The field of metamaterials and nanoplasmonics opens up many technologically important capabilities ranging from subwavelength light manipulation to unique abilities for controlling the optical response of nanostructured materials. Many of these functionalities rely on the control of the magnetic part of light at optical wavelengths utilizing plasmonic nanostructures like the canonical example of a split-ring resonator, an inductive metallic ring with a gap that is a building block of many magnetic metamaterials. However, up to now, the vast majority of such nanostructures contain metallic nanoparticles that suffer from high dissipative losses at optical frequencies that dramatically limit their performance. Thus, the big question now is: How to remove metallic components - and their intrinsic losses - but maintain the electric and magnetic response enabling low-loss metadevices at the nanoscale?
Recent developments in the physics of metamaterials stipulated a birth of a new branch of nanophotonics dealing with optical magnetism of high-index dielectric components. Unlike their metallic counterparts, all-dielectric nanoparticles do not suffer from conduction losses due to free charges in metals giving them a unique advantage over metallic nanostructures. Particularly, since high-refractive-index dielectric nanoparticles, as theoretically predicted, can also exhibit optically induced magnetic resonances, they are envisioned to provide new, competitive alternatives for optical nanoantennas and metamaterials. Very recently, these optically-induced magnetic resonances were observed experimentally for silicon nanoparticles in the whole visible spectral range from blue to infrared wavelengths and it has been shown that dielectric nanoparticles with strong magnetic response can be used as building blocks to explore new types of structures and interactions at the nanoscale like, e.g., Fano resonances predicted for all-dielectric oligomers. Furthermore, the interaction of magnetic and electric dipoles may lead to new scattering properties. In particular, interference between two optically induced dipoles in such an antenna results in azimuthally symmetric unidirectional scattering. This property can also be employed for all-dielectric metasurfaces with much higher efficiencies in comparison to their plasmonic counterparts.
This talk will give a simple introduction into the physics of all-dielectric nanophotonic and metamaterials, as well as discuss more recent developments in this field. It overviews the development of this novel research direction in nanophotonics exploring the potential of electric and magnetic response of dielectric nanoparticles originating from Mie-like resonances. We show that all-dielectric nanostructures are the best candidates for the emerging field of metadevices with unique functionalities well beyond the capabilities of currently existing devices.

Conventional optical components such as lenses and holograms rely on gradual phase shifts accumulated during light propagation to shape light beam. New degrees of freedom in optical design are attained by considering interfaces decorated by nanoscale features. During this presentation, I will discuss some of our recent works in the field. In particular, i will present our recent results on the design of nano-structured holograms. The holography principle is used as a tool to solve an inverse engineering problem consisting of designing novel plasmonic interfaces to excite either surface waves or free-space beams with any desirable field distributions. Leveraging on the new nanotechnologies to carve subwavelength features within the large diffracting apertures of conventional holograms, it is now possible to create holographic interfaces to shape both amplitude phase and polarization of light. By engineering the response of nano-structured materials, we also demonstrate the routing of SPP wakes at visible frequency in a controllable way. The ability of the new generation of ultrathin and compact optical devices to fully address light properties could find widespread applications in photonics.

Gradient metasurfaces are 2-dimensional optical elements capable of manipulating light by imparting local, space-variant phase-changes on an incident electromagnetic wave. These surfaces have thus far been constructed from nanometallic optical antennas and high diffraction efficiencies have been limited to operation in reflection mode. We describe the experimental realization and operation of dielectric gradient metasurface optical elements capable of also achieving high efficiencies in transmission mode in the visible. Ultrathin gratings, lenses, and axicons have been realized by patterning a 100-nm-thin Si layer into a dense arrangement of Si nanobeam-antennas. The use of semiconductors can broaden the general applicability of gradient metasurfaces as they offer facile integration with electronics and can be realized by mature semiconductor fabrication technologies.

Conventional single- or multi-layer antireflection coatings to eliminate the undesirable interface reflection and enhance the transmission have strict requirements in the refractive index and film thickness of the materials being coated. Alternative approaches have been developed, for example, using surface relief structures. However, they pose difficulties and severe restrictions in device integration. Taking advantage of the tailored reflection/transmission and their dispersion, metasurfaces consisting of subwavelength planar metallic structures and combining with a dielectric spacer enable ultrathin antireflection coatings without much requirement in the refractive index of the spacer material. We have pioneered this concept particularly in the development of terahertz metamaterial antireflection coatings [1,2]. In this contribution, we will present our latest progress in antireflection metasurfaces for terahertz and mid-infrared. Particularly, in contrast to our previous metal-dielectric-metal structures enabling narrowband antireflection, we found that excellent antireflection can be accomplished using much simpler metal-dielectric structures - a combination of a single-layer metasurface and an ultrathin dielectric film, achieving a bandwidth comparable to quarter-wave antireflection. Further improvement of the bandwidth or achieving multi-band antireflection is possible by using more complex unit cells of the metasurfaces [3]. We will present our numerical and experimental results, together with analytical analysis to elucidate the underlying principles [4].
[1] H.-T. Chen et al., “Antireflection coating using metamaterials and identification of its mechanism,” Phys. Rev. Lett.105, 073901 (2010).
[2] H.-T. Chen et al., “A numerical investigation of metamaterial antireflection coatings,” THz Sci. Technol.3, 66 (2010).
[3] N. K. Grady et al., “Terahertz metamaterials for linear polarization conversion and anomalous refraction,” Science340, 1304 (2013).
[4] H.-T. Chen, “Interference theory of metamaterial perfect absorbers,” Opt. Express20, 7165 (2012).

We demonstrate that polarization conversion in coupled dimer antennas, used in phase discontinuity metasurfaces, can be tuned by careful design. By controlling the gap width, a strong variation of the coupling strength and polarization conversion is found between capacitively and conductively coupled antennas. A theoretical two-oscillator model is proposed, which shows a universal scaling of the degree of polarization conversion with the energy splitting of the symmetric and antisymmetric modes supported by the antennas. This picture is supported by extensive electrodynamical simulations based on the 3D-Green's dyadic method.
Using e-beam lithography we fabricated nano-antennas consisting in orthogonal dimers of gold nanorods of fixed widths and heights of 120 and 40 nm, respectively, and with nanorod lengths between 180 and 330 nm. Using Spatial Modulation Spectroscopy on individual antennas, we measured first the “pure” bonding and antibonding states of several representative L = 230 nm antennas. We then measured the intensity of the scattered light along different polarizations for an incident optical excitation polarized along one of the antenna arms. These measurements allowed to quantify the polarization conversion achieved by these nanostructures.
We find good agreement with theory for the scaling of mode splitting and polarization conversion with gap width over the range from capacitive to conductive coupling.
Next to linear polarization conversion, we demonstrate single-antenna linear to circular polarization conversion, which could offer many interesting sensing applications.
Our results provide strategies for phase-discontinuity metasurfaces and ultracompact polarization optics.
[1] L.-J. Black, Y. Wang, C. H. de Groot, A. Arbouet, and O. L. Muskens, “Optimal Polarization Conversion in Coupled Dimer Plasmonic Nanoantennas for Metasurfaces.,” ACS Nano, May 2014.

Metasurfaces, optically thin structures with engineered diffraction, have gained attention in the past few years as new platform for ultra-compact photonics. Metasurfaces are typically based on arrays of planar optical resonators, fabricated at the interface between two dielectrics, arranged to produce diffracted waves of pre-designed amplitude and direction. The quasi-two dimensional nature of metasurfaces results in the majority of light-matter interaction occurring in the near-field proximity of the interfaces. Conventional techniques for calculating light interaction with composite systems, i.e. metamaterials, have been developed with volumetric materials in mind, and typically are inefficient in understanding the optics of metasurfaces. Moreover, since the optics of metasurfaces are more akin to the two dimensional structure graphene than to the traditional optics of volumetric composites, traditional effective medium theories cannot be used to describe the diffractive optics by characterizing the metasurface as an effective index or an effective permittivity/permeability. Although finite-difference and finite-element tools can in principle be used to characterize the optics of metasurface arrays, these techniques require volumetric meshes of the large structures with subwavelength resolution, which can be computationally intractable.
Here we propose an analog of effective medium description for metasurfaces by characterizing the metasurface by a two-dimensional polarizability. Since the typical spatial profile of a metasurface is inhomogeneous on the wavelength scale, polarizability becomes strongly nonlocal (dependent on the wavevector). Nonlocality is known to enable the propagation of multiple waves in volumetric structures; similarly, nonlocal polarization yields diffraction in two-dimensional metasurfaces. We show that adequate treatment of optics of metasurfaces requires discontinuity of both normal and tangential components of electric fields, and present comparison of the predictions of the developed formalism to full-wave solutions of Maxwell equations. The proposed formalism does not require any subwavelength volumetric meshing, or the solving of an eigenvalue problem, drastically reducing the total calculation time and required computational resources. The developed formalism can be used as a new convenient tool for understanding, design, and optimization of metasurface optics.

PT symmetric quantum mechanics involves the study of open or non-hermitian quantum systems which can still exhibit complete and real eigen spectra, a feature normally reserved for hermitian operators, if they remain invariant under the simultaneous transformations of parity and time reversal. However, unlike hermiticity, PT symmetry doesn&’t guarantee real eigenvalues and upon variation of an external parameter the spectrum can suddenly change from real to complex in a process known as spontaneous PT symmetry breaking. While the strict requirement that a physical system be PT symmetric limits experimental investigations in the quantum domain, modern fabrication techniques which allow the spatial profile of both real and imaginary parts of the refractive index to be controlled almost arbitrarily, combined with the similarity between Schrödinger&’s and Maxwell&’s equations has made PT symmetric optics a hot topic of research. The most notable demonstration of PT symmetry consists of a pair of side coupled waveguides, one with loss and the other with an equal amount of gain. On varying the coupling strength PT symmetry breaking can be observed in the light transmitted through the system, changing from lossless to amplifying or decaying depending on the excitation conditions. Importantly, PT symmetry breaking has also been observed in passive experiments without gain, which can be understood by the fact that a system with a loss contrast maps to a PT structure under a gauge transformation. As well as being of fundamental importance, PT symmetric waveguide systems have also revealed technologically beneficial properties such as anomalous transparency, power oscillations, non-reciprocal Bloch oscillations and unidirectional transparency. Recently, spatiotemporal studies have also highlighted the coexistence of coherent perfect absorption and lasing, realisable via PT symmetric resonators. Here we theoretically and experimentally explore novel polarisation phase transitions induced by PT symmetry breaking in anisotropic terahertz metasurfaces. By using terahertz time domain spectroscopy, giving both amplitude and phase information for all four transmission coefficients, we gain complete knowledge of the polarisation response of our samples. After constructing a metasurface unit cell out of coupled orthogonally orientated split ring resonators with the same resonant frequency but different absorption coefficients, a phase transition is observed for the polarisation eigen states of transmission upon varying the coupling strength. In this case PT symmetry breaking is found to cause a sudden 45° rotation of the eigen polarisation ellipses. Moreover, precisely at the boundary separating these two regimes, known as the exceptional point, the eigen modes coalesce into a single circular polarized state which is highly unusual given the metasurface&’s lack of rotational symmetry. Our study points to increased freedom for manipulating polarisation states of light.

Subwavelength manipulation and control of light with plasmon metasurfaces is one of the most intriguing current approaches in nanophotonics.¹ The key feature of such ultrathin optical components is that they provide an abrupt change of the light&’s phase, amplitude and/or polarization. To realize the full potential of metasurface devices, for example, in video-rate holography² or nanophotonic circuitry and flat-optics,³ dynamic tunability is a necessity. In this contribution, we introduce magnetoplasmonic analogue⁴ of a metasurface that can be externally controlled by a magnetic field. By making use of two coupled plasmon modes in conically symmetric bottom-up Au nanoantennas, the curling of near-field light polarization produces a tightly nano-confined circular dichroic ‘hot knot&’ at their apex. Furthermore, by positioning a ferromagnetic nanodot at the ‘knot&’ we gain dynamic magnetic control of the plasmon enhanced near-field intensity; the polarization of the out-coupled free-propagating visible light is thus controlled via an external magnetic field.
¹ N. Yu et al., Nature Mater. 13, 139 (2014)
² J. Lin et al., Nano Lett. 13, 4269 (2013); X. Ni et al., Nat. Comm. 4, 2807 (2013); L. Huang et al., Nat. Comm. 4, 2808 (2013); W. Chen et al., Nano Lett. 14, 225 (2014).
³ X. Chen et al., Nat. Comm. 3, 1198 (2012); A. Pors et al., Nano Lett. 13, 829 (2013); J. Lin et al., Science 340, 331 (2013).
⁴ N. Maccaferri et al., Phys. Rev. Lett. 111, 167401 (2013); V. Bonanni et al., Nano Lett., 11, 5333 (2011).

As the performance of photovoltaic cells approaches the Shockley-Queisser limit, appropriate optical schemes are needed to maximize both short-circuit and open-circuit voltages. To address this concern, we propose a planar absorber-mirror light trapping structure where a conventional mirror is replaced by a meta-mirror with asymmetric light scattering properties. The meta-mirror is tailored to have reflection in asymmetric modes that stay outside the escape cone of the dielectric, hence trapping light with unit probability. Ideally, the meta-mirror can be designed to have such light trapping for any angle of incidence onto the absorber-mirror structure. We illustrate the concept by using a simple gap-plasmon meta-mirror. Even though the response of the mirror is non-ideal with the unwanted scattering modes reducing the light absorption, we observe an order of magnitude enhancement compared to single pass absorption in the absorber, as shown by S4sim, a rigorous coupled wave analysis simulator. The tool is hosted on nanoHUB.org, an open-access science gateway for cloud-based simulation tools and educational resources for nanoscale science and technology. These results are also validated through MEEP, a finite-difference time domain solver also available on nanoHUB. Through these simulations, it is found that the bandwidth of the enhancement can be matched with the range of wavelengths close to the solar cell absorber band-edge where improved light absorption is required. Future prospects for further design enhancement to help approach theoretical limits will also be discussed, including free dissemination of our techniques to audience members.

Grating couplers are commonly used to convert light from free space or from an optical fiber to a guided mode in an optical waveguide. In the present work, we study grating couplers where each grating line consists of an array of subwavelength-sized antennas. We refer to such structures as meta-grating couplers, in contrast to conventional grating couplers that typically feature a one-dimensional variation in effective refractive index on a wavelength scale. This metamaterial grating approach can significantly modify the nature of the coupling and thus provides an additional design dimension when coupling light to integrated optical circuits. For a particular arrangement of sub-wavelength antennas, the meta-grating couplers can, e.g., couple light to opposite directions based on the circular polarization state of the incident light. Recently, this principle was exploited for unidirectional coupling of free-space light to surface plasmon polaritons on a metal-dielectric interface, using arrays of slot antennas in a metal film [1]. Here, we extend this principle to photonic modes and numerically study coupling to freely propagating photonic modes as well as photonic waveguide modes, for different sub-wavelength antenna arrays. We present experimental evidence of circular-polarization-selective wide-bandwidth coupling with high extinction ratio at telecom wavelengths using meta-gratings based on subwavelength metal antenna arrays.
[1] J. Lin, J.P.B. Mueller, Q. Wang, N. Antoniou, X.-C. Yuan, F. Capasso, Science 340, 331 (2013)

Shaped silver nanoparticles have recently gained prominence for their ease of fabrication and accessible surface plasmon resonance. This makes them particularly good candidates for sensors: they have a high selectivity for their local environment, and can interact with a broad range of frequencies. In particular, silver nanocubes have been found to have controllable dipolar and quadrupolar modes when coupled through a thin silicon substrate.
This result has led us to explore the coating of silver nanocubes by atomic layer deposition (ALD). Our goals in doing so are threefold: we would like to tune the dipole and quadrupole plasmon modes, we would like to anisotropically coat these cubes to allow controlled aggregation at an uncoated face, and we would like to improve the temperature stability of the cubes.
Our initial results show silver nanocubes with increased thermal stability and a single, uncoated face. The plasmon can be seen absorbing between 375nm and 400nm, depending on the average size of the silver nanocube. Alumina coated silver nanocubes retain the plasmon absorbance after being annealed at 250°C, while the uncoated cubes show loss of this absorbance, indicating that the cubes have misshapen.
This presentation will briefly cover the development of quadrupolar surface plasmons in silver nanocubes and the fabrication of anisotropically coated silver nanocubes using trimethyl aluminum and water. The effect of alumina on the plasmon and other characteristics of the system will be discussed, and sensor fabrication will be also be presented.

Plasmonic substrates are finding applications ranging from negative-index metamaterials to chemical sensing. We designed substrates of two-dimensional arrays of gold nanorings, nanocrescents, nanocrescent dimers and nanodisks on large areas with a novel and scalable fabrication method based on shaped nanosphere lithography. In our nanosphere lithography approach, a plasma treatment is used to reduce the size of two-dimentional hexagonal closed packed (2DHCP) polystyrene nanospheres and a thin layer of nickel is deposited as sacrificial layer. Next, gold is deposited and fills the area between nanospheres and nickel pits. Finally, by removing nickel layer, gold nanostructures are left and form fabricated substrate. Different types of nanostructures can be obtained readily by making some modifications in fabrication steps. Substrates were characterized by scanning electron, atomic force and infrared microscopes as well as UV/VIS spectrometer. Also, the finite difference time domain (FDTD) method was used to simulate the optical behavior of gold nanostructures. The measured surface plasmons were well reproduced by FDTD calculations. The dimensions of nanostructures ranged from 100 to 550 nm supporting plasmon resonance from visible to mid-infrared spectral range. Samples were highly uniform, densely packed (around 7×10⁸ cm^-2) and exhibited tunable plasmon resonance. We engineered appropriate substrates for specific applications, which might be used as metamaterial in visible range, plasmonic photocatalyst substrate and chemical and biological sensors.

In nanoscale electronic, photonic and plasmonic devices, feature dimensions shrink towards a critical limit, and new experimental approaches have to be explored in lithographic patterning. For this work, spherical submicrometer-sized silica particles were prepared by the sol-gel technique and deposited as a self-assembled monolayer onto high-purity silica glass plates by means of a spin coater system. This monolayer is then used as a mask to create regular arrays of nanoscale features in the sample by MeV Ag ion implantation. On the other hand, previously to the ion implantation, the masks can be modified by MeV Si ion irradiation to tailor the size and arrangement of these embedded features as a function of the ion fluence. Indeed, amorphous glassy materials like silicon dioxide can undergo extreme deformations under exposure to high-energy beams, which induce damage and structural changes in solids due to energy losses of MeV heavy ions via ionization events and atomic collisions. Some of the samples were irradiated at room temperature with Si ions at 4 and 6 MeV at several fluences perpendicularly to the sample surface. After the irradiation the silica particles turned into oblate particles, as a result of the increase of the particle dimension perpendicular to the ion beam and the decrease in the parallel direction. By this way, the mask openings of the silica particle monolayer were modified down to the nanoscale and a subsequent Ag ion implantation allowed the formation of ordered arrays of Ag nano-objects, after removal of the silica particles. The size, size distribution and shape of both the silica particles and the array of metallic deposits were determined by scanning and transmission electron microscopy as a function of the irradiation parameters. Finally, the long range order of the nanoparticle assembly and its plasmonics properties were characterized by means of a Fast Fourier Transform study and optical absorption measurements, respectively.

Film-coupled plasmonic nanoantennas forming metal-insulator-metal gap plasmon (GP) nano-resonators have recently attracted significant interest due to their very strong magnetic resonances, efficient and tunable absorption, and ability to concentrate and manipulate light at the nano-scale. Such meta-surfaces are essential for a range of applications such as controlled-emissivity surfaces for thermo-photovoltaics, tailoring infrared spectrum for controlled thermal dissipation and improved photo-detectors. While individual GP nano-cavities are extensively studied, a detailed investigation of the interaction between these plasmonic entities is still missing. Here, we experimentally and numerically study the interaction between GP nano-resonators in an array in the limit of diminishing spacing between the resonators. We show that the GP nanocavities maintain their individual resonance character, with negligible interaction, even at very small proximity between resonators. Then, we show that as soon as an electrical connection is established between cavities they form a two-dimensional MIM waveguide network and undergo a strong interaction. This leads to an abrupt shift in the absorption wavelength of the metasurface. We develop a one-dimensional mathematical model to approximately model such an MIM waveguide network and show that by adjusting the width of the electrical connections we could largely tune the absorption resonance of the metasurface. Furthermore, our numerical studies show that by using a very narrow electrical connection one could achieve squeezed gap plasmons within volumes which are one to two orders of magnitude smaller than the wavelength, in all three dimensions. With 2 nm wide inter-cavity electrical connections we theoretically achieved 20 times shrinkage of mode volume as compared to conventional gap plasmons. The novel phenomena explored here can have a host of potential applications in the field of active plasmonic metamaterials, integrated photonics circuits, plasmonic photocatalysis and ultra-sensitive sensors.

Since in most conductive material the magnetic permeability is much weaker than electric permittivity, artificial magnetic resonances can only be produced by oscillating loops of currents. It has been theoretically predicted that such loops of currents can be generated by placing conductive spherical objects in a circular pattern, closely spaced by a dielectric. [1] The boundary conditions of the dielectric dictates that light should be normal to the surface of the conductive spheres, therefore allowing displacement current to rotate slightly. In the optical spectral region, one can produce magnetic plasmons using an ensemble ofgold nanoparticles arranged in close proximity of each other [1]. When excited with a linearly polarized light in the plane of the nanoparticles, this ensemble can generate an oscillating magnetic dipole normal to the plane of particles.
Here, we demonstrated the synthesis of three-dimensional gold nanoparticle clusters (GNCs) that consist of many individual gold nanoparticles randomly closed-packed on the surface of a polystyrene core. The nanoparticles are protected from touching each other by a coating layer of surfactant. The average spacing of the nanoparticles can be tuned by the choice of the surfactant used during the synthesis. The GNCs exhibit isotropic broadband plasmon resonance in the vis-near-IR region which is easily tunable via varying the core size or the size and number of the individual nanoparticles. Both parameters can be easily tuned by varying the synthesis condition.
Finite-difference time-domain (FDTD) simulation of these nanostructures demonstrates that the broadband plasmon resonance is attributed to multiple resonances, including electric dipole, electric quadrupole and magnetic dipole peaks. Furthermore, when the overall size of the nanoparticles exceeds a certain level, a strong magnetic quadrupole is also observed that may be visible in the far-field. These nanoparticles are isotropic and present strong scattering peaks that are omni-directional. This effect is due to the large number of nanoparticles (100~800) in each GNC and close-packing of the particles (1-2 nm inter-particle distance) that leads to the strong circulating currents in various paths. Variations of the size of the surfactant, only by a few carbon links can tune or completely eliminate the effect, which is a further evidence for the importance of close-packing and spacing in the generation of magnetic plasmons. To the best of our knowledge, this is one of the pioneering works of fabricating GNCs with a strong magnetic dipole response and the first example of GNCs with a strong magnetic quadrupole response. The tunability of the synthesis method and the strong magnetic response would potentially lead to large-scale manufacturing and wide application in imaging, plasmonics and photonics.
(1) Alu, A.; Engheta, N. Optics Express 2009, 17, 5723.

A laser consists of a cavity and a gain material that emits monochromatic, coherent light; however, the size of the laser is constrained by the diffraction limit. By using a metal nanoparticle that supports localized surface plasmon resonances (SPRs) as the cavity and a semiconducting conjugated polymer (CP) as the gain material, a sub-wavelength analogue of a nanolaser could be created that amplifies SPRs instead of photons. Unlike photonic resonances, SPRs are localized on sub-wavelength dimensions, and the resulting device, termed a spaser (Surface Plasmon Amplification by Stimulated Emission of Radiation), would have a size less than the diffraction limit and have applications in biosensing and ultrahigh resolution microscopy.^1,2,3 The use of CPs as gain materials is advantageous because they are more stable than small organic laser dye molecules, they exhibit large gain cross sections, and they are easy to process. Here, we present two structures that are predicted to act as spasers: 1) CP nanoparticles with embedded gold nanorods (AuNRs), and 2) a monolayer of AuNRs coated with a CP thin film.
CP nanoparticles with embedded AuNRs were fabricated through a miniemulsion procedure: a solution of water and surfactant is injected into a solution containing the CP and AuNRs in toluene. Sonication creates nanodroplets of the organic phase in the aqueous phase, and evaporating the organic solvent leaves behind CP nanoparticles with an embedded AuNR, suspended in water. Scanning electron microscopy confirms the presence of CP nanoparticles with encapsulated AuNRs. Excitation power-dependent photoluminescence spectroscopy shows that when the green-emitting conjugated polymer F8BT (Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)]) is used, nanoparticles with and without an encapsulated AuNR demonstrate a threshold behavior. The emission intensity of the F8BT nanoparticles is quenched upon the encapsulation of AuNRs, at wavelengths corresponding to the transverse SPR of the AuNRs. Possible mechanisms include electron transfer, non-radiative losses in the AuNRs, or coupling to dark surface plasmon modes of the AuNR. While excitons in the CP must be coupled to a SPR of the AuNRs for spaser operation, re-radiation at the wavelength of the SPR must also occur. It is predicted that using the red-emitting conjugated polymer MEH-PPV (Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]) as the gain material will allow the structures to re-radiate because the emission of MEH-PPV overlaps with the longitudinal SPR of the AuNRs, which are less lossy than transverse SPRs. Both the nanoparticle and thin film structures are expected to act as spasers because they will both allow the emission of the CP to be transferred to the longitudinal SPR of the AuNRs.
References:
1) M. A. Noginov et al. Nature, 2009, 460, 1110-1112.
2) G. Shambat et al. Nano Lett, 2013, 13 (11), 4999-5005.
3) M. I. Stockman. IOP Science, 2010, 12, 024004.

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 in the areas of chemical and biomedical sensing, information and communication technologies, solar energy harvesting, lighting, cancer treatment, optical encoding of information and surface decorations among others.
A significant challenge in delivering the aforementioned devices is the materials&’ preparation method. So far, efforts have been dominated by various techniques like nanolithography, ion beam nanofabrication, atomic layer deposition, pattern transfer, and template stripping. However, while these techniques can deliver an unrivaled particle monodispersity, they are rather complex, demanding multiple steps, long processing times and/or the use of toxic agents. An alternative, less complex, fabrication and patterning scheme is that of laser annealing (LA): an ultra-fast and macroscopically cold process that provides freedom of design and fast processing times. These characteristics are well suited for large-scale low-cost manufacturing of materials and devices and also enable the use of inexpensive flexible substrates, a prerequisite for roll-to-roll (R2R) processing which is becoming nowadays the manufacturing route of choice for many emerging applications.
The research methodology pursued involves the sequential tuning of the laser wavelength into resonance with two different physical absorption processes: the interband transitions of the metal&’s d-electrons (UV frequencies), or the LSPR of the metal particles (visible frequencies). We use an excimer laser (193 and/or 248 nm) for the former and a Nd:YAG laser with an optical parametric oscillator unit (532nm and/or 633nm) for the latter. We demonstrate that each absorption process selectively targets the melting and re-solidification of different particle size groups, which under circumstances can lead to nearly monodispersed nanoparticle arrays. To get insight into the heating dynamics involved during UV and/or Vis LA, we performed optical and heat transport calculations and obtained the spatial absorption profile as well as the transient temperature profile at each nanoparticle. These resulted into recipe maps that can facilitate the design of plasmonic templates with predefined morphology. Recent scanning electron microscopy images and optical reflectance spectroscopy measurements on LA Ag films are discussed in light of these findings.
Acknowledgements: This research is supported from the People Programme (Marie Curie Actions) LASER-PLASMON of the European Union's Seventh Framework Programme (FP7/2007-2013) under REA grant agreement n° PIEF-GA-2012-330444.

Light-matter interactions depend strongly on both intrinsic and extrinsic properties of the interacting materials and are responsible for a wide variety of nanoscale optical phenomena, some of which can be described under the Mie theory. Examples of optical interactions within the domain of Mie theory include light scattering from particles with high refractive indices and sizes smaller than the wavelength of the incident light, which occurs via the resonant form of Mie scattering. In addition, Mie theory also encompasses the confinement of light into deep-subwavelength structures via cluster oscillations of free electrons, which is most pronounced in metallic nanostructures, and the observation of resonant absorption effects in high-index semiconductors. All of the above optical mechanisms are associated with a number of unique applications in nanophotonics. However, the interaction of light with low refractive-index nanomaterials, such as polymers and some glasses, has not been very well-investigated, and its characterization may reveal many unknown and potentially distinctive optical features. Even existing applications primarily rely on the intrinsic features of polymers, and little work has been performed on how the extrinsic properties of low-index nanostructures alter the optical effects associated with these materials.
We provide the in-depth characterization of light-nanowire interactions in the context of an effective Mie scattering regime associated with low refractive index materials. Properties of this regime sharply contrast with these of resonant Mie scattering, and involve the formation of strictly forward-scattered and coupling-free optical fields in the vicinity of core-shell polymer nanowires. Scattering from these optical fields is shown to be non-resonant in nature and independent from incident polarization. In order to demonstrate the potential utility of this scattering regime in one-dimensional (1D) nanostructures, we fabricate polymeric nanostructures using a novel iterative thermal drawing process that yields uniform and indefinitely long core-shell nanostructures. These nanowires are successfully engineered for novel nanophotonics applications, including efficient light capture on photovoltaics, optical nano-sensors with ultrahigh sensitivity and a mask-free photolithography method suitable for the straightforward production of 1D nanopatterns.
[1] M. Yaman, T. Khudiyev, M. Bayindir, et al., Nature Materials 10, 494 (2011).
[2] T. Khudiyev, E. Huseyinoglu, M. Bayindir, Scientific Reports 4, 4607 (2014).

Upconversion of two or more low energy photons into one high energy photon has been received vast attention for extensive application in photovoltaic, bio-imaging, and displays. However, it suffers from its low conversion efficiency due to small absorption coefficient and low collecting efficiency of sensitizer [1]. Here we demonstrate novel nanostructures with metal-insulator-metal geometry for enhanced upconversion luminescence. The nasnostructure was theoretically designed for 14 fold absorption enhancement by FDTD calculation. Photoluminescence spectroscopy showed up to 50-fold increase in 540nm at 90 mW laser power. Even at high power, upconversion efficiency increased 10 times and 6 times for 540nm and 653nm emission, respectively. Furthermore, The relationship between polarization dependency of laser light and enhancement factor of upconversion efficiency was investigated with different periodicity of top silver stripe.
[1] Y. L. Lu, X. B. Chen, Appl. Phys. Lett. 2009, 94, 193110.

Hexagonal Co antidot nanostructures with a pitch size of 470 nm have been fabricated that display plasmonic dispersion in the optical regime. By using the longitudinal and transversal magneto-optical Kerr effects one can measure the phase of the surface plasmon polaritons. These masurements are correlated to the highly modulated optical reflectivity that is measured as a function of incidence angle and wavelength in the optical range. From this one can see that the phase of the plasmons is strongly correlated to the Fano-like resonance that is following the surface plasmon polariton dispersion and that the phase can be inverted by tuning the conditions for excitation.

Electromagnetic field flow around asymmetric nanostructures present complex dynamics that are not necessarily governed by a hydrodynamic theory applied to electromagnetic fields [1]. Both polarization and plasmonic responses are highly susceptible to nanoparticle shape and structure. Circularly-polarized fields contain spin angular momentum that can interact with subwavelength nanostructures that can produce a phase singularity in the scattered fields [2]. We find that nanoparticle asymmetry produces signatory phase profile in the scattered field when illuminated with circularly-polarized light. The spin angular momentum carried by the circular polarization is partially converted to orbital angular momentum, the component carried in the field distribution. This coupling between both types of angular momentum is known as spin-orbit interactions. The phase distribution of a beam carrying orbital angular momentum spirals about a central point, the phase singularity, in which the phase is undefined and the intensity is zero. The presence of phase singularities can therefore be indicative of both the type and amount of angular momentum carried by a beam&’s electromagnetic fields [3].
Here, we investigate angular momentum in the fields in and around subwavelength nanostructures and find that as the radial symmetry breaks the phase singularities transmute. The dependence of radial asymmetry is investigated by transforming nanostructures from a disk to a D-shape. Rich vortex dynamics such as vortex/anti-vortex creation and annihilation and large vortex wandering are associated with greater degrees of nanostructure asymmetry. These dynamics are understood first in single nanoparticles and then extrapolated to nanoparticle arrays.
This work provides a novel method to probe field dynamics associated with nonlinear effects in subwavelength nanostructures. By studying the signatory phase profile after illuminating with circularly-polarized beams details, not only about the structure's symmetry can be determined, but also specifics on nonlinear responses such as plasmonic effects [4], magneto-optical responses [5] and spin-orbit interactions can be investigated.
References
[1] D. Rozas et al. “Propagation Dynamics of Optical Vortices” JOSAB 14 (1997)
[2] L. T. Vuong et al. “Electromagnetic Spin-Orbit Interactions via Scattering of Subwavelength Apertures” PRL 104 (2010)
[3] G. C. G. Berkhout et al. "Efficient Sorting of Orbital Angular Momentum States of Light" PRL 105 (2010)
[4] M. Durach et al. "Giant Surface Plasmon Induced Drag Effect in Metal Nanowires" PRL 103 (2009)
[5] M.Moocarme et al. “Ultralow-Intensity Magneto-Optical and Mechanical Effects in Metal Nanocolloids” Nano Letters 14 (2014)

We report here results of simulations and experiments demonstrating the active control of the phase of light upon reflection from graphene nanoresonators. It has been previously shown that graphene nanoresonators exhibit tunable plasmonic resonances that can be varied statically with the width of the resonators and actively by controlling the charge carrier concentration in the resonators by electrostatic gating.^[1] By using changes in carrier density to modulate the nanoresonator resonance frequency relative to the incident light frequency, it is possible to accumulate a π phase in reflection, enabling active phase modulation. Using double-layer graphene structures in which the graphene layers are separated by a thin dielectric layer, we are able to achieve phase control in an active pixel over a wide range of phase shifts. Combining many such independently controllable resonator pixels holds the potential to steer mid-infrared reflection radiation in phased arrays.
Using finite element electromagnetic simulations, we optimized our design for a predicted 2π phase accumulation upon reflection at a wavelength of 6mu;m under normal incidence plane wave illumination. The double layer structure consists of two layers of graphene resonator arrays with 40nm resonator width separated by a 10nm Al₂O₃ film on a thick sapphire substrate. The tunability range of this structure is approximately 0.85π for a graphene doping of E_F = 0.6eV. Results showing 2π phase tunability will be discussed. We have investigated the fabrication of a prototype double layer structure of patterned graphene into 40nm resonators separated by a layer of 10nm thick Al₂O₃ deposited by atomic layer deposition on 285nm SiO₂ on Si, and we will discuss experimental results.
[1] Brar, Jang, Sherrott, Lopez, Atwater NanoLetters 13, 2541 (2013)

The fascinating electronic and optical properties of nanoparticles, such as localized surface plasmon resonance with metallic nanoparticles and scattering phenomena of dielectric nanoparticles, have attracted worldwide attention to study more deeply on their structures and interaction with optoelectronic devices. Core-shell nanoparticle is a new class of nanoparticles which has multiple layer structures, typically with either metal material (Au or Ag) as core layer, and one dielectric material (SiO2, Si3N4, or Polymer, etc.) as out shell cladding layer, or vice versa. Core-shell nanoparticles with more than two layers, multilayer metal-dielectric nano cups, have also been proposed recently with very interesting properties on plasmon hybridization and field confinement. We have investigated the unique effects of the core-shell nanoparticles on the integrated micro disk resonator. By attaching the core-shell nanoparticle to the disk resonator with gold core and polymer shell, the coupling between the disk resonator and the core-shell nanoparticle results in shift of the resonance wavelength of the disk resonator, depending on the core size/shell thickness of the nanoparticle. An ‘invisibility&’ phenomena found from the coupled core-shell nanoparticle and integrated disk resonator system is emphasized: at certain core size/shell thickness ratio, compared to the original resonance wavelength without core-shell nanoparticle, there is almost no resonance wavelength shift observed. The detailed interaction mechanism between the disk resonator and the nanoparticle is revealed in the electromagnetic field intensity distribution at the resonator resonance wavelength. The dependence of the position and number of core-shell nanoparticles is also discussed. Future studies on this coupled photonic systems will stimulate wide variety of applications.

Semiconductor nanorod arrays (NRAs) are widely studied for application in a wide variety of areas, such as antireflection, self-cleaning and superhydrophobicity. NRAs with dimension from nanoscale to microscale could be fabricated using various template-assisted etching techniques. Dry etching technique is one of the most favorable fabrication methods due to its excellent controllability and compatibility with the CMOS technology. In this work, we have demonstrated a Guided Mode Resonance (GMR) device employing tapered Si3N4 Nano Rod Arrays on the glass substrate, with Si3N4 as the waveguide layer. The detailed light-matter interaction dynamics and generation of the GMR is investigated by the EM field evolution at femtosecond scale, which clearly shows the coupling of the diffracted wave and the guided mode in the tapered NRAs structures. A highly concentrated EM field with enhancement factor of 200~250 is observed with highly flexible tunability. Extended mode profiles to NRA tips and substrates with highly enhanced EM field intensity could be achieved in the higher order resonance modes. This particular EM field enhancement allows the engineering of materials to introduce active materials without disturb to the generation of the GMR. Also, the wide band-gap materials employed in the GMR could effectively avoid intrinsic loss or quenching of fluorescence signal caused by absorption. Our work will facilitate the future applications in fields like fluorescence enhancement, luminescence enhancement, SERS and bio sensing using tapered nano rod arrays.

In this talk, I present an overview of our most recent results on manipulating light with metastructures that influence the light-matter interaction at the extreme. Such “extreme” behavior may be the result of peculiarities of (a) dimensions, (b) nonreciprocity mixed with extreme near fields, (c) giant nonlinearity of phase-change materials; and (d) extreme material parameter values. In the case of dimensions, the comparison and contrast in wave interaction with 3-D structures (volumetric metamaterials) and 2-D surfaces (metasurfaces) have provided possibilities for novel devices for flat optics. In particular, the graphene can be considered as the thinnest possible metamaterials, i.e., one-atom-thick metamaterials and metastructues as ultimate metasurfaces that have become an exciting platform for light-matter interaction. As a second category of extreme optics we consider mixing nonreciprocity with near-field optics, which can lead to exciting physics of near-field optics and time-reversal symmetry breaking, e.g, rotation of optical energy flow in the near-zone proximity of plasmonic structures. Inspired by such optical flux rotation, we proposed a design for the sub-wavelength plasmonic circulators. Giant nonlinearity in the form of phase-change materials (such as VO2) is another case for the extreme optics. We theoretically and numerically show how heat-assisted phase transition, i.e., insulator to metal phase transition, may be an interesting approach to developing low-power nonlinear circuit elements for optical metatronics. Finally, materials with near-zero permittivity and/or permeability (i.e., epsilon-near-zero (ENZ), mu-near-zero (MNZ) and epsilon-and-mu-near-zero (EMNZ) materials) form another category of metastructures for extreme optics. Such media exhibit relatively long wavelengths for high operating frequencies, effectively “loosening” or approximately “decoupling” the notion of frequency from the wavelength. This leads to the phenomenon of “Static Optics” in which the electricity and magnetism in macroscopic scenarios are decoupled while they behave temporally as time-varying fields. This has interesting implications in quantum optics and quantum coupling. All the above examples point to a variety of novel phenomena and exciting physics for "extreme" interaction of electromagnetic waves with material media. I will discuss some of the advantages and challenges of such “light at the extreme” platforms.

Plasmonic and dielectric metamaterial nanostructures fabricated on nano membranes show exceptionally strong nonlinear, electro- and magneto-optical response driven by the competition of electromagnetic and elastic forces. We demonstrate nanostructures with responses several orders of magnitude stronger than in known natural materials. Such materials provide new opportunities for designing compact and highly energy efficient volatile and non-volatile photonic devices driven by light, electric and magnetic field.

Impedance-matched metamaterials with zero refractive index can be achieved by exploiting a Dirac cone at the center of the Brillouin zone. We theoretically and experimentally demonstrate an in-plane Dirac-cone metamaterial consisting of low-aspect-ratio silicon pillar arrays in an SU-8 matrix with top and bottom gold layers in the optical regime.
To verify the fact that it is proper to treat the presented Dirac-cone metamaterial macroscopically as a homogeneous bulk medium with effective constitutive parameters in the in-plane direction near the Gamma-point, we conduct a systematic homogenization analysis on two aspects: the homogenization criterion and conditions of locality.
For the homogenization criteria, we analyze the homogenization of our metamaterial within the array and at the boundary, separately. Within the array, the metamaterial can be treated as an infinite array, which can be analyzed using bandstructure. We compare bandstructures obtained using two different methods: a macroscopic method treating the metamaterial as a homogeneous bulk medium and a microscopic method regarding the metamaterial as an infinite array. Results show that those two bandstructures agree with each other well in the ranges 0<k<0.2 and 0<k<0.49 for the linear bands below and above Dirac point, respectively. At the boundary of the metamaterial, to quantitatively investigate the transition layer in which the local effective constitutive parameters vary from their value of the infinite array to their value of the background medium surrounding the metamaterial, we compare the phase of the electric field at the interface of our Dirac-cone metamaterial with that of a bulk zero-index medium with retrieved constitutive parameters. Results show that, similar to its homogenized model, the metamaterial also has a deterministic boundary in terms of local constitutive parameters even though its lattice constant is not much smaller than the free-space wavelength.
Results also show that the effective constitutive parameters of the presented metamaterial satisfy the conditions of locality: passivity, causality, and isotropy.

Controlling the phase of light through its interaction with plasmonic or photonic modes in nano - and meta - materials opens the way for new generations of meta-devices with enhanced sensing, light trapping or data encryption capabilities. Recently, efficient broadband visible and infrared interferential light trapping has been achieved in ultrathin absorbent films [1,2], based on non-trivial optical interfacial phase shifts. Sensors with ultralow detection threshold have been developed, based on the extreme environmental sensitivity of the light dephasing coefficient of plasmonic nano - antennas at the spectral vicinity of their localized surface plasmon resonance [3,4]. Nowadays, state-of-the-art fabrication routes allow designing multiscale metamaterials, in which both plasmonic and photonic modes can be excited and coupled [5,6]. This enables the so-called “plasmonic optical interference” phenomenon [6]. The potential of this phenomenon for light trapping has been demonstrated, but its influence on the phase of light remains unexplored.
In this work, we investigate the optical phase changes of visible and near-infrared light upon interaction with self-assembled lamellar block copolymer films, with a spectral response reflecting the interplay between plasmonic and optical interference effects. These films consist of intercalated layers of noble metal nanoparticles-doped polymer and layers of pure polymer. The layer thicknesses, nanoparticles sizes and volume fraction are tuned in the range of 20 nm to 100 nm, up to 10 nm and below the nanoparticles dense packing threshold, respectively, thus ensuring a broad tuning of the plasmonic and optical interference properties. Based on a detailed characterization of the films by spectroscopic ellipsometry, which gives access to amplitude and phase information, their plasmonic and interferential spectral features are correlated with their nanostructure. Finally, we explore the potential of coupling plasmonic and photonic modes for multiscale metamaterial-based phase-sensitive environmental detection at the nanoscale.
[1] Kats MA et al, Nature Materials 2013, 12, 20
[2] Kats MA et al, Optics and Photonic News 2014, 0, 40
[3] Kravets VG et al, Nature Materials 2013, 12,304
[4] Lodewijks K et al,, Nano Letters 2012,12, 1655 - 1659
[5] Hagglund C et al, Nano Letters 2013, 13, 3352 - 3357
[6] Choi D et al,, Shin Nano Letters 2014

The phase reversal that occurs when light is reflected from a metallic mirror produces a standing wave with reduced intensity near the reflective surface. This effect is highly undesirable in optoelectronic devices that use metal films as both electrical contacts and optical mirrors, because it dictates a minimum spacing between the metal and the underlying active semiconductor layers, therefore posing a fundamental limit to the overall thickness of the device. Here we show that this challenge can be circumvented by using a metamaterial-mirror whose reflection phase is tunable from that of a perfect electric mirror (phi; = π) to a perfect magnetic mirror (phi; = 0). This tunability in reflection phase can also be exploited to optimize the standing wave profile in planar devices to maximize light-matter interaction. Specifically, we show that light absorption and photocurrent generation in a sub-100-nm active semiconductor layer of a model solar cell can be enhanced by ~20% over a broad spectral band. Based on the reciprocity principle for light waves, Meta-material mirrors are expected to also enable more effective light extraction from thin light emitting devices.

Localized Surface Plasmon Resonances (LSPRs) are collective oscillations of electrons that can be excited in, for example, noble metal nanoparticles. The LSPRs are dependent on material, local refractive index of the surroundings and the geometry the nanoparticle. The far-field optical properties of nanoparticles in planar arrays can often be described as an artificial material, that is, a metamaterial with designable optical properties.
Reflections from disordered arrays of gold nanodisks on a solid support can be described as a Fano interference phenomenon that originates from the interaction between the Lorentzian polarizability of the nanoparticles and the continuous reflection from the bare interface, i.e. from the support/ambient interface without any nanoparticles present (Svedendahl et al. ACS Nano 2012). The interference can be completely destructive and lead to 100% absorption of the incident light for illumination angles above the critical angle of the support/ambient interface (Svedendahl et al. Nano Letters 2013).
Furthermore, the destructive Fano interference lead to so-called topological darkness, for a given wavelength and incidence angle, associated with extremely rapid responses in the phase of the reflected light. A small shift in the LSPR wavelength, for instance by changes in the local refractive index by the adsorption of protein molecules, therefore lead to very large phase shifts in reflection. We show that, using a simple interferometric setup, the phase shifts are about one order of magnitude more sensitive to molecular adsorption in the vicinity of the metal surface compared to regular spectroscopic interrogation of the LSPR.
Finally, if the nanoparticles in the metamaterial layer are elongated in the plane of the boundary, it is possible to achieve complete absorption of circularly polarized light. Additionally, the absorption can be extremely sensitive to the handedness of the incidence light; with 100% absorption of a certain handedness at the same time as the orthogonal polarization can experience reflections approaching unity. In other words, the phase lag between the two linear polarizations (p- and and s-polarized light, with respect to the metamaterials layer interface) can be tuned in order to yield anything from large reflection to large absorption of the layer. These metamaterial layers may be beneficial as interferometric detectors, optical modulators and for detecting optically active molecules.

One of the factors limiting solar cell efficiency is light reflection at the surface and at the multiple interfaces of the device stack. Suppressing reflections increases the amount of light transmitted into the actual absorber layer. In the recent years, many different approaches have been proposed to employ antireflection coatings that can be adapted to any material. Unfortunately, exotic materials, metals, or a fine tuning of nanostructures size and shape are usually required. As a consequence, large scale application is not easy to achieve yet. In this work, we propose the use of optical metamaterials as antireflection coatings. Solar cells will be one of the possible applications.
We develop the 1D and 2D Maxwell-Garnett theory to design effective layers with arbitrary refractive index. We use the transfer matrix method and full-field simulations to explore the optical behavior of different types of metamaterial designs. In particular, we obtain complete suppression of light reflection from a high index material by tuning the filling fraction and thickness of our layer. Moreover, we explore the practical realization of such metamaterials by using Focused Ion Beam to fabricate sub-wavelength linear grating on a silicon wafer. We measure optical reflectivity over the entire visible spectrum from our samples, demonstrating good antireflection properties. The effect of light polarization is also investigated. In fact, the effective refractive index is strongly dependent on the incident polarization. We propose a double layer linear grating in order to achieve zero reflectivity at both polarizations.
In addition, we discuss the transition between deep sub-wavelength and resonant regime. If resonant modes are supported, the effective refractive index deviates from the Maxwell-Garnett theory. We elucidate the role of light trapping in the efficiency of the antireflection properties.
Finally, we extend our approach to a realistic aSi solar cell. We propose the patterning of the transparent conductive oxide (TCO) layer atop the absorbing material, while keeping the rest of the fabrication process flow unaltered. We show an increase of the solar cell efficiency by more than 10%, reaching the single-pass absorption limit of the TCO.

Thermophotovoltaic (TPV) devices can enhance the efficiency of existing energy generation infrastructure by reclaiming heat lost during production processes. In order to maximize the efficiency of these devices, the conversion efficiency of the TPV system needs to be optimized. Most TPV systems consist of three discrete stages: a thermal emitter, a filter, and a TPV diode. One avenue to increase TPV efficiency is to replace the wide-band emitters presently in use with a selective thermal emitter. The selective emitter would absorb wide-band radiation from a heat source and convert it into narrow bandwidth peaks of radiation tailored to the rest of the system. This would dramatically reduce energy loss due to reflection, diode heating, and carrier relaxation. In recent years, metamaterials (MM) have emerged as the ideal medium for the development of engineerable, narrowband selective emitters for such applications.
Most research into metamaterials has utilized gold as the conducting metal. With a bulk melting point of 1064 °C, nano-sized gold patterns will completely degrade at, if not before, the operating temperature of most TPV cells. The most common TPV cells in use today are bulk GaSb diodes. The band gap of this material is 0.7eV, which corresponds to a light wavelength of 1.7mu;m. According to Wien&’s Law, to achieve a blackbody radiation curve with this wavelength at the peak intensity the radiating body would have to be at a temperature of 1400 °C, well above the melting point of gold. Exploring alternate materials and their viability for use in emitters will lead to great advances in current achievable efficiencies.
To operate at the high temperatures required to optimize the efficiency of such photodiodes, traditional MM conductors, such as gold and copper, need to be replaced with more thermally robust metals. Our research consists of a thorough study of potential materials and looks at the design and response of metamaterial selective emitters with emission peaks at 1, 2, 3, 4, 6, 8, and 10 microns, respectively. Designs were made using gold, to compare with current literature, as well as platinum, tungsten, and irridium. Testing will consist of absorption measurements at room temperature and emission testing at increasingly higher temperatures until breakdown is achieved.

Recently, the interaction between plasmonic metamaterials and vibrational modes of a molecule has been studied as a potential method for ultrasensitive light-matter interaction. It has been shown that coupling of radiative modes of plasmonic metamaterials can mimic electromagnetically induced transparency behavior as observed in atomic and molecular interference effects. In this work, we demonstrate coupling between a narrow molecular vibrational mode with a broad plasmonic resonance that enhances or suppresses the molecular resonance. At critical coupling it cancels out the absorption band entirely. The observed phenomenon is investigated by direct absorption using the FTIR and photothermal spectral-microscopy. Plasmonic metamaterials were fabricated using a liftoff process and e-beam lithography. A thin layer of liquid crystal was placed on top of the structures with a weak combinational band at 1912cm^-1. By tuning the length and the periodicity of the plasmonic nano-antennae array we were able to suppress, enhance and cancel out the molecular vibrational mode. A home-built mid-IR photothermal setup is used in combination with a tunable QCL, a plasmonic substrate, an inexpensive silicon photodetector and lock-in detection to measure the sensitivity and specificity of performing vibrational infrared spectroscopy on biomolecules. Mid-IR photothermal spectroscopy combined with plasmonic metamaterials has the potential to detect ultralow concentration of absorbers using low-cost photodetectors and bright tunable QCLs.

Metamaterial analogues of electromagnetically induced transparency (EIT) have been a major focus of the nanophotonics field over the past several years due their ability to produce high quality factor (Q-factor) resonances for use in potential applications such as low-loss slow light devices and highly sensitive optical sensors. The origin of such resonances is the interference between a super-radiant and sub-radiant mode, ultimately allowing suppression of radiative damping. However, plasmonic EIT structures still suffer non-radiative damping due to Ohmic loss, ultimately limiting the achievable Q-factors to values less than ~10, significantly hampering device performance. Here, we report the experimental demonstration of a classical analogue of EIT using all-dielectric silicon-based metamaterials. Due to extremely low absorption loss, a record-high Q-factor of 306 was experimentally observed. Furthermore, the unit cell of the metamaterial was designed with a feed-gap which results in strong local field enhancement in the surrounding medium resulting in strong light-matter interaction. This allows the metamaterial to serve as a refractive index sensor with a figure-of-merit (FOM) of 101, far exceeding the performance of previously demonstrated localized surface plasmon resonance sensors.

Nanophotonic structures provide new opportunities for tailoring the properties of thermal radiation, which results in significant new device and system applications in energy technology. In this talk, we will review some of our recent works in designing nanophotonic structures for the control of thermal radiation, and discuss the applications of these structures for radiative cooling applications.

Selective solar absorption is critical for efficient solar-thermal energy conversion.¹ By absorbing solar energy while suppressing thermal emission in the IR, efficient solar to thermal energy conversion can be achieved. Plasmonic and metamaterial absorbers have shown broadband absorption but are difficult to fabricate on large scales and have yet to survive high temperatures.^2,3 Previously presented metallic photonic crystal structures have shown absorption in the near infrared region and high temperature durability (>1000°C), but are limited in the visible domain due to diffraction losses.⁴ Here, we present a 6” wafer-scale fabricated 2D metallic dielectric photonic crystal (MDPhC) with an average measured absorption of 85% for 250 nm < lambda; < 1770 nm and <10% for lambda; > 3.1 mu;m. The MDPhC has a period of 780 nm with a minimum structure width of 40 nm fabricated via atomic layer deposited (ALD) sidewall lithography across the 6" wafer. Measurements, theory, and simulations are presented to explain the broadband absorption profile.
The MDPhC uses dielectric filled cavity with an anti-reflection coating (ARC) to extend the absorption bandwidth of the MDPhC from the near-UV to the near-IR domain. Finite-difference time-domain (FDTD) simulations show that the dielectric filled cavities red shift the higher order cavity modes into the visible spectrum to create a high density of modes. This overlap of the multiple low-Q cavity modes creates the broadband absorption profile. The cut-off mode of the optical cavities in the near-IR suppresses absorption/emission at longer wavelengths for efficient solar to thermal energy conversion.
The high density of optical cavity modes allows for improved absorption in the visible spectrum, but can be further enhanced via a thin ARC coating. By using the experimentally fitted complex permittivity of ruthenium, an ARC layer of HfO₂ designed for the visible spectrum is calculated via the Fresnel reflection equation to range from 22 nm to 72 nm. With the addition of such a layer, both experiments and simulations confirm the improved absorption in the visible domain with a peak measured absorption of 96% at 688 nm. Thus together with the ARC coating and the high density of optical states, a highly absorbing profile across the solar spectrum is accomplished.
The experimentally demonstrated broadband absorption in metal can lead to major improvements in thermal emitters and photoelectrolysis for advanced solar energy conversion applications.
(1) Lenert, A.; Bierman, D. M.; Nam, Y.; Chan, W. R.; Celanovicacute;, I.; Solja#269;icacute;, M.; Wang, E. N. Nat. Nanotechnol.2014, 9, 126-130.
(2) Aydin, K.; Ferry, V. E.; Briggs, R. M.; Atwater, H. A. Nat. Commun.2011, 2, 517.
(3) Cui, Y.; Fung, K. H.; Xu, J.; Ma, H.; Jin, Y.; He, S.; Fang, N. X. Nano Lett.2012, 12, 1443-1447.
(4) Yeng, Y. X.; Ghebrebrhan, M.; Bermel, P.; Chan, W. R.; Joannopoulos, J. D.; Solja#269;icacute;, M.; Celanovic, I. Proc. Natl. Acad. Sci.2012, 109, 2280-2285.

Emission of photons in the form of incoherent thermal radiation is the process that governs many energy conversion schemes and contributes to the energy efficiency of appliances and buildings. Conventional approaches to enhance or suppress thermal emission as well as shape the thermal emission spectrum has been based on searching for appropriate material candidates: natural, idealized and composite (e.g. photonic crystals and metamaterials). In this work, we explore an approach to the radiative heat extraction based on the manipulation of emission via the proper choice of morphology of active thermal emitters and/or passive thermal extractors having one or more dimensions on the scale at or below the thermal emission peak wavelength.
At the core of this approach is the engineering of the local density of confined photon states in two- and three-dimensional potential traps, which include wells, wires, and dots. By building upon the parallels with the engineering of the electronic density of states in quantum wells, wires and dots, we aim to develop a fundamental understanding to the tailoring of far-field thermal emission/absorption by the morphology-dependent local density of states in low-dimensional structures. We will present several examples of micro- and nano-object designs with tailored thermal emission/absorption spectra for applications including selective solar absorption, thermophotovoltaic energy conversion, radiative cooling and thermal up-conversion of photon energy.

Broadband plasmonic absorbers have recently become particularly important for several sunlight-harvesting applications, which exploit either light-to-heat conversion, such as in solar thermoelectrics or thermal photovoltaics, or light-to-hot electron conversion, such as in photochemical and solid-state devices. In order to fulfill the inherent requirements of solar emission, optimal manifestations of these devices need to possess multi-functionality, i.e. to be simultaneously broadband, polarization-insensitive and wide-angle. So far, despite impressive progress, most of the proposed designs fall short of combining fabrication facility and scalability with high performance. Here, we report on the design and implementation of a novel solar absorber based on an array of tapered triangles in a metal-insulator-metal (MIM) configuration, which exploits different absorption phenomena to achieve ultrabroadband (88% average absorption in the range 380-980 nm), wide-angle and polarization-insensitive absorption. Furthermore, we are able to fabricate the device with a potentially low-cost and scalable method, a must if plasmonic solar harvesting is to become a reality outside the research laboratory. Benefitting from fabrication facility of the design, we were also able to fabricate the absorbers on a cm² scale and performed their steady-state and transient thermal characterizations using a fast IR camera. We show that, due to its sub-wavelength thickness and negligible heat capacity, this plasmonic absorber has a transient response time of less than 13 ms and thermal sensitivity above 2.4*10³ K/W. Compared to an absorbing coating with a commercially used blackening spray, our plasmonic design has more than one order of magnitude faster transient thermal response while exhibiting comparable steady-state thermal sensitivity. Finally, we implemented the proposed absorber on a commercial thermoelectric radiation detector and demonstrate substantial improvement in characteristic response time of the device.
References:
[1] G. Tagliabue, H. Eghlidi, and D. Poulikakos. Facile multifunctional plasmonic sunlight harvesting with tapered triangle nanopatterning of thin film, Nanoscale5, 9957-9962 (2013).

Plasmonic circuitry is an attractive field of study that promises optical signal processing at the nanometer scale. Metals, however, exhibit high dissipative losses when they interact with light, and this ultimately challenges practical applications of plasmonic nanocircuits. To mitigate this problem, we introduced discrete optoplasmonic networks, which integrate high-Q optical microcavity (OM) resonators and plasmonic nanoantennas into well-defined hybrid structures [1]. The optoplasmonic structures achieved the directed photon transfer from fluorescent dye-tethered plasmonic nanoantennas into the OMs with a relative photon transfer efficiency as high as 44% [2]. To further demonstrate the capability of optoplasmonic circuits as long-range photon carriers, we will show resonance energy transfer based spectral coupling between quantum dot-coated OMs and fluorescence dye-functionalized nanoantennas. Through spectrally modulated excitation and emission of respective elements in the optoplasmonic nanocircuits, photons guided by the OMs were efficiently outcoupled into the free space and precisely measured using fluorescence microscopy. Finally, we will provide experimental evidences that validate the theoretically proposed concept of optoplasmonic superlenses [3], by measuring photons transferred to the metal nanoantennas that are spatially separated from the emitter by the OMs. The optoplasmonic nanocircuits will pave the way for a new class of integrated optical circuits for future energy, sensing and imaging applications.
References
[1] 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.
[2] 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.
[3] Boriskina, S. V.; Reinhard, B. M. “Spectrally and Spatially Configurable Superlenses for Optoplasmonic Nanocircuits” Proc. Natl. Acad. Sci. USA, 2011, 108, 3147-3151.

The interaction of light with graphene and related 2d materials, and its related processes such as carrier relaxation, photoconversion, energy transfer and plasmon propagation, embodies rich physics and strong potential for disruptive opto-electronic technologies. In this talk, several examples of the opto-electronic and nano-photonics response of novel 2d material heterostructures are being addressed.
First, strong interactions between graphene and nanoscale light-emitters are controlled and detected. Because graphene is gapless with tunable carrier density, it can effectively behave as a semiconductor, a dielectric, or a metal. We exploit this to electrically control of optical emitter relaxation pathways. Specifically, we control whether emitter excitations are converted into either photons, electron-hole pairs, or plasmons with confinement to the graphene sheet below 15 nm.
Second, nano-structured sandwiches of graphene with boron nitride have resulted in high quality plasmonic systems for infrared light. We discuss new configurations of these electrically tunable metamaterials and plasmonic circuits with in-situ tunable control of light at lenghtscales more than a factor 50 below the free-space wavelength. Finally, we report strong improvements of the graphene plasmon lifetime and propagation lengths and we assess the intrinsic loss mechanisms.

Among many remarkable features, the gapless and two-dimensional character of graphene has direct impact on the way it interacts with light. The strength of this interaction, together with the high electronic conductivity, makes graphene a primary material for photovoltaics and the development of novel, efficient and ultra-fast photodetectors. On the other hand, the potential of graphene in nanophotonics has yet to be fully exploited. For example, in combination with quantum emitters, graphene can reveal new functionalities.
This paper is our contribution to the development of a novel platform for quantum position nanosensors, based on the energy transfer from a single emitter to a graphene monolayer. In this work we study quantitatively the energy transfer mechanism between single long-lived quantum emitters and graphene. In particular, organic Dibenzoterrylene (DBT) molecules are embedded as impurities in thin Anthracene crystals and deposited on pristine graphene, grown by chemical vapour deposition.
The sample is well characterized by Raman spectroscopy and Atomic force microscopy. Single DBT molecules are pinpointed by detecting the characteristic anti-bunching dip at zero delay in the intensity autocorrelation function.
By measuring the excited-state lifetime of single molecules at distances (d) below 80 nm from the carbon monolayer, we estimate the transfer efficiency independently from the instrumental collection capability. We report the highest — to our knowledge — ever measured transfer efficiency from single emitters to graphene, amounting to (61 ± 21)%. Because of such interaction strength, we can detect a FRET-like effect to distances well beyond the characteristic 10 nm of standard acceptor-donor energy transfer.
We compare the statistical distribution of DBT lifetimes next to graphene with that on a reference sample and observe a net reduction of the average value, due to the opening of a novel non-radiative decay channel to the monoatomic carbon layer.
The experimental results are well described by a semi-classical model, yielding a universal d^minus;4 dependence of the coupling efficiency, ensued from the bidimensionality of the system. The presented investigation on our DBT:anthracene platform constitutes a first proof of principle for a graphene-based nano-ruler, where ideally the distance to a surface can be measured by extracting the lifetime of a well-referenced single emitter, serving as a marker.
This work offers new insights over novel effects and functionalities of the interlace between quantum emitters and graphene, as a novel nanophotonic material.

As one emerging plasmonic material, graphene can support surface plasmons at infrared and terahertz frequencies with unprecedented properties due to the strong interactions between graphene and low frequency photons. Since graphene surface plasmons exist in the infrared and terahertz regime, they can be thermally pumped (excited) by the infrared evanescent waves emitted from an object. Here, we show, for the first time, that thermal graphene plasmons can be efficiently excited and have monochromatic and tunable spectra, thus paving a way to harness thermal energy for graphene plasmonic devices. We further demonstrate that "thermal information communication" via graphene surface plasmons can be potentially realized by effectively harnessing thermal energy from various heat sources, e.g., the waste heat dissipated from nanoelectronic devices. These findings open up a new avenue of thermal plasmonics based on graphene for different applications, ranging from infrared emission control, to information processing and communication, and to energy harvesting.

We demonstrate a graphene-based Salisbury screen consisting of a single sheet of graphene placed in close proximity to a gold back reflector. The light absorption in the screen can be actively tuned by electrically gating the carrier density in graphene with a liquid electrolyte. Spectroscopic reflectance measurements in the infrared (IR) were performed in-situ as a function of the gate bias. The reflectance spectra were analyzed using a Fresnel-based transfer matrix model by treating graphene as an infinitesimally thin sheet with conductivity derived from the Kubo formalism. Temporal coupled mode theory was used to explain the absorption mechanism in the Salisbury cavity. A change of ~6% was achieved in the optical absorption of graphene by tuning the applied gate bias.

The use of two-dimensional (2D) materials in optoelectronics applications has attracted much attention due to their fascinating optical and electrical properties. For instance, graphene-based photodetectors have shown ultrafast and broadband responsivity, transition metal dichalcogenide (TMDC) based photodetectors can exhibit internal amplification, and heterojunctions involving 2D materials have succeeded in boosting photoresponsivity to up 10⁸ A/W. However, the external quantum efficiency of 2D material based photodetectors is typically limited by their poor optical absorption, a feature which is a direct consequence of their atomic thickness. Here, we experimentally demonstrate the use of a Fano-resonant photonic crystal (FRPC) structure to significantly boost absorption in atomically thin materials. The FRPC structure is composed of a photonic crystal (PC) slab separated from a silver back plane with the 2D material embedded in the high field region below the PC. Graphene is chosen as an example 2D material in both simulations and experiments due to its relatively poor absorption compared to TMDCs, thus representing the worst-case scenario. Photocurrent measurements demonstrate that graphene absorbs 77% of the incident light independent of the incident polarization, which is the highest value reported to date in the telecommunications band, to the best of our knowledge. Furthermore, the structure with the silver back plane performs equally well in the visible regime and for a wide range of 2D materials with varied absorptivity, with up to 95% absorption being achieved in the ultrathin film. In addition to the enhanced absorption, light propagates up to 16 mu;m from the incident spot within the structure when graphene is absent. This property enables one to detect light incident away from the 2D material and is particularly beneficial in harvesting light from large areas in field-effect-transistor based graphene photodetectors.

Refractive lenses are some of the most ubiquitous optical components and are widely used in applications such as imaging and focusing of light in quasi far field region. However, as the physical dimensions of these dielectric based lenses are reduced to optical wavelength scale, their focusing capabilities deteriorates. They also carry uncured problems of chromatic and spherical aberration. On the other hand 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 microlens which concentrates wavelengths of light across the entire visible spectrum to a single spot along its center axis. Numerical simulations obtained using finite difference time domain (FDTD) methods suggesting chromatic aberration correction along with concentration of incident electromagnetic waves using novel metallic devices will be presented. The proposed plasmonic lens focuses single wavelengths as well as broadband white light into small focal volume and is extraordinarily transmissive. The extraordinarily high transmission in this microlens is understood to be predominantly due to reduced metal surface area, while the surface plasmons along with diffraction is understood to be responsible for its high enhancement factor of transmission efficiency.

Hyperbolic meta-materials (HMMs) are known to have a hyperbolic dispersion contour due to their extreme anisotropy, and thus have ultrahigh photonic density of states in broad-band, leading to many applications such as enhancing spontaneous emission and thermal radiation. Van der Waals (VDW) force, or in a more general form: the Casimir interaction, which governs the surface energy as well as the wetting property of certain surface, sensitively depends on the dielectric properties of the whole system over broad frequency range. It has been widely accepted that the Casimir forces which arises from quantum and thermal electromagnetic fluctuations, are generally determined by the permittivity and permeability of all media involved and the permitted electromagnetic modes in the system. According to Lifshitz theory of VDW, the interaction free energy can be calculated as the sum of the fluctuation energies for each permitted normal electromagnetic modes of the system at every frequencies.
Based on this fact, it can be hypothesized that the wetting property can be dramatically changed by HMMs considering the hyperbolic dispersion and the associated ultrahigh photonic density of states. Here in this work, based on the Lifshitz theory we theoretically calculate the VDW forces as well as the works of adhesion with different kinds of HMMs as the substrate. In our calculation, HMMs based on Au/MgF₂ with different metallic filling ratio and structures: (a) layered metal-dielectric and (b) nanorod arrays are tested. The works of adhesion which corresponds to the contact angle of liquid on solid surface are calculated for the wetting of water on the HMMs surface. According to the calculation based on the effective medium treatment of HMMs and Lifshitz theory, both these two kinds of HMMs can indeed change the wetting property of the surface but they behave more like a diluted metal or dielectrics, for which the wetting property is mainly determined by the filling ratio instead of the structures. Further experimental studies on wetting properties of real multilayered metal-dielectric HMMs and corresponding theoretical studies considering the multilayered structures of HMMs instead of effective medium treatment have been accomplished. Both experiments and calculations show that the top layer of HMMs will dominate the short range VDW as well as the work of adhesion. One layer of dielectrics as thin as one molecular cutoff can screen at least 70% of the wetting properties of the substrate, which indicates that it is not practical to use multilayer based HMMs to changing the wetting properties.

We present a highly wavelength-tunable plasmonically induced transparency (PIT) device composed of graphene nanostrips for the mid-IR region. We demonstrate that PIT can be achieved using a single layer of graphene nanostructures. Wavelength-shift active control of the PIT resonance is realized by varying the Fermi energy of the graphene without reoptimizing and refabricating the nanostructures. Meanwhile, the three-level plasmonic system is demonstrated to well explain the formation mechanism of PIT in the graphene nanostrips. This makes graphene PIT device more useful than metallic PIT devices. We believe that this method of actively tuning the wavelength of PIT may open up avenues for the development of compact elements such as tunable sensors, switchers and slow light devices.

It has been widely reported that plasmonic effect in metallic nanostructure can enhance the light absorption in optoelectronic device such as solar energy devices, photo-detector, and spectrum imaging sensors. Especially, nanostructures with novel metal have been investigated to characterize their unique optical and electronic properties for light absorption. Nanopillar, nanohole, nanocone, and nanowire were already fabricated and proven by lithographical technique, including porous aluminum template, electron beam lithography, nanosphere lithography, photolithography, nanoimprint lithography, and so on. Recently, nanoring and nanotube like metallic structure have a lot of attention due to the unique electric and magnetic resonance and low optical loss.
In this study, we report the characteristics of the free-standing Au nanoring and nanotube fabricated using template based electrochemical deposition, which is possible to control the height, pitch, shape, and diameter of Au nanoring and nanotube. First, polymer template is formed on the ITO/glass or FTO/glass using nanoimprint lithography and reactive ion etching. Then, sacrificial metal such as Cu and Ni is selectively deposited on the substrate using electrodeposition. Then, SiO₂ is deposited on top surface of metal nanostructure and the polymer template is removed using N,N-dimethylformamide. Next, Au is deposited on the side of metal nanostructure using electroless deposition. Finally, SiO₂ and sacrificial metal nanostructure was removed, then Au nanoring or nanotube are formed on the substrate.
The Au nanoring or nanotube are analyzed using scanning electron microscopy, UV-Vis spectrometer, and Ellipsometer. In addition, FDTD simulation results are compared with experimental data. Then, multiple resonances are generated as Au nanotube due to magnetic resonance and Fabry-Perot resonance, which can ehance the light absorption. These results are important to the design of metallic nanostructure for optoelectronic devices.

Surface plasmon polaritons (SPPs) are surface waves that exist at a metal-dielectric interface resulting from the coupling of electromagnetic radiation to charge oscillations. The study of SPPs offers a richness of phenomena that has lead to a diverse range of applications in sensing, imaging, and nonlinear optics. Developments over the last decade have also led to an unprecedented ability to control light through plasmonic based nanostructures. For instance, enhanced transmission of light through a subwavelength aperture can be achieved by surrounding the aperture with surface-structures such as grooves, while their spectral properties can be tuned by scaling the underling periodicities of the surface nanostructures. However, one shortcoming with all plasmonic spectral color-filters proposed till date has been that a plasmonic device once fabricated only exhibits optical response at a certain frequency when excited at normal incidence. A change in incidence angle only results in a slight-wavelength-shift in the transmission peak; and hence cannot easily be utilized in applications requiring a full-color RGB response; which has until now required fabrication of three physically separate structures with scaled periodicities. Furthermore, the quality-factor of the optical resonances in these structures is primarily limited because of the absorption losses in the metal-film as well as scattering from the surface nanostructuring. Here, we demonstrate the design, fabrication and experimental characterization of a single aperiodic slit-groove based plasmonic device that exhibits angle-selectable RGB color response with high-Q and high optical contrast at optical frequencies. The device consists of a single-subwavelength slit surrounded by multiple grooves whose position, widths and depths are individually optimized to exhibit the desired color response at specific incident angles. It is important to note that since a single plasmonic structure exhibits all the three colors as defined by the user (RGB at 0#778;, 10#778; and 20#778; resp.), the design algorithm optimizes the spectral transmission to match these user-defined conditions at all the three incident angles simultaneously.
The optimization algorithm based SPP device design as applied to a simple slit-groove based device results in an aperiodic device with higher quality factor, optical contrast and spectral control than is not possible with standard periodic device designs primarily employed in experimental demonstrations till date. We expect the same optimization algorithm, as devised here, to be applicable to a number of other plasmonic and photonic device designs. This reverse-engineering approach of designing structures with user-defined initial conditions such as a high-Q at specific frequencies or a desired color-response at specific incident angles hold promise for next generation aperiodic nanophotonic and nanoplasmonic devices.

Subwavelength optical scatterers such as nano-holes, grooves or slits etched in a metal film are efficient sources of surface plasmon polaritons (SPPs). Interference between SPP waves can lead to unprecedented control of light at the nanoscale, with applications in higher-efficiency solar cells, surface enhanced Raman scattering, and compact biochemical sensors. Plasmonic interferometry has the potential to bring the advantages of conventional optical interferometry to the micro- and nano-scale regimes, while bearing the merits of high throughput and real time monitoring.
We have recently reported on a compact, high-throughput plasmonic sensor based on plasmonic interferometry optimized for real-time monitoring of glucose in aqueous solutions [1]. The sensor consists of a spatially dense, planar array of plasmonic interferometers (with a density >1,000/mm²)_. Each interferometer is comprised of two 200nm-wide, 10µm-long grooves flanking a 100nm-wide slit etched in a 300nm-thick silver film. The detection limit of the plasmonic sensor for glucose in aqueous solutions is 5.5mu;M, with a sensitivity of 105,000%/RIU (refractive index units) or 0.2 × 10⁵%M at 590nm.
In order to improve the sensor&’s selectivity to glucose, we incorporate a novel molecular recognition scheme that couples plasmonic interferometry with dye chemistry (specifically the Amplex Red enzyme assay). In this device, optically inactive D-glucose is rapidly consumed to oxidize Amplex Red into a red-fluorscent molecule Resorufin in a 1:1 stoichiometric ratio. The reaction is monitored in real-time by simply measuring changes in the light intensity transmitted through the slit of each interferometer, which is a precise indicator of glucose concentration in solution. This device is both highly sensitive, with a measured intensity change of 1.7 × 10⁵%M (i.e., about one order of magnitude more sensitive than without assay) and selective for glucose in picoliter samples, across the physiological range of glucose found in human saliva (20 - 240mu;M).
The plasmonic interferometer concept has a plethora of useful applications besides sensing. We can extract the dispersion of SPP excitation efficiency, optical constants of interface materials over a broad range, from the light transmission through a dense array of plasmonic interferometers within extremely small areas and sample volumes. This study enables us to predict the performance of plasmonic interferometers accordingly, which would help on the design of the best devices for sensing application.
By varying specificity scheme and tuning device parameters, plasmonic interferometry can lead to point-of-care diagnostic tools for biomedical sensing of clinically relevant analytes (such as glucose and insulin) within a very small volume (sub-picoliter) of biological fluid.
[1] J. Feng et al., Nano Lett. 2012, 12 (2), pp 602-609

Topologically ordered phases of matter, such as topological insulators and topological semimetals, represent a new class of materials, unique in their ability to support edge or surface states that are protected against disorder. Not only is the novelty of these boundary excitations interesting, but the extraordinary isolation they experience from perturbations makes them ideal candidates for overcoming dissipation and de-coherence in many quantum devices, including quantum computation. Recently the possibility of achieving one-way backscatter immune transportation of light by mimicking the key properties of these solid state systems has received a great deal of attention. Thus far however, demonstrations of photonic topological order without the aid of an external magnetic field have relied on photonic crystals with precisely engineered lattice structures, periodic on the scale of the operational wavelength and composed of finely tuned, complex materials. In this work we propose a novel and much simpler effective medium approach towards achieving topologically protected photonic surface states. Using just a few effective electromagnetic parameters to describe the propagation of plane-waves we reveal the first time-reversal-invariant metamaterial, exhibiting properties derived from artificial subwavelength structures, with photonic surface states robust against spatial disorder on all length scales and for a wide range of material parameters. In particular, we show that combining chirality with a hyperbolic permittivity tensor, containing components of mixed sign, results in the complete separation of elliptically polarised waves of opposite handedness, in k space. As with previously studied lattice systems, a region in parameter space separating sets of solutions associated with an opposite sense of rotation can be shown to possess non-trivial topology and must therefore support protected boundary modes. Using analytical calculations as well as full wave simulations we confirm the existence of unidirectional backscatter immune surface waves at the interface between a chiro-hyperbolic medium and a vacuum, both in the effective medium limit and for a realistic metamaterial structure for microwave frequencies. As our metamaterial scheme does not require the use of an external magnetic field and therefore can in principle operate at any frequency we believe that the effective medium route to topological order will pave the way for highly compact one-way transportation of electromagnetic waves in integrated photonic circuits.

Magnetic susceptibility of materials are usually near zero at optical frequencies. Therefore, one needs to pursue magnetic enhancement in the optical region through indirect approaches. The development of nanotechnology enables fabrication of subwavelength metamaterials from non-magnetic substances that exhibit strong optical magnetic response. First proposed by Engheta et al, polarization independent magnetic response can be produced from a 3D isotropic structure of mutually separated metal nano-spheres forming closed loops.[1] Experimentally, isotropic magnetic plasmon resonance has been observed in several optical metamaterials systems[2, 3], but technical challenges remain in improving the uniformity, stability, or structural tunability. Recently, isotropic gold nanoclusters from colloidal approaches shown to have decent tunability in optical and electric properties[4, 5] have opened up new approaches. The magnetic response in these systems can be significantly improved if one is able to pack more than only one layer of metallic nano-spheres on the same dielectric core.
Here, surfactant templated seed grown gold nanoclusters are investigated for their magnetic plasmonic properties. These nanoclusters are composed of a polystyrene core with numerous (100-800) gold nanobeads, tightly closed-packed on their surface. While the beads are on average distanced by about 1nm from each other, they are protected by a layer of surfactant and never touch. Experimentally, these isotropic nanoclusters showed a broad extinction profile with multiple resonance peaks, strongly dependent on the size of the cluster, which can be easily tailored by control over the relative amount between the growth and seed precursor. In order to explain the origin of these multiple resonances, Finite-difference time-domain (FDTD) simulations and multipole analysis were performed on the modeled nanoclusters. These studies confirmed the existence of a strong magnetic resonance in these particles that intensify with the increased number of the nanobeads as well as decreasing the inter-bead distance. The strong and tunable magnetic dipole resonance in these nanostructures make them strong candidates for metamaterial applications.
1 A. Alu and N. Engheta, Opt. Exp.,17, 5723-5730, 2009.
2 J. A. Fan, et al.Science, 328, 1135-1138, 2010.
3 S. N. Sheikholeslami, et al. , Nano Lett.,13, 4137-4141, 2013.
4 B. L. Sanchez-Gaytan, et al. J. Phys. Chem. C,117, 8916-8923, 2013.
5 A. T. Fafarman, et al. , Nano Lett., 13, 350-357, 2013.

Surface plasmon polariton (SPP) wave has been drawing tremendous attention since the observation of enhanced optical transmission (EOT). Now, its application covers a wide range of disparate fields: from energy harvesting to biomedical sensing to photo-detection. As the nanofabrication technology is advancing in optoelectronics, device sizes keep shrinking even to nano-scale. Therefore, manipulating light over a nano-scaled region is highly desired, for example, a nano-sized photodetectors needs a highly focused field onto the nano-sized region, which is formidable from a conventional optical point of view due to the diffraction limit. However, by taking advantage of SPP wave, we can easily find a solution. In this report, we investigated the promising application of SPP wave for in-plane light field focusing through milled holes in a metal silver film (a structure also known as the plasmonic concentrator) both in modeling and experiments. We proposed a fast and efficient method in designing the proper nanohole-structures, followed by the experimental demonstration. This study will focus on cylindrical holes, both individually and as parts of a larger array. As is well-known, SPP can be generated by corrugated nano-structures in a metal film. Particularly, a single nano-hole can be a good SPP wave source excited by a wide range of incident wavelengths. Quite fortunately, such a SPP wave can be nicely described by a simple 2D wave model. By tuning the geometry size of the hole, different SPP excitation efficiencies and launching phase can be achieved for different wavelengths, which we can further quantify through numerical simulations. Combining with the 2D wave model, we demonstrated the design of various plasmonic concentrators, with a field enhancement factor easily reaching up-to several times or more in a confined nano-sized region. The experimental methods involve fabricating nanoholes on silver films and using plasmonic interferometry to extract information about field enhancement as it relates to SPP wave propagation, interference, and scattering.

Ohmic loss in metals at optical frequencies poses a great challenge to the use of plasmonic metamaterials in many applications. Metamaterials formed from high permittivity resonators exhibit electric and magnetic Mie resonances, providing a lossless alternative to plasmonic implementations. Here, we present our work on all-dielectric metamaterials consisting of cylindrical unit cells made of silicon. By controlling the aspect ratio the spectral separation between electric and magnetic dipole resonances is maximized. This leads to maximizing the single negative band in the metamaterial, which in turn leads to near-perfect reflection. We experimentally demonstrated a single layer metamaterial with a 200 nm broad reflectance band at optical frequencies with an average reflectance over 98% and peak reflectance of 99%. The study is also extended to disordered metamaterials and it was found that near-unity reflection is preserved as long as resonator interaction is avoided. Due to the simple geometry of the unit cells and the ability to tolerate disorder, all-dielectric metamaterial perfect reflectors are amenable to self-assembly based fabrication techniques and thus open the door to large scale metamaterial implementations.

Fabrication of three-dimensional nanostructures is a challenging process, often requiring multiple cycles of patterning, deposition and planarization. Planar and three dimensional plasmonic nanostructures were fabricated through a combination of focused ion beam (FIB) milling and electron beam lithography (EBL) patterning, achieving truly chiral nanoscale patterns in a single deposition step. The plasmon resonance spectra of these structures were investigated computationally and experimentally, identifying interesting plasmon resonance phenomena in the visible and near infrared ranges of the EM spectrum. The effect of the substrate (oxide, semiconductor, or metal) is also considered, influencing the ultimate usability of the metamaterial for the selective manipulation of radiation at nanometer length scales in transmission or reflection modes.

Epsilon-Near-Zero materials often experience high losses and resulting low transmission. The recent advent of materials with simultaneously zero effective permittivity and permeability circumvent this issue with properties that include impedance matching and low losses. Such double zero refractive index materials can be achieved by taking advantage of the Dirac cone dispersion relation at the center of the Brillouin zone. We experimentally demonstrate a metamaterial structure consisting of a silicon pillar array embedded within an SU-8 matrix with top and bottom gold mirrors that has double zero refractive index within the telecom regime.
The metamaterial is designed through FDTD simulations that optimize pitch and radius through a two dimensional sweep to minimize the absolute value of complex refractive index. Such a structure offers the advantage of in-plane isotropy. We consider the dependence of transmission on incident angle for both the photonic crystal based metamaterial and its equivalent homogenous medium. Similar transmission results verify the effective homogenization of the proposed material.
To experimentally verify the zero refractive index, we fabricate a prism consisting of the metamaterial structure coupled with either a silicon or an SU-8 slab waveguide, and an input coupled silicon waveguide. Through optimization of the prism device and its associated slab waveguide, we are able to fully observe an unambiguous demonstration of the effective index of the metamaterial. The device is imaged from the top with a near infrared camera and based on the observed angle of the refracted beam, we measure an effective index for wavelengths from 1260-1680nm. We observe a linear dispersion in the positive and negative index regions, separated by a finite bandgap corresponding to a zero refractive index. The bandgap can be attributed to minor imperfections in fabrication. This design offers the potential for on-chip integration of zero refractive index materials with current optical platforms.

With the aim of gaining greater control over the passage of heat in a solid medium, the concept of a metamaterial, with an engineered sub-structure/arrangement of materials has gained much popularity. In this context, we have previously shown that the dual use of the thermal extremum principle - whereby the propagation of heat takes the path of least thermal resistance, along with suitable coordinate transformation techniques - for inducing thermal conductivity anisotropy, yields quantitative criteria for tracing and consequently manipulating the propagation of heat flux [1]. We experimentally demonstrate the bending of heat flux through positive/negative refraction at the interface of such multilayered anisotropic composites considering the respective thermal conductivity tensors (κ_ij). The angle of refraction (Phi;_r) is considered to be positive or negative , according to whether the horizontal and the vertical components of the incident and refracted heat flux vectors point in the same or opposite direction. We will show practical implementation of the anomalous refraction phenomenon in anisotropic composites through ingenious placement of two isotropic thermal conductive materials, constituted from copper with isotropic thermal conductivity (κ₁ = 391 W/mK ) and PDMS (poly-dimethylsiloxane, κ₂ = 0.17 W/mK ). We experimentally realize representative positive refraction of Phi; = 58^o and negative refraction of Phi; = -29^o at the interface of such composites. The thermal profiles of the diffusive flux were captures through infrared imaging (FLIR 320) of the metamaterial surfaces. Such phenomenon could be used to the directing of thermal energy to useful purpose, e.g., thermal cloaking/insulation, heat concentration and perhaps, a perfect thermal lens.
Reference
[1] Krishna P. Vemuri, P.R.Bandaru, Geometrical considerations in the control and manipulation of conductive heat flux in multilayered thermal metamaterials , Applied Physics letters,103,13311.

Silicon (Si) photonics can demonstrate compact, single-chip, CMOS-compatible photonic integrated circuits for optical interconnect applications. Current Si modulators, however, suffer from low efficiency and large footprint. Recently, epsilon-near-zero (ENZ) transparent conducting oxides (TCOs) have shown promise for integrated electro-absorption modulators. In this work, we propose a Si slot-waveguide electro-absorption modulator based on engineered indium tin oxide (ITO) materials, which demonstrates high extinction ratio and low insertion loss over a wide optical bandwidth.
The ITO samples were prepared by radio-frequency magnetron sputtering on Si substrates from an ITO (99.99%) disc target of four inches in diameter. The ITO target power was 200 W, and post-deposition annealing treatments were applied to tune the optical dispersion of the films. The permittivity was measured using spectroscopic ellipsometry. According to the Drude model, the permittivity of ITO can be tuned by actively changing the carrier concentration. With an appropriate bias voltage to increase the carrier concentration from N₁ (1.78E21 cm^-3) to N₂ (3.69E21 cm^-3), the magnitude of the permittivity is changed by a factor of |ε₂/ε₁| =0.29 at a wavelength of 1.31 mu;m. With such an ITO layer embedded in a dielectric waveguide, the light absorption will be modulated accordingly.
The proposed modulator consists of the following layers (from bottom up): 220-nm thick Si, 7-nm thick silicon nitride (Si₃N₄) buffer, 10-nm thick ITO, 7-nm thick Si₃N₄ buffer and 160-nm thick polycrystalline-Si. The modulator is 500-nm wide. A voltage applied across the doped Si and the ITO layer tunes the permittivity of the ITO layer by carrier accumulation at the ITO/Si₃N₄ interface. The optical loss (α) of the slot-waveguide modulator is determined by both the waveguide structure and the permittivity of the ITO layer. A smaller |ε| can lead to an enhancement of the electric field magnitude confined in the ITO layer due to the continuity of the normal component of the electric displacement field. Therefore, it is important to engineer the ENZ condition so that a minimum |ε| is achieved for a state with high optical loss.
Based on the measured permittivity of ITO, a 2D mode solver is employed to simulate the optical loss of the fundamental TM mode of the modulator. A maximum extinction ratio, ER = α(N₂) - α(N₁), of 32.2 dB/mu;m is achieved at a wavelength of 1.36 mu;m with an insertion loss, IL = α(N₁), of only 1.1 dB/mu;m. At 1.31 mu;m, the ER and IL are 27.4 dB/mu;m and 0.9 dB/mu;m, respectively. Between 1.24 mu;m and 1.50 mu;m, the ER is above 11 dB/mu;m while the IL is less than 2.0 dB/mu;m. We have also simulated structures with different buffer materials: silicon dioxie (SiO₂) and Si₃N₄, and different buffer thicknesses, 10 nm and 7 nm. With the same thickness, the structure with the Si₃N₄ buffer shows higher ER as well as higher IL. For a given buffer material, the 10-nm thickness shows lower IL and lower ER.

The nonlinear effect of a metamaterial perfect absorber (MPA) was investigated under high electric field terahertz (THz) pulses. This nonlinear MPA has potential applications of terahertz imaging or sensing. The MPA we presented in this paper is on a thin flexible substrate, which gives it possibility to be used in wearable devices. The device was fabricated using semiconductor transfer method to transfer the GaAs patches and gold electrical split ring resonators (ESRRs) to polyimide substrate to form the nonlinear metamaterials and then the backside of the polyimide is coated by gold ground plane to construct the MPA. In our design, the gap in the electrical ring resonators is 1.3mu;m to realize high field enhancement in the gap. The reflection spectrum of the MPA was measured with the THz time domain spectroscopy using different field strength. In the low field condition, the device can realize the near-unit absorption at the resonance frequency (around 0.68THz). With the increase of the incident electric field, the GaAs in the gap will be excited through the impact ionization. As a result, the resonance will be damped and the absorption at the resonance frequency will be damped gradually. The modulation depth of absorption at the frequency is about 30%. To understand the origin of the field strength dependent of the nonlinear MPA, we performed the full-wave simulation of the absorption of the device. The GaAs was modeled with the Drude response in the simulation. The results agree well with the experiment.

While the field of plasmonics has grown from initial interest in the plasmons of noble and coinage metals in various geometries, many envisioned applications call for patterning over large areas where low-cost and highly abundant materials would be most desirable. We have begun to study the properties of Aluminum as a plasmonic material, examining its plasmonic properties in simple geometries. [1] In nanostructures, the localized plasmon resonance is determined not only by the usual considerations of size, shape, and nature of dielectric environment, but also the concentration of bulk oxides that may be present in the structure. [2] The CMOS-compatibility of Al makes it quite desirable for device applications: a wavelength-sensitive photodetector that fully integrates color filtering with photosensitivity in a single device is one such example. A combination of near-field and far-field coupling effects can be used to render Al nanostructure arrays as vivid, monochromatic pixels suitable for flat-panel displays. In addition to deposition methods, Al nanostructures can also be synthesized chemically, with unique geometries and tunable plasmon resonances.
[1]M. W. Knight, Lifei Liu, Yumin Wang, Lisa Brown, Shaunak Mukherjee, Nicholas S. King, Henry O. Everitt, Peter J. Nordlander, and N. J. Halas, “Aluminum Plasmonic Nanoantennas”, Nano Letters 12, 6000-4 (2012).
[2]Nicholas S. King, Mark W. Knight, Henry O. Everitt, Peter Nordlander, and N. J. Halas, “Aluminum for plasmonics”, ACS Nano 8, 834-840 (2014).

In the last few years, there has been a burst in the study and conceiving of new devices for the generation and manipulation of electromagnetic field at the nanoscale, where radiation-matter interaction is strongly enhanced. Several fabrication methods are now available for material preparation and nano-structuring, but only few of them can ensure the stringent design control needed for the effective and reproducible device behavior.
We report, herein, novel processes of micro and nanofabrication techniques for several applications in different research fields.
During the lecture it will be presented selected topics from our research activity. In particular it will be highlighted the results on Plasmon Polariton conversion to Hot electrons [1], single molecule detection [2] and detection at atto molar concentration [3].
[1] Hot-electron nanoscopy using adiabatic compression of surface plasmons
Nature nanotechnology 8 (11), 845-852, 2013
[2] Nanoscale chemical mapping using three-dimensional adiabatic compression of surface plasmon polaritons
Nature nanotechnology 5 (1), 67-72, 2009
[3] Breaking the diffusion limit with super-hydrophobic delivery of molecules to plasmonic nanofocusing SERS structures,
Nature Photonics 5, 683-688, 2011

Recently, we reported the possibility of obtaining directional scattering using a bimetallic antenna composed of closely spaced Au and Ag NPs dimers. We here first demonstrate that directional scattering can be achieved simply by using Au NP dimers made of different height. We give a first proof-of-concept that directional scattering can be used for refractive index based sensing applications. In this case, shifts of the ratio between right and left scattered light are found to happen. This effect has been used for bulk refractive index measurements and has been tested on streptavidin-biotin molecular aggregates.
Second, we designed novel nanoparticle arrangements to investigate novel plasmonic functionalities. To this purpose we designed and realized a new plasmonic oligomer structure that supports both magnetic and dark modes in the same wavelength range and study the relation between the two. Also, we investigate the unique characteristics of a photonic quasicrystal that consists of plasmonic Ag nanodisks arranged in a Penrose pattern. The quasicrystal scatters light in a complex but spectacular diffraction pattern which allowed us to assess the excitation efficiency of the various diffraction modes. Furthermore, surface plasmon polaritons can be launched almost isotropically through near-field grating coupling when the quasicrystal is positioned close to a homogeneous silver surface. It is demonstrated that the quasicristal in-coupling efficiency is dramatically enhanced compared to a nanoparticle array with the same particle density but only short-range lateral order.
Finally, a novel structure composed by gold vertical dimers has been produced. The structures support an extended hot spot, in agreement with recent literature. The resonance is tunable over the visible and IR region and both geometry and optical characteristics are easily controllable. Remarkably, we describe a preparation method which renders the hot spot region available for sensing. Advantages in terms of sensitivity are hereby discussed.

Coupling between engineered plasmonic resonators and material absorption lines plays an important role in the fundamental behavior of biochemical sensors, surface enhanced spectroscopy techniques and active plasmonic devices. Often, the design of and interpretation of these coupled mode systems draws inspiration and intuition from analogies to atomic and molecular physics systems. They have, for example, been shown to mimic quantum interference effects, such as electromagnetically induced transparency and Fano resonances.[1] Recent work on the subject has involved engineering features such as the coupling strength to the absorption line via field enhancement or the frequency mismatch between the two resonances.
Here we focus on a number of significant effects that instead are directly determined by the composition of the loss mechanisms that characterize the damping of the plasmonic resonance.[2] These are radiative damping and intrinsic loss due to e.g. material absorption. The ratio between the two loss rates defines one of three regimes of operation for the plasmonic resonator.[2,3] We show, both theoretically and experimentally, that depending on the regime of operation, coupling between the plasmonic resonance and absorption band can manifest itself as either an electromagnetically induced transparency-like effect or alternatively strongly enhanced absorption. Notably, the overall magnitude of the effect is also a function of the ratio between the different loss rates.[2]
These results imply a number of important consequences. For example, because the composition of the loss mechanisms associated with a plasmonic mode can vary strongly depending on its resonant frequency, very different behavior can result for surface enhanced absorption measurement performed in e.g. the visible versus the infrared.[2] Alternatively, because the radiative damping rate of a plasmonic resonance can be tuned by tailoring the particle geometry, one can engineer whether coupling to a material absorption resonance will lead to either increased absorption or the opening of a narrow transparency window.[2] Finally, we show that tuning the ratio of the radiative to intrinsic loss can dramatically alter the magnitude of the response. This provides another mechanism, in addition to field enhancement, with which to optimize the response of plasmonic sensors used for surface enhanced spectroscopy applications. Overall, these results and their theoretical description provide important insights into the nature of signals observed in a range of surface enhanced spectroscopy measurements and plasmonic devices that leverage coupling to material absorption bands as well as new opportunities for their design.
References:
[1] Halas, NJ; Lal, S; Chang, W-S; Link, S; and Nordlander, P; Chem Rev. (2011) 111 3913-61
[2] Haus, HA; Waves and Fields in Optoelectronics, Prentice-Hall, NJ, 1984
[3] Adato, A; Atar, A; Erramilli, S; and Altug, H; Nano Letters (2013) 13 2584-91.

The future of plasmonics depends crucially on the realization of nano-optical devices that add functionality to existing optical elements. Novel concepts are necessary to expand the range of applications in the nano-optical world. Realizing working and reproducible prototypes is essential to demonstrate the usefulness of nano-optics to scientists and engineers alike.
We report on the successful combination of switchable elements such as phase change materials and Mott-Hubbard isolator-to-metal transition materials with complex plasmonic structures to achieve active and reconfigurable functionalities. We are going to present three examples of such combinations: (a) the demonstration of a switchable plasmonic chiral element in the mid-infrared, (b) the realization of a switchable perfect absorber in the 3-5 µm wavelength region, (c) and the use of the Mott-Hubbard insulator to metal phase transition to realize plasmonic nanoantennas in the near-infrared where the plasmonic antenna resonance can be completely turned off and reversibly turned back on by utilizing the transition between YH₂ and YH₃.
In particular, the combination of GeSbTe and multilayer plasmonic nanoantennas from gold allows for chiral plasmonic structures whose chiral resonance can be switched by inducing the phase change in the semiconductor material. By sandwiching this material between aluminum antennas and a solid aluminum film at tailored dimensions, perfect absorption with over 90% efficiency can be turned on and off in the mid-infrared at tailored wavelength bands. Evaporating nanoantennas from the transition metal yttrium and cycling hydrogen incorporation and extraction, near-infrared plasmonic resonances can be turned on and completely off in a reversible fashion.
Our experiments confirm that the hybrid combination of complex plasmonic nanostructures with switchable materials leads to novel and exciting functional nanodevices, which will expand the realm of real-world applications of nano-optics in the future.
We acknowledge funding by DFG, BMBF, GIF, ERC, BW-Stiftung and MWK Baden-Württemberg. We are grateful to Jürgen Weis (MPI FKF, Stuttgart) for continuous support.
This work has been carried out by Xinghui Yin, Andreas Tittl, Nikolai Strohfeldt at University of Stuttgart and MPI FKF Stuttgart, by Ann-Katrin Michel and Thomas Taubner, RWTH Aachen, and by Ronald Griessen, Univ. Amsterdam.

We report here an ultracompact chip-based channel plasmon slot waveguide field-effect modulator operating at telecommunication wavelengths based on gate-controlled index modulation in a transparent conducting oxide (TCO) active region. The slot waveguide is fabricated with electron beam lithography on silica [1]. The field effect modulator active region comprises of a thin layer (5 nm) of aluminum oxide gate deposited by atomic layer deposition (ALD), which provides a pinhole-free dielectric gate layer. The active plasmonic material consists of sputtered indium tin oxide (ITO) and Au and ITO layers serving as electrical contacts [2]. We demonstrate field effect dynamics giving rise to high dynamic range (3.2 dB/mu;m) modulation and low waveguide loss (~ 0.45 dB/mu;m). We further demonstrate that the large modulation strength is a result of the resonant behavior of an epsilon-near-zero (permittivity of the material close to zero) field enhancement in the voltage-tuned ITO accumulation layer.
Combining the strong TCO field-effect optical tunability and the concept of resonant guided wave networks [3], we further present a design for an electrically gate-controlled 2x2 port plasmonic network which serves as an ultrafast multi-channel logic device with more than one hundred on/off combinations. The network exploits coherent interference of forward and backward propagating light, and thus network resonances can be altered by introduction of conducting oxide-based active switching devices gated with an external bias. We have previously experimentally demonstrated that passive 2x2 networks can function as a router and logic device with the full permutation of Boolean on/off values at 2 output ports, unlike other photonic crystal and plasmonic add/drop filters in which only two on/off states are accessible. Fabrication of the 2x2 network with conducting oxide switches is underway, and network measurements will be discussed.
References:
1. A. Kriesch, S. P. Burgos, D. Ploss, H. Pfeifer, H. A. Atwater, U. Peschel. “Functional Plasmonic Nanocircuits with Low Insertion and Propagation Losses,” Nano Lett. 13, 4539-4545 (2013).
2. E. Feigenbaum, K. Diest, and H. A. Atwater, “Unity-Order Index Change in Transparent Conducting Oxides at Visible Frequencies,” Nano Lett. 10, 2111-2116 (2010).
3. S. P. Burgos, H. W. Lee, E. Feigenbaum, R. M. Briggs and H. A. Atwater, “Synthesis and Characterization of Plasmonic Resonant Guided Wave Networks,” Nano Letters 14, 3284-3292 (2014).

Plasmon drag effect and galvanic modulation of plasmons in nanostructured metal can provide a direct opportunity to incorporate plasmonic elements into electronic nanocircuits, and monitor or control them electrically. We have studied these effects in flat metal films and nanostructures, and demonstrated the possibility to control the magnitude and direction of the plasmon drag with nanoscale geometry and light illumination conditions. We showed that surface plasmon polaritons can be affected with dc electric current. The effects are discussed in terms of the hydrodynamic model considering plasmonic pressure and striction, and in terms of kinetic approach accounting for strong plasmonic wave-beam interaction.

Optical sensing at visible and infrared wavelengths is of paramount importance for a vast number of applications. Optoelectronic functionalities of photodetection and light harnessing rely on the band-to-band excitation of semiconductors, thus the spectral response of the devices is dictated and limited by their bandgap. A novel approach, free from this restriction, is to harvest the energetic electrons generated by the relaxation of a plasmonic resonance in the vicinity of a metal-semiconductor junction [1-3]. In this configuration, the optoelectronic and spectral response of the detectors can be designed ad hoc just by tailoring the topology of metal structures, which has tremendous applications in solar energy harvesting and photodetection.
Fully exploiting hot electron based optoelectronics yet requires a platform that combines their exotic spectral capabilities with large scale manufacturing and high performance. Herein we introduce the first implementation of a large area, low cost quasi 3D plasmonic crystal (PC) for hot electron photodetection [4], showcasing multiband selectivity in the VIS-NIR and unprecedented responsivity of 70 mA/W.
We will also discuss the importance of the metal-semiconductor interface in these devices, and show how this is critical for light harvesting applications to concurrently sustain high open-circuit voltages and short-circuit currents. By modifying the properties of the interface at the nanoscale, we show that the photovoltaic performance of plasmonic-hot electron solar cells can be tuned. By doing so, we report IPCE over 5% and power-conversion-efficiency under simulated solar illumination up to 0.11%.
References
[1] Y. Tiang, T. Tatsuma, “Plasmon-induced photoelectrochemistry at metal nanoparticles supported on nanoporous TiO₂”, Chem. Commun., 2004, 1810-1811
[2] M.W. Knight, H. Sobhani, P. Nordlander and N.J. Halas, “Photodetection with Active Optical Antennas Science 2011, 332, 702- 704
[3] Clavero, C. Plasmon-induced hot-electron generation at nanoparticle/metal-oxide interfaces for photovoltaic and photocatalytic devices. Nat. Photonics 8, 95-103 (2014).
[4] F. Pelayo García de Arquer, Agustín Mihi, and Gerasimos Konstantatos, “Multiband Tunable Large Area Hot Carrier Plasmonic-Crystal Photodetectors”, arXiv:1406.2875
[5] F. Pelayo García de Arquer, Agustín Mihi, Dominik Kufer, and Gerasimos Konstantatos, “Photoelectric Energy Conversion of Plasmon-Generated Hot Carriers in Metal-Insulator-Semiconductor Structures” ACS Nano 2013 7 (4), 3581-3588

While the non-radiative decay of surface plasmons was once thought to be only a parasitic process which limits the performance of plasmonic devices, it has recently been shown that it can be harnessed in the form of hot electrons for use in photocatalysis, photovoltaics, and photodetectors. Unfortunately, the quantum efficiency of hot electron devices remains low due to poor electron injection, and in some cases, low optical absorption. Here, we demonstrate how metamaterial perfect absorbers can be used to achieve near-unity optical absorption using ultrathin plasmonic nanostructures with thicknesses of 15 nm, smaller than the hot electron diffusion length. By integrating the metamaterial with a silicon substrate, we experimentally demonstrate a broadband and omnidirectional hot electron photodetector with a photoresponsivity that is among the highest yet reported. We also show how the spectral bandwidth and polarization-sensitivity can be manipulated through engineering the geometry of the metamaterial unit cell. These perfect absorber photodetectors could open a pathway for enhancing hot electron based photovoltaic, sensing, and photocatalysis systems.

Over the past decade, fascinating and compelling experiments performed on nanostructured surfaces have revealed that metamaterial physics—such as the negative index media or invisibility cloaks first explored at microwave frequencies—can be obtained at infrared and visible wavelengths. However, these demonstrations have also shown how detrimental material absorption and losses can be, rendering most of these blockbuster experiments impractical as routes for competitive devices or practical applications. For this reason, we are led naturally to alternative applications and phenomena for optical metamaterials, which provide a far more promising path to practicality. In particular, optical metamaterials that exploit surface plasmons on metal nanostructures—a phenomenon unavailable at lower frequencies—can dramatically enhance optical fields and can form the platform for novel and advantageous absorbing or nonlinear optical media, as well as new and reconfigurable diffractive optics. As an example, we will discuss the use of film-coupled nanoparticles as a system in which plasmonic field enhancements can be controlled to unprecedented levels. The film-coupled system leverages planar fabrication techniques, such as self-assembled organic layers or atomic layer deposition, to enable extreme (sub-nanometer) and reproducible spacing between nanoparticle and film. The film-coupled nanoparticle system provides just one potential route to a useful metamaterial format. We will present an overview of the prospects of scaling metamaterials to the optical (infrared and visible) regime, focusing on configurations like the film-coupled nanoparticle system that provide useful functionality while mitigating losses and other deleterious properties.

New optical materials are finding increasing applications in communications, healthcare and sensing. Combining metals, dielectric, semiconductors and soft polymers in unusual nano-architectures allows extreme control over the coupling of light and matter, producing advanced optical functionality. However constructing such nano-materials has been a huge problem, and must be overcome for applications to emerge in widespread technologies. While top-down fabrication is effective for small area 2D materials, for many applications volume production of 3D metamaterials is needed. In this talk I will present a number of new approaches to produce mass-scale visible-wavelength plasmonic- and meta-material films.
A breakthrough for 3D metallic architectures for visible wavelength metamaterials has been enabled by our exploitation of block-copolymers to create large-area 3D gyroidal nano-architectures with 30-50nm unit cells and 10nm strut dimensions [1]. We investigate their linear and nonlinear optical properties, showing such ‘holey Au&’ has tuneable plasma edge into the infrared, with massively enhanced transmission in select resonances.
An alternative route to novel woodpile photonic crystals and metamaterials manufactured over areas of several cm that operate in the visible regime, involves multiply-stacking sheets of gold wires supported by ultrathin polymer films. With 3D periodicities down to 134nm, we create strong resonant reflectors that can be flex tuned, with tuneable properties.
Nanoparticle dimers can be used as ultrasensitive plasmonic sensors, but generation of reproducible signals requires accurate control over particle separation. Here we use an innovative DNA origami design to create strongly coupled gold NP dimers in high yield (>80%) with reproducible and accessible gaps of 3nm [2]. Supercontinuum spectroscopy of many individual dimers shows the reproducibility, and demonstrates massive surface-enhanced Raman scattering. Since this is a simply scalable technology it has significant implications for building plasmonic sensors directly in the field [3].
Light can be used not only to characterize such materials, but can also be used for their structural modification. Here, we show how intense laser light can be employed to induce metallic threads between precisely assembled nanoparticles with sub-nm gaps with real time optical feedback. We also show how such controlled laser-sculpting can be extended to large scales by threading millions of nanoparticle chains in suspension simultaneously maintaining accurate control over their diameter [4]. Pulsed laser light becomes an inexpensive and effective tool to manipulate structures at the nanoscale that can easily be scaled up to meet the requirements of industrial production.
[1] Adv Mat 25, 2713 (2013)
[2] Nat. Comm. 5, 3448 (2014)
[3] ACS Nano 5, 3878 (2011)
[4] Nat. Comm. (2014)

Plasmonic architectures those direct light propagation three dimensionally are important components for constructing future 3D metamaterials and optical circuits. At nanoscale, metal nanoparticles with tunable surface plasmon is widely used to confine light energy. Further integration of metal nanoparticles into programmable 1D/2D coupled arrays directs the light propagation into prescribed pathways through surface plasmon polariton propagation. To construct a 3D architecture, DNA nanostructures, especially 3D DNA origami, are recently highlighted as promising templates to align metal nanoparticles, owing to their 3D digital shape programmability. However, limited by their intrinsic scaffold-length, 3D DNA origami structures are generally smaller than 100 nm, and mostly work only for discrete clusters with less than 10 nanoparticle componenets. It remains an open challenge for scaling the metal nanoparticle integration into large-scale 3D ordered programmable architectures.
To step-forward this challenge, we here describe a versatile strategy for scalable integration of hundreds metal nanoparticles into ordered programmable 3D architectures, ranging from several hundred nanometers to microns with sub-5 nm aligning precision. We first assemble large-scale DNA templates through the hierarchical assembly of pre-formed DNA building blocks or non-hierarchical epitaxial growth of DNA bricks. Printing gold nanoparticles, ranging from 5 nm to 30 nm, onto the prescribed positions on the DNA templates through surface DNA hybridization produces the prescribed 3D plasmonic architectures. Theoretical computation and optical measurements suggest, by tuning the vertical spacing, complicate plasmonic interactions and light propagation behaviors exist exclusively within the 3D nanoparticle architectures.
Large-scale 3D DNA templates provide a simple, high precision, and versatile strategy for 3D plasmonic architectures. Simplicity. Large-scale 3D DNA templates reduce the challenging task of exploring self-assembly conditions for local interactions among particles to a much simpler task of designing the surface binding patterns on a DNA nanostructure, which can be readily achieved by computer-aided design softwares. Additionally, each metal nanoparticles can be uniquely addressed and positioned onto prescribed spatial positions, owing to the single-strand addressability of DNA templates. High precision. Using current DNA templates, nanoparticles can be aligned at a sub-5 nm spatial positioning precision, which is beyond most of the existing top-down and bottom-up approaches for 3D architectures. Versatility. Not limited to small or symmetric shapes from traditionally bottom-up approaches, especially DNA origami-directed self-assembly, large-scale 3D architectures, particularly asymmetric structures with distinct layers, can be designed and fabricated from current method.

Plasmonic nanoparticles play an important role in biomedical applications today as they can serve as superior optically-stable bioimaging agents, be employed in biosensor devices for the early diagnosis of diseases, and exhibit promising results for their employment in vivo as therapeutic agents. For several bioapplications, however, nanoparticles that express more than one functionality are often advantageous [1]. This has led to the synthesis of multifunctional plasmonic nanoparticles that combine the attractive plasmonic properties with other functionalities like magnetism, photoluminescence, dispersibility in aqueous solutions and resistance to degradation [2,3]. Furthermore, tumor ablation by thermal energy via the irradiation of plasmonic nano-particles is a relatively new oncology treatment. Here, hybrid plasmonic-superparamagnetic nanoaggregates (50-100 nm in diameter) consisting of SiO₂-coated Fe₂O₃ and Au (asymp;30 nm) nanoparticles were fabricated using scalable flame aerosol technology. By finely tuning the Au interparticle distance using the SiO₂ film thickness (or content) the plasmonic coupling of Au nanoparticles can be finely controlled bringing their optical absorption to the near-IR that is most important for human tissue transmittance [4]. The SiO₂ shell facilitates also dispersion and prevents the reshaping or coalescence of Au particles during laser irradiation, thereby allowing their use in multiple treatments. Their effectiveness as photo-thermal agents is demonstrated by killing human breast cancer cells with a short, four minute near-IR laser irradiation (785 nm) at low flux (4.9 W cm^-2).
References
[1] G.A. Sotiriou, “Biomedical Applications of Multifunctional Plasmonic Nanoparticles”, WIREs Nanomed. Nanobiotechnol.5, 19-30 (2013).
[2] G.A. Sotiriou, T. Sannomiya, A. Teleki, F. Krumeich, J. Vörös, S.E. Pratsinis, “Non-toxic, Dry-coated Nanosilver for Plasmonic Biosensors”, Adv. Funct. Mater.20, 4250-4257 (2010).
[2] G.A. Sotiriou, A. Hirt, P.-Y. Lozach, A. Teleki, F. Krumeich, S.E. Pratsinis, “Hybrid Silica-coated, Janus-like Plasmonic-Magnetic Nanoparticles”, Chem. Mater.23, 1985-1992 (2011).
[3] G.A. Sotiriou, F. Starsich, A. Dasargyri, M.C. Wurnig, F. Krumeich, A. Boss, J-C. Leroux, S.E. Pratsinis. “Photothermal Killing of Cancer Cells by the Controlled Plasmonic Coupling of Silica-coated Au/Fe2O3 Nanoaggregates”, Adv. Funct. Mater.24, 2818-2827 (2014).

Solid plasmonic particles that provide strong circular dichroism at ultraviolet wavelengths, which could enhance secondary-structure determination in biomacromolecules, have not been possible. Here we address this by demonstrating a simple and general route to chiral colloids. We exploit anisotropic etching of non-standard high-index silicon wafers to prepare plasmonic nanopyramids with a specific handedness. The resulting particles, which are easily dispersed into liquids, present chiral pockets for molecular binding. If fabricated from gold, colloids with record molar circular dichroism (>5x10⁹ M^-1cm^-1) at red wavelengths are obtained. More importantly, we demonstrate chiral colloids from aluminum, a plasmonic metal suited to ultraviolet wavelengths. Because these aluminum nanopyramids exhibit chiral optical signatures resonant with many biomacromolecules, new methods for detecting structural chirality in chemistry and biology become possible.
McPeak et al, Complex Chiral Colloids and Surfaces via High-Index Off-Cut Silicon. Nano Lett. 2014, 14, 2934-2940.

Hyperbolic media in which the dielectric tensor has both positive and negative eigenvalues have been shown to defeat the diffraction limit, and allow features at very small wavelengths to be resolved as demonstrated through hyperlenses. Even in quasistatics the underlying equation resembles a wave equation. Whereas a circular hole in an isotropic dielectric medium has a simple dipolar field around it, we will see that a circular hole in an almost lossless hyperbolic media, has surrounding quasistatic fields which diverge along characteristic lines tangent to the hole, and which have finite total energy absorption along these lines, even as the loss in the medium tends to zero. In a hyperbolic medium a dipole with small polarizability can dramatically influence the dipole moment of a distant polarizable dipole, if it is appropriately placed. We call this the searchlight effect, as the enhancement depends on the orientation of the line joining the polarizable dipoles and can be varied by changing the frequency. For some particular polarizabilities the enhancement can actually increase the further the polarizable dipoles are apart, like the way quarks interact more strongly the further they are apart.

Light propagation is usually Lorentz-reciprocal. However, a static magnetic field along the propagation direction can break the Lorentz reciprocity in the presence of magneto-optical (MO) materials [1]. The Faraday effect in such materials rotates the polarization plane of light by an angle phi;, and when light travels backward the polarization is further rotated. The enhancement of the Faraday rotation in MO thin Films is of particular interest due to the demand of optical isolation devices in integrated optics [2] and laser engineering [3].
We hybridized thin films of magneto-optical materials with plasmonic structures and achieved 4.2 degrees of Faraday rotation for a 200 nm thick structure. This large Faraday rotation is accompanied with a reasonably high transmittance of 27 %.
Our structure consists of a 150 nm thick MO slab waveguide and a 50 nm thick gold nanowire grating on top. The gold wires were deposited by electron beam lithography [4]. The structure shows the highest enhancement for a 360 nm period. For higher periods the TM and TE waveguide modes shift away from each other and the enhancement decreases. This behavior is in good agreement with simulations and can be explained by the fact that the TE waveguide mode has no plasmonic component where the TM mode has one [5]. Thus changing the period affects the TE and TM mode differently [6]. This leads to the existence of a minimal spectral distance of the TE and TM mode where the enhancement of the Faraday rotation is maximal.
To ensure that the measured rotation is purely non-reciprocal and only due to the Faraday effect we performed B-field dependent measurements of the optical rotation. The Faraday rotation curves are highly symmetric with respect to the axis of zero degree. This indicates that the measured rotation is highly non-reciprocal and caused by the plasmonically enhanced Faraday rotation.
We acknowledge funding by DFG, BMBF, GIF, ERC, BW-Stiftung and MWK Baden-Württemberg. We are grateful to Jürgen Weis (MPI FKF, Stuttgart) for continuous support.
This work has been carried out by Dominik Floess, Akihito Kawatani, Daniel Dregely, and Jessie Y. Chin at University of Stuttgart and MPI FKF in Stuttgart. We acknowledge assistance by Hanns-Ulrich Habermeier at MPI FKF in Stuttgart.
A. De Hoop, A reciprocity theorem for the electromagnetic field scattered by an obstacle, Appl. Sci. Res. 8:135, 1960
H. Dötsch, Applications of magneto-optical waveguides in integrated optics: review, J. Opt. Soc. Am. B 22:240-253, 2005
K. Petermann, External Optical Feedback Phenomena in Semiconductor Lasers, IEEE J. Sel. Top. Quantum Electron. 1:480-489, 1995
J. Chin et al., Nonreciprocal plasmonics enables giant enhancement of thin-film Faraday rotation, Nat. Commun. 4:1599, 2013
A. Christ et al., Phys. Rev. Lett. 91: 183901, 2003
A. Christ et al., Optical properties of planar metallic photonic crystal structures: experiment and theory, Phys. Rev. B 70:125133, 2004

Magneto-plasmonics is a designation generally associated with ferromagnetic-plasmonic materials because such optical responses from nonmagnetic materials alone are considered weak. Magneto-optical responses from nonmagnetic nanoparticles are possible [1]. Such repsonses provide a novel route to applications in bottom-up self-assembly, biosensing and microfluidics, in which conventional ferromagnetic materials are unsuitable. Here, we show that there exists a switching transition between linear and nonlinear magneto-optical behaviors in noble-metal colloidal nanospheres that is observable at ultralow illumination intensities (~1W/cm²) and DC (~1mT) magnetic fields [2]. The response is attributed to polarization-dependent nonzero-time-averaged plasmonic loops and vortex power flows that ultimately lead to nanoparticle magnetization. Circularly-polarized beams and coincident DC magnetic fields act to drive plasmon-polaritons in circular loops to generate a static magnetic dipolar response. These circular loops modify the conductivity which results in a change in the refractive index of the gold. Magnetization of the nanoparticle is observed in the transmission spectra of the nanocolloidal solution which shifts in accordance with changes in the refractive index calculated analytically via a Drude-like model[2].
Furthermore, we demonstrate that significant mechanical effects result from magnetic-dipole interactions, and such effects are subject to anisotropy when nanoparticles are non-spherical. When nanopsheres exhibit slight asymmetry such as those expected in the fabrication of nanospheres mechanical effects result, as seen in minute time-scale changes in transmission spectra. Slightly asymmetric nanospheres rotate such that the magnetization of the nanoparticle maximizes with respect to the external magnetic field. Moreover, by increasing the anisotropy further to nanowires we further increase the mechanical response. Nanowires exhibit multiple resonances depending on their orientation with respect to the incident field as well as wavelength, thus interactions with an external magnetic field will be orientation-dependent [3]. Strong interactions in which free energy is minimized are preferred, which results in a alignment of the nanowires via magnetic-dipole interactions. This work demonstrates alignment and induced magnetization of nonmagnetic plasmonic nanoparticles at ultralow light intensities and DC magnetic fields.
References
[1] F. Pineider et al. “Circular Magnetoplasmonic Modes in Gold Nanoparticles” Nano Letters 13 (2013)
[2] M.Moocarme et al. “Ultralow-Intensity Magneto-Optical and Mechanical Effects in Metal Nanocolloids” Nano Letters 14 (2014)
[3] T. Ming et al.”Strong Polarization Dependence of Plasmon-Enhanced Fluorescence on Single Gold Nanorods” Nano Letters 9 (2009)

Over the last decade Ge has been proposed as one of the most promising materials for light detection, modulation, and emission in silicon-photonics architectures [1]. Its direct band-gap, which is only about 140 meV larger than the indirect one, ensures excellent absorption and promises remarkable emission properties at telecommunication wavelengths, which recently led to the realization of integrated detectors [2], electroluminescent devices [3], and to the first demonstration of optically-pumped [4] and electrically-pumped [5] Ge lasers.
Here we exerimentally investigate germanium-on-silicon Fabry-Pérot cavity resonators working around 1.55 mu;m and, by properly tuning their length, we report an almost 30-fold enhancement in the collected spontaneous emission per unit volume when compared to a Ge film of the same thickness [6]. With the help of finite-difference time-domain (FDTD) simulations we are able to gain insight into this phenomenon and reveal that it can be ascribed to the combined effect of (i) enhancement in the radiation efficiency (the nanoresonators act as efficient optical antennas), (ii) absorption enhancement at the pump wavelength, and (iii) a fluorescence emission enhancement (Purcell effect).
These results set the basis for the understanding and engineering emitting devices based on subwavelength, CMOS-compatible nanostructures operating at telecommunication wavelengths.
[1] D. Liang & J. E. Bowers, Nat. Photonics 4, 511 (2010).
[2] J. Osmond, G. Isella, D. Chrastina, R. Kaufmann, M. Acciarri, & H. von Känel, Appl. Phys. Lett. 94, 201106 (2009).
[3] X. Sun, J. Liu, L. C. Kimerling, & J. Michel, Opt. Lett.34, 1198 (2009).
[4] J. Liu, X. Sun, R. Camacho-Aguilera, L. C. Kimerling, & J. Michel, Opt. Lett. 35, 679 (2010).
[5] R. E. Camacho-Aguilera, Y. Cai, N. Patel, J. T. Bessette, M. Romagnoli, L. C. Kimerling, & J. Michel, Opt. Exp. 20, 11316 (2012).
[6] M. Celebrano, M. Baselli, M. Bollani, J. Frigerio, A. Bahgat Shehata, A. Della Frera, A. Tosi, A. Farina, F. Pezzoli, J. Osmond, X. Wu, B. Hecht, R. Sordan, D. Chrastina, G. Isella, L. Duograve;, M. Finazzi, & P. Biagioni, Submitted.

Localized surface plasmon resonance (LSPR) can help address the small absorption coefficients and limited band gaps that hinder solar energy harvesting with semiconductors. The photonic enhancement due to light trapping improves absorption above the band edge of the semiconductor. Plasmon induced resonant energy transfer and hot electron transfer allows photoconversion at energies below the band edge, extending spectral conversion. In this presentation we will examine the mechanisms of plasmon enhanced solar energy harvesting. The energy alignment, spectral overlap, and insulating barrier thickness are systematically varied in metal-semiconductor nanostructures to elucidate the switches that control and concentrate the different plasmonic enhancement mechanisms. Transient absorption and action spectrum analysis are used to map out each mechanism, as well as tie short time energy transfer to long time scale device performance using charge relaxation dynamics. The results provide fundamental insight into plasmonic energy transfer as well as guide device optimization.

Due to the great improvement of organic materials, electrode design and stack structure, internal quantum efficiency of OLED is near 100%. However, ~80% of emitted light is trapped in ITO/organic layer (~50%) and glass substrate (~30%) owing to the refractive index difference in planar OLED device so that the total out-coupled efficiency is limited to ~20%. There have been diverse approaches that enhance out-coupling efficiency of OLED including scattering wave-guided mode, removing surface plasmon technique and refracting substrate mode. In this work, we suggest a simple method that extracts substrate mode. For lightings, the light extraction structure should satisfy broad angle enhancement and spectral stability. As an external light extraction structure, microlens array (MLA) is effective method to extract substrate mode. However, OLED with typical hemispheric MLA enhances the out-coupling efficiency mainly normal direction. This drawback is unfavorable to apply to OLED lightings. Here, we have fabricated an external light extraction structure that satisfies both uniform enhancement distribution and spectral stability using polymer phase separation. Immiscible two polymers, polystyrene and polyethylene glycol are blended in toluene. By spin coating the blend solution, polyethylene glycol is phase separated randomly on the polystyrene matrix. After selectively removing the phase separated polyethylene glycol, a random hole pattern film is fabricated. The diameter and height of the holes are 1~2mu;m and 300~400nm, respectively. The random hole pattern was transferred on an adhesive plastic film by using UV curable polymer (Ormocomp). Morphology of transferred pattern on the adhesive plastic film is changed to embossed structure. Transferred random embossing structure is attached on the fabricated green phosphorescence OLED. The device characteristics of RES OLED was measured and compared with planar and MLA OLED. The radius of hemispheric MLA used in this study is ~80mu;m. The external quantum efficiency (EQE) of the planar, MLA and RES OLED on the normal direction was 21.93%, 34.96% and 27.95%, respectively, at current density level of 2.0mA/cm². The enhancement ratio of MLA and RES OLED was 59.4% and 27.45%, respectively. Enhancement ratio of MLA OLED is higher than RES OLED on the normal direction (0⁰). However, when the EQE of side direction (0⁰~70⁰) is considered, the integrated EQE of planar, MLA and RES OLED was 20.13%, 27.60% and 26.18%, respectively. The enhancement ratio of MLA and RES OLED was 37.11% and 30.05%, respectively. These results means that RES OLED can enhance out-coupling efficiency uniformly on the whole angle range, while the enhancement of MLA OLED is mainly focused on normal direction. For OLED lightings, RES is more proper light extraction technique than hemispheric MLA from the uniform enhancement point of view.

Fluorescence fluctuation measurements are a versatile tool to measure local diffusion times, to deduce the hydrodynamic radius of single fluorescently labeled species or measure the viscosity of the local environment. One of the promises of plasmonics is the improvement of fluorescence detection schemes towards the single molecule level and to reduce the detection volume to well below the diffraction limit. Especially the detection of fluorescently labeled species can profit threefold from properly designed plasmonic antennas: first, the plasmonic antenna can increase the excitation rate by enhancing the local pump-field, secondly, the antenna can funnel the fluorescence emission into a certain direction or polarization state and thirdly, for fluorophores with low quantum efficiency the offered additional photonic states of the antenna can greatly improve the quantum yield. In fluorescence fluctuation measurements the reduction of the detection volume directly allows measurements at higher concentrations. Indeed, great strides have been made using nano-apertures that provide geometric confinement. In contrast, nanoantennas localize fields in deep-sub-wavelengths volumes, but they lack a mechanism to suppress the signal from the diffraction limited excitation beam which ultimately may limit the benefits of the enhanced near field.
We unravel what is the best tradeoff between near field enhancement on one hand and hot spot volume on the other hand to obtain the best reduction in FCS detection volume, thereby providing a classification scheme for the merit for FCS of nanoantennas. We find that there is an optimal value for the hotspot volume for a given nearfield enhancement [1]. For realistic antennas the peak enhancement and the hotspot volume are strongly entangled parameters. The bowtie antenna for instance exhibits the strongest field enhancement due to its small gap size. Yet plasmon-enhanced FCS measurements with realistic antennas would benefit from a bigger hotspot volume, counter to the intuition of standard FCS. Experimentally it is very difficult to increase the hotspot volume of single antennas without reducing the field enhancement. We show how this can be effectively achieved in a multiplexed measurement scheme by exciting simultaneously several hotspots in antenna arrays without modifying the antenna design.
A severe problem in plasmon enhanced FCS is that retrieving diffusion constants requires calibrated hot spot dimensions. Since this varies not just from antenna to antenna, but even depends on fluorophore quantum efficiency, such a calibration is very difficult to obtain. We show that a designed and known antenna geometry in a polarization-resolved FCS cross correlation measurement can intrinsically provide a distance reference which allows an absolute and calibration free measurement of the local diffusion coefficient.
[1] L. Langguth and A. F. Koenderink. Opt. Express, 22, 15397 (2014)

Electrochromism (EC), the reversible change in optical properties when a material is electrochemically oxidized or reduced, has a long history of fundamental and practical interest. Application of this electrochromism, such as display, mirrors, and smart windows, has been studied while combining with other devices. Smart windows change light transmission properties in response to heat, light or electricity. WO₃ and NiO are well known typical electrochromic materials where sputtering or sol-gel method are used for obtain thin layer but those methods require high vacuum or solution and that is not an economical process. This study evaluates ATO (Antimony doped Tin oxide) film as EC materials. ATO film is known to have better transparent properties than that of WO₃ and NiO films. Films with its thickness of 500nm using ATO powders, NiO and WO₃ powders, are deposited via NPDS for its evaluation of electrochromic performance, such as EC efficiency. Nano particle deposition system (NPDS) is one of the dry depositions systems using aerosol in which particles are accelerated at supersonic velocity through a nozzle using compressed air in low vacuum condition. This method has advantages such as simple fabrication process and low manufacturing cost. Those films are optimized using cyclic voltammetry (CV) analysis with various electrolytes, and are adjusted as EC layer and ion storage layer. As a result, the fabricated device showed its change in transmittance of 50% between bleached state and colored state at its wavelength of 630#13210;. Therefore, NPDS was successfully used to deposit EC layer to be considered as a potential alternative processing method.

Photonic technologies utilizing circularly polarized light are very active research fields due to their variety of applications such as optical communication of spin information, quantum-based optical computing and information processing, and ellipsometric tomography. In order to open up opportunities offered by these technologies to their full potential, development of integrated devices that can detect the handness of circularly polarized light with high selectivity in an intended wavelength range is strongly required. The handness of circularly polarized light can be detected by utilizing its difference in refraction or absorption upon interacting with chiral media. For instance, an array of a left-handed helical metal oxide film preferentially transmits right-handed circularly polarized light with wavelength lying in the Bragg regime that is determined by the pitch and refractive index of the helical metal oxide film, called as circular Bragg phenomenon. In addition, helically arranged metal nanoparticles are capable of creating strong optical chirality. In this study, arrays of helical metal oxides decorated with noble metal nanoparticles showing the circular Bragg phenomenon and the plasmonic chiroptical effect simultaneously are presented in order to improve the selectivity in polarization state of circularly polarized light and tailor photo-responding wavelength region and its band width as desired. By using a simple oblique angle deposition method, arrays of helixes of metal oxides such as titanium dioxide (TiO₂) and tin dioxide (SnO₂), and their heterostructures were fabricated in a precisely controlled way to extend the detection ability of circularly polarized light in a broad wavelength range. Additionally, by decorating silver or gold nanoparticles along the helical metal oxide nanostructures, the selectivity in polarization state was improved up to 30 percent and active wavelength range became broad enough to cover almost all the visible spectrum, which are enabled by the combination of the metal oxides heterostructures and plasmonic chiroptical effect. Structural and optical properties of the helical films have been systemically investigated, and compared with simulations results by finite element method. Finally, promising opto-electrical characteristics of back-gated circular polarization state discriminator will be presented.

The electric holographic can show real three dimensional (3D) images without any special glasses. A key characteristic of holographic displays is a viewing angle of the reconstruction images. The viewing angle depends on the pixel pitch of spatial light modulators (SLMs). We have developed magneto-optic 3D displays (3D-MOSLMs) with submicron scaled magnetic pixels to apply wide-viewing holographic display. The 3D-MOSLMs modulated light polarization with magneto-optic (MO) effect. The magnetic pixel array was fabricated on a-TbFe films by thermomagnetic driving. 3D images reconstructed from the magnetic pixel array. The viewing angle is 30 degrees. The brightness of reconstruction image depends on MO property of magnetic film. In this paper, we focused on mono-layer magnetic garnet films and magnetophotonic crystals (MPCs) composed of magnetic garnet as defect layer.
The brightness of reconstruction image depends on the reference light intensity and optical efficiency eta;_opt of magnetic films. The eta;_opt was calculated with eta;_{opt =} %T × eta;_diff, where %T is transmissivity of magnetic films, eta;_diff is diffraction efficiency that is defined as ratio between intensity of a transmitted light and a first order diffracted light. The transmissivity depends on thickness of magnetic films. The Faraday rotation is proportional to thickness of magnetic film. Therefore, the eta;_diff has dependence of thickness. The MPC structure was (Ta₂O₅/SiO₂)²/magnetic garnet/(SiO₂/Ta₂O₅)². The thickness of magnetic garnet layer was 1.0 mu;m. This structure was designed optical thickness to localize light at 532 nm. The magnetic garnet was used poly crystalline Bi_1.3Dy_0.85Y_0.85Fe_3.8Al_1.2O₁₂ (BiDyYFeAlG) films with perpendicular magnetization. Thickness of mono-layer BiDyYFeAlG films was 0.66 mu;m, 1.2 mu;m, 1.9 mu;m and 3.7 mu;m were fabricated. For comparison, the theoretical eta;_opt for simple magnetic grating for MPC and mono-layer BiDyYFeAlG films was calculated by finite element simulation, while changing thickness of BiDyYFeAlG layer for MPC and mono-layer film.
The diffraction efficiency of magnetic films was measured with a two beam interferometer, while changing energy density of laser light for thermomagnetic recording. The highest eta;_opt of mono-layer BiDyYFeAlG was 1.8×10^-2 % at 1.9 mu;m thickness. However, simulated theoretical eta;_opt of mono-layer BiDyYFeAlG film has peak at 3.8 mu;m thickness. The reason of this difference was thermal diffusion in the thermomagnetic recording. These results mean that there is a limit to the thickness for thermomagnetic recording. The MPC with a thin defect layer could be obtained MO property corresponding to the large thickness film. The theoretical eta;_opt of MPC has peak at about 1.0 mu;m defect layer. The thickness was enough for thermomagnetic recording. As a result, the eta;_opt of MPC was 2.1×10^-2 %. The brightness of reconstruction image of MPC that was calculated by eta;_opt and reference beam power (lambda; = 532 nm, 8.6 mW/cm²) was 101 cd/m².

We demonstrate low-cost and highly scalable Silicon-based near infrared (NIR) photodetectors. We observe broad-band photoresponse up to 2000 nm wavelength with low dark current density about 50 pA/µm². The devices exhibit photoresponsivity values as high as 2 mA/W and 600 µA/W at 1.3 µm and 1.55 µm wavelengths, respectively.
We make use of Au nanoislands forming Schottky contact with Silicon to detect near infrared photons. Photons with energies less than the optical bandgap are not absorbed by Silicon, resulting in no photocurrent. However, NIR photons with sub-bandgap energies can excite plasmon resonances at Au nanoislands and generate highly energetic charge carriers (hot electrons) through plasmon decay. These hot electrons are then transferred to Silicon, resulting in photocurrent. Use of carefully designed plasmonic nanoantennas for hot electron generation has been studied by several groups. For the first time, we demonstrate the use of randomly sized and randomly shaped nanoislands for broad-band photoresponse at NIR wavelengths. The responsivity values are on the same order as the recent work in the literature using plasmonically tuned nanoantennas, however, with narrow-band photoresponse. Plasmon excitation mechanism on Au nanoislands and hot carrier collection mechanism are investigated to explain the high photoresponse observed in our devices with such simple configuration.
Au nanoislands were formed by depositing a thin Au layer on high-resistance n-type Silicon wafer and rapid thermal annealing at different temperatures. An Al-doped Zinc Oxide (AZO) layer was deposited using thermal atomic layer deposition (ALD) technique to perform as a transparent conductive oxide (TCO) and patterned using photolithography. AZO film acts as the electrical connection between the nanoislands and also makes a heterojunction to Silicon. Laser light from a super continuum laser source was monochromated by an acousto-optic transmission filter (AOTF) and mechanically chopped for photoresponsivity measurements. Resulting photocurrent of the devices were measured with a lock-in amplifier.
Simple and scalable fabrication on Si substrates without the need for any sub-micron lithography or high temperature epitaxy process make these devices good candidates for ultra-low-cost broad-band NIR imaging and spectroscopy applications.

The workhorse of today`s active light engineering technologies is optical modulators. Conventionally, optical modulation is achieved utilizing electrical modulation of free carrier concentration in semiconductors¹. Thermal modulation is considered as an alternative due to large thermo-optic coefficients²; yet, thermal stability requirements of the modulators bring additional operational costs. Hybrid schemes using liquid crystals for large index modulation, magneto-optical materials for fast switching, optically nonlinear materials for low operation power and low-loss have also been demonstrated; however, with an elevated cost in fabrication process. Alternatively, solid-state phase-change materials switching between metallic and dielectric phases through atomic scale modifications have gained significant attention due to their high modulation contrast in refractive index, fast switching speeds, low switching power and non-volatility. In this study, we introduce a novel electro-optic modulation method based on resistive switching memories by exploiting modulation of atomic scale modifications in the active layer using electrical stimuli.
Our resistive switching device is formed by stacking Al/ZnO/p-Si layers. Al layer forms the top electrode and the grating window which optical reflection measurements can be performed. The substrate is chosen as highly conductive p-type Silicon to be used as a bottom electrode. A 80nm ZnO film is deposited by RF sputtering using a ZnO target.
The resistive switching behavior of the devices is characterized through a current voltage measurement through a varying voltage bias between -6V and 6V in double sweep mode. The I-V curve of the Al/ZnO/Si device exhibits a pinched hysteresis loop associated with resistive switching behavior. The device exhibits more than 100 repeated switching cycles between OFF and ON states. We performed fourier transform infrared reflection measurements of the device while the device is electrically biased. We observed broadband modulation within the entire 5-18µm wavelength range and obtained non-volatile modulation of reflection spectrum up to 4%. Through finite difference time domain simulations, a contrast in the refractive index of more than 1 is estimated. We obtain stable resistive switching behavior in electrical measurements and verify the underlying atomic scale modifications through transmission electron microscopy (TEM) images. We develop an analytical model for the optical modulation and verify by energy dispersive X-ray spectroscopy (EDX) and energy filtered transmission electron microscopy (EFTEM) measurements. The effect is confirmed to be non-volatile such that the optical reflection spectra stay at corresponding low resistance or high resistance state values when the voltage bias is removed.
[1] J. A. Dionne et al., Nano Lett. 9 (2), 897-902 (2009).
[2] G. Cocorullo, et al., Appl. Phys. Lett., 74, 3338-3340 (1999).

The increasing awareness of industries in the commercial opportunities given by nanotechnology is driving the research and development of novel material designs with cutting edge functionalities. Nature has often been taken as example to derive smart solution to engineering problems. In the field of optics the natural world molds the flow of light with wide variety of nanometer scale modulations of materials properties, like porosity and morphology (e.g. cuticle of the beetles, butterfly wings or mother of pearls). Being able to mimic these strategies with added high-tech functionalities would open up a broad array of applications. Existing fabrication techniques of porous photonic architectures severely limit their exploitation. Here we report on photonic hierarchical nanostructures obtained by self-assembly from the gas-phase at low temperature. Periodic refractive index modulation is achieved by stacking layers with different nano-architectures. Fine control over material density and porosity allows to reach refractive index contrast above unity and thus the fabrication of high efficiency (reflectivity(lambda;) > 85% in 500 nm thick photonic crystals) single material photonic crystal with tuneable Bragg-diffraction peak. On one hand the possibility of spanning from materials with porosity from 0 to 85% and on the other being able to do so with conductive (indium tin oxide), semiconductor (titanium dioxide) and insulating (zirconium dioxide, alumina) materials makes this technique interesting for a wide range of applications. Porosity is also shown to work as film stress dumper making these photonic crystal capable of withstanding temperature (>500°C) common in in the fabrication process of photoelectrochemical devices. As an example porous crystalline TiO₂ photonic crystal is integrated as scaffold for dye chemisorption in dye-sensitized solar cells to exploit higher light-matter interaction given by controlled photon propagation within the device. The fabrication of high efficiency broad band dielectric mirrors (Rasymp;1 over the whole visible spectrum) on glass and flexible substrates, opto-fluidic switches and matrix of photonic crystal pixels with feature size < 10mu;m is envisioned. This new technique is demonstrated to be a promising route for filtering, optical-sensing, electro-optical modulation, light harvesting energy devices and photocatalysis applications.

We report growth of GaN films at room temperature facilitated by UV surface plasmon-mediated N₂H₄ decomposition to generate reactive nitride growth precursors. Conventional growth methods for GaN include CVD and MBE, which both require high temperatures (> 500 K) to create atomic nitrogen by thermally decomposing nitride precursor molecules. Alternatively, magnetron sputtering can be used to grow GaN at room temperature but the high ion kinetic energy limits the crystalline quality of the growing film. Our study is motivated by the fact that UV radiation can resonantly dissociate nitrogen bonds, through a pathway that neither produces species with high kinetic energy nor requires high temperatures. Ultraviolet surface plasmons are generated at a nanostructured aluminum surface where nitrogen precursors are adsorbed, and the resonantly excited surface plasmons generate dissociated nitrogen species that lead to GaN film growth. Additionally, because film growth can occur at lower temperatures, our method may open doors for new semiconductor materials such as indium-rich InGaN, which is normally hindered due to phase separation into InN and GaN at elevated growth temperatures.
For UV surface plasmon-mediated GaN growth, we identified N₂H₄ as a promising nitrogen precursor molecule. We have designed periodic Al nanostructure arrays optimally suited for efficient coupling of UV radiation into surface plasmons, yielding a surface field enhancement at 248 nm which corresponds to the maximum absorption cross section for hydrogen abstraction from N₂H₄. Electromagnetic simulations revealed up to a 25x field enhancement within the near field of the Al nanostructure for resonant excitation. We fabricated large area (3 mm x 9 mm) Al UV plasmonic nanostructures using nanoimprint lithography printing from a master stamp generated by e-beam lithography. Reflection spectroscopy was performed to optically characterize these Al nanostructures, confirming enhanced absorption at 248 nm. In high vacuum ambient conditions at cryogenic temperatures, mass spectrometry indicated that UV surface plasmons enhance N₂H₄ dissociation by an overall factor of 6.2x. We calibrated and matched the atomic nitrogen flux from the plasmonic substrate and atomic gallium flux from an effusion cell in a growth chamber. The nitrogen flux and gallium flux were introduced sequentially to enable layer by layer growth of GaN on gold and sapphire (001) substrates. The XPS data of the sample on gold substrate confirms the Ga-N bond formation based on the redshifts of Ga 2p and 3d electron binding energies relative to the Ga-O bond. For the films grown on sapphire substrates, we performed XRD and Raman spectroscopy to investigate the crystalline quality of the GaN film. We used AFM to study the surface morphology of the deposited film. We will discuss strategies for improving the quality of deposited films using this method, as well as other amenable materials systems such as InN and InGaN.

Realizing building blocks for long-distance quantum communication is a major driving force for the development of efficient nanophotonics devices. Significant progress has been achieved in this field with respect to the fabrication of efficient quantum dot (QD) based single-photon sources [1]. Even spin-photon entanglement has been demonstrated in a semiconductor system which is considered a crucial step towards the realization of a quantum repeater [2,3]. Up till now, the work in this field has almost exclusively been performed on statistically grown QDs and random device technology leading to process yields of a few per cent at maximum - an issue which hampers further development of the underlying concepts towards practical devices. In light of this, it is clear that further progress will rely crucially on deterministic device technologies which will for instance enable the processing of efficient single-photon sources with pre-defined emission energy. In this work we apply a novel in-situ electron-beam lithography technique to realize deterministic QD microlenses with enhanced photon-extraction efficiency. Here, the microlenses act against total internal reflection which presents a major obstacle for semiconductor based single-photon sources as it limits the photon-extraction efficiency of quantum dots in planar samples to a few percent [4]. Aligning microlenses to single pre-selected InGaAs QDs we are able to boost the photon-extraction efficiency by almost one order of magnitude. Further increase is achieved by integrating a lower distributed Bragg reflector which leads to extraction efficiencies exceeding 60 %. The experimental results are in good agreement with numerical modelling in the framework of a finite-element method. The underlying in-situ electron beam lithography technique is based on low temperature cathodoluminescence (CL) spectroscopy and leads to a process yield larger than 90% [5]. This deterministic nanoprocessing platform allows us to achieve broadband extraction efficiency, and as such is highly attractive to boost not only the extraction of single photons but potentially also of polarization entangled photon pairs from a biexciton-exciton emission cascade. The high optical quality of the fabricated structures is proven by quantum optical studies proving the quantum nature of emission with g⁽²⁾(0) < 0.01 and a high degree of indistinguishability of successively emitted photons. These quantum optical studies demonstrate the high potential of such structures to act as deterministic quantum light sources in future quantum communication networks. [1] A. Shields, Nat. Photonics 1, 215 (2007). [2] K. D. Greve, et al., Nature 491, 421 (2012). [3] W. B. Gao et al., Nature 491, 426 (2012). [4] W. Barnes et al., Eur. Phys. J. D. 18, 197 (2002). [5] M. Gschrey et al., Appl. Phys. Lett. 102, 251113 (2013).

Integrated nanophotonic systems require the development of a large set of devices with functionalities ranging from light emission, directed transport, detection, to modulation, switching and routing, with high efficiency and small footprints. Recently, semiconductor nanowires have been configured into a variety of nanophotonic devices including lasers, waveguides, photodetectors, and all-optical switches, all from a single material, CdS. Although CdS is not the only system that can provide these functionalities, it serves as a good test case to experiment the idea of obtaining integrated nanosystems from a single functional material. Another device element essential for optical circuits and other applications is a modulator, which functions by changing the absorption and refractive index of materials to route the signal. However, most of these devices are based on the linear effects or the nonlinear Kerr effect and only control the input signal. The possibility to design a nanoscale modulator based on second-order nonlinearity would be useful for future nanophotonic circuits. In this work, we design a nanowire modulator to control the second harmonic generation (SHG) signal, which converts the two photons with frequency of omega; into one photon of 2omega;. When shone on the end of a crossed nanowire device, the fundamental wave (FW) is scattered into the nanowire waveguide due to the edge-scattering effect and then produces the SHG signal as it propagates in the nanowire. By controlling the external conditions, e.g. excitation polarization, electric field and environmental temperature, we can change the conversion efficiency of the nonlinear process (e.g. chi;⁽²⁾ and phase mismatch) and finally modulate the SHG output (ON/OFF) to eventually route it in different directions. We will discuss the optical routing process in crossed-nanowire devices and for on-chip signal processing.

A large portion of the cost of solar cells lies in wafer processing, but this cost can be reduced by using thin-film cells with substantially less material. A challenge in achieving good performance with thin-film cells is low absorptance due to the short distance over which incident photons can be absorbed. This is especially true of indirect bandgap materials such as crystalline silicon. Low absorption is frequently addressed by texturing the surface of the cell so that photons are scattered into longer paths which result in greater chance of absorption. This work investigates an external cavity approach which increases absorption and may also reduce losses from radiative recombination. Absorption enhancement using the external cavity was studied through ray tracing simulations for thin-film PV cells, both with and without surface texturing. Simulation results show that use of an external cavity in conjunction with sub-wavelength surface texturing enables absorption enhancement above the Yablonovitch limit in a wide frequency range, which suggests that using the cavity could therefore be an effective way to increase absorption within thin-film photovoltaic cells. Additionally, for high temperature applications such as catalysis and thermo-photovoltaics (TPV), the same cavity approach can be used to increase efficiency by suppressing thermal emission. This work is supported by the DOE EERE SunShot Initiative under award number DE-EE0005806.

Nanoantennas, the counterparts of radiowave antennas at optical frequencies, are capable of converting localized energy into far-field propagating waves and vice versa with predesigned radiation patterns. Specifically, optical antennas that exhibit a unidirectional radiation feature are important for a wide range of applications, such as nano-imaging and biosensing. Various designs have been proposed and realized. For example, Yagi-Uda antenna has been successively scaled down to the nanoscale, and shown directional radiation at optical frequencies. Besides the excellent directivity, the enhancement of radiation is also an important concern. Plasmonic effect, well-known for its ability to generate remarkable local field enhancement, is widely employed. Typical designs focusing on this aspect include bowtie antennas and other dimer-like nanoparticles.
Recently, several novel designs have been proposed to achieve nanoantennas with both unidirectional radiation and enhanced far-field emission intensity. By properly arranging nanoparticles in proximity to an emitter, a strong electric or magnetic dipolar resonance can be excited and interferes with the source, giving rise to unidirectional radiation in the far-field when a certain phase delay is reached. Since the directivity relies predominantly on the relative phase between the participant modes, the separation of the nanoparticles can hardly be very small, usually in the same order of the particle size. Meanwhile, the radiated beam is fixed in one direction. A switch of directivity normally requires a change of particles.
In this work, we report an ultracompact nanoantenna that can provide switchable unidirectional radiation between the forward and backward directions, while it can also substantially enhance the spontaneous emission of an emitter. The antenna consists of only three nanoparticles arranged in a simple trimer configuration and can be fed by a molecule or a quantum dot at the gap. Because of the nanometric distance between the source and particles, spontaneous emission rate is enhanced over 1000 times, resulting in strongly localized fields that further contribute to the formation of a magnetic resonance. This induced mode can also interfere with the exciting dipole constructively in one direction while destructively on the opposite side, creating a unidirectional radiation pattern. In addition, different from the previous proposals, where the interference occurs between resonances of the same type, here the two modes involved are electric and magnetic respectively, with the phase difference highly sensitive to the separation of the particles. We show by simulations that within a displacement of a few nanometers, the forward-backward directionality can be switched from over 20 dB to -3.5 dB. The presented design uses basic plasmonic building blocks and can be implemented by standard fabrication techniques. We expect the performance to be further improved by optimizing the design.

Magnetooptical isolators protecting light sources from scattered and reflected light are essential components of an optical communication system. Integrated isolators based on ring resonators or Mach-Zehnder interferometers have been studied. These isolators and many other magnetooptical devices use magnetooptical garnet because of its low optical losses and large magnetooptical responses at optical communications wavelength (lambda; = 1550 nm). However, the size of the isolator is still large compared with CMOS chips. To shrink the isolator by enhancing the Faraday rotation of the garnet, a microcavity technique could be good candidate. Faraday rotation can be greatly enhanced in a microcavity comprising a magnetooptical layer sandwiched between two Bragg mirrors/gratings to form a magnetophotonic crystal (MPC). The localization of the light in the magnetooptical material contributes to the enhancement of Faraday rotation angle by analogy with a Fabry-Perot resonator. In the case that the optical thickness of the magnetooptical layer equals to lambda;/2n, where l is the localized wavelength and n is the refractive index of magnetooptical layer (the Fabry-Perot resonance condition), the light is confined to the vicinity of the magnetic layer.
However, the fabrication of MPCs working at optical communications wavelengths has been challenging because of the difficulty of obtaining good magnetooptical materials on non-garnet substrates. We show that the cerium substituted yttrium iron garnet (Ce₁Y₂Fe₅O₁₂, CeYIG) on non-garnet layers annealed in vacuum could provide large magnetooptical responses, comparable to single crystal values. CeYIG was then incorporated into a MPC to demonstrate the enhancement of the magnetooptical response in the near-IR. The MPC structure was (Ta₂O₅/SiO₂)⁸/CeYIG/(Ta₂O₅/SiO₂)⁸. The dielectric mirrors were fabricated by ion beam evaporation (IBE). The physical thickness of the Ta₂O₅ was 180 nm and that of SiO₂ was 287 nm. The 339 nm thick polycrystalline CeYIG layer was prepared by RF sputtering and annealed. This anneal step was done in 5 Pa air to suppress equilibrium phases consisting of ceria and iron oxides to obtain the pure garnet phase.
Transmission and Faraday rotation spectra of the MPC showed a photonic band gap in the spectral range of 1350-1750 nm. The localized state was located at a wavelength of 1570 nm. It is considered that the shift of resonant wavelength from the designed value is caused by the film thickness distribution. An enhanced Faraday rotation angle of -2.92 degree (86.1×10³ degree/cm) was obtained in the vicinity of 1570 nm. The reported value for single crystal CeYIG was 3.3×10³ degree/cm, therefore this microcavity has the potential to shrink the isolator by 26 times. Further optimization of the structure and increase in the Ce content would increase the Faraday rotation angle, and would open the way to the integration of optical isolators onto silicon substrates.

Two-dimensional materials such as graphene and transition metal dichalcogenides have attracted enormous interest for their potential applications in photonics and optoelectronics. However, for their unique properties to lead to practical applications, the inherently low optical absorption must be increased. Numerous efforts have focused on narrowband photon management using plasmonic enhancement and integration of absorbers into resonant cavities, but a remaining problem is to achieve absorption enhancement over a wide range of wavelengths and incident angles, which is crucial for applications such as photovoltaics and camera imagers. Here, we numerically demonstrate coherent broadband absorption by integrating two-dimensional materials in a dielectric cavity with chirped distributed Bragg reflectors. We use the Nelder-Mead simplex optimization technique to search for the optimal thicknesses of the dielectric layers. These simulations, together with analytical approaches of broadband cavity absorption, show that a single sheet of graphene can reach nearly 100% absorption over a 2nm bandwidth of incoming red light, and that an External Quantum Efficiency of 7.09% is possible for a 1nm thick graphene / MoS₂ solar cell, representing an average of 3.6-fold enhancement from near ultraviolet to near infrared. Furthermore, a photodetector configuration with monolayer MoS₂ could absorb as many as a third of above-bandgap photons over a 300nm bandwidth. The proposed layered dielectric structures operate across a wide range of incident angles and could enable applications for atomically thin photodetectors or solar cells.

The focused ion beam microscope (FIB) is useful tool for direct-write lithography of novel metamaterials structures. It has unique control in varying the depth of structure feature through beam and dose control. Because of this, FIB may be used to exploit 3-D resonant modes in metamaterial nanostructures in ways that conventional lithography may not.
In this work, we developed a protocol for patterning arrayed nanostructures that support surface phonon polariton (SPhP) modes supported in polar dielectric materials, such as SiC and hexagonal-BN. These materials are attracting interest as promising alternatives to plasmonic media, due to the long scattering lifetimes (low optical losses) associated with phonons in polar dielectrics in comparison to free carriers in metals and doped semiconductors. This results in exceptionally narrow resonance linewidths, long propagating polariton modes and high efficiency metamaterials, albeit in the spectral range between the mid-IR to the single digit THz. In order to obtain localized resonance responses, arrays of holes or pillars are necessary to fabricate to achieve high field concentrations.
We present novel metamaterial structures written in 4H-SiC and hBN using FIB patterning. By using a Cr or Al hard mask on the surface, we were able to both mitigate beam tail effects and to protect the surface from Ga implantation. To explore the prospects of polarizability of these SPhP structures, we also devised strategies to fabricate chiral metamaterials. The ability of the FIB to combinatorially write varying-depth structures in a single-write step enabled the development of smooth-sloped chiral structures. The structures show a tailorable, polarizable spectral response in the IR for both materials, which is an encouraging step towards fabricating nanoscale circular polarizers and wave plates in a critical spectral range.

Parity-time (PT) symmetry has opened a new paradigm of non-Hermitian Hamiltonians ranging from quantum mechanics, electronics, to optics. We report here the first experimental demonstration of PT synthetic lasers. By carefully exploiting the interplay between gain and loss, we achieve degenerate eigen modes at the same frequency but with complex conjugate gain and loss coefficients. In contrast to conventional ring cavity lasers with multiple modes, the PT synthetic laser exhibits an intrinsic single-frequency lasing: the thresholdless PT broken phase inherently associated in such a photonic system squeezes broadband optical gain into a single lasing longitudinal mode regardless of the gain spectral bandwidth. This chip-scale semiconductor platform provides a unique route towards fundamental explorations of PT physics and next generation of optoelectronic devices for optical communications and computing.

We demonstrate the plasmonically modulated infrared radiation by control of surface emissivity through carrier density modulation in graphene nanoresonators on thin silicon nitride substrates. We interpret the emissivity control in terms radiation from plasmonic low Q graphene nanoresonators that serve as antennas coupling radiation to free space. The nanoresonators exhibit narrow spectral emission features that can be varied in frequency and intensity by changing the graphene carrier density. At 250 C, we show that the emissivity at 7 microns of the nanoresonator-covered surface can be varied by 2% via changes in carrier density of only 3.1 x10¹²cm^-2, and we demonstrate that the total power modulation approaches 50pW in a 50 micron x50 micron area in 100 cm^-1 bandwidth. Through finite element modeling, we show that the origin of this radiation can be traced to the microscopic loss channels of the graphene plasmons. Finally, we will discuss the implications of these results on thermal power management and the
possibility of ultrafast control of blackbody radiative emission.

The integration of single photon emitters with optical buses has recently attracted great interests in the realm of photonic integration, as the generation and transfer of single photons become essential in quantum information processing (QIP) [1]. Many platforms, such as photonic crystals and cavities involving complex fabrications, have been proposed towards the collection of light from single nanoscale emitters, while only a few of them tackle the issue of effectively addressing a single photon emitter [2]. Herein we report a simple strategy to realise the localised excitation of a single photon source made of a CdSe/CdS nanocrystal using a nanowaveguide made of a single ZnO nanowire, which acts as a passive or an active sub-wavelength nanowaveguide to excite the single photon source, depending on whether we use above or below bandgap energy to couple light into this nanowire waveguide. The efficient excitation of the single photon source and the waveguiding behaviour within the nanowire in active and passive cases are characterised using a photoluminescence setup corroborated by FDTD simulations, as well as using a Hanbury-Brown and Twiss interferometer for photon correlation measurements. Combined with the intriguing properties of semiconductor nanowires, such integration can be extended to various applications, such as electrically driven single photon emitter, efficient single photon detection and quantum information interconnect between nodes.
[1] O&’Brien, J. L.; Furusawa, A.; & Vu#269;kovicacute;, J. Photonic Quantum Technologies. Nat. Photonics2009, 3, 687-695.
[2] Northup, T. E.; & Blatt, R. Quantum Information Transfer Using Photons. Nat. Photonics2014, 8, 356-363.

The successful implementation of quantum information processing will give insight into many fundamental problems, such as simulations of quantum physical processes and quantum cryptography. Quantum cryptography requires a high-rate single-photon source and quantum computing proposals involve spin readout. We have developed a new method of producing single photons using a surface acoustic wave (SAW). In a piezoelectric material a SAW consists of both an electrostatic potential and an elastic wave travelling parallel to the surface. A SAW can drag a stream of single electrons along a narrow channel to produce a quantised current. We have extended this technique to drag electrons into a region of holes at the end of the channel, where each electron recombines with a hole and emits a photon. In principle, the emission rate can be very high, above 10⁹ photon/second. Also, spin-polarised electrons can generate circularly polarised photons, providing a method of spin readout in a quantum computer. The device requires a lateral p-n junction which we form on an undoped GaAs/AlGaAs heterostructure containing a quantum well. We have developed methods of inducing closely spaced regions of electrons and holes. By self-consistent electrostatic modelling in 3D, we have designed gates above a channel such that the potential profile of the active region has a slope that is shallow enough to allow the SAW potential to drag electrons out of the induced region and move them into the region of induced holes. To characterise and investigate the properties of our single-photon source, we have also integrated a fibre-coupled scanning microscope into the top-loading probe of a 300mK ³He cryostat. Further studies will include studying the viability of such SAW-driven single-photon sources for quantum computing applications such as spin readout or the conversion of spin qubits to photon qubits.

The development of artificial electromagnetic media with topological phase transitions provide novel strategies to dramatically improve the performance metrics of miniaturized optical devices for sensing, energy harvesting, light emission, detection and nonlinear signal generation. In this talk, we will present our results on the design, nanofabrication, and optical characterization of Si-compatible light emitting dielectric metamaterials based on rare earth-doped conductive oxide nanostructures. In particular, we will discuss the fabrication, optical and electrical properties of light-emitting Zinc Oxide (ZnO) and Indium-Tin-Oxide doped with Erbium (Er) ions and demonstrate the ability to tailor their negative dielectric permittivity across the near-infrared spectral range. Based on these engineered materials, we will discuss the design and fabrication of hyperbolic metamaterials in Si-based transparent conductive oxides doped with Er, which present fascinating opportunity for device integration on a planar substrate. Finally, using spectroscopic ellipsometry in combination with dark-field scattering and photoluminescence spectroscopy we will present the optical emission properties of Erbium-doped (ITO) hyperbolic metamaterials fabricated by reactive co-sputtering and demonstrate the ability to tailor the hyperbolic regime across the broad emission spectrum of Er, thus providing unique opportunities to achieve broadband Purcell enhancement of the radiation rate within a low-loss and metal-free metamaterial structure on Si.

The generation and manipulation of spin currents in semiconductors lie at the cutting edge of Spintronics[1]. To this purpose, group-IV semiconductors represent excellent candidates for spintronic applications, due to their large spin-orbit interaction and electron spin lifetimes. Spin currents in semiconductors can be photo-induced via optical orientation, that is the generation of a spin-oriented electron population in the conduction band of the semiconductor via circularly-polarized light absorption [2]. Although optical orientation allows overcoming the drawbacks of the electrical spin-injection schemes, this approach generates electron spin-polarization vectors always parallel (or antiparallel) to the k-vector of light, preventing the generation of relevant in-plane spin-polarization projections on the sample. This feature is certainly detrimental in view of future potential applications.
Here we show that, by properly engineering the wave-front of the incoming light, using a normal incidence geometry complementary spin densities (i.e. two spin populations with opposite spin-polarization projections) can be simultaneously injected in the plane of the device, thus obtaining the spintronic equivalent of a photovoltaic cell. While a photovoltaic generator spatially separates photoexcited electrons and holes, our device exploits circularly polarized light to produce two spatially well-defined electron populations with opposite in-plane spin projections. This is achieved by modulating the phase and amplitude of the light wavefronts entering a semiconductor (germanium, Ge) through micro and nanopatterned metal (platinum, Pt) overlayers [3]. The resulting spin distribution inside the semiconductor can be then detected through inverse spin-Hall effect (ISHE) technique, where the spin current, flowing from Ge towards the Pt layer, is converted into a transverse electromotive field via spin-dependent scattering with the Pt nuclei. In presence of complementary spin populations, the driving force for the spin-oriented electron motion is only given by the spin density gradient. This allows to study spin transport and dynamics in semiconductors, totally decoupled from the electron diffusion, hence exploiting the non-dissipative features provided by pure spin transport.
We will also show that the simultaneous generation of complementary spin populations can be achieved with high spatial confinement [3], i.e. well below the spin diffusion length. By properly combining optically-induced confined spin injection to common readout blocks, such as magnetic tunnel junctions, it will be possible to achieve the mandatory versatility for the engineering of future nanoscale spintronic devices.
1. I. Zuticacute;, J. Fabian & S. Das Sarma, Rev. Mod. Phys. 76, 323-410 (2004).
2. T. Jungwirth, J. Wunderlich & K. Olejník, Nat. Mater. 11, 382-390 (2012).
3. F. Bottegoni, M. Celebrano, M. Bollani, P. Biagioni, G. Isella, F. Ciccacci & M. Finazzi, accepted in Nat. Mater.

For photovoltaics to compete with grid electricity prices very high conversion efficiencies and low production costs are required. Multi-junction solar cells offer a proven pathway to very high efficiency by absorbing different energy bands in the solar spectrum in different semiconductors with appropriate bandgaps. However, the epitaxial growth process and current-matching requirements cause the production process for such solar cells to be too expensive for large scale applications.
In this work we present a novel method to split the solar spectrum, that does not rely on sequential filtering but instead utilizes the unique optical properties of nanostructured materials. We theoretically demonstrate subwavelength spectrum splitting in two coupled core-shell nanowires comprising a silver core and a Cu₂O (2 eV bandgap) and Cu₂S (1.4 eV bandgap) shell respectively. In previous work [1] we have shown that such metal-semiconductor core-shell nanowires can exhibit absorbtion cross sections near the semiconductor bandgap that significantly exceed the geometrical cross section. 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 to free space and (2) increasing the total number of degenerate resonances near the bandgap.
The large absorption cross sections near the bandgap makes these core-shell nanowires ideal candidates for subwavelength spectrum splitting. We place the pair of nanowires very closely together, and control the flow of light such that light is absorbed in the appropriate semiconductor. Over 50% of photons in the high energy band can be absorbed in the high bandgap semiconductor (Cu₂O), while in the low energy band all photons are absorbed in the low bandgap semiconductor (Cu₂S). Assuming moderate open circuit voltages this already leads to a 30-50% efficiency increase compared to the single material systems. This concept is general for many semiconductor materials and since it is not constrained by epitaxial growth mechanisms or current matching, it is a promising alternative to high efficiency multi-junction solar cells.
Reference:
S.A. Mann, E.C. Garnett, Extreme light absorption in thin semiconductor layers wrapped around metal nanowires. Nano Letters 2013, DOI: 10.1021/nl401179h

Thermophotovoltaic (TPV) systems consisting of a thermal emitter and a photovoltaic (PV) cell can theoretically convert thermal energy to electricity with an efficiency corresponding to the Carnot cycle. However, state-of-the-art TPV systems have only managed to achieve efficiencies of about ~24%. This is due to a variety of system non-idealities including radiative losses from the high-temperature emitter, the inability of the PV cell to utilize thermally emitted photons with energies below the cell&’s band gap, and non-radiative recombination in the PV cell. In this work, we propose a new design approach to dramatically enhance the energy conversion efficiency of a TPV system in both the near-field and far-field regimes. Our approach is to spectrally engineer the photonic density of states (DOS) of both the emitter and PV cell. Specifically, we show that by using thin-films of semiconductors on both the emitter and absorber side, the number of available photon states at low photon energies is drastically reduced thus minimizing emission and absorption of sub-band gap photons. Furthermore, bulk recombination losses are simultaneously reduced in a thin-film PV cell yielding a lower saturation current. Our detailed calculations predict that these two factors will significantly increase TPV energy conversion efficiency enabling device operation at lower emitter temperatures.
Acknowledgment: This work is supported by DOE BES grant No. DE-FG02-02ER45977

The quantum efficiency of a semiconductor like silicon is limited to one electron-hole pair per photon over most of the visible spectrum, and reaches zero in the infrared. An array of “microrectennas” can be designed to respond to any desired portion of the infrared spectrum (dependent only on antenna dimensions and conductivity since there is no bandgap) with theoretically high efficiency, as long as these very high frequencies (25 - 400 THz) can be rectified. Microrectennas, consisting of Ag stripe-teeth arrays (with widths and lengths less than 100 nm) lying horizontally on an oxide-coated metal, have produced output power (e.g., open-circuit voltage of mV, short-circuit current of nA) when illuminated by visible (514 nm) light [1] - an exciting result still under investigation. In this configuration the diode (a metal-insulator-metal (MIM) diode) is vertical, and rectifies the high-frequency vertical electric field produced by the antenna at resonance. We have improved on the older Simmons model of the MIM diode [2], which is still widely used but predicts unphysical effects at large voltages, by incorporating new physics (quantum solution of image-modified trapezoidal potential using a unique pseudopotential technique, temperature, effective mass, etc.). This model of the MIM diode is successfully compared, over ten orders of magnitude in current, to four different thicknesses of NbOx during the investigation of Nb-NbOx-based MIM diodes [3], and also to low-frequency rectification data, indicating our model's improved capability for predicting MIM diode conduction and rectification.
Stripe arrays are interesting in their own right as metamaterials. Au stripe arrays, with dimensions larger than those in Ref. 1, have demonstrated near-perfect absorption in the infrared due to critical coupling [4]. Using design rules for novel microrectenna arrays based on maximizing asymmetry (and the component of the electric field pointed into the substrate), we have designed new Au/NbOx/Nb microrectenna arrays appropriate for broadband infrared absorption and rectification, using dimensions and more complex nanostructures (e.g., chirped arrays) than in Ref. 1, and discuss fabrication and measurements using infrared spectrophotometry and current-voltage measurements (which are compared to our model of the MIM diode). We compared measured and simulated absorption resonances, calculate expected THz-rectification using classical [5] and quantum [6] rectification models, and compare to measurements of direct current output, under infrared illumination.
[1] R. M. Osgood III, et. al., Proc. SPIE 8096, 809610 (2011).
[2] J. G. Simmons, J. Appl. Phys. 34 (1963) 1793.
[3] M. Chin, P. Periasamy, et. al., J. Vac. Sci. Tech. B 31 (2013) 051204.
[4] C. Wu, et. al., Phys. Rev. B 84 (2011) 075102.
[5] A. Sanchez, et. al., J. Appl. Phys. 49 (1978) 5270.
[6] J. R. Tucker and M. J. Feldman, Rev. of Mod. Phys. 57 (1985) 1055.

The development of metamaterials into a viable platform for nanophotonic applications, data processing circuits, sensors, etc. requires identification of new plasmonic materials to overcome the limitations of noble metals, in particular their high losses. Here we describe a class of topological insulator materials which support broadband plasmonic response and possess extremely appealing photonic properties ranging from mid-IR to UV.
Bi_1.5Sb_0.5Te_1.8Se_1.2 (BSTS) is a bulk insulator with robust conducting surface states protected by time-reversal symmetry, due to the strong spin-orbit coupling. BSTS single crystals were synthesized by melting high-purity Bi, Sb, Te and Se powders at 950°C in an evacuated quartz tube. The temperature was then gradually decreased to room temperature over a span of three weeks. The resulting crystals were then cleaved along the (100) family of planes to a thickness of ~0.5 mm.
BSTS dielectric constants were derived by ellipsometric measurements and appear to be in excellent agreement with first principle DFT calculations. Unlike common direct or indirect bandgap semiconductors, the anomalous dispersion region falls in the visible part of the spectrum, leading to negative values of the permittivity. This behavior of the optical response is attributed to a combination of bulk interband transitions and surface contribution of the topologically protected states.
To prove metallic behavior of BSTS, we fabricated metamaterials and gratings on crystal flakes and registered strong plasmonic response from UV to NIR. The coexistence of plasmonic response of the topological surface with dielectric properties of the semiconducting bulk enables ultrafast (t>100 fs) and broadband (to mid-IR) photo-modulation of the optical response. These findings show the potential of topological insulators as a platform for high-frequency switchable plasmonic metamaterials.

The development of metamaterials and metasurfaces with reconfigurable optical responses has become a visionary frontier, which will lead to unique functionalities or devices with unprecedented performance for applications such as sensing, imaging, and optical signal processing. Among the various tuning techniques, electrical tuning based on graphene has substantial technological potential in terms of response time, broadband operation, and compatibility with silicon technology and large scale fabrication, because graphene has high electrical and thermal conductivity, broadband widely-tunable electro-optical properties and good chemical resistance. Here we have demonstrated electrically tunable metamaterial absorbers by incorporating a tunable metasurface into an asymmetric Fabry-Perot resonator which has a total thickness less than lambda;₀/10.
The device is based on a Fabry-Perot (FP) resonator with two mirrors, i.e. a tunable metasurface reflector as a front partially-reflecting mirror and a metallic back fully-reflecting mirror. The metamaterial absorber is fabricated on a silicon substrate, with an aluminum layer as the back reflector, an aluminum oxide (AlOx) as the insulator and a metasurface on graphene (grown via chemical vapor deposition) with a broad wavelength tuning range. As the gate voltage applied on the graphene sheet changes, the metamaterial absorber can be switched in and out of critical coupling condition. Optical modulators based on such metamaterial absorbers exhibit a maximum modulation depth of more than 95% and bandwidths (modulation depth > 50%) of up to 2 µm (5 µm to 7 µm). The highest modulation speed that can be achieved in our experiment is estimated to be >1 GHz, which can be further increased by shrinking the size of the device. Our strategy provides ultra-compact solutions for light modulation over broad wavelength ranges from the near infrared to the far infrared and even Terahertz.

The strong optical response of atomic-scale systems such as nanographene, polycyclic aromatic molecules, and nanostructured thin metal films opens new possibilities for controlling the near field with extreme electro-optical tunability and large sensitivity to the environment. We will discuss the underlying physics and potential applications of these types of systems.

We present new spectroscopic techniques that enable visualization of nanoparticle phase transitions in reactive environments and light-matter interactions with nanometer-scale resolution. First, we directly monitor hydrogen absorption and desorption in individual palladium nanocrystals. Our approach is based on in-situ electron energy-loss spectroscopy (EELS) in an environmental transmission electron microscope. By probing hydrogen-induced shifts of the palladium plasmon resonance, we find that hydrogen loading and unloading isotherms are characterized by abrupt phase transitions and macroscopic hysteresis gaps. These results suggest that alpha and beta phases do not coexist in single-crystalline nanoparticles, in striking contrast with conventional first-order phase transitions and ensemble measurements of Pd nanoparticles. Then, we then extend these techniques to monitor nanoparticle reactions in a liquid environment. By constructing a flow chamber, we directly monitor growth and assembly of colloidal plasmonic metamaterial constituents induced by chemical catalysts. Lastly, we introduce a novel tomographic technique, cathodoluminescence spectroscopic tomography, to probe optical properties in three dimensions with nanometer-scale spatial and spectral resolution. Particular attention is given to reconstructing a 3D metamaterial resonator supporting broadband electric and magnetic resonances at optical frequencies. Our tomograms allow us to locate regions of efficient cathodoluminescence across visible and near-infrared wavelengths, with contributions from material luminescence and radiative decay of electromagnetic eigenmodes. The experimental signal can further be correlated with the radiative local density of optical states in particular regions of the reconstruction. Our results provide a general framework for visualizing chemical reactions and light-matter interactions in plasmonic materials and metamaterials, with sub-nanometer-scale resolution, and in three-dimensions.

Control of the radiative properties of emitters such as molecules, quantum dots, and color centers is central to nanophotonic and quantum optical devices, including lasers and single photon sources. Plasmonic cavities and nanoantennas can strongly modify the excitation and decay rates of nearby emitters by altering the local density of states. Here, we demonstrate large enhancements of fluorescence and spontaneous emission rates of molecules embedded in plasmonic nanoantennas with sub-10-nm gap sizes. The nanoantennas consist of colloidally synthesized silver nanocubes coupled to a metallic film which is separated by a ~5 nm self-assembled polyelectrolyte spacer layer with embedded molecules. Each nanocube resembles a nanoscale patch antenna whose plasmon resonance can be changed independent of its local field enhancement. By varying the size of the nanopatch, we tune the plasmonic resonance by ~200 nm throughout the excitation, absorption, and emission spectra of the embedded molecules demonstrating giant fluorescence enhancement for antennas resonant with the excitation wavelength. Next, we directly probe and control the nanoscale photonic environment of the embedded emitters including the local field enhancement, dipole orientation and spatial distribution of emitters. This enables the design and experimental demonstration of Purcell factors ~1,000 while maintaining high quantum efficiency and directional emission. Full-wave simulations incorporating the nanoscale environment accurately predict the experimentally observed emission dynamics and reveal design rules for future devices. Finally, progress on coupling colloidal CdSe/ZnS core-shell quantum dots to the plasmonic nanopatch antennas will be discussed.

The detection of single-molecule fluorescence is a key technique for numerous applications in biomedicines including DNA sequencing, diagnostics, and molecular biology. Unfortunately, the detection volume is limited to femtolitre (10^-15) in conventional diffraction-limited optics. In addition, the concentration of molecules has to be limited to pico- or nano-molar, so that on average only one molecule is excited inside the diffraction-limited optical spot. This concentration level is far below the micromolar range where many biologically relevant processes occur. Such a limitation can be overcome by using the so-called zero-mode waveguide (ZMW) where the light field is mainly confined at the bottom of the ZMW, acting as small reaction chambers. The metal film blocks the illuminating light so that only the molecule located at bottom of the ZMW can be excited and detected while leaving other molecules unaffected. The ZMW allows reduction of the observation volume by 3 to 6 orders of magnitude, from 10^-15 (with a standard confocal microscope) to 10^-18 - 10^-21 liter, allowing for single-molecule detection. The ZMW structures are commonly fabricated on Al film with light field well confined at bottom but with less field enhancement at visible spectrum compared with that using silver (Ag) or gold (Au). The poor performance for field enhancement further limits the fluorescence emission of a molecule inside these structures according to the optical reciprocity.
We designed a heterogeneous optical slot antenna (OSA) that is capable of detecting single molecule in solutions at high concentrations. The heterogeneous OSA is compose of a rectangular nanoslot fabricated on heterogeneous metallic films that are formed by sequential deposition of gold and aluminum on a glass substrate. The rectangular nanoslot gives rise to large field and fluorescence enhancement for single molecules. The near-field intensity inside a heterogeneous OSA is 170 times larger than that inside an aluminum zero-mode waveguide (ZMW), and the fluorescence emission rate of a molecule inside the heterogeneous OSA is 70 times higher than that of the molecule in free space. The proposed heterogeneous OSA enables excellent balance between performance and cost. The design takes into account the practical experimental conditions so that the parameters chosen in the simulation are well within the reach of current nano-fabrication technologies. Our results can be used as a direct guidance for designing high-performance, low-cost plasmonic nanodevices for the study of bio-molecule and enzyme dynamics at the single-molecule level.

Abstract
We will show that composite photo-catalysts combing plasmonic metallic nanoparticles of noble metals and semiconductor nanostructures exhibit improved photo-chemical activity compared to conventional photo-catalytic materials.^1,2 We will also show that plasmonic silver nanoparticles, optically excited with low intensity visible light, exhibit direct photo-catalytic activity. We will discuss underlying mechanisms associated with these phenomena.^2,3,4 We propose that this new family of photo-catalysts could prove useful for many heterogeneous catalytic processes that cannot be activated using conventional thermal processes on metals or photo-catalytic processes on semiconductors. I will show an example of such a process.⁵
D. B. Ingram, S. Linic, JACS, 133, 5202, 2011
Suljo Linic, Phillip Christopher and David B., Nature Materials,10, 911, 2011.
Ingram P. Christopher, H. Xin, S. Linic, Nature Chemistry, 3, 467, 2011.
P. Christopher, H. Xin, M. Andiappan, S. Linic, Nature Materials, 11, 1044, 2012.

When light is emitted by an electronic system, it radiates into the local modes of its optical environment with characteristic distributions in energy, momentum, and polarization. These distributions can reveal a great deal of information about the electronic structure of the emitter and its local optical environment. For example, Fourier space images can be used to identify the orientation of single fluorescent molecules, study how quantum dots interact with optical antennas, and quantify the multipolar origin of electronic transitions in solid state ions.
We will present a method that allows for the simultaneous acquisition of the complete, wavelength-dependent, angular emission at two orthogonal polarizations within a single measurement and without the need for bandpass filters or scanning optics. Specifically, with the use of a Wollaston prism and a bertrand lens we image the back focal plane of a microscope objective to obtain two-dimensional Fourier-space momentum distributions at two orthogonal polarizations. Since the radiation patterns from emitters in structured environments can be decomposed into a well-defined set of basis functions, we can readily identify their multipolar strength and order by comparison to analytical or numerical theory.
We will present experimental results demonstrating how this method improves optical throughput by orders of magnitude compared to other spectroscopic techniques. In addition, since the spatial-spectral overlap is accounted for, this technique could allow for subpixel resolution. This method could be particularly useful in systems with sharp spectral resonances that would otherwise be impossible to resolve using bandpass filters.
As an illustrative example, we show how this new technique can be used to characterize the dispersive and highly asymmetric radiation patterns for quantum emitters coupled to directional optical antennas. We will demonstrate how this method can be used to extract the 2D radiation patterns at each and every wavelength within the measurement domain from a single acquisition.

Sculpted electromagnetic beams can serve as optical tweezers, allowing small objects to be accelerated, manipulated, or trapped with light alone. In this work, we introduce a novel fiber-optical tweezer based on a coaxial plasmonic aperture. Coaxial apertures confine light in all transverse direction while producing sharp field gradients at the dielectric/metal interface. Because of this tight optical confinement, coaxial apertures can efficiently trap dielectric particles with dimensions <10nm. Here, we take full advantage of the coaxial aperture design by integrating it on an optical fiber tip. To further improve the performance, we also introduce a circular plasmonic grating at the input side of the aperture. This grating focuses light on the coaxial aperture, resulting in significant enhancement of the transmission efficiency. We first theoretically characterize the tweezer design using full-field finite difference time domain simulations. Our simulation consists of a 5um silica fiber tip coated with a 200-nm-thick gold film. A 300-nm-diameter coaxial aperture is embedded in the gold and surrounded by concentric circular grooves at the gold/silica interface. By varying the grating period, its duty-cycle, and the depth of the grooves, we achieve a 7x increase in transmission efficiency compared to a coaxial aperture without focusing grooves. Such enhancement significantly relaxes the power budget necessary to achieve efficient, sub-10-nm-particle optical trapping. Experimentally, we fabricate this optical tweezer by tapering an optical fiber and cleaving it using focused ion beam milling. Circular grooves are then patterned on the fiber tip before evaporating a 200-nm-thick gold film, patterning the coaxial aperture, and polishing the output surface. The probe is mounted on a micromanipulator fitted on top of a microscope and inserted into a fluidic flow cell filled with fluorescent particles of diameter 10-40nm. The fiber is illuminated at the resonant wavelength of the coaxial aperture (780nm), enabling the strongest optical trapping. Trapping events of the fluorescent particles are directly imaged, by exciting the particles with a 450nm laser and monitoring their photoluminescence in the visible using a CCD camera. Our presentation will discuss the potential for our optical tweezer to trap even smaller particles, and will provide design strategies for directly trapping basic biological building blocks such as proteins.

Sub-wavelength split ring resonators (SRRs) have attracted much attention in creating artificial magnetism beyond the gigahertz frequencies where natural magnetism starts to fade off. In practical, it has been accomplishing by scaling down the dimension of the SRRs. However, with the reduced dimension of the planner SRRs, the increasing self-inductance of electrons makes it challenging to further increase the resonant frequency above 100 THz. To overcome this limit, we successfully extended the artificial magnetism to the visible spectrum by designing a free-standing U-cavity array consisting of 1D metallic nanowires with U-shaped cross-section. The numerical simulation showed that the magnetic resonance at 468 THz and electrical resonance at 605 THz. The U-cavity array was fabricated using nanotransfer printing (nTP) process. The reflectance and transmission spectra were characterized by optical spectrometer, and the experimental results were in good agreement with numerical simulations. Moreover, the resonance frequency is shown to be readily tuned by the material property and dimension of the cavity. In addition, both electric field and magnetic field are very well confined within the gap in comparison with the planar SRRs. Thus, so filling the gap of U-cavity with different materials will build up a platform for low-loss and tunable metamaterial and sensor device. By changing the inserting media of the cavity structure using various dielectric solvents, the sensing ability of the U-cavity structure can be investigated and evaluated by sensitivity S = dlambda;_res / dn, and the concentration can potentially lead to higher sensitivity.

Traditional architectures for harnessing absorption rely on a three-step process. Light is first efficiently directed to the absorbing layer with the aid of an antireflex layer. Then, the interaction of the light with the absorbing matter is further enhanced with a back-reflector that provides additional light passes. Current research directions for extreme absorption management, beyond the traditional recipe, involve impedance-matched metamaterials that can totally suppress reflection thus leading to near-perfect absorption or plasmonic resonances that act as nanolenses that shape strong fields into the absorber material thus achieving large power loss rates. While these two new approaches have widened the possibilities for absorption engineering across the EM spectrum for detector and photovoltaic applications, they entail an inherent disadvantage. This is that they comprise two kinds of absorbing media, one being a metal which yields only Ohmic losses with no possibility for carrier harvesting. This stresses on the advantage of one-step absorption platforms made only of a single-kind semiconducting material that operate in the vicinity of the band-gap.
The purpose of this study is to uncover new mechanisms for absorption control in platforms utilizing only a single kind of absorbing material, although not necessarily a semiconductor operating in the band-gap regime. For this purpose we considered a paradigm made of an extreme optical material; that of SiC in the Restrahlen band which in bulk form is highly dissipative but also extremely reflecting resulting overall in a poor absorption performance. Our aim is to determine mechanisms that overcome these obstacles in this extreme material and provide transferable insight to the design of other all-semiconductor absorbing systems.
We discuss here two distinct absorption mechanisms that can yield an almost perfect absorption with an all-seminconductor architecture in a one-step process. These are:
(i) Judicious surface structuring in an all-semiconductor photonic crystal (PC) that leads to near-zero reflection at the vicinity of the photonic crystal band-gap [1]. As a result almost all incoming light couples into the lossy Floquet-Bloch mode which dies off within a PC unit cell. This is due to the near-PC-band edge operation and leads to superabsorption with a compact PC design [2]
(ii) Coupling to cascaded vortex-like cavity modes in semiconductor logs that lead to efficient and broadband superabsorption [3].
We believe these results are relevant to the design of IR sources and also will inspire new absorbing
all-seminconductor structures across the EM spectrum.
[1] G. C. R. Devarapu and S. Foteinopoulou, Opt. Exp. 20, 13040-13054 (2012).
[2] G. C. R. Devarapu and S. Foteinopoulou, J. Appl. Phys. 114, 033504 (2013).
[3] G. C. R. Devarapu and S. Foteinopoulou, “Broadband superabsorbing IR SiC moth-eyes”, in preparation.

Recently graphene has attracted intense experimental and theoretical interest [1, 2] owing to its fascinating electronic, optical, and mechanical properties. On the other hand, the zero-dimensional form of graphene, graphene nano-flakes (GNFs), has been studied much less extensively [1-3]. Graphene can be cut into a large variety of shapes and sizes, which implies greater chemical flexibility and increased optical tunability [3]. In addition, chemical doping and electrical gating can be used to control the free charge density in GNFs, which offers another route towards optical tunability [4]. Closely related to the nano-optics of GNFs, surface plasmons have also received rapidly growing attention owing to their strong confinement, increased tunability, and long lifetime [5]. These advantages can extend graphene plasmon towards the visible spectral regions, thus facilitating more applications in electronics, photonics, and sensing, etc. Therefore, the theoretical studies of the optics and quantum plasmonics in GNFs are timely and of multidisciplinary interest.
We have studied the optical and plasmonic properties of a variety of single and coupled GNFs, using time-dependent density functional theory (TDDFT) within the generalized gradient approximation. Nanometer-sized hexagonal and triangular GNFs have been investigated, in both doped and undoped cases. The computed spectra suggest the optical response and plasmonic properties of GNFs are strongly dependent on the shape, size, and charge doping. In particular, optical spectra are red-shifted when increasing the GNF size or electron doping and blue-shifted when doping holes. The weak dependence of the optical spectra of the neutral hexagonal GNF dimers on the inter-disk distance suggests a weak interaction between disks. In sharp contrast, the electron doping of the dimer leads to a much stronger interaction, primarily due to the ease of electrons tunneling between the disks. To illustrate the plasmon excitation in these GNFs, the time evolutions of the charge density, induced by a continuous-wave excitation electric field, have been calculated using TDDFT. For small GNFs (diameter ~1 nm), the edge plasmons are dominant, whereas for larger ones (diameter ~ 2 nm) multipolar-type collective charge oscillations are observed. Moreover, electron and hole doping lead to a more efficient excitation of surface plasmons, due to increased free-carrier density. This study could be instrumental in guiding the experiments towards quantum plasmons in nano-sized structures GNFs and other two-dimensional nanomaterials and to the development of advanced nano-photonic devices as well.
References:
[1] A. Castro Neto, et. al. Mod. Phys. 81, 109 (2009).
[2] F. H. L. Koppens, et. al., Nano Lett. 11, 3370 (2011).
[3] Ian Snook, et. al., Chapter 13, S. Mikhailov (Ed.), Physics and Applications of Graphene - Theory, InTech (2011).
[4] T. Low, et. al., ACS Nano 8, 1086 (2014).
[5] L. Ju, et. al., Nat. Nano. 6, 630 (2011).

#12288;Optical excitation of selected valleys is demonstrated in a multivalley semiconductor Ge.
Recently, the bulk crystalline and nanostructure Ge have attracted much attention from the optoelectronic points of view. In fact, covering the Si-transparent window in the solar spectrum, improved indirect absorption of Ge is likely to have an impact on the light harvesting applications while the recent discovery of the direct-gap luminescence at room temperature has stirred considerable interest in light emission from the otherwise indirect-gap Ge. Meanwhile, the valley-selective delivery of spin-polarized electrons is expected to lead us to on-chip spintronic integration.
#12288;In this work, an attempt is made to tune the light absorption characteristics of Ge so that electrons are efficiently launched into the target valley simply by selecting the excitation energy. Such a k-specific pumping technique with unmatched efficiency will be extremely useful in designing and implementing Ge-based thin-film devices.
A 30-band k.p perturbation calculation of the band dispersion allowed us to map the k-specific absorption strength. Consideration of (i) vertical transitions, (ii) bunched van Hove singularity and (iii) subsequent relaxation of electrons and holes allowed us to identify the off-peak resonant absorption pathways. The use of the higher-lying bands has opened up the possibility of the selective pumping of remote valleys.
For example, the 1064-nm excitation allowed selective excitation of the direct G-valley in Ge while excitation with a more convenient 532-nm light source allowed selective population of electrons in the indirect L-valley and holes in the G-valley. This occurs primarily because electrons and holes are separated in k-space due to intra-valley relaxation. Specifically, the newly developed excitation pathway was estimated to be at least 106 times as efficient as standard indirect-gap absorption involving phonons.
#12288;The valley-selective optical pumping scheme promises in-depth study on the intervalley scattering of electrons that controls the spin relaxation in Ge, and hence is applicable to diverse multivalleyed systems including compound semiconductors like GaAs and InP.

Efficient control of light-matter interaction at deeply subwavelength scales is key to many photonics applications such as detectors, sensors and novel light sources. Enhancing and funneling light efficiently through nanoscale channels can dramatically improve the performance of such devices by making them compact and more efficient. Currently, this is accomplished by utilizing the extraordinary optical transmission phenomenon wherein structural surface plasmon resonances are excited in perforated nanostructured metal films. As a result the phenomenon is inherently narrowband with low transmission. Here, we introduce a new paradigm structure consisting of a double-grooved metallic nanostructure platform that can outperform extraordinary optical transmission structures while operating nonresonantly across broadband (Phys. Rev. Lett. 107, 163902(2011)). Our platform consists of a continuous periodic metallic nanostructure composed of an array of connected large (~100-200nm) and small (~ 15-20nm) rectangular slits. The key feature of our platform is that the optical power can be channeled through an area as small as ~ (lambda;/500)² associated with optical field enhancement and high transmission while operating across a broad wavelength band in the mid-infrared (~ 2- 20 mu;m). We will discuss the nonresonant mechanism underlying this phenomenon based on a simple quasistatic picture that shows excellent agreement with our numerical simulations. We will also show experimental implementations of this platform and discuss pertinent results.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Diamond&’s extreme properties are of intense interest in optical applications such as quantum information processing, spin sensing, and Raman lasers. However, there is a lack of effective techniques for creating structures with the required resolution and without simultaneously introducing collateral damage to the surrounding material. Recently, it has been shown that exposure of diamond surfaces to sub-ablation fluences of ultraviolet radiation desorbs carbon from the surface at well defined rates and without inducing damage to the crystal structure [1,2]. The phenomenon forms the basis for a prxomising novel method manipulation of the surface shape and properties of one of the most challenging of materials to process. Also, it enables exploration of one of the few opportunities in nature in which an optical process is able to selectively remove atoms from a surface [3,4]. However, many of the details of the process have not been investigated including the nano-scale properties of the machined surface that are critical in many applications.
Here we report a summary of this novel phenomenon and present a detailed study into the morphology and roughness of the UV treated surfaces as a function of laser parameters. Electron microscope imaging reveals that regular nano-structured features are produced that have morphologies strongly dependent on the polarization of the incident beam and insensitive to the pulse fluence provided it is below the ablation threshold. From a scaife-polished as-supplied surface, the etched patterns appear to be initiated from noise and show increasingly defined nano-structures. For polarizations along directions of high symmetry, the patterns are highly periodic and have good long range order. For small etch depths, the smallest feature size visible using the SEM was approximately 30 nm. Facetted ridge, grid, and wave-like patterns, are obtained for polarizations parallel to low-miller index directions on {100} and {110} diamond surfaces. We show that the etching provides a rapid, controllable, and area-scalable method for nano-patterning diamond surfaces. Of more fundamental significance, these observations comprise mesoscopic evidence for polarization dependent coupling of photons with localized and directional surface states corresponding to the carbon bonds. We show that the phenomenon may enable nano-scale manipulation of atoms on diamond and potentially other covalently bonded materials.
[1] V. V. Kononenko, M. S. Komlenok, S. M. Pimenov, and V. I. Konov, Quantum Electron., vol. 37, no. 11, pp. 1043-1046, 2007.
[2] R.P. Mildren, J. E. Downes, J. D. Brown, B. F. Johnston, E. Granados, D. J. Spence, A. Lehmann, L. Weston, and A. Bramble, Opt. Mat. Express 1, 576-585 (2011).
[3] T. Vondrak and X.-Y. Zhu, Phys. Rev. Lett., vol. 82, no. 9, pp. 1967-1970, Mar. 1999.
[4] A. Lehmann, C. Bradac, and R. P. Mildren, Nat. Commun., vol. 5, p. 3341, Jan. 2014.

Electron energy-loss spectroscopy (EELS) is an efficient tool for investigating the local density of optical states in single and coupled nano-systems in a transmission electron microscope (TEM) [1]. In EELS, a relativistic electron inelastically interacts with a sample, and hence loses energy by pumping the sample to a higher photonic state. The amount of energy loss of the electron is detected, providing us with information about the resonant energies of the sample. Mapping EELS by parallel acquisition, energy-filtered transmission electron microscopy (EFTEM) is a fast and efficient detection tool for mapping the optical modes in two spatial dimensions. Here, this is made possible by the in-column MANDOLINE energy filter in the Zeiss SESAM microscope [2].
Using EFTEM, we have analyzed the plasmon modes of hexamer and pentamer nanocavities, especially with respect to their symmetry and topology rules. The pentamer nanocavity is composed of 5 holes drilled into a silver slab. By utilizing a peak-finding algorithm [3], the spatial distribution of the excited modes can be efficiently investigated. Four distinguished plasmon resonances at the mapped energy range are excited, which is interpreted in terms of electric dipoles oriented along the radial or azimuthal directions. To precisely analyse the plasmon eigen-energies, the reported EELS spectra at certain electron impacts are compared to theoretically computed EELS spectra, utilizing a three dimensional FDTD method with an embedded electron source [4]. The bulk plasmon exhibits a broad resonance centred at 3.5 eV for silver, and the radially polarized plasmon mode has a resonant energy of 3.4 eV. The interference of these two modes becomes visible in the calculated EELS spectra by a typical Fano-shaped resonance.
EFTEM series were acquired for a 6-hole oligomer nanocavity drilled in a silver slab. A comparison between the peak maps with those of the pentamer nanocavity shows that again the same classification of the modes into longitudinally and radially polarised modes is possible. There is no evidence of a toroidal mode as reported in reference [5] for a heptamer nanocavity with a central hole, which is a proof of a strong dependence of this mode on the topology of the structure. The experimental and calculated EELS spectra show a blue shift of the resonances with respect to the pentamer nanocavity, which is a specification of the shorter spacing, and hence a stronger coupling of the nanoholes.
[1] Garcia de Abajo, Rev. Mod. Phys.82 (2010) 209
[2] Uhlemann and H. Rose, Ultramicroscopy63 (1996) 161
[3] Talebi et al., Langmuir 28 (2012) 8867
[4] Talebi et al., New J. Phys.15 (2013) 053013
[5] Ögüt et al., Nano Lett.12 (2012) 5239
[7] NT acknowledges the Alexander-von-Humboldt Foundation for financial support. The research leading to these results has received funding from the European Union Seventh Framework Programme [FP7/2007-2013] under grant agreement n°312483 (ESTEEM2).

We demonstrate significant graphene enhancement of external quantum efficiency (EQE) of thin film silicon in visible bandwidth from 4.3% to 65%, with efficient charge collection through lateral p-i-n junction at room temperature. Through electrical contact to silicon substrate, the high mobility of graphene boosts drift current and thus the photocurrent. The enhancement factor (EF) of photocurrent is correlated to the graphene morphology on active region. Graphene photocurrent EF is ruled by the carrier mobility-density relation in graphene, saturating at high optical injected carrier densities.

Recently, intense effort of research has been done in order to realize materials with enhanced plasmonic response in the wavelength range not covered by the noble metals [1]. Poor metals, metal nitrides or TCOs, among others, have been identified as interesting alternatives to silver and gold especially in the UV and IR [1,2]. The plasmonic response of these materials, which is driven by their Drude carriers, is permitted in the spectral regions where their dielectric function e presents a negative real part e₁ and a small enough imaginary part e₂.
Actually, a material only requires such criteria on e to be fulfilled in order to be suitable for plasmonics, regardless of the underlying physical phenomena from which originates such behaviour. Therefore it has been shown that the criteria can be met without sizeable contribution of Drude carriers, provided electronic transitions with high oscillator strengths can be excited [1, 3-4] so that e₁ takes negative values and e₂ remains small enough at the spectral vicinity of the transitions.
In this presentation, we show that the dielectric function of bismuth (Bi) thin films as determined from spectroscopic ellipsometry shows negative values in a broad wavelength range (300-1400 nm), and fulfils the criteria to present plasmonic properties in the visible thanks to the excitation of strong interband transitions in the near IR. The plasmonic behaviour of Bi is confirmed by the realization of nanocomposite films with embedded size-controlled Bi nanoparticles, their absorption spectrum being dominated by well defined resonances. Similarly to localized surface plasmon resonances, they can be tuned in the whole visible range as a function of the nanoparticles shape, size and environment [5]. In addition, due to their interband origin, these resonances could be further tuned upon tailoring the electronic structure of the nanoparticles. In this context, we investigate for the first time the effect of geometrical confinement on the visible - near IR dielectric function of bismuth down to the semi-metal to semi-conductor transition regime. These results open the way to a new generation of plasmonic nanostructures with unprecedented versatility and tunability.
[1] G.V. Naik, V. Shalaev, A.Boltasseva Adv. Mat. 25, 3264 (2013).
[2] M.G. Blaber, M.D. Arnold, M.J.., Ford J. Phys.C: Cond. Mat, 22, 143201 (2010)
[3] L. Gu, J. Livenere, G. Zhu, E. E. Narimanov, M. A. Noginov Appl. Phys. Lett., 103, 021104 (2013)
[4] M. J. Gentil, S. Nuñez-Sanchez, W.L. Barnes,. Nano Letters 4 , 2339 (2014)
[5] J. Toudert, R. Serna, and M. Jiménez de Castro, J. Phys. Chem. C 116, 20530 (2012).

Integrated diffractive optics has many applications in beam shaping and control on the micro-scale. Fabrication using lithography is limited to planar or layered geometries. We demonstrate fabrication of diffractive elements via direct laser writing. We have tested 3D diffraction gratings and zone plates designed for operation at visible wavelengths. Direct laser writing is a promising technique to fabricate integrated 3D and multi-layer diffraction optics.
We have previously developed a laser writing technique that enables fabrication of disconnected metal structures in a polymer matrix, which we now apply to 3D diffraction optics. Ultrafast laser pulses centered at 795 nm are focused into a thick (100-250 micrometer) polymer film doped with silver nitrate. Multi-photon absorption prompts reduction of metal ions and formation of silver structures at the focal point. Using a long-travel, high precision 3D translation stage and a 0.8-NA, long working distance objective, we have demonstrated feature sizes below 100 nm and resolution of 0.5 micrometers on samples spanning several millimeters. Features can be positioned freely in the horizontal and z-directions and we have produced structures with over 10 layers in the z-direction. The polymer is left in place after fabrication, providing a dielectric matrix transparent in the visible and near infrared wavelengths for the disconnected structures.
We fabricate 3D gratings which are analogs of crystallographic structures (simple and body-centered cubic and structures with a two-atom basis with different scattering strengths). Scatterers are spaced by 5-40 micrometers in order to observe multiple diffraction orders using visible wavelength illumination. Samples span several millimeters in plane and consisted of 2-12 layers in the z-direction. Diffraction patterns measured in transmission using a 633-nm HeNe laser are in good agreement with calculated Laue diffraction patterns.
Zone plates with focal lengths ranging from 4 to 50 micrometers at 633 nm are fabricated. Devices are tested in a transmission microscope using a white LED light source and focal spots for different wavelengths are imaged by adjusting the microscope focal plane. Wavelength selectivity using pairs of laser-written zone plates and pinholes has been observed, showing that multi-layer structures with flexible spacing in the z-direction can be successfully fabricated.

We present highly luminescent carbogenic dots produced by using precisely pore-size controlled subnano-porous silicas as templates. In the field of recent nanotechnology, photoluminescent carbon nanoparticles (or carbogenic dots) are gathering interests in their unique and efficient optical properties, such as high photoluminescence quantum yields and excitation-dependent photoluminescence. The templating method using the pores of mesoporous silicas is one of the most simple and efficient technique for the synthesis of carbogenic dots. However, the pore size dependency of their photoluminescence property was still unknown, because precise pore-size control in supermicropore region (0.5-2.0 nm) had not been accomplished adequately.
In this study, we synthesized series of porous silica having controlled pore sizes in the range of 3.0 to 0.6 nm and used them as templates for the synthesis of the carbogenic dots. The aqueous solution of citric acid was impregnate into the pore and followed by calcination in air at optimized temperature to obtain carbogenic dots.
The obtained carbogenic dots exhibit bright blue luminescence under UV irradiation without further surface passivation. We discovered that the photoluminescence quantum yields of carbogenic dots were increased with decreasing the pore size of template porous silicas, and it was enormously enhanced for the subnanometer pores. Addition of the alkali metal ions to the as-prepared carbogenic dots greatly enhanced the quantum yields up to 40%. Unlike most of carbogenic dots, the present carbogenic dots exhibit no excitation-dependent photoluminescence behavior. This absence of excitation dependency was probably attributed to the narrow particle size distribution of present carbogenic dots. In addition, the present carbogenic dots were able to isolate from porous silica by simple extraction using conventional solvents such as ethanol and acetone. The extracted dots could be able to re-disperse in various medium, and retain original luminescence. In summary, the present templating method using subnano-porous silica was highly effective for the syntheses of size tuned and highly luminescent carbogenic dots.

The optical and electronic properties of suspensions of inorganic nanoparticles of WS₂/MoS₂ are studied through light absorption and zeta potential measurements, and compared to those of the corresponding microscopic platelets. The total extinction measurements show that, in addition to excitonic peaks and the indirect bandgap transition, a new peak is observed at 650-800 nm. This spectral peak has not been reported previously for WS₂/MoS₂. Comparison of the total extinction and decoupled absorption spectrum indicates that this peak largely originates from scattering. Furthermore, the dependence of this peak on nanoparticle size, shape, and surface charge, as well as solvent refractive index, suggest that this transition arises from a plasmon resonance.

We have realized thin-film structures based on vertical cavity architectures that are capable of producing quantized thermally emitted light in the mid-IR spectral range. In essence, by quantizing the optical modes which are available in the vicinity of the emitter via a microcavity, we restrict the radiation modes to one specific wavelength of light and therefore transform the “black-body emitter” into one optimized for a specific color. This concept was proposed and simulated by Celanovic et al [Physical Review B 72, 075127 (2005)] for a microcavity consisting of one distributed Bragg reflector (DBR), a non-active cavity spacer layer, and then a second metallic mirror. By heating the metal layer, directed narrowband IR emission should be generated from the cavity. Towards this aim, we have developed low-loss DBRs for the mid-IR spectral range by thermally evaporating Ge, ZnS, and CaF₂ thin films of appropriate thickness, with reflectivity values approaching 99%. We have also demonstrated mid-IR microcavity optical resonators with wavelength tunability and relatively high quality factor microcavities with Q = 150. We have shown that thin films of metals, even from good electrical conductors like gold and chromium, can act as resistive black-body emitters. The thicknesses and morphology of the layers were characterized by high-resolution scanning electron microscopy (HRSEM). Upon applying a voltage, the devices emitted IR light, which was detected using an MCT photodiode and IR spectrometer. When devices were packaged in a glove box, they could operate stably for several hours with no discernable degradation. In comparison to thin metal films, the cavity devices emit a narrower spectrum. Moreover, the cavity devices appear to have a threshold power level that must be surpassed before light is detected, which perhaps is due to the cut-off frequency associated with a microcavity, and a signature of quantized thermal light emission. Using the architecture we have realized, it should be possible to achieve wavelength tuned emission covering the mid-IR spectral range of 3µm < lambda;_c < 8µm.

Any real surface is rough at some length scale. Surface roughness plays an important role in various physical properties including light scattering phenomenon. We have demonstrated some facile ways to fabricate the large area polymer surfaces with varying roughness followed by studying their anti-reflective properties.
One of the approaches discussed here is based on electrospun nanofibers. In electrospinning, polymer nanofibers are deposited on a substrate in an uneven non-woven matrix. Electrospun fabric which offers a variable roughness surface has been considered in this work as a master template to fabricate the negative replica of the fibers in polydimethylsiloxane (PDMS) using soft lithography. By controlling the fiber morphology from long uniform fibers to only beads, we have fabricated PDMS surfaces with surface roughness varying over an order of magnitude length scale.
Biomimicking has been demonstrated as another way to fabricate rough polymer surfaces over a large area. Here we have chosen flower petals of a new flower plant, Euphorbia milii, as a master template. Euphorbia milii petal shows the superhydrophobicity due to the presence of surface features with hierarchical roughness. These surface features were then successfully transferred using replica molding to prepare large area rough PDMS surfaces.
As-fabricated polymer surfaces with varying roughness have then be tested for their anti-reflective properties using UV-Vis spectroscopy over a wide range of wavelengths (400-700nm) and at incidence angles ranging from 30° to 70°. These measurements show that reflectance has been reduced significantly (less than 0.1%) as compared to PDMS surfaces with no surface texturing. This omnidirectional broadband anti-reflection behavior of polymer surfaces may be attributed to multiple internal reflections and light trapping within the micro textures present on their surfaces as source of this extra roughness.
We believe that facile approach as depicted here to fabricate large area rough polymer surfaces may provide cost effective solution in the manufacturing of anti-reflective surfaces for wide variety of engineering applications including in solar cells.

Heat-assisted magnetic recording (HAMR) is a promising technique to extend the areal density of hard drives. In HAMR, localized optical spots are obtained via plasmonic transducers and these plasmonic transducers are utilized to heat the magnetic medium during the recording process. One potential challenge in a HAMR system is the heating of plasmonic transducers and performance reduction due to such heating. The heating of the plasmonic transducers can result in both performance and reliability issues in a HAMR system, including structural distortions of the slider and transducer. In this study, to overcome the aforementioned performance and reliability issues in HAMR, we designed heat-sinks for plasmonic transducers and reduced the temperature of the plasmonic transducer and surroundings by cooling techniques. We discuss various plasmonic transducers and provide heat-sink designs to reduce their heating.

Plasmonic metamaterials have been a burgeoning area of research in recent years, where surface plasmon polaritons (SPPs) can manipulate light on the nanoscale. Typically, noble metals (e.g. Ag, Au) have been the key materials in this field of research, but suffer drawbacks (e.g. high loss) especially in the mid- and near-infrared (NIR) spectral ranges. Recently, wide bandgap semiconductors, such as Al-doped ZnO (AZO), have been shown to hold great potential in surpassing the tunability and flexibility of traditional noble metals in nanoplasmonic applications. Generally, these transparent conducting oxides have been extremely important for various optoelectronic applications due to the coexistence of high conductivity and high transparency, which can be tuned through doping. Recent studies have shown that these wide bandgap semiconductors, in particular AZO, are also efficient nanoplasmonic materials in the NIR due to their metallic behavior, strong confinement of SPPs, and low loss. AZO has been studied extensively using a multitude of deposition techniques, especially atomic layer deposition (ALD), which is particularly useful to grow uniform and conformal films with a high degree of thickness control on complex three-dimensional topographies because it is based on a binary sequence of self-limiting surface chemical reactions. Furthermore, the doping concentration can be precisely controlled by adjusting the ALD cycle ratios of the host and dopant materials, thus making ALD a unique and powerful method to deposit AZO into high aspect ratio structures for nanoplasmonic applications. Recently, it has been shown that ALD-grown AZO offers extreme tunability that can be utilized for many applications, including plasmonic components for epsilon-near-zero metamaterials. This extreme tunability is exploited here in metal-dielectric multilayered structures in order to manipulate and control light in subwavelength volumes for various optical applications.

Holographic display is a realistic three-dimensional (3D) display because holography retrieves exactly light wave front of the corresponding 3D object [1]. The 3D display with spatial light modulator (SLM) provides the next viewing angle#12288;theta;#12288;for diffracted rays: theta;=2Sin^-1(lambda;/p), where lambda; is a wavelength of diffracted light and p is a period of a two-dimensional (2D) array of pixels. The viewing angle q of holographic displays based on conventional SLMs (pixel size rages from 10 to 100 mu;m) is less than three degrees. We have developed thermomagnetic driven magneto-optic SLMs (3D-MOSLMs) with submicron magneto-optic pixels for the wide-viewing-angle holographic displays. Magnetization direction of pixels on a magneto-optic layer is controlled by focused laser. This display has the submicron-scaled magnetic pixel array on amorphous TbFe (a-TbFe) film. Color of the reconstructed images was only green color.
In this study, we developed colorized 3D-MOSLM by magnetic garnet films. The magnetic garnet films have high optical efficiency on a visible light wavelength.
The colorized 3D-MOSLM used optical space division method that compounded images of each color reconstructed from the BiDyYFeAlG films. The optical space division method can reconstruct the full-colorized images by synthesizing reconstructed images of each color. This method uses some SLMs to show single color components of a full-colorized image. In this study, blue, green and red reconstruction images were synthesized to represent magnetic holographic images with intermediate colors. The optical efficiency of magnetic films has wavelength-dependence and depends on thickness of magnetic films. In order to obtain the optical efficiency of the equivalent red, green and blue colors, the thickness of BiDyYFeAlG films was 1.1 mu;m for wavelength of 532 nm, 2.6 mu;m for 633 nm and 1.2 mu;m for wavelength of 450 nm. The optical efficiency of fabricated films that 1.1 mu;m thickness was 2.5×10^-2 % at 532 nm, 2.6 mu;m thickness was 2.8×10^-2 % at 633 nm, 1.2 mu;m thickness was 3.9×10^-3 % at 450 nm. The reconstructed images were captured by a camera that conform to the sRGB standard, and plotted on xy chromaticity gamut map. We demonstrated a 3D image with intermediate color. The reconstructed image of red, green and blue colors from each BiDyYFeAlG film contains colorized three spheres respectively.
In this study, we developed the colorized 3D-MOSLM was synthesized red, green and blue reconstructed images by optical space division method. These results suggest that 3D-MOSLM can represent colorized 3D images.
References
[1] D. Gabor, “A new microscopic principle”, Nature, 161, 777 (1948).

Quantum optics involves the coupling of quantum emitters to their electromagnetic environment. Because this coupling is related to the concentration of the optical field, it is typically constrained by the diffraction limit of light. One way to circumvent this is by moving to quantum plasmonics, which uses surface plasmon polaritons (SPPs) instead of photons. SPPs are surface waves that arise from the coupling of photons to electronic oscillations. They allow light to be concentrated well below the diffraction limit. However, despite this capability, quantum plasmonics has not yet been thoroughly exploited. This can be attributed to challenges in fabricating high-quality systems that combine robust efficient emitters with well-defined plasmonic structures. To address this, we have recently fabricated quantum-plasmonic systems that consist of active colloidal quantum dots precisely placed in the mode of a passive plasmonic resonance. Since material quality is critical, we optimized both the active and passive components. For the active part, quantum dots were synthesized with stable fluorescence quantum yields above 90%. For the passive part, finite-element simulations were used to identify suitable structures. These were then fabricated by combining template stripping with a cryogenic deposition process to yield ultrasmooth silver patterns with previously unattainable performance. Countable numbers of quantum dots were placed into the plasmonic modes of these structures with nanometer precision using an electrohydrodynamic printing technique. The result is an ideal system for exploring fundamental physics in quantum plasmonics.

Surface plasma waves are typically quantized by direct analogy to electromagnetic waves in free space. As a result, the quantum theory of these waves predicts that their quanta—surface plasmons—should exhibit the same quantum phenomena that photons do.
Here we report on two experiments that test this analogy between photons and surface plasmons. The first is a plasmonic version of the Hong-Ou-Mandel experiment in which we observe two-photon quantum interference (TPQI) between plasmons with a visibility of 93%, comparable to what we observe using dielectrically-guided photons. To make this measurement, we produce pairs of single photons by spontaneous parametric down-conversion and couple them into low-loss silicon nitride waveguides that deliver them to and collect them from plasmonic directional couplers. This hybrid dielectric-plasmonic platform enables us to couple single photons into and out of plasmonic components with relatively high efficiency, resulting in high count rates and error bars of order 1%.
In the second experiment, we extend this platform to investigate path entanglement in circuits that involve plasmonic elements. We use TPQI at a dielectric 50-50 directional coupler to prepare a path-entangled two-photon state, then send the photons through plasmonic waveguides, and finally let them interfere at a second dielectric coupler to determine whether they remain entangled. Unlike previous experiments that converted polarization- or frequency-entangled photons into plasmons, in our experiment any information about the mere presence or absence of a plasmon could potentially distinguish between the components of the entangled state and cause decoherence. We have observed path entanglement in a dielectric circuit with 90% contrast and will discuss measurements in plasmonic circuits as well.

We discuss and present quantum calculations using the random-phase approximation with effective single-particle states, from either a tight-binding Hamiltonian or the Dirac equation, for doped graphene nanodisks. Recent numerical tight-binding calculations have indicated that the plasmon resonance of such disks exhibits a redshift in the quasi-classical size-regime [1]. The existence of edge states due to zigzag boundaries was suggested as the origin of this redshift.
Using a Dirac equation approach, we demonstrate explicitly and analytically that this redshift, and also to some extent a concomitant broadening, is indeed due to the presence of edge states. In particular, we derive a simple, radius-dependent correction to the classical bulk conductivity, accounting for these effects by allowing interaction between edge and bulkstates. The impact of such an edge conductivity is significant even in disks of radius exceeding R = 10 nm, introducing additional dispersion, and allowing damping via non-vertical Landau transitions from the Dirac point to the Fermi energy. The necessary momentum for these interactions is supplied by the structural truncation of the disk, with a strength proportional to v_F/R. A phenomenological loss-mechanism of similar scaling has been applied successfully for metallic nanospheres, usually referred to as Kreibig damping [2], and was recently derived in the context of nonlocal diffusion mechanics [3]. The consideration of edge states in graphene nanostructures offers a complementary and novel pathway to v_F/R response-components.
In addition, we automatically account for the effects of nonlocal response through the use of a real-space formulation of the random-phase approximation. By comparison, we show that a simple hydrodynamic model for graphene reproduces the effects of nonlocality well. Furthermore, within a hydrodynamic description, we derive simple analytical corrections to account for the consequences of nonlocality.
In conclusion, we offer a semi-quantitative analytical description of the dominant nonclassical features of graphene nanodisks, in the quasi-classical size-regime beyond the realm of molecular plasmons. Specifically, we find that the plasmonic redshift arises from the competing effects of edge states and nonlocal response, shifting to the resonance toward the red and blue, respectively.
[1] S. Thongrattanasiri, A. Manjavacas, and F.J. García de Abajo, ACS Nano 6, 1766 (2012).
[2] U. Kreibig and C. Fragstein. Z. Physik. 224, 307-323 (1969).
[3] N.A. Mortensen, S. Raza, M. Wubs, T. Soslash;ndergaard, and S.I. Bozhevolnyi, Nat. Comm., 5, 3809 (2014).

Quantum emitters, such as fluorescent molecules, quantum dots, and nitrogen-vacancy centers in diamond, have received considerable attention as robust solid-state single photon sources in a wide range of applications ranging from fundamental studies of light-matter interactions to emerging quantum information technologies. In both cases, large scale positioning of individual quantum emitters needs to be achieved, while maintaining the compatibility for integration with nanostructures. Many recently developed nanoparticle positioning techniques are limited to positioning clusters of emitters or individual emitters one at a time, whereas the large-scale placement of many individual emitters remains a challenge. Moreover, once the quantum emitters become photo-bleached or if different emitters need to be investigated on the same nanostructures, new samples have to be fabricated. This is problematic because it is time-consuming and wasteful to keep remaking the same nanostructure, and there will always be unknown variations between samples. Thus, it can be difficult to perform statistical characterizations of nanostructures where variations in the emission can be deconvolved from variations of the nanostructures themselves. As a result, it is very important to realize the reproducibility of positioning new quantum emitters on the same nanostructure samples.
Here, we will describe a new, inorganic, and reusable electrostatic self-assembly method which allows for precise positioning of individual quantum emitter nanoparticles at large scales and also easy integration with nanostructures. By taking advantage of the opposite surface charges caused by the isoelectric point difference of colloidal nanoparticles and predefined template materials, we show precise and large scale self-assembly of silica-clad QDs and NV center nanodiamonds on predefined dielectric pad arrays. Then, we demonstrate the reusability of these dielectric templates by cleaning and re-depositing silica-clad QDs and nanodiamonds on the same pad arrays multiple times while maintaining precise single nanoparticle placement. To verify single nanoparticle placement we also perform single photon anti-bunching measurements using a Hanbury-Brown and Twiss interferometer. Lastly, to demonstrate the ease of nanostructure integration, we position silica-clad QDs at specific predetermined positions relative to gold rod antennas.

The prevailing model of a network capable of complex quantum information processing consists of solid-state quantum memory and processor bits (qubits) connected via flying qubits in photonic channels. The negatively charged nitrogen-vacancy defect in diamond (NV) has an electronic spin state with long coherence times that can be optically initialized, manipulated, and measured - unique properties that allow the NV to act as an optically accessible solid-state qubit. Unfortunately, the scalability of quantum networks fabricated entirely in diamond is limited due to the stochastic nature of NV placement and the difficulty of large-scale high-quality diamond patterning. In contrast, silicon nitride (SiN) based photonics is a mature field allowing for the fabrication of complex photonic circuits.
Here we present a scalable method for the bottom-up fabrication of a hybrid photonic circuit combining the superior quantum properties of single NVs with the mature fabrication of SiN photonic elements. Using transfer-mask nanolithography developed by our group, we fabricated arrays of single-mode micro-waveguides from diamond membranes with a low density of naturally occurring NVs. These micro-waveguides were optically characterized with a confocal setup. We selected individual diamond micro-waveguides with optimal NV placement and deterministically placed them over air gaps in single-mode SiN waveguides. Simulations indicate that with an adiabatic-like transition between the diamond and SiN waveguide modes, up to 82% of the photons emitted from the NV can be collected into the single mode SiN waveguide. This high collection is crucial for high fidelity spin measurements and network scalability.
We demonstrate efficient coupling of fluorescence from a single NV into a single-mode SiN photonic circuit with a rate of more than 1.4 million photons collected into a single direction of a single-mode SiN waveguide at saturation. The single-photon statistics of signal collected through the waveguide are preserved below saturation with antibunching of photon arrivals below 0.5. Moreover, the NV electron spin coherence time is measured to be comparable to previous bulk measurements, indicating that our nano-fabrication technique and device assembly do not degrade the nuclear and electronic environments around the NV centers.
In conclusion, we have demonstrated the high-yield integration of diamond micro-waveguides containing NV centers with high-quality spectral and spin properties into SiN photonic circuitry. Scaling to multiple, high-quality quantum bits integrated into a complex photonic circuit is made possible by pre-screening arrays of diamond micro-waveguides for optimal coupling of fluorescence. This provides a scalable architecture for the creation of a complex photonic circuit for quantum information processing.

I will discuss application of plasmonic nanostructures and metasurfaces that can be used to control light-matter interaction over wide wavelength range, spanning visible, mid-IR and microwave. For example, using metallic nanocavities and gratings we were able to engineer radiative decay rate of quantum emitters in diamond, including nitrogen-vacancy (NV) centers, emitting in red and near-IR part of the spectrum. Our structures are based on metallic apertures formed by surrounding sub-wavelength diamond nanoposts with a silver film, which can enhance the spontaneous emission rate of embedded quantum emitters. Addition of grating structure enables extraction of emitted surface plasmon modes and improves the overall photon collection efficiency. I will also present our work on the development and utilization of a double split-ring microwave resonator operating at ~3GHz frequency, for uniform and efficient coupling of microwave magnetic field into NV centers over a large area. By performing the Rabi nutation experiments on arrays of diamond nanowires with ensemble NV centers, we obtained nearly ten-fold improvement in microwve delivery efficiency, over a mm² area, compared to traditional techniques. Finally, I will present our efforts towards the realization of integrated, zero-refractive-index metamaterials based on an array of Si pillars, and discuss their application in control of light propagation on semiconductor chip.

Diamond is found in a very wide field of applications from thermal management to mechanical applications, from quantum technologies to optics. For many of these fields of study, a controlled shaping of single-crystal and polycrystalline diamond to the nanoscale is crucial. Using CVD processes it is possible to grow polycrystalline diamond film varying thickness, morphology and surface functionalization. Film properties and shaping can be modified both during or post synthesis. In particular in this talk it will be illustrated some examples of nanostructures obtained starting from diamond films by H-etching processes with a custom-made dual-mode MW-RF plasma reactor.
Nanodiamond interest for optical application is related to the possibility of the formation of fluorescent defects in the diamond lattice, called color centers. These centers display remarkably optical properties as single photon sources and have fluorescence energy range suitable for biological applications. Ion implantation is up to now the most used technique for the formation of color centers, but it does not allow a very high yield of doping, due to the deterioration of the diamond lattice. We use CVD processes to drive the inclusion of color centers during synthesis of diamond films. The CVD process guarantees the formation of good crystalline quality diamonds which represents the basic requirement to enhance the optical properties of the color centers. Three methods to control insertion and position of color centers will be presented. The first method utilizes lithographed SiO₂-NbN substrates and brings to the growth of diamond film with a very intense emission of the Si color center in correspondence of the diamond grown on the SiO₂ compared to the NbN. The second method, supported by a thermodynamic model, consist in a substrate treatment with nickel nanoparticles to obtain unimaginable highly brilliant isolated single crystals or ultra-bright polycrystalline diamond films as compared to the usual CVD or irradiation methods.Finally the last method is related to the use of a modified CVD apparatus able to introduce metallic species in the diamond growth area. This species are able to introduce a significative conductivity in the diamond matrix together with the presence of exotic color centers as Ti, Ta, Cr and Ge.

Quantum repeaters require efficient interfaces between photons in optical fibers and quantum memories. There has been particular interest in quantum memories based on spin states in the negatively charged nitrogen-vacancy (NV^-) defect center in diamond. Here we demonstrate the integration of diamond micro-waveguides containing individual NV^- centers directly with silica optical fibers. Our micro-waveguides are single-mode, and are fabricated from single crystal diamond with a rectangular shape of 200 nm x 200 nm x 10 mu;m. We fabricate adiabatically tapered fibers with a diameter of approximately 500 nm to couple light efficiently to and from these diamond waveguides. A single micro-waveguide is selected, based on its optical properties, and subsequently placed directly on the tapered region using a nanomanipulator. Finite-difference-time-domain simulations suggest efficient energy transfer from the NV^- center into the micro-waveguide and finally the tapered fiber. This coupling efficiency is substantially higher than previous fiber-integrated systems that rely on coupling to point emitters such as nanodiamonds and colloidal quantum dots. In our experiments, we observe a raw single photon count rate of (712.3 ± 23.6) x 10³ sec^-1 at saturation pump power from two fiber ends combined, prior to correcting for system inefficiencies. Strong photon anti-bunching is observed in both auto- and cross-correlation measurements. We also present preliminary data on spin manipulation using optically detected magnetic resonance of the NV^- electron spin. The demonstrated experimental system can find applications as a robust room-temperature single photon source and as a fiber-integrated quantum memory for quantum information networks.

Solid-state qubits with long spin coherence times are promising for the realization of integrated quantum information applications. Several different systems are presently under investigation. Among them, color centers in diamond, the negatively charged nitrogen-vacancy centre (NV) is a promising candidate for such solid-state qubits. Recently, the NV has been proposed and applied to realize several quantum entanglement experiments that indicate the prospects of this system. However, by coupling an NV to a nanophotonic cavity its capabilities for quantum information applications can in principle be enhanced. Some examples include long-distance quantum entanglement, high-fidelity quantum interference, quantum repeaters and quantum memories. An important hurdle for these diamond-based quantum technologies is to efficiently couple a long-lived NV spin qubit with the mode of an optical nanocavity.
In this Abstract, we demonstrate cavity enhancement of a single NV embedded within high-purity diamond with a one-dimensional photonic crystal cavity. We achieve spin control of the NV by integrating the NV-cavity structure onto lithographically patterned strip lines. The enhancement lies in the strong Purcell regime and the NVs in the structures are shown to have long spin coherence times exceeding 200 mu;s using an on-chip opto-electronics architecture. These milestones will enable solid-state based quantum information devices that operate faster and more efficiently than previous implementations.

Diamond based quantum spin systems like the negatively charged nitrogen-vacancy center (NV) represent a promising platform for precision measurements and quantum information applications. Most of these applications rely on the long electron spin coherence time of the NV ground state of up to 600 ms in bulk. Solid-state cavity systems, on the other hand, have attracted much interest for enhancing light-matter interaction on the nano-scale. Cavity-enhanced light matter interaction can enable the speed up of established quantum protocols and the implementation of novel concepts for quantum networks. A first important step towards more complex systems is the Purcell induced spontaneous emission rate enhancement of an NV inside a cavity. First realizations of cavity-coupled NVs have been reported recently. In these realizations, NVs were randomly distributed with respect to the cavity mode thereby prohibiting a high NV - cavity-mode overlap, hence limiting the spontaneous emission rate enhancement, while spin coherence times were below 1 us.
Here, we present the spatially deterministic creation of NV centers inside L3 photonic crystal cavities using an implantation mask leading to a high NV-cavity mode overlap. ¹⁵N implantation through circular apertures and subsequent annealing result on average in about 1.1 (0.2) NVs per cavity. Furthermore, we demonstrate electron spin coherence times of several hundred microseconds of cavity coupled NVs. We furthermore show strong Purcell enhancement of the NV spontaneous emission rate paving the way to advanced quantum network implementations.

When an array of small metallic particles is embedded into a dielectric matrix one should expect a pole in the polarizability of the medium at certain energy, when the negative real part of the dielectric function of the metal compensates the double value of the positive real part of the dielectric function of the surrounding dielectric material. This gives rise to so called Froelich resonance in the optical properties of such metamaterial.
We discovered and investigated a resonance in optical absorption, which originates from localized plasmon excitations in a self-organized system of metal AsSb nanoparticles embedded in a semiconductor AlGaAs matrix.
The AsSb-GaAlAs metamaterial was produced by a low-temperature molecular-beam epitaxy on the (001) GaAs substrates followed by a high-temperature annealing. Our transmission electron microscopy study revealed a system of almost spherical AsSb inclusions in the crystalline AlGaAs matrix. The average diameter of the inclusions was 6 nm. The filling factor was about 0.2%.
The Froelich plasmon resonance in our metamaterial was revealed at 1.48 eV with a bandwidth of 0.18 eV. The absorption coefficient within the resonant band was as large as 9000 cm-1. The observed optical properties were theoretically considered in terms of either Mie scattering or Maxwell-Garnett effective medium. We used well documented data for the dielectric properties of AlGaAs and Drude model for the electron system in the metal AsSb nanoinclusions. A reasonably good description was achieved in both approaches.

Recently a bunch of groups have been seeking and demonstrating metamaterials and photonic crystal (PhC) structures realizing double zero index material (DZIM) for novel device applications such as super coupling, super bending, cloaking, and etc. However, there are still no suitable media for on-chip planer devices with telecom regime. In this paper, we propose and demonstrate on-chip DZIM given by intended degeneracy of photonic bands at the gamma point, which would be a useful platform enabling new functional devices. And in the case of using DZIM, we can obtain finite impedances for the efficient optical coupling, and minimize the insertion loss of the devices.
As a basic structure, we employ square lattice Si pillar array PhC with 500-nm tall since DZIM behavior owing to the intended degeneracy at the arbitrary operation wavelength can be obtained if we carefully tune the spacing and diameter of the pillars. When we look at TM-like polarization for that, each pillar has a loop of magnetic (electric) flux at the wavelength of the air (dielectric) guided resonance mode along the in-plane (out-of-plane) direction of the PhC structure. In that case, the loops inside the pillar array cancel out the neighbor loops each other, then mu; or ε becomes zero effectively inside the material - we can optimize the structure to have “Dirac-cone” shape band structure at the gamma point, which means both mu; and ε become zero simultaneously at the single wavelength. And the effective wavelength becomes infinite at the gamma point. Therefore the PhC structures can be treated as bulk media. However, we have a considerable scattering loss at the gamma point. In order to compensate this, our DZIM is sandwiched by 50 nm thick gold mirrors at the top and bottom side of the material. The Si pillars are covered by ~ 600 nm thick SU8 polymer to have the top mirror.
To investigate the DZIM behavior, we designed a PhC prism with SU8 slab waveguide which couples to Si waveguide coupler to observe the refraction angle through an objective lens and a NIR camera from the top side of the device. From that angle of the refraction, we can estimate the effective index of the prism by using Snell&’s law. In the measurement, we first measured the SU8 prism without PhC for the control measurement. In that case, we couldn&’t see any refraction. In the case of the prism with PhC, we could observe the clear diffraction indicating its zero index. However, that sample shows the zero index band around 1340 - 1500 nm, which means there is the obvious photonic bandgap since our structure still doesn&’t have proper pillar diameter, gold mirror structure, and SU8 thickness inside the prism. But we&’ve already observed that the bandgap width depends on the pillar diameter. By optimizing the fabrication, we would get near DZIM characteristic to be presented.

Magnetically responsive photonic crystals (MRPCs) have the merits of high refractive index, fast and fully reversible photonic response across the visible spectrum under external magnetic field when comparing to the traditional photonic crystals composed of monodispersed SiO₂ or polymer particles.¹ They are promising for the fabrication of chemical or biomolecule sensors. Although they have been solidified in various responsive polymers in the forms of films or balls, the relatively high content of polymers always increases the diffusing length of analytes leading to the failure of real-time sensing. Photonic nanochains are the smallest structure of MRPCs, however, up to now, only SiO₂ or carbon has been used to fix the nanochains which are not suitable for sensing.^{2, 3} Thus, we have fixed individual photonic nanochains in responsive polymers by dispersing steric-stabilized Fe₃O₄@PVP particles (previously fabricated by our group⁴) in solutions of monomers which are subsequently polymerized under external magnetic field and UV irradiation.⁵ It has been found that the interparticle distance of the as-obtained responsive photonic nanochains can be responsive to external field like pH, solvents or other analytes. Most importantly, the responsive photonic crystal chains show a fast response rate when the external environment changed due to tens nanometers of responsive polymer thickness. Our result will significantly shorten the diffusion length of analytes, accelerating the response rate of photonic crystal sensors for chemical sensing or biosensing devices.
REFERENCES
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We experimentally demonstrate for the first time on-chip negative diffraction of SPPs in an array of gap-plasmonic waveguides for the spectral range lambda;₀ = 1200-1800 nm, which is caused by negative mutual coupling of the waveguides [1]. We also observe negative refraction [1] on the array&’s interface with an adjacent metal film and subsequent refocusing of SPPs on this film. These findings are supported by band structure calculations proving negative refraction over the broadest spectral range reported up to date [2]. Since the propositions of negative refraction, first by V. Veselago [1], later by J. Pendry, different concepts have been developed to increase the range of incidence angles and the spectral bandwidth. In contrast to our approach, most designs have a narrow bandwidth as they are based on resonant excitations in metamaterials [2].
To achieve our goal we transferred the concept of discrete diffraction from arrays of dielectric waveguides [3] to arrays (nanoscale confinement, ca. 300 nm asymp; lambda;₀ / 5) of coupled plasmonic gap waveguides [4] (pitch Γ asymp; 370 nm) 25 x 25 µm in size. To inject light into the array we excite a single waveguide of the array via a connected Yagi-Uda nanoantenna (15% efficiency) [4] and monitor the discrete diffraction inside the array [3].
Measuring the intensity distribution at the array end we determined the coupling constant c = W_a / (2 L Γ) asymp; 0.34 mu;m^-1 for lambda;₀ = 1550 nm, resulting in an extremely large anisotropy of the metasurface&’s effective refractive index Δn_eff = 2 lambda;₀c / π asymp; 3.14. The spectral dependence of the wave spreading turns out to be opposite to the known dispersion in dielectric structures. Diffractive wave spreading is reduced for larger wavelengths, consistently indicating a negative coupling process. Light leaving the array is converted to SPPs while experiencing negative refraction at the array interface. SPPs propagating further on the adjacent metal film refocus spontaneously thus imaging the incoupling spot.
In conclusion this work introduces a new and extremely broadband concept for negative refraction in a chip-based hyperbolic metasurface. It offers the opportunity to build and combine arbitrarily shaped hyperlenses made from positively and negatively diffracting metamaterials.
References
1. V. G. Veselago, Sov. Phys. Uspekhi10, 509-514 (1968).
2. A. Poddubny, I. Iorsh, P. Belov, Y. Kivshar. Nat. Photonics 7, 948-957 (2013).
3. U. Peschel, T. Pertsch, and F. Lederer, Opt. Lett.23, 1701-1703 (1998).
4. A. Kriesch, S. P. Burgos, D. Ploss, H. Pfeifer, H. A. Atwater, U. Peschel. Nano Lett. 13, 4539-4545 (2013).

Transformation optical designs provide unprecedented control of electromagnetic fields within optical devices. Typically, the device functionality is defined by a coordinate transformation, which is translated into the constitutive material properties of the device. While powerful and versatile, this method often results in extreme material requirements, which restricts the range of devices that can be realized. In order to achieve realistic designs, we present an inverse approach to transformation optics. Rather than starting with a coordinate transformation, we consider the range of material properties that can be achieved experimentally. Within this parameter space, we can generate a customized coordinate transformation to achieve the desired functionality using a facile split-step method. This design is well suited to fabrication.
To verify our approach, we design a mid-infrared invisibility cloak based on split-ring resonators using realistic material properties. We use finite-difference time-domain simulations to characterize the scattering properties and compare the cloak performance to standard transformation designs. This investigation reveals an inherent non-uniqueness of coordinate transformations. In particular, there may exist multiple coordinate transformations within the given parameter space, which all qualify as invisibility cloaks. We can leverage these additional degrees of freedom to improve the performance of inverse transformation optical designs. In the case of cloaks, we present designs that are optimized for easier fabrication, reduced loss, and wider bandwidth of operation.

We demonstrate a frequency and dispersion-tunable planar metamaterial structure with field effect gating to electrically modulate the permittivity in transparent conductive oxides (TCO) via changes in carrier density [1]. It exhibits optical dispersion that undergoes a transition from elliptical to hyperbolic with variation of carrier density in the TCO layers. We describe the parameter retrieval to obtain both in-plane and out-of-plane effective permittivities that goes beyond the Maxwell-Garnett approximation and is non-local. We find that active permittivity modulation results in tunability of the optical bandgaps.
We use the transfer matrix formalism to calculate the transmission and reflection coefficients of the metamaterial stack and relate them to the analytical expressions for the transmission and reflection of a single isotropic slab and retrieve effective permittivities. We correct for the anisotropy using the normal surface equation for uniaxial crystals and retrieve the in-plane and out-of-plane permittivities [1]. To experimentally verify the accuracy of our model we fabricate a Ag/SiO₂ planar metamaterial with e-beam evaporation and perform ellipsometric measurements which are in agreement with our model.
Next we investigated a tunable metamaterial consisting of two 20 nm layers of Au, separated by a 15 nm active layer of ITO with carrier concentration 5*10²⁰/cm³. The two materials are isolated from each other by 5nm Al₂O₃ layers. Under applied field between the Au and the ITO, a 5nm accumulation layer is formed at the Al₂O₃-ITO interface [2]. We consider the case where the carrier concentration of the accumulation layer of ITO is 5*10²¹/cm³ according to previous experimental results [2]. Our calculations show that in the spectral region from 300nm up to 452nm the metamaterial exhibits an elliptical dispersion. At 452nm the in-plane permittivity becomes negative giving rise to the hyperbolic regime up to 510nm where the out-of-plane permittivity also becomes negative, giving rise to an optical bandgap, up to 744nm when it returns to positive values, giving rise to a second hyperbolic regime.
We calculate the optical band structure of the metamaterial and surprisingly find that, apart from the predicted forbidden region in the range 510nm-744nm, there exists another narrow band gap at the wavelength regime where the dispersion transits from elliptical to hyperbolic. Active tuning of the metamaterial shifts the plasma wavelength of both permittivities by at least 30nm in the visible, translating to active control over the spectral position of the optical band gaps.
We are fabricating tunable metamaterials and ellipsometric measurements for demonstrating the active tuning of the effective permittivities will be discussed, as well as cathodoluminescence measurements to map the density of optical states in the optical band gap regions.
G.T. Papadakis et al., Spring MRS 2014, II8.06
E. Feigenbaum et al.,, Nano Lett. 10, 2111-2116 (2010)

The research in nanoplasmonics and metamaterials has demonstrated that scores of physical phenomena ranging from Raman scattering to negative index of refraction can be enabled or modified by the presence of metallic nanostructures and surfaces. In particular, it has been shown that the rate of spontaneous emission of dye molecules and quantum dots can be enhanced in vicinity of metals and metamaterials with hyperbolic dispersion. In the latter case, change of non-local dielectric environment causes an enhancement of the photonic density of states that leads to increase of the spontaneous emission rate. We infer that the range of processes, which critically depend on non-local dielectric environments and which can be controlled by metamaterials and plasmonic systems, extends far beyond the traditional scope of electrodynamics to include van der Waals interactions, Förster energy transfer and chemical reactions. The latter phenomenon is reported in this communication.
Poly-3-hexylthiophene, (P3HT), is the semiconducting polymer of choice commonly used in organic photovoltaic devices. However, this material is prone to photo-degradation in presence of oxygen. According to the Marcus theory, rates of chemical reactions can depend on dielectric environments at microwave and optical frequencies. The idea of this study was to control the rate of photo-degradation of thin p3ht films by a variety of dielectric environments provided by metallic, metamaterial and metal/dielectric substrates.
We have found that metallic or metamaterial films brought to close vicinity to the p3ht film (with thin dielectric spacer separating p3ht and metal) inhibit photo-degradation, in qualitative agreement with the arguments of the Marcus theory. However, removal of the dielectric spacer accelerates the photo-degradation reaction, probably due to chemical catalysis that overpowers the effect of dielectric environment.

Semiconductors with proper doping densities and carrier mobilities can support surface plasmon resonances in the terahertz (THz) region allowing semiconductor THz metamaterials (MM). Semiconductor MMs bring many advantages due to their tunable characteristics. Besides tunability, they exercise nonlinearities at high THz fields via various mechanisms such as impact ionization or intervalley scattering, which is important for the development of dynamic devices such as modulators, absorbers, sensors, and imagers. Similar to semiconductor electronics, thick substrates limit the application fields of semiconductor MMs.
We developed a solely semiconductor flexible terahertz (THz) metamaterial (MM). The device was fabricated using transfer printing method. InAs ring structures (diameter ~55 µm, thickness ~2 µm) were transferred to a 25 µm thick polyimide substrate and the transmission spectrum of the device was collected using THz time domain spectrometry (TDS) with two different polarizations. Dips in the transmissions were observed due to particle surface plasmon resonance. The resonance dip frequencies were found as 1.3 THz and 1.4 THz for the electric field polarization aligned with long axis and short axis, respectively. A full-wave solver was used to interpret the MM transmission. InAs was modeled with the Drude response and a good agreement with the experiment was observed. Future possibilities with the InAs nonlinear behavior is investigated.

The term "photonic bandgap" (PBG) refers to a frequency region where light cannot propagate in a certain material. The possibility that such 3-D periodic structures may exist was first pointed out by Yablonovitch [1]. A number of crystalline metamaterials were subsequently proposed and fabricated, including the so-called Yablonovite, the inverse opal [2] and the woodpile structure [3]. However, these structures exhibit intrinsic anisotropies in their photonic properties which makes it difficult to engineer true stopbands in all directions of light propagation. A step towards more isotropic PBGs was made by Ledermann et al. [4], who managed to fabricate silicon quasicrystals in the infrared region. Subsequently Florescu et al. [5] suggested that for a PBG to open up crystalline or quasi-crystalline structures are not required. Their work suggests that three conditions must be met: uniform local topology, short-range order and hyperuniformity. A particular point pattern is termed hyperuniform if it has vanishing long wavelength density fluctuations. The authors of [5] suggested a way to translate such pattern into a 2D dielectric structure, which in the limit of high refractive index possesses a pronounced PBG. The idea was expanded to 3D networks of dielectric rods by Liew et al. [6]. However, experimental realizations of such materials have only appeared very recently and were limited to 2D and to millimeter length scales.
Here I will present our successful efforts to fabricate a 3D polymer template [7-8] for such metamaterial by means of 3D laser nanolithography in a polymer photoresist. I will point to challenges connected with the micro/nanofabrication process of a demanding 3D network structure of this type and show (via light scattering, confocal microscopy and SEM experimental results) that our structure indeed follows the design rules [8]. Finally, I will demonstrate how to transform the templates into an isotropic metamaterial with a photonic bangap in the shortwave infrared [9] by infiltration with titanium dioxide and silicon, followed by the removal of polymeric material.
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[2] Y. A. Vlasov, X-Z. Bo, J. C. Sturm, D. J. Norris, Nature 414, 289-293 (2001)
[3] I. Staude, M. Thiel, S. Essig, C. Wolff, K. Busch, G. von Freymann, M. Wegener, Opt. Lett. 35, 1094-1096 (2010)
[4] A. Ledermann, L. Cademartiri, M. Hermatschweiler, C. Toninelli, G. A. Ozin, D. S. Wiersma, M. Wegener, Nature Materials 5, 942-945 (2006)
[5] M. Florescu, S. Torquato, P. J. Steinhardt, PNAS, vol. 106 no. 49 20658-20663 (2009)
[6] S. F. Liew, J. K. Yang, H. Noh, C. F. Schreck, E. R. Dufresne, C. S. O&’Hern, H. Cao, Phys. Rev. A 84, 063818 (2011)
[7] J. Haberko, F. Scheffold, Opt. Express 21, 1057-1065 (2013)
[8] J. Haberko, N. Muller, F. Scheffold, Phys. Rev. A 88, 043822 (2013)
[9] N. Muller, J. Haberko, C. Marichy, F. Scheffold, Adv. Opt. Mat, vol. 2, iss. 2, 115-119 (2014)

Nature offers a great diversity of well-optimized photonic engineering designs, which often represent a compromise between several potentially conflicting purposes and can maintain such a diverse array of functions as mate attraction, UV protection, water repulsion, camouflage and sensory enhancement. During the last decade, advancements in nanoscience and modern fabrication methods have enabled the detailed investigation and functional mimicry of these natural designs, especially with regards to the structural coloration effects found in biological systems. The physical phenomena responsible for structural coloration have also received considerable attention, as these structures derive their colors from a number of optical effects, such as interference, scattering, photonic crystal effects, or a combination thereof. Photonic crystals in living systems are especially notable for their iridescence and exceptionally bright coloration, which may assist in camouflage, communication, sensing and other purposes that still remain largely unexplored. The imitation of these nanostructures represents significant advances in the area of nano-optics, and promotes the design of novel photonic configurations. However, current fabrication methods cannot satisfactorily mimic the architectural complexity found in natural systems without greatly compromising control capacity, ease of fabrication and/or production costs.
We investigate and successfully imitate a peculiar 2D photonic scheme observed on the neck feathers of mallard drakes. Our bioinspired 2D photonic crystals successfully replicate not only the optical properties, but also the material features and the architectural complexity (i.e. ribbon-like flat platform) of the original structure. A novel top-down approach, called iterative size reduction (ISR) is used to avoid potential issues associated with the fabrication of solid core 2D photonic crystals and to produce a biomimetic design that displays the same structural complexity and functionality as mallard feather barbules with low fabrication costs, short processing times and minimal labor intensity. In addition to its optical properties, the barbule surface is also shown to be strongly hydrophobic, and this property is also successfully replicated in our bio-inspired 2D photonic crystals. The present work represents the first successful fabrication of an all-polymer 2D solid-core photonic crystal capable of functioning at optical frequencies. We further demonstrate that biomimetic fiber arrays displaying a great range of colors can be obtained by minor alterations in a single fabrication step, without necessitating individual process optimization procedures for each desired color.
[1] M. Yaman, T. Khudiyev, M. Bayindir, et al., Nature Materials 10, 494 (2011).
[2] T. Khudiyev, T. Dogan, M. Bayindir, Scientific Reports 4, 4718 (2014).

Metallic nanoantennas allow focusing from the far field to near-field hot spots of heightened electromagnetic field density, enabling applications in surface enhanced spectroscopies, solar light harvesting, and nonlinear nanophotonics. Recently the focus has shifted from all-metallic to hybrid systems, including dielectric, semiconducting, or oxide materials.
This talk will look at the process of far- to near-field focusing from a materials perspective, contrasting metallic and dielectric nanoantennas in terms of focusing performance and optical losses, as well as materials systems for applications in the near- and mid-infrared regime of the spectrum. Examples from surface enhanced spectroscopy, control over nanoscale light emitters, photovoltaics, and higher harmonic generation will be discussed.

Transparent Conductive Oxides (TCOs) are a promising silicon-compatible material platform for a variety of applications ranging from enhanced light sources to energy harvesting. TCOs have attracted significant attention in the last few years as alternative materials for the engineering of metamaterials and plasmon resonances with reduced optical losses compared to noble metals such as gold and silver in the near-infrared spectral range. Recently, it has been theoretically predicted that metamaterials with near-zero permittivity (Epsilon Near Zero, ENZ) exhibit enhanced optical nonlinearities. However, the potential of TCOs, such as Indium Tin Oxide (ITO), as ENZ media still remains to be addressed.
We fabricated ITO thin films featuring the ENZ condition by magnetron sputtering. We show by spectroscopic ellipsometry that the ENZ wavelength can be largely tuned in the range between 1000nm and 2000nm, depending on the post deposition annealing conditions. The optical tunability of the fabricated ENZ materials is directly correlated to the modulation of their measured optical bandgaps E_g, which largely shift by post-deposition annealing due to the increased free carrier density in the material.
We addressed the potential of the fabricated ITO thin films as ENZ nonlinear materials by performing ultrafast second- and third harmonic generation spectroscopy (SHG and THG, respectively) in a wide spectral range of incident wavelength from 1000nm to 1700nm and for different polarizations. We show that both the SHG and the THG spectra exhibit maximum nonlinear generation when pumped at the ENZ wavelength, leading to a significant nonlinear signal enhancement. Our results directly demonstrate that the enhanced electric field achieved at the tailored ENZ condition can be utilized to boost the optical nonlinearities in homogeneous ENZ media. These findings open the way to the engineering of silicon-based active materials operating in the near-infrared spectral range, without the need of specialized sub-wavelength nanofabrication process.

The ability to drive photochemical reactions using sub-bandgap photons is highly sought after in order to increase the usable solar spectrum for energy applications. We propose a novel approach that seeks to initiate photochemical processes utilizing two-photon absorption (2PA) in an integrated photonic device to enable two sub-bandgap photons to excite an electron-hole pair. This approach allows us to utilize sub-band gap photons to drive photochemical reactions that would otherwise require higher photon energy. In this presentation, we report our progress toward this goal. In the first part, we present our fabrication work toward developing an optical microchip from titanium dioxide (TiO₂). TiO₂ is one of the most well-known photocatalysts and its high refractive index (n > 2.5) lends itself small devices and dense integration. These features make it an ideal model material to explore 2PA-induced photochemistry. In the second part, we will discuss our theoretical results that compare geometries, and identify loss and pump power requirements. Lastly, we will present our current experimental results and discuss the potential impact of this approach.

Over the last few years, metallic nanostructures showing resonant behavior in the optical regime have gained a lot of attention. When interacting with light, they offer the possibility of enhancing and concentrating electric fields to a subwavelength volume due to the excitation of localized surface plasmon polaritons. Although these nanoantennas cannot be simply scaled down from the radio wavelength regime to operate at optical frequencies, it is possible to tailor their optical response - as known from their macroscopic counterparts - by an architecture going beyond what is possible with colloidal chemistry approaches, e.g. Yagi-Uda antenna designs. A lot of effort has been invested in the investigation of gold optical antennas.
Here, we report on the two-photon laser excitation and subsequent plasmonic mode relaxation of aluminum resonant optical antennas. We observe plasmon mode relaxation spectra of gap-mediated antenna structures of different geometries, fabricated on a non-conductive glass substrate using electron-beam lithography, electron-beam evaporation of aluminum and a subsequent liftoff process.
The photon emission from single two-arm gap structures as well as from their single-arm counterparts is observed by the use of a raster scanning piezo stage. A single-photon-counting avalanche photodiode detects the plasmon emission intensity while the response spectrum is observed with a spectrometer with an attached electron multiplying CCD camera.
We demonstrate that the underlying multi-photon absorption process is indeed a two-photon process and show that the plasmon spectra are very similar to the linear scattering resonances for the same structures showing a spectral red shift for increasing arm lengths.
In contrast to e.g. gold optical antennas, there is a major difference for the given excitation laser wavelength as we approach the interband transition of aluminum at about 1.5 eV. The nonlinear absorption mechanism yields different radiative relaxation channels due to relaxed optical selection rules. As a result, radiative contributions from formerly forbidden transitions can be brought out leading to a splitting of emission peak positions when comparing linear and nonlinear excitation routines.
The overall spectral blue shift for aluminum optical antennas compared to gold and silver in combination with the large field enhancement in the antenna gap region offers new playgrounds for e.g. UV and deep-UV Raman applications.

Recently, Figotin and Vitebskiy introduced novel non-reciprocal photonic crystals, using alternating layers of magnetic-dielectric materials, to create slow-wave periodic structures in which certain Bloch modes transport energy at near-zero velocity (i.e., slow light). This is possible near a frequency in the interior of a spectral band because the non-reciprocal nature of the layered structure allows one to tune it to create a stationary inflection point of the Bloch-wave dispersion relation relating frequency to wavenumber. This allows significant energy of incident light to be transmitted across an interface between air and a slow-wave medium which is almost completely converted into slow light. Our study concerns an ambient slow-wave layered medium in which a defect layer is embedded. This layer can support frozen-light defect modes. Unlike traditional guided modes of a dielectric slab in air, which decay exponentially into the ambient medium, a frozen-light defect mode does not decay. Yet neither does it radiate energy because it connects perfectly to the zero-energy-flux mode of the ambient medium on each side of the defect. Near a frequency that admits a zero-flux mode, the scattering of incident fields in the ambient space by the defect layer is pathological and defies standard notions of scattering, especially in the presence of a frozen-light defect mode. This is because at such a frequency, the scattering problem becomes undefined as the notion of forward and backward waves degenerates. The degeneration of the scattering problem is reflected in the degenerate eigenvalues and Jordan normal form of the transfer matrix of the non-reciprocal layered medium. The perturbation analysis of the scattering problem in this frozen-light defect-mode regime is discussed where the perturbation parameters are frequency and wavevector parallel to the layers. This is joint work with Stephen P. Shipman (LSU).

A reconfigurable plasmonic nanosystem combines an active plasmonic structure with a regulated physical or chemical control input. Such plasmonic devices hold great promise for applications in adaptable nanophotonic circuitry and optical molecular sensing. There have been considerable efforts on integration of plasmonic nanostructures with active platforms using top-down techniques. The active media include phase-transition materials, graphene, liquid crystals, and carrier modulated semiconductors, which can respond to thermal¹, electrical², and optical stimuli^3-5. However, these plasmonic nanostructures are often restricted to two-dimensional substrates, showing desired optical response only along specific excitation directions. Also, realization of structural reconfigurability in the visible wavelength range remains challenging due to the static nature of top-down techniques. Alternatively, bottom-up techniques offer a new pathway to impart reconfigurability and functionality to passive systems. In particular, DNA has proven to be one of the most versatile and robust building blocks^6-9 for construction of complex three-dimensional architectures with high fidelity^10-14. Here we lay out a multi-disciplinary strategy to create reconfigurable 3D plasmonic metamolecules, which execute DNA-regulated conformational changes on the nanoscale. In one role, DNA serves as construction material to organize plasmonic nanoparticles in 3D. In the other role, DNA is used as fuel for driving the metamolecules to distinct conformational states. Simultaneously, the 3D plasmonic metamolecules can work as optical reporters, which transduce their conformational changes in situ into circular dichroism changes in the visible wavelength range. The experimental results show an overall good agreement with theoretical predictions. We believe that our work will advance the perspective of plasmonics towards synthetic plasmonic machinery for tailored optical response and active functionality.

Fabrication processes of metal nanorod array have been attracted due to the availability of their anisotropic optical properties based on localized surface plasmon resonance (LSPR). Various functional optical devices based on metal nanorod arrays have been proposed, for example, photovoltaic cells, biosensors, nonlinear optical devices, meta-surface, and so forth. Various fabrication processes of metal nanorod arrays have been proposed. However, the process to efficiently fabricate nanorod arrays has yet to be established.
The process based on self-organizing materials is widely applied to fabricate an ordered array of shape-controlled nanostructures. Anodic porous alumina is a typical self-organizing material and obtained by anodizing Al in acidic solutions. Anodic porous alumina has an ordered array of circular nanoholes. One of the advantageous points of using anodic porous alumina for nanofabrication is controllability of the geometrical structures of the nanoholes. By the fabrication process based on the anodic porous alumina, it becomes possible to efficiently fabricate geometrically-controlled nanostructure array [1,2]. To fabricate anisotropic-shaped metal nanostructures using the anodic porous alumina, the shape control of nanoholes in the porous alumina is essential. However, it is impossible to form anisotropic-shaped nanoholes by typical anodization process. In the presentation, it is presented that the formation of the anodic porous alumina with rectangular nanoholes by the anodization process applying the texturing process, and the fabrication of Au nanorod arrays using the anodic porous alumina as an evaporation mask. It is also shown that the application of Au nanorod arrays to the substrate for the surface-enhanced Raman scattering (SERS) measurements. In the results, it was observed that the formation of the anodic porous alumina with rectangular-shaped nanoholes. Aspect ratio of nanoholes could be controlled by adjusting the texturing and anodizing conditions. And, Au nanorod arrays were fabricated by thermal evaporation technique using the porous alumina membrane as an evaporation mask. The shape of Au nanorods was agreed with the shape of nanoholes of the anodic porous alumina. Au nanorod array showed anisotropic LSPR properties originating from its geometrical structures. The SERS spectra of pyridine moleculces adsorbed on Au nanorods were measured. SERS peaks originating from pyridine were observed at 1014 and 1040 cm^-1. It was observed that the SERS activity was dependent on the polarization direction of the incident light. It is expected that the present fabrication process could be used for the fabrication of not only SERS substrates but also functional optical devices requiring the ordered array of metal nanorods.
[1] T. Kondo, H. Masuda, K. Nishio, J. Phys. Chem. C, 117, 2531 (2013).
[2] T. Kondo, K. Nishio, H. Masuda, Appl. Phys. Express, 6, 1024011 (2013).

Within the past a few years, transformation optics has emerged as a new research area, since it provides a general methodology and design tool for manipulating electromagnetic waves in a prescribed manner. Using transformation optics, researchers have demonstrated a host of striking phenomena and devices, among which invisibility cloaks are the benchmark example.
Here, we apply the transformation optics approach to design photonic devices with desired functionalities and high efficiency. First, Photonic Crystal (PC) devices that consist of dielectric rods with varying size are investigated. Based on the TO technique, the original device model is transformed into an equivalent model that consists of uniform and fixed-sized rods, with parameterized permittivity and permeability distributions. Therefore, different from the conventional optimization process, mesh refinement around small rods can be avoided, and PC devices can be simulated much more efficiently. In addition, gradient of the optimization object function is calculated with the Adjoint-Variable Method (AVM), which is very efficient for optimizing devices subject to multiple design variables. We successfully implement a PC waveguide coupler and a PC waveguide bend with almost perfect transmission over a broad bandwidth at telecommunication wavelengh. Comparing with tapered PC coupler that normally have 7sim;10 layers of rods, the proposed PC coupler and bend are very compact, consisting of only 3 layers of dielectric rods along the light propagation direction. Furthermore, we design a nanostructured Luneburg lens that can focus incoming plane wave from any direction into a focal point at the surface of the lens on the opposite direction. Although the Luneburge lens comprises a large number of nanorods, the optimization process is very quick. We compare the performance of the optimized nanostructured Luneburg lens with the original gradient-index Luneburg lens, which show excellent agreement. In summary, our method opens up a new avenue to design and optimize a variety of photonic devices with complex structures for optical computing and information processing.

In this talk, we will present the recent findings in our group on optical metasurfaces and 2D atomic sheets for light processing and devices creation. We will start with the concept of metasurfaces designed from plasmonic and dielectric nanoantennas and exploit how to tailor amplitude, phase, and polarization locally and desirably in subwavelength scale. The idea will be used to achieve flat optical platforms manipulating guiding, networking, radiation and absorption characteristics. We will then extend the concept to 2D sheets and study how integration with organic molecules and gate-voltages can engineer the permittivity in atomic scale and on the surface. This will be a multiscale study linking atomic detail with macro-electromagnetic phenomena. A comprehensive study will be performed and novel characteristics-applications for optics processing on the surfaces will be presented. It will be demonstrated how to obtain a mathematical functionality of interest with metasurfaces. Our modeling is based on a powerful computational scheme that can synthesize large-array metasurfaces and of complex building blocks. This will be also of great benefit to guide fabrication and allow required tolerances in such complex platforms, fabrication by design computation.

Carbon nanotubes and graphene are known to exhibit some exceptional thermal (K~2000 to 4400 W.m^-1K^-1 at 300K) and optical properties. Here, we demonstrate preparation and testing of multiwalled carbon nanotubes and chemically modified graphene-composite spray coatings for use on thermal detectors for high-power lasers. The synthesized nanocomposite material was tested by preparing spray coatings on aluminum test coupons used as a representation of the thermal detector&’s surface. These coatings were then exposed to increasing laser powers and extended exposure times to quantify their damage threshold and optical absorbance. The graphene/carbon nanotube (prepared at varying mass% of graphene in CNTs) coatings demonstrated significantly higher damage threshold values at 2.5 kW laser power (10.6 µm wavelength) than carbon paint or MWCNTs alone. Electron microscopy and Raman spectroscopy of irradiated specimens showed that the composite coating endured high laser-power densities (up to 2 kW.cm^-2) without significant visual damage.