Magnetically tuned resonant photon-assisted tunneling

We present evidence of a new channel for electronic conduction in a multi-quantum well type superlattice in the presence of a magnetic field applied parallel to the quantum well layers. Electrons in one potential well that are photo-excited to anti-crossings of the dispersion curves of the conduction subbands tunnel resonantly to the neighboring wells without intrasubband relaxation. This tunneling mechanism must be distinguished from tunneling involving states located around the energy minimum of the conduction subbands. We show that the new tunneling channel can only be understood from a nonperturbative calculation of the energy levels of the multi-quantum well structure in the presence of the magnetic field, since first order perturbation theory predicts the same energy dispersion for all conduction subbands, thus making this tunneling channel indistinct from standard intersubband excitation. Measurements of THz photocurrent show that both tunneling processes co-exist in a GaAs∕AlGaAs multi-quantum well superlattice.