Femtosecond study of exciton tunneling in (Zn,Cd)Se/ZnSe asymmetric double quantum wells

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