Homodyne phase sensitive terahertz spectrometer

Abstract

We present the theory of a field effect transistor (FET) operating as a THz or far infrared detector. We demonstrate that the detected signal is enhanced by orders of magnitudes by using a homodyne detection scheme involving the interference of a weak incoming signal and a strong signal of a local oscillator with the close frequency. We developed a theory valid for the arbitrary relationship between the amplitude of the local oscillator and the gate voltage swing. Remarkably, the response saturates at a high local oscillator intensity at the value which depends on the phase difference between the signals. The observed gain in this regime is over 100, and the predicted maximum gain in this operating regime is on the order of 105. These results show that a FET could be used as a sensitive spectrometer and/or interferometer when exposed to a strong tunable local oscillator signal with the varying frequencies and phases. This regime of the detector operation is very promising for the interferometric and spectroscopic applications in the subterahertz and terahertz ranges.