Minimal group refractive index dispersion and gain evolution in ultra-broad-band quantum cascade lasers

Quantum cascade (QC) lasers [1] are based on optical intersubband transitions between quantized states in the conduction band of multiple coupled quantum wells. Four distinct characteristics contribute to the uniqueness of the QC-laser. First, the peak wavelength of the active region can be chosen freely over a very wide range by bandstructure engineering [2], secondly, the strength of the optical dipole matrix element can similarly be tailored. The semiconductor heterostructure is transparent outside the customarily narrow bandwidth of the intersubband transition, and finally, being unipolar by nature, many active regions can be stacked upon each other, interleaved with injector regions, in the so-called cascading scheme. We exploit these four characteristics to demonstrate the first QC-laser with an engineered ultrabroadband gain spectrum [3], designed to provide flat net-gain from 5 to 8 mum wavelength. Yet, the concept is more general, allowing to generate composite gain spectra with near arbitrary shape.