FEMTOSECOND STUDY OF EXCITON TUNNELING IN (ZN,CD)SE ZNSE ASYMMETRIC DOUBLE-QUANTUM WELLS/

We have studied, both experimentally and theoretically, exciton tunnel ing in a (Zn,Cd)Se/ZnSe asymmetric double quantum-well structure. The large exciton binding energy of these structures allows us to elucidat e the role of the electron-hole interaction. Subsequent to resonant fe mtosecond pulse excitation, we observe fast (1 ps) exciton tunneling o ut of the narrow well, although LO-phonon scattering is forbidden for holes in a single-particle picture. Our theoretical analysis shows, ho wever, that tunneling of the exciton as a whole entity with the emissi on of only one LO phonon is very slow. Instead, the exciton tunnels vi a an indirect state in a two-step process whose efficiency is dramatic ally enhanced by Coulomb effects.