Bloch gain in quantum cascade lasers

Esaki and Tsu's superlattice(1), made by alternating two different semiconductor materials, was the first one-dimensional artificial crystal that demonstrated the ability to tailor semiconductor properties. One motivation of this work was the realization of the Bloch oscillator(2,3) and the use of its particular dispersive optical gain(4,5) to achieve a tuneable source of electromagnetic radiation. However, these superlattices were electrically unstable in the steady state(6). Fortunately, because it is based on scattering-assisted transitions, this particular gain does not arise only in superlattices, but also more generally in semiconductor heterostructures(7,8) such as quantum cascade lasers(9) (QCLs), where the electrical stability can be controlled(10). Here, we show the unambiguous spectral signature of Bloch gain in a special QCL designed to enhance the latter by exhibiting laser action in the condition of weak to vanishing population inversion.