Graphene Parallel-plate Waveguide Research Articles

Dispersion characteristics of surface plasmon polaritons in a graphene–plasma–graphene waveguide structure

Theoretical investigations are carried out to study surface wave propagation for a graphene parallel-plate waveguide structure filled with isotropic plasma. The extended wave propagation theory is used. The Kobo formula is used to determine graphene conductivity. Maxwell’s equations (differential form) are used to solve the analytical problem. It is concluded that surface wave propagation can be tuned and controlled by tuning the plasma parameters (plasma frequency and collisional frequency) as well as chemical potential and relaxation time of graphene. Furthermore, the effect of plasma frequency and collisional frequency on the wave attenuation is discussed, and the effect of graphene’s chemical potential, plasma frequency, and collisional frequency on propagation length are also analyzed. The normalized field distribution of plasma medium is also studied. These results may lead to potential applications in optical sensing, communication, and plasma-based optical integrated devices in the gigahertz frequency regime.

Characteristics of dispersion modes supported by Graphene Chiral Graphene waveguide

A theoretical formulation is obtained for a chiral filled waveguide formed by two parallel graphene layers. The thin graphene layers are extended infinitesimally and Kubo formula is used for surface conductivity. The fields expressions in the model are obtained using Maxwell’s equations. It is shown that a chiral filled graphene parallel-plate waveguide can guide hybrid mode instead of quasi-transverse electromagnetic modes. The influence of chirality and or chemical potential on the properties of electromagnetic wave propagation in a planar graphene-chiral-graphene waveguide is discovered under realistic material parameters. A dispersion expression is derived, which covers the achiral graphene-chiral-graphene case. It is observed that when the chirality is raised to certain value the propagating mode is vanished. It is also revealed that chirality and chemical potential might modulate the propagation mode in the waveguide. This work may enrich the electromagnetic theory which is of great importance for Electronic devices. The presented results are compared with published under special cases.

Nonlinear TE-polarized SPPs on a graphene cladded parallel plate waveguide

We consider the transverse electric (TE) surface plasmon polaritons (SPPs) supported by a graphene parallel plate waveguide bounded by Kerr-type nonlinear media in the mid-infrared and terahertz frequencies. Through theoretical analysis of the exact dispersion relations, we reveal the existence conditions of the even mode and odd mode of nonlinear TE SPPs in this system. To be specific, if the linear permittivity of the nonlinear cladding is larger than the permittivity of the core, it only supports the even mode and two branches of the dispersion curve exist. However, when the linear permittivity of the nonlinear cladding is smaller than the permittivity of the core, both even and odd modes can be supported. Moreover, it is found that the propagation constant of even and odd modes decreases with the increasing Fermi energy of graphene.

Phase-Matched Coupling and Frequency Conversion of Terahertz Waves in a Nonlinear Graphene Waveguide

In this paper, we propose a novel technique for the phase-matched coupling and frequency conversion of terahertz (THz) waves. The principle of operation is based on nonlinear THz wave interaction in a graphene parallel plate waveguide filled with lithium niobate. The THz waves induce nonlinear polarization that modulates the effective permittivity of the waveguide by modifying the graphene conductivity. To model the evolution of the THz waves, small modulation amplitudes are presumed and a perturbation analysis is performed. First, two THz inputs (the pump and the signal) with distinct frequencies, that is, ${\boldsymbol{f}_1}$ and ${\boldsymbol{f}_2}$ , are considered. The two waves are coupled in a phase-matched fashion and show significant signal gain. Second, a third THz input with low intensity is considered at frequency ${\boldsymbol{f}_3}$ . Frequency conversion from ${\boldsymbol{f}_3}$ to a new frequency, that is, ${\boldsymbol{f}_4}$ , is attainable, assisted by the permittivity modulation induced by the co-propagating waves ${\boldsymbol{f}_1}$ and ${\boldsymbol{f}_2}$ . The generated frequency has a $\boldsymbol{\Omega } = {\boldsymbol{f}_1} - {\boldsymbol{f}_2}$ frequency detuning from the ${\boldsymbol{f}_3}$ frequency, that is, ${\boldsymbol{f}_4} = {\boldsymbol{f}_3} - ({{\boldsymbol{f}_1} - {\boldsymbol{f}_2}})$ . Thus, tunable frequency conversion can be achieved by controlling ${\boldsymbol{f}_1}$ , ${\boldsymbol{f}_2}$ , or ${\boldsymbol{f}_3}$ . Our numerical estimations show viable gain and conversion efficiency over the entire THz range considered, from $\text{0.1}$ to $\text{2}$ THz. The proposed technique is compact, tunable, and (in principle) spans the entire THz range.

Tunable spatial mode converters and optical diodes for graphene parallel plate waveguides.

We introduce compact tunable spatial mode converters between the even and odd modes of graphene parallel plate (GPP) waveguides. The converters are reciprocal and are based on spatial modulation of graphene's conductivity. We show that the wavelength of operation of the mode converters can be tuned in the mid-infrared wavelength range by adjusting the chemical potential of a strip on one of the graphene layers of the GPP waveguides. We also introduce optical diodes for GPP waveguides based on a spatial mode converter and a coupler, which consists of a single layer of graphene placed in the middle between the two plates of two GPP waveguides. We find that for both the spatial mode converter and the optical diode the device functionality is preserved in the presence of loss.

Nonreciprocal Graphene Devices and Antennas Based on Spatiotemporal Modulation

A new class of magnet-free nonreciprocal plasmonic devices operating at terahertz (THz) frequencies is introduced based on the spatiotemporal modulation of graphene’s conductivity. The proposed components are based on graphene parallel-plate waveguides with double-gated electrodes, which allow independent manipulation of graphene properties in both space and time. We employ this structure for the design of plasmonic isolators and leaky-wave antennas at THz frequencies and study the effect of graphene and modulation parameters on their response. We envision that this technology may pave the way towards silicon-compatible fully planar nonreciprocal plasmonic components and antennas with enhanced functionalities at THz, with important applications in biosensing, imaging, and intra/interchip communications.

Actively controlled plasmonic Bragg reflector based on a graphene parallel-plate waveguide

We investigate theoretically and numerically a graphene parallel-plate waveguide structure with two alternate chemical potentials (which can be realized by alternately applying two biased voltages to graphene). A plasmonic Bragg reflector can be formed in infrared range because of the alternate effective refractive indexes of SPPs propagating along graphene sheets. By introducing a defect into the Bragg reflector, and then the defect resonance mode can be formed. Thanks to the tunable permittivity of graphene by bias voltages, the central wavelength and bandwidth of SPPs stop band, and the wavelength of the defect mode can be tuned.

Surface plasmons of a graphene parallel plate waveguide bounded by Kerr-type nonlinear media

The exact dispersion relations of the transverse magnetic surface plasmons (SPs) supported by a graphene parallel plate waveguide (PPWG), surrounded on one or both sides by Kerr-type nonlinear media, are obtained analytically. It is shown that if self-focusing nonlinear materials are chosen as the surrounding media, the SPs localization length (LL) is decreased, while their propagation length (PL) remains unchanged, as compared to those of a typical graphene PPWG. Moreover, PL and LL of the SPs are considerably affected by adjusting nonlinear parts of the dielectric permittivities of the nonlinear media. It is found that using an appropriate defocusing nonlinear material as a substrate of the graphene PPWG, which is surrounded on one side by the nonlinear medium, leads to noticeable enhancement of the propagation and localization characteristics of the surface plasmons. The results presented here can be useful for enhancing capabilities of plasmonic devices based on the graphene PPWG for sensing and waveguide applications.

Optimizing terahertz surface plasmons of a monolayer graphene and a graphene parallel plate waveguide using one-dimensional photonic crystal

Through analytical calculations, the transverse magnetic surface plasmon (SP) dispersion relations for a monolayer graphene and a graphene parallel-plate waveguide (PPWG) in the presence of a one-dimensional photonic crystal (1D PC) as a substrate and a symmetric/asymmetric cladding medium are obtained. For the monolayer graphene case, we show that the presence of the 1D PC leads to significant modification in propagation length (PL) and localization length (LL) of THz SPs, as compared with the SPs of monolayer graphene on SiO2 substrate. And the SPs with largest PL and small LL, named as optimized SPs, could be supported when the 1D PC is used only as a substrate. For the graphene PPWG case with the plate separation “D” (here, 10 nm≤D≤1 μm), in addition to support of typical upper and lower branches of coupled THz SPs, presence of the 1D PC leads to support of an extra branch when 10 nm≪D≤1 μm. Moreover, as far as supporting optimized 0–2 THz SPs are concerned, the case in which the graphene PPWG with D = 1 μm is sandwiched between two symmetric 1D PC could be the best candidate. Whereas the graphene PPWG on the 1D PC, with D = 10 nm, supports optimized SPs in a frequency range from 2 to 4.5 THz. Therefore, using 1D PC improves the capability of monolayer graphene and also graphene PPWG for sensing and waveguide applications in THz frequencies.

Terahertz Antenna Phase Shifters Using Integrally-Gated Graphene Transmission-Lines

We propose the concept and design of terahertz (THz) phase shifters for phased antenna arrays based on integrally-gated graphene parallel-plate waveguides (GPPWGs). We show that an active transmission-line may be realized by combining GPPWGs with double-gate electrodes, in which the applied gate voltage can control the guiding properties of the gated sections. This may enable the realization of THz electronic switches and tunable loaded-lines for sub mm-wave antenna systems. Based on these active components, we theoretically and numerically demonstrate several digital and analog phase shifter designs for THz frequencies, with a wide range of phase shifts and small return loss, insertion loss and phase error. The proposed graphene-based phase shifters show significant advantages over other available technology in this frequency range, as they combine the low-loss and compact-size features of GPPWGs with electrically-programmable phase tuning. We envision that these electronic phase shifters may pave the way to viable phased-arrays and beamforming networks for THz communications systems, as well as for high-speed, low-RC-delay, inter/intra-chip communications.

Quasi-transverse electromagnetic modes supported by a graphene parallel-plate waveguide

A model is developed for a parallel-plate waveguide formed by graphene. The graphene is represented by an infinitesimally thin, local two-sided surface characterized by a surface conductivity obtained from the Kubo formula. Maxwell’s equations are solved for the model fields guided by the graphene layers. It is shown that despite the extreme thinness of its walls, a graphene parallel-plate waveguide can guide quasi-transverse electromagnetic modes with attenuation similar to structures composed of metals, while providing some control over propagation characteristics via the charge density or chemical potential. Given the interest in developing graphene electronics, such waveguides may be of interest in future applications.