https://journals.aps.org/prb/accepted/3b074Of7Pa2Ea210634e99e143a55ddc10d0d1a0b
The possibility of excitation and amplification of terahertz plasmons in a structure containing a layer of graphene with charge carrier drift and a layer of graphene without carrier drift is theoretically investigated. The plasmon excitation in the graphene structure by an incident terahertz electromagnetic wave is calculated in the attenuated total reflection geometry. It is shown that, in graphene with charge carrier drift, it is possible to achieve negative values of the real part of graphene conductivity for the phase velocity of plasmons exceeding the charge carriers drift velocities, which indicates a non-Cherenkov amplification of terahertz plasmons in the double-layer graphene structure for practically achievable direct electric current values. Terahertz amplification originates due to the variation of the carrier mass density in graphene with terahertz electric field. In the case of weak deceleration of terahertz wave incident onto graphene structure, the value of the negative conductivity of graphene does not depend on the direction of the direct current in graphene (only the co-directional and counter-directional currents with respect to the direction of plasmon propagation are considered). This is due to the insignificant spatial dispersion of the hydrodynamic conductivity of graphene in the case of weak deceleration of terahertz waves. The results of this work can be used to create compact terahertz radiation amplifiers operating at room temperature. The investigations of plasmonic properties of graphene structures for amplification, generation, detection and modulation of terahertz (THz) radiation an actively exploring area of nanophotonics in recent years [1-10]. Graphene conductivity in THz frequency range can be described by using a hydrodynamic approach in the case when the frequency of acting electromagnetic field and the electron momentum relaxation rate in graphene is smaller than the frequency of carrier-carrier collisions [11, 12]. The hydrodynamic regime of graphene at THz frequencies is confirmed in experimental works, in which the current vortices in graphene are discovered [13, 14.]. In general, the carrier drift in graphene, the carrier pressure forces, as well as the influence of the Doppler effect in graphene leads to the spatial dispersion of the graphene hydrodynamic conductivity. The Doppler effect results in Cherenkov amplification, which is difficult to achieve in two-dimensional isotropic parabolic materials [15, 16]. The amplification of THz radiation in single-layer and double-layer graphene structures due to the radiative recombination of charge carriers and obtaining the negative dynamic conductivity of graphene by different methods of graphene pumping were investigated [17-19]. However, creating a long-lived inversion in graphene is a difficult task due to the strong non-radiative recombination of charge carriers in graphene [12, 20] and strong heating of the structures by high pumping power. The problem of graphene heating can be solved by using the diffusion pumping of graphene on a black-As substrate [21]. Experimental observation of optical (2.0–3.5 eV range) emission from graphene induced by an intense THz pulse is reported in [22]. An alternative mechanism of amplification of THz radiation in graphene is the Cherenkov plasmon instability, which can occur in graphene when a direct electric current is passed in the plane of graphene [23].
