Optical emission between excited subbands of asymmetric coupled GaAs/AlGaAs quantum wells is investigated. The structures are designed to present an energy separation between the ground subband and the first excited subband close to the LO-phonon energy in order to enhance the non-radiative relaxation between those states. Optical pumping at 10 μm is used to excite electrons in the third subband. A mid-infrared emission at 14 μm is shown to take place between the third and second subbands. The spontaneous emission is observed up to room temperature. The radiative efficiency is ≈60 nW/W in lightly doped samples. We also predict that this situation is very effective for achieving almost complete population inversion as well as large stimulated gains. The possibility of mid-infrared intersubband emission under band-to-band optical pumping is also discussed.
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Acknowledgements—This work was supported in part by EC program d CI1*-CT93-0072. J.W. and J.-P.L. acknowledge funding by Engineering Research Center for Compound Semiconductor Electronics through Grant d NSF EEC 89-43166.