Design and development of nonstationary THz spectrometers
Department 160

Design and development of nonstationary THz spectrometers

The essence of a nonstationary spectroscopy is as following: interaction of electromagnetic radiation with ensemble of absorbing molecules results in induction of macroscopic dipole. After a radiation impact is over, absorbed energy is reradiated coherently at the frequency of molecular transition. The operation of nonstationary spectrometers can be realized in a phase switching or fast passage regime.

Phase-switching spectrometer based on BWO

Frequency range

118-178 GHz (can be broaded up to 37-720 GHz)

Sensitivity with absorption cell length = 1 m and averaging time = 1 s 5×10-10 cm-1
Minimal measurement time resolution 1×10-6 s
Accuracy of the absorption line intensively (without preliminary calibration) ≤ 5%
Microwave frequency accuracy 10-9
Microwave radiation power ≥ 10mW

Phase-switching spectrometer based on solid state generator

Frequency range

1-2.8 THz

Frequency set accuracy 10-9
Frequency range of the Gunn oscillator 97.5-117.5 GHz
Instrumental linewidth of generation < 10 kHz

5×10-7 cm-1
Min. time of measurement 1×10-6 s
Accuracy of measuring the intensity of absorption line ≤ 10%

Fast scan fourier-transform spectrometer with frequency-locked BWO

Frequency range

37-180 GHz

Ramp ratio 105-107MHz/s
Single scan range 10-1000 MHz
Time of the scan 0.01-1 ms
Resolution Sub-Doppler
Sensitivity (l = 1 m, Tmeas/Npoint = 1 ms) ~ 10-8 cm-1

The integrated spectrometer based on SIS-mixer and FFO oscillator

Together with IRE RAS (Moscow) and SRON (Netherlands) we elaborated a high-sensitive spectrometer for registration of atmospheric gases in the framework of the ТELIS (TErahertz and sub-millimeter Limb Sounder) project. It was a new remote sensing instrument measuring atmospheric trace gases from a stratospheric balloon platform in the submillimeter and far infrared spectral region. On March, 11th, 0:18 local time a 400.000 m3 helium balloon was launched in Kiruna / Sweden for a 12 hour flight reaching a maximum altitude of 36 km.

Substances detected in the first TELIS flight
FFO Frequency, GHz Substances (High priority)
495.04 H218O
496.88 HDO
505.6 BrO (ΔТ = 0.3 К !!)
507.28 ClO
515.25 O2
519.25 BrO (ΔТ = 0.3 К !!)
607.78 O3 isotopes
619.1 HCl (HOCl, ClO)

Phase-switching spectrometer combined with supersonic molecular beams technique

We propose an approach to increase a sensitivity of microwave and THz spectroscopy, that involves application of supersonic molecular beams. The key advantage offered by such an approach is that a gas temperature can be decreased along with an increase in the gas density, which results in a much greater number of molecules interacting with radiation and, hence, in a higher absorption coefficient. This effect has been demonstrated experimentally on supersonic CO and NO beams, using a phase manipulation microwave spectrometer. The absorption coefficient was found to be three orders of magnitude higher than the value of gas absorption coefficient in a standard 1-m long cell at room temperature.

Mixture Gain in absorption coefficient
(experiment)
Gain in absorption coefficient
(theory)
NO (10% argon mixture) 1.3×103 9×102

CO (0.1% mixture)

1.2×103 1.5×103
CO (0.3% mixture) 1.5×103
CO (3% mixture) 1.5×103

Our devices were delivered to many scientific centers all over the world: Technical faculty of St. Albrecht University, Kiel, Germany; Insight Product Company, Newton, USA; Chalmers University, Gothenburg, Sweden; Texas A&M University, USA; University, Tokio, Japan; Justus Liebig University, Giessen, Germany.

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