Symmetric Surface Plasmon Polariton Research Articles

Enhanced second-harmonic generation in monolayer MoS2 on suspended metallic nanostructures by plasmonic resonances

Abstract Monolayer transition metal dichalcogenides (TMDs) possess large second-order nonlinear responses due to the broken inversion symmetry, which can extend their intriguing applications in nonlinear nanophotonics and optoelectronics. However, the atomic thickness of monolayer TMDs severely decreases the interaction length with free light with respect to bulk materials, leading to rather low second-harmonic generation (SHG) conversion efficiency. Here, we demonstrate a hybrid structure consisting of a monolayer MoS2 on a suspended perforated silver film, on which the SHG signal emitted from the monolayer MoS2 is enhanced by more than three orders of magnitude at room temperature. The pronounced SHG enhancement is attributed to the distinct electric field amplification nearby the nanoholes, which is induced by the symmetric surface plasmon polaritons (SPPs) existing in the ultrathin suspended silver grating. Our results reported here may establish the substrate-free engineering of nonlinear optical effects via plasmonic nanostructures on demand.

Cherenkov terahertz radiation from Dirac semimetals surface plasmon polaritons excited by an electron beam**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0701000), the National Key Scientific Instrument and Equipment Development of China (Grant No. 2018YFF01013001), and the National Natural Science Foundation of China (Grant Nos. 61701084 and 61505022).

We demonstrate a physical mechanism for terahertz (THz) generation from surface plasmon polaritons (SPPs). In a structure with a bulk Dirac semimetals (BDSs) film deposited on a dielectric substrate, the energy of the asymmetric SPP mode can be significantly enhanced to cross the light line of the substrate due to the SPP-coupling between the interfaces of the film. Therefore, the SPPs can be immediately transformed into Cherenkov radiation without removing the wavevector mismatch. Additionally, the symmetric SPP mode can also be dramatically lifted to cross the substrate light line when a buffer layer with low permittivity relative to the substrate is introduced. In this case, dual-frequency THz radiation from the two SPP modes can be generated simultaneously. The radiation intensity is significantly enhanced by over two orders due to the field enhancement of the SPPs. The radiation frequency can be tuned in the THz frequency regime by adjusting the beam energy and the chemical potential of the BDSs. Our results could find potential applications in developing room temperature, tunable, coherent, and intense THz radiation sources to cover the entire THz band.

Near-Field Plasmonic Probe with Super Resolution and High Throughput and Signal-to-Noise Ratio

Near-field scanning optical microscopy (NSOM) enables observation of light-matter interaction with a spatial resolution far below the diffraction limit without the need for a vacuum environment. However, modern NSOM techniques remain subject to a few fundamental restrictions. For example, concerning the aperture tip (a-tip), the throughput is extremely low, and the lateral resolution is poor; both are limited by the aperture size. Meanwhile, with regard to the scattering tip (s-tip), the signal-to-noise ratio (SNR) appears to be almost zero; consequently, one cannot directly use the measured data. In this work, we present a plasmonic tip (p-tip) developed by tailoring subwavelength annuli so as to couple internal radial illumination to surface plasmon polaritons (SPPs), resulting in an ultrastrong, superfocused spot. Our p-tip supports both a radial symmetric SPP excitation and a Fabry-Pérot resonance, and experimental results indicate an optical resolution of 10 nm, a topographic resolution of 10 nm, a throughput of 3.28%, and an outstanding SNR of up to 18.2 (nearly free of background). The demonstrated p-tip outperforms state-of-the-art NSOM tips and can be readily employed in near-field optics, nanolithography, tip-enhanced Raman spectroscopy, and other applications.

Plasmonic coupled modes in metal-dielectric multilayer structures: Fano resonance and giant field enhancement.

We provide an overview of Fano resonance and plasmon induced transparency (PIT) as well as on plasmons coupling in planar structures, and we discuss their application in sensing and enhanced spectroscopy. Metal-insulator-metal (MIM) structures, which are known to support symmetric and anti-symmetric surface plasmon polaritons (SPPs) arising from the coupling between two SPPs at the metal-insulator interfaces, exhibit anticrossing behavior of the dispersion relations arising from the coupling of the symmetric SPP and the metal/air SPP. Multilayer structures, formed by a metal film and a high-index dielectric waveguide (WG), separated by a low-index dielectric spacer layer, give narrow resonances of PIT and Fano line shapes. An optimized Fano structure shows a giant field intensity enhancement value of 106 in air at the surface of the high-index dielectric WG. The calculated field enhancement factor and the figure of merit for the sensitivity of the Fano structure in air can be 104 times as large as those of the conventional surface plasmon resonance and WG sensors.

Decay and propagation properties of symmetric surface plasmon polariton mode in metal–insulator–metal waveguide

Decay and propagation properties of symmetric surface plasmon polariton (SPP) mode based on metal–insulator–metal (MIM) waveguide are investigated numerically. SPP mode is excited through a dipole embedded in Al2O3 layer of Au/Al2O3/Au structure. We demonstrate that the distance between the dipole and Al2O3/Au interface is an important tunable parameter to influence the decay properties. The electric/magnetic field intensity horizontal and vertical decay lengths of symmetric SPP mode are 19nm and 24nm, respectively. Moreover, the propagation length along Al2O3/Au interface of symmetric SPP mode depends on Al2O3 layer thickness. The maximal propagation length reaches 0.608μm with Al2O3 layer thickness of 100nm. These values can provide a theoretical reference for designing a high-performance SPP source using Au/Al2O3/Au structure.

Significantly increased surface plasmon polariton mode excitation using a multilayer insulation structure in a metal-insulator-metal plasmonic waveguide.

In this paper, we propose a novel multilayer insulation structure in a metal-insulator-metal (MIM) plasmonic waveguide to explore the possibility of increasing surface plasmon polariton (SPP) mode excitation. Numerical investigations show that the effective refractive index of the multilayer insulation structure affects symmetric SPP mode excitation. The significant enhancement of electric field intensity in horizontal and vertical profiles with a dipole in SiO2 compared with in Al2O3 is observed in the proposed MIM plasmonic waveguides due to a combination of the improved optical density and dipole radiation intensities under a low refractive index. The Au/SiO2/Al2O3/SiO2/Au geometry shows the best enhancement performances, which can serve as an excellent guideline for designing and optimizing a high-performance SPP source using a multilayer insulation structure.

Extraction of optimized parameters for Si0.6Ge0.4 material and SPP mode propagation through Si0.6Ge0.4/Ag/Si0.6Ge0.4 waveguide

The Lorentz model and modified Debye model (MDM) parameters for Si0.6Ge0.4 are presented. A nonlinear optimization algorithm is developed. The obtained parameters are used to determine the complex relative permittivity of Si0.6Ge0.4, and compared with the experimental data for validation. Finally the obtained parameters are used to analyze the properties of symmetric surface plasmon polariton (SPP) mode propagation in a dielectric-metal-dielectric (DMD) material constructed with silver (Ag) and Si0.6Ge0.4 for further verifying the extracted parameters.

Numerical investigation of coupling efficiency between a magnetic line dipole emitter and surface plasmon polariton modes of a nanometer thin metallic film

In this paper we considered a line magnetic dipole emitter near a 25nm thick silver film embedded in a homogeneous dielectric environment. By using a two dimensional magnetic Green's function, the Purcell factor and coupling efficiency factor (β factor) for symmetric and antisymmetric surface plasmon polaritons (SPP) and out of plane (OUP) propagating decay channels were calculated. For a fixed wavelength emission, the effects of source position and refractive index of the dielectric medium on the Purcell factor and β factors are investigated.The numerical results show that, where the Purcell factor is maximum (minimum) the antisymmetric (symmetric) SPP is dominant decay channel and the selective excitation of symmetric SPP is better achievable in the case in which the dielectric medium has the higher refractive index. The wavelength dependency of β factors is also investigated for different source locations. This investigation shows that how the wavelength position for the peak of β factors depends on source location. The results of this paper could have implications for metamaterials research and many lanthanides based photonic devices.

Erratum to: Characteristics of Symmetric Surface Plasmon Polariton Mode in Glass–Metal–Glass Waveguide

Erratum to: Characteristics of Symmetric Surface Plasmon Polariton Mode in Glass–Metal–Glass Waveguide

Characteristics of Symmetric Surface Plasmon Polariton Mode in Glass–Metal–Glass Waveguide

The propagation characteristics of symmetric surface plasmon polariton mode in a glass–metal–glass waveguide are presented. Gallium lanthanum sulfide has been taken as the glass and silver (Ag) has been used as the metal. The analysis has been done both numerically and analytically. A two-dimensional finite-difference time-domain-based simulation model has been developed in order to analyze the propagation characteristics numerically. The obtained results using numerical and analytical methods have been compared and a very good agreement has been found.

Influence of Insulator Layer Width on Propagation Properties of Symmetric Surface Plasmon Polariton Mode in Metal-Insulator-Metal Waveguide

Influence of insulator layer width on propagation properties of symmetric surface plasmon polariton (SPP) mode, which is excited through a dipole embedded in SiO2 layer of Au/SiO2/Au structure in metal-insulator-metal (MIM) waveguide, has been investigated. The symmetric SPP mode has a propagation length along SiO2/Au interface that depends on SiO2 layer width. Its maximal value is 0.61 μm with SiO2 layer width of 100 nm. These values provide a theoretical reference for designing a high-performance SPP source using Au/SiO2/Au structure.

Coupling light into and out from the surface plasmon polaritons of a nanometer-thin metal film with a metal nanostrip

Scattering of light by a metal nanostrip placed near a nanometer-thin metal film in a homogeneous dielectric environment is considered. For a plane wave incident on the geometry from outside we study the scattering cross sections governing the excitation of symmetric and antisymmetric surface plasmon polariton (SPP) waves and out-of-plane (OUP) propagating waves. For a symmetric (long-range) or antisymmetric (short-range) SPP propagating along the metal film and being incident on the nanostrip, we study the fraction of power coupled into forward and backward propagating symmetric and antisymmetric SPP waves, and into OUP propagating waves, respectively. Resonance peaks in the scattering cross sections, or dips in the relative power going into all scattering channels due to resonant absorption, are related to standing-wave resonances of counterpropagating gap SPP waves in the gap between the metal nanostrip and the metal film. The resonances are studied with respect to the width of the metal nanostrip and the size of the gap between the metal film and nanostrip. For a 15-nm-thick film it is found that coupling of light in and out of the symmetric SPP is weak compared with coupling to and from OUP and antisymmetric SPP waves. Improving the in and out coupling of power in the symmetric SPP waves with thicker films (50 nm) and larger scattering objects is considered.

Propagation Properties of Symmetric Surface Plasmon Polaritons Mode in ${\rm Au}/{\rm Al}_{2}{\rm O}_{3}/{\rm Au}$ Waveguide

Propagation properties of symmetric surface plasmon polariton (SPP) mode excited through a dipole embedded in Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer in Au/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /Au waveguide have been investigated. The skin effect of symmetric SPP mode vertical to Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /Au interface is presented. The skin depths in the Au layer and in the intermediate Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer are 12 and 24 nm, respectively. Moreover, the propagation properties of symmetric SPP mode along Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /Au interface are also analyzed. The propagation lengths of electric and magnetic field component are 0.4 and 0.8 μm, respectively. It is expected that this study will provide additional understanding to the coupling between SPP sources and plasmonic waveguides.

Effect of dipole location on profile properties of symmetric surface plasmon polariton mode in Au/Al2O3/Au waveguide

This study uses a dipole embedded in Al2O3 layer to excite a symmetric surface plasmon polariton (SPP) mode in Au/Al2O3/Au waveguide to investigate its profile properties by using finite-difference time-domain (FDTD) method. The excited dipole decay radiatively direct near-field coupling to SPP mode owing to thin Al2O3 layer of 100 nm. The effects of electric and magnetic field intensity profiles and decay length have been considered and characterized. It is found that dipole location is an important factor to influence the horizontal and vertical profile properties of symmetric SPP mode in Au/Al2O3/Au waveguide. The amplitudes of electric and magnetic field intensity and the wavelengths of metal-insulatormetal (MIM) SPP resonance mode can be tuned by varying dipole location. The horizontal and vertical decay lengths are 19 and 24 nm, respectively. It is expected that the Au/Al2O3/Au waveguide structure is very useful for the practical applications of designing a SPP source.

Surface plasmon polaritons assisted transmission in periodic superconducting grating

Transmission properties in periodic superconducting grating, with a dispersive dielectric function governed by system temperature and frequency of incident light, have been numerically studied. Sharp transmittance peaks are observed with a transmission intensity of 100%, which are identified as associated with the formation of symmetric surface plasmon polaritons (SPPs) on the interface between the superconducting grating and the vacuum areas. More than six resonances originating from the SPP assisted transmissions can be sustained by increasing the diameter of the circular superconducting strip to approach the period of the superconducting grating. In addition, there exists a cut-off frequency that is almost independent of the diameter of the superconducting strip. The transmission peaks, as well as the cut-off frequency, are found to be very sensitive to the system temperature, giving rise to wide-ranging tunability of the transmission properties of the superconducting grating.

Theoretical Analysis of Long-Range Dielectric-Loaded Surface Plasmon Polariton Waveguides

A structure for guiding surface plasmon polaritons (SPPs) over millimeter distances with tight mode confinement is presented and analyzed in detail using the finite element method. The proposed long-range plasmonic waveguide consists of a dielectric ridge deposited on a narrow metal stripe supported by a dielectric buffer layer covering a low-index substrate. It is shown that such an asymmetric waveguide structure can be designed to support a long-range symmetric SPP mode, featuring a propagation length of ≈ 3.1 mm and lateral mode width of ≈ 1.6nμ m at telecom wavelengths of ~ 1.55 μm. Our analysis covers a broad spectrum of parameters: ridge dimensions, buffer layer parameters (refractive index and thickness), as well as metal stripe width, considering in detail the underlying mechanisms of SPP waveguiding in this configuration. The suggested configuration offers easy connection to electrodes enabling, e.g., thermo-optic or electro-optic control, and is technologically simple, making fabrication possible using only a few lithography steps. Additionally, a new figure of merit is introduced, which is related to a number of plasmonic components allowed for a given mode confinement and propagation loss, aiming thereby at the evaluation of the application potential of plasmonic waveguides.

Characterization of the terahertz near-field output of parallel-plate waveguides

We experimentally characterize the field confinement properties of various parallel-plate waveguide (PPWG) geometries in the terahertz spectral range. In contrast to infinite-width PPWGs with free-space diffraction along the unshielded direction, finite-width (and also tapered) PPWGs show well-confined THz fields at the output facet. Both the transverse field component, perpendicular to the inside surfaces, and the longitudinal component, parallel to the propagation direction, exhibit strong lateral confinement. We also observe an antisymmetric longitudinal field distribution across the air gap, analogous to the symmetric surface plasmon polariton mode observed in optical slot waveguides.

Near-Field Localization in Plasmonic Superfocusing: A Nanoemitter on a Tip

Focusing light to subwavelength dimensions has been a long-standing desire in optics but has remained challenging, even with new strategies based on near-field effects, polaritons, and metamaterials. The adiabatic propagation of surface plasmon polaritons (SPP) on a conical taper as proposed theoretically has recently emerged as particularly promising to obtain a nanoconfined light source at the tip. Employing grating-coupling of SPPs onto gold tips, we demonstrate plasmonic nanofocusing into a localized excitation of approximately 20 nm in size and investigate its near- and far-field behavior. For cone angles of approximately 10-20 degrees , the breakdown of the adiabatic propagation conditions is found to be localized at or near the apex region with approximately 10 nm radius. Despite an asymmetric side-on SPP excitation, the apex far-field emission with axial polarization characteristics representing a radially symmetric SPP mode in the nanofocus confirms that the conical tip acts as an effective mode filter with only the fundamental radially symmetric TM mode (m = 0) propagating to the apex. We demonstrate the use of these tips as a source for nearly background-free scattering-type scanning near-field optical microscopy (s-SNOM).

Active modulation of plasmonic signal with a subwavelength metal/nonlinear dielectric material/metal structure

Light propagation through a metal/nonlinear dielectric material/metal (M-NL-M) structure is numerically studied. The design parameters of the M-NL-M structure are found with the waveguide theory so that the structure only supports the symmetric surface plasmon polaritons (SPP(0)) mode and the antisymmetric surface plasmon polaritons (SPP(1)) mode. The coupling between the two modes within the M-NL-M structure is exploited. Through controlling the propagation constants of the two modes with the intensity-dependent dielectric constant of the nonlinear Kerr material, an effective all-optical control of plasmonic signal modulator can be realized with this M-NL-M structure.

Enhanced photoluminescence from dye layers embedded in metal–insulator–metal structures

Photoluminescence spectra of a dye layer embedded in a metal–insulator–metal structure are investigated. Peculiar behaviors of the emission peaks observed can be well explained by a photoluminescence mechanism mediated by a symmetric surface plasmon polariton mode supported by the metal–insulator–metal structure. The enhancement in the photoluminescence intensities relative to those of a reference sample is clearly demonstrated. Theoretical analyses suggest that the enhancement in emission process is dominant.