Dual-comb spectroscopy may not be as well known as other types of spectroscopy, but it has found its niche across many applications where the detection and analysis of multiple gas species is required. In this article, we look at how dual-comb spectroscopy has been used by a team of researchers from the US to measure and analyse the gaseous elements found in laser-induced plasmas.
Dual-comb spectroscopy has become the method of choice for applications that require a broad spectral coverage and high-frequency resolution to be measured. Experiments to date have mainly been used to determine the elemental composition of multiple gaseous molecules, commonly under semi-static conditions. Applications to date have included environmental monitoring of greenhouse gases and high-resolution molecular spectroscopy.
Dual-comb spectroscopy now offers an alternative technique to laser-induced breakdown spectroscopy (LIBS) in the analysis of laser-induced plasma gases. Laser-induced plasma gases are produced when the intensity of a laser pulse is very high. This process also ablates a small amount of material above the plasma, and the atoms, molecules, and ions within the plasma plume can then be detected.
LIBS has been widely used to detect the emission spectrum of laser-induced plasmas, until now. The team of US-based researchers have taken a different approach to optically probing these plasmas by using dual frequency comb spectroscopy, to demonstrate broadband, high-resolution, and time-resolved measurements within the plasma.
The researchers have demonstrated this technique by simultaneously analyzing the trace amounts of potassium and rubidium in solid samples. To do this, the researchers used a single laser ablation shot whilst providing a spectral resolution to observe the isotopic and ground state hyperfine splitting of the rubidium ions.
The researchers used a model PMAM-780-TR-DCS-0.120 GHz dual-comb spectrometer with two Kerr lens mode-locked Ti:sapphire lasers. The researchers centered the bandwidth of the lasers to between 760 and 800 nm, and a transfer oscillator (Vescent D2-100) was used to establish phase coherence between the comb modes.
To measure the elements in real-time, the researchers had to measure a rubidium reference material. This took the form of measuring the rubidium D2 around 780 nm in a glass cell using a natural abundance of rubidium atoms (72% 85Rb, 28% 87Rb). The reference hyperfine states were easily resolved and created a pathway for atoms to be determined in laser-induced plasmas.
Using this approach, and after performing all reference tests, the researchers were able to detect trace amounts of rubidium and potassium in the solid samples. The researchers found that the transitions were separated by more than 6 THz (13 nm); and that the spectral resolution was sufficient enough to resolve the isotopic states, and the ground states, for the hyperfine splitting in the D2 line of the rubidium ions. In addition, this spectroscopy approach was also found to simultaneously measure the potassium ions in the plasma, as well being able to provide rapid measurements. The spectral coverage also showed that it could extend to almost any part of the electromagnetic spectrum.
This new spectroscopic approach, with a sub-GHz resolution, provides the broad spectral coverage found with powerful detection techniques such as laser-induced breakdown spectroscopy (LIBS). In addition to this, dual-comb spectroscopy also provides a high resolution and accuracy similar to that of cw laser-based spectroscopies. In short, it is a powerful technique for measuring plasmas and can be used to identify and track multiple ionic, atomic, and molecular species in an evolving plasma.
Based on its potential capabilities from this research, future studies involving dual-comb spectroscopy could exploit the properties of femtosecond (fs) frequency combs to probe laser-induced plasmas for use in the optical analysis of solid materials. Dual-comb spectroscopy could also be further extended to studying the kinetic dynamics in chemical reactions and pulsed-detonation combustion.
“Dual-comb spectroscopy of laser-induced plasmas”- Bergevin J., et al, Nature Communications, 2018, DOI: 10.1038/s41467-018-03703-0
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