Excitation of Surface Plasmon Polaritons in an Inhomogeneous Graphene-Covered Grating

Plasmonics - Highly confined waves of surface plasmon polaritons (SPPs) in monolayer graphene are efficiently excited using an etched diffractive grating on silicon. In this paper, an inhomogeneous...     

Efficient and Directional Excitation of Surface Plasmon Polaritons by Oblique Incidence on Metallic Ridges

For many years, the search for efficient surface plasmon polariton (SPP) excitation mechanisms has been a recurring matter in the development of compact plasmonic devices. In this work, we excited SPPs illuminating a subwavelength metallic ridge with a focused spot to characterize the coupling efficiency by varying the incidence angle of the excitation beam from ??50 to 50°. The intensity distribution of the excited SPPs was measured using leakage radiation microscopy to determine the relative coupling efficiency in the wavelength interval from 740 to 840 nm. We modeled the excitation efficiency as a function of the incidence angle using a simple analytical diffraction model. Two ridges of different width (200 and 500 nm) were used to compare results and validate the model. The experimental results show a higher coupling efficiency at oblique incidence, where the coupling was enhanced by factors of 2× for the 500-nm-wide ridge, and 3× for the 200-nm-wide ridge, as well as unidirectional SPP excitation. The experimental results are in good agreement with the proposed model.     

An Efficient Large-Area Grating Coupler for Surface Plasmon Polaritons

We report the design, fabrication, and characterization of a periodic grating of shallow rectangular grooves in a metallic film with the goal of maximizing the coupling efficiency of an extended plane wave (PW) of visible or near-infrared light into a single surface plasmon polariton (SPP) mode on a flat metal surface. A PW-to-SPP power conversion factor >45% is demonstrated at a wavelength of 780?nm, which exceeds by an order of magnitude the experimental performance of SPP grating couplers reported to date at any wavelength. Conversion efficiency is maximized by matching the dissipative SPP losses along the grating surface to the local coupling strength. This critical coupling condition is experimentally achieved by tailoring the groove depth and width using a focused ion beam.     

Omnidirectional Surface Plasmon Polaritons Concentration in 3D Metallic Structures

Plasmonics - 3D metallic structures with symmetrically curved surfaces are proposed for surface plasmon polaritons (SPPs) deflection and concentration. Two-photon polymerization (2PP) and a...     

Optimization on Plasmonic Lenses Based on Generation Efficiency of Surface Plasmon Polaritons at Metallic Nanoslit

The generation efficiency of surface plasmon polaritons at metallic nanoslit is theoretically analyzed, and a novel plasmonic lens with two semiannular nanoslits is proposed in this paper. Based on the analysis results, the focusing performance of the proposal is optimized with a maximum field intensity enhancement factor of 7.69 and the full width at half maximum is 132 nm (~0.2λ _i), far beyond theoretical diffraction limit. Meanwhile, some other classical plasmonic lenses are also optimized through improving generation efficiency of surface plasmon polaritons at nanoslit and the focusing performances are consequently greatly enhanced.     

Characterization of Grating Coupled Surface Plasmon Polaritons Using Diffracted Rays Transmittance

A method to sense the excitation of surface plasmon polariton (SPP) on metallic grating device using the transmitted signal will be presented. The grating transmittance signal will be fully characterized varying the light incident angle and azimuthal grating orientation by means of the SPP vector model and rigorous coupled-wave analysis simulation. Simulation results will be compared with experimental measurements obtained with a 635 nm wavelength laser in the transverse magnetic polarization mode. The laser will light grating devices in contact with either air or water through a customized microfluidic chamber. A characterization of the diffracted rays will show the relationship between the grating coupling configuration and the Kretschmann one. In fact, the diffracted ray affected by SPP resonance is transmitted with an output angle which is the same incident angle that should be used to excite SPP in Kretschmann configuration. Lastly, the grating parameters (amplitude and metal thickness) impact on transmittance signal will be analyzed with respect to the order zero reflectance signal.     

Combination of Surface Plasmon Polaritons and Subwavelength Grating for Polarization Beam Splitting

Plasmonics - A polarization beam splitter (PBS) based on the plasmonic subwavelength grating (PSWG) is proposed and investigated. The PBS is composed by a directional coupler with a PSWG as the...     

Directional Excitation of Surface Plasmon Polaritons by Circularly Polarized Vortex Beams

We investigate the excitation of surface plasmon polaritons (SPPs) using a metallic nanoaperture array illuminated by circularly polarized Laguerre-Gaussian (LG) vortex beams. The direction of SPP excitation is tunable by changing the circular polarization and topological charge of LG beams. The left- or right-handed circular polarization determines SPP propagation on either side of the nanoaperture array. Furthermore, varying the topological charge of LG beam will result in beam splitting of SPPs. We also utilize a composite nanoaperture array with different periods to achieve unidirectional excitation of SPPs. The study provides an interesting approach to control the excitation direction of SPPs and may find great applications in SPP generators and optical switches.

     

Simultaneous Excitation of Surface Plasmon Polaritons on Both Interfaces in an Asymmetric Dielectric-Metal Corrugated Structure

The simultaneous excitation of plasmon polaritons on both surfaces of metal film was studied for asymmetric dielectric-metal-dielectric corrugated structures. Due to the small resonant absorption of the incident light on the transmission side of the structure, we investigated the enhancement of the surface plasmon polaritons on the mentioned side by controlling the structure parameters. When the illuminate light changes from normal incidence to non-normal incidence, the resonant absorption peak splits into a doublet. The simultaneous excitation of surface plasmon polaritons on both surfaces of the metal film can be achieved by controlling the incident angle. Since the wave vector matching condition is not satisfied, there is no coupling between the plasmon polaritons modes on the two surfaces of the corrugated metallic film. The excitation and control of the non-coupled surface plasmon polartions simultaneously propagating on the different interfaces of one metallic film have potential applications for designing novel compact and tunable nano-photonic devices at visible frequency.     

Design of a Slit-Groove Coupler for Unidirectional Excitation of the Guided Surface Plasmon Polaritons Through a Plasmonic Slot Waveguide

In this paper, a plasmonic-photonic nanostructure has been introduced for efficient unidirectional coupling of free-space radiation to surface plasmon polariton (SPP) waves under normal illumination on a subwavelength slit. The structure consists of a conventional metallic slit-groove nanostructure integrated with a plasmonic waveguide to support SPP waves along the desired direction with a remarkable lateral confinement. The unidirectional coupling is achieved by using an integrated plasmonic distributed reflector designed under Bragg condition. This reflector basically distributes part of the light coupled through the slit into the SPP modes of the waveguide. Numerical simulations show that up to 26 % of the normally incident light couples to the transversely localized field of the surface plasmon. In addition, the ratio of mode current density of the surface plasmon, launched in the desired direction, to that in the opposite direction can reach about 23 times. This structure shows a 2.5-fold improvement in coupling efficiency relative to a standard slit-groove structure. Also, the transmission distance for the new nanostructure is shown to be more than 8 times greater than that of the standard nanostructure.     

Nanofocusing of Surface Plasmon Polaritons on Metal-Coated Fiber Tip Under Internal Excitation of Radial Vector Beam

We theoretically present the nanofocusing of the metal-coated fiber tip under internal excitation of the radial vector beam within visible band based on the finite difference time domain (FDTD) analysis. The electric field intensity enhancement factor of the localized surface plasmons (LSP) mode at the tip apex is quantitatively shown in relation with incident wavelength, coating material, conical angle of tip, and coating film thickness/length. Specially, the evolution of fiber radial vector mode to surface mode with respect to the radius of metal-coated fiber tip is calculated under typical excitation wavelengths of 633 nm and 785 nm. Furthermore, the reason of the tip eliminating far-field background signal is explained, and the transverse electric field distributions of LSP mode and the tip-substrate coupling are also given at the optimal excitation wavelength. These calculation results will be a good reference for the fabrication of metal-coated fiber tips and for the experimental design of the tip-enhanced spectroscopy (TES) system.

     

Impact of Propagative Surface Plasmon Polaritons on the Electromagnetic Enhancement by Localized Gap Surface Plasmons Between Metallic Nanoparticles and Substrate

Plasmonics - The nanoparticle-on-mirror system as a surface-enhanced Raman scattering substrate is sufficient for single molecule detection and possesses advantages of high reproducibility and ease...     

Plasmonic Notch Filter Design Based on Long-range Surface Plasmon Excitation Along Metal Grating

A single notch plasmonic spectral filter design using evanescently coupled resonant ultrathin metal grating is numerically studied in this article. Due to excitation and coupling of long range surface plasmon between the metal grating nanowires, a deep and narrow reflection spectrum dip can be obtained. Narrower spectral bandwidth is achieved through decreased damping from the existence of large dielectric gaps between the grating nanowires. This physical explanation is confirmed by the field distribution calculation. As an example, a single notch filter design with full width half maximum band width less than 3 nm centered at 808 nm is presented.     

Compact Color Filter and Polarizer of Bilayer Metallic Nanowire Grating Based on Surface Plasmon Resonances

Surface plasmon resonances on bilayer aluminum nanowire gratings are studied in both theory and experiment. It is found that there are two kinds of surface plasmon on the bilayer metallic gating: longitudinal aluminum/dielectric/aluminum slit and lateral aluminum/dielectric interface waveguide mode. The surface plasmon waveguide mode resonance in the slits makes the grating act as a transverse magnetic (TM)-passing polarizer. With the lateral waveguide mode resonance, certain wavelengths of the incident TM light are translated to aluminum/air or aluminum/substrate waveguide light, and the grating acts as a color filter. With both resonances, the bilayer nanowire grating can be a compact-integrated polarizer and color filter.     

The Tunability of Surface Plasmon Polaritons in Graphene Waveguide Structures

The tunability of propagation properties of surface plasmon polariton (SPP) modes in a waveguide formed by two parallel graphene layers separated by a dielectric layer is studied. For this purpose, the dispersion equation of the structure is numerically solved and the effects of applied bias voltage, the role of effective structural parameters, and electron–phonon scattering rate on the propagation of symmetric and antisymmetric SPP waves are investigated. The results of calculations show that considering the electron–phonon scattering rate as a function of Fermi energy and temperature leads to a considerable decrease in the propagation length of SPPs. As the main result of this work, tuning the propagation characteristics of SPPs is possible by varying any of the parameters such as applied voltage, thickness of insulating layer between two graphene layers and permittivities of dielectric layers, and finally the temperature. It is found that antisymmetric mode benefits from a larger propagation length in comparison with that of the symmetric mode.

     

Mode Controlling of Surface Plasmon Polaritons by Geometric Phases

Metasurfaces are made of two-dimensional arrays of subwavelength nanostructures that form a spatially varying optical response, to control the wave fronts of optical waves. As the feature size of its constituent materials is nanoscale, investigation of the light-nanostructure interactions in the near field is critical for understanding the novel properties of metasurfaces. Here, we used a scanning near-field optical microscope (SNOM) to observe the near-field distribution of surface plasmon polaritons (SPPs) from a ring-shaped metasurface under illumination of circularly polarized light. It was found that with an additional degree of freedom of the geometric phase provided by the regularly arranged metamolecules, control over the near-field interference of the SPPs can be achieved, which is governed by the metasurface geometric symmetry that can be tuned by its topological charge. Meanwhile, the planar chiral character of the metamolecules exerts a deep influence on the near-field interference patterns. Our results can pave the way for active control of SPP propagation in near fields and have potential applications in highly integrated optical communication systems.

     

Metasurface Lens for both Surface Plasmon Polaritons and Transmitted Wave

Metasurface lenses which could simultaneously focus both surface plasmon polaritons (SPPs) and transmitted wave are designed. This kind of device is composed of slit antennas and is optimized with the simulated annealing algorithm to realize a single-focus or double-focus lens. Interestingly, the focusing of SPPs is polarization dependent while the focusing of the transmitted wave is immune from the polarization of incident light. The proposed methodology may inspire more designs of device steering both surface wave and transmitted wave.