Modern resonator spectroscopy at SubMM wavelengths

Abstract

IntroductionThe resonance methods of investigations are widely used in precision spectroscopy in all frequency ranges due to their high sensitivity. But the resonance systems have different specific features in each range. In the millimeter (MM) and submillimeter (SubMM) ranges, classical open Fabry-Perot resonators with an emptyresonator Q-factor of about 10 6 (for reasonable sizes) are used for the gaseous and condensed media investigations [1][2][3][4][5][6][7][8].The present work based on the previous development is to our knowledge the first advancement of highsensitivity classical resonator spectroscopy to frequencies of up to 520 GHz. The resonator spectrometer earlier developed at the IAP RAS, methods of measurement, and the results of studying various materials from 40 GHz up to 370 GHz are well described in [2][3][4][5][6]12]. So in present paper, a detailed description of the resonator excitation system and the frequency phase lock system, which were essentially modified for operation at the 350-520 GHz, is presented.Also the first results obtained using the spectrometer in the extended frequency range are reported. Resonator spectrometer excitation system and frequency stabilization systemBackward wave oscillators (BWOs) are used as the sources of radiation with continuous frequency tuning. The sources are equipped with a precise digital frequency stabilization and control system based on the phase lock-in loop (PLL) system with the centimeters synthesizer harmonics used as the reference signal. It is worth mentioning that for broadband high-sensitivity MM and SubMM spectroscopy, BWOs remain the most convenient and reliable sources of radiation. A quasi-optical line forming the Gaussian beam is used for the resonator excitation. Fundamental TEM00q modes of the Fabry-Perot resonator are employed. A small-angle round smooth horn is used for transformation of the main generator waveguide mode TE01 to a quasi-optical Gaussian beam. Our quasi-optical line makes it possible to excite the parasitic TEMmnq modes at a level less than 10% of the TEM00q modes at all operation frequencies.The main characteristic of the frequency ranges above 370 GHz are, first of all, the decreasing (down to several milliwatts) BWO output power and the point-contact detectors sensitivity reducing. For the frequency range 350-520 GHz covered by an OB-32 BWO, a quasi-optical line Fig. 1. was developed. There are: (1) BWO in the magnet. (2) Waveguide junction from output waveguide with 1.2 ×2.4 mm 2 cross section to the waveguide with 0.55 × 1.1 mm 2 cross section. (3) Horn. (4) Beam splitter for the PLL system. (5) Attenuator. (6) 3-dB divider. (7) Terminating loads. (8) Transmission line of resonator response to bolometer. (9) Horn. (10) Multiplier-mixer of the PLL system. (11) Multiplier-amplifier of the cantimetre synthesizer signal in the PLL system. (12) Isolator 8-18 GHz. (13) Preamplifier (HEMT) of intermediate frequency (IF) in the PPL system.We would like to note that components 4-8 have been assembled from the excellent S...