Limitation of parasitic absorption in designs of three-state terahertz quantum cascade lasers with direct-phonon injection

Abstract

To maximize the population inversion in terahertz quantum cascade lasers for high-temperature operation, the scheme of electron injection following direct-phonon emission is more attractive than the common way via resonant tunneling. If this direct-phonon injection can be realized within a simple quantum structure (only three quantum states), much higher optical gain is expected even at room temperature. However, the attempts of this design in experiments are full of challenges. In this work, the non-equilibrium Green’s function method is introduced to draw a full picture of the quantum transport in this design. It demonstrates that the optical gain is strictly limited by a parasitic absorption from one specific high-lying state. Moreover, a strategy is proposed to avoid this limitation.