  In this paper the photon-assisted electron motion in a multiquantum well
(MQW) semiconductor heterostructure in the presence of an electric field is
investigated. The time-dependent Schrodinger equation is solved by using the
split-operator technique to determine the photocurrent generated by the
electron movement through the biased MQW system. An analysis of the energy
shifts in the photocurrent spectra reveals interesting features coming from the
contributions of localized and extended states on the MQW system. The
photocurrent signal is found to increase for certain values of electric field,
leading to the analogue of the negative-conductance in resonant tunneling
diodes. The origin of this enhancement is traced to the mixing of localized
states in the QWs with those in the continuum. This mixing appears as
anticrossings between the localized and extended states and the enhanced
photocurrent can be related to the dynamically induced
Landau-Zener-Stuckelberg-Majorana transition between two levels at the
anticrossing.
