Reed Center for Photonics, University of Wisconsin, Madison, WI 53706, U.S.A.
University of Florida, Gainesville, FL 32611, U.S.A.
Semiconductor lasers operating continuous wave (cw) at or near room temperature (RT) and emitting in the mid- and far-infrared wavelength ranges, 3–13 μm, are critically needed for a vast array of applications. Intersubband (IS) transition emitters are the most likely solution. The first implementation of the concept for using IS transitions for laser action,1 was realized in early 19942 and named the quantum cascade (QC) laser. Current QC devices have demonstrated room-temperature cw operation at 4.3 μm and 4.8 μm,3,4 but with very low wallplug-efficiency values (< 2.5%) due to inherently high operating voltages (10–11 V). Furthermore, these devices have extremely temperature-sensitive characteristics3,4 at and near RT, due to thermal runaway triggered by the backfilling effect,5,6 which raises serious issues of device reliability. In fact, no device reliability has been demonstrated to date for any type of QC laser.
Intersubband-QC lasers have fundamentally poor radiative efficiencies since the nonradiative, LO-phonon-assisted relaxation time for electrons in the upper laser states is about 1.8 ps,5 whereas the radiative relaxation time is 4.2 ns. That is, nonradiative processes are about 2300 ...