We review the exciting field of optical metamaterials (MMs) and transformation optics (TO) and outline the recent progress in developing tunable, active and loss-free MMs as well as all semiconductor-based MMs and their applications for TO devices. We also discuss a new approach for broadband light bending and negative refraction by using symmetry-breaking meta-interfaces with plasmonic nanoantennas.

A three-dimensional negative index metamaterial designed via transformation optics Ashwin C Atre, Aitzol García-Etxarri, Hadiseh Alaeian, Jennifer A. Dionne With applications including negative refraction, invisibility cloaks, and perfect lensing, metamaterials have gained increasing attention in recent years. Due to their subwavelength constituent structures, metamaterials exhibit artificial electromagnetic properties not possible with naturally occurring materials. While many incredible and unnatural optical functions of metamaterials have been demonstrated both theoretically and experimentally, these unique capabilities are often limited to specific orientations and polarizations of incident light, severely restricting their practical applicability. In order to realize a truly three-dimensional invisibility cloak, for example, a metamaterial is needed which can be excited from any direction. In this presentation, we introduce a new and simple approach for the design of resonator-based metamaterials with optical properties that are insensitive to the orientation and polarization of incident light. Using a previously described conformal transformation, we transform a planar periodic metal-insulator-metal waveguide, known to support both capacitive and inductive modes, into a two dimensional metallodielectric crescent structure that exhibits both electric and magnetic resonances. The practicality of this metamaterial is extended by introducing rotational symmetry to the structure, resulting in a three-dimensional spherical crescent resonator which exhibits similar resonances and thus spectral tunability of both electric permittivity and magnetic permeability. By calculating the phase and amplitude of light transmitted through periodic arrays of 2D and 3D crescents in full-field simulations, we determine that both structures exhibit a negative refractive index over a broad spectral range. In two dimensions, negative index behavior spans wavelength bandwidths of nearly 250 nm, with an index of -1.89 at 975 nm. Negative refraction is confirmed through simulations of a wedge of the two-dimensional crescent-based metamaterial. The three-dimensional crescent exhibits negative index behavior over a 115 nm range, with an index of -1.3 at 805 nm. Both metamaterial structures show an isotropic response over a broad range of incident angles, while the spherical crescent metamaterial provides an additional degree of isotropicity with respect to the polarization of incident light due to the extra degree of symmetry of the meta-atom. Furthermore, the index of the material can be tuned from layer to layer by changing the relative orientations of the crescents within each layer. Our results illustrate the power of transformation optics techniques in designing new metamaterials, and may enable broadband, three-dimensional superlenses or invisibility cloaks at visible frequencies.

The optical response of a plasmonic nanoparticle is significantly affected by its surroundings, such as the dielectric environment and the proximity of secondary nanoparticles. In this work, we present experimental measurements on gold nanoring dimers fabricated via electron beam lithography with a variety of interparticle separations. The particle-to-particle distance of the two nanorings determines the degree of near-field coupling between the particles, which is manifested in a measureable spectral red-shift in the peak extinction energy of the dipole resonance. In addition, a distinct Fano line shape is observed for the dipole resonance peak of the dimer. Rigorous FDTD modeling was performed on the nanoring dimer at various interparticle separations. Both ideal and experimentally realistic nanoring geometries were used in the modeling efforts to fully understand the role of nanoparticle asymmetry in determining ling shape. The Fano response is attributed to a combination of asymmetry in the nanoring shape and the anisotropic dielectric environment of the high index substrate.

Advances in self-assembly have enabled development of nanocrystal superlattices: periodic arrangements of metallic or semiconducting nanoparticles. To date, nanocrystal superlattices mimicking almost every Bravais lattice can be assembled over centimeter-scale areas. Moreover, preliminary studies of the electronic and optical properties of these superlattices reveal properties distinct from the constituent nanocrystals. Accordingly, these self-assembled materials may provide the framework for new, bottom-up plasmonic metamaterials. In this presentation, we investigate the optical properties of nanoparticle superlattices using a generalized rigorous coupled wave analysis. Attention is given to investigating the properties of superlattices composed of gold and/or silicon nanoparticles. Through the concept of homogenization, the effective electromagnetic properties â?" including the real and imaginary permittivity and permeability - of these lattices have been derived. By varying the nanoparticle size, separation, and lattice array, we demonstrate the broad tunability of the superlattice optical properties. For example, a lattice composed of a three-dimensional cubic array of 60-nm-diameter Au nanoparticles with a 2-nm interparticle spacing exhibits an electric permittivity ranging from -3 to 10 and a magnetic permeability ranging from 0.7 to 1.2 at visible frequencies. Replacing half the Au particles with silicon nanoparticles allows the permeability to drop below zero. Further tunability can be achieved by varying the lattice from cubic to rectangular, hexagonal, or more complex arrangements. Our calculations indicate that the retrieved optical parameters are nearly polarization and angle-independent over a broad range of incident angles (0-90 degrees). Accordingly, nanocrystal superlattices behave as isotropic bulk materials. The emergent magnetism of non-magnetic nanoparticles may enable new functional metamaterials at visible-frequencies, with applications ranging from sub-diffraction limited imaging to three-dimensional visible-frequency invisibility.

In recent years terahertz technology has become an optimistic candidate for numerous sensing, imaging, and diagnostic applications. Nevertheless, THz technology still suffers from a deficiency in high-power sources, efficient detectors, and other functional devices ubiquitous in neighboring microwave and infrared frequency bands, such as amplifiers, modulators, and switches. One of the greatest obstacles in this progress is the lack of materials that naturally respond well to THz radiation. The potential of metamaterials for THz applications originates from their resonant electromagnetic response, which significantly enhances their interaction with THz radiation. Thus, metamaterials offer a route towards helping to fill the so-called â?oTHz gapâ?. Here, we present a series of novel THz metamaterials. Importantly, the critical dependence of the resonant response on the supporting substrate and/or the fabricated structure enables the creation of active THz metamaterial devices. We show that the resonant response can be controlled using optical or electrical excitation and thermal tuning, enabling efficient THz devices which will be of importance for advancing numerous real world THz applications.

We report an invisibility carpet cloak device, which is capable of making an object undetectable by visible light. The cloak is designed using quasi conformal mapping and is fabricated in a silicon nitride waveguide on a specially developed nano-porous silicon oxide substrate with a very low refractive index. The spatial index variation is realized by etching holes of various sizes in the nitride layer at deep subwavelength scale creating a local effective medium index. The fabricated device demonstrates wideband invisibility throughout the visible spectrum with low loss in contrast to the previous demonstrations that were limited to infrared or red light. This silicon nitride on low index substrate can also be a general scheme for implementation of transformation optical devices at visible frequencies. We use the concept of transformation optics (TO) to route electromagnetic waves so that the existence of objects does not perturb light propagation. The device cloaks an object hidden under a reflective layer (the carpet) by conformal mapping of the raised protrusion in the carpet (the bump) to a flat plane. The result of the transformation is a spatially variable refractive index profile. We implemented the index profile in a silicon nitride waveguide on a nano-porous silicon oxide substrate by drilling holes of different sizes in the nitride layer to locally form an effective medium with variable composition. The very low index nano-porous substrate (n=1.25) is specially developed to provide a large index contrast with the waveguide, which is crucial for confining the light inside of the waveguide in low index regions of the TO device. Different size holes were formed by electron beam lithography on polymer resist and the pattern was then transferred through the polymer to a silicon oxide etch mask, which was then used to complete the transfer to the nitride layer by reactive ion etching. The back side of the bump was covered with silver using directional electron beam evaporation while the sample was mounted vertically. Gaussian beam output of a Ti-sapphire laser and an optical parametric oscillator was used to couple light with different wavelengths to the TO device, through a dark field objective, and the out-coupled reflection from the bottom of the device was imaged using a CCD. By comparing to control samples, it was verified that the implemented cloak device reconstructs the beam incident on the bump, as if it is reflected off a flat mirror, and as such, successfully cloaks the bump.

Plasmonic metamaterials have attracted wide interest for applications ranging from subwavelength optical components to molecular sensing to photovoltaics. For photovoltaic applications, wavelength-selective response to light combined with robustness under variations in angle of incidence and polarization is of particular interest. We have designed, fabricated, and characterized plasmonic multilayer metasurfaces, with each layer consisting of arrays of subwavelength metallic elements embedded in a dielectric matrix, that provide strongly wavelength-selective reflection and transmission characteristics in the optical regime that are largely invariant under changes in polarization and angle of incidence. Furthermore, with appropriate design, these characteristics can be achieved in multilayer metasurface structures in the presence of misalignment between successive layers and disorder within an individual layer. In single-layer metasurface structures, we show, both experimentally and via simulation, that optical transmission spectra contain a strong transmission minimum (reflection maximum) that is associated primarily with the plasmon resonance of the individual metallic elements within the metasurface array. The wavelength at which this feature occurs can be tailored via selection of different metals, e.g., Au, Ag, or Al, changes in size and shape of each metallic element, and to a lesser degree by changes in array periodicity. Measured and simulated transmission characteristics are found to be independent of the polarization of incident light, and very weakly dependent on angle of incidence within a range between 0 (normal incidence) and 60 degrees. Disorder within the array appears to have minimal effect on this response. Secondary features associated with higher-order plasmonic resonances and coupling to laterally propagating modes are also observed and analyzed. Extension of this concept to multilayer structures allows multiple plasmonic resonance behaviors to be combined, resulting in improved ability to achieve specific wavelength-dependent transmission and reflection characteristics. Transmission minima at wavelengths centered from 500nm to 850nm, with FWHM ranging from 100nm to 250nm, have been demonstrated. Furthermore, for structures in which the separation of individual metasurface layers is greater than the near-field coupling distance for individual metallic elements, optical characteristics are found to be independent of variations in alignment between the layers, and to retain their robustness to variations in angle of incidence from 0 to ~30 degrees. Additional features that arise when the separation between layers in a multilayer metasurface structure is varied, such as hybridizations of resonance states and Fabry-Perot interference have been observed and analyzed. Potential applications of these types of structures for solar energy harvesting and color filtering will also be discussed.

We demonstrate the first optical metamaterial based on coupled plasmonic waveguide arrays which exhibits a three-dimensional negative index of refraction in the blue/UV spectral range. Previous negative index metamaterial designs have utilized subwavelength resonant elements such as split rings to engineer the magnetic response. Due to their resonant properties these structures show poor performance in the visible regime, caused by increased ohmic losses when operating close to the plasma frequency. Theoretically we have shown that a metamaterial comprised of coupled plasmonic waveguides can yield an isotropic negative index [1], with significantly reduced losses when compared to other resonator based geometries. The coupled plasmonic waveguide arrays are comprised of a double periodic unit cell, which improves the incoupling efficiency of incident light and is crucial for facilitating omnidirectional negative refraction. In order to probe the isotropy of the index, we investigate two types of structures. The first structure consists of metal-insulator-metal waveguides embedded within a Si3N4 membrane fabricated by focussed ion beam (FIB) milling followed by infiltration with silver. In this geometry, the waveguides are oriented perpendicular to the membrane interface, and propagation of light is governed by coupled surface plasmon polaritons, which have a negative mode index. Optical refraction experiments were performed on miniature Ag/Si3N4 metamaterial prisms sculpted in this material and demonstrate a negative index equal to n=-1.0 at wavelengths as short as 360 nm. Using UV interferometry, we find that light experiences a -130 degree phase retardation across our metamaterial, further verifying the negative sign of the index. For the second structure, electron beam physical vapour deposition was used to fabricate a multilayer stack with a unit cell: 50 nm Ag, 44 nm TiO2, 32 nm Ag and 44 nm TiO2. This technique allows a conformal deposition over a large area, whilst maintaining accurate control over thickness. We characterize the optical constants and thicknesses of individual layers using variable angle spectroscopic ellipsometry. Light propagating normal to the interfaces of the layers can be described as a series of Bloch harmonics, of which the fundamental mode is characterized by a negative index. Refraction simulations of a prism show that higher order harmonics are also present in the angular distribution of the transmitted light. Miniature prisms were sculpted in this layered geometry using FIB milling, and the refraction angle of the transmitted light was determined by imaging the back focal plane of the collection objective onto a CCD camera. With knowledge of the refraction and prism angles, we retrieve the effective index of the multilayer stack, and compare this with analytic calculations and numerical simulations. [1] E. Verhagen, R. de Waele, L. Kuipers and A. Polman, Phys. Rev. Lett. 105, 223901 (2010)

Localized surface plasmons (LSPs) supported by metal nanoparticles (NPs) can concentrate optical fields into subwavelength volumes. Locally enhanced optical fields are important for many applications, ranging from surface enhanced Raman spectroscopy to plasmonic nanolasers. To amplify the localized optical fields in a plasmonic system, the most important factor is to overcome radiative loss and to slow down the depletion of plasmon energy. This work reports a new type of subradiant out-of-plane lattice plasmon (OLP) resonance that is only found in 2D arrays of large (> 100 nm, all three dimensions) nanoparticles (NPs). Unlike either isolated NPs or thin or small NP arrays, arrays of large NPs support out-of-plane dipolar interactions between NPs that can be so strong that radiative damping of plasmon oscillations is significantly suppressed. At the resonant wavelength, the incident light is trapped in the plane of the NP array, and the accumulated plasmon oscillation energy results in strong (near-field) nano-localized optical fields at the surface of each NPs in the array. Our work has also clarified the dispersion effects on both far-field and near-field optical properties of OLPs. We experimentally demonstrate that the dispersion relation of OLPs can be tailored simply by controlling the NP height. Because of their unusual spectral tunability, narrow resonance linewidths, and high local optical field enhancements, delocalized lattice plasmons are promising for applications ranging from ultra-sensitive plasmon sensors to nonlinear nano-optical devices.

In this talk, I will present some of our most recent results in the field of metatronics, which we have been developing in recent years [N. Engheta, Physics World, 23(9), 341 (2010); N. Engheta, Science, 317, 1698-1702 (2007).] I will discuss some of our ideas on the tunable and active elements involving nonlinear materials, providing metatronic elements with non-Foster functionalities. In such structures the input signal with frequency omega can be mixed with the input pump with frequency of 2*omega, leading to the variation of the effective linear permittivity. With the proper levels of nonlinearity and pump intensity, one can in principle change and tune the real and imaginary parts of the permittivity of such structures. We will discuss the conditions under which such tunability would be possible. Moreover, we will discuss some of our ideas on tunable metatronic elements involving the phase-change materials, in which a 3-port element in the paradigm of metatronic may become a possibility. Finally, if time permits, I will discuss some of our work on non-metallic metatronics, in which high-dielectric elements may behave as epsilon-negative structures, opening up the possibility for all-dielectric metatronic circuitry

Surface plasmon polaritons (SPP) can confine optical fields beyond the diffraction limit, which makes them very attractive for miniaturized optical devices. Several passive plasmonic systems have been successfully studied in the last decade, but the implementation of active configurations remains still a challenge. Among the different control agents that manipulate the SPPs, the magnetic field is a strong candidate since it is able to directly modify the dispersion relation of SPPs with a high switching speed. This modification lies on the non-diagonal elements of the dielectric tensor, ε_ij. For noble metals, the standard plasmonic ones, these elements are unfortunately very small at reasonable field values. Ferromagnetic metals, in contrast, have large ε_ij values but are optically too absorbent. Thereby, a smart system would be multilayers of noble and ferromagnetic metals. Magnetoplasmonic modulation of SPP wavevector in these hybrid Au/Co/Au multilayer films has been recently demonstrated. These magneto-plasmonic modulators are based on plasmonic micro-interferometers formed by a tilted slit-groove pair. In those systems, interference between incident light and light decoupled from the SPP is measured. When an external oscillating magnetic field is applied, both real and imaginary parts of the SPP wavevector are modified therefore changing the interference intensity synchronously with the applied magnetic field. This intensity modulation has therefore two contributions: a Ï?/2 phase-shifted term proportional to the product Î"k_rÃ-d, where Î"k_r is the real part of the SPP wavevector modification induced by the magnetic field (Î"k) and d the groove-slit distance; and an in-phase term proportional to Î"k_iÃ-d, with Î"k_i the imaginary part of Î"k. This double contribution is interesting as it opens the possibility of designing modulators based on optical path interference or SPP attenuation, depending on the specific weight of each one of them. Here we will present a detailed study, both theoretical and experimental, of the spectral dependence of these two parameters, Î"k_r and Î"k_i. From our analysis, we have obtained a simplified relation where the system plasmonic and MO properties are separated, which can be useful to optimize the magnetoplasmonic interferometers performance. For Au/Co/Au multilayers, the plasmonic properties are the dominant ones. Moreover, from this simplified relation a straightforward approach to increase the modulation can be inferred: to cover the metallic multilayer with a dielectric media of higher permittivity ε_d. By covering our interferometers with a thin layer of PMMA (ε_d=2.22) a fourfold enhancement of Î"k_r has been achieved. As the SPP propagation distance decreases with the use of dielectrics of higher ε_d , we have defined a relevant figure of merit in this system, the product Î"kÃ-L_SP. Our theoretical results show that an optimized thickness of the polymer overlayer can almost double this product.

The dynamics of two-level systems have attracted the attention of researchers from the most diverse fields ranging from chemistry to photonics. Perhaps one of the simplest two-level systems is a hydrogen molecule, with two identical atoms presenting the two characteristic energy levels. In spite of the simplicity of two-level systems they have become extremely important in quantum optics, nuclear magnetic resonance and quantum computation. Hybrid plasmonic waveguides have been previously introduced as an efficient way to achieve subwavelength light confinement and management and they are the base of deep-subwavelength plasmon lasers. In this work we analyze the eigenmode dynamics in a system where a pure optical microcavity is coupled to a hybrid plasmonic microcavity. We observe a strong coupling behavior between the eigenmodes that leads to a periodic excitation of the plasmonic hybrid mode in analogy to a plasmonic pulsar. Optical microcavities in Si-photonic waveguides are the equivalent of a Fabry-Perot resonator, where the mirrors are replaced by distributed Bragg reflectors (DBRs). In this case, the DRBs are gratings consisting of a series of periodic grooves on top of the waveguide. The DBRs lead to a characteristic photonic bandgap determined by the shape, period and depth of the gratings. Whenever the separation of two DBRs equals an integer number of effective semi-wavelengths, a photonic confined mode will appear in the minigap. The proposed structure in this work is based on a standard Si waveguide on a Si-oxide substrate. Two concatenated cavities are created in the structure using three DBRs separated by two spacers (similar to a photonic molecule). To create a hybrid plasmonic cavity, we added a thin Si-oxide+Ag bilayer on top of one of the cavity spacers. This structure presents two coupled modes for which the electric field is mainly concentrated in the thin Si-Oxide film underneath the metal as well as in the pure optical cavity spacer of the Si waveguide. The separation between the modes is determined by the central DBR while the FWHM is controlled by changing the number of periods forming the two external DBRs. In this coupled hybrid-plasmonic/photonic microcavity system, the energy associated to each of the eigenmodes is distributed between a hybrid plasmonic cavity and a pure photonic resonator. When these modes are excited, the energy oscillates between the two cavities with a period determined by the splitting of the modes; particularly, the energy in the plasmonic cavity will oscillate with the same period. We introduce the concept of a plasmonic pulsar and analyze the engineering parameters of this structure. The design of the DBRs, cavity spacers, and materials lead to highly optimized plasmonic structures where the plasmon dynamics can be manipulated. The confinement characteristics could provide the platform to perform enhanced Raman scattering and nonlinear experiments in Silicon-compatible waveguides.

Plasmonics offers unique possibilities to confine and direct light at the nanoscale, with broad applications anticipated in sensing, imaging, and information technology. After many years of toolbox development in research programs worldwide, the timing has never been better for the commercialization of innovative products based on plasmonic engineering. Integrated Plasmonics Corporation was formed in early 2011 to create a new biochemical sensing platform based on the integration of plasmonic nanostructures with CMOS imaging sensors. We will discuss the progress of our proof-of-concept technology development program, which has largely focused on the modification of low-cost commercially-available CMOS sensors, typically used in surveillance camera applications, with nanostructured gold films patterned using focused ion beam methods. Our technique allows us to simultaneously study the transmission properties of several hundred different patterns of nanohole arrays, efficiently reproducing in a single experiment a large body of past work in extraordinary optical transmission. We have observed and classified a variety of interesting features related to the interaction of the nanostructured gold film and the underlying front-illuminated CMOS sensor. We will also discuss studies of subwavelength nanoscale resonators and a systematic evaluation of misalignment effects in coupling such resonators to CMOS pixels.

As plasmonics enters its second decade, new phenomena are continually being discovered. In this talk I will describe plasmonic broadband absorbers and their use in solar and thermoelectric energy conversion. I will also discuss the plasmoelectric effect, a plasmonic analog to the thermoelectric effect. Further, I will discuss new opportunities in quantum plasmonics in which quantum measurement techniques can be used to investigate plasmon creation and annihilation.

The strong optical field confinement in optical micro-/nano-cavities leads to many fundamental studies and exciting applications in nanophotonics. The sizes of these conventional dielectric cavities cannot be smaller than wavelength scale for efficient photon confinement, which is limited by the low refractive indices of nature materials. We have investigated theoretically indefinite medium with hyperbolic dispersion can be used to miniaturize optical cavities due to the supported unbounded large wave vectors. [1] Here, by incorporating multilayer indefinite metamaterials, we experimentally demonstrate deep-subwavelength optical cavities with sizes down to ~λ/12. [2] The metamaterial structure consists of alternating thin layers of silver (Ag) and germanium (Ge). These cavities are highly anisotropic, resulting in a hyperbolic iso-frequency contour (IFC). The hyperbolic dispersion permits the access to wave vectors much larger than that in air and the tremendous momentum mismatch between the metamaterial and the air causes the total internal reflection at the interface. By cutting the metamaterial into a nanoscale cube, 3D optical Fabry-Pérot cavity can be formed with a high effective refractive index equals to k/k_0. Since arbitrary large wave vectors can be reached along the unbound IFC hyperbolic curve, the cavity size can be arbitrarily small. Multilayered metamaterial with 20nm Ag and 30nm Ge is fabricated in experiments Cavities with different sizes are aligned at the identical resonant frequency of 191 THz for the (1, 1, 1) modes, which exemplifies one of the unique features of indefinite cavities. For the same cavity, the (1, 1, 2) mode has a lower frequency compared to the (1, 1, 1) mode, showing the anomalous mode dispersion. Also, the transmission depth is different for each cavity mode, indicating the coupling between the excitation plane wave and the cavity mode strongly depends on the cavity size and the mode order, which is related to the resonating wave vector supported inside the cavity. The retrieved radiation quality factor extracted from the measured transmission spectra, based on the coupled mode theory, shows vertical and total radiation quality factor is proportional to the fourth power of effective index. Also, the indefinite metamaterial cavities allows unnaturally high effective refractive index and a maximum value of 17.4 is demonstrated for the (1, 1, 2) modes. The demonstrated nanoscale metamaterial optical cavities significantly differ from the conventional dielectric cavities, showing anomalous mode dispersion and unique universal fourth power law between the radiation quality factor and the resonating wave vector. This new type of optical cavities will greatly extend the ability to manipulate light at deep-subwavelength scale for potential applications including light-matter interactions. References: [1] Yao, J. et al. PNAS 108, 11327 (2011). [2] Yang, X.,* Yao, J.,* Rho, J.* et al. Submitted to Science (2011)

A continuous flow of hot electrons that are not at thermal equilibrium with the surrounding metal atoms is generated by the absorption of photons. Here we show that hot electron flow generated on a gold thin film by photon absorption (or internal photoemission) is amplified by localized surface plasmon resonance. This was achieved by direct measurement of photocurrent on a chemically modified gold thin film of metal-semiconductor (TiO2) Schottky diodes. Photons coupled into the modified gold thin film excite surface plasmon resonance, which enhances hot electron flows going over Schottky barrier between the gold film and TiO2. The short-circuit photocurrent obtained with low-energy photons is consistent with Fowlerâ?Ts law, confirming the presence of hot electron flows. The morphology of the metal thin film was modified to a connected gold island structure after heating such that it exhibits surface plasmon. Photocurrent and optical measurements on the connected island structures revealed the presence of a localized surface plasmon at 550 ± 20 nm. The results indicate an intrinsic correlation between the hot electron flows generated by internal photoemission and localized surface plasmon resonance. We discuss the effect of dye molecules adsorbed on gold film in the efficiency of internal photoemission.

Plasmonics promises ultra-fast and ultra-compact components for the next-generation integrated optical circuit, since it simultaneously combines the fast dynamics of photonic processes and the capability of achieving tight optical confinement beyond the diffraction limit. To achieve an on-chip plasmonic footprint, it is crucial to scale down individual building blocks, including the plasmon light source, into nanometer scale. Here we demonstrate a highly compact, efficient plasmonic source that can generate unidirectional surface plasmon polaritons (SPPs). This novel device consists of two subwavelength magnetic resonators with detuned resonant frequencies. At the magnetic resonance frequency, an incident optical wave can be efficiently channeled into SPP mode. By tailoring the relative resonance phase and the separation between two resonators, we can steer SPPs to propagate predominantly along one direction owing to the constructive and destructive interference of SPPs. The dimension of the device is smaller than the wavelength in all three dimensions. Furthermore, due to the strong resonance nature, the generation efficiency of SPPs is as high as 125%, which is at least three times larger than the single slit case and six times larger than the single aperture case. We employ leakage radiation microscopy and conoscopic microscopy on single antennas to experimentally demonstrate the unidirectional excitation of SPPs at the air-gold interface. The experimental results are in good agreement with the full-wave finite element simulation and magnetic dipole approximation. Similar to optical antennas in regards to free-space propagating waves, our nanostructure paves a new way to manipulate near field optical waves, which can be used as an efficient nanoscale plasmonic directional antenna. Such antennas may also be useful for nonlinear applications, active modulation and wireless optical interconnects based on surface plasmons.

Recent advances in nanofabrication, electron microscopy imaging and near field microscopy allowed metallic structures to be defined and characterized with sizes of several nanometers and opened up the possibility of engineering optical antennas. Nanoantennas enable optimum conversion of propagating light into sub-wavelength localized optical fields by resonant oscillations of free electrons in metallic structure. Since nanoantennas provides strong near field enhancement at specific wavelength, they are used for many technological applications that include single molecule spectroscopy, optical trapping, tumor therapy, detection, manipulation and generation of light. Optical nonlinear generation, the creation of new frequency components from narrow band and high energy input pulses is required for all optical signal processing and spectroscopic optical applications. Initiating nonlinear generation with low threshold is critical issue for device applications. Thus, optically nonlinear materials within the nanoantenna near field can be proposed as an efficient solution for generation of new frequency components with low threshold. In this study, stripe gold nanoantenna on As₂Se₃ substrate structure is simulated with finite difference time domain (FDTD) technique and nanoantennas are produced using anodized aluminum oxide (AAO) templates. Chalcogenide glasses such as As₂Se₃ are attractive materials for optical nonlinear applications due to their high nonlinear refractive index; it is about three orders of magnitude greater than silica. By simulations, sizes of gold nanoantennas are optimized and nonlinear response of the structure is investigated when it is excited with 150 fs 1550 nm laser pulse. Simulation results show that metallic antenna enhances incident light about 5000 times around the sharp corners because of localized surface plasmons and reduces threshold intensity value for nonlinear generation. Moreover, 500 nm FWHM spectral broadening around the incident wavelength and third harmonic generation is achievable with proposed plasmonic antenna and chalcogenide glass structure. Since the simulation results are so promising we fabricate the structure by using AAO membranes. AAO membranes provide us fabrication of rod shaped gold nanoantennas with desired lengths and diameters. As a future work, this fabricated nanoantenna on As₂Se₃ substrate structure will be used in experimental setup for realization of simulation results.

ABSTRACT TO BE DETERMINED

Here we present the results of a study demonstrating that plasmonic resonances in metallic nanostructures can cooperate with multilayer interference effects to produce wavelength-tunable absorption enhancements in a semiconductor photoelectrode structure, and that these effects can be engineered to enhance the efficiency of solar fuel generation [1]. We chose iron oxide (α-Fe2O3; hematite) as a prototype system that shares many features with other candidate materials for future large-scale solar fuel production, and therefore anticipate that the results obtained in this study will be applicable to other materials systems as well. Hematite has relatively weak absorption in the 500-600 nm range (0.1 â?" 1 μm absorption length), very long compared to its minority carrier diffusion length on the order of 2-4 nm or 20 nm. To enhance the efficiency of solar fuel generation, we have designed thin film sunlight absorbing structures with embedded metal nanoparticles to guide and concentrate sunlight close to an absorber/water interface, such that the generated photocurrent can be used more efficiently to split water into hydrogen and oxygen. Metal nanoparticles have long been recognized to influence and sometimes even to intensify chemical reactions. However the disentanglement of different effects leading to the intensification of chemical reactions when metal nanoparticles are embedded into sunlight-absorbing structures has been scientifically much more challenging. Here, we have devised a strategy to establish that plasmonic effects indeed are responsible for the observed photocurrent enhancements. Specifically, we have provided a detailed comparison of the spectral features in the measured photocurrent to full field electromagnetic simulations for gold nanoparticle/ hematite model systems. We then investigated silica-coated gold spheres/ hematite systems, and we could show an increase of the total wavelength-integrated photocurrent by 10% in an operating photoelectrochemical cell. We have provided further insight by showing that it is possible to tune the energy of maximum absorption enhancement by exploiting the interplay between plasmon resonances and Fabry-Perot resonances. These results open the door to the optimization of a wide variety of photochemical processes by leveraging the rapid advances in the field of plasmonics. [1] I. Thomann et al., â?oPlasmon Enhanced Solar-to-Fuel Energy Conversionâ?, Nano Letters 11, 3440â?"3446, (2011).

We examine how light absorption in a plasmonic grating nanosurface can be calculated by means of a simple, analytical model based on a transmission line equivalent circuit. The nanosurface is a one-dimensional grating in silver metal film covered by a silicon slab. The transmission line model is specified for both transverse electric and transverse magnetic polarizations of the incident light, and it incorporates the effect of the plasmonic modes diffracted by the ridges of the grating. Under the assumption that the adjacent ridges are not strongly coupled, we show that the approximate, closed form expression for the reflection coefficient at the air-silicon interface can be used to evaluate light absorption of the solar cell. We will also show the utility of the circuit theory for understanding how the peaks in the absorption coefficient are related to the resonances of the equivalent transmission model as well as how this can help in designing more efficient structures.

Surface plasmon resonance (SPR) phonemenon has been attracting tremendous attention in nano- and bio-technological fields including photovoltaics, photocatalysis, controlled light emission, optical sensing, plasmonic waveguides, biomedical engineering, etc. It has been widely applied to observe the surface chemistry due to the fact that it is extremely sensitive to the changes in the refractive index of the dielectric medium close to metal surface. Thus, it is recognized that SPR-based sensing is considered as one of the most powerful and efficient platforms for label-free biomolecular detection. Here, we propose an extremely simple SPR coupling-based setup via incorporation of metal NPs in a DNA sensing assay without any sophisticate and complicated design. In brief, biotin-streptavidin system was employed to immobilize probe DNA on basal Au substrates and three different types of models were proposed employing Ag colloids or AuNPs with different size in different configurations. The sensitivity enhancement factor was compared experimentally using a Kretschmann configuration type SPR spectrometer. Next, we discuss about the development of a surface plasmon induced visible light active photocatalyst system composed of silica-titania core-shell (SiO2@TiO2) nanostructures decorated with Au nanoparticles (Au NPs). TiO2 has been considered as one of the apt materials for the production of clean energy resources and environmental remediation. Nevertheless, the large band gap of 3.2 eV limits it use under UV light, which constitutes only 3% of the total solar spectrum. Absorption of Au nanostructures in the visible region can also been exploited to extend photocatalytic activity of TiO2 to visible region by utilizing their plasmonic properties. We report herein the SPR induced visible light photocatalysis of TiO2/Au nanostructures with tailored configurations and desired properties for the degradation of organic pollutants. Core@shell nanoparticles have demonstrated distinctly different properties and potential uses in electronics, magnetics, catalysts, optics, and sensors. They have also been utilized as a platform for the extensive study for integration of functionalities into both the core and shell. We report the fabrication of P4VP decorated AuNRs as pH-responsive nanocomposite materials by SI-ATRP on a Au surface. The obtained AuNR@PVP nanocomposites are pH responsive at a critical point of pH 3.2 revealed by SPR changes of AuNPs. Such environmentally responsive nanocomposites with effective coordinating pyridyl segments provide a smart supporter or carrier to transition metal ions and NPs to construct novel bimetallic nanocomposites, especially in catalyst applications. At the latter part of this presentation, I will discuss about plasmonic-coupling induced highly selective sensing for anions/cations using tagged AuNPs.

We analyze a new strategy for generation of DC electrical power from resonant optical absorption in plasmonic nanostructures. Power conversion results from the dependence of the plasmon resonance frequency on electron density. Electrically connecting two nanostructures with resonant absorption maxima at distinct frequencies, and irradiating both structures with an intermediate frequency, induces electron transport from the high frequency plasmonic resonator to the low frequency resonator. This phenomenon, termed the â?~plasmoelectric effectâ?T, is entropically driven by the increase of the total absorbed radiation, which is characteristic of the shifted plasmon resonance induced by the new charge density configuration. Our proposed geometry requires no thermal gradients or semiconductor components, distinguishing it from all previously described thermoelectric generators, rectennas, photovoltaics, or â?~hot electronâ?T optical energy convertors. Using Mie theory and a modified complex dielectric function for silver that accounts for changes in electron density, we construct a detailed analysis of the plasmoelectric effect for electrically coupled spherical silver nanoparticles. We predict a 12.7% single frequency power conversion efficiency for a device consisting of 10 nm radius silver nanoparticles, irradiated with 379 nm light at a power density of 1 kW/m^2 under ambient conditions. This corresponds to an open circuit voltage of 130 mV, and a spectral shift of the absorption maximum by 3.5 nm. We extend this analysis to device geometries optimized for broadband absorption across the AM1.5G solar spectrum, and discuss strategies for further efficiency gains that take advantage of the remarkable spectral tailorability of plasmonic structures. Our results indicate the plasmoelectric effect is a powerful tool for the conversion of solar energy as well as an entirely new class of optoelectronic devices.

Controlling light-matter interactions on a subwavelength scale is a pivotal focus of nanophotonics. Advances in this field have enabled powerful applications including near-field microscopy, surface enhanced spectroscopy, biosensing, and solar energy harvesting. To date, most research efforts have focused on engineering the interaction of matter with the electric field of light. It has recently been discovered, however, that closely packed metal nanoparticle assemblies can support both electric and magnetic modes at optical frequencies. Establishing methods to engineer the optical magnetic modes and their interplay with electric modes will be crucial for advanced nanophotonic applications. In this work, we utilize fully rigorous generalized Mie theory calculations to explore the near and far field properties of silver nanoparticle trimers. We theoretically demonstrate Fano-like interference effects between the fields radiated by the electric and magnetic modes of symmetric nanoparticle trimers (radius = 30 nm, interparticle spacing = 2 nm). Breaking the symmetry of the trimer system leads to a strong interaction between the modes, which can be readily understood in terms of group theoretical arguments. The near and far field electromagnetic properties of the broken symmetry trimer are tunable across a large spectral range. We exploit this Fano-like effect to demonstrate spatial and temporal control of the localized electromagnetic hotspots in the plasmonic trimer. Displacing one of the spheres in the symmetric nanoparticle trimer by 1 nm results in a 30-fold increase in the near field enhancement and changes the relative phase by Ï?/8 at the junction between the other two spheres. Our experimental efforts towards biologically templated assembly of nanoparticle clusters and their characterization with polarization-selective single particle spectroscopy will also be discussed.

Lithographic photopatterning to fabricate 2D and 3D structures is typically a static process in the sense that a solid block of photoresist is exposed to spatially varying light intensity that is constant in time. The ability to grow inorganic materials whose morphologies respond dynamically to changing illumination conditions can enable the development of complex architectures with tailored optical responses. We have developed a method for the light-mediated formation of nanoscale lamellar patterns during the electrodeposition of photoresponsive selenium-tellurium (Se-Te) alloys. While no pattern is observed for Se-Te films prepared in the dark, those deposited under illumination display a lamellar structure that is continuous over the entire growth substrate. The angle and polarization of the incident light determine the growth direction of the lamellae and their orientation on the substrate, respectively. The illumination wavelength controls the lamellar size and pitch, where the periodicity can be varied continuously from ~150 nm for ultraviolet illumination to ~350 nm for near-infrared light. Because the patterns are dynamically responsive, structural complexity can be built into the films by changing the illumination conditions during the electrodeposition process. For instance, branching is induced in the lamellae by switching the illumination wavelength. We attribute the light-induced pattern formation to interference between scattered fields at the surface of the Se-Te film, which produce a periodic modulation in light intensity. Full-wave simulations of the interference patterns formed by varying illumination wavelengths correspond well with the observed lamellar periodicities. The mechanism by which the local light intensity affects the growth of the SeTe alloy is a subject of further investigation. This light-mediated growth technique opens up the possibility for optimizing the absorption and transmission characteristics of a material by using light in a feedback loop to direct the filmâ?Ts structure and morphology.

With recent advancements in nanoscale fabrication, significant effort has been focused on developing large-area plasmonic optical sensors such as those for surface-enhanced Raman scattering (SERS). Most of these efforts have focused upon making periodic or aperiodic arrays of structures via electron beam lithography or nanoimprint lithography and then coating via traditional methods such as electron beam evaporation or sputtering of silver or gold. These efforts have led to large-area, highly reproducible and uniform enhancements, which for SERS have surpassed 1x10⁸ with typical variations on the order of 15-30% across a large-area array. However, when plasmonic nanoparticles are placed in close proximity to one another (<20 nm) the plasmonic field intensities between the nanoparticles are increased dramatically due to interparticle near-field plasmonic couping, which are the cause of SERS 'hot-spots'. Additionally, a red-shift in the surface plasmon resonance (SPR) condition is observed. It has long been anticipated that if large-area structures of coupled plasmonic nanoparticles could be achieved that the most sensitive detectors could be created. This implies that a very dense network of nanoparticles be created, with each nearest neighbor being spaced at less than 20 nm gap from its neighbor following metal deposition, something that is difficult for most nanolithography approaches and extremely difficult for most standard metal coating techniques. However, the recent development of atomic layer deposition (ALD) of silver has now enabled the fabrication of such structures. Using Ag ALD, we have demonstrated that large area arrays (20-60 um on a side) of Ag-coated, Si nanopillars with controllable, reproducible and uniform interpillar gaps as small as 2 nm can be achieved using Si nanopillar templates with interpillar gaps (50-100 nm) that are attained using standard ebeam lithography techniques. Using a combinatorial approach, a complete diameter and gap dependence, with the post-metal diameters ranging from 196-397 nm and the interpillar gaps ranging from 2-199 nm was fabricated. SERS measurements were carried out on a self-assembled monolayer of thiophenol to ensure an accurate accounting of the number of molecules could be ascertained. Enhancement factors in the 10⁷-10⁸ range were observed for the best responding structures with interpillar gaps >20 nm. In all cases, as the interpillar gap was reduced, a slight decrease in SERS intensity was observed, until the gap was reduced below 20 nm, where the onset of plasmonic coupling is anticipated. At this point, over an order of magnitude increase in SERS intensity was observed over the uncoupled arrays. It should be noted that in the case of the uncoupled arrays, this translated to 1 sec acquisition times, while acquisition times of 0.1-0.5 sec, depending on incident wavelength, were required for the coupled arrays to ensure saturation of the detector did not occur.

Nanoscience has supplied us with a wealth of new strategies to construct high performance devices such as solar cells, thermoelectrics, sensors, transistors, and transparent electrodes via a bottom-up synthetic approach. This fabrication route offers the benefits of large-scale chemical synthesis, high throughput and circumvents the need for etching processes that waste material and create surface defects. One of its greatest challenges is finding suitable assembly and contacting procedures that allow for complex device fabrication without costly patterning steps. Here, we demonstrate a light-induced plasmonic nanowelding technique to assemble metallic nanowires into complex interconnected networks. The small gaps that form naturally at nanowire junctions cause local light focusing and heating predominantly at the point where the wires need to be joined together. The extreme sensitivity of the heating efficiency on the junction geometry causes the welding process to self-limit when a connection between the wires is made. At each junction point where nanowelding occurs, the bottom nanowire recrystallizes epitaxially onto the top nanowire, consistent with the simulated heat generation profile. This talk will use results from full-field simulations, electron microscopy, optical scattering and single junction resistance measurements performed before and after optical illumination to understand this self-limited plasmonic nanowelding process. Large-area nanowire mesh networks have also been fabricated and tested as transparent electrodes, connecting the microscopic to macroscopic properties and demonstrating one potential application. Moving beyond transparent electrodes to complete solar cells, the ability to control light and heat at the nanoscale becomes even more important. We have already demonstrated that these metal nanowire transparent electrodes can be integrated into organic solar cells and have used laser beam induced current (LBIC) measurements to map out their local effects. By optimizing the optical and electrical interactions, we hope to use these metal nanowire transparent electrodes as both efficient current collectors and active light focusing elements to enhance solar conversion efficiency.

Optical imaging and manufacturing at deep sub-wavelength scale promises vastly new applications in nano-electronics, metrologies, and single-molecule biology. The diffractive nature of the light, however set a fundamental limit in optical resolving power. Approaches including near-field microscopy and plasmonic lens (PL) have been developed to circumvent the diffraction limits by accessing the electromagnetic field at near-field, but confronted the formidable limitations due to the momentum mismatch induced trade-off between resolution and energy coupling efficiency. Through adiabatic transformation, converting the propagating surface plasmon (PSP) modes efficiently into localized surface plasmon (LSP) has been theoretically proposed. Here we report a new high-throughput optical focusing scheme based on a compound multi-stage plasmonic lens (MPL) that is capable of compressing the optical energy to deep sub-wavelength scale with high efficiency through progressive coupling of both PSPs and LSPs. Combined with a pulsed laser source and an advanced airbearing surface (ABS) technology, the MPL provides an unprecedented spatial resolution with extremely high speed writing at the order of 10 m/sec. As a result, 22nm half-pitch features are successfully recorded on an inorganic resist, which corresponds to 1/16 of the wavelength.

Optical antennas have successfully been used to modify the emission from optically excited quantum emitters by enhancing the local density of optical states via the Purcell effect. To this end, a wide variety of nanometallic antennas have been implemented to enhance and control key emission properties such as the decay rate, directionality, and polarization state. Optical antennas have also been employed in receiving mode to concentrate light in deep subwavelength volumes and to enhance light absorption in semiconductor nanomaterials. Here, we build on this work to realize antenna-electrodes which simultaneously function as electrodes for electrical field and current manipulation as well optical antennas for light manipulation in nanoscale LEDs and solar energy harvesting applications.

Optical engineering of organic solar cells (OSCs) utilizing localized surface plasmon polaritons (LSPPs) can effectively enhance optical absorption by OSCsâ?T active layers. LSPP, collective electron oscillation excited by light in metal nanoparticles, both widely scatter the incident light and concentrate the electromagnetic fields near the nanoparticleâ?Ts surface. Furthermore, metal nanoparticlesâ?T optical properties can be tuned by adjusting their size and shape. While many studies have found that the power conversion efficiency of OSCs is improved by inserting metal nanoparticles into the OSCs, little has been reported on the use of size- and shape-controlled metal nanoparticles for fine-tuning the optical properties. Here, we investigate the dependence of silver nanoparticlesâ?T size and shape, at various concentrations, on the power conversion efficiency of OSCs. We incorporate silver nanoparticles of various sizes (10nm-70nm) into the anodic buffer layer, PEDOT:PSS, to induce light scattering in the PCDTBT:PC70BM bulk heterojunction active materials. The power conversion efficiency of OSCs with optimal concentration of silver nanoparticles embedded tends to increase as the size of the silver nanoparticles increases up to 50nm because the scattering efficiency increases as the size of the silver nanoparticle increases [1]. The maximum short circuit current enhancement is achieved with nanoparticles at the size of 50nm, and the power conversion efficiency is improved by up to 15.1%, from 5.18% to 5.99%. When the nanoparticles are larger than 50nm, the enhancement of short-circuit current and fill factor are shown to be limited due to possible increase in the reflection by the nanoparticles and carrier recombination on the surface of the nanoparticles penetrating into the active layers. We will also present the shape effects of light scattering by incorporating silver nano-prism in the OSCs. [1] J. -Y. Lee, P. Peumans, Optics Express, 18, 10078 (2010)

There is a significant scientific and technological interest in zero-reflectivity substrates for enhanced light harvesting in applications ranging from photovoltaics[1] to enhanced photocatalysis to photocathodes for the next generation light sources[2]. A scheme for the complete absorption of light using a subwavelength nano-grooves (NGs) on a metallic surface has been proposed by Le Perchec et. al.[3]. The dimensions of these grooves are very small compared to the wavelength of light. For example, for a gold structure designed to trap 720 nm light, the NGs would be 14 nm wide by 45 nm tall. We have confirmed the light trapping effect of such structures in a recent experimental demonstration[4,5]. For many applications it is desirable to operate at wavelengths deeper into the NIR compatible with high power lasers such as Ti:sapphire (800 nm) and Ytterbium-based fiber lasers (1064 nm). A structure resonant at such wavelengths would have NGs less than 8 nm wide, which is a major fabrication challenge. In this work we present a new method for tuning the resonance of a subwavelength metallic grating in post-fabrication by coating the structure with a known dielectric. This method is well suited for tuning the absorption resonance from the visible to the near IR spectrum. Fine control over the resonance position can be readily achieved via atomic layer deposition (ALD) that allows sub nanometer control of the dielectric layer thickness. Yet another challenge in designing an efficient metal absorber is the angle-of-incidence dependence. While for a grating-coupled systems the angular bandwidth is only a few degrees, the NGsâ?"as described aboveâ?"open up a new area of possible applications due to their extreme angular bandwidth exceeding 150 degrees. In this work, we present an experimental demonstration of the ultimate tuning for the plasmonic subwavelength gratings ranging from the narrow spectral and angular bandwidth to the > 99% absorbing extreme spectral (> 400 nm) and angular (> 150 degrees) bandwidths metallic absorber. [1] V. E. Ferry et al., Adv. Mater. 22(43), 4794 (2010). [2] A. Polyakov et al., Proc. of SPIE Vol. 8094, 809407 (2011). [3] J. L. Perchec et al., Phys. Rev. Lett. 100(6), 066408â?"4 (2008). [4] A. Polyakov et al., App. Phys. Lett. 98(20), 203104 (2011). [5] A. Polyakov et al., J. Vac. Sci. Technol. 29, 06FF01 (2011).

Nanoplasmonic cavities have the unique ability to localize light on the nanoscale, and drastically alter the photonic density of state in active materials. This talk will encompass our recent efforts in the design of nanoscale broadband light harvesters based on transformation optics, plus a full numerical model of plasmonic solar cells, linking electromagnetic simulations with semiconductor device modeling for an accurate description of the induced photo current, taking into account the finite minority carrier lifetime in the device. Lastly, the concept of plasmonic sinks will be introduced, a paradigm for the removal of unwanted long-lived states in organic light emitters. It will be shown that plasmonic sinks can lead to a drastic reduction in photo bleaching and an increase in achievable repetition rates in organic light emitting devices, due to selective triplet state quenching.

The integration of plasmonic nanostructures with solar cells offers the ability to guide and confine light in nanoscale volumes. Enhanced light absorption is critical for both fundamental and practical reasons, potentially enabling better performance through improved open-circuit voltages while also reducing the cost and fabrication time of devices. Moreover, reduced device thickness enables the use of unusual, collection-limited semiconductors. Here we present the design, simulation, and fabrication of plasmonic solar cells, which couple incident sunlight into guided and localized resonant modes of a solar cell. The plasmonic nanostructures are integrated into the solar cell through patterning of the back metallic contact. The nanopatterns are fabricated via an inexpensive nanoimprint method, which allows for both large-area patterning and precise positioning of the nanostructures. The ability to control the position and shape of each nanostructure in the substrate allows us to construct surfaces with engineered power spectral density. While light trapping surfaces in a-Si:H cells are typically fabricated via a textured deposition process which results in a random surface, our plasmonic surfaces possess controlled spatial curvature of individual nanostructures and engineered spacings to produce a tunable power spectral density. By optimizing the spatial frequencies of the pattern to match the modes of the a-Si:H solar cell, we experimentally demonstrate significant broadband and isotropic photocurrent response over cells containing conventional random substrates. Control of the nanostructure shape allows us to avoid metal features of high spatial curvature that result in parasitic metal losses. Electromagnetic simulation of absorption in these a-Si:H devices agrees closely with experimental measurements of the external quantum efficiency. For a more complete model, we link the optical absorption in a-Si:H solar cells to device simulations to fully calculate the cell efficiencies. Using this method we explore how the spatial modification of absorption due to nanostructures influences carrier collection. We find that directing light absorption to the intrinsic region of an n-i-p device results in higher efficiency than directing light absorption to the doped regions, and demonstrate nanostructures that efficiently couple to the i-region. Finally, I will discuss recent progress on integrating plasmonic electrodes into both organic photovoltaics and quantum dot photovoltaics. Both of these material systems are collection limited, and would benefit from reductions in layer thickness, making them interesting candidate materials for plasmonic light trapping design.

GaAs thin film solar cells currently hold the 1 Sun AM1.5G single junction solar cell efficiency record of 28.2% [1]. Further advances in cell efficiency are possible with the use of optimal nanoscale light trapping structures. A broadband and wide-angle response anti-reflection (AR) coating is essential for optimum solar cell performance. Conventional surface textures and AR coatings suffer from strong angle dependence. Here we demonstrate a wide-angle broadband AR coating consisting of metallic and dielectric nanostructures. We find that angular response of the short-circuit current density can outperform the optimized two-layer conventional AR coating even for a 1um thick GaAs film, for which we obtain a 10% increase at 30 degrees relative to the two-layer AR coating. We present a combined computational/experimental study optimizing the size, shape, and position of the nanostructures with respect to the active layer. A properly designed metallic/dielectric nanostructure can act as a strong resonator-antenna. If it is placed in the close proximity of a thin semiconductor film, it will preferentially couple light into the semiconductor waveguide modes, increasing the optical path length in the active material. Using full-field electromagnetic simulations we demonstrate a wide-angle enhancement in the short-circuit current simulated for a thin GaAs film on a silica or Cu substrate with periodic arrays of surface-applied silicon nitride and Ag structures. The size, shape and position of the nanostructures are designed to efficiently couple light into waveguide modes and suppress the reflection loss at particular wavelengths in order to achieve a broadband increase in the photocurrent. To understand different enhancement mechanisms we investigated GaAs structures with varying thicknesses (100 nm - 1μm) with optimized grating parameters. We find that the active layer thickness determines the dominant enhancement mechanism. In ultra-thin (100 nm - 200 nm) films the plasmonic and the dielectric Mie resonances that couple light into waveguide modes are significant, whereas for thicker films (500 nm â?" 1 μm) both particle resonances and Fabry-Perot resonances need to be optimized in order to maximize the performance. In order to experimentally verify our theoretical predictions we fabricated thin-film GaAs cells using epitaxial lift-off techniques. With these techniques, an epitaxial thin GaAs device layer is transferred onto an composite metal/glass substrate. The nanoscale light trapping surface structures have been fabricated using electron beam lithography, soft conformal imprint lithography (SCIL), and PECVD growth techniques. The wavelength-dependent optical and electrical response were measured using a tunable white light source. Results of the characterization of these thin film GaAs cells will be discussed and compared with model predictions. 1. H.A. Atwater and B. Kayes PVSC 2011, 37

Dye-sensitized solar cells (DSSCs) differ from other photovoltaics in that they rely on a large area nanoparticle network to achieve sufficient absorption of solar radiation. Although highly successful to date, this approach limits the opportunities to reduce the potential loss inherent in conventional DSSC design. We will describe a resonantly coupled cavity scheme that demonstrates precise multilayer DSSCs with a 30-fold increase in monochromatic incident photon to current efficiency compared to the planar control. Upon prism coupling in the Kretschmann configuration, light can be efficiently coupled into the surface plasmon and higher order guided modes supported by the structure. Under resonance conditions we observe record open-circuit voltages that approach the theoretical limit set by the traditional Ru-dye and iodide-based electrolyte. These results provide insight into the large number of co-dependent system components that govern dye-sensitized solar cell performance.

Nanoplasmonics show a great promise for enhancing power conversion efficiency of polymer solar cells, which is mainly ascribed to the light-concentrating effect caused by the plasmonic scattering or near-field enhancement. Here, we demonstrate that the improved optical absorption and exciton dissociation efficiency in a narrow band gap organic solar cell leads to significant improvement in short-circuit current. The plasmonic Au nanopyramid structures are prepared via the self-assembled nanosphere lithography. The plasmonic building blocks are optimized by controlling the Au nanostructure size and surface passivation, which shows power conversion efficiencies up to 20% improvement under AM 1.5 solar illumination via plasmonic near-field enhancement. Our approach can be applied to a wide range of nanostructured solar cells incorporating nanoplasmonic effect and is compatible with conventional solution processing, thereby offering a general method for the fabrication of highly efficient polymer solar cells.

The influence of plasmonic effects on the performance of amorphous silicon thin film solar cells with metal back contacts was studied experimentally and numerically. Randomly textured transparent conductive oxides (TCO) are widely used to achieve light-trapping in thin film solar cells since they improve the incoupling of light in the solar cell and diffract / scatter the incident light. However, the interaction of light with the rough metal back contact results in parasitic optical losses in the back contact. We investigated the influence of different surface textures on the quantum efficiency and short circuit current of amorphous silicon solar cells with ZnO/Ag and ZnO/Al back reflectors. The investigations were carried out experimentally and compared to optical simulations. The amorphous solar cells were prepared on 1.4 m2 large glass substrates using plasma enhanced chemical deposition (PECVD). The solar cells exhibit conversion efficiencies up to 9.6% using industry compatible processess. The optical wave propagation was simulated in 3D using a Finite Difference Time Domain (FDTD) approach. Introducing a zinc oxide layer between the amorphous silicon p-i-n diode and the metal back contact lowers the parasitic losses and increaes the reflection and quantum efficiency. The influence of different back contact designs on the quantum efficiency and the optical losses in the individual layers of the solar cell will be discussed.

Downscaling antennas into the optical domain is possible thanks to the resonant interaction between far-field radiation and metal nanostructures. A typical design for an optical antenna combines several particles with spacings in the nanometer range to allow efficient electromagnetic coupling. When an antenna is excited from the far-field, phase differences between the induced dipoles in each nanoparticle determine if the incoming light interacts with a dark or bright mode. When excited in the near field by a quantum emitter, far-field interferences between the coherent dipoles of the antenna promote directional radiation, as observed in Yagi-Uda geometries. In general, the particle polarizabilities and the optical path are the only parameters used to modify the phases of the induced dipoles in order to tune the brightness or the directionality of an optical antenna. However, we recently demonstrated that, at sub-wavelength scales, dipole-dipole interactions play a major role and strongly modify the expected behavior of nanoantennas [1,2]. In the case of metal particle pairs with diameters of the order of 100 nm, we investigated the influence of dipole-dipole coupling on the scattering cross-section. Using generalized Mie theory calculations, we showed that the transversely coupled mode with opposite phases is brighter than the in-phase longitudinal mode for touching spheres [1], even though it is considered a dark mode in the quasi-static hybridization model. This effect was recently verified experimentally by reconstructing the three dimensional electromagnetic field scattered by nanodisk pairs using digital heterodyne holography [3]. These measurements indicate that an opposite phase transverse mode exhibits a scattering cross-section similar to an in-phase longitudinal mode excited at resonance, but has a completely different angular scattering pattern [3]. Sub-wavelength phase effects are also essential in the directivity of optical antennas coupled to quantum emitters. Analytical calculations show that an 80 nm silver particle coupled to an electric dipole emitting at 600 nm can act as an efficient light collector or reflector if the emitter-particle distance is tuned by only λ/20 [2]. Modifying this distance by λ/100 is sufficient to go from a strong directional emission (91 % light emitted into one half-space) to a symmetric emission (50 % power in both half-spaces). This strong dependence of the directivity with respect to the emitter-particle distance is obtained when the phase of the particle polarizability is lower than Ï?/4 (emission red-shifted with respect to the plasmon resonance) [2]. By tuning the emitter-particle distances and the particle diameters, it is possible to design ultracompact and unidirectional optical antennas with low ohmic losses [4]. [1] B. Rolly et al, Phys. Rev. B 84, 125420 (2011) [2] B. Rolly et al, Opt. Lett. 36, 3368 (2011) [3] S. Y. Suck et al, submitted [4] N. Bonod et al, Phys. Rev. B 82, 115429 (2010)

Gold nanoparticle groupings can act as optical antennas: they couple efficiently in the near field to photon emitters in order to enhance their radiation to the far field. A typical way to design nanoantennas is to couple the plasmonic modes of gold particles positioned a few nanometers apart. We demonstrate here how optical antennas synthesized by the programmed assembly of DNA functionalized gold nanoparticles can be used to strongly increase the emission rate of a single molecule. This bottom up strategy allows us to tune particle sizes and spacings while fully controlling the chemical environment of the antenna in order to introduce dye molecules at specific positions. In practice, a known number of trithiolated DNA single strands, as short as 10 nm, are grafted on the surface of polyethylene glycol stabilized gold particles with diameters of 27 or 36 nm. Hybridization of complementary DNA sequences drives the assembly of well defined nanoparticle groupings with spacings ranging between 7 and 18 nm [1]. The optical properties of self assembled dimers, inserted in microfluidic chambers, are studied by confocal scattering spectroscopy of single groupings. Shortening the DNA linker induces a clear red shift of the plasmon resonance wavelength. A statistical analysis of scattering spectra is performed over dozens of groupings and correlated with cryo-electron microscopy images and theoretical calculations. This analysis indicates that the particle dimers are stretched by electrostatic interactions in buffer solutions with low ionic strengths [1]. The interaction of the antenna with a single chromophore added in the center of the DNA scaffold is studied using confocal fluorescence lifetime measurements. Experiments are performed with either single 36 nm gold particles or dimers and two spacing values (13 and 18 nm). The dye molecule is chosen with a fluorescence wavelength (670 nm) red-shifted with respect to the dimer plasmon resonance (around 560 nm) to minimize non-radiative decay channels in the gold nanostructure. A statistical analysis of the fluorescence lifetimes is performed on several hundred single molecules. The measured distributions agree quantitatively with theoretical values of the emission rates estimated in Mie theory using nanoparticle sizes and spacings observed in electron microscopy. Emission rate enhancements of one order of magnitude are typically observed with single 36 nm gold particles. These enhancements can reach up to two orders of magnitude for dimers with a 13 nm spacing when the emission dipole of the molecule is parallel to the dimer axis. These experiments demonstrate that it is possible to reproducibly assemble single quantum emitters in optical antennas in order to influence substantially their photophysical properties. [1] M. P. Busson et al, Nano Lett. (2011), DOI: 10.1021/nl2032052

The propagation and emission of light in plasmonic nanostructures and metamaterials is determined by an intricate interplay of electromagnetic field components that often vary on a deep subwavelength scale. Standard optical techniques cannot be used to spatially resolve these variations and thus the detailed working mechanism of many plasmonic nanostructures has remained an open question. Here we use a tightly focused 30 keV electron beam in a scanning electron microscope as a nanoscale excitation source of surface plasmons to investigate the nanoscale local field distribution in plasmonic antennas. The ridge antennas, with lengths ranging from 100-2000 nm, are carved into a single crystalline gold substrate using focused ion beam milling and support plasmonic standing wave resonances in the visible/near-infrared. By collecting the optical radiation emitted by the antenna in the electron microscope using a half-parabolic mirror, and collecting the emitted light using a CCD imaging detector, we determine the antennaâ?Ts angular emission pattern for different wavelengths. These measurements are done for different electron beam positions on the antenna at a spatial resolution of only 10 nm. The electron beam effectively acts as a point dipole source that couples strongly to components of the resonant modes that are parallel to the electron trajectory. By precisely positioning the electron beam at a well-defined antenna position and collecting the optical radiation in the far-field, the antennaâ?Ts resonant mode field components can be accurately determined We find that the shortest antennas show a 0â?"th order resonance with a polarization normal to the surface. Furthermore, we observe several higher order resonances, depending on the antenna length, with in-plane polarization. By accurately positioning the beam on an antinode for a resonant standing wave, we are able to measure the radiation pattern for each resonance order. The radiation patterns can be fully modeled by assuming that the two end facets of the antenna radiate as in-plane dipoles where the phase difference between the dipoles is determined by the phase lag experienced by the corresponding surface plasmon polaritons (SPPs) travelling along the antenna. Secondly, we study the electron-beam induced radiation patterns for single plasmonic scatterers composed of 50-nm-diameter Au nanocylinders on a Si substrate. While intuitively it would be expected that cylindrical nanoparticles would show an isotropic radiation profile in the azimuthal direction, we find strongly peaked emission profiles that depend on the position of the electron beam. We find that the incident electron beam accesses different dipole orientations in these particles by going from central excitation to almost grazing excitation. The strong beaming of light observed from these symmetric particles is then due to the fact that by exciting the particle off-center a combination of in- and out-of-plane dipoles is excited.

We demonstrate that the coupling of bright and dark plasmonic Surface Lattice Resonances (SLRs) in nanoantenna arrays, which leads to an anti-crossing behavior in the Far-Field (FF) extinction, may lead to a crossing in the Near-Field (NF) response. At this point, the array behaves as a strongly scattering medium when excited locally, but displays a local minimum in its response to FF excitation, i.e., the NF is resonant whilst the FF is anti-resonant. We observe this experimentally as an extremely narrow (both angularly and spectrally) emission of quantum dots in the proximity of the array with a simultaneous minimum in extinction of an incident plane wave. We name this new phenomenon Diffractively Induced Transparency (DIT) in light of its similarity with Electromagnetically Induced Transparency (EIT), albeit there is an important distinction. Whereas EIT is associated with a minimum in absorption, DIT is associated with a local maximum in absorption but a minimum in extinction. DIT finds its origin in the interference nature of extinction, which allows for the scattering to be minimized while the medium behaves as a coherent absorber. DIT holds remarkable properties for modified light emission and sensing, both of which depend on the local field rather than on the global properties of the array determining the FF extinction.

We present an overview of 2D and 3D plasmonic oligomers [1], metamaterials, and nanoantennas which are utilized for different purposes. Stacked 3D metamaterials can be used as perfect absorbers, which give angle and polarization independent absorption beyond 90% in the visible and near-infrared region [2]. Utilizing transition metals as well as plasmonic induced transparency schemes [3], the application of sensors for liquids and gases becomes feasible [4,5]. Also, 3D plasmon rulers become possible [6]. Cavity enhancement allows for tailoring the spectral resonances of plasmonic systems and results in very high figures of merit for the sensor schemes [7,8]. Also, 3D plasmonic rulers become feasible. Arranging plasmonic substructures in 3D geometries, chirality can result as optical property. Using this method allows for the construction of novel broadband circular polarizers with large angle acceptance angles. Nanoantennas can aid the sensing and nonlinear properties of plasmonic nanostructures as well [9]. We are going to discuss applications in this area as well. References [1] Transition from isolated to collective modes in plasmonic oligomers M. Hentschel, M. Saliba, R. Vogelgesang, H. Giessen, A. P. Alivisatos, and N. Liu Nano Lett. 10, 2721 (2010). [2] Infrared perfect absorber and its application as plasmonic sensor N. Liu, M. Mesch, T. Weiss, M. Hentschel, and H. Giessen Nano Lett. 10, 2342 (2010). [3] Planar metamaterial analog of electromagnetically induced transparency for plasmonic sensing N. Liu, T. Weiss, M. Mesch, L. Langguth, U. Eigenthaler, M. Hirscher, C. Sönnichsen, and H. Giessen Nano Lett. 10, 1103 (2010). [4] Hydrogen sensor based on metallic photonic crystal slabs D. Nau, A. Seidel, R.B. Orzekowsky, S.-H. Lee, S. Deb, and H. Giessen Opt. Lett. 35, 3150 (2010). [5] Palladium-Based Plasmonic Perfect Absorber in the Visible Wavelength Range and its Application to Hydrogen Sensing A. Tittl, P. Mai, R. Taubert, D. Dregely, N. Liu, and H. Giessen Nano Lett. 11, 4366 (2011). [6] Three-Dimensional Plasmon Rulers N. Liu, M. Hentschel, Th. Weiss, A. P. Alivisatos, and H. Giessen Science 332, 1407 (2011). [7] Cavity plasmonics: Large normal mode splitting of electric and magnetic particle plasmons induced by a photonic microcavity R. Ameling and H. Giessen Nano Lett. 10, 4394 (2010). [8] Cavity-enhanced localized plasmon resonance sensing R. Ameling, L. Langguth, M. Hentschel, M. Mesch, P. V. Braun, and H. Giessen Appl. Phys. Lett. 97, 253116 (2010). [9] Nanoantenna-enhanced gas sensing in a single tailored nanofocus N. Liu, M. L. Tang, M. Hentschel, H. Giessen, and A. P. Alivisatos Nature Materials 10, 631 (2011).

Recently, optical hydrogen-sensing using palladium-based plasmonic structures has attracted considerable attention, and many sensor geometries such as nanoparticles, nanodisks, or subwavelength hole arrays have been proposed. Palladium is used because of the strong change of its dielectric function upon hydrogen incorporation. Optical detection is of particular importance since hydrogen forms an explosive mixture with air at concentrations ranging from 4 to 75%. Furthermore, the investigation of plasmonic sensors down to the single particle level[1] allows for the study of Pd hydriding/dehydriding kinetics on the nanoscale. We will present experimental results for a perfect absorber hydrogen sensor[2] working in the visible wavelength range that utilizes a design consisting of palladium nanowires stacked above a dielectric spacer layer and a gold mirror to reliably and reproducibly detect hydrogen. The mechanism of perfect absorption is excitation of a particle plasmon in the Pd wire which interacts with its mirror image in the gold film and leads to a magnetic mode in the spacer layer. By varying the wire width and the spacer height, we can tailor both the dielectric and magnetic response of the structure to obtain optical impedance matching and consequently zero reflectance. Combined with the suppression of transmittance by the gold mirror, this leads to almost complete absorption of the incident light for a fixed design wavelength. Incorporation of hydrogen causes a change of the Pd dielectric function and leads to an impedance mismatch which in turn allows reflected light to be detected. Utilizing this design we are able to reliably and reproducibly detect concentrations of hydrogen down to 0.5% in nitrogen. Using fabrication methods such as interference lithography, this sensor could be realized over large areas and should be suitable for industrial applications in areas such as hydrogen mobility and chemical engineering. In order to access smaller detection volumes and concentrations a move to single particle measurements and especially antenna-enhanced geometries is desirable. A previously published experimental work utilizes a single gold bowtie antenna situated next to a palladium nanodisk to detect hydrogen in extremely small sensing volumes[1]. We performed extensive numerical FDTD calculations and will show how these experimental results can be modeled and understood by considering two competing effects: a small spectral blueshift of the resonance caused by the change of the dielectric function from Pd to PdH and a substantial redshift caused by the expansion of the Pd lattice and in turn of the Pd nanodisk. This allows us to reproduce the experimental results qualitatively and paves the way towards the optimization of present designs and ultimately the realization of extremely sensitive plasmonic hydrogen sensors. References: [1] N. Liu et al., Nat. Mater. 10, 631-636 (2011) [2] A. Tittl et al., Nano Lett. 11, 4366-4369 (2011)

Surface plasmon polariton and related phenomena in metallic nanostructures are currently being exploited for a variety of applications including molecular sensing for medical diagnostics and environmental monitoring, focusing of light for photovoltaics application, near-field optical microscopy, subwavelength photonics and optical metamaterial. In this work, arrays of plasmonic nanocavities with very low volumes, down to λ³/1000, have been fabricated by Soft UV Nanoimprint Lithography [1] on large surfaces up to 1 cm². Nearly perfect omnidirectional absorption (30° - 70°) is demonstrated for the fundamental mode of the cavity (λ = 1.15 μm). The second-order mode exhibits a sharper resonance with strong angular dependence and total optical absorption when the critical coupling condition is fulfilled. It leads to high refractive index sensitivity and state of the art figure of merit (Î"I/Î"n â^¼ 20) and offers new perspectives for efficient biosensing experiments in ultralow volumes [2]. Moreover, the wavelength and width of the broad fundamental resonance can be tuned by carefully designing the resonant nanoantenna array, and for 2D arrays, the total omnidirectional absorption obtained is independent of the polarization of light, making this structure ideal for the design of efficient photovoltaic devices in which the absorbed light leads to electron-hole pair generation in an ultrathin inorganic or organic semiconductor layer placed in the ultrasmall cavity volume. [1] Cattoni et al., Microelectron. Eng., 87, 1015 (2010) [2] Cattoni et al., Nano Letters 11, 3557 (2011)

Optical trapping provides a powerful tool for particle manipulation at the micro and nanoscale, based on forces due to intensity gradients. However, at the nanoscale, optical trapping becomes challenging due to the diffraction limit of light. This limit can be overcome using plasmonic nanostructures. In this presentation, we introduce metal-dielectric coaxial cavities as efficient nano-optical traps. Using analytic and experimental techniques, we determine the modal characteristics of these structures and the forces exerted on nanoparticles close to their apertures. The mode profiles of these cavities exhibit high field intensities in the dielectric channel with strong field gradients near the channel interfaces. Such strong gradients produce a deep trapping potential that is essential for stable nanoparticle trapping. Full-field finite difference time domain simulations were used to determine the position-dependent optical forces on a dielectric nanoparticle near the coaxial aperture. By calculating the field distribution at the aperture, Maxwellâ?Ts stress tensor formalism can be used to evaluate the force acting on the particle. Attention is given to coaxial cavities formed from nanoscale silica rings embedded in a 300 nm thick silver slab. The slab is illuminated by a plane wave with perpendicular incidence. Our results show that the nanoparticle experiences strong forces in both the transverse and the longitudinal directions. For example, at a wavelength of 540 nm, a 10 nm dielectric sphere of refractive index 2 experiences a peak force of 900 fN/W in the longitudinal direction and 400 fN/W in the transverse direction when it is 20 nm above a coaxial aperture with 60 nm inner core radius and 25 nm dielectric channel thickness. The longitudinal force pulls the particle toward the aperture, while the transverse force creates a trapping potential that confines the particle around the center of the coaxial aperture dielectric channel. This trapping potential is deep enough to stably trap the nanoparticle. Trapping forces can be further increased at longer illumination wavelengths, due to the reduction in coaxial modal losses. Further, the width and the depth of the trapping potential can be engineered by changing the size of the metallic core. Experimentally, nano-plasmonic traps are investigated using fluorescence microscopy. Ring apertures are patterned using focused ion beam milling of a 300-nm-thick Ag substrate. The structure is then integrated into a flow cell consisting of CdTe nanocrystals with diameters ranging from 3nm to 8nm. By varying the illumination wavelength, we can selectively trap or release various sized nanoparticles. Our results illustrate the potential for sub-diffraction-limited optical manipulation, with promise for manipulating objects down to the single-nanometer regime.

Ag nanoparticles (NPs) have been reported possessing an extremely high enhancement effect for the detection sensitivity in surface plasmon resonance (SPR) spectroscopy. One of the fundamental issues impeding the further development of Ag enhancer for SPR is the lack of stability of Ag NPs. Here, we present a novel method to stabilize Ag nanoplates by depositing a thin Au layer on the plates in solution instead of etching the plates. The coated Ag@Au structures maintain the plasmonic property of original Ag nanoplates. Thanks to thin gold layer, Ag@Au nanoplates now can be stored in water, PBS buffer or H2O2 solution for several days without any changes. The deposited Au layer also makes the post-modification of Ag nanoplates easy to manipulate and very efficient. A layer of protein-nanoplate complexes are formed after flow-injection of biotinylated Ag@Au nanoparticle, which has led to about 20-fold increase in the SPR resonance angle shift compared with that enhanced by Au NPs. The versatile biotin-streptavidin interaction used here should allow adaptation of Ag@Au nanostructures to many other systems that include DNA, RNA, peptides, and carbohydrates, opening new avenues for ultrasensitive analysis of biomolecules with flow-injection assay and SPR spectroscopy.

Metamaterials have provided new and interesting linear media that have provided a venue to explore otherwise inaccessible concepts¹. Analogously, nonlinear metamaterials based on metals are appealing for potential use in nonlinear optical applications at infrared or visible wavelengths. Plasmonic field enhancement, along with the intrinsic nonlinearity of metals, makes metal-based nonlinear optical metamaterials a concrete possibility. In particular, we investigate second order nonlinear phenomena. Although metals are centrosymmetric and do not possess an inherent Ï?⁽²⁾ nonlinearity, the surface of a metal can break the symmetry and provide a mechanism for an effective Ï?⁽²⁾ nonlinearity. This homogenizedÏ?⁽²⁾ nonlinear response thus becomes highly dependent on the metal geometry, making it inherently a metamaterial construct. Moreover, the origin of nonlinearity in metals arises from both volume and surface contributions. Nonlinear surface contributions are strictly related to the response of the electrons within a Fermi wavelength (~5Ã.) from the metal boundaries. In this sub-nanometer realm, electron-electron interactions become non-negligible and non-local effects must be taken into account. We present an analysis of second-harmonic generation in plasmonic systems of arbitrary shape². The nonlinear optical response of the metal is described by a hydrodynamic model, which includes the effects of quantum pressure associated with the electron gas³. In particular, plasmonic systems are investigated, in which metal nano-structures are strongly coupled to a metal film. As the gap region reaches distances of fractions of a nanometer, non-local effects become predominant and a saturation of the electric field enhancement occurs. The nonlinear effects are thus intrinsically connected to the linear non-local properties of the system. The free electron limit, in which the pressure is completely neglected, is also investigated. In this limit nonlinear surface contributions are expressed in terms of the polarization vector in the bulk regions², thus avoiding tackling more complex, 3D equations. We then numerically investigate second-harmonic generation arising from U-shaped metal nanoparticles and show that its basic characteristics may be explained solely by the electric properties of the structure rather then its magnetic response, in contrast to previous works⁴. References 1J. B. Pendry, D. Schurig, and D. R. Smith, â?oControlling Electromagnetic Fields,â? Science 312, 1780â?"1782 (2006). 2C. Ciracì, E. Poutrina, M. Scalora, and D. R. Smith, â?oSecond-Harmonic Generation in Plasmonic Metamaterialsâ?, submitted. 3M. Scalora, M. A. Vincenti, D. de Ceglia, V. Roppo, M. Centini, N. Akozbek, and M. J. Bloemer, â?oSecond- and third-harmonic generation in metal-based structures,â? Phys. Rev. A 82, 043828 (2010). 4 M. W. Klein, C. Enkrich, M. Wegener, and S. Linden, "Second-harmonic generation from magnetic metamaterials," Science 313, 502â?"504 (2006).

Surface chemistry can become pronounced in determining the optical properties of colloidal metal nanoparticles as the nanoparticles become so small (diameters <20 nm) that the surface atoms, which can undergo chemical interactions with the environment, represent a significant fraction of the total number of atoms although this effect is often ignored. For instance, formation of chemical bonds between surface atoms of small metal nanoparticles and capping molecules that help stabilize the nanoparticles can reduce the density of conduction band electrons (i.e., free electrons) in the surface layer of metal atoms. This reduced electron density consequently influences the frequency-dependent dielectric constant of the metal atoms in the surface layer and, for sufficiently high surface to volume ratios, the overall surface plasmon resonance (SPR) absorption spectrum. The important role of surface chemistry will be discussed by carefully analyzing the classical Mie theory and a multi-layer model will be presented to produce more accurate predictions by considering the chemically reduced density of conduction band electrons in the outer shell of metal atoms in nanoparticles. Calculated absorption spectra of small Ag nanoparticles quantitatively agree with the experimental results for our monodispersed Ag nanoparticles synthesized via a well-defined chemical reduction process, revealing an exceptional size-dependence of absorption peak positions: as particle size decreases from 20 nm the peaks blue-shifts but then turns over near ~12 nm and strongly red-shifts. A comprehensive understanding of the relationship between surface chemistry and optical properties will be beneficial to exploit new applications of small colloidal metal nanoparticles, such as colorimetric sensing, electrochromic devices, and surface enhanced spectroscopies. Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

The plasmon resonances of individual and coupled metallic nanoparticles have received considerable attention for their applications in nanophotonics, biology, sensing, spectroscopy, and solar energy harvesting. While individual spheres larger than 10 nanometers and dimers with gaps greater than 1-2 nm have been thoroughly characterized, their properties at smaller sizes (entering the quantum regime) have been historically difficult to describe. Quantum-sized individual plasmonic particles exhibit very low extinction cross-sections while closely-spaced colloidally-synthesized dimers have been challenging to control. Such difficulties preclude experimental analysis of quantum-plasmonic systems, which are highly relevant to many natural and engineered processes. In this presentation, we investigate the plasmon resonances of individual ligand-free silver nanoparticles using aberration-corrected transmission electron microscope (TEM) imaging and monochromated scanning TEM electron energy-loss spectroscopy (STEM EELS). This technique allows direct correlation between a particle's geometry and its plasmon resonance. As the nanoparticle diameter decreases from 20 nm to less than 2 nm, the plasmon resonance exhibits a blue-shift from 3.3 eV to 3.8 eV, with particles smaller than 10 nm showing a substantial deviation from classical predictions. We present an analytical quantum-mechanical model that well describes the plasmon resonance shift due to a change in particle permittivity. Our results highlight the unique quantum plasmonic properties of small metallic nanospheres, with direct application to understanding and exploiting catalytically-active and biologically-relevant nanoparticles. Furthermore, using TEM EELS, we can observe the plasmonic properties of multi-particle systems. Using excitation from the electron beam, ligand-free silver particles are capable of moving on silicon nitride substrates, allowing dynamic monitoring of plasmonic resonances as the particles approach each other and coalesce. This strategy provides a straightforward method for studying dimer interactions at variable separation distances, including quantum-sized separations. Because individual sets of particles can simultaneously imaged and spectrally analyzed, we can directly probe the crossover from classical to quantum plasmon resonances in particle dimers.

The great asset of plasmonic systems is their ability to concentrate and enhance electromagnetic fields in nanometer sized dimensions. The increase in light-matter interactions associated with the highly confined fields is of great importance for sensing , quantum and non-linear optics. Structures with gaps are of particular interest because in these systems the opposite charge accumulation on both sides of the gap can lead to very high electric fields. We experimentally investigate the plasmon resonances of side-by-side aligned single-crystalline gold nanorod dimers. Robust gaps between the particles reaching well below 1 nm are formed by reproducible self-assembly. For such atomic-scale gaps extreme splitting of the symmetric and anti-symmetric dimer eigenmodes is observed in white-light scattering experiments. Besides providing evidence for atomic-scale gap modes at visible wavelengths with correspondingly small mode volumes, our experimental results can serve as a benchmark for electromagnetic modeling beyond local Maxwell theory [1, 2]. References: [1] García de Abajo F.J. J. Phys. Chem. C, 112, 17983-17987 (2008) [2] Zuloaga, J.; Prodan, E.; Nordlander, P. Nano Lett.9, 887-891 (2009)

During last twenty years the field of nanophotonics and metamaterials has advanced dramatically. Up to the recent times, metamaterials have been considered as classical electromagnetic structures, and the research has been mainly focused on classical aspects of light-metamaterial interactions. However, interesting phenomena stem from the quantum nature of interaction of nanostructures and light. We report here a theoretical investigation of the cooperative behavior of quantum emitters embedded in nanostructured materials. In the recent years, the interest in the Dicke model [1] has been revived since it is a simple model system in which one can find multi-partite entanglement. Inspired by the Dicke model, we first address collective spontaneous emission process of a dense ensemble of radiating two-level quantum objects embedded in a metamaterial. The radiative coupling between quantum emitters causes the emergence of collective modes whose lifetimes are longer or shorter compared to those of uncorrelated independent quantum emitters. Whereas in the original Dicke model of quantum emitters in free space spontaneous emission takes place in a timescale inversely proportional to the number of radiating atoms N and the emission intensity is proportional to N squared, in our case we observe a modified power-law dependence of the emission timescale and intensity. We also show that the collective damping parameter is an oscillatory function of inter-emitter spacing. Further, we analyze the dynamics of the ensemble of the two-level emitters under continuous pumping. For the both cases we derive analytical expression for the electric field operator in the far field and calculate first- and second-order correlation functions of the emitted light. This enables us to define physically measurable quantities such as the emission spectrum and intensity of the emitted light. We also analyze photon statistics and nonclassical properties of the radiation field. From the analysis of long-range coupling, we derive conditions for the superradiance, which may play a significant role for ultrafast applications. Interestingly, the proposed system allows for post fabrication tuning of the emission properties of the metamaterial. We will discuss routes to modified emission, e.g., by applying external electric fields that can dynamically modify the transition energy of the quantum emitters via an induced Stark shift. References [1] R.H. Dicke, Phys. Rev. 93, 99 (1954).

We present a novel technique, Angle-Resolved Cathodoluminescence Imaging Spectroscopy (ARCIS), that enables, for the first time, measurements of both the local optical density of states (DOS) and the angular radiation profile of nanophotonic structures at deep subwavelength resolution. The ARCIS instrument is composed of a 30 keV field-emission SEM with a half-parabolic mirror integrated between the microscopeâ?Ts pole piece and the sample. Light is collected and spectrally analyzed to determine the spatial-resolved DOS, and, in a parallel geometry, directed onto a 1024x1024 pixel CCD array that images the beam profile emanating from the mirror. Using this Fourier imaging geometry, the angle-resolved emission pattern can be determined at an angular resolution of 1 degree, over a solid angle as large as 1.4 sr. (NA=0.99). With the SEM in high-resolution mode, the spatial resolution of the ARCIS technique is as small as 1-10 nm. The collected photon count rate over the 400-950 nm spectral band is between 10 and 100 million counts/sec, depending on the sample and the beam current. The ARCIS technique is a unique tool to characterize photonic structures in which the local DOS varies on a subwavelength scale, and is thus ideally suited for studies in plasmonics and metamaterials. Such studies cannot be made using conventional microscopy due to the limitations imposed by the diffraction limit. The ARCIS technique also surpasses the resolution of conventional near-field microscopy, with the added advantage that the geometry under study is not affected by a fiber probe. When applied on optically doped nanomaterials such as e.g. rare earth doped geometries, the ARCIS geometry can also be used for spatially and angle-resolved fluorescence lifetime imaging. In this presentation, we give detailed insight in the design of the ARCIS system and its measurement characteristics. We will present several successful applications of the technique to obtain insight in the photonic density of states and radiation profiles for metal nanoparticles, nanoparticle dimers, Yagi Uda antennas, metal nanorod antennas, planar elliptical plasmonic antennaâ?Ts, and epsilon-near-zero materials. We also present an entirely new range of applications of the ARCIS technique in dielectric materials, resolving the modal distribution of resonant modes in silicon surface Mie scatterers and their radiation profile with deep-subwavelength resolution. Finally, we will demonstrate the fluorescence lifetime imaging capabilities of the system using rare-earth doped silica Mie scatterers.

Central to modern nanophotonics is spontaneous emission control which can be attained by local density of states (LDOS) engineering in dielectric and metallic nano structures. We report nanoscale mapping of the local density of states by extending cathodo-luminescence microscopy, recently developed for plasmonic excitation, to dielectric structures. The method is a combination of electron-beam scanning and optical spectroscopy; it relies on scanning a transient dipolar emitter induced by electron beam bombardment with respect to its photonic environment while measuring the total emitted power. Each individual electron traversing the photonic structure generates a nanoscale transient dipole by the accelerated charge which we exploit as a local probe of the LDOS. With unprecedented resolution (â^¼10 nm) we image localized photonics crystal cavity modes in a nano-structured silicon nitride membrane, over the visible spectrum into the near-IR. We identify individual cavity modes that are spatially different and we map their LDOS. Also, our measurements reveal extended Bloch modes which are delocalized over the crystal and periodically modulated. Moreover, by momentum spectroscopy, we resolve the angular emission pattern of the radiation emitted, which exhibits complex diffraction patterns. In addition, we image the LDOS for random gold films as their topology approaches percolation. Thanks to the high resolution imaging we are able to observe single-particle resonances localized at the gold particle trasforming into extended modes when the cluster merge into a network. We report a study of the rich spectral dynamics of the local hot-spot of the LDOS through all the visible range. We also observe a long-tailed distribution of the LDOS at percolation in agreement with recent Purcell factor studies. Through the combination of energy and momentum LDOS imaging we demonstrate full tomography of the optical modes density.

Controlling the far field emission pattern of nanoscale objects is one of the central goals of optical antennas. In most cases, the desired pattern is a beam of light in the far field, which can couple a nanoscale source or sink of light to a distant object. Such optical beaming could improve efficiency in a variety of important applications, such as LEDs, photodetectors, sensors, and photovoltaics. For macroscopic applications there is one well-known shape that easily accomplishes this goal based on simple ray optics principles: the parabolic reflector. Parabolic reflectors can efficiently direct a far field beam of light either into or out of a focal point. These reflectors are intrinsically broadband, working equally well for microwave and radiowave applications for satellite dishes and antennas; to infrared radiation for simple space heaters; and finally at optical frequencies in applications like automotive headlights and flashlights. It would seem natural then that parabolic directors would be useful as well for small scale optical antennas; however, fabricating complex three dimensional surfaces is not generally possible with traditional nanofabrication tools like electron beam lithography and focused ion beam etching. We will demonstrate that a line cross section of a parabolic surface, namely an elliptically-shaped depression in a smooth metal surface, can provide the same broadband unidirectional emission as a full parabolic structure with a total footprint similar to that of existing optical antennas, while being much simpler to fabricate. We will report on the fabrication of wavelength-scale elliptical â?~arenasâ?T in a gold surface, and the characterization of their wavelength dependent local density of optical states as well as their far-field radiation patterns using cathodoluminescence. These structures show a high maximum directivity for emitters near the elliptical foci, and also function as optical resonators, enhancing emission at certain wavelengths and positions tunable by changing the resonatorsâ?T physical dimensions.

We demonstrate for the first time an opto-mechanical system composed of a sub-wavelength plasmonic nanocavity coupled to a silicon-nitride nanomechanical oscillator. Doubly clamped silicon nitride nanobeams, 500-nm-wide and 20-micron-long, were made in 50-nm-thick silicon nitride membranes using focused ion beam milling and then coated with a 120 nm thick gold layer. The beams act as coupled nanomechanical oscillators, spaced by a narrow gap of 10-50 nm width. The metal-coated gap region supports metal-insulator-metal (MIM) plasmons of which the dispersion is extremely sensitive to the gap width and thereby induces a large optomechanical coupling between the plasmon mode and mechanical oscillator. The optical transmission spectrum of the gap region is determined by Fabry-Perot oscillations of MIM plasmons confined in the MIM cavity. Optical transmission measurements on the MIM plasmonic nano-mechanical resonator were performed using a narrowband (linewidth < 1kHz) CW laser at a wavelength of 1550 nm, focused onto the plasmonic nanocavity using a microscope objective. The transmitted light is collected using a fiber-coupled InGaAs photodiode and recorded by a spectrum analyzer. We find that the nano-mechanical motion of the MIM oscillator is efficiently transduced to the optical field and is measurable in the frequency spectrum of the time-dependent transmitted laser light intensity. The plasmonic nanocavityâ?Ts transmission spectrum shows a strong resonance at a frequency of 4 MHz, corresponding to the fundamental nanomechanical resonance of the silicon nitride beam. Doubly clamped beams with varying length in the range 10-30 micron show a decreasing resonance frequency with length following the expected 1/L frequency dependence. The measured mechanical eigenfrequency of the beams is in good agreement both with finite element simulations and with calculations based on measurements of the beamsâ?T spring constant using atomic-force microscopy. Polarization-dependent measurements of the resonant mechanical mode spectrum confirm the plasmon-mechanical coupling occurs through TM-polarized plasmons. Due to the very strong local field enhancement in the MIM cavity and the large sensitivity of the coupling to gap size, the optical coupling strength in these cavity plasmo-mechanical nanosystems is as high as 2 THz/nm, a more than two orders of magnitude improvement over any cavity opto-mechanical system to date. Taking advantage of this extreme sensitivity we demonstrate that the mechanically modulated transmission through the plasmonic Fabry-Perot cavity can be used to detect the thermal motion of the mechanical resonator at room temperature with high signal to noise ratio. The demonstrated readout method provides a rapid, effective and ultra-sensitive readout of nanoscale mechanical motion that could be of use in both fundamental and applied studies, in which nanomechanical systems act as transducers, sensors or active elements.

We present results of a combined scanning and photoemission electron microscopy (SEM and PEEM) investigation of plasmonic structures such as core-shell nanoparticles. To determine the electromagnetic enhancement, due to excitation of localized surface plasmons (LSPs), we measure relative photoelectron yields from individual silver coated polystyrene (PS) nanoparticles under 3.1 eV femtosecond laser irradiation using PEEM. Silver thin films are grown by magnetron sputtering on polystyrene nanoparticles supported on mica substrates. We compare the photoelectron yield emitted from single particles to the surrounding silver thin film surface to determine enhancement factor distributions for collections of nominally identical particles with average diameters ranging from 200 to 800 nm. A correlated SEM analysis demonstrates that particles possessing the greatest enhancements are connected with defects present in the silver coating or composed of aggregate particle structures. At higher PEEM magnifications the spatial distribution of the local electric fields emitted from single particles and aggregates is resolved and compared with the near-field intensity predicted by finite-difference time domain (FDTD) simulations. One consequence of exciting the LSP is generation of locally enhanced electromagnetic (EM) fields at the surface of the nanoparticle. Plasmon enhanced regions on roughened metal surfaces are often referred to as hot spots. PEEM images such hot spots by spatially resolving the electron emission which is a direct measure of the EM field enhancement. By correlating emission yields with specific nanostructures, as revealed by correlated SEM measurements, one can relate the magnitude of plasmon-induced field enhancement with nanoparticle structure. A variety of interesting plasmonics structures, such as particle aggregates and metal gratings, show photoemission enhancement. Furthermore, when metal structures are coated with thin films of insulating metal oxides, a hybrid material, with unique new properties, is created. Hybrid materials often display dramatic reductions in work function, increases in quantum yield, and changes in electron angular distribution, when compared to bare metal structures. Understanding the dynamics of electronically excited materials is essential to developing mechanistic models relevant to applying such materials to photocatalysis, radiation chemistry, and charge and energy transfer.