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.

Magnetic Plasmon Networks Programmed by Molecular Self-Assembly.

Nanoscale manipulation of magnetic fields has been a long-term pursuit in plasmonics and metamaterials, as it can enable a range of appealing optical properties, such as high-sensitivity circular dichroism, directional scattering, and low-refractive-index materials. Inspired by the natural magnetism of aromatic molecules, the cyclic ring cluster of plasmonic nanoparticles (NPs) has been suggested as a promising architecture with induced unnatural magnetism, especially at visible frequencies. However, it remains challenging to assemble plasmonic NPs into complex networks exhibiting strong visible magnetism. Here, a DNA-origami-based strategy is introduced to realize molecular self-assembly of NPs forming complex magnetic architectures, exhibiting emergent properties including anti-ferromagnetism, purely magnetic-based Fano resonances, and magnetic surface plasmon polaritons. The basic building block, a gold NP (AuNP) ring consisting of six AuNP seeds, is arranged on a DNA origami frame with nanometer precision. The subsequent hierarchical assembly of the AuNP rings leads to the formation of higher-order networks of clusters and polymeric chains. Strong emergent plasmonic properties are induced by in situ growth of silver upon the AuNP seeds. This work may facilitate the development of a tunable and scalable DNA-based strategy for the assembly of optical magnetic circuitry, as well as plasmonic metamaterials with high fidelity.

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.

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.