Long-wavelength (15.5 microns) quantum fountain intersubband laser in GaAs/AlGaAs quantum wells

A new type of semiconductor unipolar laser operating in the mid-infrared spectral region, the Quantum Fountain intersubband laser, is demonstrated. It is based on optical pumping of a three-bound-state coupled quantum well structure in the GaAs/AlGaAs material system. The lasing transition occurs between the two excited states. Population inversion can be achieved by benefiting from LO-phonon resonance between the two lower subbands. The optical pumping scheme enables a simpler design of the active region than electrically pumped intersubband lasers. Moreover, because doped layers and metallic contacts are not necessary for the operation of the Quantum Fountain laser, free-carrier and plasmon absorptions can be minimised, thus allowing long-wavelength operation. Large optical gains are measured using pump-probe experiments with a free-electron laser. Lasing action under optical pumping by a pulsed CO2 laser has been achieved at a record long wavelength of 15.5 mu m with an output peak power of the order of 0.6 W at low temperatures.