Superlattices and Microstructures, Vol. 21, No. 4, 1997 Mid-infrared intersubband emission from optically pumped asymmetric coupled quantum wells A. Sa’ar, Y. Lavon Division of Applied Physics, The Fredi and Nadine Herrmann School of Applied Science, The Hebrew University of Jerusalem, Jerusalem 91904, Israel F. H. Julien, P. Boucaud Institut d’Electronique Fondamentale, URA 22 CNRS, Bat 220, Universite Paris XI, 91405 Orsay Cedex, France J. Wang, J.-P. Leburton Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, U.S.A. R. Planel Laboratoire De Microstructures et Microelectronique, CNET-Bagneux, 196 Av. H. Ravera, 92225 Bagneux Cedex, France (Received 15 July 1996) Mid-infrared optical emission due to intersubband transitions between excited conduction subbands of a coupled quantum well structure is studied. The emission process is based on optical pumping of free carriers from the ground subband into the third subband followed by a radiative transition from the third subband into the second subband and a fast phonon assisted relaxation into the ground subband. We have observed spontaneous emission at 14 µm that persists up to room temperature. Our results indicate that population inversion between conduction subbands and large stimulated gain can be achieved. c  1997 Academic Press Limited Key words: intersubband transitions, optical properties of quantum wells, semiconductor lasers. 1. Introduction Since the first demonstration of optical transitions between quantum well (QW) conduction subbands (intersubband transitions) by West et al. [1] , there has been a significant progress in the understanding of the underlying physical principles of these transitions [2 , 3] and the exploration of potential applications. Intersubband transitions (ISBT) provide an effective way to engineer transition energies between subbands, oscillator strengths and subband’s population. Linear [4 , 5] and non-linear optical properties [6 – 8] of these 0749–6036/97/040517 + 09 $25.00/0 sm960171 c  1997 Academic Press Limited