Coupling Of Surface Plasmon Modes Research Articles

Spectrum enhancement of extraordinary terahertz transmission through metal-dielectric multilayer compound annular hole array metamaterial

This study presents, what we believe to be, a novel approach to enhance the spectrum of extraordinary terahertz transmission utilizing surface plasmon polaritons (SPPs) mode coupling in metal-dielectric compound annular hole array metamaterial. The transmitting properties of the structure are thoroughly assessed through a combination of theoretical analysis and numerical simulation, with a particular focus on understanding the surface plasmon mode coupling. Our investigation revealed the presence of propagating surface plasmon polaritons (PSPP), localized surface plasmon resonance (LSPR) and Fabry-Perot (FP) resonances within the compound layer. We find that the coupling of PSPP and LSPR modes plays a crucial role in determining the broadband nature of the structure. Interference theory is utilized to mitigate the FP resonance, while further optimization of the transmission bands is attained by implementing the impedance matching hypothesis. By adjusting the structure parameters, the effective coupling of the SPPs modes and numerous reflections in the dielectric cavity produced a remarkably smooth and transparent character, leading to a significant enhancement of transmission spectrum and a reduction in the incident loss of terahertz wave.

Unified Scattering and Photoluminescence Spectra for Strong Plasmon-Exciton Coupling.

The strong coupling between excitons and single plasmonic nanocavities enables plexcitonic states in nanoscale systems at room temperature. Here we demonstrate the strong coupling of surface plasmon modes of metal nanowires and excitons in monolayer semiconductors, with Rabi splitting manifested in both scattering and photoluminescence (PL) spectra. By utilizing the propagation properties of surface plasmons on the nanowires, the PL emitted through the scattering of plasmon-exciton hybrid modes is extracted. The analytically calculated scattering and PL spectra well reproduce the experimental results. These findings unify the scattering and PL spectra in the plexcitonic system and eliminate the ambiguities of PL emission, shedding new light on understanding the rich spectral phenomena in the plasmon-exciton strong coupling regime.

Electrofluidynamic Patterning of Tailorable Nanostructured Substrates for Surface-Enhanced Raman Scattering.

The unique attributes of surface enhanced Raman spectroscopy (SERS) make it well suited to address the challenges associated with portable diagnostics. However, despite the remarkable progress in this field, where the instrumentation has made great strides forward providing a route to the miniaturization of sensing devices, to date producing three-dimensional low-cost SERS substrates which simultaneously fulfill the multitude of criteria of high sensitivity, reproducibility, tunability, multiplexity, and integratability for rapid sensing has not yet been accomplished. Successful implementation of SERS requires readily fine-tuned nanostructures, which create a high enhancement. Here, an advanced electrofluidynamic patterning (EFDP) technique enables rapid fabrication of SERS active topographic morphologies with high throughput and at a nanoresolution via the spatial and lateral modulation of the dielectric discontinuity due to the high electric field generated across the polymer nanofilm and air gap. The subsequent formation of displacement charges within the nanofilm by coupling to the electric field yield a destabilizing electrostatic pressure and amplification of EFDP instabilities enabling the controllable pattern formation. The top of each gold coated EFDP fabricated pillar generates controllable high SERS enhancement by coupling of surface plasmon modes on top of the pillar, with each nanostructure acting as an individual sensing unit. The absolute enhancement factor depends on the topology as well as the tunable dimensions of the nanostructured units, and these are optimized in the design and engineering of the dedicated EFDP apparatus for reproducible, low-cost fabrication of the three-dimensional nanoarchitectures on macrosurfaces, rendering them for easy integration in further lab-on-a-chip devices. This unique combination of nanomaterials and nanospectroscopic systems lay the platform for a variety of applications in chemical and biological sensing.

Enhancement of extraordinary optical transmission and sensing performance through coupling between metal nanohole and nanoparticle arrays

The extraordinary optical transmission (EOT) of sub-wavelength nanohole array is significantly enhanced through introducing nanoparticles, including nanospheres and nanocylinders, into the centers of the nanoholes. Maxima of 56% and 48% for nanocylinder and nanosphere matrices are achieved, compared with that of 37% for a none-in-hole nanohole array gold film. The mechanism behind the phenomena is discussed, indicating that surface plasmon mode coupling between nanoholes and nanoparticles rather than Fabry–Pérot resonance is the cause for the EOT enhancement. High near-field intensity enhancement also leads to the interaction between analytes and optical field increasing, therefore an improved sensitivity and figure of merit for biosensing. The modified structures are highly promising in practical sensing applications due to the incident angle independence. Similar results are obtained for a hexagonal array of nanohole gold film and nanoparticle modifications thereof.

Coupling of Surface Plasmon Modes and Refractive Index Sensitivity of Hollow Silver Nanoprism

Localized surface plasmon (LSP) modes depend strongly on the morphology of nanoparticle and the surrounding dielectric medium. The hollow nanostructure provides a new way to modulate the surface plasmon modes due to the additional cavity surface. In this work, we study systematically the multipolar surface plasmon modes of hollow silver nanoprism (HSN) by simulation of electron energy loss spectroscopy (EELS) spectra based on the boundary element method (BEM). Herein the effects of the cavity size and position are taken into account. The LSP modes of HSNs are compared with those of perfect silver nanoprism (SN). The red-shift behaviors of multipolar modes can be found as increasing the cavity size. Modes A and C have similar red-shift tendency and obey the plasmon ruler equation, which can be explained by dipole-dipole coupling mode. Meanwhile, the degenerate modes will be split by changing the cavity position, and opposite shift tendencies of split degenerate states are observed. These are caused by different coupling nature of degenerate modes. Moreover, high refractive index sensitivity (RIS) can be obtained for HSN by changing the cavity size and position.

Light emission via surface plasmon modes in metal semi-shell cavity shaped antennas and film coupled structure

The dipole emission into surface plasmon in a semi-shell cavity shaped metal antenna and metal film coupled structure is investigated numerically. The decay rate of the dipole emitter is found to be greatly enhanced at certain wavelength of surface plasmon resonance mode. Noradiative decay shows a near-field (∼10nm) effect, and radiative decay dependence on the emitter-antenna distance is determined by the dipole emitter location. In particular, light emission in metal antenna-metal film coupled structures have shown a further accelerated radiative decay and a directional far-field radiation pattern due to the surface plasmon mode coupling in complex structures.

Surface-Plasmon-Mediated Gradient Force Enhancement and Mechanical State Transitions of Graphene Sheets

Graphene, a two-dimensional material possessing extraordinary properties in electronics as well as mechanics, provides a great platform for various optoelectronic and optomechanical devices. Here, we theoretically study the optical gradient force arising from the coupling of surface plasmon modes on parallel graphene sheets, which can be several orders stronger than that between regular dielectric waveguides. Furthermore, with an energy functional optimization model, possible force-induced deformation of graphene sheets is calculated. We show that the significantly enhanced optical gradient force may lead to mechanical state transitions of graphene sheets, which are accompanied by abrupt changes in reflection and transmission spectra of the system. Our demonstrations illustrate the potential for broader graphene-related applications such as force sensors and actuators.

Hyperbolic metamaterial-based near-field thermophotovoltaic system for hundreds of nanometer vacuum gap.

Artificially designed hyperbolic metamaterial (HMM) possesses extraordinary electromagnetic features different from those of naturally existing materials. In particular, the dispersion relation of waves existing inside the HMM is hyperbolic rather than elliptical; thus, waves that are evanescent in isotropic media become propagating in the HMM. This characteristic of HMMs opens a novel way to spectrally control the near-field thermal radiation in which evanescent waves in the vacuum gap play a critical role. In this paper, we theoretically investigate the performance of a near-field thermophotovoltaic (TPV) energy conversion system in which a W/SiO2-multilayer-based HMM serves as the emitter at 1000 K and InAs works as the TPV cell at 300 K. By carefully designing the thickness of constituent materials of the HMM emitter, the electric power of the near-field TPV devices can be increased by about 6 times at 100-nm vacuum gap as compared to the case of the plain W emitter. Alternatively, in regards to the electric power generation, HMM emitter at experimentally achievable 100-nm vacuum gap performs equivalently to the plain W emitter at 18-nm vacuum gap. We show that the enhancement mechanism of the HMM emitter is due to the coupled surface plasmon modes at multiple metal-dielectric interfaces inside the HMM emitter. With the minority carrier transport model, the optimal p-n junction depth of the TPV cell has also been determined at various vacuum gaps.

Multipole surface plasmons in metallic nanohole arrays

The quasibound electromagnetic modes for the arrays of nanoholes perforated in thin gold film are analyzed both numerically by the rigorous coupled wave analysis (RCWA) method and semianalytically by the coupled mode method. It is shown that when the size of the nanohole occupies a large portion of the unit cell, the surface plasmon polaritons (SPPs) at both sides of the film are combined by the higher order waveguide modes of the holes to produce multipole surface plasmons: coupled surface plasmon modes with multipole texture on the elec-tric field distributions. Further, it is revealed that the multipole texture either enhances or suppresses the couplings between SPPs depending on their diffraction orders and also causes band inversion and reconstruction in the coupled SPP band structure. Due to the multipole nature of the quasibound modes, multiple dark modes coexist to produce a variety of Fano resonance structures on the transmission and reflection spectra.

Surface Plasmon Mode Coupling in Photonic Crystal Fiber Symmetrically Filled with Ag/Au Alloy Wires

The surface plasmon mode coupling in photonic crystal fiber filled with Ag/Au alloy wires into two symmetrical cladding air holes have been numerically investigated using the finite element method. For the cases of pure noble metal silver and gold, they exhibit similar polarization characteristics. The coupling strongly depends on the complex indices of the core mode and surface plasmon polaritons mode. Ag/Au alloy decreases the refractive index of surface plasmon mode and significantly increases the loss at longer wavelength. What should be paid a special attention is that there is a surge in loss of y-polarized direction in the case of very low Au-doped alloy, loss up to 309.14 dB/cm, and the full width half maximum of 9 nm can be achieved. The results match well with the approximate interpretation.

Tunable Microstructured Surface-Enhanced Raman Scattering Substrates via Electrohydrodynamic Lithography

Readily fine-tuned structures are an important requirement for the optimization of surface-enhanced Raman scattering (SERS) to obtain the highest enhancements. Here, a lateral modulation of an electric field applied to a dielectric interface enables the rapid replication of nearly any topographic morphology with micrometer resolution by electrohydrodynamic lithography (EHL). Gold-covered periodic EHL-generated arrays yielded the reproducible enhancement of adsorbed SERS-active molecules. Periodic arrays of micropillars with square and circular cross sections give rise to the effective coupling of surface plasmon modes, which generate enhanced SERS signals. The overall enhancement factors depend on the geometry of the gold-coated structures, and intriguingly, a strong correlation is found with the gap-to-width ratio of the square pillar morphology. A numerical simulation of the EHL-based SERS substrates is consistent with this dependence. The EHL surface architectures can be easily tailored at micrometer-to-submicrometer dimensions, allowing the fabrication of reliably engineered and cost-effective highly sensitive SERS substrates.

Coupled surface plasmon modes of graphene in close proximity to a plasma layer

Coupled surface plasmon modes of graphene on a plasma layer are studied. Three coupled plasmon modes are found, and analytical equations for their dispersion characteristics are given in the long-wavelength limit (q→0). Symmetric and antisymmetric modes are identified; between two strongly coupled modes, the antisymmetric mode is found to be more energetic than the symmetric mode. The great tunability of plasmon characteristics is shown by changing the surrounding dielectric constant, the coupling distance, and the plasma frequency of the substrate.

Radiative and Nonradiative Properties of Single Plasmonic Nanoparticles and Their Assemblies

A surface plasmon is the coherent oscillation of the conduction band electrons. When a metal nanoparticle is excited to produce surface plasmons, incident light is both scattered and absorbed, giving rise to brilliant colors. One available technique for measuring these processes, ensemble extinction spectroscopy, only measures the sum of scattering and absorption. Although the spectral responses of these processes are closely related, their relative efficiencies can differ significantly as a function of nanoparticle size and shape. For some applications, researchers may need techniques that can quantitatively measure absorption or scattering alone. Through advances in single particle spectroscopy, researchers can overcome this problem, separately determining the radiative (elastic and inelastic scattering) and nonradiative (absorption) properties of surface plasmons. Furthermore, because we can use the same sample preparation for both single particle spectroscopy measurements and electron microscopy, this technique provides detailed structural information and a direct correlation between optical properties and nanostructure morphology. In this Account, we present our quantitative investigations of both radiative (scattering and one-photon luminescence) and nonradiative (absorption) properties of the same individual plasmonic nanostructures employing different single particle spectroscopy techniques. In particular, we have used a combined setup to study the same structure with dark-field scattering spectroscopy, photothermal heterodyne imaging, confocal luminescence microscopy, and scanning electron microscopy. While Mie theory thoroughly describes the overall size dependence of scattering and absorption for nanospheres, our real samples deviate significantly from the predicted trend: their particle shape is not perfectly spherical, especially when supported on a substrate. Because of the high excitation rate in laser based single particle measurements, we can efficiently detect one-photon luminescence despite a low quantum yield. For gold nanoparticles, the luminescence spectrum follows the scattering response, and therefore we assigned it to the emission of a plasmon. Due to strong near-field interactions the plasmonic response of closely spaced nanoparticles deviates significantly from that of the constituent nanoparticles. This response arises from coupled surface plasmon modes that combine those of the individual nanoparticles. Our correlated structural and optical imaging strategy is especially powerful for understanding these collective modes and their dependence on the assembly geometry.

Cadmium Sulfide Silver Nanoplate Hybrid Structure: Synthesis and Fluorescence Enhancement

Semiconductor–noble metal hybrid nanostructures are of current interest in the development of bioimaging agents. To enhance photoluminescence of cadmium sulfide nanoparticles via surface plasmon mode coupling, cadmium sulfide conjugated silver nanoplates (denoted as CdS-Ag NP) have been prepared by a forward–reverse cation exchange method. The morphology, crystallinity, and atomic composition of the CdS-Ag NP have been investigated by energy dispersive X-ray spectroscopy and high-resolution transmission electron spectroscopy, and the CdS-Ag NP can be described as Cd2+-rich CdS nanoparticles covalently bonded to the surfactant of Ag nanoplates. Photoluminescence enhancement of CdS measured is attributed to the matching between the emission bands of cadmium sulfide and the tailor-made localized surface plasmons resonance bands provided by the Ag NP. Application of the CdS-Ag NP in cell imaging has also been demonstrated.

Tunable hybridization in metal nanoshell chains

We have studied the coupled surface plasmon (SP) modes in periodic metal nanoshellchains by including long range electromagnetic interactions. The eigen-decompositionmethod is used to analyze the dispersion and dissipation of the SP modes. The resultingband structure can be understood as a hybridization between a hole band and a particleband with a structurally tunable band gap in the middle of the first Brillouin zone. Themode quality, which is defined as the imaginary part of the generalized polarizability,increases as the shell thickness decreases. This indicates a larger energy loss and anincreasing coupling between the bands. Through the manipulation of the band structures,the propagation of the coupled SP modes in the nanoshell chain can be controlled.

Sensitivity enhanced all-optical switching using prism-grating coupled surface plasmon modes

Abstract A sensitivity enhanced all-optical-switch design using prism-sinusoidal grating coupled surface plasmon modes is demonstrated numerically. Owing to the simultaneous excitation of surface plasmons for the pump and signal at the interface between a nonlinear waveguide layer and a metal layer, a sensitivity enhanced bistability effect in the structure can be obtained. The nonlinear refractive index of the waveguide layer experiences an abrupt change of the local pump field owing to this bistability effect. Such a characteristic greatly improves the performance of optical switch by reducing the threshold pump intensity. An all-optical-switch design with a pump threshold as low as 0.9 GW/cm2 is presented.

A disposable polymer sensor chip combined with micro-fluidics and surface plasmon read-out

This paper describes a new type of disposable polymeric sensor chip based on the grating coupling of surface plasmon modes combined with a micro-fluidic channel system. A specifically designed silicon stamp with nano-structure (grating) on the micro-structures (micro-channel) was fabricated by combining a holographic method and photolithography. By using such a stamp the micro-channels, the grating coupler and the gold which was first thermally evaporated onto the stamp were transferred to the polymeric substrate successfully in one step. It is demonstrated that the grating profile in the micro-channels allowed a very efficient coupling of the laser light to the surface plasmons propagating at the bottom of the micro-channels. The transferred gold exhibits properties of a freshly cleaned surface, and the self-assembly of a functional thiol derivative (mercapto-PEG) onto the sensor chip can be monitored by surface plasmon spectroscopy. The results obtained in this sensor chip show no difference from those obtained on a regular grating-coupled SPR sensor chip.

Surface plasmons on gratings: Coupling in the minigap regions

We present results of an experimental study of grating coupling to surface plasmons in minigap regions. These results are interpreted, qualitatively, in terms of grating coupling to coupled surface plasmon modes. The observed minigap sizes and the cancellations of the coupling oscillator strengths are related to the coupled mode matrix elements.

Light scattering study of surface plasmon resonances in very thin silver films

The surface plasmon modes of Ag thin films were studied by the light scattering method in the thickness range down to 100 Å. The observed thickness dependence of the resonance peaks was analysed in detail by the theory of Kretschmann. It is shown that the coupled surface plasmon modes in the very thin films can be detected only in modified forms by the light scattering experiment.

Light Emission from Inelastic Electron Tunneling

We report the discovery of a new method for the generation of light. This technique yields a broad-band light source with a high frequency linear cutoff which is dependent only upon the applied voltage through the quantum relation $h{\ensuremath{\nu}}_{\mathrm{co}}=|\mathrm{eV}|$. The light source consists of a metal-insulator-metal tunneling junction. The effect can be interpreted in terms of inelastic tunneling excitation of optically coupled surface plasmon modes.