A quantum cascade laser is a very unique semiconductor laser that has been first demonstrated at Bell Laboratories in 1994.
Wikipedia has some nice information on QCLs: Quantum cascade laser
This figure shows the calculated electron density in a THz QCL.
This figure shows the calculated gain of a THz QCL.
nextnano software for QCLs
Download nextnano.QCL including an evaluation license until 2016-12-31.
nextnano3 & nextnano++
The following two papers give a very basic introduction to QCL modeling.