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Terahertz plasmonic devices using graphene-based 2D materials

T. Otsuji¹, A. Satou¹, V. Ryzhii¹, and M.S. Shur²

¹Research Institute of Electrical Communication, Tohoku University, Sendai, Japan

²Rensselaer Polytechnic Institute, Troy, NY, USA

Terahertz (THz) electromagnetic waves are still under-explored frequency range often referred to as the THz gap [1]. Using Graphene Dirac plasmons (GDPs), quanta of collective charge density waves of graphene Dirac fermions (GDFs), has the potential to bridge the THz technological gap [2-4]. This paper reviews recent advances in the research and development of THz plasmonic devices using graphene-based 2D materials.

Graphene, a monoatomic layer of carbon with a honeycomb lattice structure, has exceptional carrier transport, photonic, and plasmonic properties. Its gapless, linear, and symmetric energy band structure enables extremely strong light-matter interaction [2]. Both electrons and holes in graphene lose their effective mass and behave as back-scattering-free relativistic particles called graphene Dirac fermions (GDFs) [2]. GDPs exhibit unique properties such as extremely low decay rates, extremely high viscosity, and extremely high nonlinearities [3, 4]. As a result, the quantum efficiency of linear and nonlinear interactions between photons and electrons in graphene can be dramatically enhanced via excitation of the GDPs, leading to highly efficient coherent oscillations and amplification of THz electromagnetic waves, fast and sensitive detection of THz radiation, and enabling ultra-broadband frequency conversion of interacting light waves, THz waves, and microwaves [5,6].

One of the key advantages of the GDPs is its extremely low damping rate in high-quality graphene approaching 10¹¹ s^-1 even at room temperature because of weak scattering dominated only by the optical phonons [7]. This makes promising the realization of resonant THz detection and plasmon instability-driven THz luminescence. Due to a strong nonlinearity and low attenuation of GDPs, GDP rectification of a graphene-channel field effect transistor (GFET) resulted in a resonant THz detection with higher harmonics observed from low temperatures up to room temperature [8].

Frequent electron-to-electron scattering in graphene due to many-body Coulomb interactions results in highly viscous electronic liquid-like behavior [9]. The viscosity of the electronic fluid in graphene strongly depends on temperature and electron sheet density and could exceed 0.1 m²/s, which is much higher than the viscosity of honey.

The temperature dependence of the resistivity change in the GDF at different densities has been measured by injecting current in the diagonal direction into a single-layer graphene Hall bar and observing the voltage response in the opposite diagonal direction. The hydrodynamic modeling of GDPs shows that the extremely high viscosity of GDFs results in Eddy currents in the GDF flow causing the generation of a backward wave flow in the opposite direction of the potential gradient. [10] If an appropriate resonator structure is provided, the negative conductivity related to this effect can support self-oscillations in the THz band. However, the negative conductivity disappears at temperatures above 200 K due to decoherence caused by acoustic phonon scattering [10].

DC current could excite resonant plasmons in a confined two-dimensional electron system. The system could become unstable due to plasmon generation when the electron drift velocity exceeds a certain critical value causing self-excited oscillations at THz frequencies at plasmon resonances [11]. This phenomenon is called plasmon instability and is caused by the conversion of the DC energy of electrons into plasmon energy near the resonance frequencies [11]. There are several different mechanisms to promote plasmon instability including the Dyakonov-Shur (D-S) instability (Doppler shift type) [11], Ryzhii-Satou-Shur (R-S-S) instability (electron velocity modulation type), and the plasmonic boom instability [12]. These instability mechanisms are expected to become the operating principles for the realization of coherent THz radiation devices [6,12].

The authors succeeded in room-temperature amplification of stimulated emission of THz radiation in a current-driven asymmetric dual-grating-gate graphene-channel field-effect transistor (ADGG-GFET) structure by promoting the GDP instability [13]. The maximal amplification gain of 9% was obtained from monolayer graphene, which is four times larger than the quantum mechanical limit of 2.3% when THz photons interact directly with electrons without GDPs [13]. We expect that room-temperature high-intensity graphene laser transistors can be realized by incorporating a seed section that emits spontaneous THz radiation by current-injection pumping [14] and a gain section that induces and amplifies that spontaneous THz radiation emission by the GDP instability [12].

The authors have recently predicted a new type of GDP instability called “Coulomb-drag instability” in which the injection of ballistic GDFs from the source end to the channel mediates the drag motion in the quasi-equilibrated highly dense electronic fluid in the gated region. Under a pertinent drain bias, this effect leads to the inverted potential in the drain-side ungated region resulting in a gigantic negative dynamic resonant conductivity in the THz frequency range [15,16]. This new instability mechanism is expected to enable the development of room-temperature high-intensity THz laser transistors.

A fast and sensitive THz detector device based on the hydrodynamic nonlinear rectification action of GDP in the GFET channel is promising as a receiver front-end device for 6G/7G THz wireless communications. The authors have fabricated a prototype Asymmetric Dual Grating Gate GFET (ADGG-GFET) demonstrating a 10-ps class fast photoresponse with a rather high responsivity of 0.3 mA/W, and an equivalent noise power of 166 nW/√Hz at room temperature [17]. The ADGG-GFET structure also enables photothermoelectric THz detection with a fast response speed comparable to plasmonic detection [17].

By utilizing the high carrier-mobility graphene and band engineering through hetero stacking with other 2D materials, such as h-BN insulator and semiconductor transition metal chalcogenides, high-performance THz functional devices have been demonstrated [18-20]. The authors have recently proposed new high-speed bolometric THz detectors and intense THz oscillators by van der Waals hetero stacking of graphene with black phosphorus (b-P) and b-As_1-xP_x (b-AsP). The bandgap energy of these materials is well aligned to the graphene Dirac point and is a number-of-layer-dependent tunable [21-23]. GDFs become hot by absorbing the THz radiation and escape into the cool reservoir of a metal electrode over the graphene-b-AsP barrier modulating the graphene conductivity. This works as a new type of fast-response bolometric THz detector. Its experimental verification is undergoing.

In conclusion, THz plasmonic devices using graphene-based 2D materials have promise to bridge the THz technological gap, which is important for a sustainable, resilient, future smart society.

Acknowledgments
The authors thank V. Mitin, W. Knap, S. Boubanga-Tombet, H. Fukidome, T. Suemitsu, D. Yadav, C. Tang, T.T. Lin, K. Tamura for their contributions. The devices were fabricated in the Nano-Spin Facility, RIEC, Tohoku University. This work was supported by JSPS KAKENHI # 21H04546 and #21H01380, Japan, and NICT #JPJ012368C01301, Japan.

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