Intersubband dynamics below the optical phonon energy for single and coupled quantum well systems

We have made direct measurements of the intersubband lifetime in single GaAs quantum wells below the optical phonon energy at 30 meV. At low temperatures and excitations LO phonon emission is inhibited and acoustic phonon emission dominates; a sharp cross-over exists between the two regimes. Rate equation calculations have been performed for the single well samples with no fitting parameters and these agree well with experiment. The calculations have been extended to coupled quantum wells and we find that by careful tuning of the well widths the LO phonon rate can be tuned by up to an order of magnitude.