Room temperature, THz photomixing sweep oscillator and its application to spectroscopic transmission through organic materials

An all-solid-state continuous-wave sweep oscillator has been developed that provides high-resolution, transmission measurements between ∼30 GHz and 3 THz. It is based on difference-frequency generation between two cw frequency-offset lasers driving an ultrafast photoconductive mixer (photomixer). The output power around 100 GHz is approximately 10 μW, falling to about 1 μW around 1 THz and 0.1 μW around 3 THz. The sweep oscillator is used with two types of detectors: a hot electron bolometer for high-sensitivity measurements of weak absorption features below 1 THz, and a room temperature Golay cell for coarse measurements anywhere between 30 GHz and 3 THz. The sweep oscillator facilitates the rapid characterization of a broad variety of materials including inorganic solids, biological materials, liquids, and gases with far greater resolution (∼10 MHz), frequency accuracy (∼0.1 GHz), and spectral density (∼1 μW/MHz) than competitive wideband instruments such as Fourier-transform or time-domain spectrometers. To demonstrate the versatility of the sweep oscillator, results are presented for two diverse cases of interest: (1) fast scan, broadband (>1 THz) absorption profiles from biomaterials such as polysaccharides, and (2) slow scan, narrow-band (∼1 GHz) atmospheric lines from water vapor.