Analysis of the slope efficiency for terahertz quantum-cascade lasers

The slope efficiency is, in addition to the threshold current density and operating voltage, a decisive target value for the operation of quantum-cascade lasers (QCLs) in order to achieve an optimal total efficiency for the conversion of electrical input power into optical output power. We analyze the light-current characteristics for a set of similar, high-power, bound-to-continuum terahertz QCLs. The calculated internal slope efficiency shows a clear dependence on the height and thickness of the barriers. In contrast to the simulations, we found experimentally a significant difference in the threshold current densities and slope efficiencies for QCLs with nominally identical layer structures, which we mainly attribute to different line broadening. (C) 2010 American Institute of Physics. [doi:10.1063/1.3511470]