Carrier escape tunnelling 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 escape tunnelling out of the individual quantum wells. We perform transient four-wave mixing experiments on two shallow GaAs/AlxGa1-xAs multiple quantum well samples with x=0.04 and x=0.06. We find that the electron tunnelling times out of the quasibound n=1 quantum well states into the continuum states decrease with increasing electric field, and lie in the subpicosecond regime at high fields. In addition, for the x=0.06 sample, the experimentally determined escape rate is resonantly enhanced when the quasibound n=1 quantum well state couples to the n=2 state embedded in the continuum above the second-nearest-neighbour well.