A NEW TERAHERTZ PHOTOMIXER SPECTROMETER: APPLICATION TO $SO_{2}$ SELF BROADENING

Abstract

We will describe the characteristics of a new tunable cw far-infrared difference-frequency spectrometer obtained by mixing two single-mode visible lasers separated by terahertz frequencies in an ultra-high-speed photoconductor. The photomixer device is fabricated at MIT Lincoln laboratory with submicron interdigital electrodes driving a broadband self-complementary spiral antenna on epitaxial low-temperature-grown GaAs material. The LTG-GaAs material has subpicosecond recombination lifetimes, facilitating the ultrafast response of the mixer. The far-infrared radiation is is coupled out of the GaAs substrate by an aplanatic Si lens and is detected by a conventional Si-composite bolometer at T = 4.2 K. We have obtained acceptable signal-to-noise ratios for radiation from 0.1 to 1 THz using 584 nm dye laser pumps with prospect to 2 or 3 THz with pump wavelengths closer to the GaAs bandgap. The instrumental linewidth is currently ${\sim} 3$ MHz FWHM, limited primarily by the jitter of one of the two dye lasers. We will describe baseline normalization techniques to compensate for the amplitude fluctuations of the asers and the strong fringes arising from standing waves between the source and detector and within the detector dewar. Application to the measurement of self broadening of the pure rotational transitions of $SO_{2}$ will be discussed.