Laser action by a weak to vanishing population inversion is observed in mid-infrared (7 μm) quantum cascade
lasers. The origin of the optical gain is the same than for the Bloch oscillator predicted in superlattices. This
special gain does not only relax the threshold condition for lasing, but also underlines the major role played by
coherent processes in semiconductor heterostructures, as it is based on scattering assisted optical transitions.
Two families of photonic crystal based Terahertz Quantum Cascade Laser structures are demonstrated. The first one uses Bragg mirrors that relies on a two-dimensional photonic lattice. Single mode lasing emission is observed from the edge of the structure for particular lattice constants. Moreover mode pinning of the laser is obtained along the whole dynamic range. In the second part a vertically emitting THz Quantum Cascade Laser device that exploits in-plane resonator based on a two-dimensional photonic crystal is demonstrated. Stable single mode lasing is reported. Simulations based on block-iterative frequency-domain methods on a plane wave basis account for the observed results.