PHOTOCONDUCTANCE OF 2-DIMENSIONAL BALLISTIC MICROSTRUCTURES

We calculate the photoconductance of two-dimensional ballistic microst ructures subject to a high-frequency electromagnetic field. First, we study a simple quantum point contact. Absorption of photons is due to electronic transitions between different modes. A transition between a propagating and a nonpropagating mode results effectively in a backsc attering process, and gives a negative or positive contribution to the current, depending on the gate voltage; the total quantized conductan ce acquires an additional, quite pronounced step-structure. Then, we d emonstrate a new effect where the electron-photon interaction in a str ucture of slightly more complex geometry plays the same role as impuri ty scattering does in a ''dirty'' system. All relevant photons of the external electromagnetic field are coherent and spatial interference e ffects in electron-photon scattering become possible in spite of the i nelastic nature of the collisions. These interference effects can be c ontrolled by the gate voltage or the frequency of the electromagnetic field. As an illustration we calculate the photoconductance of a doubl e point-contact geometry.