Transverse-magnetic Surface Plasmon Polariton Research Articles

Conditions for photonic bandgaps in two-dimensional materials

Conditions that define the spectral location of bandgaps in the quasi-transverse magnetic surface plasmon polariton modal dispersion for 2D/quasi-2D materials with a tensor response function, embedded in a simple isotropic medium, are obtained. In the isotropic case, transverse magnetic surface plasmon polariton modes propagate if the surface conductivity is inductive. However, in the anisotropic case considered here, we find that quasi-transverse magnetic modes are supported by surfaces with an inductive effective conductivity seen by the wave along the direction of propagation (written as a weighted sum of the diagonal elements). Examples of natural anisotropic 2D/quasi-2D materials are presented to demonstrate the effectiveness of the method.

Plasmonic all-optical switching based on metamaterial/metal waveguides with local nonlinearity

We present a plasmonic all-optical switch based on Mach–Zehnder interferometer (MZI) with local nonlinearity. The design of the miniaturized all-optical switch is possible by employing surface-plasmon polaritons (SPPs) that confine the energy of electromagnetic (EM) waves at sub-wavelength scale. The coupling between sub-wavelength plasmonic waveguides in MZI and a control beam generates ‘ON’ and ‘OFF’ states at the switch and switching between ‘ON’ and ‘OFF’ states happens when the control beam is turned on or off. The waveguides cladding in the switch structure is made of lossy media including metamaterials with positive and negative EM susceptibilities and metals; the core consists of nonlinear and dielectric media. Employing materials with negative EM susceptibilities in the switch structure facilitates the propagation of both transverse electric and transverse magnetic SPPs along the waveguides. Our all-optical switch design enables multi-frequency switching with low-intensity control field. Ascertaining the capabilities of multi-frequency plasmonic all-optical switches facilities their applications in miniaturized photonic circuits and in biosensors.

Equivalent Voltage and Current Waves for TM SPP in Graphene Sheets

We present a theoretical model to describe the propagation of a transverse magnetic surface plasmon polariton in graphene based on equivalent voltage and charge current waves, which includes the spatial dispersion effect. Electrons and holes in graphene are governed by the Boltzmann equation in the particle conserving relaxation time approximation. First, we deduce expressions for the non-equilibrium distributions when there are charge oscillations in graphene as a response to the electromagnetic field applied to it. These distribution functions are used in the Boltzmann equations to derive other equations for the following four local macroscopic averages: the oscillating electron and hole densities, and the electron and hole current densities. Then, for a specific structure, we solve the wave equations for the electric and vector magnetic potentials to obtain the relations between the charge oscillations and the potentials. So, we reach a homogeneous system of four coupled equations relating the amplitudes of the voltage and the current waves. The non-trivial solutions of the system allow us to compute the dispersion and loss curves for such waves. As it is already known, for a given frequency, we can see that the higher the Fermi level is, the lesser the spatial-dispersion effect is. Following the analysis, a distributed-element circuit for the equivalent transmission line in which would propagate the waves, is developed. Finally, we analyze the dependence of these circuit elements and the impedance on both the frequency and Fermi level.

Quantum plasmonic excitation in graphene and loss-insensitive propagation

We investigate the excitation of quantum plasmonic states of light in graphene using end-fire and prism coupling. In order to model the excitation process quantum mechanically, we quantize the transverse-electric and transverse-magnetic surface plasmon polariton (SPP) modes in graphene. A selection of regimes are then studied that enable the excitation of SPPs by photons and we show that efficient coupling of photons to graphene SPPs is possible at the quantum level. Furthermore, we study the excitation of quantum states and their propagation under the effects of loss induced from the electronic degrees of freedom in the graphene. Here we investigate whether it is possible to protect quantum information using quantum-error correction techniques. We find that these techniques provide a robust-to-loss method for transferring quantum states of light in graphene over large distances.

Complete dispersion relations of surface plasmon polaritons at a metal and birefringent dielectric interface

The complete dispersion relations of transverse magnetic surface plasmon polaritons (TM SPPs) at a metal/birefringent dielectric interface are studied using a rigorous electromagnetic treatment. The exact expression of the wave vector normal to the boundary is derived. The propagation properties and components of the Poynting vectors of four types of TM SPPs are demonstrated with real materials in a wide range of photon energies. On the basis of this analysis, new types of TM SPPs are predicted. Furthermore, a perfect surface wave is realized with real materials in this simple structure. Our results may be useful for plasmonic-based devices associated with anisotropic media.

A Loss-Controllable Absorbing Boundary Condition for Surface Plasmon Polaritons Propagating Onto Graphene

The development of a robust terminating boundary scheme for the transverse magnetic surface plasmon polaritons, supported on graphene, is introduced in this paper and incorporated in the finite-difference time-domain (FDTD) method. First, the 2-D FDTD algorithm is adjusted to efficiently model the graphene sheet as a surface conductivity, and the effect of the scattering rate—the main loss mechanism of graphene—on the surface wave propagation properties, is thoroughly studied. Then, the new scheme is optimally formulated via the prior scattering rate and combined with the FDTD algorithm. Its enhanced performance is successfully validated for several 2-D setups and a 3-D configuration with a surface wave traveling on a graphene microribbon waveguide.

Soft-boundary graphene nanoribbon formed by a graphene sheet above a perturbed ground plane: conductivity profile and SPP modal current distribution

An infinite sheet of graphene lying above a perturbed ground plane is studied. The perturbation is a two-dimensional ridge, and a bias voltage is applied between the graphene and the ground plane, resulting in a graphene nanoribbon-like structure with a soft boundary (SB). The spatial distribution of the graphene conductivity forming the SB is studied as a function of the ridge parameters and the bias voltage. The current distribution of the fundamental transverse magnetic surface plasmon polariton (SPP) is considered. The effect of the ridge parameters and shape of the SB on the current distributions are investigated, and the conditions are studied under which the mode remains confined to the vicinity of the ridge region.

Surface plasmon polaritons on soft-boundary graphene nanoribbons and their application in switching/demultiplexing

A graphene sheet gated with a ridged ground plane, creating a soft-boundary graphene nanoribbon, is considered. By adjusting the ridge parameters and bias voltage a channel can be created on the graphene which can guide transverse magnetic surface plasmon polaritons. Two types of modes are found; fundamental and higher-order modes with no apparent cutoff frequency and with energy distributed over the created channel, and edge modes with energy concentrated at the soft-boundary edge. Dispersion curves, electric near-field patterns, and current distributions of these modes are determined. Since the location where energy is concentrated in the edge modes can be easily controlled electronically by the bias voltage and frequency, the edge-mode phenomena is used to propose a voltage controlled plasmonic switch and a plasmonic frequency demultiplexer.

Beam-Scanning Planar Lens Based on Metal–Dielectric–Metal Waveguide Arrays

Under specific illumination conditions, periodic arrays of metal–dielectric–metal (MDM) waveguides act as uniform optical phased-array antenna where the phase of the radiating optical wave can be controlled by modifying the refractive index distribution of the dielectric material. Based on this property, we propose a planar gradient index MDM-based lens which can transform spherical waves of the transverse-magnetic surface plasmon polariton waves to plane waves with specific beam deflections by adjusting the refractive index configurations. Using numerical simulations based on two-dimensional finite-difference time-domain method, it is confirmed that beam focusing and splitting with multiple dflection angles can also be achieved.