Abstract

Excitons in III-V quantum wells (QW) are strongly coupled to polar longitudinal optical (LO) phonons. The binding energies of the quasi 2D excitons (E_b ≈ 1-10 meV) are smaller than the energies of the LO-phonons (ℏΩ_LO=30-40meV). Excitons are thus unstable against LO-phonon collisions which can ionize them and release a free electron hole pair with substantial excess energy. It is clear that this process is temperature dependent. At high temperature the collision rate increases and the excitonic life time can be reduced significantly. An experimental investigations of the dynamics of excitonic nonlinearities in GaAs QWs using femtosecond spectroscopic techniques, was able to time resolve the room temperature exciton ionization . An unexpected finding of these experiments was that a population of excitons is more efficient than electron-hole (e-h) plasma in reducing the strength of the exciton resonances. The experimental findings in GaAs QWs were qualitatively explained by a theory² that accounts only for the effects of the Fermi statistic and completely neglects the long range screening. An extension of this theory was recently presented by Zimmermann³ which uses an elegant technique to performe exact infinite summations over all excitonic states (bound and unbound).

© 1989 Optical Society of America