  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 (THz) 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 fixed magnetic field. The emission frequency detected at
1.5 Tesla is centered on 2.2 THz and can already be electrically tuned over 250
GHz. With an optimized gate and a realistic permanent magnet of 1.0 Tesla, 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.
