High-performance superlattice quantum cascade lasers

Superlattice quantum cascade (QC) lasers based on optical transitions between conduction minibands are unipolar semiconductor lasers with high-current-carrying capability and attendant high optical power due to miniband transport in the active and in the injector regions. Other advantages include the intrinsic population inversion associated with the large interminiband-to-intraminiband relaxation time ratio and the high oscillator strength of the laser transition at the superlattice Brillouin zone boundary. This oscillator strength is significantly larger than that of intersubband transitions in double-quantum-well active regions of conventional cascade lasers, particularly at long infrared wavelengths (greater than or equal to 10 mu m). Following a brief review of results on conventional QC lasers, design considerations for superlattice cascade lasers are discussed, along with recent advances in long-wavelength (11 mu m) structures with doped active regions. Dopants broaden the gain spectrum and increase the laser threshold. Two laser designs that avoid doping of the active regions without causing electric-field-induced localization of the superlattice states are then presented, along with experimental results. In the first one, modulation doping creates a space-charge electric field that compensates the voltage drop across the undoped superlattice active regions. In the second scheme, the latter are designed with quantum wells of varying thickness (chirped superlattice), so that under application of the external field, the localized quantum well states overlap, forming minibands. Both schemes lead to considerably lower threshold current densities than devices with doped active regions, as well as to much higher peak optical power and to room-temperature operation. A record peak power of 500 mW at lambda = 7.6 mu m at room temperature is obtained with the chirped design. The latter also. leads to the longest operating wavelength of any other QC laser (17 mu m). The last section of the paper describes superlattice cascade lasers operating simultaneously at two or more widely different wavelengths.