GaN-based Intersubband Structures and Mid-Infrared Cascade Lasers with Heterogeneous Cascades

We review two recent developments in the field of quantum cascade lasers. First, early studies of the GaN/AlGaN material system demonstrate the suitability of the latter for the communications wavelength renge. Intersubband absorption measurements at wavelengths as short as lambda >= 1.35 mum are reprorted in multiple coupled quantum wells. An electron scattering time of few 100 fs between the subbands is obtained. Secondly, in InP-based quantum cascade lasers, the recent development of lasers with heterogeneous cascades, i.e., lasers with stacks of many intentionally dissimilar active regions interleaved with injectors, is leading to novel concepts for multi-wavelength operation. We report broadband optical gain from 5 to 8 mum and Fabry-Perot laser action covering all wavelengths from 6 to 8 mum simultaneously in a single device.