Gate tunable terahertz cyclotron emission from two-dimensional Dirac fermions

Two-dimensional Dirac fermions in HgTe quantum wells close to the topological phase transition can generate significant cyclotron emission that is magnetic field tunable in the terahertz frequency range. Due to their relativistic-like dynamics, their cyclotron mass is strongly dependent on their electron concentration in the quantum well, providing a second tunability lever and paving the way for a gate-tunable, permanent-magnet Landau laser. In this work, we demonstrate the proof-of-concept of such a back-gate tunable THz cyclotron emitter at a fixed magnetic field. The emission frequency detected at 1.5 T is centered at 2.2 THz and can already be electrically tuned over 250 GHz. With an optimized gate and a realistic permanent magnet of 1.0 T, we estimate that the cyclotron emission could be continuously and rapidly tunable by the gate bias between 1 and 3 THz, that is to say on the less covered part of the THz gap.