POPULATION-INVERSION IN OPTICALLY PUMPED ASYMMETRIC QUANTUM-WELL TERAHERTZ LASERS

Intersubband carrier lifetimes and population ratios are calculated fo r three- and four-level optically pumped terahertz laser structures. L aser operation is based on intersubband transitions between the conduc tion band states of asymmetric GaAs-Ga1-xAlxAs quantum wells. It is sh own that the carrier lifetimes in three-level systems fulfill the nece ssary conditions for stimulated emission only at temperatures below 20 0 K. The addition of a fourth level, however, enables fast depopulatio n of the lower laser level by resonant longitudinal optical phonon emi ssion and thus offers potential for room temperature laser operation. (C) 1997 American Institute of Physics.