Two-photon Absorption Spectroscopy in GaAs Quantum Wells.

We report an experimental investigation of two-photon absorption spectroscopy in GaAs quantum wells with photon energy near half gap. The transition selection rules are found to be different between the two polarization configurations (epsilon hat), perpendicular and parallel to the confinement direction z hat. For epsilon hat Z hat. For epsilon hat Z hat, the onset of two-photon absorption occurs at a two-photon energy 2h omega equal to the 2P exciton of the lowest-energy heavy-hole (H1) to conduction (C1) subband transition. The transition rate increases roughly linearly with 2h omega reflecting the k-dependence of the intraband transition matrix element. For epsilon hat || Z hat, the onset, of two-photon absorption occurs at a higher energy corresponding to the 1S exciton of the second light- hole (L2) to C1 subband transition. The spectrum is stepped and is superimposed with sharp exciton resonances of odd DELTA N (N is the subband quantum number) L-C transitions. These results are used to accurately determine exciton binding energy and band offsets.