Terahertz quantum-cascade lasers for high-resolution absorption spectroscopy of atoms and ions in plasmas

Abstract

We report on terahertz (THz) quantum-cascade lasers (QCLs) based on GaAs/AlAs heterostructures, which exhibit single-mode emission at 3.360, 3.921, and 4.745 THz. These frequencies are in close correspondence to fine-structure transitions of Al atoms, N+ ions, and O atoms, respectively. Due to the low electrical pump power of these THz QCLs, they can be operated in a mechanical cryocooler in continuous-wave mode, while a sufficient intrinsic tuning range of more than 5 GHz is maintained. The single-mode operation and the intrinsic tuning range of these THz QCLs allow for the application of these lasers as radiation sources for high-resolution absorption spectroscopy to determine the absolute densities of Al atoms, N+ ions, and O atoms in plasmas.

Terahertz absorption spectroscopy for measuring atomic oxygen densities in plasmas

“…4.744 777 49 THz) [20], a tunable QCL exhibiting single-mode operation was developed based on GaAs/AlAs heterostructures. More details on the development of this QCL can be found in the work of Lü et al [34]. Stable laser operation could be realised in continuouswave mode with optical output powers ranging from 1 to 8 mW at operating temperatures below 70 K. The QCL was therefore operated in a Stirling cryocooler (Ricor K535), as schematically illustrated in figure 1.…”

“…Tuning of the laser output across a frequency range of approximately 3 GHz was achieved by linearly ramping the input current. Continuous ramping instead of discrete stepping of the current (as was done in the work of Hübers et al [28]) was enabled by the stable and mode-hop-free behaviour of the used THz QCL [34]. It allowed for faster measurements with a much better spectral sampling rate (with frequency 'steps' down to 1 × 10 −7 cm −1 , i.e.…”

“…Therefore, THz QCLs have recently been developed for the purpose of detecting atomic oxygen in plasmas by using the fine structure transition at approximately 4.75 THz [33,34]. This paper reports on the first implementation of these lasers for high-resolution absorption spectroscopy, and it demonstrates the suitability of THz absorption spectroscopy as a diagnostic technique for measuring atomic oxygen densities.…”

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

“…The absorption spectroscopy measurements were performed on a reference gas cell (RGC) filled with ammonia gas to a precisely known pressure. 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 first QCL with an emission frequency in the THz range was reported in 2002 by Köhler et al [4]. THz QCLs cover, at present, the emission frequency range of about 1 THz to 5.5 THz [5][6][7]. In this spectral region, QCLs are the only available powerful cw radiation sources, with optical output powers of up to a few mW and extremely narrow linewidths of a few MHz down to kHz.…”