Superlattice transport in terahertz electric fields

The transport properties of semiconductor superlattices and quantum wells in the presence of AC electric fields has been the focus of much theoretical work for the past twenty years. Among the many theoretical predictions have been photon-assisted tunneling, dynamic localization, and absolute negative conductivity (ANC). Until recently, however, few of these theories have been tested experimentally.; We have studied the transport properties of semiconductor superlattices coupled to the terahertz radiation of UCSB's Free Electron Lasers. The structures studied were MBE grown semiconductor multiple quantum well superlattices. Two different coupling schemes were used to couple the superlattices to the terahertz radiation. The first series of experiments involved wire bonded mesas in which the bonded wire acted as an antenna to couple the devices to the terahertz radiation. These experiments enabled the first observation of photon-assisted tunneling in semiconductors as well as lend some support to the Tien and Gordon model of photon-assisted tunneling.; Several new observations were made using semiconductor superlattices fabricated into bow-tie antennas. These observations include dynamic localization, absolute negative conductivity (ANC), stimulated emission, multi-photon assisted tunneling, photon induced electric field domains and an oscillatory dependence of the induced current on laser power.; Perhaps the most remarkable result was found in the power dependence of the current-voltage (I-V) characteristics near zero DC bias. As the laser power is increased the current decreases towards zero and then becomes negative. This result implies that the electrons are absorbing energy from the laser field, producing a net current in the direction opposite to the applied voltage.; ANC around zero DC bias is a particularly surprising observation considering photon-assisted tunneling is not expected to be observable between the ground states of neighboring quantum wells in a semiconductor superlattice. Contrary to this we believe these results are most readily attributable to photon absorption and multiphoton emission between ground states of neighboring wells.