Image Number 10 for United States Patent #8121164.
A quantum cascade laser (QCL) having a bias-neutral design and a semiconductor with multiple layers of Al.sub.xIn.sub.1-xAs/In.sub.yGa.sub.1-yAs. The first active region barrier has a thickness of less than fourteen angstroms, and the second active region barrier has a thickness of less than eleven angstroms. The lower active region wavefunction overlaps with each of the injector level wavefunctions. Also, the laser transition is vertical at a bias close to roll-over. The injector level 3' is above a lower laser level 3, the injector level 2' is below the lower laser level 3, and the active region level 2 is confined to the active region. The lower laser level 3 is separated from the active region level 2 by the energy of the LO phonon. The remaining active region states and the remaining injector states are either above the lower laser level 3 or significantly below the active region level 2.