (t-bt)2Ir(acac) emitter with inherently narrow bandgap (2.3 eV) and low triplet energy level (2.2 eV), had outstanding property of trapping and recombining charges to emitting light, which was evidenced from the high photoluminescence (PL) efficiency and high electroluminescent (EL) efficiency of (t-bt)2Ir(acac). One WOLED showed high current efficiency of 79.0 cd/A (at 1550 cd/m2) and power efficiency of 40.5 lm/W (at 1000 cd/m2) with a maximum external quantum efficiency (EQE) of 22.01% by using ultrathin (t-bt)2Ir(acac) layer and doped iridium(III) bis(4&’,6&’-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) blue EML, which were separated by an interlayer. Similarly, another WOLED using the host-free system of ultrathin (t-bt)2Ir(acac) layer and host-guest doping system of bis[(4,6-difluorophenyl)-pyridinato-N,C2&’](picolinate) iridium (III) (FIrpic) blue EML showed a high current efficiency of 65.3 cd/A and a maximum EQE of 19.04% without any interlayer. The optical and electrical characteristics of these two devices demonstrated efficient energy transfer and proper energy distribution inside the devices. Moreover, we fabricated a stable white emission OLED by combined the three emitting materials, based on the blue and yellow complementary spectrum characteristic. The device performed pure white light emission with the Commissions Internationale de l&’Eclairage (CIE) coordinates varying from (0.29, 0.34) to (0.30, 0.33) while the bias voltage was changed from 6 V to 14 V. The WOLED with triple-phosphor-element EMLs had lower efficiency roll-off, owing to stable recombination zone. The high performance of the three WOLEDs showed the novel (t-bt)2Ir(acac) material was an efficient yellow emission material for white organic light-emitting devices.

Chirality is one of the most intriguing structural properties often exhibited by organic and biomolecular constructs. Their chiroptical activity, measured by Circular Dichroism (CD), typically appears in the UV range, and it is broadly used to probe a molecular stereometry. We have discovered that shaped non-chiral plasmonic nanoparticles, namely gold/silver core/shell nanocubes, induce a giant CD signal that is two orders of magnitude stronger for molecules (DNA) attached to the particles than for their free form. The enhanced chiroptical activity presents itself in the plasmonic band of nanoparticles, while a native molecular CD signal in the UV range is unaffected. Based on the experimental and theoretical comparison of nanoparticles of other shapes and materials, we demonstrated a uniqueness of silver nanocube geometry for the CD enhancement. Using the plasmon-induced CD measurements we investigated the changes in DNA configurations due to the environmental factors and hybridization. The discovered phenomenon opens novel opportunities in ultrasensitive probing of chiral molecules and for optical applications based on the chiral nano-elements.

The plasmonic response of solid, hollow, and stacked Au nanorectangles (260 nm x 200 nm x 30 nm) with separations of 0-430 nm has been investigated and correlated with their surface-enhanced Raman scattering (SERS) properties to determine how unique structural parameters can be utilized for enhanced sensing. Nanorectangles were fabricated in single and dimer forms using electron beam lithography (EBL) and arranged in an array format to allow for the characterization of specific nanostructures with a variety of techniques. The investigation of individual nanostructures made it possible to determine how their specific geometry (solid, hollow, stacked, slanted stacked) and dimer combinations (bridged, open-open, open-stacked, stacked-stacked) affected their plasmonic and SERS enhancement properties. The isolation of transverse and longitudinal plasmonic modes was achieved through polarized hyperspectral mapping and was corroborated with finite difference time domain electromagnetic modeling. Surface mapping of the nanostructures using Raman spectroscopy was also performed to determine how various structural properties like the stacked or bridged construction affected SERS hot spot formation and the corresponding SERS enhancement. This work provides insight into how unique structural parameters can be employed in EBL nanostructures to tailor their plasmonic properties and to rationally improve SERS sensor design.

Metal nanoparticles on semiconductors are of interest because
of the tunable effect of the surface plasmon resonance on the physical
properties of the semiconductor. Nanoparticles plasmon and dielectric
properties of the semiconductor are considered to have a mayor effect on
the luminescence of these hybrid systems. In this work, colloidal gold
nanoparticles were place on the top of porous silicon single layers. We
show that plasmon effects, refractive index and also surface chemical
modification have effect on the photoluminescence the porous silicon-gold
nanoparticle hybrid system.

Chiral plasmonic nanoparticles have been predicted to exhibit both strong circular dichroism in the visible spectral range and non-linear optical effects. These properties open up applications in sensing, enantiomer separations, and non-linear optics. Unfortunately, to date the fabrication of colloidal nanoparticles with chiral shape has posed a significant challenge due to a very limited ability to transfer chirality from chiral template molecules to nanoparticles. Here we report a novel route to enantiopure chiral colloidal gold nanoparticles. Importantly, we do not use in principle any chiral molecules or biomolecules and therefore create chiral shapes artificially. Template stripping is applied to anisotropically etched off-cut silicon wafers to prepare nanoparticles with chiral shape of only one handedness. Chiral colloidal suspension of 170 nm gold nanoparticles exhibit an extraordinarily high molar circular dichroism of 5x10⁹ cm^-1M^-1 and a record high anisotropy factor of 5.5%.

With the ever increasing need for ultra-fast devices, photonic devices have emerged as a viable option. Although major efforts have been made in designing this transformation, issues like metallic losses and unidirectionality still need to be addressed. In nano-regime, plasmons can efficiently guide and localize electromagnetic waves of visible-near infrared region at metal-dielectric interfaces. Here we report, using Finite Difference Time Domain (FDTD) simulations, a novel design of optical antenna for efficient communication at micro scale. Designed optical antenna emits highly unidirectional radiation as compared to the previously reported designs. Emitter, when excited by strategically placed quantum dot, channels the electromagnetic energy towards the receiver. The device has enhanced power transmission capabilities and better unidirectionality of radiation. This energy was found to be sufficient to excite chromophore and hence, confirm data transfer. This arrangement should be a strong contender for nano optoelectronic devices

We present an experimental study of the optical behavior of novel nanostructures by using a modification of the leakage radiation microscope (LRM) [1]. This technique has demonstrated that it is possible to quantify the near-field interactions between light and nanostructures with a diffraction-limited system. In particular, we observe the resonance modes in a deep circular cavity and the radiation pattern produced by a nanocoaxial structure. We found, in both cases, that the LRM can resolve the characteristic optical features associated with these nanostructures, particularly the plasmonic drumhead modes that was recently reported by our group [2]. Comparison with images obtained by near-field scanning optical microscopy (NSOM) confirms that our results are consistent and show the predicted physical effects. This modified LRM can be used in a wide range of small-scale optical investigations.
[1] A. Drezet, A. Hohenau, D. Koller, A. Stepanov, H. Ditlbacher, B. Steinberger, F.R. Aussenegg, A. Leitner, J.R. Krenn, Mat. Sci. and Eng. B 149, 220 (2008).
[2] F. Ye, M. J. Burns, M. J. Naughton, Nano Lett., 13, 519 (2013).

We have investigated localized surface plasmon resonance (LSPR) of silicon nanowires fabricated by metal assisted etching (MAE) technique. It was found that upon the use of hydrogen peroxide in MAE method degenerates the surface of nanowires and creates oxygen related defect sites. Surface morphology of the nanowires obtained through TEM and optical properties are investigated through PL and diffuse reflectance measurements. The surface characteristics obtained through FTIR. The defect sites obtained upon use of hydrogen peroxide during etching enhance the infrared absorption at higher frequencies and emit visible light by band to band radiative recombinations. Moreoever, almost a 30-fold enhanced LSPR was obtained for 36 nm thick Ag deposited 3 mu;m long Si NWs compared one-step etched Si NWs. This enhancement ratio increases to 100-fold for 24 nm thick Ag deposited samples. This enhancement was attributed to the change in the dielectric constant of effective medium created by defect states at the surface of the nanowires. Furthermore, it was also found that to get an overall enhancement, NW length as well as particle size are the two parameters. Enhancement in LSPR for silver nanoparticle decorated silicon nanowires can be useful for optoelectronic plasmonic devices.

Photonic quasicrystal in Fibonacci arrays are structures with
long-range order and interesting properties such as self similar spectra. When
this structures are made of active materials they may give rise to
resonant and strongly coupled when the exciton frequency
coincides with an energy pseudo-gap. In this work we construct 1D
quasiperiodic photonic arrays by the join of two Fibonacci substructures.
Each Fibonacci substructure follows the well-known recursive rule but in
the second substructure dielectric layers A and B are exchanged. These
quasiperiodic arrays gives rise to multiple perfect transmission states. The Fibonacci array are made of luminescence porous silicon. We analyze
experimentally the reflectance and photoluminescence and find a
remarkable increase in the photoluminescence from porous silicon at the
resonance modes.

In this study, green-colored remote-type light-emitting diodes (LEDs) were realized using an AgIn₅S₈-ZnS nanocrystals (NCs) film sandwiched with short-wavelength pass dichroic filter (SPDF) and long-wavelength pass filter (LPDF). The LPDF, which transmit green wavelength and block blue wavelength, was capped on top of the NCs film to prevent color mixing with blue and green color. The SPDF, which transmit blue wavelength and recycle green wavelength, was located bottom of the NCs film to enhance the green color conversion.
The AgIn₅S₈-ZnS NCs, which were synthesized by a hot-injection method, were dissolved in a mixed solvent (toluene and dimethylformamide (DMF) with a volume ratio of 2 : 1) and deposited on a glass or SPDF substrate using an electrospray (e-spray) method. The AgIn₅S₈-ZnS NCs film was solvent annealed with chloroform to enhance the uniformity and transmittance. The LPDF was capped on top of the AgIn₅S₈-ZnS NCs film and measured with a blue LED. TheAgIn₅S₈-ZnS NCs and NCs thin films were characterized by photoluminescent (PL), electroluminescent (EL), UV-Vis spectroscopy, X-ray diffraction and atomic force microscopy (AFM).

Recently, there has been significant interest in integrated optical isolators using magneto-optical (MO) materials, especially those based on yttrium iron garnet (YIG or Y3Fe5O12). Garnets have a mismatch in lattice parameter and thermal expansion coefficient with Si and this hampers the integration of magnetoopticalgarnets into silicon photonic circuits. However, there has been recent progress in integrating garnet films onto various substrates which has facilitated the development of a range of MO opticalisolator designs.
Here, we present pulsed laser deposition of Ce:YIG and YIG films on silicon nitride films and integration of magnetic oxides with optical resonators for obtaining strong magneto-optical response and nonreciprocal wave propagation. The measured Faraday rotation of the Ce:YIG films grown on silicon nitride exceeds 1500 deg/cm at lambda;=1550nm, while bulk Faraday rotation of Ce:YIG is ~4500 deg/cm. Both YIG and Ce:YIG films were measured to have saturation magnetic moment of 135 emu/cm3, rms surface roughness below 3 nm and coercivities between 5 Oe to 50 Oe for different samples. Refractive indices of magneto-optical YIG and Ce:YIG films was 2.19 and optical loss was 40 dB/cm.
While the process parameters developed here strictly apply only to Ce:YIG films grown on silicon nitride, the same process flow with different deposition parameters can be applied to the integration of any crystalline film onto any amorphous substrate that can withstand anneals up to 900°C.
We also present the integration of these oxide films on slab-coupled optical waveguides (SCOW) for inducing non-reciprocal loss or phase shift. Numerical simulations indicate that optical isolation can be achieved with isolation ratio above 20 dB using SCOW integrated with ring resonators. This concept can be applied for optical isolation, circulation, and modulation.
We grew YIG and Ce:YIG on ring resonator-loaded waveguides and measured 2 pm shift in resonance wavelength of the ring resonator for transverse magnetic polarization (TM) when the applied magnetic field was reversed. This shift is in agreement with the expected resonance shift calculated from the measured optical and magnetooptical parameters of the films. This study represents the first report of the theoretical and experimental demonstration of microring coupling into a resonant MO film for optical isolation.

Silicon photovoltaic devices have demonstrated improved power conversion efficiency with the inclusion of plasmonic Ag nano-islands above active layers [1,2]. However, these islands were produced by physically depositing Ag films onto stiff substrates at elevated temperatures, a process probably incompatible with roll-to-roll solar cell manufacturing. We investigate Ag nanoparticles, and their incorporation into thin polymer films, which could more easily be applied to large-area flexible substrates. Strong plasmon resonances have been demonstrated in monolayers of Ag nanoparticles with diameter d < 300 nm, when coated with thin polymer layers having thicknesses < d [3,4] on small batch samples. We study the enhancement of the short-circuit current density (Jsc) and solar spectrum-averaged efficiency in photovoltaic devices having thin (< 1 um) silicon layers. These devices are combined with a monolayer of plasmonic Ag nanoparticles having spherical, core-shell, and “nano-urchin” shapes, and different average inter-particle spacings ranging from less than one radius to several radii. We employ spectrophotometry, a solar simulator, and lasers to characterize the Ag nanoparticle-coated thin silicon films. We measure power conversion efficiency, and interpret it in terms of plasmonically-enhanced enhanced scattering, absorption, and light trapping. We model the effect of Ag nano-urchins, nano-triangles, and core-shell particles on Jsc and the open-circuit voltage, to predict the optimum particles shape, size, and arrangement for maximizing the power conversion efficiency in thin silicon photovoltaic films.
[1] F. Luekermann, et. al., Appl. Phys. Letts. 100 (2012) 253907.
[2] “Plasmon Enhanced Absorption in Photovoltaic Cells”, PhD Thesis of Jeffrey Philip Clarkson, University of Rochester (2010).
[3] M. K. Kinnan and G. Chumanov, J. Phys. Chem. C.114 (2010) 1796.
[4] R. M. Osgood III, et. al., Proc. SPIE 8096, 809610 (2011).

The hybrid nanostructures of light-emitting materials including metal nanostructures have been intensively studied because of an improvement and control of intrinsic properties through a nanoscale hybrid-junction of two different materials. For example, a promising approach for achieving high luminescence efficiency of light-emitting materials is the use of plasmonic resonance coupling effect by metal nanoparticles (NPs). Extensive researches have attempted to enhance the luminescence efficiency of light-emitting materials using nanoscale metals, through an energy and/or charge transfer effect in a surface plasmon (SP) resonance coupling. The photoluminescence (PL) of light-emitting organic materials incorporated with metal nanostructures can be enhanced when the energy of the SPs in the nanoscale metals matches the energy of the photons emitted by the organic materials. Herein, we introduce the plasmon enhanced phosphorescence emission using pure organic phosphors by incorporation of Ag NPs. We detect a phosphorescent enhancement about 100 times, which is originating from the activating triplet state by incorporation of Ag NPs. The plasmon band of Ag was well-matched with the conduction band of phosphors crystal, which can be generate strongly activate spin-orbit coupling and plasmon energy or charge transfer effect. It is worth note that the pure organic phosphors provide material for the plasmon enhanced phosphorescence.

We will show the novel chemical structures based on dual core chromophores for blue emission in OLEDs. One of the derivatives, TP-AP-TP, exhibits a high luminance EQE value of 7.51% and twice the lifetime of a commercialized material, MADN. The dual core chromophore materials have basically narrower PL and EL spectra and better color purity than single core chromophore compounds. Also, it has higher thermal property than the single core chromophore materials. The molecular design and synthesis as well as the device performance of novel organic molecules for highly efficient blue emission will be discussed.

Plasmonic materials and devices aim to exploit the unique optical properties of metallic nanostructures to enable routing and manipulation of light at the nanoscale. Lately this field has enabled exciting applications involving chemical and biomedical sensing, information and communication technologies, photovoltaics, lighting, optical encoding and surface decorations. One of the most significant challenges in delivering the aforementioned devices is the material&’s preparation methods in order to produce nanostructures with tunable plasmonic properties. In this work we present a novel engineering approach, namely Laser Annealing (LA), which is capable of subsurface modification of metal 2D structures alternating with dielectrics and meets the requirements of industry, such as fast and large-area processing, spatial selectivity and compatibility with sensitive organic substrates and reel-to-reel production.
We have recently demonstrated the photosensitivity of pulsed laser deposited (PLD) a-AlN doped with Ag nanospheres enabling the reconstruction of both the metal and the dielectric matrix, producing core/shell complexes (consisting of Ag nanoparticles surrounded by a thin layer of w-AlN in an a-AlN matrix) capable of encoding optical information. We further develop this approach by proving the suitability and effectiveness of LA for LSPR manipulation in metal-dielectric stratified media with well-defined sharp interfaces grown by magnetron sputtering; a technique that permits high volume production and is already incorporated in several industrial applications.
Multilayers consisting of alternate layers of AlN (initial layer) and Ag were developed. The bilayer thickness was kept constant at 15 nm and we varied the individual layer thicknesses of Ag and AlN. On top of the repeated structure we applied a cup layer of AlN in all cases, ensuring that the metal phase is not directly exposed to irradiation and ambient contamination, when the samples subjected to LA.
Upon LA and by varying the processing parameters, we delivered Ag nanospheres of various size distribution embedded in a hard, inert and durable ceramic material; suitable for applications under harsh environment. We quantitatively analyze the effects of LA (number of pulses, laser wavelength and fluence) in the processed stratified media not only in a prototype substrate, such as Si wafers, but also in two polymeric substrates: a flexible biaxially oriented polypropylene film (BOPP) and a rigid polycarbonic one. Our findings qualify these metal nanostructure arrays as potential candidates for core nanostructures in plasmonic devices with applications in optical recording/encoding, decorated optical lenses, packaging labeling, among others.
Acknowledgement: Co-funded by the Operational Program "Competitiveness and Entrepreneurship" and the Regional Operational Programs of the Greek National Strategic Reference Framework (NSRF) 2007-2013, Activity "SYNERGASIA II".

Metamaterials can produce new functional devices in which the optical response of a metamaterial can be engineered by manipulating designs and composition, in which their unique optical properties are not seen in individual components. Extreme tunability of metamaterials is limited by materials fabrication. Here we demonstrate the extreme tunability of the crossover wavelength ~0.45 to 0.7 micron in the near-infrared region using both external voltage as well as varying thickness of artificially engineered multilayered gradient films of Al:ZnO fabricated by atomic layer deposition. We anticipate that the multilayer explored here will open up a plethora of applications in the field of nano-plasmonic and metamaterials based devices for telecom and transformative optic applications in the NIR region. We will use several models, including electro-optic effect, and simulation to explain the extreme tunability in this transparent conducting oxides. We believe that the multilayer explored here will open up a plethora of applications in the field of subwavelength scaled electro-optic modulator and metamaterials based devices for telecom and transformative optic applications in the near-infrared region.
Acknowledgments: This work is supported by the DoD (CEAND) Grant Number W911NF-11-1-0209 (US Army Research Office), NSF-CREST (CNBMD) Grant number HRD 1036494.

Gold nanodots have received great attention in photovoltaics because they lead to enhanced local electromagnetic field intensity and thus improve the visible light absorption due to plasmonic effects. Moreover, they represent the preferred catalyst for the metal-induced growth of quasi-1D nanostructures, such as nanowires, proposed as novel architectures to improve the cell efficiency. Gold nanodots are usually deposited by sputtering because it is an easy, reproducible and low cost technique. A large quantity of papers present in literature deal with the morphological and structural characteristics of the Au nanodots after the sputtering and their interaction with the substrate, but their time evolution after the deposition during room temperature storage is less known. Our study focuses on the morphological and structural characterization of gold nanodots deposited by sputtering during their time evolution along a period of 14 weeks while stored at room temperature. Au nanodots have been deposited by radio-frequency sputtering on a <100> Si substrate pre-treated to remove the SiO₂ native oxide, and have been analyzed by using high resolution TEM. In order to avoid any modification of the material during the preparation for the TEM analysis, the samples have been thinned before the Au sputtering. A large statistical analysis on the distribution in size of the dots has been obtained by digital processing of the TEM images. The results indicated that, after the deposition, a percentage of the deposited gold forms nanodots with high surface density. The remaining part diffuses into the sub-surface regions forming amorphous agglomerates with lateral dimension of a few nanometers. These latter evolve, both in morphology and structure, while the Au nanodots instead preserve their characteristics all over the time. The evolution also involves that the amorphous nanophases undergo to ripening and tend to align to some crystallographic preferential orientations of the substrate.

In the last few years, great efforts have been dedicated to study the surface plasmon resonances in metallic nanostructures. This physical phenomenon consists in the collective oscillation of the conduction electrons of a metal excited by an electromagnetic radiation. Plasmonic properties of metal nanoparticles attract great interest owing potential applications in different fields such as electronics, photonics, biotechnology and Raman spectroscopy. The energy of surface plasmon resonances are known to be determined by the size, shape and composition of the nanostructures, therefore it is essential to be able to directly correlate the surface plasmon resonances with the structural properties at the nanoscale. In order to move one step further, we have nanoengineered the morphology of complex metal nanoparticles in order to obtain new plasmonic properties. By means of low loss EELS in an aberration corrected STEM, equipped with a monochromator, we have obtain the in-plane 2D distribution of the plasmonic resonances with sub-eV and sub-nanometer resolutions. The studied complex nanoparticles are different individual AuAg single and double nanoboxes.[1] We have investigated their local plasmonic properties at different parts (corners, edges, etc) and the effects of present voids in their walls. In addition to the mapping of individual nanoboxes, plasmon coupling between different nanoboxes is also studied.
[1] E. González, J. Arbiol, V. F. Puntes, Science, 334, 1377 (2011).

Near-field scanning photocurrent microscopy (SPCM) of graphene and indium arsenide nanowire field effect transistors (FET) with high spatial resolution has been performed. In SPCM, a laser is rastered across the surface of the device and the photocurrent is measured as a function of the laser spot position. Such measurements are extremely useful in studying the mechanisms involved in light-harvesting device such as solar cells and photodetectors and in elucidating fundamental physics of these materials.
The measurements have been performed using a home-made near-field scanning optical microscope constructed by modifying a commercial atomic force microscope. The standard optical detection of cantilever deflection was replaced by a quartz tuning fork based arrangement with an optical fiber tapered to approximately 80 nm acting as the tip. The measurements were done in shear-force mode with the direction of oscillation of the tip parallel to the scanned surface. A closed loop flexural piezoelectric stage was used to scan the sample. This yields a simultaneous map of the photocurrent and the surface topology and the two can be overlaid for comparison.
We estimate the spatial resolution of the photocurrent map to be less than 200 nm, this being one of the key advantages of near field illumination. The resolution is no longer diffraction-limited by the wavelength of light being used. This allows us to explore the photocurrent response of structures with dimensions much smaller than the dimension of the wavelength. We also present far-field SPCM data to contrast the near field measurements and to highlight the improved spatial resolution.
In this study, we perform near-field SPCM on two kinds of devices - graphene and indium arsenide nanowire FETs. Graphene is fabricated by mechanical exfoliation of graphite and indium arsenide nanowires are grown by the vapor-liquid-solid (VLS) growth mechanism. Source and drain contacts are fabricated by conventional electron beam lithography. The variation of the photocurrent with the source-drain and gate voltages has been studied which lets us to investigate the mechanisms responsible for generation of photocurrent such as the photovoltaic and photothermal effect.
Preliminary experiments have already been performed on InAs nanowire devices. The wires resulting from VLS growth mechanism are inherently tapered and one of the key observations from these measurements has been a variation in photocurrent signal along the length of the wire, with a stronger signal in the thicker part of the wire and a weaker signal at the other end.

Metal nanoparticles, such as gold and silver, present a fascinating research field due their surface plasmon absorption band, caused by the collective oscillation of the free conducting electrons at nanoparticle surfaces, upon interaction with the electric field of light. These plasmons can interact with surrounding luminescent centers leading to an enhancement of light emission from the centers. In our work we are investigating whether plasmons in noble metal nanoparticles can be used to also enhance the light emission from radiation detector materials based on optically stimulated luminescence (OSL) from radiation-induced defects. This dosimetric technique is largely used for radiation detection and dosimetry in medical procedures such as cancer radiation therapy and radiation protection. Its physical principle is similar to the classical thermoluminescence (TL) process, and is based on the emission of light by a previously irradiated insulator or semiconductor material. However, the stimulation of the irradiated detector occurs upon light exposure instead of heat, as in the case of TL (1). Electrons trapped in the band gap as a result of the ionizing radiation, are then excited by the stimulation light (usually green or blue light), and the relaxation produces emission in the ultra-violet region (2).
In our studies, we have produced ZnO films deposited on glass substrates with previously implanted ZnO seeds, in an aqueous solution containing several concentrations of silver nanoparticles. Zinc oxide has been chosen as an emitter because its band gap is very close to the plasmon resonance of Ag. Furthermore, ZnO by itself has already been shown to be capable of detecting ionizing radiation using several techniques such as OSL, TL and radioluminescence (3). XRD patterns of our nanoparticle films reveals them to be highly oriented but as the concentration of the AgNPs increases, the orientation becomes more random. At the same time, photoluminescence spectra reveals that the increase in the silver nanoparticle concentration decreases the intensity of the green emission band peaked at 630 nm but strongly enhances the near band gap emission band at 385 nm. The enhancement of the UV emission band we have seen makes the ZnO/AgNp films potential candidates for application in optically stimulated luminescence dosimetry. These and other properties, including microstructures and other optical properties, of these materials will be described.
1. Yukihara EG, McKeever SWS. Optically stimulated luminescence (OSL) dosimetry in medicine. Physics in Medicine and Biology 2008;53:R351-R379.
2. Eduardo G. Yukihara, McKeever SWS. Optically Stimulated Luminescence: Fundamentals and Applications. 1st ed: John Wiley & Sons Ltd; 2011.
3. Gorokhova EI, Anan'eva GV, Demidenko VA, et al. Optical, luminescence, and scintillation properties of ZnO and ZnO:Ga ceramics. Journal of Optical Technology 2008;75:741-746.

Highly luminescent metal-organic complexes of Eu3+ ions are finding applications in many fields of science including biotechnology and medicine. We explore a possibility of engineering optical properties of such complexes at nanoscale, by coupling them to plasmonic elements. Strong effects of coupling are observed in emission spectra, polarization and radiation patterns. Significant modification of absorption spectra in vicinity of silver is demonstrated as well. Experimental findings are compared with numerical simulations.

Surface-enhanced Raman scattering (SERS) and localized surface plasmon resonance (LSPR) spectroscopy have emerged as viable analytical tools for the detection of analytes. It has been suggested that large SERS enhancements are achieved when the analyte of interest is placed near a noble metal gap or crevice feature. However, the lack of reproducible high-throughput fabrication techniques with nanometric control over the gap size has limited practical applications. Here, we report a novel method for inexpensive, high-throughput fabrication of a periodic array of ring-shaped nanocavities with 10 nm gap size by combining nanosphere lithography with straightforward batch processing steps, namely, atomic layer deposition (ALD) and ion milling. Using the batch fabrication techniques, we have produced the nanogap structures in cm²-sized areas with the nanogap dimension below 10 nm. In the resulting hybrid nanostructure, incident light resonantly excites ring-shaped 10-nm-gap cavities formed alongside the curvature of Ag nanospheres. The resonant coupling between the Ag nanospheres and the ring-shaped nanocavity boosts light-coupling efficiency into the ultra-thin gap. Compared to conventional Ag films on nanosphere (AgFON) substrates, the addition of nanoring cavities improves the SERS enhancement factor by at least an order of magnitude, and the resonance wavelength can be readily tuned by changing the size of nanospheres used. After optical characterization of the nanoring cavity substrates, we also employ the nanogap structures for the detection of adenine, a commonly detected molecule in intrinsic DNA sensing experiments, and demonstrate improved detection limits as well as the use of the same substrate for concurrent LSPR biosensing. Considering the reproducibility, high-throughput fabrication, and high SERS enhancement factor, the developed method will have significant implications for the practical application of engineered SERS substrates.

Metal nanocluster (NC) have some attractive behaviors such as size dependent fluorescence, superparamagnetic properties and nonlinear carrier transport. Recently, highly fluorescent Au NCs are reported, which are important to applicant for biosensors, imaging labels and optoelectronic devices. Optical coupling between NCs is considered to be importance both fundamentally and technologically. In this study, we investigate the inter-dot distance dependence of photoluminescence (PL) of human serum albumin protected Au₂₅ (HSA-Au₂₅) NCs. We measured PL spectra of HSA-Au₂₅ NCs varying inter-dot distance. The inter-dot distance controlled by changing the density of NCs in water solvent. We observed strong luminescence of HSA-Au₂₅ at 1.9 eV with small swell at 1.7 eV. Photoluminescence excitation (PLE) spectra revealed that absorption band of 1.9 eV emission was at around 2.5 and 3.5 eV. With reduction of the inter-dot distance, the PL intensity was decreased and the peak position was slightly red-shifted. On the condition, the intensity of higher absorption band at 3.5 eV of PLE spectrum was relatively decreased. This shows that non-radiative transition probability of higher absorption band increased with decreasing inter-dot distance. The reason of this is not yet cleared, but one possible reason is energy or charge transfer to non-radiative intermediate state of adjacent Au₂₅ NCs. The results indicate that inter-dot distance plays an important role in the luminescent properties of HSA-Au₂₅ NCs.

We demonstrated a highly responsive photodetector based on novel two-dimensional In2Se3 nanosheets. Few-layer nanosheets of alpha-phase Indium(III) Selenide (α-In2Se3) are prepared by micromechanical exfoliation. The derived nanosheets are shown to be of single phase, highly crystalline, and nearly stoichiometric with low-level selenium defect density. Nanosheets with sub-ten-layer thickness were prepared and used to fabricate a photodetector device with intriguing properties. When operated in the photoresistor functional mode the In2Se3 nanosheet photodetector exhibits a response time of 1.8x10^-2 seconds, a responsivity of 1.04x103 A/W in the UV spectral region (400 nm wavelength) under 5 V bias, and a corresponding External Quantum Efficiency greater than 8.9x10^4 % with specific detectivity on the order of 9.4x10^12 Jones. The response of photodetector extended into visible and near-infrared spectral range. The exceptional optoelectronic responsivity of the In2Se3 nanosheet (with reference to its bulk counterpart) could be attributed to largely increased carrier lifetime and nano-size effects in the highly crystalline 2D material system. The results point to the potential pathway towards 2D semiconductor-based light sensors capable of real time imaging and detection in low-light conditions.

Dye-sensitized solar cells (DSSCs), which are one of the most potential photovoltaic devices, have attracted considerable attention as an alternative platform to the future renewable energy production owing to their high solar energy conversion efficiency as well as relatively low fabrication cost and simple process. The photon management is a crucial factor in solar cells to enhance the light absorption and thus their performance. In order to increase the capability of light harvesting, diverse efforts have been devoted by many research groups. Recently, photonic elements such as 1-dimmentional Bragg stacks and 3-dimmentional inverse opal structures are successfully introduced into photoelectrode to absorb the non-absorbed photons by light active materials. Another promising strategy to improve the light trapping effect can be coupling of surface plasmon resonance (SPR) effects of metallic nanostructures, which will lead to further enhancement of the photocurrent. Localized SPR (LSPR) phenomenon, collective oscillations of electrons in metallic nanostructures, is induced by excitation of hot-electrons that can lead to the optical field enhancement at the resonance wavelength. LSPR has very high sensitivity that is influenced by the size and shape of nanoparticles as well as the index of surrounding environment.
In this work, we introduce functional tailored hybrid nanostructures into the TiO₂ nanoparticles based DSSCs to improve the light harvesting of dye molecules and thus the performance of the cells. First, periodic organic-inorganic multilayer films are constructed by stepwise buildup of block copolymer (BCP) inverse micelles and included in the counter electrode as light reflecting layers. The multilayered BCP films exhibit strong reflective color and well-defined photonic stop bands. The layered films are integrated into the non-conducting side of counter electrodes and thereby remarkably enhanced overall power conversion efficiency is achieved, which can be attributed to the increased light harvesting by sensitized dye molecules. Second, plasmonic DSSCs containing tailor-designed Au-TiO₂ core-shell nanostructures with unique LSPR property are demonstrated and improved performance is achieved. Enhancement in efficiency can be attributed to not only the increase of the dye excitation by LSPR effects associated with near-filed enhancement and scattering but also generation of additional photocurrent induced by direct hot-electron transfer from plasmonic core-shell structures to TiO₂ conduction bands. The simple yet novel approaches presented in this work can be utilized as a generalized protocol and applied to other energy conversion devices to improve the device capability.

To set up a true ubiquitous environment, the optoelectronic devices, such as solar cell, touch screens, and displays, should be not only be portable; they must be able to change their shapes freely. In other word, these optoelectronic devices should be flexible. In order to embody fully flexible optoelectronic devices, conventional transparent anode materials represented by indium-tin oxide (ITO) should be replaced with any other flexible anode materials due to its brittleness. Furthermore, the increasing price due to scarcity of indium makes it difficult for its use in low-cost, large-area optoelectronics. Here, we embody ITO-free organic light-emitting diodes (OLEDs) using self-organized conducting polymers as transparent anodes without any hole injection layer (HIL). The self-organized conducting polymers are prepared based on poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) in which 5wt% DMSO is added to enhance their conductivity. As a key component, perfluorinated polymer (PFI) is included to achieve a high work function and hydrophobicity by inducing surface-enriched PFI layer. They show excellent tuning of work-function. The maximum work-function of the anode is 5.8 eV which is the highest value among the flexible anodes which have been reported until now. Moreover, transparency is higher than 90 % in all the visible range. Also, additional silver nanoparticle layer under our polymeric anode makes it possible to utilize the surface plasmon resonance effect. Our OLED devices based on our polymeric anodes with silver nanoparticles showed highly improved current efficiency and stability even with a simplified structure compared to conventional ITO/HIL devices. Even the stability of OLEDs based on our polymeric anodes was improved. Furthermore, we analyzed hole-injection characteristics between the modified polymeric anode layer and a hole-transport layer using dark injection space-charge limited current (DI-SCLC) measurement. We concluded that the improved properties of our polymeric anode based devices come from highly efficient hole injection and surface plasmon resonance effect.

Raman scattering in Si and SiGe nanowires (NWs) presents antenna effects strongly dependent on the electromagnetic coupling of the system laser/NW/substrate. The antenna effect of the Raman signal was measured in individual NWs deposited on either Al or Ge substrates, and also in air. The one phonon Raman band intensity in NWs, either Si or SiGe, can reach high values depending on the system configuration; values of Raman intensity per unit volume more than a few hundred times with respect to bulk Si were observed. The Raman intensity measured depends on the light polarization with respect to the NWs axis. The complex refractive index of the substrate plays a major role in the Raman enhancement, which was also observed to depend on the laser wavelength. The Raman intensity is determined by the electric field inside the NW; because of the sub-wavelength dimension of the NW diameter, the distribution and intensity of the electric field in the NW presents a marked dependence with the diameter. It adopts different distributions according to the dimensions, the wavelength and the surrounding media. Local spots of the electric field are observed inside the NW.
We present herein the calculation of the electric field in the NWs using finite element analysis (FEM) for different substrates. The theoretical solutions are illustrated with the experimental Raman spectra of individual Si and SiGe NWs. The results are discussed in terms of the electromagnetic interaction between the laser beam, the NWs and the substrate.

We studied the effect of annealing temperatures of gallium (Ga)-doped zinc oxide (ZnO) electron transport layer (ETL) on the electrical and optical properties of inverted quantum dot (QD) light-emitting diodes (QLEDs) with red-emitting CdS@CdZnS QDs. Metal oxide nanoparticles, recently, have been used as the ETL of QLEDs because of their superior processibility, electrical and optical properties. However, their electron mobility is limited up to 10-3 cm2/Vs. To increase the electron transporting ability, we adopted Ga-doped ZnO films deposited by sol-gel based spin-coating process as the ETL of QLEDs, and investigated the performances of QLEDs with different annealing temperatures from 150°C to 350°C. When the Ga-doped ZnO layer was annealed at 350°C, QLEDs showed the best performance in terms of low turn-on voltage and high external quantum efficiency of 5%. We found that the device performance was affected by the morphology changes of Ga-ZnO films as the annealing temperature was varied. The AFM analysis indicated that higher annealing temperature resulted in larger grain size of Ga-doped ZnO films while lower annealing temperature leaded to smaller grain size. The electron mobility of Ga-doped ZnO film with larger grains is higher than that with smaller grains since carriers are scattered at grain boundaries.

Gold nanoparticles can be utilized as plasmonic elements in making of an absorber from near infrared to visible region. Such absorbers have a lot of applications in photovoltaics and imaging. In this work we present a large area fabrication of gold nanoparticles based absorber using a thermal method on quartz and silicon substrates.
To make the gold nanoparticles, a thin layer of gold was deposited on the substrate (1.5 cm×1.5 cm) using DC sputtering then samples were annealed in the nitrogen ambient for 30 minutes at different temperature between 300, 550 and 750 °C. Gold nanoparticles are formed on substrate after annealing. Samples was fabricated with annealing gold layers with different thickness of 6 nm, 9 nm and 14 nm. The morphology of nanoparticles was studied using scanning electron microscopy (SEM). According to our result, the size of nanoparticles is dependent on the thickness of the gold seed layer prior to annealing, and the shape is dependent on the annealing time and temperature. In the next step nanoparticles are coated with the 130 nm of indium tin oxide (ITO) followed with deposition of 200 nm of silver layer as the back metal. The reflection light was measured using a UV-Vis reflectometer in the range of 400-800 nm wavelengths. Our results shows more than 70% decrease in the reflection at the resonance frequency (540, 580 and 670 nm) and between 40-60% decreases at all over the wavelength band. Absorber was also simulated using CST microwave studio software and simulation shows strong agreement with measurement results.

Au nanoparticles (NPs) coated composite microspheres have received considerable attention due to various applications in the fields of electronics, photonics, and biotechnology. Owing to the localized surface plasmon resonance (LSPR), Au NPs exhibit unique optical and electronic properties. Since these features can be tuned by controlling their sizes, shapes, and interparticle distances, the arrangement of Au NPs on the microsphere is one of the important factors to realize their unique properties. By fabricating close-packed Au NPs arrays on microsphere that have high surface-to-volume ratio and dispersibility in various solvents, such composites can be used in various applications, such as surface enhanced Raman scattering (SERS), medical imaging, photothermal therapy, and drug delivery. However, it remains difficulties to fabricate close-packed Au NPs arrays on the microspheres. In this study, we successfully fabricated close-packed Au NPs arrays with nanoscale gaps on polymer particles by simple mixing of pre-formed polymer particles and Au NPs via electrostatic interactions.
Aqueous dispersion of submicron-sized core-shell type polymer particles comprised of polystyrene (PS) cores and amino-terminated poly (butadiene) (PB-NH2) shells was prepared by evaporating tetrahydrofuran from a polymer solution containing PS, PB-NH2, and water. Since the surfaces of the polymer particles were composed of ionized PB-NH3+, the polymer particles had positive surface charges. When the aqueous dispersions of the polymer particles and negatively charged citrate-stabilized Au NPs with a diameter of 20 nm were mixed at room temperature, the Au NPs adsorbed to the polymer particles via electrostatic interactions. Transmission electron microscope (TEM) observation revealed that the Au NPs were incorporated into the PB-NH2 shell. Since the PB-NH2 has a lower glass transition temperature (Tg) than room temperature, the PB-NH2 shell has low viscosity. When Au NPs were adsorbed onto the surface of the core-shell particles, positively charged PB-NH2 chains attached to the negatively charged Au NPs to form ion pairs by electrostatic interactions. The adsorbed PB-NH2 chains formed core-shell like polymer blush structures around the Au NPs, as a result, Au NPs were encapsulated into the PB-NH2 matrix. When the shell thickness of the core-shell particle was compatible with the diameter of Au NPs, highly ordered Au NPs arrays were formed at the shells. Scanning electron microscope (SEM) and transmission electron microtomography (TEMT) observations revealed that close-packed Au NPs arrays with about 5 nm interparticle spacings were formed at the PB-NH2 shells of the core-shell type polymer particles. From the ultraviolet-visible absorption (UV-Vis) measurement, the fabricated composite particles exhibited a broad absorption band in 600~700 nm regions due to the plasmon coupling among adjacent Au NPs.

Many experimental systems consist of large ensembles of uncoupled or weakly interacting elements operating as a single whole; this is the case in many experimental systems in nano-optics and plasmonics including colloidal solutions, plasmonic nanoparticles, dielectric resonators, antenna arrays, and others. In such experiments, measurements of the optical spectra of ensembles will differ from measurements of the independent elements even if these elements are designed to be identical as a result of small variations from element to element, known as polydispersity. In particular, sharp spectral features arising from narrow-band resonances will tend to appear broader and in extreme cases can be washed out completely. Here, we explore this effect of inhomogeneous broadening as it occurs in colloidal nano-polymers comprising self-assembled nanorod chains in solution. Using a technique combining finite-difference time-domain (FDTD) simulations and Monte-Carlo sampling, we predict the inhomogeneously-broadened optical spectra of these colloidal nano-polymers, and observe significant qualitative differences compared to the unbroadened specta. The approach combining an electromagnetic simulation technique with Monte-Carlo sampling is widely applicable for quantifying the effects of inhomogeneous broadening in a variety of physical systems, including those with many degrees of freedom which are otherwise computationally intractable.

Interests in patterned polymer based nanodevices and sub-100 nm metal and transparent conducting nanostructured electrodes have led us to modify the traditional imprint lithography technique to enable fabrication of an array of sub-100 nm diameter electrode structures [1]. In this approach, an e-beam lithographed hard Si or SiC master is used to directly imprint a large area nanopattern onto polyacrylonitrile (PAN) film. The PAN film is then cured at ~ 200 °C to synthesize nanostructures. Metal and metal oxides can be coated with different thickness onto the PAN structures to create nanostructured electrodes. ITO nanostructured electrodes with nanopillars of 170 ± 8.8 nm diameter and 442 ± 10.6 nm high [2] encompassing a 1 cm × 1 cm area were fabricated. E-beam deposition of ITO resulted in thinner (up to 15 nm) sidewalls and a thicker coating on the top and bottom of the pillars. A thin layer of Ag at the center of ITO layer was deposited to increase sample conductivity, resulting in a multilayer sample with 70 nm ITO on the bottom, followed by 10 nm Ag and then 70 nm ITO on top. By optimizing the thickness of each of these layers one may optimize the trade-off between conductivity and transparency of the sample. The addition of the Ag also leads to localized and tunable surface plasmon resonances. At plasmon resonance the reflection of sample is minimized and absorption in the sample will dominate [3]. Experimental and simulated reflection spectra of this structure are in good agreement, including the appearance of sharp spectral features that are absent in a simple planar analog. The simulated Brewster angle of the nanopillars decreases compared to the planar reference sample by up to 10-13 degrees depending on the height of the pillars and appears to an indication of a reduced refractive index. The depolarization factor that was obtained by ellipsometry is about 0.05 and as expected from the ellipsoidal shape of the pillars [4].
References:
[1] J. Thomas et. al., “Nanoimprinting by melt processing: An easy technique to fabricate versatile nanostructures”, Advanced Materials 23, 4782-4787 (2011).
[2] A. A. Khosroabadi et. al., “Fabrication, electrical and optical properties of silver, indium tin oxide (ITO), and indium zinc oxide (IZO) nanostructure arrays” Phys. Status Solidi A, 210: 831-838. doi: 10.1002/pssa.201329129
[3] A. A. Khosroabadi et. al., “Hybrid Nanostructured plasmonic Core-Shell Electrodes”, manuscript under preparation.
[4] A. A. Khosroabadi et. al., “Spectroscopic ellipsometry on metal and metal oxide multi-layer hybrid plasmonic nanostructures”, manuscript under preparation.

Inverted polymer light-emitting diodes (IPLEDs) has been expected as a flexible display due to easy to fabrication, low process cost, large area application and possible roll-to-roll process. However, the air-sensitivity of Cs2CO3, electron injection interlayer (IL), limits their feasibility as a flexible devices and diffused Cs+1 ions reduce efficiency by inducing exciton quenching site in emitting layer. In order to solve these problems, we studied two air stable polymer interlayers (ILs), branched polyethyleneimine (PEI) and polyethyleneimine ethoxylated (PEIE), which can increase electron injection into emitting polymer more than Cs2CO3 by forming strong interface dipole. We also studied the effect of [N+]/[C] ratio of two polymer ILs on work function change of underlying ZnO by X-ray photoelectron spectroscopy (XPS). We can find that, with polymer ILs thickness, work function of underlying ZnO tends to increase by ultraviolet photoelectron spectroscopy (UPS) and exciton quenching tends to decrease by photoluminescence (PL). Finally, in IPLEDs with optimum ILs thickness, 8-nm PEI, we achieved high current efficiency (12 cd A-1) and power efficiency (4.5l m W-1) that were quiet higher than those with Cs2CO3 (8cd A-1 and 1.9lm W-1). The impedance spectroscopy is also used to study the roll of polymer ILs which block hole injection into ZnO in low bias and facilitate electron injection into emitting layer in high bias.

Fabrication processes of metal nanostructure arrays have attracted increasing attention because of its applicability of enhanced electric field of incident light based on localized surface plasmon resonance (LSPR) [1,2]. Metal nanowire is one of the candidates to enhance electric field of the light. In addition, the nanowires have large surface area. Various types of applications based on nanowires, such as photovoltaic cells, nonlinear optics and sensing devices, have been proposed. Precise control of geometrical structures of the nanowires is important because the properties of LSPR are substantially dependent on the shape and arrangement of the nanowires. However, processes to fabricate the array of the geometrically controlled metal nanowires have not been established. In the present work, we studied the fabrication of metal nanowires by mechanical deformation process using anodic porous alumina, and its application to a substrate for the surface-enhanced Raman scattering (SERS) measurement. One of the advantageous points of using the anodic porous alumina to fabricate nanostructures is that the shape and arrangement of the nanostructures can easily be controlled by changing the geometrical structures of the porous alumina. Electrodeposition process using anodic porous alumina as a template is usually used to fabricate the metal nanowires. There are several metals that have difficulties to electrodeposit. Al nanowire is attracted attention due to the properties of LSPR. However, it is difficult to fabricate the Al nanowires because of its difficulties of the electrodeposition. By the present process based on mechanical deformation, it becomes easy to fabricate the nanowires composed of various metals including Al.
Anodic porous alumina was fabricated by anodizing Al in acidic solution. The porous alumina was pressed onto a surface of the metal (Au, Ag, Al etc.) by using an oil press. The metal was mechanically deformed, then, filled each nanohole of the porous alumina. After removing the alumina, the metal nanowires were obtained. The diameter and length of the nanowires were precisely controlled by changing the geometrical structures of the porous alumina. The metal nanowire arrays were applied to the substrate for the SERS measurements. SERS spectra were measured using a Raman microscope (excitation wavelength: 633 nm). The obtained structure was dipped in a pyridine solution and dried in air before the measurement. The SERS peaks originating from the adsorbed pyridine molecules were observed at 1014 and 1040 cm-1. The intensity was strongly dependent on the geometrical structures of the nanowires. It is expected that the present process can be applied to fabricate functional optical devices, solar cell, chemical and biological sensing devices.
[1] T. Kondo, H. Masuda, K. Nishio, J. Phys. Chem. C, 117, 2531 (2013).
[2] T. Kondo, H. Miyazaki, K. Nishio, H. Masuda, J. Photochem. Photobiol. A, 221, 199 (2011).

Deposition of TiO₂ by reactive High Power Impulse Magnetron Sputtering (HiPIMS) was extensively researched during last years in terms of factors that influence film morphology as well as in terms of factors that affect state of plasma and thus pulse shape and deposition process and remarkable results were reported recently. HiPIMS as-grown polycrystalline TiO₂ films (rutile phase) have quality far superior to as-grown by DC magnetron sputtering in terms of roughness, phase homogeneity, refractive index and microstructure, and perform exceptionally as a part of high contrast Bragg mirrors, where it was demonstrated that transmission through metal film can be substantially enhanced by resonant plasmon tunneling making those heterostructures promising as transparent contacts for light-emitting devices. The major difference of HiPIMS from DC magnetron sputtering is the high energy density that is delivered to the growing film by the high voltage pulses at a certain repetition frequency. Since HiPIMS is relatively new modification of DC magnetron sputtering there is an ongoing research activity attempting to determine the factors that affect the plasma and deposition process in order to optimize its performance and efficiency. The shape of the pulse current contains a lot of information on the physics of deposition process and its dependence upon energy density, strength of magnetic field and factors that alter target oxidation was reported.
In this work we describe the effect that shutter state (open/close) has on target oxidation, and the pulse waveform during reactive deposition of TiO₂. This results in time dependent behavior of the deposition rate. TiO₂ films were grown by reactive HiPIMS in Ar/O₂ gas mixture at 500 C from metallic Ti target. The growth was studied during initial 10 min after shutter was opened, after the target was pre-sputtered for a long period with the shutter closed. With shutter opened the pulse current shape and process energy changed. The deposition rate which was high initially, 0.15 nm/sec, dropped to 0.115 nm/sec (>23 %) during the first 2 minutes and continued to decrease for the remaining 10 minutes of deposition. Film thickness and growth rate were determined from X-ray reflectivity. Simultaneously the integrated energy involved in deposition process was monitored. Film thickness was found to have a linear relation on the integrated deposition energy. Thus for materials where deposition rate instability is observed it is possible to calibrate and control the film thickness by monitoring the integrated pulse energy and end growth when the desired integrated pulse energy level has been reached.

Scanning thermal lithography (SThL) was demonstrated with the addition of energetic peroxides for synthesis and patterning of silver nanoparticles in polymer films. SThL samples were prepared by spin-coating poly(ethylene terephthalate) (PET) thin films preloaded with silver nitrate precursors and energetic peroxides, cumene hydroperoxide (CHP). Localized thermal analysis of these samples indicated the energetic peroxide decomposed at the temperature as low as 80 C. Above these temperatures, SThL thermal probe initiated the peroxide decomposition, which caused the localized discharge of exothermal energy close to the SThL thermal probe. This additional energy effectively compensated the joule shortage and the rapid cooling in the thin film samples. As a result, SThL fabrications can be operated at the relatively lower temperature, and the extra and localized joule energy significantly enhanced the chemical synthesis of silver nanoparticles, which were SThL-patterned and embedded in the polymer thin films. Polymer thin films were carefully characterized in terms of the dispersion of the energetic peroxide and silver precursor prior to the SThL process. After the SThL patterning, surface plasmon resonance (SPR) scattering of these silver nanoparticles were observed by the dark-field optical microscopy, as well as the transmission electron microscopy. Variations in the peroxide concentrations, SThL temperatures, and SPR signatures were investigated.

The phosphor screens used in current x-ray detectors are micron-sized powder phosphors and the spatial resolution is limited due to strong light scattering. The resolution limit restricts the usefulness of x-ray in various medical applications. Transparent scintillator screens can produce higher spatial resolution and have the potential to significantly enhance current medical and biological imaging techniques if the efficiency is improved. A photonic crystal layer on the surface of the scintillator can improve the light output efficiency. It can enable the extraction of photons which would otherwise be reflected within the scintillator due to total internal reflection.
In this study, we investigate the use of a photonic crystal layer on transparent nanocrystalline scintillators. The nanophosphors, terbium doped gadolinium fluoride nanoparticles, are embedded in an aluminosilicate glass-ceramic matrix. The scintillators were prepared by a melt-quench method followed by annealing. The gadolinium fluoride nanophosphors were precipitated within the glass-ceramic matrix during this process. The photonic crystal pattern was directly created on the top layer of scintillators material by electron beam lithography followed by reactive ion etching. The photonic crystal pattern and the scintillators were physically characterized by scanning electron microscopy, atomic force microscopy and transmission electron microscopy. Their luminescence and scintillation properties such as photoluminescence, photoluminescence excitation, reflectance and transmission were examined and compared with scintillators without the photonic crystal layer.

Ultra-low-reflective and super-hydrophobic surfaces are required for solar cells, large outdoor display panels, and large road signs to increase their performance and durability through self-cleaning in harsh environments.
It has been developed for well designed and micro/nano fabricated surface which has reflectance of less than 1-2 % in visible resign. However, it was still challenge to provide the solution for large area with low-cost and uniform properties. Here, we have developed a novel method to fabricate ultra-low reflective surfaces on Si with hierarchical nano-structures by the combination of Si and AlOOH (boehmite) nano-structures.
Using CF4 plasma etching, Si surfaces were nano-structured with pillar-like structures by selective etching with self-masking by fluorocarbon residues. AlOOH nano-flakes were formed by coating Al thin films on nano-structured Si surfaces and subsequent immersion in boiling water.
This process is relatively simple and available in large scale. Most of all, any kind of surface can be modified by this process as long as the materials can be survived in boiling water for 5 minutes. We tried this method on PDMS, glass, and silicon. As long as the adhesion of thin Al surface is stable, we could achieve the ultra-low-reflectance by the hierarchical thin film structures.
These hierarchical structures can be also coated with a low-surface-energy material which has higher water wetting angle (over 150 degree) and a very low contact angle hysteresis, implying a self-cleaning surface.
As a result, these surfaces have reflectance of less than 1.75 % in the wavelength of 600 nm and more.

Metamaterials have shown their remarkable applications in biosensing, bioimaging, and nanophotonics. Current studies regarding metamaterials as biosensor were only restricted in probing the extinction spectra (transmittance or reflectance), but however lack the complete understanding of metamaterials about its phase detection, limiting the performance in biosensing and bioimaging. Here, we experimentally demonstrate that a highly sensitive metamtaerial refractive index sensor by extrapolating its optical phase change instead of resonance spectra shift as the surrounding refractive index changes. To fabricate the metamaterial with resonance within near infrared, we use conventional electron beam lithography (ELS-7800, Elionix) and electron beam evaporation to deposit metallic layer followed by lift-off process. Next, to extrapolate the phase difference between TE and TM mode excitations, we apply a home-made optical setup and to excite metamaterials with different polarizations of incident light by Kretschmann-Raether configuration. With the freedom of scaling the metamaterials, we optimized the resonant wavelength of them to be close to the exciting laser so that the phase difference here becomes highly sensitive to the surrounding refractive index. The sensitivity of our metamaterials is around 17.1 rad/RIU, which is beyond the previous results of localized surface plasmonic resonance (LSPR) and metamaterial cases. We believe that phase interrogation is an alternative method in probing metamaterial resonance, and is able to complement the sensitivity insufficiency of metamaterial that interrogated by extinction spectra.

Sub-100 nm rings and dots, made of metal materials, are of considerable interest due to the unique properties from their peculiar morphologies and to their diverse applications in nanoscience and nanotechnology including optical devices, magnetic data storage, biological sensors, and catalysts for one-dimensional nanostructure growths [1-5]. For their practical implementation, the ability to fabricate uniformly controlled metal nanoring and nanodot arrays should be firmly established in effective processes. In this research, we report an efficient strategy to form sub-100 nm metal rings and dots by a printing method using new stamping platforms. Vertically aligned 1-dimensional carbon nanostructures of the ring-shaped and dot-featured tips, which are supported by the porous channels of anodic aluminum oxide templates, are used as stamps for transferring metal nanoring and nanodot patterns. Using the methodology, hexagonally oriented nanoscale metal ring and dot arrays such as gold, silver, platinum, copper, nickel, and aluminum are successfully printeded on various substrate substances in the laboratory environments. Moreover, the size, density, and interval of the printed metal nanorings and nanodots are able to be precisely controlled by modifying the geometries of the mother stamps. To this end, the characterization of the surface plasmonic resonance of the metal rings and dots are explicitly demonstrated by applying the printed metal nanoring and nanodot arrays.
Acknowledgement
This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea (0420-20120126) and by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2013029776 (Mid-career Researcher Program)).
References
[1] Aizpurua, J.; Hanarp, P.; Sutherland, D. S.; Kall, M.; Bryant, G. W.; de Abajo, F. J. G. Phys. Rev. Lett. 2003, 90, 057401.
[2] Banaee, M. G.; Crozier, K. B. Opt. Lett. 2010, 35, 760.
[3] Lodewijks, K.; Roy, W. V.; Borghs, G.; Lagae, L.; Dorpe, P. V. Nano Lett. 2012, 12, 1655.
[4] Lee, S. H.; Jo, G.; Park, W.; Lee, S. K.; Kim, Y. -S.; Cho, B. K.; Lee, T.; Kim, W. B. ACS Nano 2010, 4, 1829.
[5] Lee, S. H.; Cho, B.; Yoon, S.; Jeong, H.; Jon, S.; Jung, G. Y.; Cho, B. K.; Lee, T.; Kim, W. B. ACS Nano 2011, 5, 5543.

Titanium dioxide (TiO2) is an attractive material for optical applications for its high refractive index and transmittance in the visible spectral range. PECVD is a low temperature deposition method which is compatible with silicon based semiconductor technologies and allows the accurate tuning of film properties. However, one of the main issues in the case of PECVD TiO2 films used for optical applications is to modify the columnar or heterogeneous morphologies which are usually observed [1-4].
In this study, the effect of ion energy from oxygen rich O2/titanium tetraisopropoxide (TIPT) inductively coupled radiofrequency plasmas on TiO2 film properties has been recently investigated. In addition, it is also shown that incorporating SiO2 phase in the TiO2 network from O2/TIPT/Hexamethyldisiloxane plasmas leads to more homogeneous films. The properties of TixSiyOz layers including morphology, crystallization and chemical composition were analyzed by TEM, SEM, AFM, XRD, Raman, FTIR, XPS and WDS. The film optical properties and additional structural information were extracted by properly modeling of the spectroscopic ellipsometry measurements, while the film growth process was monitored by kinetic ellipsometry.
TiO2 films of ~300 nm deposited at floating potential (Vf) exhibit a columnar morphology which consists of a dense layer near substrate, an intermediate gradient layer and a top roughness layer. A small amount of anatase is identified in this film. By appling a d.c. substrate bias of -50 V, the columnar structure can be eliminated, with a more homogenous morphology along the growth direction, and the appearance of the rutile phase. The refractive index (at 633 nm) of the whole film has been greatly increased up to 2.49 for the case of -50 V. Then for the composite film Ti0.35Si0.65O2, no crystalline phase can be evidenced, whereas a more homogenous morphology and smoother top surface are observed. The formation of new phase, Ti-O-Si indicates the strong interaction for chemical bonding. Although the refractive index is reduced to 1.71 compared to pure TiO2 films, the optical properties of mixed oxide layers are improved on the whole for waveguide corelayers.
Finally channel waveguides with TixSiyOz core layers have been fabricated and their optical propagation losses characterized by the cut-back method.
[1] D. Li, M. Carette, A. Granier, J. P. Landesman, A. Goullet, Thin Solid Films 522 (2012) 366-371.
[2] A. Granier, T. Begou, K. Makaoui, A. Soussou, B. Becirc;che, E. Gaviot, M.P. Besland, A. Goullet, Plasma Process. Polym., 6 (2009) S741-S745.
[3] A. Borras, J. R. Sanchez-Valencia, R. Widmer, V. J. Rico, A. Justo, A. R. Gonzalez-elipe, Crystal Growth & Design, 9 (2009) 2868-2876.
[4] F. Gracia, F. Yubero, J.P. Holgado, J.P. Espinos, A.R. Gonzalez-Elipe, T. Girardeau, Thin Solid films 500 (2006) 19-26.

Ag is one important metal which has novel optical properties in the nanoscale size regime. Ag nanoparticles show a wide range of colors corresponding to their localized surface plasmon resonance. The combination of Ag and other precious metals in a single particle has become an attractive technique to limit its disadvantages, such as easy oxidation, difficulty in aqueous synthesis, sensitive to ionic environment, etc. One of the primary highlights for this combination is the Pt-Ag system for not only plasmonic properties but also electro-catalytic activity. Among several heterostructures, the core@shell structure could be very interesting and beneficial in combination of the catalytic properties of Pt and plasmonic properties of Ag. However, there are still many challenges in the synthesis of core@shell structures. Galvanic replacement reaction poses a challenge to synthesizing Ag@Pt core@shell structures, while successful formation of Pt@Ag core@shell must overcome lattice mismatch and oxidation of the silver shell. This presentation focuses on our recent results in synthesis techniques and surface treatments of Pt@Ag core@shell NPs with controllable size and shell thickness. Moreover, the electronic-transfer effect is found to be a potential tool in designing and enhancing plasmonic properties of this system. Understanding the electronic structure and behavior of Pt@Ag nanoparticles will provide valuable insights into the fundamental mechanism of electron-transfer. Results are discussed in terms of UV-Vis, XRD, TEM, HR-TEM, EDS, XPS, and HAADF-STEM.

Plasmon excitation of metal nanoparticles (MNPs) is widely studied in recent years. The plasmonic materials formed by MNPs can support subwavelength phenomena near optical frequencies and have strong near-field enhancement, promising for many plausible applications. To understand the underlying physics of the collective plasmonic response in the MNP array, an eigendecomposition method together with the free-standing Green function can be utilized to obtain the dispersion relations [1]. Localization characteristics and symmetric properties of a free-standing quasicrystalline MNP array [2, 3] have been investigated using the eigendecomposition method. It is found that spatial localized modes can either be leaky with out-of-plane radiation loss, or be very high fidelity. Apart from the free-standing case, the eigendecomposition method can also be applied to the MNP array sitting on multilayer, when we employed the layered Green function computed by Sommerfeld integration [4]. An effective eigenpolarizability, involving the collective effects of both the scattering of free-standing MNPs and the interaction between particles and layered structure, is well defined to characterize the dispersion relation and the mode quality of the plasmonic modes. In particular, we apply the new method to study the eigenmode of a one-dimensional periodic MNP chain near a metallic surface. It is demonstrated that the interplay between the surface plasmon at metal-dielectric interface and the localized plasmon in the chain enables strong modes splitting. The upper and lower modes show the in-phase and anti-phase properties between the MNPs and the metallic surface, respectively. In addition, for the polarization perpendicular to layer surface, high quality modes can be present inside the light cone even if the chain is open to air surrounding. A slow-light band is also predicted to exist as long as the layered medium supports surface plasmon mode that can couple to the chain mode.

Reference
1.K. H. Fung, and C. T. Chan, "Plasmonic modes in periodic metal nanoparticle chains: a direct dynamic eigenmode analysis," Opt. Lett., Vol. 32, No. 8, 973-975, 2007.
2.Jian-Wen Dong, K. H. Fung, C. T. Chan, and H.-Z. Wang, "Localization characteristics of two-dimensional quasicrystals consisting of metal nanoparticles," Phys. Rev. B, Vol. 80, No. 15, 155118, 2009.
3.Zi-Lan Deng, Z.-H. Li, Jian-Wen Dong, and H.-Z. Wang, "In-Plane Plasmonic Modes in a Quasicrystalline Array of Metal Nanoparticles," Plasmonics, Vol. 6, No. 3, 507-514, 2011.
4.Jian-Wen dong, and Zi-Lan Deng, "Direct eigenmode analysis of plasmonic modes in metal nanoparticle chain with layered medium," Accepted by Opt. Lett., Vol. xx, No., xx, 2013.

We search to amplify a signal into a strip-waveguide comprising an active layer co-doped with silicon nanograins and rare-earth ions stacked on a silica cladding layer and topped by a silica strip. The signal wavelength corresponds to the transition wavelength between two energy levels of a rare-earth ions. To achieve an efficient population inversion, the rare earth ions benefit from the indirect excitation via silicon nanograins which show an absorption cross section fourfold as high as the one of rare-earth ions. In order to study the influence of the type of rare earth on the optical gain, we consider two rare earth ions : neodymium ions Nd³⁺ for a signal wavelength at 1,06 mu;m and erbium ions Er³⁺ for a signal wavelength at 1,54 mu;m. The optical gain is expected to be different between Er³⁺ and Nd³⁺ doped waveguides since the former ion has a three-level electronic structure favoring the ground state re-absorption on contrary to the latter ion showing a four-level electronic structure.
To estimate the gain, we solve the Maxwell equations coupled to population rate equations and Lorentz-type polarization equations. We introduce an original algorithm based on the auxiliary-differential equation and finite difference time domain method (referred to as ADE-FDTD method). Using the conventional ADE-FDTD method we cannot have access to the steady state populations because there is a huge difference between characteristic time scales: the decay times of level populations (in the mu;s-ms range) and the period of the electromagnetic field (10^-15s). Our algorithm allows to know the steady state levels populations as well as the spatial distribution of the guided field in the waveguide.
Populations and electromagnetic field maps in the waveguide will be presented considering two pumping schemes: co-propagation pumping and top pumping. The optical gain obtained with erbium ions and neodymium ions for those two pumping schemes will be discussed.

A plasmonic concentrator is a device that couples incident light to plasmon oscillations in noble metals with the goal of increasing the intensity of electromagnetic fields in local regions. One example is the nanohole array (NHA), a long-range pattern of shallow sub-wavelength holes that have been etched onto the surface of the metal. In an NHA, the incident light is converted to a surface plasmon polariton (SPP) mode that propagates along the dielectric / metal interface. Thin films resting on top NHA-etched silver films have demonstrated broadband absorption enhancement (compared to the case of an uncorrugated surface). The nature of the enhancement depends on the pattern, symmetry, and the characteristic hole-to-hole distance of the array, as those properties determine the nature of the constructive / destructive interference between counter-propagating SPP modes. This has immediate applications for thin film photovoltaics, as including an NHA beneath a thin absorbing layer will enhance absorption without sacrificing the benefits of using a thin film.
The authors&’ previous work studied absorption enhancement in terms of the entire array. However, there were still numerous issues related to SPP generation, propagation, and interference that were not well-understood; for example, how different parameters (wavelength, hole dimensions) can affect the scattering efficiency of the nanohole, how the SPP behaves in and around its parent nanohole, how an SPP reacts if it encounters a different nanohole, how SPPs interfere in short-range patterns, etc. To answer these questions, this report studies the plasmonic behavior of NHAs as they are built from the ground up - starting with a single hole, then moving to simple patterns, then to short-range arrays, then to long-range arrays. Both experimental and simulated data was utilized to characterize these systems. For the experimental part, NHAs were etched onto silver films (~10² nm thick) using Focused Ion Beam (FIB) milling; numerous characterization techniques were considered, including plasmonic interferometry and localized absorption / fluorescent measurements. These systems were also simulated using Finite Difference Time Domain (FDTD) software, as well as homemade simulation programs. In addition to providing a stronger understanding of the optical phenomena in plasmonic concentrators, this research will undoubtedly have a positive impact in real-world applications where efficient scattering and strong intensity concentrations are required.

Nanoscale dipole antennas are interesting systems to study electric field enhancement effects. Within the gap between both antenna arms, a very strong electric field arises, which can stimulate molecules present in the gap. This enhanced electric field is well suited for the optical characterization of molecules, such as in surface enhanced Raman scattering.

We study gold antennas which are lithographically fabricated on thermally oxidized silicon. A confocal laser microscope was used to investigate their single-photon photoluminescence (PL). The photoluminescence of a monopole antenna follows well a model proposed by Boyd _{et al}. [1] if the energy peak position (E_max) is above the interband absorption edge (1.7-1.8 eV) of gold. The PL spectra of dipole antennas show a red-shift of E_max for decreasing gap sizes. We relate this behaviour to the increased coupling of individual arms via their optical near-field. The PL spectra are highly dependent on the shape of the antenna. For increased length or aspect ratio, we observe a decrease in the energy value E_max. We can accurately fit these spectra to the model [1] and estimate the dielectric properties of the environment. Interestingly for long antennas the spectra deviate from the model and show an additional peak. The latter peak show no geometrical dependence and remains at the energy value of the interband absorption edge. Our studies set the basis for future experiments on antennas embedding optically-active molecular compounds.

[1] G. T. Boyd et al., Physical Review B (33), 7923, 1986

Thin monocrystalline silicon derived from wafer-based source materials represents a promising materials platform for high performance photovoltaic systems as it can provide superior materials properties compared to polycrystalline or amorphous silicon but also form ultrathin, microscale devices in a fashion that allows versatile choices of module substrates as well as high degrees of mechanical flexibility. The performance of ultrathin silicon solar cells, however, is inherently limited owing to their weak optical absorption associated with the indirect bandgap of silicon. Consequently, strategies of photon management to enhance the absorption of incident solar radiation and therefore improve the performance and cost-effectiveness of silicon solar cells have been of significant recent interest. Here we present experimental and computational studies of ultrathin, monocrystalline silicon solar cells with engineered photonic nanostructures as new materials platforms for high performance flexible photovoltaics, which can be deposited over large area on soft, deformable substrates to form lightweight, mechanically flexible solar modules. Arrays of ultrathin, nanostructured silicon solar cells as ‘printable&’ forms are fabricated directly from high quality bulk <111> wafer, in which various types of subwavelength photonic nanostructures are implemented softlithographically on the surface of silicon solar cells for optimized photon management and therefore enhanced photovoltaic performance with significantly reduced consumption of active materials.
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Optical interconnection is heavily investigated as an alternative to electrical interconnection for data communication between and on processor chips[1]. Optics is recognized as probably the only technologically-viable solution to overcome physical limitations of electronics in fulfilling the future chip scale data communication performance metrics. Nevertheless, despite intense research efforts, it is still unclear how the currently-proposed silicon-compatible optical solutions can meet the size, driving voltage, and energy consumption requirements for chip-scale interconnection. Due to weak electro-optical interaction, optical modulation schemes appear to require high applied voltages, long interaction length, and/or precise temperature control.
In this context, we theoretically and experimentally investigate a novel modulation concept on silicon (Si) based on the combination of quantum confinement and plasmon enhancement effects, with a view to realizing very efficient light modulation around a telecommunication wavelength of 1.3 µm. We exploit the strong modulation mechanism based on quantum-confined Stark effect (QCSE) in Ge/SiGe quantum wells (QWs) and use plasmonic enhancement to enable a large overlap between optical field, active Ge/SiGe materials, and electrical bias voltages. Firstly, we experimentally study the suitability of Ge/SiGe QWs on Si as the active material for a plasmon-enhanced optical modulator. We demonstrate that in QW structures absorption and modulation of light with transverse magnetic (TM) polarization is greatly enhanced due to favorable selection rules. A very sharp absorption peak, and strong light modulation in a Ge/SiGe QW waveguide are reported from photocurrent and transmission measurements. Later, we theoretically and experimentally study the plasmon propagation of a metal-Ge/SiGe QW interface. We design and fabricate a novel Ge/SiGe QW structure that allows maximized overlap between the plasmonic mode and the underlying Ge/SiGe QWs. Specifically, we employ 5 periods of a 10 nm Ge QW and a 10 nm Si0.15Ge0.85 barrier embedded in a p-i-n junction with only a 10 nm thick n type (~4×1019 cm 3) top layer, on which an Al contact is realized in order to support both TM plasmon mode propagation and electrical bias for Ge/SiGe QW absorption coefficient modulation. Electrical simulation and experimental I-V measurements confirm good electrical properties of the diodes based on this Ge/SiGe QW design. Finally, from the simulation, the structure gives a very effective optical modulation performance of 1.5 dB/µm with only 1 V bias. The optical loss is 2.5dB/µm which is largely due to metal absorption loss in the near infrared regime, and could be further improved through emerging plasmonic metals [2]. The latest experimental results on the propagation and modulation characteristics of the tested devices will be discussed at the conference. [1] D Miller, Proc IEEE 97 2009. [2] G Naik et al., Opt. Mat. Ex. 2 2012.

GaAs is the material of choice for a wide variety of optoelectronic application. The light-matter interaction in this material can be additionally enhanced by using metallic nanoparticles, which increase the local electromagnetic field. Creation of such nanoparticles is a big challenge, since the semiconductor technology, such as molecular-beam epitaxy (MBE) that is appropriate for mass production, does not allow one to directly grow the metallic nanoparticles in the film bulk.
In this paper we show the ability to create both random and periodic arrays of metallic nanoparticles in epitaxial AlGaAs via a two-step procedure based on the technology of low-temperature MBE. In this procedure an oversaturated crystalline AlGaAs medium is first grown with built-in precursors for the subsequent heterogeneous precipitation of the pure arsenic or AsSb alloy. The second step is the post-growth annealing, which allows us to control the size and density of the metallic nanoparticles.
Optical study of the samples shows a strong impact of the metallic nanoparticles on the reflectivity of the hybrid medium. It is important that the target Al content in the AlGaAs matrix and AsSb composition of the nanoinclusions have been adjusted to match the frequencies of the Bragg and plasmonic resonances within the optical window of the host matrix. In the case of a random system of the nanoparticles we observe an enhanced optical absorption, whereas in the case of the regular plasmonic lattices we recorded a strong Bragg reflection of the incident light.

Optical antenna is widely used for biosensing at its resonant frequency since resonant antenna can efficiently convert propagating electromagnetic wave into localized energy. The energy conversion efficiency is determined from antenna&’s resonant natures and sizes. In this study, we propose a multiresonant metamaterial simultaneously sensing the refractive index change and molecular vibration signal at fingerprint bands. We amplify the molecular signal by manipulating incident polarized light; and the measured angle selective enhancement is 1.4 of part;ΔR/part;theta; . We improve the signal collective ability by utilizing an metamaterial array; and the sensing performance of refractive index sensor gains to 2547 nm/RIU, as against 1180 nm/RIU of single meta-atom. In addition, this designed metamaterial can be considered as a multifunctional plasmonic pixel, tracing the pictures of the distribution of refractive indices and molecules with resolution of 20000 d.p.i. Thus, an image of label-free and coupler-free cell observation is demonstrated structurally and functionally, providing new feasibility for multifunctional biosensor and bioimaing design.

Over the past decade, organic semiconductors have attracted significant attention as a potential alternative of prevalent silicon based semiconductor for achieving low cost fabrication, easy processing, good mechanical property, flexibility, and so on. Aiming at the realistic performance, wide range of organic semiconductor materials have been designed and synthesized. In particular, heteroarene-containing fused aromatic systems have been intensively investigated as a backbone unit, for organic field-effect transistors (OFETs) and organic solar cells (OCSs), due to their high charge carrier mobility and environmental stability. Among various heteroaromatics, nitrogen-atom-containing fused aromatics are one of the most promising materials to ensure easy functionalization, high solubility and strong electron donating nature, which are of great importance in view of energy level tailoring for various organic electronic applications.
In this presentation, we report on a high-performance bulk heterojunction small molecule organic solar cell (SMOSC) using novel class of nitrogen containing fused aromatic system, indolo(3,2-b)indole (IDID). Among various IDID-based molecules synthesized in this work, ODIDID4T-OCA, which employs IDID as strong electron donor and octylcyanoacetate as strong electron acceptor, with PC₆₁BM blend showed the organic photovoltaic device performance with 0.83 V of open circuit voltage (V_oc), 9.03 mA cm^-2 of short circuit current (J_sc), and 41% of fill factor (FF), resulting in 3.06% of power conversion efficiency (PCE) under the illumination of AM 1.5G, 100 mW cm^-2.

Nonreciprocal photonic devices play a key role in optical communication and computing technologies because of their ability to eliminate cross-talk and feedback. In this perspective, topologically nontrivial phenomena, such as photonic chiral edge states, also attract increasing interest because they offer unique opportunities ensuring reflection-free one-way transport of light even in the presence of strong disorder. In general, spectral nonreciprocity is encountered in composite media that lack both space-inversion and time-reversal symmetries. Therefore, the development of accurate full electrodynamic computational methodologies that can describe such low-symmetry media is of crucial importance. The layer-multiple-scattering method is a rigorous and versatile method for studying photonic crystals consisting of nonoverlapping scatterers in a homogeneous host material. Contrary to traditional band-structure or time-domain techniques, it solves Maxwell equations in frequency domain and thus one can allow the electric permittivity and magnetic permeability of any of the constituent components to depend on the frequency, including also dissipative losses. A further advantage of the method is that it does not require periodicity in the direction perpendicular to the layers, which must only have the same two-dimensional periodicity. As a result, architectures without any inversion center, such as chiral structures, photonic crystal slabs on homogeneous plates and substrates, semi-infinite photonic crystals, etc. can be treated in a straightforward manner. On the other hand, time-reversal symmetry is absent in gyrotropic materials under the action of a static, uniform magnetic field. In the present communication we present an extension of the layer-multiple-scattering method to photonic crystals of gyrotropic spheres and demonstrate its applicability on different nonreciprocal architectures, namely three-dimensional chiral structures, as well as surfaces and layers of fcc crystals of plasma spheres in a static, uniform magnetic field. A detailed analysis of relevant photonic dispersion diagrams and transmission spectra, in the light of group theory, provides a consistent interpretation of some intriguing features and effects like Dirac points, polarization-dependent transmission, as well as band splitting and nonreciprocal optical response. In particular, it is shown that, in a bulk helical structure of plasma spheres, nonreciprocal light transport takes place along the helix axis in the Faraday geometry, i.e., under a magnetic field oriented along the propagation direction. On the other hand, at the (001) surface of a semi-infinite fcc crystal of plasma spheres or at a monolayer of it deposited on a substrate, under an in-plane magnetic field, nonreciprocal propagation is demonstrated in the Voigt (Cotton-Mouton) geometry, i.e., perpendicular to the direction of the magnetic field.

In this work we compared two different optical techniques to sense Volatile Organic Compounds (VOC). VOC are organic chemicals with high vapor pressure at room temperature. This results in a large evaporation or sublimation from the liquid or solid form into the surrounding air. Some VOCs are dangerous to human health or cause harm to the environment.
Herein we present plasmonic VOC sensors, working at room temperature, obtained coupling TiO2-Pt nanocomposite films with Au nanoparticles (NPs) or depositing the same TiO2-Pt nanocomposite film on a sinusoidal surface plasmon metallic grating (SPG).
In the first case the Au Localized Surface Plasmon Resonance (LSPR) peak is used as an optical probe for the VOC detection as it is well know that its position, shape and intensity depends on the environments and/or the electron densities.
In the second case the sensitive material is deposited on a sinusoidal grating constituted by a gold thin layer embedded between two dielectric, one of which is the sensitive layer. The surface plasmon polaritons (SPPs) generated at the two metal-dielectric interfaces, result in two physical coupled modes: the long range SPPs (LRSPPs) and the short range SPPs (SRSPPs). The metal layer is in contact with the environment through the sensitive material, which changes its dielectric properties upon interaction with the target gases. Any change in the refractive index of the dielectric environment modifies the resonance conditions to excite the SPP.
Experimental and theoretical characterizations of the optical properties of the plasmonic devices are reported.
The aim of the comparison between localized and propagating SPR based technique with VOC is to investigate if the change of the refractive index of the sensitive film in response to the analyte is only due to the substitution of the air in the pores with the VOC, which has an higher refractive index, or can be connected to an exchange of electrons between the VOC molecules and the metal oxide matrix. We also want to find out which one of the two techniques is the best to sense this electron interaction, avoiding the refractive index effect.

Plasmonic nanostructures show great technological promise with applications including bioimaging, chemical sensing, solar cell enhancement, and fast hybrid photonic-electronic devices. Realizing their full potential will require convenient means for characterizing their optical and electronic properties and comparing those properties to theoretical predictions. Conventional optical imaging techniques can provide important spectroscopic information about plasmonic nanostructures but they lack the spatial resolution to probe nanometer-scale morphological and geometric features that have been shown to affect plasmonic behavior strongly. Nonlinear optical behavior, on the other hand, has been shown to be highly sensitive to those sub-wavelength size features [1].
Nonlinear optical imaging of metallic nanoparticles was previously demonstrated through second harmonic generation (SHG) [1-3], third harmonic generation (THG) [4,5], and two-photon luminescence (TPL) [6]. The nonlinearity stems from the localized surface plasmons resonant with the optical field. The frequencies at which the resonances occur within individual nanoparticles depend on the nanoparticle size, shape, and orientation, as well as the refractive index of the surrounding medium. The plasmon resonance can be strongly affected by interactions with nearby plasmonic nanoparticles. This coupling has been shown to enhance SHG signals from nanoantennas [1], periodic arrays of gold nanospheres [7], and gold nanospheres with controllable separations [8].
Through a combination of total internal reflection darkfield microscopy and nonlinear optical microscopy and spectroscopy, we survey the SHG, THG, and TPL signals from self-assembled gold nanorod structures. We then use scanning electron microscopy to correlate nonlinear optical signatures with the morphologies of individual nanorods and the geometries of nanorod assemblies. The nonlinear optical measurements are made using an ultrafast near-infrared laser whose second and third harmonics are resonant with the gold nanorods&’ longitudinal and transverse plasmons, respectively.
[1] J. Butet, K. Thyagarajan, and O. J. F. Martin. Nano Lett. 13 (4), 1787-1792 (2013).
[2] C. Hubert, L. Billot, P. M. Adam, R. Bachelot, P. Royer, J. Grand, D. Gindre, K. D. Dorkenoo, and A. Fort. Appl. Phys. Lett. 90 (18), 181105-1-3, (2007).
[3] E. J. Smythe, E. Cubukcu, and F. Capasso. Opt. Exp. 15, 7439-7447 (2007).
[4] M. Lippitz, M. A. van Dijk, and M. Orrit. Nano Lett. 5 (4), 799-802 (2005).
[5] O. Schwartz and D. Oron. Nano Lett. 9 (12), 4093-4097, 2009.
[6] D. Yelin, D. Oron, S. Thiberge, E. Moses, and Y. Silberberg. Opt. Exp. 11, 1385-91 (2003).
[7] B. L.Wang, M. L. Ren, J. F. Li, and Z. Y. Li. J. Appl. Phys. 112, 083102 (2012).
[8] A. Slablab, L. Le Xuan, M. Zielinski, Y. de Wilde, V. Jacques, D. Chauvat, and J. F. Roch. Opt. Exp. 20, 220-227 (2012).

The optical properties of small complex metal nanostructures, gold and silver, have been investigated in great detail over the past two decade. Specifically, these two metals exhibit localized surface plasmon resonance (LSPR), which is a collective oscillation of the conduction electrons when excited by electromagnetic radiation within the visible spectrum.
However, metal nanostructures with defects are hardly studied, due to that the ability to resolve the effects of the defects on the optical properties depends crucially on having a computational method in which the discrete positions of all atoms are explicitly represented, which cannot be addressed by conventional numerical methodologies(Discrete Dipole Approximation, Finite Element Method, Finite-Difference-Time-Domain Method, etc).
In this presentation, we report a computational atomistic electrodynamics investigation of the effects of planar defects on the optical properties of silver nanocubes, where the planar defects we considered are different surface orientations, twins, partial dislocations and full dislocations. Our simulation results show that for nanocubes smaller than about 3 nm, the optical response is very sensitive to the specific surface structure resulting from the defects. However, the sensitivity, as measured by shifts in the plasmon resonance wavelength, is strongly reduced at larger sizes, due to the decreasing importance of surface effects, even when the majority of the atomic deformation due to the crystal defects is contained within the interior of the nanocube. Overall, this systematic study suggests that the effects of individual crystalline defects on the optical properties of nanostructures can be safely ignored for nanostructure sizes larger than about 5 nm. This optomechanical coupling effects will have wide application in biosensing, mechanical strain sensing, and optical tagging and detection.

We have achieved the production of white light by illuminating nominally pure Y2O3 nano-powder samples with the beam of a laser diode operating at 803.5 nm with an output power of 3 watt. Some nano-samples (~20 nm) were prepared by us with a method based on the thermal decomposition of alginate gels. Some other samples were commercially available Y2O3 nano-powders (<50 nm) purchased from Sigma Aldrich. The white light spectrum ranged from 450 to 850 nm. Its intensity was not depending on the temperature of the samples, but was strongly dependent on pressure, with higher values at low pressure. We shall present some hypothesis on the mechanism responsible for this effect.
1. Department of Physics, Istanbul Technical University, Istanbul, Turkey
2. Department of Physics, Wheaton College, Norton, MA USA
3. Department of Physics, Boston College, Chestnut Hill, MA, USA

Dyakonov Surface Waves (DSWs) are novel surface waves capable of propagating at interfaces between two lossless dielectric media in which at least one medium exhibits optical anisotropy and where the relative refractive indices of the two media meet specific criteria. DSWs were first predicted to exist by M. I. Dyakonov in 1988 at an interface between two dielectric media where the first is an isotropic cladding material and the second is an uniaxial anisotropic substrate material meeting the DSW propagation condition εs,o < εc < εs,e. Since their first prediction, DSWs have been theoretically shown to exist in a number of additional dielectric media interface systems. However, to date their exploration has remained largely theoretical and limited to basic principles rather than their implementation in applications. As a first attempt to advance the basic science of DSWs into applications, this study explores the desirable low-loss, highly localized, and self-collimating waveguiding character of DSWs for the development of highly sensitive next-generation optomechanical sensors. The inherent extreme sensitivity of DSWs on the dielectric constants of the constituent system media results in optically observable changes in the DSW propagation characteristics upon the application of an external mechanical stimulus, instituting the functional mechanism for the proposed novel stress sensing devices. Such an effect is analogous to piezoelectric transduction, yet being optical in read-out, the proposed DSW based mechanism can be thought of as novel ‘piezo-optic&’ sensing. As such it breaks an inherent speed limit originating from the high impedance of electrical read-out in piezoelectric transducers. After establishing the necessary conditions for DSW propagation and the essential DSW propagation properties, a number of feasible isotropic / uniaxial birefringent DSW structures of interest for use in stress sensing applications are introduced and thoroughly investigated. Through theoretical analysis and simulations, the effects of dynamic stress stimuli on the DSW propagating characteristics are explored and presented. To achieve this, in-depth theoretical modeling of the proposed sensing device structures is performed, merging DSW and tensor analysis to predict the properties, benefits, and possible limitations of these systems. Furthermore, discussion of the proposed DSW based sensors is extended to practice and eventual device implementation. The expected benefits of DSW based sensing devices over current piezoelectric sensing devices, including increased sensitivity and speed, miniaturization in scale, and lower power consumption, are also presented.

Surface plasmon-based optics allows the manipulation of optical properties of matter at the nanoscale with length scales far below the diffraction limit. Recently, the resonant interfacing of surface plasmons with active materials in confined nanocavity architectures resulted in highly non-thermalized and efficient light emission from semiconductor nanowires. Here, we present results on photoluminescence studies performed at variable excitation energies on large (~150 nm) silicon nanowires coupled with metal nanocavities, which provides strong evidence for a hot-photoluminescence process in which individual hot (non-thermalized)-states are independent of the excitation wavelength, but the overall emission bandwidth is restricted by silicon&’s electronic structure. Size-dependent and excitation energy-dependent experimental photoluminescence measurements are coupled with FDTD simulations of the cavity modes. By examining cavity modes of the silicon-based nanocavities, at the excitation and emission channels, we are able to gain insights into the role of both surface plasmon polariton modes and localized surface plasmon resonances in modulating the broad spectral characteristics of the emission. Our results show that nanocavity plasmons can be utilized to efficiently produce and extract light from a “dark” material such as silicon, which typically has a very low emission quantum yield.

Devices in which sunlight is used as energy source to produce photon-to-electron conversion have been continually growing in importance in the last years (i.e. dye solar cells). The use of new compounds needs the proposal of novel optical architectures to improve the efficiency[1] or to modify the external characteristics of these devices.[2] We employ theoretical models based on transfer matrix method to describe light behavior into these types of layered structures[3] and to design new ones in which we can selectively control light confinement. The use of porous one dimensional photonic crystals allows us not only to spectrally enhance the photocurrent resonances but also to change the aspect.[4] We foresee the structures that we present could allow the development of photo-electrochemical devices with finer spectral control over light absorption and different aesthetic performances.
References
[1] Colodrero, S.; Forneli, A.; Lopez-Lopez, C.; Pelleja, L.; Miguez, H.; Palomares, E.; Adv. Func. Mat. 2012, 22, 1303-1310.
[2] Lozano, G.; Colodrero, S.; Caulier, O.; Calvo, M.E.; Miguez, H.; J. Phys. Chem. C 2010, 114, 3681-3687.
[3] Colonna, D.; Colodrero, S.; Lindstron, H.; Di Carlo, A.; Miguez, H.; Energy Environ. Sci. 2012, 116, (21), 11426-11433.
[4] Anaya, M; Calvo, M.E.; Luque, J.M.; Miguez, H.; J. Am. Chem. Soc. 2013, DOI: 10.1021/ja401096k.

Femtosecond laser micromachining has been used to induce permanent changes in the bulk of transparent materials in order to produce three-dimensional microstructures. Since the process involves nonlinear optical interactions, the material modification occurs only around the focal volume. Thus, surface damages are avoided by focusing the laser beam into the material bulk. Moving the sample regarding the laser focus it is possible to change the material properties with high spatial accuracy. Such proprieties can be effective changed by the formation of metallic nanoparticles during the irradiation process. Once metallic nanoparticles (NPs) exhibit ultrafast response times and high third-order nonlinearities, their spatial control in the micrometer scale can be essential for the development of all-optical devices. Although waveguides, splitters, amplifiers and resonators have been demonstrated using different techniques and materials, 3D-protonic microstructures containing metallic NPs have received little attention despite their importance for integrated optics. In this sense, the purpose of the current study was to produce waveguide containing metallic NPs in its core using femtosecond laser micromachining. To achieve this goal, we used copper doped borosilicate glass which was irradiated with a Ti: Sapphire laser, operating at 5 MHz with pulses of 50 fs at 800 nm (40 nm bandwidth). The laser pulses were focused beneath the sample surface through a microscope objective, while the sample was moved by a xyz stage at constant speed. Using an input energy of 74nJ and 10 mu;m/s of scan speed, we indentified the formation of metallic copper nanoparticles in the irradiated regions through the surface plasmon resonance band at 570 nm in the absorption spectrum. By doing so, cylindrical waveguides with a core of metallic copper NPs were obtained throughout the sample (3 mm), which have a transversal profile of 5 mu;m of diameter. An optical system, based on a He-Ne laser and microscope objectives, was used to characterize the propagation modes and losses of the light guided. Near-field output images showed a multimode waveguide at 633 nm. In this case, the propagation loss was estimated to be about 0.4 dB/mm. Single mode waveguides were also produced by changing the irradiation conditions. In summary, we have shown the fabrication of waveguides containing metallic copper NPs in glass using femtosecond laser micromachining. Low losses of the multimode and single mode propagation were obtained, which is a promising result for practical applications.

Rare-earth (RE) ions are well-known phosphorescent materials, which are practically used for many kinds of commodities, such as lighting, display, laser and so on. However, the optical absorption of RE ions is very weak due to the forbidden nature of intra-4f transitions. Sensitization by an energy transfer process is an attractive way to obtain efficient emission from RE ions. Recently, Eichelbaum et al. reported that photoluminescence (PL) of RE ions in silicate glasses is significantly enhanced by metal clusters precipitated by synchrotron X-ray irradiation. The enhancement was found to be caused by energy transfer from metal clusters to RE ions. Their results indicate that metal clusters have a great potential to enhance the effective excitation cross section of RE ions.
In this work, we focus on zeolite as a host material to study the sensitization of RE ions. Zeolite, possessing a cage structure consisting of orthosilicate ion (SiO₄ ^4-) and orthoaluminate ion (AlO₄ ^5-) structural units, is one of the most suitable host materials for the formation of metal clusters. Metal clusters are known to be chemically stable in the nano-sized pores in the cage, and can be formed by a simple ion exchange process followed by a thermal treatment. In this study, the effect of silver (Ag) clusters on the PL properties of RE ions in zeolite was investigated for the first time. We show that PL of europium (Eu) ions in the visible region can be significantly enhanced by Ag clusters in zeolite Y, due to the photosensitization effect. We also studied the sensitization of dysprosium (Dy) and samarium (Sm) in zeolite A and observed similar PL enhancement by Ag co-doping. The results suggest that zeolite is a promising host material for the sensitization of RE ions by metal clusters or ions.

When a molecule near the surface is excited by the large localized electric field, its Raman and/or fluorescence intensities are enhanced dramatically. These processes have generally occurred on the noble metal surface with nanostructure. However, one of the most crucial issues for fluorescence intensity enhancement is how to reduce fluorescence-intensity quenching via inherent energy transfer from excited molecules to the substrate metal. That is, when molecules are within a few tens nanometer from the metal surface, the energy of the molecules in an excited electronic state has to be effectively transferred to an electronic state in the metal band structure, which has a high density of state. Therefore, to obtain a large EF, it is extremely important to inhibit energy-transfer and fluorescence-quenching processes. One of the good candidates for reducing energy transfer is to use an indirect semiconductor as an enhancement substrate. Silicon (Si) is a typical indirect-transition semiconductor. In addition, there are the other good properties; (1) it is nontoxic, (2) it exists in great abundance, and (3) it is inexpensive. Therefore, if the enhancement effect is observed with Si, the development of economical enhancement substrates formed of a nontoxic quench-free material would be possible. Here, we show whether the fluorescence intensity of dye molecules can be enhanced using a silicon microarray substrate, synthesized using chemical etching together with silver ion. The fluorescence intensity of crystal violet (CV) solution was investigated using the Si array substrate. As a result, we obtained an enhancement factor of 30, which is as large as typical enhancement factors of noble-metal nanoparticles for fluorescence intensity enhancement of dye molecules. In addition, the repeatability for enhancement effect was very good.

Dye sensitized solar cells are one of the youngest members of the solar cell family and have been inspired by mother nature&’s process of photosynthesis, which involves separate process for light absorption and charge separation. Sensitizers play a crucial role in higher photon to electricity conversion which in turn dictates the efficiency of the dye sensitized solar cells. Classical ruthenium based dyes have been used and a lot of effort has been made to enhance the efficiency of these solar cells by modifying these dyes. Recently influence of mesityl, di-tert-butyl and/or tri-iso-propyl phenyl moieties with or without conjugation of ancillary bipyridyl ligand for improved photovoltaic parameters of dye sensitized solar cells have been reported and a record efficiency of 11.4% has been claimed and certified. The underlying mechanism resulting in these enhanced efficiencies is not well understood. In this pursuit results providing insight into charge transfer mechanism for different ruthenium based complexes obtained using first principles time dependent density functional theory will be discussed.

The harmful effects of particulate matter (PM) on health have manifested as immunological disorders like asthma, tumorigenicity, and heavy-metal toxicity, and, therefore, air pollution is a very serious issue. The World Health Organization (WHO) has compiled air quality guidelines, wherein acceptable environmental levels of various air pollutants are reoprted. PM is classified according to size. PM2.5 and PM10 are defined as fine particles which are permeated through a grading device with a 50% of sampling efficiency at a diameter of 2.5 mu;m (median diameter; 2.5 mu;m) and which are permeated through a grading device with a 50% of sampling efficiency at a diameter of 10 mu;m (median diameter; 10 mu;m), respectively. Additionally, fine particles with a median diameter ca. 7 mu;m, which are permeated through a grading device with a 100% of sampling efficiency at a diameter of 10 mu;m, are referred to as suspended particulate matter (SPM) in Japan. Environmental limit for the mass concentration of PM2.5 has been strictly established at less than 15 mu;g/m3 as an annual average with less than 35 mu;g/m3 as a daily average, and that of SPM has been established at less than 0.10 mg/m3 as a daily average with less than 0.20 mg/m3 as an hourly average. A rapid analytical technique with a wide detection range for PM is required.
In the 1960s, metal mesh devices (MMDs) with two-dimensional periodic structures were found to exhibit frequency selectivity, and could be used as band-pass filters. The frequency selectivity depends on the size characteristics of the MMDs. The electric field is localized around the surface of the MMD, and as a result a dipped structure is observed in the transmittance infrared (IR) spectrum of the MMD. Intrusion of matter into the electric field localized on the surface of the MMD leads to a shift in the dipped frequency. Therefore, MMDs is applied as a sensor. In this study, the detection of PM in air was performed using MMD sensing, where the MMDs functioned both as a membrane filter for the collection of PM and as a band-pass filter for the detection of PM.
A 100 THz-operated MMD with a 2.6 mu;m grid interval and a 1.8 mu;m opening length was used for the collection and detection of SPM. After the recovery of air through the MMD, the collection of SPM on the MMD was observed by optical microscope. Changes in dipped frequency in the transmittance spectra of the MMD containing SPM were found to have a linear relationship to the mass concentration of SPM, which was determined by the Ministry of the Environment in Japan, with a high coefficient of determination. The MMD sensor enabled the collection and detection of PM.

Effective light trapping inside active layer is essential to enhance the performance of the photovoltaic devices at lower active layer thicknesses. We took the advantage of light scattering by metal nanoparticle and strong local field enhancement due to localized surface plasmon resonance around the metal nanoparticle by embedding inside a semiconducting material. By placing spherical silver (Ag) nanoparticles inside hydrogenated amorphous silicon (a-Si:H), we measured the photo-current enhancement due to enhanced optical absorption. We analyzed the effect of size and position (depth) of metal nanoparticles on photo-current enhancement. 15nm, 25nm and 50nm silver (Ag) particle are embedded in a:Si:H and photo current are measured. The simulation results were also matching with the experimental results. We found that 15nm particles are enhancing the photo current about 2 times the reference structure when placed in the a-Si:H. Enhancement may be larger for larger particles by keeping constant coverage area rather than constant particle density.

Metallic nanostructures are of high interest because their optical properties can be tuned throughout the visible and near-infrared portions of the electromagnetic spectrum by adjusting nanoparticle shape, size, composition, and local dielectric environment. In particular, the localized surface plasmon resonance (LSPR) properties of plasmonic nanoparticles have shown promising applications in chemical and biological sensing, waveguiding, single particle tracking, and surface enhanced Raman spectroscopy (SERS).
Since inhomogeneous broadening from nanoparticle size and shape variations causes an increase in the linewidth of bulk LSPR spectra, monodispersity of the nanoparticle population is critical for optimal performance in LSPR-based devices and sensors. Towards that end, previous studies have focused on obtaining more homogeneous nanoparticle populations via post-synthetic separation techniques including size exclusion chromatography, gel electrophoresis, and centrifugation. Among these methods, density gradient centrifugation has proven to be particularly successful, resulting in narrow distributions of gold nanoparticle diameters, shapes, and aggregation state.
The shape-dependent optical properties of metal nanostructures have also motivated efforts to realize new nanoparticle shapes via novel synthetic routes and to correlate nanoparticle structure with plasmonic behavior. Among available plasmonic nanoparticle shapes, gold bipyramids (BPs) are of interest due to their sharp tips that lead to strong localized field enhancement and high sensitivity to the surrounding dielectric environment. In particular, {110}-faceted gold BPs consisting of two triangular pyramids joined by a (111) twin plane, have shown high stability and resistance against oxidative etching compared to other structures with near-infrared resonances. Despite their potential, relatively few reports have studied the optical properties of small BPs (i.e., BPs with edge lengths in the sub-100 nm range that show LSPR at visible wavelengths) due to their relatively low synthetic yields (~30% BPs).
Herein, we have developed a centrifugal route for separating sub-100nm, polyhedral plasmonic nanostructures, namely rhombid dodecahedra (RD) and triangular BPs, which form simultaneously during synthesis and cannot be separated via conventional filtration methods. Centrifugation in a shallow density gradient allows the enrichment of the original minority BP species, which enables a 2.5 times increase in refractive index sensitivity compared to the as-synthesized, unsorted mixture of plasmonic structures. Furthermore, these ensemble measurements are further corroborated by single-particle LSPR spectra with the distribution of the plasmon resonance energies of the sorted particles quantified via dark-field optical microscopy.

The phase reversal that occurs when light reflects from a metallic mirror produces a standing wave profile that features a reduced light intensity and thus light-matter interaction near its surface. This is highly undesirable in optoelectronic devices that employ metal films as electrical contacts and optical mirrors; it dictates a minimum spacing between the metal and active semiconductor layers in a device and poses a fundamental limit to their practical thickness. Here, we illustrate how a metamaterial-mirror can be created in the visible spectral range whose reflection phase is tunable from that of a perfect electric mirror (phi;=π) to a perfect magnetic mirror (phi;=0). This is accomplished by nanopatterning a conventional mirror surface with subwavelength grooves. We then demonstrate the benefits of implementing such a mirror in a real device. Specifically, we show how light absorption and photocurrent generation in a sub-100-nm active semiconductor layer can substantially be enhanced over a broad spectral band.

Biology enables solutions to many problems encountered in materials science in the form of exceptional molecular and genetic level control, chemical specificity, and the availability of a large diversity of highly functional biological templates. Therefore, to date, there has been extensive effort aimed at mimicking biological systems for the synthesis, functionalization, and self-assembly of extended nanomaterial structures. Notably, one approach has been to utilize peptide amphipiles for the controlled synthesis and self-assembly of 1-D nanoparticle helical structures. In this study, we have extended this approach by including gold-binding peptide dendrimer amphiphile with multiple peptide copies. Using this configuration of peptides and alkyl chains, we have increased the binding affinity to gold and created self-assembled helical nanoparticle ribbons which exhibited tunable plasmonic, chirooptical, and catalytic activity.

The plasmonic response of metallic nanoparticles depends upon the particle composition, size, shape, surrounding medium, and electromagnetic field coupling to neighboring particles and surfaces [1]. Localized surface plasmon (LSP) resonances are sensitive to changes in the surrounding dielectric medium, a phenomenon that has been widely studied and successfully used as an ultra-sensitive sensing platform [2], [3]. Here we present the mechanical components of a MEMS-based, micron-scale plasmonic microscope which utilizes the LSP response to map local variations in the dielectric properties of a sample.
We developed a MEMS actuator that can precisely scan a pair of plasmonic nanoparticles over a sample. By optically exciting plasmon modes, performing a scan, and measuring the change in intensity of the plasmonic response, we will map local changes in the dielectric properties of the sample. The device consists of two polycrystalline silicon probes, each containing a plasmonic nanoparticle. Using electrostatic comb drives and plates, we can independently actuate each tip and control the interparticle separation with a precision of several nm [4]. We can then move the pair of nanoparticles along two orthogonal axes, one in the plane of the device (x) and one out of the plane (z). A moveable stage below the nanoparticles enables us to position the sample relative to the nanoparticles along the y-axis. Combining these degrees of freedom, the device enables us to spatially scan the nanoparticles over a sample area and forms the mechanical basis of a micron-scale plasmonic microscope.
[1] Murray, A.; Barnes, W. L., Nano Lett. 19, 3771-3782 (2007).
[2] Mock, J. J.; Smith, D. R.; Sheldon, S. Nano Lett. 3 (4), 485-491 (2003).
[3] Roh, S.; Chung, T.; Lee, B. Sensors. 11, 1565-1588 (2011).
[4] Imboden, M.; Han, H.; Chang, J.; Pardo, F.; Bolle, C.; Lowell, E.; Bishop, D. J.; Nano Lett. (2013) to be published.

Zinc oxide (ZnO) is an attractive semiconductor for UV optoelectronic devices such as LEDs, photodetectors, and laser diodes, benefiting from its large bandgap (3.37eV) and high exciton binding energy (60meV). In order to fully realize the potential of ZnO for these applications, great efforts have been put into the study of fundamental optoelectronic properties since 1940s. However, in most cases, the light emission efficiency is low as a large number of carriers are trapped by defects or impurities inside luminescence centers. Recently, the resonant coupling between spontaneous emission of ZnO and surface plasmons opens a new route to enhance the UV emission and simultaneously suppress defects related visible emission.
In this work, we have demonstrated enhanced photoluminescence of ZnO nanorods by 1D and 2D metal gratings. 300 nm period gratings with different metals, i.e. Au, Ag, and Al were patterned on silicon (100) by laser interference lithography. ZnO nanorods were synthesized by hydrothermal method at 85 deg C on the patterned substrate. It is shown that the enhancement in luminescence strongly depends on metal materials and the height of ZnO nanorods. Preliminary results show the near band edge (NBE) emission at room temperature is enhanced by seven fold, which is attributed to the resonant coupling of spontaneous emission in ZnO with surface plasmons supported by the metallic nano-strucutres. The method presented in this work possesses advantages of low cost, large area periodic nano-patterns achievable and good controllability. It is believed that the luminescence enhancement will improve the efficiency of ZnO based optoelectronic devices.

We apply ultrafast optical probes to explore the nature of exciton polaritons in organic-semiconductor-based optical microcavities. Cavity polaritons are half-light half-matter quasi-particles arising when a photon confined in a cavity strongly interacts with an exciton. In practice this is obtained by incorporating a thin film of a semiconductor in a Fabry-Perot microcavity with an excitonic transition degenrate with the optical mode. The study of the dynamics of cavity polaritons in organic semiconductors is still at its infancy and this is remarkable because organic semiconductors represent a very promising frontier in this branch of physics: Their energy gap ranges from the visible to the near infrared and their oscillator strengths and excitonic bonding energies (sim;0.5 eV) are much higher than in inorganic semiconductors. This set of properties makes organic semiconductors in optical microcavities extremely promising for ground-breaking applications such as organic polariton lasers.
Here we present images of the far-field emission of a J-aggregates-based Fabry-Perot microcavity at 4 K. Such images were obtained by imaging the Fourier plane of an objective lens that collects the photons leaking out of the microcavity and they allow us to identify the energy dispersion of the polariton system. With the aim of clarifying the coherence properties of the polariton gas, we implemented an experiment of optical coherent control of the polariton state by resonantly exciting the polariton branches with a sequence of two femtosecond phase-locked pulses. The coherence state of the system can be controlled by cycling the relative phase of the two pulses at a given relative delay that is short with respect to the decoherence time. The resulting visibility is measured as a function of delay, thereby revealing coherence dynamics throughout the polariton dispersion curve.

A new method for trapping light in bulk-heterojunction (BHJ) solar cells is the incorporation of metallic nanostructures that support surface plasmons onto the electrodes of a device. A nanostructured metallic electrode can couple both photons and surface plasmons supported at the metal/semiconductor interface, where light may be converted into photocarriers. When applied to organic photovoltaics, this technique may allow considerable shrinkage of active layer thickness, while keeping effective optical absorption length constant.
However, for effective carrier collection the workfunction and electrode type (anode or cathode) must be carefully chosen. The results of theoretical calculations of open circuit voltage (Voc) establish that for inverted devices, the contacts that optimize Voc are copper (Cu), copper (II) oxide (CuO), and gold (Au), whereas silver (Ag) and aluminum (Al) are optimal for the conventional configuration.
In addition to this theoretical work, arrays of metal nanoparticles have been fabricated by thermally evaporating metal through nanoporous alumina masks to form nanostructured optically-active electrodes. Preliminary bright-field and dark-field spectroscopy measurements have been carried out on the arrays with and without the presence of a thin (<150 nm) film of poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methylester (P3HT:PCBM), to determine the enhancement in absorption and scattering of the solar spectrum. These measurements showed that uncoated silver nanostructures on a planar silver film enhanced extinction (sum of incident light absorption and scattering) by a factor of 2.3 relative to a planar silver film. In addition, dark-field scattering was enhanced by a factor of 0.7. Dark-field spectroscopic measurements of the silver nanoparticles on a planar silver film coated with P3HT:PCBM showed dark-field scattering enhancement of up to 7.5 relative to a planar coated silver film.
Current research has been focused on the fabrication of copper nanoparticle arrays on copper films so that enhancement in absorption and scattering can be determined and subsequently compared to the results for silver. So far, the challenges include the relatively fast oxidation of copper, the roughness of copper films when deposited via thermal evaporation, and the subsequent inability to distinguish nanoparticles from the rough particles that form the planar copper film. Possible solutions include the use of e-beam rather than thermal evaporation to achieve smoother films, so that deposited nanoparticles can be more easily distinguished. Ultimately, these measurements will establish which nanostructured material results in the largest absorption enhancement. This will determine their potential for use as “optically-active” metal electrodes in bulk-heterojunction solar cells. Successful developments in optically-active electrodes have the potential to push organic photovoltaics closer to competitive efficiencies.

In recent years, bimetallic nanoparticles with complex structure (heterostructures) have received considerable attention because of their novel optical, electronic, magnetic and catalytic properties. These heterostructures offer the opportunity to tailor the properties through manipulation of the particle composition as well as structure. For example, the plasmonic properties of traditional gold nanoparticles can be manipulated or tuned by coating with an additional plasmonic metal such as copper. Such a particle structure is expected to display unique optical properties as a function of the thickness of the copper shell, however there is still considerable concern about the stability of the deposited copper in terms of oxidation. It was recently discovered however that in the case for gold coated by silver nanoparticles, the silver gained electron density from the gold, resulting in resistance to oxidation of the silver shell. In light of the observation, we also expect that the properties of copper can be modified by coating on gold nanoparticle cores. To test the idea, we created gold nanoparticles via the citrate reduction method and then deposited an additional copper shell through seed mediated growth. The technique allowed the copper shell thickness to be finely controlled. The resulting particles display tunable plasmonic properties and enhanced chemical state as a result of electronic transfer between the gold and copper in the core shell structure. The presentation relates our recent findings in the synthesis and characterization of gold coated by copper nanoparticles with controllable structure and plasmonic properties and will discuss the role of electron transfer in modifying the properties of the copper shell. The results are discussed in terms of synthesis technique, and characterization of the optical properties using techniques such as TEM, EDS, XRD, XPS, UV-Vis as well as others.

The field of biomolecule detection has been rapidly advanced with the advent of gold nanoparticles (NP)s as sensing probes because these materials provide a highly sensitive mode of detection (plasmonic properties) and have a strong interaction with biomolecules through sulphur functionality. Gold NPs have been used as sensitive probes to detect molecules such as DNA, RNA, Protein, amino acids etc., enhancing our ability to identify and treat serious disease. Despite these advancements however, gold NPs still display some drawbacks, such as inability to manipulate the plasmonic properties of the material for a desired application. The synthesis of bimetallic NPs with sophisticated structures offers an opportunity to overcome this challenge by incorporating the properties of an additional metal to the NP structure. For example, gold coated by silver NPs have demonstrated remarkable oxidation resistance as a result of a unique electron transfer phenomenon that occurs at the interface of the two metals. Such an observation demonstrates the feasibility of integrating other metals with gold core NPs in the search for control of the plasmonic properties for these materials. To probe the plasmonic properties of this class of heterostructured NP system, we first created gold particles coated in copper. These particles displayed interesting electronic and plasmonic properties that arose as a result of electronic transfer between the gold and copper in the core shell structure. We characterized the electronic transfer using XPS analysis and studied the impact on the resulting particle stability and plasmonic properties. The results will be discussed in terms of the synthetic technique of the NPs, the structural and chemical characterization of the particles as well as the study on the electronic properties. The results provide insight into how to control the properties of plasmonic NP probes for sensing and diagnostics applications.

Molybdenum disulphide (MoS2), a layered quasi-2 di-mensional (2d) chalcogenide material[1], is subject of intense research because of its electronic[2] and optical properties[3].The mono- and Bi-layer (1L or 2L, for short) MoS2 crystals are produced by micromechanical cleavage of bulk MoS2 90nm SiO2 characterized by Atomic Force Microscopy (AFM) and Raman scattering analysis . Photoluminescence was obtained from the 1L 2L-and bulk MoS2 at room temperature peaked around 630 and 680 nm originated from the well known A1 and B1 excitons.[3] Electroluminescence was realized by the Au/Ti Schottky contact with single layer MoS2 in the three terminal device with a 2mu;m channel.
Surface Plasmon effect was usually used to enhance the luminescence, we synthesized Gold and Silver nanoparticles with the resonant absorption peak around 514nm and 680nm which are corresponding to the excitation laser wavelength and the peak of the luminescence from the 1L MoS2, tentatively we obtained the maximum enhancement of the luminescence around 30 times due to the surface plasmon effect. Optimal enhancement condition is still under investigation. The latest results and the luminescence enhancement mechanism will be reported in the conference.
References:
[1] Qing Hua Wang, Kourosh Kalantar-Zadeh, Andras Kis, et.al. : Electronics and optoelectronics of two-dimensional transition metal dichalcogenides Nat. Nanotech. Vol 7 (2012) p 699
[2]Hao Qiu, Xinran Wang et. al. : Electrical characterization of back-gated bi-layer MoS2 field-effect transistors and the effect of ambient on their performances . Appl.Phys.lett. Vol 100 (2012) p 123104
[3] Andrea Splendiani, Liang Sun,et.al.: Emerging Photoluminescence in Monolayer MoS2 Nano.Lett. Vol 10 (2010) p 1271.

Complete photonic integrated circuits on Si will strongly benefit from the development of appropriate direct band-gap group IV semiconductors. It has been predicted that direct band-gap can be achieved in Ge with 2% tensile strain without a need for heavy n-type doping and relaxed GeSn alloys are direct gap for Sn concentrations of about 8-10%. However, the use of strained Ge(Sn) layers in optoelectronic applications like semiconductor lasers demands the development of suitable barrier layers. SiGeSn ternaries are ideal solutions for forming promising quantum-well structures since their band-gap and lattice constant can be tuned separately.
In this contribution we first present a group IV semiconductor laser design based on a SiGeSn/strained GeSn/SiGeSn double heterostructure. Their calculated electronic band structures indicate type I band alignment and conduction (Γ-valley) and valence (HH-band) band off-sets sufficient to sustain population inversion and hence make lasing possible. We show that lower strain levels are required to induce direct transition in low Sn content GeSn alloys. For the proposed quantum well laser structure, we discuss via optical net gain calculations the optimum conditions regarding tensile strain, Sn content and n-doping for a maximized laser gain. Especially, the role and the evolution of intra-valence band absorption vs. carrier injection concentration are addressed. Then, we demonstrate, using an industry compatible RP-CVD tool with showerhead technology, the epitaxial growth of such GeSn/SiGeSn double heterostructures on Ge virtual substrates on Si(100) as indicated by simulations. In this context, we present the single crystalline growth of high Sn content virtual substrates (Sn content ~14%) which (may) induce a direct-gap in Ge and GeSn binaries with lower Sn contents as well as to tensile strain direct gap Ge0.9Sn0.1 laser active layers. Aiming for an electrically pumped laser, we present the growth of highly n- and p-type doped SiGeSn layers to be used for carrier injection.

The plasmonic resonances of metallic nanoparticles in the quantum size regime are not well understood owing to their complex physical/chemical behavior and ensemble inhomogeneity. For example, there are contradictory experimental reports on whether the local surface plasmonic resonances are red-shifted or blue-shifted as the particle dimensions are reduced [1-2]. Theoretically, the classical electromagnetic theory cannot capture the quantum resonances, while the Kohn-Sham density functional theory (DFT) cannot deal with particle sizes more than 2 nm. Here we propose a time-dependent orbital-free density functional theory with a dynamic kinetic energy potential term and local pseudopotentials, which provide an exact description of frequency-dependent linear response of uniform electron gas. With this method, we can afford to study size-dependent surface plasmonic resonances of Na nanoparticles with diameters ranging from 0.7 nm to 10 nm. We find that the surface plasmonic excitation energy in fact depends on the particle size in a non-monotonic manner and three resonance modes could coexist. Other novel features of the quantum plasmonic behaviors will also be discussed.
[1] J. A. Scholl, A. L. Koh, J. A. Dionne, Nat. 483, 421 (2012)
[2] S. Peng, J. M. McMahon, G. C. Schatz, S. K. Gray, Y. Sun, Proc. Natl Acad. Sci. USA 107, 14530 (2010)

Graphene has become a subject of great interest because of its unique electronic structure and optical properties [1]. Graphene has been also demonstrated to serve as a good exciton sink dominantly through Förster-type nonradiative energy transfer (NRET), e.g., from quantum dots (QDs) and dye molecules to graphene [2,3]. To date, high-quality epitaxial graphene has been widely studied. However, this requires epitaxial growth at elevated temperatures. On the other hand, solution-processed graphene variants including graphene oxide (GO) and reduced graphene oxide (RGO) offer attractive advantages that cannot be provided by the epitaxial graphene including ease of processing with low cost, large area coverage and tunable energy gap [4,5]. Therefore, NRET into GO and RGO, which may exhibit an exciton quenching range comparable to graphene [3,6,7], is particularly important to explore and understand the extend and feasible capacity of exciton sink as well as its required conditions.
NRET efficiency is strongly dependent on the distance between the exciton donor and acceptor pair [8]. To date, the efficiency of the exciton transfer into GO and RGO has not been studied in terms of the critical parameter of donor-acceptor separation distance. In this work, we studied systematically the exciton transfer distance in QD-GO and QD-RGO hybrid films using time-resolved measurements and modeling. These allowed us to analyze the excited state dynamics as a function of the donor-acceptor separation in detail. The time-resolved fluorescence spectroscopy results, reported here for the QD-GO and QD-RGO for the first time, confirm steady state fluorescence quenching. In our samples, thin QD coatings were achieved over uniform fully covered thin GO (1-2 ML) and thermally reduced thin RGO films. The NRET rates were found to be strongly dependent on the degree of reduction of the graphene based films. The separation of the QDs to the GO and also to the RGO was systematically controlled using dielectric thin films with thicknesses precisely varied by atomic layer deposition, demonstrating the spatial behavior of the resulting NRET. The NRET efficiencies were found to be around 97% and 89% for the closest separation of the QD-RGO pair and the QD-GO, respectively. These results reveal that the strong NRET from QDs into thin layer GO and RGO makes these solution-processable graphene-based thin films promising candidates to be used in excitonic light harvesting and detecting systems.
References:[1] K. S. Novoselov et al. Science 306, 666-9 (2004).[2] Z. Chen et al. ACS Nano 4, 2964-8 (2010).[3] L. Gaudreau et al. Nano Letters 13, 2030-2035 (2013).[4] K. P. Loh et al. Nature Chemistry 2, 1015-24 (2010).[5] S. Park et al. Nature Nanotechnology 4, 217-24 (2009).[6] J. Kim et al. Journal of the American Chemical Society 132, 260-7 (2010).[7] E. Morales-Narváez et al. Carbon 50, 2987-2993 (2012).[8] C. Kagan et al. Physical Review. B, Condensed Matter 54, 8633-8643 (1996).

Strong chiral optical activity can be achieved for nanoscale structures with three dimensional (3D) tetrahedral or helical out-of-plane geometries. Their preparation on solid substrate was achieved in the past using complex lithographic processes capable of producing the 3D plasmonic architectures which have limitations in respect to both spectral window of chiral optical effects and the size of substrates. Here we present a facile route to 3D chiral plasmonic nanostructures (CPNs) with controllable optical activity in the visible range without using any lithographic process or chiral templates. Out-of-plane 3D geometry of the coatings is obtained using ZnO nanopillars grown perpendicularly to the substrate. The chirality of the nanopillars is achieved via two-steps depositions of gold carried out according to a specific protocol: (1) evaporation direction was inclined in respect to the plane of the substrate resulting in one-sided deposition of gold shells on nanopillars; and (2) substrate was rotated between the deposition steps by an angle α resulting in the symmetry breaking. Circular dichroism peaks of the prepared films were observed at 680 nm, i.e. in the plasmonic region of gold and had opposite signs for α = +/- 90 deg. Polarization rotation originates from the coupling of incoming light comparable in wavelength with the nanopillars. Theoretical calculations of the optical properties confirm the experimental observations. The described lithography-less technique to obtain 3D chiral plasmonic materials represents general platform to obtain optically active chiral films for metamaterials, optoelectronics, biomedical, and chiral catalysis.

Enhancement of photonic resonator and nanoparticle interaction has been playing increasing important roles in next generation sensing, biomedical device and renewable energy fields. In this work, we demonstrate the applicability and universality of the phenomenological approach to the photonic resonance-nano particle interaction by focusing on resonances of a planar disk resonator interacting with a subwavelength, also planar, particle. In particular, we demonstrate that the response of this system to a TE-polarized excitation consists of two resonance peaks, both of which are shifted toward longer wavelengths from the initial single-disk resonance. This prediction, which is in contradiction with previous popular phenomenological model, is further confirmed by direct experimental observation of the peaks in the optical response of a silicon microdisk interacting with Si nanoparticles functionalized on its surface. Our results indicate that the assumption of direct coupling of the resonance modes of the resonator to the propagating modes of free space made is not sufficient to capture the essential mechanism of photonic resonance-particle interaction. A more appropriate model requires instead consideration of interaction between photonic resonance and modes of the particle, which are naturally coupled to the propagating modes. Our work will have a variety of applications for the performance enhancement of integrated optoelectronic materials and devices.

Cerium-doped lutetium-yttrium oxyorthosilicate (LYSO) with advantages of high light output (~30 000 ph/MeV), good energy resolution (~10%), and short decay time (~40 ns) has become an excellent scintillator for the applications in medical imaging and wavelength conversion optoelectronic devices. In spite of the high internal quantum efficiency of LYSO, the light extraction efficiency is rather low due to its high refractive index (1.83), which leads to a small critical angle. The extraction efficiency from one side of the crystal-air interface is as low as 7.5%. In order to achieve a significant increase of light extraction, photonic crystals have been widely used in inorganic semiconductor light-emitting diodes (LEDs) and organic electroluminescence devices. Although the research on the enhancement light extraction efficiency in LEDs has attracted great attention, the application of photonic structures to the field of scintillators is scarcely reported. For practical purposes in scintillation detection systems, large-area patterned photonic crystals fabricated in low cost are highly desirable. Therefore, two-dimensional photonic crystals consisting of a monolayer of self-assembled hexagonal-close-packed (hcp) dielectric spheres prepared in a very economical and effective way holds a great promise. In this work, monolayers of arrays of periodic polystyrene (PS) spheres are designed to couple onto the surface of cerium-doped lutetium-yttrium oxyorthosilicate scintillator to improve the light extraction efficiency. The enhancement of extraction efficiency up to 38% relative to the reference case without polystyrene spheres is achieved. Combining with the simulation for the transmission as well as its dispersion relation, detailed analysis of the effect of whispering gallery modes and diffraction on the extraction mechanism are given. As a result, the optimal diameter of 414 nm is obtained based on a trade-off between the transmission loss and the diffraction enhancement.

The conventional white light emitting diodes (WLEDs) combine a blue LED chip and a yellow [Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce)] powder phosphor coating with a silicone resin on the LED chip surface to obtain white emission. However, micro-size powder phosphor leads to limited conversion efficiency due to scattering and reflection loss of the emission. The transparent polycrystalline YAG:Ce ceramic plate phosphors (CPPs) have been used to achieve improved conversion efficiency. However, YAG:Ce CPP shows a low light extraction efficiency by waveguide effect and total internal reflection (TIR) because it is film-type phosphor.
In order to address this problem, a TiO₂ nanohole structure as two-dimensional (2D) photonic crystal was introduced on the YAG:Ce CPP by a combination of e-beam evaporation, thermal evaporation, nanosphere lithography (NSL) and reactive ion etching (RIE) processes.
First, a TiO₂ film was coated on the YAG:Ce CPP using E-beam evaporation process. Subsequently, the PS monolayers with various diameters (580, 720, 850, 960nm) were coated on the TiO₂-coated CPP via a scooping transfer technique, forming the PS/TiO₂/CPP structure. PS monolayer was thinned by RIE etching process to be used as deposition mask. A Cr mask layer for selective TiO₂ etching was coated on thinned PS/TiO₂/CPP by Thermal evaporator. After elimination of PS nanosphere, the Cr hole/TiO₂/CPP was etched by RIE process. The remained Cr mask layer was eliminated by wet-etching process. Finally, the TiO₂ nanohole-coated CPP was annealed at 450°C. The effects of the TiO₂ nanohole structures with various lattice constants (580, 720, 850, 960nm) were investigated on the extraction efficiency of YAG:Ce CPP. The structural, morphological and optical properties of 2D TiO₂ nanohole PCL-assisted YAG:Ce CPPs on top of a blue LED chip were investigated by scanning electron microscopy (SEM), photoluminescence (PL) measurements.

For almost 50 years, stimulated emission has been stronger and far more important than spontaneous emission. Indeed spontaneous emission has been looked down upon, as a weak effect. Now a new science of enhanced spontaneous emission is emerging, that will make spontaneous emission stronger and faster than any possible stimulated emission. This new science depends upon the use of nanoscale metallic optical elements, as antennas for spontaneous emission.
We call this antenna-enhanced effect “Spontaneous Hyper-Emission” (SHE). The overall increase in spontaneous emission rate can be up to 8 orders of magnitude, but will ultimately be limited the optical anomalous skin effect, which imposes extra Ohmic dissipation as quantum dots approach within nano-meters of metal surfaces. Currently we are using bare “naked” InGaAsP quantum dots, which have the remarkable property of radiating well, even without surface passivation, and even in direct contact with metallic optical antenna electrodes.
We will present the evidence for 35X spontaneous emission enhancement by an optical antenna.

Optically pumped organic microcavities rely on the quite broad gain spectrum of excited molecules, allowing to investigate particular photonic resonances of these microcavities. At sufficient pump power, distinct spectral regions with a particularly large photonic density of states show stimulated emission and lasing even at room temperature.

The systems analyzed in the present contribution rely on a lambda;/2 layer of Alq3:DCM (2 wt.%) embedded between two distributed Bragg reflectors (DBRs). With an additional 40 nm Ag layer inside this microcavity, the single resonance is replaced by two Tamm states [1], and for one of them, the quality factor remains sufficiently high to show lasing, with a pump threshold only about a factor of 3 above a metal-free lambda;/2 microcavity [2].
According to the Bloch theorem, photolithographic structuring of the metal layer into a stripe pattern with period a induces periodicity in wave vector space with 2π/a, so that each Brillouin zone contains a replica of the original microcavity mode. The crossing points of the microcavity dispersion branches enhance the photonic density of states at the center and edge of each Brillouin zone, allowing for lasing along wave vectors of 0, π/a, and 2π/a [3]. The Tamm resonances from the metal-covered regions are quantized into standing waves with discrete energies. Superimposed to the microcavity and Tamm resonances, the periodically patterned Ag stripes induce seemingly linear dispersion branches. These arise from Bragg scattering of surface plasmons up to very high orders, allowing eventually to detect these plasmonic resonances inside the air light cone [3].
Transfer matrix calculations of the multilayer system reveal the dispersion relation of the microcavity resonance, the Tamm states and the surface plasmons together with their envelopes across organic gain medium, metal layer, and DBR region. The plasmonic dispersion branches do not correspond anymore to the well known surface plasmon at a single interface, but they become merely a property of the entire arrangement with particular continuity conditions of electric field and dielectric displacement at each interface.
[1] R. Brückner, M. Sudzius, S. I. Hintschich, H. Fröb, V. G. Lyssenko, and K. Leo, Phys. Rev. B 83, 033405 (2011).
[2] R. Brückner, A. Mischok, A. a. Zakhidov, S. I. Hintschich, H. Fröb, V. G. Lyssenko, M. A Kaliteevski, and K. Leo, submitted (2013).
[3] R. Brückner, A. a. Zakhidov, R. Scholz, M. Sudzius, S. I. Hintschich, H. Fröb, V. G. Lyssenko, and K. Leo, Nature Photonics 6, 322 (2012).

The use of silicon nanocrystals (SiNC) in optoelectronic devices has risen from a decade thanks to the discovery of photoluminescence in porous silicon in 1991 [1]. However SiNCs exhibit a low quantum yield, which prevent from currently using them in optoelectronic devices. This problem can be bypassed thanks to the use of localized surface plasmons (LSP). Indeed LSPs can modify emitters&’ photoluminescence by changing the optical local density of states and/or increasing the local excitation field. Numerous studies have been realized with different emitters but the study of LSP coupled with SiNC is scarce. Seminal studies by Biteen and coworkers [2,3] have paved the way for increasing SiNCs&’ performances with LSPs. In these preliminary works only the SiNCs&’ emission wavelength was coupled to LSPs.
In this work, we present a study where LSPs resonances are coupled simultaneously with the absorption and emission wavelengths of SiNCs. Our original fabrication method gives us the control of all the geometrical parameters that modify the SiNCs-LSPs coupling. The use of nanorods with different shapes and metals allows us to have two plasmon modes. One is located at the SiNC&’s absorption wavelength and the other is located at the SiNC&’s emission wavelength. The SiNC&’s photoluminescence enhancement is thus due to two physical phenomena. To understand these phenomena, we study the influence on SiNC&’s photoluminescence intensity, polarization and spatial redirection. We also extract the SiNCs&’ radiative quantum yield enhancement in presence of these rods.
[1] A. G. Cullis, L. T. Canham, Nature 1991, 353, 335-338.
[2] J. S. Biteen, D. Pacifici, N. S. Lewis, H. A. Atwater, Nano Lett. 2005, 5, 1768-1773.
[3] J. S. Biteen, N. S. Lewis, H. A. Atwater, H. Mertens, A. Polman, Appl. Phys. Lett. 2006, 88, 131109.

It has been postulated that for an ensemble of dipoles situated at the surface of a plasmonic nanoparticle (NP) the interaction between emitters is of plasmonic character, rather than radiative, leading to the formation of collective states known as “superradiant modes”.[1] These results are an extension of the previous work by Dicke, in which he postulated the existence of such states for an ensemble of dipoles confined in a volume smaller than the radiation wavelength.[2] To the best of our knowledge, there has been no experimental demonstration of the plasmon-mediated superradiance near metal nanoparticles, this being a consequence of the difficulty to create a system in which the number of dipoles per plasmonic core and the distance between them are precisely controlled.
In our work, we have synthesized Au-CdTe nanohybrids as model system. This composite has been created attaching water-soluble CdTe quantum dots (QDs) at the surface of Au nanoparticles, while the distance between the emitters and the plasmonic core has been finely controlled using polyelectrolytes or silica as spacers. Moreover, the concentration of CdTe QDs in solution has been varied to obtain different populations of these NPs attached at the surface of the Au core. Time-resolved fluorescence spectroscopy has been used to study the light-matter interactions within the system, both in solution and in single particle measurements. This will allow us to determine the collective emission rates as a function of wavelength, concentration of quantum dots and QD-NP distance to finally compare the experimental data with the theoretical predictions.
[1] V. N. Pustovit, T. V. Shahbazyan Phys. Rev. Lett., 102 (2009), 077401.
[2] R. H. Dicke, Phys. Rev., 93 (1954), 99.

Surface plasmon (SP), due to collective oscillation of conduction-band electrons in noble metals, can be confined within a sub-optical-wavelength dimension and propagate at the interface between a conductor and a dielectric medium. It has received great attention due to the potential applications in ultrafast optical computation and communication. The integration of nanoemitters with plasmon waveguides for enhanced optical devices is of particular research interest. Great efforts are focused on assembling such devices, but it remains extremely difficult.
In this work, we used electric tweezers - our recently invented nanomanipulation technique - to assemble large ordered arrays of tip enhanced QD-plasmonic-nanoantenna hybrid device. By applying an optimized AC field on our designed microelectrode chip, randomly dispersed plasmonic nanowires can be readily aligned and self-assembled with essentially uniform interspacing on the top of microelectrodes. Then, QDs which were dispersed in this suspension were swiftly attracted to the tips of plasmonic nanowires. By coating a variable thickness silica layer on plasmonic nanowires, the fluorescent intensity and life time of QDs as a function of thickness were systematically investigated when QDs attached to the tips of the modified plasmonic nanowires. This work may inspire a new general paradigm for assembling large order arrays of tip enhanced QD-plasmonic nanoantenna hybrid device.

Advanced photonic materials are essentially based on structures sculpted to feature sizes comparable to or shorter than the wavelength of light. Even single-material structures may present interesting features such as light diffraction and photonic band-gaps; diffusion and localization, etc.
When several components are incorporated the properties expand and offer new possibilities. This is the case, for example, when passive dielectrics are mixed with chromophores or metals are combined with dielectrics. Beside the choice of materials the interplay between order and disorder occupies a central position in designing functions.
These structures can be turned into thermal switches, humidity sensors, ordinary or random lasers, etc. that can be prepared by inexpensive selfassembly and templating techniques. They most often involve water at some stage which imparts interesting properties whose nanoscale study links optics with mechanical properties. Some materials aspects presented by opals at micrometer scale can help understand seemingly as unrelated systems as wet granular materials. [1]
Lasers created upon these photonic structures can benefit from the possibilities offered by FRET phenomena such as in DNA-supported dyes. Such a strategy improves the performance of single dyes and expands their potential through tuneable coupling.[2]
Additional laser functionalities are enabled if beside the material and morphology it is possible to engineer the pumping: addressing selected modes and mode locking,[3] mode portation, switching, amplification, [4] as well as non-local collective processes.[5]
References
[1] Gallego-Goacute;mez, F. et al., Nano Letters, 12, 4920 (2012).
[2] Ibisate, M et al., Adv. Opt. Mater.in press (2013)
[3] Leonetti, et al., Nature Photonics, 5, 615 (2011).
[4] Leonetti, M et al., Nature Communications, 4, 1740 (2013).
[5] Leonetti, M et al., Light: Science & Applications, in press (2013).

We have fabricated a multilayered metamaterial consisting of alternate noble metal (Ag or Au) and dielectrics (TiO2, ZnO or Silicone) which has capability to operate in the visible region. Optical properties and both real and imaginary dielectric permittivity were measured. We demonstrated the tunability of the multilayer films which depend of the nature of the dielectrics as well as their layer thickness. The crossover wavelengths were measured using ellipsometry technique for each set of multilayer films in order to quantitatively determine the tunability (basically the dependence of crossover wavelength from positive to negative permittivity), which is found to match with our simulations. The absolute transmission reduces with increasing number of multilayer pairs due to metal absorption. We have optimized the system to fabricate grating structure for designing nanoscale emitters for light emission control. The detail results will be presented.

Three dimensional (3D) photonic crystals, defined by 3D periodic dielectric structures whose periodicity matches the wavelength of photons, have attracted much attention due to their potential applications as ultra-low threshold lasers, single photon sources, and nonlinear photonics devices. In this work, we report a new kind of a 3D photonic crystal structure, which consists of a sphere coated by multi-layers. When the optical thickness of each layer coincides with a quarter wavelength, the stack works as a Bragg mirror and can confine photons three-dimensionally inside the core. Due to the complete point symmetry about the center of the sphere, a high quality factor and a wide photonic band gap can be obtained. We investigate both experimentally and theoretically the optical properties of a multi-layered sphere.
Theoretically, we calculate the radiative decay rate of an oscillating dipole placed in a core of a multi-layered sphere.[1] The periodic layers are assumed to be yttrium oxide (Y₂O₃) and silicon oxide (SiO₂), possessing refractive indices of 1.91 and 1.45, respectively. The thickness of each layer is fixed to be a quarter wavelength. We obtained a sharp resonant peak of radiative decay rate and a broad photonic band gap beside them. The quality factor of the resonant peak increases exponentially with increasing the number of layers.
Experimentally, we prepare three layered sphere by using chemical solution processes in three steps. The first one is the preparation of core spheres of cerium-ion doped Y₂O₃ by a homogeneous precipitation method. The second one is the coating of SiO₂ layer on the surface of core particles by a sol-gel method. The final one is the coating of Y₂O₃ layer on the surface of the SiO₂-coated Y₂O₃ particles by a homogeneous precipitation method. The thickness of each layer can be estimated from the difference between the size distributions in each step and each layer thickness almost coincides with a quarter optical wavelength. We measured the photoluminescence properties of these particles. By coating Y₂O₃ layer on the surface of SiO₂-coated Y₂O₃ particles, spectral width of luminescence peak became narrow and the modification of PL spectra well coincides with the calculated result. This result suggests that the photonic mode density can be controlled by using multi-layered structure. [1] K. Imakita, et al., Opt. Express. 21, 9 (2013).

Green electrophosphorescent inverted top-emitting organic light-emitting diodes with a
Ag/1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) anode are demonstrated.
A high current efficacy of 124.7 cd/A is achieved at a luminance of 100 cd/m2 when an optical outcoupling layer of N,N;-di-[(1-naphthyl)-N,N;-diphenyl]-1,1'-biphenyl-4,4'-diamine (alpha-NPD) is deposited on the anode. The devices have a low turn-on voltage of 3.0 V and exhibit low current efficacy roll-off through luminance values up to 10,000 cd/m2. The angle dependent spectra show deviation from Lambertian emission and color change with viewing angle. Hole-dominated devices with Ag/HAT-CN electrodes show current densities up to three orders of magnitude higher than devices without HAT-CN.

Plasmonic hybrid materials incorporating metal nanoparticles and semiconducting materials such as conjugated polymers and colloidal quantum dots have attracted intense interests for applications such as optoelectronics and biosensing. Engineering of such hybrids via simple and cost-effective self-assembly methods is sought in order to control the plasmon-exciton interaction between components towards improved performance and depending on the foreseen application.
Here we demonstrate self-assembly of core/shell Au/SiO2 nanoparticles with a cationic conjugated polymer (polythiophene derivative) onto plasmonic hybrids with tunable photoluminescence output. By varying the SiO2 shell thickness in the rage 5-30nm and for Au nanoparticles of 50nm core size, we demonstrate the ability to tune the conjugated polymer photoluminescence from regimes of efficient PL quenching to PL enhancement of up to five fold. We do this by using alternating laser single particle confocal photoluminescence spectroscopy by exciting outside and onto the Au surface plasmon resonance. The utility of the proposed hybrids for optoelectronic applications is demonstrated by coating the Au/SiO2/conjugated polymer plasmonic hybrids with fullerene acceptor molecules for which we observe plasmon assisted enhancement in the charge transfer rate between polymer and fullerene.

Due to an exponential increase of information processing and communications traffic requirements, there are needs for active devices for photonic integrated circuits (PIC) that operate at 50 GHz and above. We have taken an approach of using ferroelectric oxide thin films with high electro-optic (EO) coefficient for use as modulators in PIC. Thin film barium titanate (BaTiO3) has shown promise due to its intrinsically large EO coefficients and its potential for integration with silicon. Nevertheless early on it was believed that dielectric loss would limit the high speed operation of the modulator. Furthermore, the high dielectric constants of BaTiO3 would make it impractical to phase velocity match. By using thin ferroelectric composites, however, the effective microwave index of the composite structure is significantly lowered and operation of modulator at gigahertz frequency is possible. To lower the operating voltage, recently we have demonstrated that two-dimensional photonic crystals of BaTiO3 have resulted in EO modulator that operates at 17 GHz with transistor-transistor logic level driving voltage for a 3 mm long device. Simulation of the modulator&’s frequency response indicated that reduction of microwave losses due to metal contacts and the dielectric medium would result in bandwidths of greater than 50 GHz. One way to reduce the losses of the modulator is to decrease its size. Here, we report the simulation, design, fabrication and characteristics of a millimeter scale, EO modulator operating in the V-band at a wavelength of 1550 nm based on BaTiO3 thin film platform. 50 GHz devices were demonstrated using two-dimensional photonic crystal arrays and optimized electrode geometry based on recent EO simulations. The modulators at high frequencies of 20 to 50 GHz showed peaks in the EO response that were attributed to microwave resonances. While these prototype devices were fabricated on dielectric substrates, it should be noted that BaTiO3 films can be integrated with silicon, making ultra-wide bandwidth silicon photonic circuits possible.

Active control of light at the nanoscale with plasmonic waveguides is a key approach for future photonic/electronic chip integration. We combine the high modal confinement achievable in plasmonic waveguides and large changes plasmonic mode effective index field-effect induced changes in transparent conductive oxides (TCO) active regions to design modulators with potentially high dynamic range and low loss modulation [1]. We report here an ultracompact chip-based channel plasmon slot waveguide field-effect modulator operating near 1630 nm wavelength based on a transparent conducting oxide active region. Numerical simulations show a significant enhancement of modulation depth by introducing a 70nm x 400 nm nanocavity with a localized mode along the waveguide path. Resonant interference between the waveguide mode and the localized mode to increases the modulation strength to more than 30 dB. Changing the width/length of localized mode cavity enables the resonant interference wavelength to be tuned by 150 nm, paving the way to design of wavelength-specific plasmonic add-drop filters [2]. In this work, we investigate the integration of TCO material into recently developed plasmonic slot waveguides [3] to form an efficient modulator. The slot waveguide is fabricated with electron beam lithography on silica, consisting of 200 nm Au layers with a waveguide slot width of 300 nm. To form a modulator, a thin layer of aluminum oxide is deposited for isolation and indium tin oxide (ITO) is sputtered on top as an active plasmonic material, with the Au and ITO layers serving as the electric contacts for the modulator. To demonstrate the modulation ability of the cavity structure, a 5 nm accumulation layer with different carrier concentration is introduced between the gold and ITO. Simulating the structure with FDTD electromagnetic calculations [1], we find that the resonance wavelength shifts to shorter wavelength for more than 50 nm for stronger carrier modulation layers (from ITO carrier concentration n = 1 x 1019 to 2.5 x 1020 cm-3). In addition, for the case of a double nanocavity system (cavity separation of 180 nm), the modulation strength is increased to 30.4 dB at lambda; =1630 nm (compared to 1 dB for the structure without cavity)(stub-induced insertion loss ~ 3.8 dB), showing a remarkable resonance-based modulation efficiency. Currently, we are fabricating the nanocavity slot waveguide structure with e-beam lithography and sputter ITO into the slot, measurements are currently in progress and will be presented at the conference. This work provides insight towards developing ultra-compact ultrafast nanoscale modulators for integrated nanophotonic circuits.
1. E. Feigenbaum, K. Diest, and H. A. Atwater, Nano Lett. 10, 2111-2116 (2010).
2. M. Rezaei et al., IEEE J. Sel. Top. Quantum Electron 18, 1692-1699 (2012).
3. A. Kriesch, D. Ploss, U. Peschel, S. P. Burgos and H. A. Atwater, Nanometa conference (2013).

The properties of nanophotonic devices and structures can to a large extent be derived from the distribution of the local (or photonic) density of states (LDOS) around the structure. We report on our efforts to spatially map the LDOS in the vicinity of nanostructures by measuring the fluorescent lifetime in quantum dots and other fluorophores that are separated from the structure by a polymer film of variable thickness. Measurements performed on planar gold and silicon surfaces show good agreement with theoretical predictions for the LDOS. The polymer film deposits conformally on complicated geometries, which makes it possible to extend the technique to intricate non-planar structures. The polymer spacer film can also be made responsive to parameters such as temperature and pH, which means that a single structure could be used to map out the LDOS in three dimensions. Such stimulus-responsive films can also be used to dynamically regulate nanometer sized gaps in plasmonic and other nanophotonic structures, making possible post-fabrication tuning of the LDOS in the gap. For example, we have shown that the wavelength of the plasmon resonance in a gold nanoparticle can be dynamically shifted by as much as 250 nm if it is separated from a gold surface by a pH-responsive polymer film.

A perfect absorber is an optical element capable of completely absorbing the incident light . There has been considerable research on perfect absorbers based on the resonant excitation of localized surface plasmons (LSPs) in various metallic nanostructures, such as disks, squares, and cruciform pads. These structures have resonance wavelengths determined by geometrical design parameters, and changing these parameters enables one to tailor their resonance properties. However, none of these works has reported dynamic tuning of this type of absorption resonance. In this presentation, we show that an ultrathin transparent conducting oxide (TCO) film sandwiched by a planar metal substrate and patterned metal pads can demonstrate a substantial change in the resonant absorption properties under electric bias.
We first elucidate how the optical properties of a thin TCO film can be modified by electrical gating, inducing either accumulation or depletion of mobile carriers. The incorporation of TCO material into a plasmonic cavity enables active electrical tuning of its resonance properties. Here, the metals forming the cavity can be used as electrical gates. Arrays of this type of cavity placed on a planar surface facilitate near perfect absorption at a target resonance wavelength and electrical modulation of the absorption up to 80% in the infrared (IR) regime. We believe that the proposed device can pave the way to further research on various optical applications such as electro-optic modulation and thermal emission control.

The concept of a metamaterial (MM) absorber utilizes an array of suitably-designed periodic metal-insulator-metal (MIM) patterns to reduce reflection of radiation from a surface at a resonant frequency. With an optically-thick lower metal ground plane yielding zero transmission, and reflection reduced toward zero, such an MM is claimed to be perfectly absorbing. In the language of RF technology, the reflection is identically zero when the impedance of the MIM pattern is matched to that of the incident medium, usually air.
Considering this phenomenon in more depth, one finds that freely-propagating photons are first coupled into surface plasmon polaritons (SPP), whose energy is then dissipated into heat due to Drude heating in the metal and/or dielectric loss in the insulator layer. The generated SPPs can propagate along the metal/insulator interfaces in the form of surface plasmon waves, or can form standing (localized) waves due to the symmetry and periodicity of the MIM structure. So far in the literature, MM absorbers are designed on symmetric MIM patterns. The symmetry of these patterns causes the generated SPPs to form standing waves, yielding localized surface plasmons (LSP). The small effective cavity size for the LSP is also the main reason for the relatively low Q factors observed for current MM resonant absorbers [1].
Here, we propose symmetry-broken MIM patterns that, instead of localizing the SPPs, cause them to travel along a preferred direction. At the resonance frequency where the impedance of the MM surface matches that of air, these devices bend the incident light 90 degrees, without reflecting back to the incident direction, yielding a quasi-perfect absorber. This phenomenon can be regarded as an extreme case of the recently published work by Pfeiffer and Grbic on wave front tailoring with reflectionless MM sheets [2]. Due to the considerably larger effective cavity size of the propagating SPPs, the symmetry-broken MM patterns can have much larger Q-factors than their symmetric counterparts. We demonstrate the above phenomena in both experiment and simulations in the optical and infrared regions. These novel, asymmetric SPP couplers have the advantage of high coupling efficiency (theoretically reaching 100%), good directionality and large quality factor.
[1] X. Liu, T. Starr, A. F. Starr, and W. J. Padilla, Phys. Rev. Lett. 104, 207403 (2010).
[2] C. Pfeifer and A. Grbic, Phys. Rev. Lett. 110, 197401 (2013).

Surface plasmons provide the ability to confine electromagnetic energy to sub-wavelength sizes and have potential applications in fields such as active photonics, biosensing and telecommunications. The high density of optical energy confined in small volumes in the vicinity of a metal is a strong channel of optical loss but can also be utilized to heat semiconducting materials. In the current work efficient laser-induced heating has been observed on silicon nanowires coated with SiO2 and gold to form a highly confined plasmon nanocavity. Temperature estimations done using the intensities of Stokes and anti-Stokes scattering lines and also from the peak shift of the Stokes lines reveal that temperature as high as 1200 K can be reached for this integrated system at low laser powers. Simulations of the system showed increased absorption for the integrated Si-SiO2-Au system in comparison to bare silicon nanowire with a broad absorption. Since the bare silicon nanowires in our measurements show very little increase in temperature, the extraordinary heating can be attributed to plasmonic effects from the gold nanocavity. The current system provides an efficient means to obtain high temperatures in localized regions at the nanoscale. This system has potential applications in the field of high temperature catalysis and thermoelectrics.

The resonant optical absorption and optothermal response of plasmonic nanostructures enables local concentration of optical energy that can be employed in catalysis and photochemistry, for example, but that can also be detrimental for the operation of plasmonic-based optoelectronic devices. As yet, there is little quantitative examination of the magnitude of such optically induced electrochemical potentials. Here, we provide a comprehensive theoretical and experimental analysis of the nanoscale electrochemical environment defined by an individual colloidal Au nanoparticle, both in the dark and during absorption in the nanoparticle localized surface plasmon resonance.
Finite element methods (COMSOL) are used to model the electrostatic environment of a 60 nm Au spherical particle connected to ground via a transparent indium tin oxide (ITO) substrate. Experimentally, Kelvin probe force microscopy (KPFM) corroborates the electrostatic simulations. KPFM measures the work function difference between a sample surface and a conductive AFM tip, providing a nanoscale map of the surface potential in the vicinity of individual 60 nm Au nanoparticles. During dark studies we observe up to an ~100 mV shift of the work function of the Au particles based on their surface preparation.
Comparison of similarly prepared particle ensembles using X-ray photoelectron spectroscopy (XPS) verifies that the large shift of the measured work function can be controlled by conversion of the as-synthesized citrate ligand shell of the nanoparticle into a capping layer of gold oxide formed by etching with oxygen plasma, or by conversion to elemental Au via thermal annealing under vacuum. Modification of the electronic and dielectric surface structure is also studied optically. We observe a 40 nm shift of the plasmon resonance depending on the surface treatment, in agreement with full wave optical simulations (FDTD method) of the experimental geometry.
Analysis of the power and frequency-dependence of the single particle KPFM signal reveal three distinct optically induced electrochemical phenomena. We identify excitation of long-lived electronic trap states in the substrate surface, with a characteristic relaxation time of several hours. Additionally, in agreement with the analytically modeled heat flow and particle temperature, we also measure resonant optothermal excitation of thermoelectric potentials of a few mVs under 100 mW cm-2 illumination power. Finally, during scanned monochromatic excitation we also observe a newly described ‘plasmoelectric&’ potential [1], manifest as a characteristic change of sign of the optically induced surface potential from negative to positive, based on excitation to the blue or red, respectively, of the particle plasmon resonance. Our analysis provides detailed insight into the complex electrochemical behavior of metal nanostructures resulting from resonant plasmonic absorption.
[1] Sheldon et al. (2012) The Plasmoelectric Effect, arXiv

Localization of ultraviolet radiation via surface plasmons can provide high concentration of optical energy for resonant dissociation of nitrogen bonds in surface-adsorbed hydrazine and ammonia. UV surface plasmon induced photochemistry therefore enables nonthermal resonant dissociation pathways for deposition of refractory nitride materials at low growth temperatures.
Here, we report observations of surface plasmon-mediated oxidation of hydrazine at cryogenic temperatures. A pulsed 10 W m^(-2) KrF laser operating at 248 nm excited surface plasmons on Al nanostructures after a layer of hydrazine was cryogenically adsorbed to their surface. The adsorbed hydrazine was athermally decomposed via excitation of ultraviolet surface plasmons, with the Al nanostructure substrates in low pressure vacuum. Gas phase mass spectrometry characterized the decomposition products of hydrazine, indicating a > 20x enhancement of the hydrazine oxidation rate for layers adsorbed on Al plasmonic nanostructures compared with laser induced oxidation on dielectric control substrates. We also performed numerical simulations of surface plasmon modes and coupling efficiency into the Al nanostructure substrates. We observe a near-field enhancement factor up to 25x in optimized designs. Experimental ultraviolet reflection and transmission characterization of fabricated Al nanostructures show absorption maximum consistent with simulations. We note that at ultraviolet wavelengths, Al exhibits a surface plasmon resonance between 100 nm and 300 nm, and is about eight times less lossy at 200 nm compared with Au.
Conventional methods for growing nitride films require high temperatures (> 700 K) in order to dissociate high-energy nitrogen bonds in precursor molecules such as ammonia and hydrazine. Alternatively, ultraviolet surface plasmon induced photochemistry can provide a new method for deposition of nitride materials at ambient temperature. Our results indicate that surface plasmon mediated oxidation of hydrazine is a promising route to enable low temperature growth of nitride semiconductor films.

Gas species recognition through fully optical devices is currently a raising trend over the well-established conductometric approach, as it opens new possibilities especially for in situ recognition of flammable and/or toxic species such as CO, H2 NO2 or volatile organic compounds (VOC). Noble metal nanoparticles (NPs) dispersed inside a porous semiconductive matrix constitute an effective design for a gas sensor&’s active material, thanks to their catalytic and localized surface plasmon resonance (LSPR) properties. In fact, metal NPs can show catalytic properties and therefore modify the chemical interactions between the oxide surface and the target analyte, improving the sensing process. Moreover, if the metal NPs show LSPR peak in the visible range (like Au), the nanocomposites can be used as selective optical gas sensor. In fact, the variation of the dielectric constant around the LSPR bands will differ for different gas species; this leads to a diverse variation of the optical properties at different wavelengths.
Some strategies for the synthesis of highly reproducible semiconductors thin layers (TiO2 and ZnO) containing elevated concentrations (up to 12% wt) of Au and/or Pt NPs for optical gas sensing are presented. Both spherical and elongated Au NPs have been used.
TiO2(anatase) thin films with embedded Au and/or Pt NPs were obtained synthesizing high quality metal and oxide colloids and spinning directly a nanocrystal ink made of colloidal solutions on glass substrate. The effect of sample composition and annealing temperature on the optical and microstructural properties has been assessed and used to explain the gas sensing results. A nanocrystal ink has been obtained also for the realization of ZnO containing Au nanorods (NRs) and Pt NPs.
TiO2-Au samples showed fast and reversible change in optical absorption when exposed to H2 and CO species at 200°C-350°C with high sensitivity despite small thickness, while TiO2-Au-Pt films showed room temperature response to H2 and VOC (alcohols).
In the case of ZnO-Au samples the presence of Pt is essential to catalyze the reaction with the target gasses at low temperature and hence avoiding the spheroidization of the Au NRs which occurs above 200 °C.

: Hydrogen atom H (equiv; proton + electron) transfer (HAT), is the most common reaction that involves the transfer of two elementary particles, a proton and an electron. Antioxidant, proton-coupled electron transfer (PCET) reactions involve also the transfer of two elementary particles, a proton and an electron. These constitute the fundamental step in a wide range of processes, chemical energy technologies, which rely on e-/H+ transfer from combustion and aerobic oxidations, to enzymatic catalysis and the destructive effects of reactive oxygen species in vivo. Here we describe a novel phenomenon, plasmonically-enhanced PCET, using nanothin silica-coated plasmonic Ag nanoparticles [1], functionalized with gallic acid, a natural antioxidant molecule [2], that is able to perform PCET. These nanoparticles can transfer rapidly electrons and protons to stable radicals. The kinetics and yield of these PCET reactions can be enhanced by plasmonic resonance modes excited by low-power, near-Infrared (785nm) laser irradiation. The demonstration that these plasmonic nanoparticles can enhance the HAT rates for both electrons and protons expands the traditional view of interfacial PCET. The occurrence of interfacial plasmon-enhancement of PCET brings together so far unrelated domains of nanoplasmonics, electron/proton translocation with significant impact on a variety of applications and most notably in theranostics.
1. G.A. Sotiriou, T. Sannomiya, A. Teleki, F. Krumeich, J. Vörös, S.E. Pratsinis, “Non-toxic, dry-coated nanosilver for plasmonic biosensors”, Adv. Functional Mater., 20, 4250-4257 (2010).
2. Y. Deligiannakis, G.A. Sotiriou, S.E. Pratsinis, “Antioxidant and Antiradical SiO2 Nanoparticles Cova-lently Functionalized with Gallic Acid”, ACS Appl. Mater. & Interf., 4, 6609-6617 (2012).

Hybrid optoplasmonic structures that combine high-quality optical microcavity (OM) resonators and plasmonic nanostructures achieve much narrower linewidths of photonic modes than those of plasmonic resonances, which can improve sensing capabilities of a plasmonic sensor beyond its limits. In addition, photonic microresonators offer great opportunities for cascaded hot-spot intensity enhancement, long distance energy transfer, adaptive switching of spatial light distributions, and resonant manipulation of radiative rates of embedded emitters [1]. Using our previously reported novel nanofabrication approach that enables a precise and controllable vertical and horizontal positioning of plasmonic elements relative to embedded microspheres by self-assembling microspheres on lithographically patterned, dried-etched surfaces [2], now we demonstrate efficient photon transfer in this discrete optoplasmonic molecule comprising gold nanoparticle (NP) dimer antennas located in the evanescent field of a 2 mu;m diameter polystyrene bead, which served as an OM. Cy3 and Cy5.5 dyes were tethered to the plasmonic antennas through oligonucleotide spacers to act as on-chip light sources. The intensity of Cy3 was found to be increased relative to that of Cy5.5 in the vicinity of the plasmonic antennas where strongly enhanced electric field intensity and optical density of states selectively increase the excitation and emission rates of Cy3 due to spectral overlap with the plasmon. The fluorescent dyes preferentially emitted into the OM, which efficiently trapped and recirculated the photons. We experimentally determined a relative photon transfer efficiency of 44% in non-optimized self-assembled optoplasmonic molecules in this proof-of-principle study [3].
[1] Boriskina, S. V.; Reinhard, B. M., Proc. Natl. Acad. Sci. USA 2011, 108, 3147-3151.
[2] Ahn, W.; Boriskina, S. V.; Hong, Y.; Reinhard, B. M. “Photonic-Plasmonic Mode Coupling in On-Chip Integrated Optoplasmonic Molecules” ACS Nano 2012, 6, 951-960.
[3] Ahn, W.; Hong, Y.; Boriskina, S. V.; Reinhard, B. M. “Demonstration of Efficient On-Chip Photon Transfer in Self-Assembled Optoplasmonic Networks” ACS Nano 2013, 7, 4470-4478.

Ultra-thin metal films are conductive and transparent. Those properties, separately and in combination, result in interesting phenomena attracting researchers&’ attention, particularly to “good plasmonic metals” such as gold and silver. Potential application areas include transparent contacts, plasmonic waveguides, layered meta-materials etc. Unfortunately, factors such as polycrystallinity, interface roughness, poor adhesion, unwanted self-organization, surface energy, surface mobility, and mechanical instability pose significant challenges to fabrication, performance and reproducibility of devices involving ultra-thin metal films. In spite of this, there has been relatively little focus on fundamental materials science within the plasmonics research community. We show that significant improvements in quality of ultra-thin gold films can be achieved via surface modifications by using carefully selected optical polymers, which have received little attention in spite of their advantages, such as optical transparency over a wide range of wavelengths, patternability, possibility to cover conformally various substrate geometries, and a wide range of optical and mechanical properties.
We succeeded to suppress surface diffusion of Au toward formation of continuous smooth and conductive films which are <8 nm thick and exhibit 95% mass density as measured by X-ray reflectivity. By comparison, gold of similar thickness deposited on untreated SiO2 forms isolated islands, with a surface coverage of around 50% and strong optical absorption in the visible range due to localized plasmon resonances. We demonstrate that X-ray reflectivity is a very accurate method to determine surface coverage of ultra-thin films on non-wetting substrates. Accuracy of the results was verified with optical transmission measurements and SEM. Another important outcome of this method is ability to determine actual thickness of the metal films which depends on their mass density. We demonstrate its importance for estimation of optical losses in plasmonic waveguides and show how our achievements can benefit manufacturing of hyperbolic meta-materials.
We believe that our results are of high interest to a wide audience of researchers, in particular because our methodology can be easily brought both into the research lab and to industry.