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 th e two-dimensional electron gas in the GaAs-GaAlAs system. Delta I-dc W as induced by photoexcitation of the QPC in the terahertz regime. To o ur knowledge, these are the first measurements in which the induced cu rrent has been measured as a function of the excitation frequency omeg a. Spectra of Delta I-dc versus omega were obtained with a time-domain interferometer based on photoconductive (Auston) switch sources of pu lsed terahertz radiation. The pulses were coupled to the gate that def ines 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 sug gests that photon-assisted transport is a possible mechanism for the i nduced de current. Measurements of the QPC response demonstrate the ap plicability of this technique to a variety of other mesoscopic structu res, such as single and multiple quantum dots.