A TERAHERTZ PHOTOMIXING SPECTROMETER - APPLICATION TO SO2 SELF-BROADENING

A new tunable cw far-infrared difference-frequency spectrometer has be en developed by mixing two single-mode visible dye lasers separated by terahertz frequencies in an ultrahigh-speed photoconductor. The photo mixer 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 subp icosecond recombination lifetimes, facilitating the ultrafast response of the mixer. The coherent far-infrared radiation is coupled out of t he rear surface of the GaAs substrate using an aplanatic Si lens and i s detected by a conventional Si-composite bolometer at T = 4.2 K. We h ave 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 instrumen tal linewidth is currently about 2 MHz, limited by the jitter of the d ye lasers. Baseline normalization is necessary to compensate for ampli tude fluctuations of the lasers and for the strong fringes arising fro m standing waves between the source and detector and within the detect or dewar. The instrument has been used to obtain SO2 self-broadening c oefficients for pure rotational transitions in several Q branches. (C) 1996 Academic Press, Inc.