We report a new mechanism of photoluminescence up-conversion in undoped GaAs-AlGaAs asymmetric coupled quantum wells. Under band-to-band excitation, resonant excitation of the £i -• Ei intersubband transition is shown to result in a finite nonthermal occupation of the E2 subband, giving rise to an Ei-> HH\ photoluminescence. The induced luminescence closely follows the polarization selection rules and the spectral dependence of the intersubband absorption. We show that this up-conversion scheme can be used to estimate the intersubband scattering time, which is found to be 2.5 ± 0.7 ps.PACS numbers: 78.55.Cr, 73.40.KpBand-to-band photoluminescence (PL) from higher conduction subbands in undoped semiconductor multiple quantum wells (MQW) is usually difficult to observe, since the electron scattering time to the ground conduction subband is much shorter than the band-to-band recombination time. Nonthermal occupation of higher subbands can be provided by sequential resonant tunneling of the electrons in the MQW structure under application of an electric field perpendicular to the layers. The application of a perpendicular electric field was demonstrated to induce an interband luminescence from the second and third electronic subbands in undoped superlattices [1]. It was also shown that an intersubband emission can take place [2]. For this type of experiment, the lifetime of the electrons in the excited subbands is of critical importance. It is now well established that the intersubband relaxation of the electrons is dominated by the emission of optical phonons when the energy separation between the subbands is larger than the longitudinaloptical (LO) phonon energy [3]. However, the value of the intersubband scattering time is controversial, since the experimental values range from a few hundreds of femtoseconds to a few picoseconds [1][2][3][4][5][6][7]. Recent theoretical calculations of two-dimensional electron-LOphonon scattering rates give values on the order of 1 ps or less for wells with infinite potential barriers, although some variation is predicted with the well width, the finite barrier potential, and the respective localization of the envelope wave functions of the initial and final subbands [8][9][10].In this Letter, we report the first observation of interband photoluminescence from the second electronic subband to the first heavy-hole subband (£2-*HH\) induced by an intersubband absorption from the first to the second electronic subband iE\-+Ej) in undoped GaAs-AlGaAs asymmetric coupled quantum wells (ACQW). The intersubband excitation is shown to result in a nonthermal occupation of the E2 subband. The induced luminescence closely follows the polarization selection rules and the spectral dependence of the intersubband absorption. As in Ref.[1] the ratio of population between the second and first subbands deduced from the corresponding PL intensities provides an estimate of the intersubband scattering time T21 without requiring subpicosecond time-resolution techniques. Since the absorption of photons a...

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Efficient all-optical modulation of mid-infrared radiations based on photoinduced intersubband absorption of a GaAs/AlGaAs multiquantum well structure is demonstrated at room temperature. The sample designed for mid-infrared waveguiding was grown by molecular beam epitaxy. A double-resonance spectroscopy of both interband and intersubband transitions of the quantum wells is presented. The intersubband resonance is found at 9.56 μm. An AlGaAs laser diode is used to optically pump the quantum wells. On/off modulation ratios as high as 150:1 are obtained with only 45 mW pump power and for mid-infrared input powers up to 0.5 W. The frequency bandwidth of the device is presently comparable to that of acousto-optic modulators available at mid-infrared wavelengths.

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