Asymmetric quantum wells generate intersubband stimulated emission
Optical pumping of a GaAs/AlGaAs quantum-well structure with a free-electron laser enabled researchers at the Universitè Paris-Sud (Orsay, France), in collaboration with others in France and the USA, to obtain intersubband stimulated emission at 77 K with a gain of approximately 80 cm-1 at 12.5 µm. The device differs from quantum cascade lasers--it consists of a pair of asymmetric-coupled quantum wells with three conduction-bound levels. The layer thicknesses of the quantum wells control phonon scattering of electrons between subbands, determining the carrier lifetime in each band to achieve population inversion.
To increase interaction length, the quantum wells were integrated into 0.7- and 2-mm-long IR waveguides grown on n-doped GaAs substrates. The devices consist of a 5-µm-thick AlGaAs cladding layer, a 2.5-µm-thick GaAs core layer, the active region, and a second GaAs core layer, which is 2.9 µm thick. The active region contains 30 periods of coupled quantum wells, 7.7 and 4.7 nm thick, respectively. A 1.1-nm-thick AlGaAs coupling barrier separates the wells, and the periods are separated by a 21-nm-thick AlGaAs barrier. The device was pumped at 9.2 µm from the free-electron laser, and a 12.5-µm probe beam simultaneously monitored the stimulated emission. Waveguide losses were too high for laser oscillation, but the grou¥expects a 5-mm-long device to lase in the near future.