Quantum-cascade lasers with emission wavelength 3-5 μm

Abstract

Quantum-cascade lasers (QCLs) operating in the 3-5 μm spectral region are increasingly finding application in a number of areas including gas sensing for both environmental and medical uses, communication, and infrared countermeasures. QCLs emitting at wavelengths near 4 μm and below have been especially challenging, requiring a very large conduction band discontinuity, a small electron effective mass, but also a relatively mature materials system. The focus of this contribution is on our own QCL designs based on the use of strain compensation with very high levels of strain in the individual layers; barriers based on AlAs, wells on In_0.73Ga_0.27As, and the entire structure on average lattice-matched to InP. For more flexibility to control both strain and conduction band potential, "composite barriers" are used, composed of AlAs and A_l0.5In_0.5As. Indirect valleys within the well material that can limit the photon energy to the energy difference between these valleys and the lower laser state are also pushed to higher energy by using strained In_0.73Ga_0.27As wells. Combining these design components, we have produced QCLs emitting at wavelengths covering the entire range down to 3 μm. These lasers have demonstrated high power in narrow stripes at cryogenic as well as room temperatures together with excellent beam quality.