INTERWELL EXCITONS IN GAAS MULTIPLE-QUANTUM WELLS AND SUPERLATTICES

The optical properties of multiple quantum wells in the transition fro m isolated wells to a superlattice are investigated theoretically and experimentally. For superlattices with a miniband width similar to the binding energy of the 2s exciton of the isolated quantum well we find an exciton state energetically between the 1s exciton state and the o nset of the miniband absorption. We find this state to be an interwell exciton, with the electron confined to one well and the hole to the n eighboring well. The interwell exciton resembles the first Wannier-Sta rk localized exciton of a biased superlattice. However, the localizati on in the present ease is mediated by the Coulomb interaction of the e lectron and hole. The state has considerable oscillator strength and i s observed experimentally in linear and nonlinear experiments.