Multiple Plasmon Modes Research Articles

Mirror-enhanced directional out-coupling of SERS by remote excitation of a nanowire-nanoparticle cavity

We report on the experimental observation of mirror-enhanced directional surface-enhanced Raman scattering (SERS) from a self-assembled monolayer of molecules coupled to a nanowire-nanoparticle (NW-NP) junction on a mirror in a remote excitation configuration. Placing the NW-NP junction on a metallic mirror generates multiple gap plasmon modes that have unique momentum space-scattering signatures. We perform Fourier plane imaging of the SERS from the NW-NP on a mirror to understand the effect of multiple hotspots on molecular emission. We systematically study the effect of the ground plane on the directionality of emission from the NW-NP junction and show that the presence of a mirror drastically reduces the angular spread of emission. The effect of multiple hotspots in the geometry on the directionality of the molecular emission is studied using 3D numerical simulations. The results presented here will have implications in understanding plasmon hybridization in the momentum space and its effects on molecular emission.

Hybrid nanowire-hyperbolic metamaterial based broadband absorber for the visible and near-infrared regions

In this work, spectral absorbance of hybrid nanowire-hyperbolic metamaterial was investigated numerically by using transfer-matrix method. The hybrid nanowire-hyperbolic metamaterial is consisted of two-dimensional periodically arranged arrays of subwavelength-sized hybrid nanowires (with circular cross-section) embedded in silicon dioxide host matrix. Each hybrid nanowire is made by core-shell structure used silicon as a core and gold as a shell. The hybrid nanowire-hyperbolic metamaterial is used as a top layer of a broadband absorber. The top layer is over on a planar silicon dioxide layer, which is backed by a nano-sized layer of gold as a perfect reflector for guaranteeing high absorbance. When electromagnetic waves in the range of 400–1600 nm wavelength are obliquely incident on the top hybrid nanowire-hyperbolic metamaterial surface, the absorption spectra of the designed absorber structure are analyzed for different incident angles, and radii of hybrid nanowire components or fill-factor of the hyperbolic metamaterial layer. The multiple absorption peaks, which are continuously generated from multiple longitudinal and transverse surface plasmon modes and are supported by the hybrid nanowires, are observed directly. The absorption feature can be also tuned by various operational conditions. The proposed structure is made by in-complex nano-fabricating techniques comparing with the another and it provides acceptable broadband absorptivity.

Wavelength-Dependent Photothermal Imaging Probes Nanoscale Temperature Differences among Subdiffraction Coupled Plasmonic Nanorods.

Plasmonic structures confine electromagnetic energy at the nanoscale, resulting in local, inhomogeneous, controllable heating, but reading out the temperature using optical techniques poses a difficult challenge. Here, we report on the optical thermometry of individual gold nanorod trimers that exhibit multiple wavelength-dependent plasmon modes resulting in measurably different local temperature distributions. Specifically, we demonstrate how photothermal microscopy encodes different wavelength-dependent temperature profiles in the asymmetry of the photothermal image point spread function. These asymmetries are interpreted through companion numerical simulations to reveal how thermal gradients within the trimer can be controlled by exciting its hybridized plasmon modes. We also find that plasmon modes that are optically dark can be excited by focused laser beam illumination, providing another route to modify thermal profiles beyond wide-field illumination. Taken together these findings demonstrate an all-optical thermometry technique to actively create and measure nanoscale thermal gradients below the diffraction limit.

Surface-enhanced Raman scattering by the composite structure of Ag NP-multilayer Au films separated by Al2O3.

In the present study, a nanoparticle-multilayer metal film substrate was presented with silver nanoparticles (Ag NPs) assembled on a multilayer gold (Au) film by employing alumina (Al2O3) as a spacer. The SERS performance of the proposed structures was determined. It was suggested that the SERS effect was improved with the increase in the number of layers, which was saturated at four layers. The SERS performance of the structures resulted from the mutual coupling of multiple plasmon modes [localized surface plasmons (LSPs), surface plasmon polaritons (SPPs), as well as bulk plasmon polaritons (BPPs)] generated by the Ag NP-multilayer Au film structure. Furthermore, the electric field distribution of the hybrid system was studied with COMSOL Multiphysics software, which changed in almost consistency with the experimentally achieved results. For this substrate, the limit of detection (LOD) was down to 10-13 M for the rhodamine 6G (R6G), and the proposed SERS substrate was exhibited prominently quantitatively detected capability and high reproducibility. Moreover, a highly sensitive detection was conducted on toluidine blue (TB) molecules. As revealed from the present study, the Ag NP-multilayer Au film structure can act as a dependable SERS substrate for its sensitive molecular sensing applications in the medical field.

Plasmon coupling in topological insulator multilayers

Topological insulators (TIs) house two-dimensional Dirac plasmons on their surfaces. In TI thin films, plasmons on the top and bottom surfaces couple electrostatically, giving rise to an acoustic and an optical plasmon mode. By extension, a superlattice comprising TI layers and trivially insulating layers could house multiple complex plasmon modes which could be used to create a new type of Dirac metamaterial. In this paper, we synthesize TI superlattices, fabricate them into stripe arrays, characterize their optical and plasmonic properties, and model the samples using transfer matrices. We excite plasmon modes that couple to the phonons in the superlattice, resulting in hybrid plasmon-phonon polaritons. These modes are modeled using an analytical Fano resonance model and the extracted resonant positions are reproduced by transfer matrix modeling. We also excite an epsilon near-zero mode in the top dielectric material ${({\mathrm{Bi}}_{0.5}{\mathrm{In}}_{0.5})}_{2}{\mathrm{Se}}_{3}$. Understanding the behavior of the polariton modes in this complex system will lend insight into the many-body interaction in low-dimensional systems.

Nonlinear plasmon-exciton coupling enhances sum-frequency generation from a hybrid metal/semiconductor nanostructure

The integration of metallic plasmonic nanoantennas with quantum emitters can dramatically enhance coherent harmonic generation, often resulting from the coupling of fundamental plasmonic fields to higher-energy, electronic or excitonic transitions of quantum emitters. The ultrafast optical dynamics of such hybrid plasmon–emitter systems have rarely been explored. Here, we study those dynamics by interferometrically probing nonlinear optical emission from individual porous gold nanosponges infiltrated with zinc oxide (ZnO) emitters. Few-femtosecond time-resolved photoelectron emission microscopy reveals multiple long-lived localized plasmonic hot spot modes, at the surface of the randomly disordered nanosponges, that are resonant in a broad spectral range. The locally enhanced plasmonic near-field couples to the ZnO excitons, enhancing sum-frequency generation from individual hot spots and boosting resonant excitonic emission. The quantum pathways of the coupling are uncovered from a two-dimensional spectrum correlating fundamental plasmonic excitations to nonlinearly driven excitonic emissions. Our results offer new opportunities for enhancing and coherently controlling optical nonlinearities by exploiting nonlinear plasmon-quantum emitter coupling.

A multi-wavelength SPASER based on plasmonic tetramer cavity

Surface plasmon (SP) amplification by stimulated emission of radiation (SPASER) as nanoscale coherent light source has great applications in biomedical sensing, imaging, optical communications, and ultrafast spectroscopy. Shrinking coherent light sources to subwavelength regions can provide new insight into light-matter interactions. In this theoretical work, we utilize a multi-resonance plasmonic substrate as a cavity to design a multi-wavelength SPASER. The plasmonic substrate consists of four crisscrossed sub-elements with symmetric disposition, the tetramer can support multiple plasmonic resonance modes. With the appropriate gain layer, the different SPs modes can be stimulated, and SPASER with different wavelengths can be obtained.

Spectral-band replication phenomenon in a single pair of hybrid metal-organic nanospheres and nanodisks caused by plexcitonic coupling.

We study an unusual effect of spectral-band replication in the optical spectra of dimers, consisting of spherical nanoparticles or nanodisks with a silver core and a J-aggregate shell of TDBC-dye. It consists in the emergence of a doubled number of plexcitonic spectral bands compared to the case of a plasmonic dimer and in narrow peaks associated with the resonances of the J-aggregate shell. The plexcitonic bands can be divided into two groups: the "original" bands, accurately reproducing plasmonic peaks, and their "replicas," with a specific mutual arrangement and intensity distributions. The effect is interpreted using the multi-state effective Hamiltonian model describing a strong coupling between the quasi-degenerate Frenkel excitonic modes in the organic shells and multiple plasmonic modes in the pair of Ag-cores. We quantitatively explain some available experimental data on the optical properties of nanodisks and suggest a way for the observation of the replication effect. Our results extend the understanding of the nature of plexcitonic coupling to more complex systems compared to individual metal/J-aggregate nanoparticles.

Silver Nanotowers: Lift-Up Architecture and Plasmonic Applications

Plasmonic cuboidal Ag nanoparticles were grown vertically on a solid substrate by a seed-mediated growth method using Au nanospheres or nanocubes as growth seeds. Each nanocuboid, namely Ag nanotower, is grown at its bottom face, so that the seed is lifted up during the growth. Its size and aspect ratio can be controlled by changing seeds, growth time, and temperature. The nanotower shows different localized surface plasmon resonance (LSPR) modes including distal mode, proximal mode, a higher-order multipole mode, as well as a vertical mode. Higher order resonances tend to contribute to some of those modes for higher nanotowers. Intensity of some resonance modes depends on the incident angle of p-polarized light. Such structure with multiple plasmon modes would be applied to high-density data storage, polarizers, and nanoantenna ensembles for enhancement of Raman scattering, photovoltaics, and photocatalysis.

Enhanced Second Harmonic Generation in Double‐Resonance Colloidal Metasurfaces

AbstractA key challenge for optical circuits is the ability to integrate nonlinear optical signal processing components such as optical modulators and frequency mixers at the chip scale. Optical antennas that focus light into nanoscale volumes can be utilized to shrink the footprint and increase the efficiency of these components. Multiresonant antennas that enhance both optical absorption and emission process are recently demonstrated to enable efficient nonlinear frequency conversion at the nanoscale and are promising as structures for second harmonic generation (SHG) and upconversion. Here, the ability of colloidal metasurfaces fabricated by self‐assembly as on‐chip platforms for enhanced SHG is demonstrated. These metasurfaces exhibit high spatial overlap of multiple surface plasmon modes whose frequencies can be independently tuned through appropriate design of colloidal and metasurface geometries. It is demonstrated that these bottom‐up structures rival lithographic nonlinear optical antennas in SHG efficiency, suggesting the potential for these colloidal metasurfaces in integrated on‐chip architectures.

Scalable and Tunable Periodic Graphene Nanohole Arrays for Mid-Infrared Plasmonics.

Despite its great potential for a wide variety of devices, especially mid-infrared biosensors and photodetectors, graphene plasmonics is still confined to academic research. A major reason is the fact that, so far, expensive and low-throughput lithography techniques are needed to fabricate graphene nanostructures. Here, we report for the first time a detailed experimental study on electrostatically tunable graphene nanohole array surfaces with periods down to 100 nm, showing clear plasmonic response in the range ∼1300-1600 cm-1, which can be fabricated by a scalable nanoimprint technique. Such large area plasmonic nanostructures are suitable for industrial applications, for example, surface-enhanced infrared absorption (SEIRA) sensing, as they combine easy design, extreme field confinement, and the possibility to excite multiple plasmon modes enabling multiband sensing, a feature not readily available in nanoribbons or other localized resonant structures.

Localized Surface Plasmons of Supershape Nanoparticle Dimers

In this work, the all plasmonic bands of supershape nanoparticle dimers were investigated using finite difference time domain simulations. The localized surface plasmons of dimers were extracted from extinction spectra. The effects of different surrounding materials and dimer gaps were reported on dimer plasmons. The obtained results showed that there exist the multiple plasmonic modes in supershape nanoparticle dimers. The electric field distribution of supershape nanoparticle dimers was studied at resonance wavelength of dimer plasmons. We found that formation bonding modes in dimers can be related to creation hot spots in low dimer gap. This study can be a base for the characterization of multiple dimer plasmons in plasmonic devices for sensing applications.

Tri-band absorption enhancement in monolayer graphene in visible spectrum due to multiple plasmon resonances in metal–insulator–metal nanostructure

Suspended graphene has a very low light-absorption efficiency, of approximately 2.3% from the visible to near-infrared region, which is defined by the fine-structure constant. For many applications in optoelectronic devices (e.g., photodetectors), light-absorption enhancement in graphene is desired. We numerically investigate a tri-band absorption enhancement in graphene at optical frequencies, resulting from the excitations of multiple electric and magnetic plasmon modes in a metal–insulator–metal nanostructure. The position of this tri-band absorption enhancement can be easily controlled by tuning the resonance wavelengths of the plasmon modes.

Multiple plasmonic resonance excitations on graphene metamaterials for ultrasensitive terahertz sensing

A three-dimensional graphene metamaterial structure consisting of dielectric pillar arrays covered by graphene monolayer with metal layer underneath is proposed and investigated, which can realize the efficient excitations of multiple plasmonic resonance modes under incident terahertz (THz) wave. The multiple plasmonic resonance modes are highly sensitive to the change of analyte refractive index and thickness, and the frequency sensitivity of each resonance mode can reach up to 1.402, 1.687 and 1.643 THz per refractive index unit, respectively. By controlling the Fermi level of graphene, the resonance frequency can be tuned dynamically. Moreover, the proposed structure has three absorption peaks with absorptivity more than 95%, it is polarization-insensitive and can perform very well in a wide incident angles for transverse electric and transverse magnetic waves. This proposed structure can be utilized for active THz plasmonic devices such as multiple channel sensors and absorbers.

Plasmonic Spectral Splitting in Ring/Rod Metasurface

We report spectral splitting behaviors based on Fano resonances in a novel simple planar metasurface composed of gold nanobars and nanorings. Multiple plasmonic modes and sharp Fano effects are achieved in a broadband transmittance spectrum by exploiting the rotational symmetry of the metasurface. The transmission properties are effectively modified and tuned by modulating the structural parameters. The highest single side Q-factor and FoM which reaches 196 and 105 are observed at Fano resonances. Our proposed design is relatively simple and can be applied for various applications such as multi-wavelength highly sensitive plasmonic sensors, switching, and slow light devices.

Lattice-induced modulators at terahertz frequencies.

We measured the transmission spectra of an array of split-ring resonators (SRRs) up to 10 terahertz for parallel and perpendicular polarizations. Calculations of the lattice and plasmon mode dispersion relations, in combination with electromagnetic simulations, confirm the presence of multiple higher-order lattice and plasmon modes. We modify the quality factor of higher-order plasmon resonances by modulating the lattice-plasmon mode coupling via changes in the period of the array. We also propose single frequency switches and a broadband dual-state amplitude modulator based on structured illumination that actively modifies the period of the SRR array.

Localized surface plasmon resonance properties of Ag nanorod arrays on graphene-coated Au substrate

Localized surface plasmon resonance (LSPR) on silver nanorod (SNR) arrays deposited on a graphene-coated Au substrate is investigated by the discrete dipole approximation (DDA) method. The resonance peaks in the extinction spectra of the SNR/graphene/Au structure show significantly different profiles as SNR height, and refractive index of the surrounding medium are varied gradually. Numerical simulation reveals that the shifts in the resonance peaks arise from hybridization of multiple plasmon modes as a result of coupling between the SNR arrays and graphene-coated Au substrate. Moreover, the LSPR modes blue-shifts from 800 nm to 700 nm when the thickness of the graphene layer in the metal nanoparticle (NP) - graphene hybrid nanostructure increases from 1 nm to 5 nm, which attribute to charge transfer between the graphene layer and SNR arrays. The results provide insights into metal NP-graphene hybrid nanostructures which have potential applications in plasmonics.

Near-field spectral properties of coupled plasmonic nanoparticle arrays

We investigated the grating effect in complex gold dolmen structures, in which multiple plasmon modes are present due to plasmon hybridization, experimentally from both the far field and the near field. In particular, the near-field properties were investigated using photoemission electron microscopy, and it was demonstrated that two hybridized plasmon modes on the dolmen structures could be influenced by the grating effect. For comparison, we also investigated the grating effect in arrays of simple nanoblocks and heptamer structures, which were supposed to support a strong bright plasmon mode and a strong dark plasmon mode, respectively, in the near field. We found that the spectral responses of the two hybridized modes on the dolmen structures as the pitch size changed evolved in a manner similar to that of the bright dipole mode on the nanoblocks, whereas the dark mode on the heptamer structures is less sensitive to the pitch size.

Ultra-broadband Tunable Resonant Light Trapping in a Two-dimensional Randomly Microstructured Plasmonic-photonic Absorber

Recently, techniques involving random patterns have made it possible to control the light trapping of microstructures over broad spectral and angular ranges, which provides a powerful approach for photon management in energy efficiency technologies. Here, we demonstrate a simple method to create a wideband near-unity light absorber by introducing a dense and random pattern of metal-capped monodispersed dielectric microspheres onto an opaque metal film; the absorber works due to the excitation of multiple optical and plasmonic resonant modes. To further expand the absorption bandwidth, two different-sized metal-capped dielectric microspheres were integrated into a densely packed monolayer on a metal back-reflector. This proposed ultra-broadband plasmonic-photonic super absorber demonstrates desirable optical trapping in dielectric region and slight dispersion over a large incident angle range. Without any effort to strictly control the spatial arrangement of the resonant elements, our absorber, which is based on a simple self-assembly process, has the critical merits of high reproducibility and scalability and represents a viable strategy for efficient energy technologies.

Symmetry breaking induced excitations of dark plasmonic modes in multilayer graphene ribbons.

Multilayer graphene can support multiple plasmon bands. If structured into graphene ribbons, they can support multiple localized plasmonic modes with interesting optical properties. However, not all such plasmonic modes can be excited directly due to the constrains of the structural symmetry. We show by numerical simulations that by breaking the symmetry all plasmonic modes can be excited. We discuss the general principles and properties of two-layer graphene ribbons and then extend to multilayer graphene ribbons. In multilayer graphene ribbons with different ribbon widths, a tunable broadband absorption can be attained due to the excitations of all plasmonic modes. Our results suggest that these symmetry-broken multilayer graphene ribbons could offer more degrees of freedom in designing photonic devices.