Phase-Coherent Heterodyne Detection in the Terahertz Regime With a Photomixer

A low-temperature-grown GaAs photomixer with a spiral antenna has a quadratic response to the optical fields and an intrinsic nonlinear IV characteristic that allow the detection of THz radiation. A transmission line coupled to the antenna provides the photomixer with direct-current access in the microwave domain. The non-ohmic response of the photomixer is probed by rectification. The optical beat between two continuous-wave lasers illuminating the photomixer serves as a local oscillator with an adjustable difference-frequency in the THz domain. The beat between a THz field modulated at a radiofrequency rate with a radiofrequency-modulated optical beat is exploited in a phase-sensitive detection scheme.