Laser-induced Plasma Formation Research Articles

Investigation of Laser-Induced Cavity and Plasma Formation in Water Using Double-Pulse LIBS

This paper deals with double-pulse laser-induced breakdown spectroscopy (LIBS) underwater, which is a promising analytical method for elemental analysis in the deep sea up to a water depth of 6000 m. A double-pulse laser with a wavelength of 1064 nm is used, which provides a pulse energy of up to 266 mJ for each laser pulse (in single pulse mode), a pulse width of 5–7 ns and a pulse delay in the range of 0.5 to 20 µs. In the double-pulse LIBS method, the first laser pulse creates a cavity on the material surface, and then the second laser pulse forms the plasma in this cavity. It is expected that the plasma is affected by the cavity’s size and lifetime. For this reason, the influence of focus position, pulse energy and pulse delay on the cavity and plasma formation at shallow water depth has been investigated.

Characteristics of laser-induced plasma generated in water and in air with different nanosecond laser pulse durations

The characteristics of laser-induced plasma generated in water and in air were compared with two laser pulse durations of 6 ns and 17 ns.

Laser-induced plasma formation in water with up to 400 mJ double-pulse LIBS

Double-pulse LIBS is a promising technique for deep-sea applications. LIBS measurements in shallow water with up to 400 mJ each pulse were done to select laser parameters which promote optimized spectral line emission from plasma even at elevated pressures, where line broadening until loss of most of the spectral information can occur. Optical emission spectroscopy, using a Czerny-Turner spectrometer, has been applied to investigate the dependence of the emitted radiation on laser parameters and hydrostatic pressure. It has been found, that higher laser pulse energies, especially with short pulse delay as required in high water pressure, can also have an adverse effect on the measured spectrum.

Propagation characteristics of laser-induced acoustic sources in hybrid anechoic wind tunnels

The propagation characteristics of an acoustic point source generated using laser-induced plasma (LIP) were investigated experimentally. Experiments were performed in a Kevlar-walled hybrid anechoic wind tunnel (HAWT) where the sound of the LIP was measured using a 251-element microphone array, while the flow speed in the empty test section was varied. The time instant of the LIP formation was also captured. The far field sound pressure was assessed through arrival times (source to microphones) and pressure correction levels, and these quantities were compared against a commonly used shear layer refraction model. A detailed uncertainty assessment is presented on the arrival times and pressure levels. It was found that the time domain analysis was limited by the sampling rate of the analog-to-digital converter regardless of the flow speed. The uncertainty of the pressure levels was limited by the uncertainty of the microphones at low flow speeds, while they increased with flow speed at shallow observer angles. The high-speed Schlieren imaging of the LIP was performed, which revealed that the sound of the LIP reaches the far field microphones over a shorter time duration than modeled because the wave speed was initially supersonic. The discrepancy was found to be comparable to the temporal resolution of the aeroacoustic experiments. The discrepancy between the experimental and theoretical arrival times was found to increase with flow speed, and they were nearly independent of the azimuth angles. The discrepancy between the experimental and theoretical pressure correction ratio was found to be uniform for most observer locations. With an increase in flow speed, the discrepancy became positive at large, and negative at low polar angles. The sound refraction at the Kevlar wall did not change the frequency content of the sound over the investigated range of frequencies (1–10 kHz).

Micro-pinch formation and extreme ultraviolet emission of laser-induced discharge plasma* *Project supported by the Basic and Applied Basic Research Major Program of Guangdong Province, China (Grant No. 2019B030302003).

Extreme ultraviolet (EUV) source produced by laser-induced discharge plasma (LDP) is a potential technical means in inspection and metrology. A pulsed Nd:YAG laser is focused on a tin plate to produce an initial plasma thereby triggering a discharge between high-voltage electrodes in a vacuum system. The process of micro-pinch formation during the current rising is recorded by a time-resolved intensified charge couple device camera. The evolution of electron temperature and density of LDP are obtained by optical emission spectrometry. An extreme ultraviolet spectrometer is built up to investigate the EUV spectrum of Sn LDP at 13.5 nm. The laser and discharge parameters such as laser energy, voltage, gap distance, and anode shape can influence the EUV emission.

Terahertz aqueous photonics.

Developing efficient and robust terahertz (THz) sources is of incessant interest in the THz community for their wide applications. With successive effort in past decades, numerous groups have achieved THz wave generation from solids, gases, and plasmas. However, liquid, especially liquid water has never been demonstrated as a THz source. One main reason leading the impediment is that water has strong absorption characteristics in the THz frequency regime.A thin water film under intense laser excitation was introduced as the THz source to mitigate the considerable loss of THz waves from the absorption. Laser-induced plasma formation associated with a ponderomotive force-induced dipole model was proposed to explain the generation process. For the one-color excitation scheme, the water film generates a higher THz electric field than the air does under the identical experimental condition. Unlike the case of air, THz wave generation from liquid water prefers a sub-picosecond (200-800 fs) laser pulse rather than a femtosecond pulse (~50 fs). This observation results from the plasma generation process in water.For the two-color excitation scheme, the THz electric field is enhanced by one-order of magnitude in comparison with the one-color case. Meanwhile, coherent control of the THz field is achieved by adjusting the relative phase between the fundamental pulse and the second-harmonic pulse.To eliminate the total internal reflection of THz waves at the water-air interface of a water film, a water line produced by a syringe needle was used to emit THz waves. As expected, more THz radiation can be coupled out and detected. THz wave generation from other liquids were also tested.

Origin of strong-field-induced low-order harmonic generation in amorphous quartz

Kerr-type nonlinearities form the basis for our physical understanding of nonlinear optical phenomena in condensed matter, such as self-focusing, solitary waves and wave mixing1–3. In strong fields, they are complemented by higher-order nonlinearities that enable high-harmonic generation, which is currently understood as the interplay of light-driven intraband charge dynamics and interband recombination4–6. Remarkably, the nonlinear response emerging from the subcycle injection dynamics of electrons into the conduction band, that is from ionization, has been almost completely overlooked in solids and only partially considered in the gas phase7–10. Here, we reveal this strong-field-induced nonlinearity in a-SiO2 as a typical wide-bandgap dielectric by means of time-resolved, low-order wave-mixing experiments, and show that, close to the material damage threshold, the so far unexplored injection current provides the leading contribution. The sensitivity of the harmonic emission to the subcycle ionization dynamics offers an original approach to characterize the evolution of laser-induced plasma formation in optical microprocessing. Strong-field-induced nonlinearities from the injection of electrons into the conduction band contribute to harmonic generation in amorphous quartz. Close to the damage threshold, they dominate over intraband and interband contributions.

Plasma formation and relaxation dynamics in fused silica driven by femtosecond short-wavelength infrared laser pulses

Laser-induced plasma formation and subsequent relaxation in dielectric solids is the precursor to structural modifications serving as the basis for direct laser writing of functional optical micro- and nanostructures. Based on an experimental arrangement combining a time-resolved transmission measurement with a cross-phase modulation measurement, we isolate the plasma formation and relaxation dynamics in the bulk of amorphous fused silica excited by femtosecond short-wavelength infrared (λ= 2100 nm) laser pulses. Whereas the relaxation time of the generated subcritical electron-hole plasma was so far assumed to be constant, our findings indicate an intensity-dependent relaxation time. We attribute this intensity dependence to vibrational activation of the medium, leading to detrapping of trapped carriers and a reduced trapping probability.

Effect of sample temperature on time-resolved laser-induced breakdown spectroscopy

Increasing sample temperature is an effective method for improving emission intensity of laser-induced breakdown spectroscopy (LIBS). An increase in the sample temperature requires only simple modification to LIBS system, and does not require sample pretreatment. In addition, the formation and decay of laser-induced plasma (LIP) are a dynamic physical process. Time-resolved spectroscopy can enrich the dynamic information of LIBS and increase data dimension. The further analysis of the time-resolved LIBS data has the potential to understand the physical process. In this study, a beam of nanosecond laser was used to excite brass sample to produce plasma. The time-resolved spectroscopy of the LIP was measured at different sample temperatures. The results indicated that increasing sample temperature improved optical emission intensity in LIBS, and increased the plasma persistence. We also obtained plasma temperature and electron density at different sample temperatures. The time-resolved plasma temperature and electron density in the case with higher sample temperature were higher than those in the case with lower sample temperature. Moreover, the decay of electron density at higher sample temperature was faster than that at lower sample temperature.

Ultrafast third harmonic generation imaging of microplasma at the threshold of laser-induced plasma formation in solids

We present a method of ultrafast laser-induced microplasma mapping based on the third harmonic generation at tightly focused laser beams in solids. The technique gives a submicron resolution and can be applied for two-dimensional imaging of laser-induced microplasma with the electron density as low as 10−5 ncr produced by a femtosecond laser driver at the plasma formation threshold. High sensitivity and contrast are dictated by nonlinear behavior of the third harmonic yield with the laser field. The demonstrated method gives an opportunity to increase the precision of plasma volume determination in the field of material microstructuring.

Plasma emission during time-resolved laser-induced incandescence measurements of aerosolized metal nanoparticles

There has been speculation that several commonly observed anomalies in time-resolved laser-induced incandescence (TiRe-LII) measurement data may be caused by bremsstrahlung emission from a laser-induced plasma, a phenomenon known to occur at higher fluences typical of laser-induced breakdown spectroscopy. This paper presents the first theoretical framework to investigate a laser-induced plasma formation under LII measurement conditions, and explores how this plasma may affect time-resolved spectral intensity measurements. At fluences greater than 0.8 J/cm2, the absorption cross-section of the laser-energized nanoparticle is enhanced due to inverse bremsstrahlung absorption, and bremsstrahlung emission results in an overestimation of the nanoparticle temperature due to the corruption of the incandescence signal. Under these conditions, neutral bremsstrahlung emission is more prevalent than electron–ion bremsstrahlung due to the weak nature of the induced plasma. Nevertheless, the current model does not predict laser-induced plasma phenomena below ~ 0.5 J/cm2, typical of low-fluence TiRe-LII measurements.

An analytical model for the prediction of temperature distribution and evolution in hybrid laser-waterjet micro-machining

A hybrid laser-waterjet micro-machining technology was developed for near damage-free micro-ablation recently. It uses a new material removal concept where the laser-softened material is expelled by a pressurised waterjet. The temperature field in this hybrid machining process is an essential quantity for understanding the underlying material removal mechanism and optimizing the process conditions. This study presents a three-dimensional (3-D) analytical model for the temperature field in this hybrid laser-waterjet micro-machining process. The interaction among the laser, waterjet, and workpiece is considered in the model. The absorption of laser by water, the formation of laser-induced plasma in water, the bubble formation and the laser refraction at the air-water interface are discussed. DuHamel’s principle is used to determine a closed-form temperature equation and a solution in a variable separation form is obtained. A calculation for silicon carbide is conducted. The results are illustrated by a group of 3-D temperature profiles intuitively and visually. It is shown that the temperatures are below the melting point during the process due to the cooling action of waterjet. The almost damage-free micro-machining can be achieved. Besides, the maximum temperature increases with the increased average laser power and waterjet offset distance and decreased nozzle exit diameter where the average laser power takes a major action.

Single-photon sensitive light-in-fight imaging.

The ability to record images with extreme temporal resolution enables a diverse range of applications, such as fluorescence lifetime imaging, time-of-flight depth imaging and characterization of ultrafast processes. Recently, ultrafast imaging schemes have emerged, which require either long acquisition times or raster scanning and have a requirement for sufficient signal that can only be achieved when light is reflected off an object or diffused by a strongly scattering medium. Here we present a demonstration of the potential of single-photon detector arrays for visualization and rapid characterization of events evolving on picosecond time scales. The single-photon sensitivity, temporal resolution and full-field imaging capability enables the observation of light-in-flight in air, as well as the measurement of laser-induced plasma formation and dynamics in its natural environment. The extreme sensitivity and short acquisition times pave the way for real-time imaging of ultrafast processes or visualization and tracking of objects hidden from view.

Model for ultrashort laser pulse-induced ionization dynamics in transparent solids

A comprehensive model of ultrafast laser-induced plasma generation intended for coupling with pulse propagation simulations in transparent solids is introduced. It simultaneously accounts for the changing spectrum of a propagating ultrashort laser pulse while coupling to the evolution of the energy-resolved nonequilibrium free-carrier distribution. The presented results indicate that strong pulse chirps lead to ionization dynamics that are not captured by the standard monochromatic treatment of laser-induced plasma formation. These results have strong implications for ultrafast laser-solid applications that depend on ionization in a strong nonlinear focus.

Chromium oxide dissolution in steels via short pulse laser processing

Changes of microstructure, chemical and phase compositions in thin surface layers of low carbon steel saturated by chromium oxide have been studied by TEM, XPS and XRD methods. Ultrafine chromium oxide powder was spread on a steel surface and subjected to laser processing with nanosecond pulses. It was found that such conditions of processing as overheating of small volume of metal, high temperature gradient, rapid solidification and laser-induced plasma formation lead to dissolution of chromium oxide in the metal matrix. As a result of laser processing the surface layers contain chromium oxide, chrome-spinel FeO $$\cdot $$ Cr $$_{2}$$ O $$_{3}$$ and chromium in metal state dispersed in alpha and gamma iron. The processing technique allows to obtain surface layers whose chemical composition might be equivalent to the composition of stainless steels.

Functional impairment with minimal macular damage in femtosecond laser plasma injury: case report

A 26 years old female patient was examined twenty-four hours after observing laser-induced plasma formation in a process of nanoparticle production complaining of bilateral central scotoma. The ophthalmologic evaluation included dilated fundus observation, fluorescein angiography, and optical coherence tomography (OCT). In the first assessment, visual acuity was 20/20 in the right eye and 20/25 in the left eye. Ophthalmologic evaluation revealed colour changes in the macular region of both eyes. Optical coherence tomography showed a central interruption of the photoreceptor layer in both eyes and fluorescein angiography was normal. In subsequent appointments acuity was always 20/20 in both eyes. Abnormal optical coherence tomography findings disappeared in less than 5 months, but subjective complaints of scotoma in the left eye remained. Extra care must be taken in this type of experiment by, for example, reducing the time that the retina is directly exposed to the plasma radiation.

Laser-induced breakdown spectroscopy measurements of uranium and thorium powders and uranium ore

Laser-induced breakdown spectroscopy (LIBS) was used to analyze depleted uranium and thorium oxide powders and uranium ore as a potential rapid in situ analysis technique in nuclear production facilities, environmental sampling, and in-field forensic applications. Material such as pressed pellets and metals, has been extensively studied using LIBS due to the high density of the material and more stable laser-induced plasma formation. Powders, on the other hand, are difficult to analyze using LIBS since ejection and removal of the powder occur in the laser interaction region. The capability of analyzing powders is important in allowing for rapid analysis of suspicious materials, environmental samples, or trace contamination on surfaces since it most closely represents field samples (soil, small particles, debris etc.). The rapid, in situ analysis of samples, including nuclear materials, also reduces costs in sample collection, transportation, sample preparation, and analysis time. Here we demonstrate the detection of actinides in oxide powders and within a uranium ore sample as both pressed pellets and powders on carbon adhesive discs for spectral comparison. The acquired LIBS spectra for both forms of the samples differ in overall intensity but yield a similar distribution of atomic emission spectral lines.

Plasma detection and control requirements for CO2 laser cutting

This paper reveals the underlying mechanisms for laser-induced plasma formation in fusion laser cutting of stainless steel and aluminium, by means of experimental observations, analytical calculations and FEA simulations. The camera-based monitoring system which has been developed and used for real-time plasma detection is explained, as well as the results achieved for different material/thickness/geometry combinations. Requirements for real-time plasma suppression control strategies are derived and the effectiveness of such control algorithm are illustrated.

Effect of phonon spectrum heating on the formation of laser-induced electron plasma in wide-gap insulators

The effect of lattice heating by laser pulses on the dynamics of electron plasma generation in transparent solids has been theoretically studied. Several ways of taking into account the contribution of the phonon spectrum heating to the electron avalanche dynamics, depending on the type of the effective (with respect to the field energy transfer to electrons) phonons and laser pulse duration, have been proposed. A comparative analysis of the results of Monte Carlo computation of electron gas heating in the laser pulse field, which were obtained for cold and heated lattices, has been performed. It is shown that the consideration of the effect of lattice heating on the probabilities of electron-phonon and electron-phonon-photon scattering leads to an increase in the avalanche rate, which is more pronounced at longer wavelengths of the incident radiation and under longer laser pulses. Some qualitative features of the redistribution of the energy, absorbed during a pulse, between the electron plasma and lattice are revealed, which suggest initiation of irreversible microscopic changes in the insulator. In particular, the ratio R of the energy accumulated in the electron subsystem to the excess (with respect to the initial equilibrium state) energy in the phonon subsystem has been calculated for different initial lattice temperatures. It is shown that this ratio increases with a decrease in the laser wavelength in the computation scheme with lattice heating disregarded and decreases at all pulse durations when the lattice heating is taken into account.

On Plasma Formation in CO2 Fusion Laser Cutting of Stainless Steel

This paper discusses the mechanisms for emergence of laser-induced plasma (LIP) in fusion CO2 laser cutting of stainless steel plates. Experimental observations and simulation results are presented to explain why cutting at high velocity increases the risk of LIP formation despite the depleted energy input per unit length.