A terahertz photomixing spectrometer: Application to SO2 self broadening

A new tunable cw far-infrared difference-frequency spectrometer has been developed by mixing two single-mode visible dye lasers separated by terahertz frequencies in an ultrahigh-speed photoconductor. The photomixer device is fabricated with submicrometer interdigital electrodes driving a broadband self-complementary spiral antenna on epitaxial low-temperature-grown (LTG) GaAs material. The LTG-GaAs material has subpicosecond recombination lifetimes, facilitating the ultrafast response of the mixer. The coherent far-infrared radiation is coupled out of the rear surface of the GaAs substrate using an aplanatic Si lens and is detected by a conventional Si-composite bolometer at T = 4.2 K. We have obtained usable signal-to-noise ratios for radiation from 0.1 to 1.0 THz using 584-nm dye laser pumps with the prospect of reaching 2 or 3 THz with pump wavelengths closer to the GaAs bandgap. The instrumental linewidth is currently about 2 MHz, limited by the jitter of the dye lasers. Baseline normalization is necessary to compensate for amplitude fluctuations of the lasers and for the strong fringes arising from standing waves between the source and detector and within the detector dewar. The instrument has been used to obtain SO2 self-broadening coefficients for pure rotational transitions in several Q branches. (C) 1996 Academic Press, Inc.