Nima Chamanara

Non-reciprocal magnetoplasmon graphene coupler

The non-reciprocity of the edge magnetoplasmon modes of a graphene strip is leveraged to design a non-reciprocal magnetoplasmon graphene coupler, coupling only in one direction. The proposed coupler consists of two coplanar parallel magnetically biased graphene strips. In the forward direction, the modes along the adjacent strip edges of the strips have the same wavenumber and therefore couple to each other. In the backward direction, the modes along the adjacent strip edges have different wavenumbers and therefore no coupling occurs.

Terahertz magnetoplasmon energy concentration and splitting in Graphene PN Junctions

Terahertz plasmons and magnetoplasmons propagating along electrically and chemically doped graphene p-n junctions are investigated. It is shown that such junctions support non-reciprocal magnetoplasmonic modes which get concentrated at the middle of the junction in one direction and split away from the middle of the junction in the other direction under the application of an external static magnetic field. This phenomenon follows from the combined effects of circular birefringence and carrier density non-uniformity. It can be exploited for the realization of plasmonic isolators.

Optically Transparent and Flexible Graphene Reciprocal and Nonreciprocal Microwave Planar Components

The sheet resistance of graphene has been recently reduced below the level of previous optically transparent conductive materials, making graphene, with its additional advantage of mechanical flexibility, the best candidate for future transparent electronics. We investigate here the viability of microwave planar components using graphene sheets as conductors towards optically transparent and flexible radio systems. Specifically, we study the waveguiding and nonreciprocal properties of such components, through the arbitrary example of a coplanar waveguide (CPW) structure, using the 2-D finite difference frequency domain (FDFD) technique. It is shown that reciprocal graphene-based components of acceptably low loss levels are achievable using graphene sheets of the lowest available resistivity, while nonreciprocal components with graphene conductors still exhibit prohibitive loss due to a fundamental trade-off between nonreciprocity and carrier density in graphene.

Nonreciprocal electromagnetic scattering from a periodically space-time modulated slab and application to a quasisonic isolator

Scattering of obliquely incident electromagnetic waves from periodically space-time modulated slabs is investigated. It is shown that such structures operate as nonreciprocal harmonic generators and spatial-frequency filters. For oblique incidences, low-frequency harmonics are filtered out in the form of surface waves, while high-frequency harmonics are transmitted as space waves. In the quasisonic regime, where the velocity of the space-time modulation is close to the velocity of the electromagnetic waves in the background medium, the incident wave is strongly coupled to space-time harmonics in the forward direction, while in the backward direction it exhibits low coupling to other harmonics. This nonreciprocity is leveraged for the realization of an electromagnetic isolator in the quasisonic regime and is experimentally demonstrated at microwave frequencies.

Optical isolation based on space-time engineered asymmetric photonic band gaps

Nonreciprocal electromagnetic devices play an important role in modern optical and microwave technologies. Conventional methods for realizing such systems are incompatible with integrated circuits. With recent advances in integrated photonics, the need for efficient on-chip magnetless nonreciprocal devices is more urgent than ever. This paper leverages space-time engineered asymmetric photonic bandgaps to generate optical isolation. It is shown that a properly designed space-time modulated slab is highly reflective/transparent for opposite directions of propagation. The proposed method requires a low modulation frequency, is magnetless and can achieve very high isolation levels. Experimental proof of concept at microwave frequencies is provided.

Spacetime processing metasurfaces: GSTC synthesis and prospective applications

The paper presents the general concept of spacetime processing metasurfaces, synthesized by generalized sheet transition conditions (GSTCs). It is shown that such metasurfaces can perform multiple simultaneous spatio-temporal processing transformations on incident electromagnetic waves. A time-reversal space-generalized-refraction metasurface and a multi-time-space-differentiating metasurfaces are presented as applications of the general spacetime processing metasurface concept.

Theoretical Investigation of Traveling-Wave Amplification in Metallic Carbon Nanotubes Biased by a DC Field

Traveling-wave amplification along a carbon nanotube (CNT) under dc-ac fields is theoretically investigated. The ac conductivity of a metallic CNT is found with respect to the applied dc bias. For this purpose, the Boltzmann transport equation (BTE) is solved within the relaxation time approximation (RTA) by separating the ac and dc distributions. The problem is solved both exactly and approximately by semianalytical and analytical means, respectively. It is shown that an absolute negative ac conductivity accompanies a negative differential conductivity beyond a threshold dc field of 3 × 105 V/m. The complex propagation factor of the allowed surface wave modes is found by coupling the BTE current with Maxwells equations and solving a transcendental equation. The slow-wave factor and attenuation steadily increase with the dc field amplitude. Beyond the threshold field, amplification occurs, which is a promising result toward enabling traveling-wave amplifiers using CNTs. The amplification is shown to be a result of Bloch-type oscillations.

Integral equation for guided-wave problems and application to magneto-plasmonics in graphene

A novel modal electric field integral equation (EFIE) is introduced for the analysis of waveguiding problems for which the Green function is available in closed form. This method is much more efficient than finite-difference and finite-element techniques. The proposed EFIE is applied to simulate magneto-plasmons in a magnetically biased graphene strip.

Hybrid spatial-spectral integral equation for periodic guided wave problems and applications to magnetoplasmonics in graphene

A hybrid spatial-spectral integral equation (IE) is proposed for the eigenmode analysis of guided-wave periodic structures. The proposed IE uses the spatial guided-wave traveling Green function over the transverse cross section of the conducting sheet and Fourier series coefficients in the longitudinal direction. In this way, the original three-dimensional problem is reduced to a system of one-dimensional integral equations. The resulting equations are solved using the method of moments.

Graphene transverse electric surface plasmon detection using nonreciprocity modal discrimination

We present a magnetically biased graphene-ferrite structure discriminating the TE and TM plasmonic modes of graphene. In this structure, the graphene TM plasmons interact reciprocally with the structure. In contrast, the graphene TE plasmons exhibit nonreciprocity. This nonreciprocity is manifested in unidirectional TE propagation in a frequency band close to the interband threshold frequency. The proposed structure provides a unique platform for the experimental demonstration of the unusual existence of the TE plasmonic mode in graphene.