Coherent Emission in the Vicinity of 10 THz due to Auger-Suppressed Recombination of Dirac Fermions in HgCdTe Quantum Wells

The discovery of Dirac fermions in a number of 2D and 3D materials boosted the solid-state research in an unprecedented way. Among the many hopes of using their exceptional physical properties, it has been argued that their reduced nonradiative losses would allow graphene to compete with quantum cascade lasers (QCLs) in the race for terahertz (THz) emitters. Unfortunately, the nonradiative Auger recombination (AR) process is still active for massless fermions in gapless graphene. However, for massive Dirac fermions, AR can be entirely suppressed below a certain threshold of the carrier's kinetic energy that depends on the nonparabolicity and the symmetry of the electron and hole dispersions. In this work, by finely tuning the band structure of HgCdTe quantum wells hosting massive Dirac fermions, we set the electronic system below this threshold and demonstrate that the carrier recombination is purely radiative. A coherent interband emission reaching 9.6 THz, that is to say outside the spectral range of current QCLs, is measured under these conditions, opening the way to lossless interband THz emitters.