Far-infrared photon-assisted transport through quantum point-contact devices

We analyze theoretically the phenomenon of photon-assisted quantum transport in split-gate quantum point-contact devices. Both the transverse and longitudinal polarization configurations for the ac photon field are considered. We predict that ministeps should appear in the drain/source conductance versus gate voltage relation, as well as in the I-${\mathit{V}}_{\mathrm{DS}}$ curve in the nonlinear regime, for a quantum point contact irradiated with a coherent far-infrared radiation. The width of the ministeps is proportional to the radiation frequency and the height of the ministeps is a function of the radiation power. We then calculate the current responsivity for this photon-assisted process in the limit of a small radiation signal and show that it is quite comparable to the quantum efficiency e/ħω at and above 1 THz if the rf impedance of the device can be matched to that of a planar antenna.