Terahertz detection with quantum point contacts

A current focus of terahertz (THz) research concerns the applications of semiconductor nanostructures, whose characteristic energies lie in the THz (i.e., meV) range. Quantum Point Contacts (QPCs) are an important example of such devices, which are realized in a high-mobility two-dimensional electron gas by applying a depletion bias to Schottky gates on top of a semiconductor. We have studied the characteristic energy scales of these nanodevices, employing a new technique of current-voltage spectroscopy. This technique yields the full subband structure, and effective barrier height, of the QPCs, With the aid of the QPC device as the main circuit element, and utilizing a planar broadband antenna, we demonstrate clear THz photo-response in the conductance of QPCs under 1.4- and 2.5-THz radiation. We show that this THz photo-response is related to QPC characteristics and not due to an electron heating effect arising from the THz field. By fitting our experimental results to developed model, we show that the observed THz photo-response is excellently described by a classical gate-voltage rectification. This mechanism leads to ranges of gate voltage for which the photo-response of the QPC is strongly pronounced. These are the transition regions between quantized plateaus, where a small modulation of gate voltage causes a large induced photo-signal. Finally, we explore the suitability of our QPC device for use as a THz sensor that makes use of rectification mechanism. In addition to demonstrating the configuration of the QPC device that provides optimal THz sensitivity, we also determine the noise equivalent power and responsivity of the sensor. Our studies suggest that QPCs can provide a viable approach to broadband THz sensing in the range above 1 THz.