Dual-Comb Spectroscopy of Laser-Produced Plasmas

Abstract

Dual-comb spectroscopy (DCS) represents a novel method of using absorption spectroscopy as a diagnostic tool for time-resolved multispecies analysis of excitation temperatures and column densities in laser-produced plasmas (LPPs). DCS utilizes two stabilized modelocked lasers to generate a pair of mutually-coherent frequency combs and enables broadband spectroscopic measurements with high spectral and temporal resolution that are well-suited for studying the quickly evolving conditions of LPPs. The ablation plume of an LPP evolves both spatially and temporally and, when combined with optical diagnostics, has proved useful both as a means for preparing high-temperature gas-phase atomic/molecular species and for non-contact elemental analysis of solid materials. Temperature and number density studies involving ionic, atomic, and molecular species present in the LPP are applicable to quantitative analysis of sample composition as well as plasma diagnostic research focused on plume formation and expansion, molecular formation, diffusion rates, and condensation processes, both spatially and temporally. LPPs can be studied by DCS with both the necessary time and spectral resolutions required to probe many absorption transitions within the timescales of late-time LPP evolution. Recent work has shown that the technique’s high spectral resolutions enable measurements of congested optical spectra, such as those from heavy elements and molecules, to be resolved and more accurately analyzed. Broadband detection of multiple transitions, combined with Boltzmann-plot style analysis commonly used in laser-induced breakdown spectroscopy (LIBS), provides the ability to determine time-resolved excitation temperatures and total column densities of atomic species. Using efficient harmonic conversion in nonlinear crystals (e.g., second harmonic generation), DCS can easily access multiple wavelength regions. The ability to measure spectrally and temporally resolved broadband spectra within many wavelength regions makes DCS an effective optical technique for studying LPPs as well as additional spectroscopic applications.