Escape tunneling out of shallow multiple quantum wells studied by transient four-wave mixing

We demonstrate that transient optical dephasing experiments on electrically biased, shallow multiple quantum wells offer the unique possibility of studying the first step in carrier sweep-out, namely, the tunneling escape out of the individual quantum wells. We find that the electron tunneling times out of the quasibound n=1 quantum well states into the continuum states lie in the subpicosecond regime at high electric fields. In addition, the experimentally determined escape rate is resonantly enhanced when the quasibound n=1 quantum well state couples with the resonant n=2 continuum state originating from the second-nearest neighbor well.