Direct measurement of resonant and nonresonant tunneling times in asymmetric coupled quantum wells

We have directly measured the tunneling times in an asymmetric coupled double-quantum-well structure using subpicosecond luminescence spectroscopy. We observe a strong dependence of the nonresonant tunneling times on the barrier thickness. The most striking feature of the data is a drastic reduction of the tunneling time and then a further increase as an applied electric field sweeps electron levels in the two wells through a resonance. The measured resonant tunneling time is considerably longer than the estimated coherent tunneling time.