A quasi-optical multiplier for terahertz spectroscopy

The rotational K-structure (K = 0,... 6) of the a-type transition (J = 65 <-- 64) of the symmetric top molecule methylcyanide, CH3CN, has been recorded at 1.2 THz employing a newly designed quasi-optical frequency multiplier. A planar Schottky diode is used as non-linear element for producing the harmonic power and is mounted in the focus of an off-axis antenna across a short single ridged waveguide. The 2nd harmonic of the fundamental frequency is filtered with a dichroic plate and is focused onto the InSb bolometer after passing an absorption cell. The multiplier is driven by a high power backward wave oscillator with a fundamental frequency near 600 GHz.