Dynamic response of a quantum point contact

Abstract

We report measurements of the dc current ΔIdc generated across a nanometer-scale constriction, or a quantum point contact (QPC), in the two-dimensional electron gas in the GaAs–GaAlAs system. ΔIdc was induced by photoexcitation of the QPC in the terahertz regime. To our knowledge, these are the first measurements in which the induced current has been measured as a function of the excitation frequency ω. Spectra of ΔIdc versus ω were obtained with a time-domain interferometer based on photoconductive (Auston) switch sources of pulsed terahertz radiation. The pulses were coupled to the gate that defines the QPC by a combination of quasi-optical techniques and a novel antenna–transmission-line structure. The spectra are a strong function of the height of the barrier that creates the constriction, which suggests that photon-assisted transport is a possible mechanism for the induced dc current. Measurements of the QPC response demonstrate the applicability of this technique to a variety of other mesoscopic structures, such as single and multiple quantum dots.