Spectroscopy of GaAs∕AlGaAs quantum-cascade lasers using hydrostatic pressure

The authors have measured the output spectrum and the threshold current in 9.2μm wavelength GaAs∕Al0.45Ga0.55As quantum-cascade lasers at 115K as a function of hydrostatic pressure up to 7.3kbars. By extrapolation back to ambient pressure, thermally activated escape of electrons from the upper lasing state up to delocalized states of the Γ valley is shown to be an important contribution to the threshold current. On the other hand leakage into the X valley, although it has a very high density of states and is nearly degenerate with the Γ band edge in the barrier, is insignificant at ambient pressure.