Localized Plasmon Modes Research Articles

Enhancing solar cells with localized plasmons in nanovoids

Localized plasmon resonances of spherical nanovoid arrays strongly enhance solar cell performance by a factor of 3.5 in external quantum efficiency at plasmonic resonances, and a four-fold enhancement in overall power conversion efficiency. Large area substrates of silver nanovoids are electrochemically templated through self-assembled colloidal spheres and organic solar cells fabricated on top. Our design represents a new class of plasmonic photovoltaic enhancement: that of localized plasmon-enhanced absorption within nanovoid structures. Angularly-resolved spectra demonstrate strong localized Mie plasmon modes within the nanovoids. Theoretical modelling shows varied spatial dependence of light intensity within the void region suggesting a first possible route towards Third Generation plasmonic photovoltaics.

Movable Fiber-Integrated Hybrid Plasmonic Waveguide on Metal Film

A waveguide structure consisting of a tapered nanofiber on a metal film is proposed and analyzed to support highly localized hybrid plasmonic modes. The hybrid plasmonic mode can be efficiently excited through the in-line tapered fiber based on adiabatic conversion and collected by the same fiber, which is very convenient in experimentation. Due to the ultrasmall mode area of plasmonic mode, the local electromagnetic field is greatly enhanced in this movable waveguide, which has the potential for enhanced coherence light emitter interactions, such as waveguide quantum electrodynamics, single emitter spectrum, and nonlinear optics.

Increasing the efficiency of photoelectric cells by nanostructuring their surface

A method of fabrication of nanostructured objects, which increases the efficiency of the photo-electric system used in the conversion of energy of electromagnetic radiation into electric energy is suggested. The method is based on the formation of metal nanodots on the basis of the surface layer of porous silicon, which concentrates the energy of electromagnetic radiation due to localized plasmon modes.

Real time spectroscopic ellipsometry of Ag/ZnO and Al/ZnO interfaces for back-reflectors in thin film Si:H photovoltaics

Real time spectroscopic ellipsometry (RTSE) has been applied to analyze the optical characteristics of Ag/ZnO and Al/ZnO interfaces used in back-reflector (BR) structures for thin film silicon photovoltaics. The structures explored here are relevant to the substrate/BR/Si:H(n-i-p) solar cell configuration and consist of opaque Ag or Al films having controllable thicknesses of microscopic surface roughness, followed by a ZnO layer up to ~3000Å thick. The thicknesses of the final surface roughness layers on both Ag and Al have been varied by adjusting magnetron sputtering conditions in order to study the effects of metal film roughness on interface formation and interface optical properties. The primary interface loss mechanisms in reflection are found to be dissipation via absorption through localized plasmon modes for Ag/ZnO and through intraband and interband transitions intrinsic to metallic Al for Al/ZnO.

Cavity Plasmonics: Large Normal Mode Splitting of Electric and Magnetic Particle Plasmons Induced by a Photonic Microcavity

We couple localized plasmon modes in nanowire pairs with resonator modes of a microcavity. Depending on the position of the nanowire pair in the resonator, the electric (symmetric) or magnetic (antisymmetric) plasmon mode is coupled, manifested by a huge anticrossing in the dispersion diagram. We explain this behavior by taking the symmetry and spatial distribution of the electric fields in the resonator into account. Experimental spectra verify the predicted mode-splitting due to the resonant coupling and agree well with theory. Our work can serve as a model system for far-field plasmon-plasmon coupling and paves the way toward enhanced localized plasmon-plasmon interaction in photonically coupled three-dimensional Bragg structures.

Impurity-assisted nanoscale localization of plasmonic excitations in graphene

A nonlocal quantum-mechanical model is employed to compute plasmonic excitations of graphene in the presence of an impurity potential. A full diagonalization of the polarization operator is performed, allowing the extraction of all its poles. It is demonstrated that impurities induce the formation of nanoscale localized plasmonic modes. It is also shown that the chemical potential and impurity strength can be tuned to control target features of the localized modes, making graphene an intrinsic plasmonic material. These predictions can be tested by scanning tunneling microscopy experiments.

A Surface Plasmon Enhanced Infrared Photodetector Based on InAs Quantum Dots

In this paper, we report a successful realization and integration of a gold two-dimensional hole array (2DHA) structure with semiconductor InAs quantum dot (QD). We show experimentally that a properly designed 2DHA-QD photodetector can facilitate a strong plasmonic-QD interaction, leading to a 130% absolute enhancement of infrared photoresponse at the plasmonic resonance. Our study indicates two key mechanisms for the performance improvement. One is an optimized 2DHA design that permits an efficient coupling of light from the far-field to a localized plasmonic mode. The other is the close spatial matching of the QD layers to the wave function extent of the plasmonic mode. Furthermore, the processing of our 2DHA is amenable to large scale fabrication and, more importantly, does not degrade the noise current characteristics of the photodetector. We believe that this demonstration would bring the performance of QD-based infrared detectors to a level suitable for emerging surveillance and medical diagnostic applications.

Universal scaling of local plasmons in chains of metal spheres

The position, width, extinction, and electric field of localized plasmon modes in closely-coupled linear chains of small spheres are investigated. A dipole-like model is presented that separates the universal geometric factors from the specific metal permittivity. An electrostatic surface integral method is used to deduce universal parameters that are confirmed against results for different metals (bulk experimental Ag, Au, Al, K) calculated using retarded vector spherical harmonics and finite elements. The mode permittivity change decays to an asymptote with the number of particles in the chain, and changes dramatically from 1/f(3) to 1/f(1/2) as the gap fraction (ratio of gap between spheres to their diameter), f, gets smaller. Scattering increases significantly with closer coupling. The mode sharpness, strength and electric field for weakly retarded calculations are consistent with electrostatic predictions once the effect of radiative damping is accounted for.

Doubly resonant optical nanoantenna arrays for polarization resolved

We report that rhomb-shaped metal nanoantenna arrays support multiple plasmonic resonances, making them favorable bio-sensing substrates. Besides the two localized plasmonic dipole modes associated with the two principle axes of the rhombi, the sample supports an additional grating-induced surface plasmon polariton resonance. The plasmonic properties of all modes are carefully studied by far-field measurements together with numerical and analytical calculations. The sample is then applied to surface-enhanced Raman scattering measurements. It is shown to be highly efficient since two plasmonic resonances of the structure were simultaneously tuned to coincide with the excitation and the emission wave- length in the SERS experiment. The analysis is completed by measuring the impact of the polarization angle on the SERS signal.

Large spectral extinction due to overlap of dipolar and quadrupolar plasmonic modes of metallic nanoparticles in arrays

We explore the optical response of two-dimensional (2D) arrays of silver nanoparticles, focussing our attention on structures for which the individual particles in isolation support both dipolar and quadrupolar localised surface plasmon modes. For individual spheres we show that when dipolar and quadrupolar modes are excited simultaneously, interference leads to most of the scattered light being radiated in the forward direction. This is in contrast to what happens when each mode is excited on its own. We further show, using finite-element modelling that when such particles are assembled into square 2D arrays, the dipolar and quadrupolar modes can combine to produce a single peak in the optical density of the array. By simulating the field distributions associated with these modes we are able to illustrate the dual-mode character of this feature in the optical density. We have extended our examination of this effect by considering how the optical density of these arrays changes with incident angle for two polarisations (s and p).

Self-assembly: mastering photonic processes at nanoscale

AbstractSupramolecular ordering happens as an important parameter for the control of light emission processes. In this review paper, we discuss several examples of application of self-assembly to the realization of nano-structures designed in view of mastering specific photonic processes. This comprises the formation of highly localized plasmon modes in self-organized 2D assemblies of metal nanoparticles, the immobilization of dyes inside highly homogeneous 2D alveolar self-assembled molecular matrices and molecular 3D building blocks designed to combine in-plane periodicity and off-plane π-conjugated protrusions. Finally, we will discuss 3D self-assembly in solution with the example of fluorescent labelling of DNA.

Plasmonic and magnetic effects accompanying optical second-harmonic generation in Au/Co/Au nanodisks

Optical second-harmonic (SH) generation in Au/Co/Au sandwich-like nanodisks is studied, using several experimental techniques, at SH frequency being close to that of the plasmon resonance in the system. The dependences of the relative amplitude and phase of the magnetization-induced SH component on the angle of incidence are found to significantly differ from those measured for a continuous Au/Co/Au trilayer film. A phenomenological description of the observed effects is proposed, with special emphasis on resonant excitation of the local plasmon modes by the nonlinear sources induced in the nanodisks.

Anomalous near-perfect extraordinary optical absorption on subwavelength thin metal film grating

We demonstrate analytically and numerically that the anomalous extraordinary optical absorption through thin metal grating with air slit arrays can be excited simultaneously by long and short range surface plasmonic modes supported on the surfaces of thin metal film in weak coupling while this absorption can only be excited by short range surface plasmonic mode in strong coupling. In particular, we predict localized plasmonic mode inside air slits holding double-effect to generate the anomalous extraordinary optical absorption when only this mode is present and to suppress this absorption when the pathways excited by localized and surface plasmonic modes are present simultaneously. Furthermore, we present that double-layer metal grating consisting of two identical single-layer metal gratings can be exploited to enhance absorption efficiency more than 99% and 90% for excited localized plasmonic modes and surface plasmonic modes respectively.

Low loss negative index metamaterials with one type of meta-atom

Metamaterial building blocks from microwave to optical range are mainly based on metal-dielectric composites. In almost all structures with true negative index (not coming from losses), two kinds of meta-atoms (electric and magnetic) are mixed to drive simultaneously the effective permittivity and permeability to negative values leading in turn to a negative index of refraction. In this paper, we show that two coupled structures with localized plasmons modes (e.g. cut-wires or split ring resonators) can exhibit negative refractive index by their own, by appropriately controlling the hybridization scheme of the plasmons modes. Because of small metal filling factor and reduced optical losses, the resulting structures may pave the way to realistic applications of metamaterials at optical frequencies.

Optical Properties of Metal Tips for Tip-Enhanced Spectroscopies

We develop a full 3D approach to simulate the optical properties of sharp metal tips that could be quantitatively compared with experiments and provide accurate predictions. Similar to metallic wires, elongated metal tips support a series of extended mode resonances whose properties are largely determined by the tip geometry and are essentially independent of the tip material. For sufficiently long tips, these resonances are energetically well separated from a broader, high-energy feature, whose position and line shape are independent of the tip length but vary strongly with the tip material. The latter is a localized plasmon mode characterized by the hemisphere terminating the tip.

The evolution of the localized plasmon modes in a semi-infinite superlattice with cap layer

Using a transfer matrix method, we study the properties of the localized plasmon modes in a semi-infinite semiconductor superlattice (SL) with cap layer. The results show that in such a structure there exist three localized plasmon modes. They may appear either in the gap between two bulk plasmon bands, or below and above the bulk plasmon bands. The evolution from the extended plasmon modes to the localized plasmon modes can clearly be tracked with the change of the structural parameters or the relative electron density. A brief analysis of these results is given. It is suggested that the localized plasmon modes can be artificially controlled by adjusting the parameters of the proposed micro-structures.

Nanoscale optical field localization by resonantly focused plasmons

We experimentally demonstrate use of plasmonic resonant phenomena combined with strong field localization to enhance efficiency of confining optical fields in a Si waveguide. Our approach utilizes a plasmonic resonant nano-focusing-antenna (RNFA), that simultaneously supports several focusing mechanisms in a single nanostructure, integrated with a lossless Si waveguide utilized with silicon-on-insulator (SOI) technology, to achieve a sub-diffraction limited focusing with a nanoscale (deeply subwavelength) spot size. The metallic RNFA effectively converts an incoming propagating waveguide mode to a localized resonant plasmon mode in an ultrasmall volume in all 3 dimensions. The near-field optical measurements of the fabricated RNFA using heterodyne near-field scanning optical microscope (H-NSOM) validate the theoretical predictions showing strong optical field localization.

Tip-enhanced Raman scattering: Influence of the tip-surface geometry on optical resonance and enhancement

The tip-sample distance ( z) dependence of tip-enhanced Raman scattering (TERS) has been investigated. The intensities of both, the Raman lines and the broad TERS background, exhibit strong decays with increasing z, which are nearly complete within 10 nm withdrawal of the STM tip in z direction. Interestingly, the maximum of the broad Lorentzian-shaped TER background is substantially blue shifted in energy with z. This effect is ascribed to a corresponding blue shift of the energies of localized plasmon modes upon tip retraction. Both experimental results fit very well data of a simple theoretical near-field model.

Photoluminescence Up-Conversion of Bioconjugated Hybrids on CdTe and Au Nanoparticles

Semiconductor nanomaterials have attracted considerable attention in the design of high efficiency PL up-conversion in heterojunctions or nanostructures at extremely low continuous wave (cw)-excitation intensity. In this study, bioconjugated hybrids were constructed using CdTe and Au nanoparticles (NPs), where two-fold PL enhancement was observed in the solution state. These results are in accordance with theoretical predictions of the local-field effects associated with the combined influence of strong localization of the collective plasmon modes in metallic-semiconducting hybrids and multi-photon absorption into its localized plasmon modes. The feasibility of the nanohybrids as sensors was demonstrated by breaking the bioconjugation through thermal stress, which induced a rapid decrease in luminescence intensity. It is believed that the phenomena is applicable to high-compacted optoelectronic devices and sensing systems that take advantage of both quantum confinement effects and nonlinear optical properties.

Quasi-periodic distribution of plasmon modes in two-dimensional Fibonacci arrays of metal nanoparticles

In this paper we investigate for the first time the near-field optical behavior of two-dimensional Fibonacci plasmonic lattices fabricated by electron-beam lithography on transparent quartz substrates. In particular, by performing near-field optical microscopy measurements and three dimensional Finite Difference Time Domain simulations we demonstrate that near-field coupling of nanoparticle dimers in Fibonacci arrays results in a quasi-periodic lattice of localized nanoparticle plasmons. The possibility to accurately predict the spatial distribution of enhanced localized plasmon modes in quasi-periodic Fibonacci arrays can have a significant impact for the design and fabrication of novel nano-plasmonics devices.