Nonequilibrium Green's function calculation for four-level scheme terahertz quantum cascade lasers

We have calculated the performance of a recently proposed four-level scheme terahertz quantum cascade laser (4L terahertz-QCL) with the nonequilibrium Green's function method. The calculation result for 40 K showed that the 4L QCL has a larger terahertz gain than the conventional resonant phonon QCL. This is because a large number of electrons accumulate in the upper lasing level and contribute to lasing in the new scheme. When the temperature is increased, the advantage of gain decreases due to thermally activated phonon scattering.