Continuous-wave two-photon terahertz quantum cascade laser

We report on the implementation of a terahertz two-photon quantum cascade laser operating in a continuous wave mode. Lasers that can emit two photons as a result of the relaxation of a single electron between two states of the same parity have been discussed since the early days of the laser era, but implementation has been hampered by the lack of a suitable gain medium. The semiconductor structure of a quantum cascade laser seems to be an ideal medium for realizing such two-photon emission. Our work demonstrates dual-band laser radiation in the range of 3.1–3.9 THz (104–130 cm−1) at temperatures up to 90 K.