Temperature-Driven Enhancement of the Stimulated Emission Rate in Terahertz Quantum Cascade Lasers

We analyze the temperature dependence of the light output power of terahertz quantum cascade lasers (THz-QCLs) and demonstrate an enhancement of the stimulated emission rate triggered by thermally activated electron leakage from the lower laser level into the continuum. Contrary to common sense expectation, we find that this leakage channel contributes positively to the temperature performance of THz-QCLs. We show that this leakage mechanism is the missing component to explain the full temperature dependence of the light output power as it counteracts the population inversion reduction that arises from non-radiative electron scattering from the upper into the lower laser state. We analyze experimental light output power versus temperature data for a 3.87-THz-emitting device with highly diagonal optical transition over a wide temperature range (10-180 K) and show how thermally activated leakage of electrons from the lower laser level leads to a nearly constant output power up to a temperature of ~120 K. These results open the question if new design approaches that exploit this effect can be developed in order to demonstrate devices with higher maximum operating temperature.