  We investigate the effects of nonequilibrium phonon dynamics on the operation
of a GaAs-based midinfrared quantum cascade laser over a range of temperatures
(77--300 K) via a coupled ensemble Monte Carlo simulation of electron and
optical-phonon systems. Nonequilibrium phonon effects are shown to be important
below 200 K. At low temperatures, nonequilibrium phonons enhance injection
selectivity and efficiency by drastically increasing the rate of interstage
electron scattering from the lowest injector state to the next-stage upper
lasing level via optical-phonon absorption. As a result, the current density
and modal gain at a given field are higher and the threshold current density
lower and considerably closer to experiment than results obtained with thermal
phonons. By amplifying phonon absorption, nonequilibrium phonons also hinder
electron energy relaxation and lead to elevated electronic temperatures.
