Metal-dielectric Interface Research Articles

Enhanced in-plane and out-of-plane photonic spin Hall effect via surface plasmon resonance

In this paper, the impacts of surface plasmon resonance on the photonic spin Hall effect (PSHE), both in-plane (parallel to the plane of incidence) and out-of-plane (perpendicular to the plane of incidence), are investigated theoretically. The expressions of the spatial and angular shifts of the in-plane and out-of-plane spin splitting for the p-polarized Gaussian beam reflected from dielectric-metal film interface are derived. These shifts under different metal film thicknesses are calculated as a function of the angle of incidence. The simulation results reveal that both of the in-plane and out-of-plane PSHE are significantly enhanced when surface plasmons are strongly excited. The largest spatial in-plane photonic spin splitting (IPPSS) shift can reach 13.499 μm under the optimal parameter conditions. It is almost up to its upper limitation (half of the waist of the incident beam) and larger than the values reported previously. The largest angular IPPSS shift can be up to 2.462 × 10−3 rad that is almost equal to half of the divergence angle of the incident beam in our system. It is also found that the directions of spin accumulation and spatial propagation of photons in both in-plane and out-of-plane can be switched by slightly adjusting the angle of incidence or film thickness under certain conditions. The findings may provide a new way for photon manipulation and open another possibility for the development of new nanophotonic devices.

Spin and momentum of the light fields in inhomogeneous dispersive media with application to surface plasmon-polariton waves

Following to the recently published approach [Phys. Rev. Lett. 119, 073901 (2017); New J. Phys., 123014 (2017)], we refine and accomplish the general scheme for the unified description of the momentum and angular momentum in complex media. The equations for the canonical (orbital) and spin linear momenta, orbital and spin angular momenta in a lossless inhomogeneous dispersive medium are presented in the compact form analogous to the Brillouin's relation for the energy. The results are applied to the surface plasmon-polariton (SPP) field, and the microscopic calculations support the phenomenological expectations. The refined general scheme correctly describes the unusual SPP properties (transverse spin, magnetization momentum) and additionally predicts the singular momentum contribution sharply localized at the metal-dielectric interface, which is confirmed by the microscopic analysis. The results can be useful in optical systems employing the structured light, especially for microoptics, plasmophotonics, optical sorting and micromanipulation.

Analytical Investigation of In-plane Focusing Surface Plasmon Modes by a Dielectric Lens

A dielectric lens placed on a metal-dielectric interface can be used for in-plane focusing of surface plasmon (SP) modes. The propagation behind the lens is usually studied using scalar diffraction theory which is not accurate in nanometer regime and cannot explain the propagation and energy flow of SP modes. Here, we have used vectorial derivation of Huygens- Fresnel principal to calculate the diffracted fields behind the lens in order to study the energy flow of SP modes in a focusing system using the concept of higher order modes.

Refractive index sensor and filter of metal-insulator-metal waveguide based on ring resonator embedded by cross structure

Continuous improvement in nanofabrication and nano-characterization capabilities have changed projections about the role that metals could play in developing the new optical devices. Surface plasmon polaritons are evanescent waves that propagate along a metal-dielectric interface. They can be laterally confined below the diffraction limit by using subwavelength metal structures, rendering them attractive to the development of miniaturized optical devices. A surface plasmon polariton refractive index sensor and filter which consist of two metal-insulator-metal (MIM) waveguides coupled to each other by a ring resonator embedded by cross structure are proposed. And the transmission characteristics of surface plasmon polaritons are studied in our proposed structure. The transmission properties of such a structure are simulated by the finite element method, and the eigenvalue wavelengths of the ring resonator are calculated theoretically. The sensing characteristics of such a structure are systematically analyzed by investigating the transmission spectrum. The results show that there are three resonance peaks in the transmission spectrum, that is, three resonance modes corresponding to the eigenvalue solutions of the first, second and third-order Bessel eigen-function equations, and each of which has a linear relationship with the refractive index of the material under sensing. Through the optimization of structural parameters, we achieve a theoretical value of the refractive index sensitivity (S) as high as 1500 nm/RIU, and the corresponding sensing resolution is 1.3310-4 RIU. More importantly, it is sensitive to none of the parameters of our proposed structure, which means that the sensitivity of the sensor is immune to the fabrication deviation. In addition, by the resonant theory of ring resonator, we find a linear relationship between the resonance wavelength and the radius of ring resonator. So the resonance wavelength can be easily manipulated by adjusting the radius and refractive index. In addition, the positions of transmission peaks can be easily modulated by changing the radius of the ring, which can be used to design band-pass filter for a large wavelength range. Moreover, the transmission intensity and the transmission bandwidth decrease as spacing distance between the MIM waveguide and ring cavity increases. These results would be helpful in designing the refractive index sensor of high-sensitivity and band-pass filters, and have guiding significance for biological sensor applications.

Design and Simulation of a New Coaxial Probe for Near Field Scanning Optical Microscopy

Design of the new photonic devices having attractive optical properties such as subwavelength confinement of light has a numerous applications in developing nanoscale sensors, photonic integrated circuits, solar cells and etc. Abbe’s diffraction limit restricts the use of subwavelength scale photonic elements due to their size and therefore new approaches are required in order to achieve desirable nanometer scale properties of these devices. Delivering light energy to nanometer scale regions can be achieved by surface plasmon polariton (SPP) waves that propagate in metal-dielectric interface. The paper presents results in FDTD (finite-difference time domain) simulation of a new coaxial probe of nanometer-scale size.Based on FDTD simulations of the coaxial probe field localization and enhancement in the region around the tip of coaxial probe has been obtained. Configuration of the probe consists of the truncated Au cone as a core, truncated dielectric (SiO2) cone in the middle and outer metallic cladding. Simulations of the probe revealed that increasing the radius of Au core structure could lead to the enhanced field around the tip. Additionally, increasing radius of outer cladding layer doesn’t affect field enhancement around the tip. Different configurations of the probe have been analyzed in the range of visible spectrum of the source and as a result the most promising configuration has been selected

Adiabatic description of superfocusing of femtosecond plasmon polaritons

A surface plasmon polariton is a collective oscillation of free electrons at a metal–dielectric interface. As wave phenomena, surface plasmon polaritons can be focused with the use of an appropriate excitation geometry of metal structures. In the adiabatic approximation, we demonstrate a possibility to control nanoscale short pulse superfocusing based on generation of a radially polarized surface plasmon polariton mode of a conical metal needle in view of wave reflection. The results of numerical simulations of femtosecond pulse propagation along a nanoneedle are discussed. The space–time evolution of a pulse for the near field strongly depends on a linear chirp of an initial laser pulse, which can partially compensate wave dispersion. The field distribution is calculated for different metals, chirp parameters, cone opening angles and propagation distances. The electric field near a sharp tip is described as a field of a fictitious time-dependent electric dipole located at the tip apex.

Controllable multiple plasmonic bending beams via polarization of incident waves.

Plasmonic bending beams, which preserve their spatial shapes while propagating along curved trajectories in metal-dielectric interface, offer important applications in the fields of fiber sensor, optical trapping, and micro-nano manipulation. In this work, circular hole array, as a local point-like sources of surface plasmon polaritons, is designed on the metal film to generate multiple plasmonic bending beams. The electric field intensity of multiple plasmonic bending beams is controlled by polarization angle of input light. In addition, the electric filed intensity of multiple plasmonic bending beams relies on circle hole radius. These findings provide guidance in the design and optimization of plasmonic bending beam generators.

Strong Light–Matter Interaction in Quantum Emitter/Metal Hybrid Nanostructures

Surface plasmon polaritons (SPPs) are spatially confined electromagnetic field modes at a metal-dielectric interface capable of generating intense near-field optical forces on ultrafast time scales. Within the field of photonics, SPPs carry significant potential for guiding and manipulating light on the nanoscale. The intense SPP fields substantially enhance light–matter interactions with quantum emitters (QEs). Thus, hybrid systems comprised of SPP resonators and various types of QEs constitute key components of the modern photonics applications. Recent advances in nanotechnology have enabled fabrication of high quality QE/metal hybrid nanostructures, in which several aspects of light–matter interactions, including those in the quantum regime have been demonstrated and extensively studied. The present Perspective explores the central phenomenon in light–matter interaction in emitter/metal hybrid nanostructures, namely, the strong dipole coupling between QEs and SPPs, particularly between excitons (Xs) an...

Superanomalous Skin Effect for Surface Plasmon Polaritons.

It is commonly assumed that surface plasmon-polariton (SPP) excitations on a metal-dielectric interface decay exponentially inside the metallic sample. Here, we show that in a wide spectral interval the SPP field decays much slower, being inversely proportional to the distance to the interface modified by an additional logarithmic factor. This dependence differs from the standard anomalous skin effect and is provisionally referred to as superanomalous. Its origin is the nonlocality and the logarithmic singularity of the dielectric permittivity in metals. This type of decay is pronounced for SPP modes of higher frequencies, but it is suppressed for light waves.

Flat top surface plasmon polariton beams.

Surface plasmon polaritons (SPPs) have emerged as powerful tools for guiding and manipulating light below the diffraction limit. In this context, the availability of flat top SPP beams displaying a constant transversal profile can allow for uniform excitation and coupling scenarios, thus opening the door to developing novel applications that cannot be achieved using conventional Gaussian SPP beams. Here, we present a rigorous theoretical description of flat top SPP beams propagating along flat metal-dielectric interfaces. This is accomplished through the use of Hermite-Gaussian SPP modes that constitute a complete basis set for the solutions of Maxwell's equations for a metal-dielectric interface in the paraxial approximation. We provide a comprehensive analysis of the evolution of the transversal profiles of these beams as they propagate, which is complemented with the study of the width and kurtosis parameters. Our results serve to enlarge the capabilities of surface plasmon polaritons to control and manipulate light below the diffraction limit.

Metal-dielectric waveguide structure supporting an evanescent wave with long propagation distance in an aqueous environment

A metal-dielectric waveguide (MDW) structure working in an aqueous environment was investigated. The energy of the MDW mode is primarily confined near the interface between two dielectric layers, therefore significantly lowering the mode loss comparing with a conventional surface plasmon polariton (SPP), whose energy is concentrated to a metal-dielectric interface. The transfer matrix method was used to analyze the mode characteristics of the MDW structure, showing good agreement with the results obtained by directly solving Maxwell’s equations. Further experiments revealed that the propagation length of evanescent wave accompanying with the MDW mode is approximately one-order of magnitude larger than the SPP mode. This MDW structure in an aqueous environment is expected to be able to overcome the limitations associated with the traditional SPP modes, and to have significant potential for bio-sensing and imaging applications.

Detection of guided-wave plasmon polariton modes in a high-index dielectric MIM structure

Surface plasmon polaritons (SPPs) are surface charge density oscillations localized to a metal-dielectric interface. In addition to being considered as promising candidates for a variety of applications, structures that support SPPs, including metal-insulator-metal (MIM) multilayers, are of fundamental interest because of the variety of collective plasmonic modes they support. Previously, a particular class of “forbidden” plasmon polariton modes (PPMs) was proposed that includes plasmon polariton modes confined to a region of dispersion space not typically accessible to surface-constructed collective excitations. Specifically, for these modes, known as Guided Wave PPMs (GW-PPMs), due to the dielectric asymmetry of the central layer, the solution to the wave equation in the center insulator layer is oscillatory while remaining surface bound both to the supporting substrate and the exposed surface. These modes are supported by a simple physical structure that results from a minor symmetry modification of the traditional MIM structure, specifically the use of a central insulator layer with a higher refractive index than the supporting substrate. However, they display fundamental properties that are distinctly different from those of standard SPPs and from recently reported hybrid plasmonic modes. While GW-PPMs have been explored theoretically, they have not yet been realized experimentally. In this article, we present the first experimental demonstration of GW-PPMs. Specifically, we excite and detect GW-PPMs at visible frequencies and match model predictions to experimental results with remarkable accuracy using minimal parameter fitting. In addition to the experimental detection, we calculate and report on other interesting and relevant features of the detected modes, including the associated electric field profiles, confinement values, and propagation lengths, and discuss in terms of the applications-relevance of GW-PPMs.

Label-free and real-time monitoring of single cell attachment on template-stripped plasmonic nano-holes

Leveraging microfluidics and nano-plasmonics, we present in this paper a new method employing a micro-nano-device that is capable of monitoring the dynamic cell-substrate attachment process at single cell level in real time without labeling. The micro-nano-device essentially has a gold thin film as the substrate perforated with periodic, near-cm2-area, template-stripped nano-holes, which generate plasmonic extraordinary optical transmission (EOT) with a high sensitivity to refractive index changes at the metal-dielectric interface. Using this device, we successfully demonstrated label-free and real-time monitoring of the dynamic cell attachment process for single mouse embryonic stem cell (C3H10) and human tumor cell (HeLa) by collecting EOT spectrum data during 3-hour on-chip culture. We further collected the EOT spectral shift data at the start and end points of measurement during 3-hour on-chip culture for 50 C3H10 and 50 HeLa cells, respectively. The experiment results show that the single cell attachment process of both HeLa and C3H10 cells follow the logistic retarded growth model, but with different kinetic parameters. Variations in spectral shift during the same culture period across single cells present new evidence for cell heterogeneity. The micro-nano-device provides a new, label-free, real-time, and sensitive, platform to investigate the cell adhesion kinetics at single cell level.

Polarization-controlled asymmetric excitation of surface plasmons

Free-space light can be coupled into propagating surface waves at a metal–dielectric interface, known as surface plasmons (SPs). This process has traditionally faced challenges in preserving the incident polarization information and controlling the directionality of the excited SPs. The recently reported polarization-controlled asymmetric excitation of SPs in metasurfaces has attracted much attention for its promise in developing innovative plasmonic devices. However, the unit elements in these works were purposely designed in certain orthogonal polarizations, i.e., linear or circular polarizations, resulting in limited two-level polarization controllability. Here, we introduce a coupled-mode theory to overcome this limit. We demonstrated theoretically and experimentally that, by utilizing the coupling effect between a pair of split-ring-shaped slit resonators, exotic asymmetric excitation of SPs can be obtained under the x-, y-, left-handed circular, and right-handed circular polarization incidences, while the polarization information of the incident light can be preserved in the excited SPs. The versatility of the presented design scheme would offer opportunities for polarization sensing and polarization-controlled plasmonic devices.

Unidirectional Excitation of Radiative-Loss-Free Surface Plasmon Polaritons in PT-Symmetric Systems.

We investigate the excitation and propagation of surface plasmon polaritons (SPPs) at a geometrically flat metal-dielectric interface with a parity-time (PT) symmetric modulation on the permittivity ϵ(x) of the dielectric medium. We show that two striking effects can be simultaneously achieved thanks to the nonreciprocal nature of the Bloch modes in the system. First, SPPs can be unidirectionally excited when light is normally incident on the interface. Secondly, the backscattering of SPPs into the far field is suppressed, producing a radiative-loss-free effect on the unidirectional SPPs. As a result, the lifetime and propagation distance of SPPs can be significantly improved. These results show that PT symmetry can be employed as a new approach to designing transformative nanoscale optical devices, such as low-loss plasmonic routers and isolators for efficient optical computation, communication, and information processing.

Vectorial analysis of a surface plasmon mode diffracted by a thick dielectric lens

Using the vectorial form of the Huygens–Fresnel principle, we have studied the diffraction of a surface plasmon mode propagating on a metal–dielectric interface as it passes through a dielectric thick lens. This method, unlike the scalar diffraction theory, provides the interference and focusing patterns of the surface mode in the near field. Also, we have calculated the phase delay of the surface plasmon mode behind the lens without using the thin lens assumption. Based on the developed equations, we have analyzed the effects of lens thickness and aperture size on in-plane focusing of the surface plasmon.

Revisiting the boundary conditions for second-harmonic generation at metal-dielectric interfaces

We study second-harmonic generation (SHG) arising from surface nonlinearity at a metal-dielectric interface using a spectral decomposition method. Since our method avoids the need to consider the generalized boundary condition across the metal-dielectric interface in the presence of a perpendicular surface source, we retrieve the known discontinuity of the tangential component of the electric field ($E_{\parallel}^ {2\omega}$) for a general geometry, based on a purely mathematical argument. Further, we reaffirm the standard convention of the implementation of this condition, namely, that the surface dipole source radiates as if placed outside the metal surface for arbitrary geometries. We also study and explain the spectral dependence of the discontinuity of the tangential component of the electric field at second harmonic. Finally, we note that the default settings of the commercial numerical package COMSOL Multiphysics fail to account for the $E_{\parallel}^ {2\omega}$-discontinuity. We provide a simple recipe that corrects the boundary condition within these existing settings.

Plasmonic metamaterial-based chemical converted graphene/TiO2/Ag thin films by a simple spray pyrolysis technique

Graphene based hybrid nanostructures have received special attention in both the scientific and technological development due to their unique physicochemical behavior, which make them attractive in various applications such as, batteries, supercapacitors, fuel cells, solar cells, photovoltaic devices and bio-sensors. In the present study, the role of plasmonic metamaterials in light trapping photovoltaics for inorganic semiconducting materials by a simple and low cost spray pyrolysis technique has been studied. The plasmonic metamaterials thin film has been fabricated by depositing chemically converted graphene (CCG) onto TiO2-Ag nanoparticles which has a low resistivity and a low electron–hole recombination probability. The localized surface plasmon resonance at the metal-dielectric interface for the Ag nanoparticles has been observed at 403nm after depositing chemical converted graphene (CCG) on the TiO2-Ag thin film. The results suggest that the stacking order of the CCG/TiO2/Ag plasmonic metamaterials samples did not change the band gap of TiO2 while it changed the conductivity of the film. Thus the diffusion of the noble metals in the glass and TiO2 matrices based thin films can trap the light of a particular wavelength by mean of plasmonic resonance and may be useful for superior photovoltaic and optoelectronic applications.

Domain Decomposition CN-FDTD Method for Analyzing Dispersive Metallic Gratings

In this paper, an efficient domain decomposition (DD) technique is originally introduced into the unconditionally stable Crank–Nicolson finite-difference time-domain (CN-FDTD) method for analyzing the extraordinary optical transmission (EOT) phenomenon of periodic metallic gratings. Being caused by the evanescent waves propagating along the metal-dielectric interface in the visible and near infrared regions, the dispersion of the considered metal in this case is expressed by the Drude model and solved with a generalized auxiliary differential equation (ADE) technique. The periodic boundary condition is applied to the two-dimensional periodic metallic grating structures due to their periodicity. Then, the standard unsplit-field perfectly matched layer is derived as the absorbing boundary condition for CN-FDTD based on the ADE concept. In DD structures, the whole computational domain is divided into several subdomains to reduce the matrix size and save calculating time. Furthermore, the reverse Cuthill–Mckee scheme for the lower–upper decomposition is applied to the subdomain matrices individually to improve the efficiency of DD-CN-FDTD. Finally, two numerical examples are calculated and the physical mechanism of the EOT phenomenon is investigated. The results from the proposed method demonstrate its accuracy and efficiency for the nanostructures.

Localized optical states in a liquid-crystal structure adjacent to a metal

The spectrum of light transmission through a structure consisting of a metallic layer and a cholesteric liquid crystal with an induced planar defect is calculated. The possibility of existence of localized states at the metal–dielectric interface of this system is demonstrated. The transmission spectra at different positions of the defect in the structure are studied.