Dual-comb spectroscopy of laser-induced plasmas

Nature Communications, Mar 2018

Dual-comb spectroscopy has become a powerful spectroscopic technique in applications that rely on its broad spectral coverage combined with high frequency resolution capabilities. Experiments to date have primarily focused on detection and analysis of multiple gas species under semi-static conditions, with applications ranging from environmental monitoring of greenhouse gases to high-resolution molecular spectroscopy. Here, we utilize dual-comb spectroscopy to demonstrate broadband, high-resolution, and time-resolved measurements in a laser-induced plasma. As a demonstration, we simultaneously detect trace amounts of Rb and K in solid samples with a single laser ablation shot, with transitions separated by over 6 THz (13 nm) and spectral resolution sufficient to resolve isotopic and ground state hyperfine splittings of the Rb D2 line. This new spectroscopic approach offers the broad spectral coverage found in the powerful techniques of laser-induced breakdown spectroscopy (LIBS) while providing the high-resolution and accuracy of cw laser-based spectroscopies.

A PDF file should load here. If you do not see its contents the file may be temporarily unavailable at the journal website or you do not have a PDF plug-in installed and enabled in your browser.

Alternatively, you can download the file locally and open with any standalone PDF reader:

https://www.nature.com/articles/s41467-018-03703-0.pdf

Dual-comb spectroscopy of laser-induced plasmas

Abstract Dual-comb spectroscopy has become a powerful spectroscopic technique in applications that rely on its broad spectral coverage combined with high frequency resolution capabilities. Experiments to date have primarily focused on detection and analysis of multiple gas species under semi-static conditions, with applications ranging from environmental monitoring of greenhouse gases to high-resolution molecular spectroscopy. Here, we utilize dual-comb spectroscopy to demonstrate broadband, high-resolution, and time-resolved measurements in a laser-induced plasma. As a demonstration, we simultaneously detect trace amounts of Rb and K in solid samples with a single laser ablation shot, with transitions separated by over 6 THz (13 nm) and spectral resolution sufficient to resolve isotopic and ground state hyperfine splittings of the Rb D2 line. This new spectroscopic approach offers the broad spectral coverage found in the powerful techniques of laser-induced breakdown spectroscopy (LIBS) while providing the high-resolution and accuracy of cw laser-based spectroscopies. Introduction Laser-induced plasmas provide a versatile and non-contact means to apply the powerful tools of optical spectroscopy in the analysis of solid materials. When the intensity of an incident laser pulse is sufficiently high, a plasma is generated, ablating a small amount of material above the sample surface. The resulting ions, atoms, and molecules within the evolving plasma plume can be detected optically using either emission or absorption techniques. The most common optical spectroscopy technique for laser ablation plumes is optical emission spectroscopy, typically called laser-induced breakdown spectroscopy (LIBS)1,2. In this technique, the emission spectrum from electronically excited ions, atoms, and molecules is recorded. LIBS has had a significant impact in a broad range of fields. For example many geological applications exist for studying the composition of rocks, minerals, and soils3, including the planetary exploration of Mars by the NASA rover Curiosity4. LIBS has also been applied for the characterization of nuclear materials and their isotopic concentrations5, needed for monitoring nuclear waste and nuclear fuel production for its use in both civilian and military applications. Other applications are found in areas ranging from pharmaceutical quality control6, monitoring of industrial processes7, forensic science8, and for trace detection of nanoparticles used in medical applications9, to name a few. In this work, we apply the technique of dual frequency comb spectroscopy to optically probe laser-induced plasmas. Dual-comb spectroscopy (DCS) has become a valuable tool for broadband and high-resolution spectroscopic measurements. We demonstrate the potential impact of this approach in the analysis of solid materials by simultaneously detecting trace amounts of Rb and K in solid samples using a single laser ablation shot while still providing sufficient spectral resolution to observe the isotopic and ground state hyperfine splittings of the Rb D2 line. The broadband, sensitive, and high spectral and temporal resolution of this approach can enable the capability to optically identify and track multiple atomic, ionic, and molecular species present in the dynamic plasma environment. Results Laser ablation spectroscopy To obtain the maximum amount of information about the composition of a solid sample via optical probing of a laser ablation plume, it is desirable to utilize a large spectral bandwidth so that multiple ionic, atomic, molecular, and continuum transitions can be identified. At the same time, a high spectral resolution is needed to distinguish closely spaced transitions, to resolve small isotopic shifts, to measure intrinsic linewidths of transitions, and to minimize spectral interferences from multiple species. Other desirable aspects of an optical spectroscopic measurement are rapid detection, time and space resolution, and simultaneous acquisition of multiple wavelengths. Unfortunately, it is difficult to satisfy all requirements simultaneously and thus compromises must be made based on available spectroscopic instrumentation. In LIBS, a dispersive spectrograph combined with an intensified-CCD (ICCD) camera is often used to record the optical emission. It is straightforward to measure time-gated emission spectra from laser ablation plasmas with the ICCD. However, this technique requires a trade-off in spectral bandwidth versus spectral resolution based on diffraction grating selection and ICCD array size. Thus, a low- to moderate-resolution spectrum can be obtained over a large spectral bandwidth, or a high-resolution spectrum can be obtained over a reduced spectral bandwidth. For example, a typical LIBS spectrum measured using a 0.5 m Czerny-Turner spectrograph with 2400 g/mm grating could provide a spectral resolution of ≈20 pm with ≈5 nm spectral bandwidth. If the emission intensity is high enoug (...truncated)


This is a preview of a remote PDF: https://www.nature.com/articles/s41467-018-03703-0.pdf

Jenna Bergevin, Tsung-Han Wu, Jeremy Yeak, Brian E. Brumfield, Sivanandan S. Harilal, Mark C. Phillips, R. Jason Jones. Dual-comb spectroscopy of laser-induced plasmas, Nature Communications, 2018, Issue: 9, DOI: 10.1038/s41467-018-03703-0