Terahertz absorption spectroscopy for measuring atomic oxygen densities in plasmas

Abstract

This paper describes the first implementation of terahertz (THz) quantum cascade lasers for high-resolution absorption spectroscopy on plasmas. Absolute densities of ground state atomic oxygen were directly obtained by using the fine structure transition at approximately 4.75 THz. Measurements were performed on a low-pressure capacitively coupled radio frequency oxygen discharge. The detection limit in this arrangement was found to be 2∗1013 cm-3, while the measurement accuracy was within 5%, as demonstrated by reference measurements of a well-defined ammonia transition. The results show that the presented method is well suited to measure atomic oxygen densities, and it closes the THz gap for quantitative atomic density measurements in harsh environments such as plasmas.

Performance of a High-Speed Pyroelectric Receiver as Cryogen-Free Detector for Terahertz Absorption Spectroscopy Measurements

“…The distance in wavenumber between the fringes is given by 1/(2nL) and equal to 0.016 cm −1 , resulting in a tuning coefficient of 7.8 × 10 −4 cm −1 /mA. This value is in good agreement with results of previous measurements performed with a bolometer as the THz detector [15,16]. The same value for the tuning coefficient was obtained for various laser tuning frequencies up to 281 Hz.…”

“…The repetition frequency of the sawtooth current waveform is the so-called laser tuning frequency. In previous works [15,16], laser tuning frequencies of 10 Hz or 11 Hz were used; however, faster measurements were hampered by the limited bandwidth of the used bolometer (Infrared Laboratories, Tucson, AZ, USA, 4.2K Bolometer; 3 dB roll-off frequency of 500 Hz), leading to an asymmetric deformation of the observed absorption features [15]. The larger bandwidth of the pyroelectric receiver (Laser Components Germany, Olching, Germany, PR No1; 3 dB roll-off frequency of 8 kHz) should allow to increase the laser tuning frequency without causing a deformation of absorption features.…”

“…A tunable, continuouswave (cw) THz QCL with an output power of approximately 4 mW was used as the THz source [7]. The same THz QCL has been used in previous work to detect the 3 P 1 ← 3 P 2 fine structure transition of ground-state atomic oxygen at approximately 4.75 THz, which corresponds to approximately 158 cm −1 and a wavelength of approximately 63.2 µm [15,16]. The QCL was operated in a Stirling cryocooler (Ricor K535) at a temperature of 43.3 K while temperature fluctuations were minimised to be below 5 mK by an additional temperature controller (Stanford Research Systems, Sunnyvale, CA, USA, CTC100).…”

“…In particular, the detection of fine structure transitions in atoms is of high interest, as this could provide a means of developing absolute detection methods for atoms based on direct absorption spectroscopy. Recent examples of this are the detection of atomic oxygen by heterodyne spectroscopy in astronomy and atmospheric research [12][13][14] and the determination of the absolute densities of oxygen atoms in a plasma reactor [15][16][17]. There have been a few reports on the application of THz QCLs for absorption spectroscopy in the gas phase as well [18][19][20][21].…”

“…In addition to the requirement of cryogenic cooling, they typically have a limited bandwidth in the order of up to a few hundred Hz for the 3 dB roll-off frequency, which significantly reduces the laser tuning frequencies that can be applied and thus leads to longer measurement times when recording an absorption spectrum. For example, in previous work [15,16], laser tuning frequencies of 10 Hz or 11 Hz had to be used to avoid any asymmetric deformation of the observed absorption features, which, in turn, also limited the spectral range that could be probed.…”