Heterodyne phase sensitive terahertz spectrometer

Abstract

We demonstrate that a field effect transistor (FET) operating as THz or far infrared sensor could be used as a sensitive spectrometer when exposed to a strong tunable local oscillator signal with the varying frequency and phase. The FET nonlinearity results in the detected signal being enhanced by the local oscillator power and proportional to the cosine of the phase difference between the local oscillator and the signal. In contrast to the conventional heterodyne regime, the output signal becomes independent of the local oscillator power at large local oscillator powers when the induced local oscillator voltage becomes large compared to the gate voltage swing at the operating point. The observed gain in this regime is over 90 and predicted maximum gain is on the order 100,000 to 500,000. This regime of the detector operation is very promising for applications in the terahertz interferometry and spectroscopy.