Efficient exciton energy transfer between widely separated quantum wells at low temperatures

Energy transfer between quantum wells is of fundamental interest and also contributes to the dynamical response of devices based on multiple quantum wells. We report the observation of efficient energy transfer at low temperatures between unequal GaAs quantum wells separated by a thick (10-30 nm) Al0.3Ga0.7As barrier. The transfer efficiency is about 30% for transfer from the narrow well to the wide well (Stokes transfer), about 10(-2)% for the anti-Stokes transfer, and nearly independent of temperature (2-80 K) and barrier thickness. Tunneling, thermal excitation, and impurity-related transitions cannot explain these observations. We present a calculation for transfer efficiency using Forster-type dipole-dipole interaction between excitons and between excitons and free carriers in quantum wells, and show that this dipole-dipole transfer process can reproduce the observed temperature dependence and the magnitudes of the transfer efficiency. This process has not been considered previously for energy transfer between quantum wells.