DYNAMIC-RESPONSE OF A QUANTUM POINT-CONTACT

We report measurements of the de current Delta I-dc generated across a nanometer-scale constriction, or a quantum point contact (QPC), in the two-dimensional electron gas in the GaAs-GaAlAs system. Delta I-dc 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 omega. Spectra of Delta I-dc versus omega 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 de 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.