Maximizing the population inversion by optimizing the depopulation rate in far-infrared quantum cascade lasers

Abstract

Electron–LO phonon and electron–electron transition rates are calculated for a three-level triple quantum well system to be employed as a laser operating in the far-infrared (30–300 μm) or terahertz (1–10 THz) region. The population ratio is determined from the ratio of the carrier lifetimes in levels |3〉 and |2〉. The most effective way of depopulating the lower laser level is found to be by LO phonon scattering to a strongly coupled state, as occurs at an anticrossing. Back scattering of carriers from level |1〉 to level |2〉 is significant at room temperature, but a population ratio of approximately 5 is possible nonetheless.