Intersubband scattering processes are investigated in selectively doped asymmetric tunnel-coupled GaAs/AlGaAs quantum wells by infrared pump probe spectroscopy. Various subbands are excited by resonant absorption of a mid-infrared pump pulse and the carrier relaxation is observed via timeand frequency-resolved absorption changes monitored by a subsequent infrared probe pulse. The experimental data show that most of the carriers remain in the quantum well in which the excitation process mainly takes place. Only a minor fraction of the carriers is transferred between excited non-resonant subbands of the neighbouring quantum wells. The situation is different for electrons in the two lowest subbands which are separated only by 10 meV. Here a rapid and efficient transfer is observed. The scattering time is in the order of a picosecond at room temperature; it increases to values between 4 ps and 20 ps at 80 K. The data are interpreted with detailed model calculations taking into account phonon and impurity scattering. Both experimental and calculated data demonstrate the possibility of population inversion between excited states of quantum wells under optical or current electron injection.
The spectral and polarization dependences of intraband light absorption in n-and p-doped structures with InAs/GaAs quantum dots covered by InGaAs layers were studied experimentally and theoretically. The transmittance of p-and s-polarized mid-infrared radiation was studied in a multi-pass sample geometry. Elastic strain, piezoelectric fields and a complex valence band structure were taken into account in calculations of the energy spectrum, the carrier wavefunctions and intraband optical matrix elements. The experimentally determined value of the absorption cross section corresponding to |0 0 0 →|0 0 1 intraband electron transitions is 4.1 × 10 −15 cm 2 . Light absorption related to intraband hole transitions was found to be significantly smaller. The dispersion of the dot sizes was evaluated using the experimentally determined spectral widths of intraband absorption peaks.
Optical phenomena in quantum dot and quantum well nanostructures aimed at the development of mid-infrared lasers based on intraband electron transitions are investigated in the conditions of interband optical pumping. Evolution of photoluminescence spectra and interband light absorption with intensity of interband optical excitation is investigated in InAs/GaAs quantum dot structures. Optically induced intersubband mid-infrared light absorption is studied in undoped tunnel-coupled GaAs/AlGaAs quantum wells. The stabilization of the electron concentration at the ground level of tunnel-coupled quantum wells under increasing pumping level is established.
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