Femtosecond Laser-induced Plasma Research Articles

Lagrangian perspective on the expansion dynamics and shielding effect of femtosecond laser-induced copper plasma plumes

Femtosecond laser ablation of metals generates a strongly ionized plasma plume near the irradiated surface. The resulting plasma shielding effect can reduce subsequent laser energy deposition and lower nanomachining efficiency, especially during multi-pulse irradiation. Understanding the spatiotemporal evolution of the laser-induced plasma and its associated shielding effect is, therefore, crucial. A hybrid two-temperature and direct simulation Monte Carlo (TTM-DSMC) computational model is developed in this study, which synergistically couples the ultrafast laser–metal interaction physics and the plasma collisional transport. The model simulates the plasma properties including electron density, temperature dynamics, reflectivity, and energy attenuation throughout the plume expansion process from femtosecond to nanosecond timescales. A complex “penguin-shaped” plasma plume with internal shockwaves is observed due to the effects of double-pulse irradiation. Significantly enhanced plasma reflectivity and reduced laser energy deposition demonstrate the accumulated shielding effect, which increases with higher plasma density accumulation when the pulse separation is insufficient. Our model provides valuable theoretical guidance for optimizing processing parameters to enhance efficiency and precision in femtosecond laser machining. The integrated TTM-DSMC approach could also facilitate the study of laser-induced plasmas in other contexts like material characterization and nanoparticle synthesis.

Two-dimensional high resolution electron properties of femtosecond laser-induced plasma filament in atmospheric pressure argon

This work reports the measurement of two-dimensional electron properties over a nanosecond scale integration time across a femtosecond laser-induced plasma filament in atmospheric pressure argon. Radial electron properties across the ∼100\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 100$$\\end{document} μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu$$\\end{document}m diameter filament are obtained at discrete axial locations at 2.5 mm steps by one-dimensional high-resolution laser Thomson scattering with a spatial resolution of 10 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu$$\\end{document}m. These measurements reveal plasma structural information in the filament. The Thomson spectral lineshapes exhibit clear spectral sidebands with an α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha$$\\end{document} parameter ∼1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 1$$\\end{document}, enabling the measurement of both electron temperature and density profiles. These measurements yield electron densities on the order of 1022\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^{22}$$\\end{document}/m3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3}$$\\end{document} and electron temperatures of ∼2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 2$$\\end{document} eV. Heating from the probe laser due to inverse bremsstrahlung is taken into account to correct the Thomson scattering electron temperature measurements. Under these conditions, electron-neutral collision induced bremsstrahlung becomes the dominant laser-induced plasma heating process associated with the probe laser. The measurements reveal structural features of the filament, including an asymmetrically skewed density structure in the axial direction and reversed radial distributions of electron density and temperature.

Single-shot local measurement of terahertz correlated second harmonic generation in laser air plasma filaments.

We present a single-shot detection method of terahertz correlated second harmonic generation in plasma-based sources by directly mixing an optical probe into femtosecond laser-induced plasma filaments in air. The single-shot second harmonic trace is obtained by measuring a second harmonic generation on a conventional CCD with a spatiotemporally distorted probe beam. The system shows a spectrometer resolution of 22 fs/pixel on the CCD and a true resolution on the order of the probe pulse duration. With considerable THz peak electric field strength, this formalism can open the door to single-shot THz detection without bandwidth limitations.

Optical vortex coronagraphy for high-contrast imaging of femtosecond laser filaments

A high-contrast imaging technique for phase objects based on the optical vortex coronagraph (OVC) is proposed. This method offers the unique advantage of background-free imaging due to the introduction of azimuthal phase in the Fourier plane. We employed the OVC method to detect femtosecond laser-induced air plasma and compared it with the classic diffractometry and fluorescent imaging methods. We achieved a phase sensitivity of ∼0.035 waves that surpassed the capabilities of the other two methods. The combination of this highly sensitive imaging technique with the pump–probe method holds promise for applications in ultrafast imaging of laser–material interactions.

Investigation of Overflow-Water-Assisted Femtosecond Laser-Induced Plasma Modulation of Microchannel Morphology

Laser-induced plasma micromachining (LIPMM) process is an effective approach to create microfeatures with high aspect ratio (AR) and reduced heat affected zone (HAZ). Therefore, LIPMM plays a crucial role in improving the morphology of microchannels. In this study, microchannels were fabricated using a femtosecond laser with two distinct sets of process parameters under three different processing methods: overflow-water-assisted laser-induced plasma micromachining (OF-LIPMM), laser direct writing (LDW), and static water laser-induced plasma micromachining (S-LIPMM). Furthermore, single-factor experiments were conducted to systematically analyze the effects of four parameters, namely single-pulse energy, scanning speed, scanning times, and frequency, on the HAZ, AR, and material removal rate (MRR) of the microchannels. Finally, the optimized parameters determined from the single-factor experiments were applied for large-scale grid fabrication on a surface. The experimental results revealed that OF-LIPMM enables the creation of two different kinds of microchannel surfaces: one microchannel was fabricated with a higher AR of 3:1 and a larger HAZ, while another microchannel was created with a lower AR of 1:1 and a reduced HAZ. Moreover, the parameters investigated in the single-factor experiments can be applied to large-scale processing. The results also indicate that variations of the scanning speed, frequency, and single-pulse energy have similar effects on the machining characteristics of the three processing methods. The findings enable the generation of microchannels with favorable morphological characteristics and have significant implications for the large-scale production of both types of microchannels.

Single-shot spatiotemporal plasma density diagnosis using an arbitrary time-wavelength-encoded biprism interferometer

Single-shot ultrafast imaging reveals the transient dynamics of ultrafast phenomena. Herein, we introduce a new approach for capturing ultrafast micron-scale dynamics using an arbitrary time-wavelength-encoded biprism interferometer (TWEBI). In TWEBI, a time-wavelength-encoded pulse train is generated using a set of cascaded frequency-doubling crystals with slightly different phase-matching angles and independent delay lines, which have the advantages of independent temporal resolution, time window, and frame interval parameters. Phase measurement of TWEBI is achieved using a plug-and-play biprism interferometer. In the experiment to capture the transient dynamic of femtosecond laser-induced plasma filament, we demonstrated that the TWEBI setup can capture a total of 12 frames in a single shot, and its effective frame rate is 5 trillion frames per second (Tfps), corresponding to a temporal resolution of 200 fs and a spatial resolution of 4 μm. Moreover, the time window of the TWEBI setup can be adjusted from sub-picosecond to nanosecond timescales. Thus, TWEBI has the advantages of high-precision temporal-spatial resolution, high frame rate, adjustable time window, and no reference arm, thus providing a practical and feasible diagnostic scheme for complex transient dynamics.

Temporal-spatial dynamics of electronic plasma in a femtosecond laser-induced sapphire microstructure.

In this study, the time-spatial evolution of single-pulse femtosecond laser-induced plasma in sapphire is studied by using femtosecond time-resolved pump-probe shadowgraphy. Laser-induced sapphire damage occurred when the pump light energy was increased to 20µJ. Based on its shadowgraphy image, the threshold electron density can be estimated to be about 2.48×1020 c m -3. The evolution law of the transient peak electron density and its spatial position as femtosecond laser propagation in sapphire were researched. The transitions from single-focus to multi-focus as the laser focus shifted from the surface to a deeper part were observed from the transient shadowgraphy images. The focal point distance in multi-focus increased as the focal depth increased. The distributions of femtosecond laser-induced free electron plasma and the final microstructure were consistent with each other.

Single-Shot Multi-Frame Imaging of Femtosecond Laser-Induced Plasma Propagation.

Single-shot ultrafast multi-frame imaging technology plays a crucial role in the observation of laser-induced plasma. However, there are many challenges in the application of laser processing, such as technology fusion and imaging stability. To provide a stable and reliable observation method, we propose an ultrafast single-shot multi-frame imaging technology based on wavelength polarization multiplexing. Through the frequency doubling and birefringence effects of the BBO and the quartz crystal, the 800 nm femtosecond laser pulse was frequency doubled to 400 nm, and a sequence of probe sub-pulses with dual-wavelength and different polarization was generated. The coaxial propagation and framing imaging of multi-frequency pulses provided stable imaging quality and clarity, as well as high temporal/spatial resolution (200 fs and 228 lp/mm). In the experiments involving femtosecond laser-induced plasma propagation, the probe sub-pulses measured their time intervals by capturing the same results. Specifically, the measured time intervals were 200 fs between the same color pulses and 1 ps between the adjacent different. Finally, based on the obtained system time resolution, we observed and revealed the evolution mechanism of femtosecond laser-induced air plasma filaments, the multifilament propagation of femtosecond laser in fused silica, and the influence mechanism of air ionization on laser-induced shock waves.

Tuning size and shape of gold nanoparticles using seed-mediated growth by unfocused femtosecond laser-induced plasma.

Gold nanoparticles with tunable size and shape are effectively synthesized with a femtosecond laser using the seed and growth method by reducing a KAuCl4 solution with polyvinylpyrrolidone (PVP) surfactant as a stabilizer. The sizes of gold nanoparticles, including 7.30 to 9.90, 11.0, 12.0, 14.1, 17.3, 22, 23.0, 24.4, and 27.2 nm, are effectively altered. In addition, the initial shapes of gold nanoparticles (quasi-spherical, triangular, and nanoplate) are also successfully changed. While the reduction effect of an unfocused femtosecond laser can control the size of nanoparticles, the surfactant can influence the growth of nanoparticles and determine their shape. This technology represents a breakthrough for nanoparticle development by not employing strong reducing agents and instead using an environmentally friendly synthesis technique.

Time- and Position-Dependent Breakdown Volume Calculations to Explain Experimentally Observed Femtosecond Laser-Induced Plasma Properties

Focusing of femtosecond laser pulses in gases can produce different gas breakdown phenomena depending on the focusing conditions: from simple optical breakdown like laser “sparks” to a nonlinear optical breakdown like filamentation. The dynamics of such plasmas after the pulse exposure is dependent on the energy deposited by the laser in the breakdown volume. Using a time- and position-dependent breakdown model, we estimate the breakdown volume and show that the energy deposited by the laser in this breakdown volume determines the characteristics of laser-induced plasma in the post-pulse exposure regime. Experimentally we find that for different focal lengths there exists a threshold value of the energy density beyond which a transition from an ellipsoidal shape to a spherical shape can be observed, followed by a toroidal expansion of the produced plasma. When electron density and electron temperature are expressed as a function of the energy density, deviations from the parabolic dependence on irradiance are observed. They imply additional ionization by multiphoton ionization in the plasma volume that occurs when the peak power of the laser pulse is above the critical power for self-focusing in air. The relevance of this experimental and theoretical study is to prevent undesired self-focusing conditions during material processing, a step toward well-controlled laser-plasma etching without laser ablation.

Lifetime visualization of femtosecond laser-induced plasma on GaP crystal.

Gallium phosphide (GaP) is a widely used and promising semiconductor material for photonics devices and we suppose the ultrafast laser can be a competitive tool for GaP processing. We used an 800 nm centered femtosecond (fs) laser with a pulse duration of 50 fs to irradiate the GaP crystal. The ablation threshold was first determined, and then the ultrafast dynamics including plasma expansion, shockwave formation and propagation, and spectral evolution were acquired and analyzed. The evolution of ejected plasma in the initial stage changed from cylindrical to planar propagation with the augment of laser fluence. The study on the propagation properties of shockwaves showed that the energy of propelling shockwaves accounted for 12% to 18% of the laser pulse energy at all fluences above the ablation threshold. A prominent plasma splitting was observed at a fluence slightly higher than the threshold, and a phenomenon that the plasma protruded out of the shockwaves was also found. Finally, the transient temperature and density of electron at different fluences were calculated. The temperature difference between the plasma and the shockwave proved the heating effect of the plasma during ablation.

Assay measurement of gaseous UF6 using femtosecond laser-induced breakdown spectroscopy in the 424.4 nm spectral region

Most modern uranium enrichers use centrifuges to separate 235U [Uranium 235] and 238U [Uranium 238] in the gaseous form, UF6 [uranium hexafluoride]. Monitoring the enrichment level of the material being produced in a Gaseous Centrifuge Enrichment Plant (GCEP) or in a single centrifuge is important to the commercial enricher as well as those tasked with monitoring the enrichment activities of declared facilities, such as the International Atomic Energy Agency (IAEA). Here, methods for determining UF6 assay using a femtosecond laser-induced plasma are reported. These techniques resulted in assay measurements within 0.28% of the actual 235U content with a standard deviation of 0.90%, demonstrating the potential effectiveness and applicability of using femtosecond lasers in actual enrichment plants to measure UF6 assay through Laser-Induced Plasma Emission Spectroscopy (LIBS).

Enhancement of spectral line intensity and lifetime of femtosecond laser-induced Cu plasma in a cylindrical cavity

The spatial confinement effects in the femtosecond laser-induced Cu plasma contained within a cylindrical cavity are studied by optical emission spectroscopy. The Cu atomic emission intensity and spectral line lifetime are enhanced significantly when a cylindrical cavity with an inner diameter of 6 mm confined the femtosecond laser-induced Cu plasma. The enhancement factor of the emission intensity under the spatial confinement depends on the upper level energy of the spectral line, and the maximum enhancement factors is about 2.48 for the Cu I 529.3 nm spectral line. The spatial confinement can effectively prolong the lifetime of the femtosecond-induced Cu and the persistence time of the plasma is near to 7 μs. The enhancements of the spectral line intensity and lifetime are attributed to the increased electron temperature and density owing to the spatial confinement, and the mechanism for the enhancement effects with the cylindrical cavity are discussed in terms of the shockwave interactions.

Spectral-width variation of N2+ lasing

The complicated collisional processes inside a femtosecond laser-induced plasma column can show a great impact on the light emissions from it. A known example is that the lifetime of noncoherent fluorescence emitting from gas plasma can be greatly decreased due to the fierce ultrafast plasma collisions. However, for coherent stimulated emissions in plasma surrounding, to what extent it can be influenced by the collisional process has not been fully understood. Herein, we report an experimental spectral width analysis on coherent ${\mathrm{N}}_{2}^{+}$ lasing that is generated in weakly ionized gas plasma with a frequently adopted pump-probe scheme. It is found unexpectedly that the spectral width of ${\mathrm{N}}_{2}^{+}$ lasing evidently shortens with the increasing delay between the pump and the probe pulses at a pressure of 40 mbars, whereas an invariable spectral width appears when the gas pressure is decreased below 10 mbars. Moreover, it was also observed that the spectral width narrows with the increment of polarization angle between the pump and the probe pulses. These observations update the understandings of molecular spectral manipulation on the basis of the previous that the spectral modulations mainly occur in the vicinity of molecular alignment. Our results indicate that by combining the spectral width and intensity analysis, a more comprehensive understanding regarding the impact of ultrafast electronic plasma-ion collisions on the ${\mathrm{N}}_{2}^{+}$ lasing generation can be obtained. These findings also shed light on the spectral manipulation for coherent irradiations produced during strong laser-matter interaction.

Enhancing Gas Solubility in Water via Femtosecond Laser-Induced Plasma.

The generation of laser-induced plasma at the gas–liquid interface provides many fundamental and interesting scientific phenomena such as ionization, sharp explosion, shock wave radiation, bubble creation, and water splitting. However, despite the extensive research in this area, there is no reference on the effect of the surrounding environment on the chemical processes that occur during the laser-induced plasma–water interaction. In this work, we investigate the effect of the surrounding gas environment on femtosecond laser-induced plasma when generated at the pure water–gas interface. Ultrashort laser pulses were applied to water in the presence of air and N2 and Ar gas environments. Formation of a significant number of nitrate-based species in water was observed after exposure to femtosecond laser-induced plasma in air and N2 environments. The detected NO3 ions formed in the laser-treated water led to the appearance of an absorption peak in the UV range, a significant decrease in the water pH value, and a significant increase in water’s electrical conductivity. All induced properties of water were stable for 3 months of monitoring after laser treatment. Our work shows that the generation of laser-induced plasma in water propagating into a gaseous medium facilitates the interaction between the two media, as a result of which the compositions of substances present in the gaseous medium can be dissolved in water without increasing the gas pressure. The presented approach may find applications in areas such as water purification, material synthesis, and environmental stewardship.

The key role of the semiconductor property of water in femtosecond laser-induced plasma spectroscopy

We investigate femtosecond laser-induced plasma spectroscopy (fs-LIPS) of water, and find that at different input laser energies the measured characteristic fs-LIPS emissions from trace metal elements in water exhibit different dependences on the chirped pulse duration. As the pulse is temporally stretched from 50 fs to longer durations up to 500 fs, the emission intensity from Al I on the 3s24s 2S1/2 → 3s23p 2S1/2, 3/2 transitions experiences three typical variation regions, i.e., the increasing, plateau, and decreasing regions, respectively, but their temporal range varies as a function of the input laser energy. By comparing with the optical emissions of fs-LIPS of different materials including ambient air, solid metal Aluminum, and the two semiconductor materials with different bandgaps, i.e., silicon and sapphire, we ascribe the observed dependence of fs-LIPS emission of trace Al element in water on the pulse duration to the tradeoff between the strong field ionization (SFI) and the subsequent avalanche ionization (AI), both of which are determined by the wide bandgap semiconductor property of water.

Influence of distance from lens to sample surface on spectral sensitivity of femtosecond laser-induced breakdown spectroscopy with NaCl water film

Increasing the emission intensity of laser-induced breakdown spectroscopy (LIBS) is an effective way to improve the sensitivity of LIBS technology to elements analysis in liquid samples. In this work, the influence of the distance from lens to sample surface (DFLS) on the spectral emission of femtosecond laser-induced plasma of NaCl water film was studied by measuring Na atomic line. The results showed that the emission intensity of the spectral line presented the phenomenon of double peaks with an increase in the DFLS. The position for the highest spectral intensity was not geometric focal point of focusing lens, but was located in front of the geometric focal point. In addition, we carried out quantitative analysis on Na element with different concentrations, showing a calibration curve of Na element, and calculating limit of detection (LOD) and relative standard deviation (RSD) for different DFLSs. Subsequently, the changes in the LOD and RSD with the DFLS are discussed. With the increase of the DFLS, the LOD and RSD were greatly reduced. At the DFLS of 99.0 mm, the line intensity was highest, and the LOD and RSD were better. Therefore, it can be confirmed that femtosecond LIBS of water film can be optimized by changing the DFLS, the experiment shows great potential in real-time water quality monitoring.

Time-Resolved Imaging of Femtosecond Laser-Induced Plasma Expansion in a Nitrogen Microjet

We report on the study of ultrafast laser-induced plasma expansion dynamics in a gas microjet. To this purpose, we focused femtosecond laser pulses on a nitrogen jet produced through a homemade De Laval micronozzle. The laser excitation led to plasma generation with a characteristic spectral line emission at 391 nm. By following the emitted signal with a detection system based on an intensified charge-coupled device (ICCD) we captured the two-dimensional spatial evolution of the photo-excited nitrogen ions with a temporal resolution on the nanosecond time scale. We fabricated the micronozzle on a fused silica substrate by femtosecond laser micromachining. This technique enabled high accuracy and three-dimensional capabilities, thus, providing an ideal platform for developing glass-based microfluidic structures for application to plasma physics and ultrafast spectroscopy.

Time-resolved spectroscopy of collinear femtosecond and nanosecond dual-pulse laser-induced Cu plasmas

In this paper, we investigate the time-resolved spectroscopy of collinear femtosecond (fs) and nanosecond (ns) dual-pulse (DP) laser-induced plasmas. A copper target was used as an experimental sample, and the fs laser was considered as the time zero reference point. The inter-pulse delay between fs and ns laser beams was 3 μs. First, we compared the time-resolved peak intensities of Cu (I) lines from Cu plasmas induced by fs + ns and ns + fs DP lasers with collinear configuration. The results showed that compared with the ns + fs DP, the fs + ns DP laser-induced Cu plasmas had stronger peak intensities and longer lifetimes. Second, we calculated time-resolved plasma temperatures using the Boltzmann plot with three spectral lines at Cu (I) 510.55, 515.32 and 521.82 nm. In addition, time-resolved electron densities were calculated based on Stark broadening with Cu (I) line at 521.82 nm. It was found that compared with ns + fs DP, the plasma temperatures and electron densities of the Cu plasmas induced by fs + ns DP laser were higher. Finally, we observed images of ablation craters under the two experimental conditions and found that the fs + ns DP laser-produced stronger ablation, which corresponded to stronger plasma emission.

Enhancing terahertz radiation from femtosecond laser filaments using local gas density modulation

We present a method to enhance the terahertz (THz) wave radiation from a femtosecond laser-induced plasma filament by controlling the local gas density within the filament. We develop a theoretical model for THz generation from a laser-induced air plasma filament and the subsequent propagation process, to account for a varying local gas density. By adjusting the local gas density along the filament, the transient current distribution along the filament and the resulting coherent superposition of terahertz waves can be controlled. The location of the gas jet nozzle and the relative phase between multicolor light fields both affect the transient current distribution and thus the strength of the generated THz field. Compared with the conventional terahertz generation by a two-color filament in a homogeneous gas, a three-color filament can realize an increase by 6.12 times in the generated THz pulse energy, with optimized local gas density modulation. Our results suggest that the THz amplification via local gas density modulation can be further improved with well-designed multicolor pulses.