Unipolar mid-infrared semiconductor lasers

The quantum cascade (QC) laser is an excellent example of how quantum engineering can be used to design new laser materials and related light sources. It is based on intersubband transitions between excited states of coupled quantum wells and on resonant tunneling as the pumping mechanism. The population inversion between the states of the laser transition is designed by tailoring the electron intersubband scattering times. This design of scattering rates adds an important dimension to quantum engineering. In QC lasers, unlike all other semiconductor lasers, the wavelength is essentially determined by quantum confinement, i.e. by the layers´ thickness of the active region rather than by the bandgap of the material. As such it can be tailored over a very wide range using the same heterostructure material. Since the initial report of QC lasers in 1994 QC laser wavelengths in the 4 to 11 μm range using AlInAs/GaInAs heterostructures grown by MBE lattice matched to InP have been demonstrated. In the original QC laser the optical transition is between states centered in adjacent quantum wells, i.e. with reduced spatial overlap (diagonal or photon assisted tunneling transition). Although this design strongly reduces tunneling of electrons from the upper excited state of the laser transition into the continuum, a penalty is paid in terms of increased threshold current. This is because the width of the luminescence spectrum is broadened by the additional interface roughness scattering associated with the diagonal transition. To circumvent this problem a QC laser was designed with double quantum well active regions in which the optical transition occurs between excited states with strong spatial overlap in one well