Mid-infrared intersubband emission and lasing in optically pumped coupled quantum wells structures

Long wavelength infrared emission and lasing due to population inversion, induced by optical pumping, between the conduction subbands of coupled quantum well structures is studied. The excitation process involved optical pumping of the carriers into the third subband. The emission involved a radiative transition between the third and the second subbands followed by a fast relaxation into the ground subband due to vertical LO phonon emission. Indeed we observed intersubband photoluminescence at 14.5 mu m in a coupled quantum well structure. However, we found that the phonon bottleneck effect prohibits vertical phonon transitions at the button of the subbands thus giving rise to appreciably carriers heating and thermal population of the second subband. We present results of our recent study of a four levels like structure for which the phonon bottleneck effect can be avoided. In this structure the population inversion is achieved between the forth and the third subbands. The emission process is followed by a cascade of two resonant LO-phonon relaxation into the ground subband. Population: inversion as well as lasing are expected in these structures.