Terahertz emission and detection in double-graphene-layer structures

Double-graphene-layer (D-GL) heterostructures with thin inter-GL barrier Boron Nitride (h-BN), Tungsten Disulfide (WS₂), and other barrier layers (see, for example, [1]) have recently attracted much attention due to their potential applications in high speed modulators of terahertz (THz) and infrared (IR) radiation, transistors, and THz photomixers. In this work, we demonstrate that the photon-assisted resonant radiative inter-GL transitions enable the applications of the D-GL heterostructures for THz/IR lasers and photodetectors (PDs) [2-4]. The structures of these devices are schematically shown in Figs. 1(a) and 2. The main element of both the lasers and PDs under consideration is a D-GL core-shell heterostructure with the independently contacted GLs separated by the thin transparent tunnel-barrier layer. The bias voltage V applied between the GL's contacts induces the electron and hole gases in the opposing GLs. The electron and hole densities in GLs are also controlled by the gate voltage V_g. The outer gate-stack structures at both sides of the D-GL core shell also serve as the metal-metal (MM) waveguide, as shown in Fig. 1(a). The voltage-dependent band-offset energy (designated with 'Δ' in Fig. 3) between the Dirac points of the GLs and the depolarization shift determine the energies of the photons emitted (in the lasers) or absorbed (in the PDs) in the resonant-tunneling inter-GL transitions. For comparison, in the intra-GL transition-type device implementation shown in Fig. 1(b), both GLs are electrically contacted with both electrodes, so that electrons and holes are injected laterally to the GLs from the side contacts. In this device, both GLs form the lateral p-i-n junctions. Figs. 3(a)-(c) show the radiative inter-GL transitions in D-GL laser/PD structures and intra-GL laser structures, respectively.