Spectral and temperature dependences of equilibrium and non-equi-intersubband light absorption in the mid-infrared spectral range were studied in selectively doped asymmetrical tunnel-coupled GaAs/AlGaAs quantum wells. The temporal evolution of the absorption studied by means of a picosecond pump-probe technique was found to have a biexponential character. The fast decay times are determined by intersubband electron relaxation due to electron scattering by optical phonons and impurities. The presence of long decay times in transient mid-infrared absorption is probably connected with electron transitions from the states in barrier (X and L valleys as well as deep centers) to the states of the quantum well. Experimentally determined intersubband scattering times are compared with the calculated ones.
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