Single-shot, double-pulse determination of the detonation energy in nanosecond-laser ablation using the blast model.

We demonstrate a novel single-shot method to determine the detonation energy of laser-induced plasma and investigate its performance. This approach can be used in cases where there are significant shot-to-shot variations in ablation conditions, such as laser fluctuations, target inhomogeneity, or multiple filamentation with ultrashort pulses. The Sedov blast model is used to fit two time-delayed shadowgrams measured with a double-pulse laser. We find that the reconstruction of detonation parameters is insensitive to the choice of interpulse delay in double-pulse shadowgraphy. In contrast, the initial assumption of expansion dimensionality has a large impact on the reconstructed detonation energy. The method allows for a reduction in the uncertainties of blast wave energy measurements as a diagnostic technique employed in various laser ablation applications.

Physical conditions for UO formation in laser-produced uranium plumes.

We investigate the oxidation of uranium (U) species, the physical conditions leading to uranium monoxide (UO) formation and the interplay between plume hydrodynamics and plasma chemistry in a laser-produced U plasma. Plasmas are produced by ablation of metallic U using nanosecond laser pulses. An ambient gas environment with varying oxygen partial pressures in 100 Torr inert Ar gas is used for controlling the plasma oxidation chemistry. Optical emission spectroscopic analysis of U atomic and monoxide species shows a reduction in the emission intensity and persistence with increasing oxygen partial pressure. Spectral modelling is used for identifying the physical conditions in the plasma that favor UO formation. The optimal temperature for UO formation is found to be in the temperature range of ∼1500-5000 K. The spectrally integrated and spectrally filtered (monochromatic) imaging of U atomic and molecular species reveals the evolutionary paths of various species in the plasma. Our results also highlight that oxidation in U plasmas predominantly occurs at the cooler periphery and is delayed with respect to plasma formation, and the dissipation of molecular species strongly depends on oxygen partial pressure.

Optical emission from ultrafast laser filament-produced air plasmas in the multiple filament regime.

We perform optical emission spectroscopy of ultrafast laser filament-produced air plasmas in the multiple filament regime at driving wavelengths of 400 nm and 800 nm. The spatiotemporal structure of the emission from the plasmas are observed and the emission spectra are used to estimate plasma temperature and density for a range of laser parameters. Plasma temperatures are determined from the molecular nitrogen fluorescence, while the electron densities are estimated from Stark broadening of the oxygen-I 777.19-nm line. Electron temperatures are determined to be in the range of 5000-5200 K and they do not vary significantly along the length of the filament, nor are they sensitive to incident laser energy or wavelength. Electron densities are on order of 1016 cm-3 and show a greater variation with axial position, laser energy, and laser wavelength. We discuss mechanisms responsible for spatial localization of emitting species within the filament. Optical emission spectroscopy offers a simple, non-perturbing method to measure filament properties, that allows the information on the associated molecular transitions and excitation/ionization mechanisms to be extracted.

Intense laser filament-solid interactions from near-ultraviolet to mid-infrared.

Studies of high-power ultrashort laser pulse interaction with matter are not only of fundamental scientific interest, but are also highly relevant to applications in the domain of remote sensing. Here, we investigate the effect of laser wavelength on coupling of femtosecond laser filaments to solid targets. Three central wavelengths have been used to produce filaments: 0.4, 0.8, and 2.0 µm. We find that, unlike the case of conventional tight focusing, use of shorter wavelengths does not necessarily produce more efficient ablation. This is explained by increased multi-photon absorption arising in near-UV filamentation. Investigations of filament-induced plasma dynamics and its thermodynamic parameters provide the foundation for unveiling the interplay between wavelength-dependent filament ablation mechanisms. In this way, strategies to increase the sensitivity of material detection via this technique may be better understood, thereby improving the analytical performance in this class of applications.

Electrical conductance of near-infrared femtosecond air filaments in the multi-filament regime.

Electrical conductive properties of femtosecond laser filaments are of significant interest for applications such as remote arc suppression and discharge guiding. We transmitted electrical current through a DC-biased air plasma channel formed in the wake of an energetic femtosecond laser pulse and observed an increased rate of change of the charge transmitted through the ionized channel with laser energy when crossing from the single- to multi-filament regimes. This behavior is attributed to the confluent effects of greater electron density and an increased cross-sectional area of the multi-filament plasma structures. As the laser energy is increased, the formation of additional conductive channels in the multi-filamentation regime becomes a significant contributor to the rapid increase of conductivity. These observations suggest a potential path to attractive applications such as efficient energy transfer in air mediated by femtosecond laser-produced filaments.

Tracking of oxide formation in laser-produced uranium plasmas.

We use a spatially and temporally resolved emission tracking technique based on optical emission spectroscopy to map the evolution of emission features from uranium and its compounds in a plasma produced by a nanosecond laser. We observe quenching of the emission from neutral uranium (591.538 nm) and uranium monoxide (593.55 nm) species with increasing oxygen concentration and discuss possible reaction pathways for dissociation or formation of higher uranium oxides (UxOy). We further identify spectral features between 320 nm and 380 nm and between 520 nm and 640 nm, which we attribute to UxOy.

Ultrafast Laser Filament-induced Fluorescence Spectroscopy of Uranyl Fluoride.

Uranyl fluoride (UO2F2) is a compound which forms in the reaction between water and uranium hexafluoride, a uranium containing gas widely used for uranium enrichment. Uranyl fluoride exhibits negligible natural background in atmosphere; as a result, its observation implies the presence and active operation of nearby enrichment facilities and could be used as a tracer for treaty verification technologies. Additionally, detection of UO2F2 has a potential application in guiding remediation efforts around enrichment facilities. Laser-induced fluorescence (LIF) has been proposed in the past as a viable technique for the detection and tracking of UO2F2. We demonstrate that ultrafast laser filamentation coupled with LIF extends the capabilities of standard LIF to enable remote detection of UO2F2. An intense femtosecond laser pulse propagated in air collapses into a plasma channel, referred to as a laser filament, allowing for the extended delivery of laser energy. We first investigate the luminescence of UO2F2 excited by the second harmonic of an ultrafast Ti:sapphire laser and subsequently excite it using the conical emission that accompanies ultrafast laser filamentation in air. We measure the decay rates spanning 4.3-5.6 × 104 s-1 and discuss the characteristics of the luminescence for both ultrafast- and filament-excitation. Larger decay rates than those observed using standard LIF are caused by a saturated component of prompt decay from annihilation of dense excited states upon excitation with an ultrafast source. The reproducibility of such decay rates for the given range of incident laser intensities 1.0-1.6 × 1011 W cm-2 is promising for the application of this technique in remote sensing.

Transition of Femtosecond-Filament-Solid Interactions from Single to Multiple Filament Regime.

High-peak-power fs-laser filaments offer unique characteristics attractive to remote sensing via techniques such as remote laser-induced breakdown spectroscopy (R-LIBS). The dynamics of several ablation mechanisms following the interaction between a filament and a solid determines the emission strength and reproducibility of target plasma, which is of relevance for R-LIBS applications. We investigate the space- and time-resolved dynamics of ionic and atomic emission from copper as well as the surrounding atmosphere in order to understand limitations of fs-filament-ablation for standoff energy delivery. Furthermore, we probe the shock front produced from filament-target interaction using time-resolved shadowgraphy and infer laser-material coupling efficiencies for both single and multiple filament regimes through analysis of shock expansion with the Sedov model for point detonation. The results provide insight into plasma structure for the range of peak powers up to 30 times the critical power for filamentation P cr . Despite the stochastic nucleation of multiple filaments at peak-powers greater than 16 P cr , emission of ionic and neutral species increases with pump beam intensity, and short-lived nitrogen emission originating from the ambient is consistently observed. Ultimately, results suggest favorable scaling of emission intensity from target species on the laser pump energy, furthering the prospects for use of filament-solid interactions for remote sensing.