Burst-mode dual-comb spectroscopy.

We introduce a new, to the best of our knowledge, modality of dual-comb spectroscopy (DCS) that enables a simplified and powerful new approach for time-resolved measurements with increased acquisition rates. This "burst mode" form of DCS relies on the multiplexing of each probe pulse into a short train of pulses. With this approach we demonstrate a time-resolved series of absorption-based spectroscopic measurements of a laser-induced plasma using only a single laser ablation shot and identify 22 Nd lines not previously reported in the literature. The transmission spectra spanned 3.1 THz and were acquired at an effective acquisition rate of 25 kHz with 40 µs time resolution. This simple modification to ∼100MHz level dual-comb systems provides a flexible approach for studying transient and low-duty-cycle events such as laser-induced plasmas, combustion, and explosive reactions.

Standoff chemical plume detection in turbulent atmospheric conditions with a swept-wavelength external cavity quantum cascade laser.

Rapid and sensitive standoff measurement techniques are needed for detection of trace chemicals in outdoor plume releases, for example from industrial emissions, unintended chemical leaks or spills, burning of biomass materials, or chemical warfare attacks. Here, we present results from 235 m standoff detection of transient plumes for 5 gas-phase chemicals: Freon 152a (1,1-difluoroethane), Freon 134a (1,1,1,2-tetrafluoroethane), methanol (CH3OH), nitrous oxide (N2O), and ammonia (NH3). A swept-wavelength external cavity quantum cascade laser (ECQCL) measures infrared absorption spectra over the range 955-1195 cm-1 (8.37- 10.47 µm), from which chemical concentrations are determined via spectral fits. The fast 400 Hz scan rate of the swept-ECQCL enables measurement above the turbulence time-scales, reducing noise and allowing plume fluctuations to be measured. For high-speed plume detection, noise-equivalent column densities of 1-2 ppm*m are demonstrated with 2.5 ms time resolution, improving to 100-400 ppb*m with 100 ms averaging.

Expansion dynamics and chemistry evolution in ultrafast laser filament produced plasmas.

Laser ablation in conjunction with optical emission spectroscopy is a potential non-contact, stand-off detection method for all elements in the periodic table and certain isotopes such as radionuclides. Currently, significant development efforts are on-going to use ultrafast laser filaments for remote detection of materials. The application of filaments is of particular interest in extending the range of stand-off capability associated with elemental and isotopic detection via laser-induced breakdown spectroscopy. In this study, we characterize the expansion dynamics and chemical evolution of filament-produced uranium (U) plasmas. Laser filaments are generated in the laboratory by loosely focusing 35 femtosecond (fs), 6 milli Joule (mJ) pulses in air. Time-resolved, two-dimensional plume and spectral imaging was performed to study hydrodynamics and evolution of U atomic and UO molecular emission in filament-produced U plasmas. Our results highlight that filament ablation of U plasmas gives a cylindrical plume morphology with an appearance of plume splitting into slow and fast moving components at later times of its evolution. Emission from the slow-moving component shows no distinct spectral features (i.e. broadband-like) and is contributed in part by nanoparticles generated during ultrafast laser ablation. Additionally, we find U atoms and U oxide molecules (i.e. UO, UxOy) co-exist in the filament produced plasma, which can be attributed to the generation of low-temperature plasma conditions during filament ablation.

Time-resolved dual-comb measurement of number density and temperature in a laser-induced plasma.

We utilize time-resolved dual-comb spectroscopy to measure the temporal evolution of the population number densities and absorption excitation temperature of Fe in a laser-induced plasma. The spectra of three excited-state transitions of Fe around 533 nm are simultaneously measured at different time delays following laser ablation of a stainless steel sample. This Letter probes late-time behaviors of laser-induced ablation plumes during plasma cooling. The high spectral resolution and broad spectral coverage of the dual-comb technique, combined with the time-resolved measurement capability shown here, will aid in the characterization of laser induced plasmas, including species identification and molecule and particle formation that can occur at later times in the plasma evolution.

Dual-comb spectroscopy of laser-induced plasmas.

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.