PHOTOCONDUCTANCE OSCILLATIONS IN A 2-DIMENSIONAL QUANTUM POINT-CONTACT

We have calculated the photoconductance of a quantum point contact subject to a high-frequency electromagnetic held. We find very pronounced steplike oscillations of the photoconductance as a function of gate voltage. The absorption of the electromagnetic held, which is polarized in the transverse direction, is due to transitions between different modes of the quantum point contact. A transition between a propagating and a nonpropagating mode results effectively in a backscattering process, and gives a negative or positive contribution to the current, depending on the gate voltage. When the number of propagating modes through the point contact exceeds a cutoff value, the photoconductance disappears. The cutoff value depends on the frequency of the electromagnetic field. We also find that the oscillation amplitude increases with the number of propagating modes. As a result of the electron-photon interaction, the total quantized conductance acquires an additional step structure.