Non-equilibrium longitudinal and transverse optical phonons in terahertz quantum cascade lasers

We report on the experimental observation of non-equilibrium longitudinal (LO) and interface transverse-like (IF-TO) optical phonons populations associated with electron transport in resonant-phonon THz quantum-cascade lasers. The measured LO-phonon occupation numbers are in excellent agreement with the result of Monte Carlo simulations at a phonon in-plane wave number q = 4.2 x 10(+4) cm(-1), and they remain about 40% larger than the IF-TO ones.