Laser-induced Air Plasma Research Articles

Investigations of laser-induced plasma in air by Thomson and Rayleigh scattering

Laser Thomson scattering and Rayleigh scattering methods were applied to investigate the dynamics of laser-induced plasmas in atmospheric air. Laser-induced plasma was generated by a 1064 nm, 200 mJ Nd: YAG laser. Another nanosecond Nd: YAG laser (532 nm, 50 mJ) was used as the probe laser. The temporally and spatially resolved electron number density and temperature distributions of the laser-induced plasma were determined from the Thomson scattering spectra. From 1 μs to 21 μs after plasma generation, the electron number density around the centre of the plasma decreased from 4.96 × 1023m‐3 to approximately 1.1 × 1021m‐3, while the electron temperature dropped from approximately 51,500 K to 6900 K. The plasma images and the measured distribution of the electron number density and temperature indicated the formation of a toroidal structure approximately 18 μs after plasma generation. The Rayleigh scattering results show that the Taylor-Sedov model cannot well describe the early evolution of the shockwave in radial directions.

Third harmonic generation in double-pulse laser induced air plasma

We found a fast hydrodynamic process in air double pulse Laser Induced Plasma (LIP) during the first 50 ns of plasma life. This process leads to strong inhomogeneity in the refractive index of air LIP, disturbance in the Gouy phase shift, and an increase in Third Harmonic Generation (THG). The efficiency of THG strongly depends on the confocal parameter of the focusing beam. The decrease in the focus of the pumping lens, despite the increase in the pump intensity, leads to a decrease in TH intensity. This can be attributed to the change from weak to tight focusing conditions in THG.

Effect of distance between the laser spot and the cavity center on spatially confined laser-induced copper plasma

We investigated the effect of distance between the laser spot and the cavity center on spatially confined laser-induced copper plasma in air. Spatial cylindrical cavities with various diameters (2mm, 3mm, 4mm and 5mm) and heights (2mm, 4mm, 6mm and 8mm) were used to confine the plasma, and different enhancements were observed when the laser spot at different locations in the same cavity. The maximum enhancement factor for the spectral intensity of Cu atomic line to be around 9 was measured at a time delay of 2.5μs when the distance was 0mm. The plasma temperature was calculated by the Boltzmann plot method, including five Cu atomic lines (324.75nm, 327.40nm, 510.55nm, 515.32nm and 521.82nm). The enhancement of the plasma temperature had a similar trend with the spectral intensity and also showed a strong dependence on the distance. The results demonstrated that the enhancement effect at the center of the cavity was the most significant for the uniform and balanced compression of the reflected shock waves in all directions.

Comparative study on self-absorption of laser-induced tungsten plasma in air and in argon.

The onset of self-absorption of laser-induced plasma poses a problem for converting emission line intensities to concentrations, which is one of the main bottlenecks in quantitative laser-induced breakdown spectroscopy (LIBS) measurements. In this paper, the effects of atmosphere and laser fluence on self-absorption reduction of the plasma induced on tungsten-copper alloy target were investigated with nanosecond infrared (1064 nm) laser pulse over a range of 2.9 to 18.2 J/cm2. The time-resolved features of self-absorption, and temperature and electron density of the plasma were characterized in atmospheric air and argon, respectively. The experimental results show the effect of self-absorption can be significantly reduced by increasing the laser pulse energy. The argon atmosphere is more helpful for self-absorption reduction. The time-resolved diagnostics of emission spectra in the early stage of the plasma formation are very effective to prevent self-absorption to improve the LIBS analytical performance.

Influence of humidity on the characteristics of laser-induced air plasma

The dependence of laser-induced breakdown spectroscopy (LIBS) on ambient conditions has not been clearly clarified as yet for element qualitative and quantitative analysis. To fill this gap, a laser-induced air plasma was produced by focusing a 1064-nm laser beam of Q-switched Nd:YAG pulse laser and the influence of relative humidity on the air plasma was systematically investigated by varying humidity at different laser irradiances. In contrast to the early findings, i.e., weak dependence of plasma properties on the humidity, some distinctive characteristics have been observed and explained. Examining the time-resolved emission spectra shows that the temporal evolution of spectral lines almost remains unchanged with the humidity and the influence of humidity on emission spectra can be neglected under a short gate width less than several hundred nanoseconds. As for the time-integrated emission spectra obtained at a higher laser irradiance, the increase in the emission intensity of H and O with the humidity is attributed to the increasing mole fraction of H and O and more laser energy that can be absorbed by the increasing water vapor. A faster growth of emission intensity of H and a slower increase of emission intensity of O respectively result from the relatively large increase of mole fraction of H and the relatively small increase of mole fraction of O with the humidity. Observations indicate that a shorter gate width or a lower laser irradiance is preferable in reducing the humidity influence and a certain magnitude of emission intensity of H and O spectral lines should be subtracted from the measured spectra for a more accurate and more reliable LIBS examination.

Laser guided ionic wind

We report on a method to experimentally generate ionic wind by coupling an external large electric field with an intense femtosecond laser induced air plasma channel. The measured ionic wind velocity could be as strong as >4 m/s. It could be optimized by increasing the strength of the applied electric field and the volume of the laser induced plasma channel. The experimental observation was qualitatively confirmed by a numerical simulation of spatial distribution of the electric field. The ionic wind can be generated outside a high-voltage geometry, even at remote distances.

An evaluation of equilibrium conditions and temperature-dependent speciation in a laser-produced air plasma

A laser-produced air plasma is a dynamical system with fundamental physical parameters that change significantly during its lifespan. The evolution of the spectral features from an air plasma shows broadband radiation at early times followed by discrete electronic transitions from ions, atoms, and rotational and vibration bands from molecules. The molecular band emission from an air plasma typically appears at times ⪞5 μs and persists for 100's of μs. In this article, an evaluation is made on the temperature evolution and speciation in a laser-produced air plasma. The air plasmas are generated by focusing 1064 nm, 6 ns pulses from an Nd:YAG laser. High-resolution emission spectra of various molecules such as N2, N2+, CN, OH, NH, and NO are acquired and fit using spectral models. Fitting of atomic and molecular emission features permits tracking of the air spark temperature evolution from 1 to 200 μs. Though the excitation and molecular temperatures show a good overlap at times ⪝10 μs, the molecular temperatures obtained from different species show a discontinuity at times ∼30 μs and this is related to shock collapse and subsequent changes in hydrodynamics and chemistry of the plume. The fitting of multiple species in broadband spectra has permitted calculation of the relative concentrations of various molecules as a function of temperature that provides insight into the air spark speciation. The measured relative mole fractions from laser-induced air plasma are within factors of 2–6 of the values estimated by speciation model under the assumption of thermal and chemical equilibrium across the temperature ranges studied. Probable reasons for the observed deviation in the relative fractions are discussed.

Influence of target temperature on H alpha line of laser-induced silicon plasma in air

This study mainly discusses the effect of target temperature on the emission intensity of the H alpha line in laser-induced silicon plasma in an atmospheric environment. The sample was uniformly heated to temperatures ranging from room temperature (22 °C) to a high temperature (300 °C) with laser energy ranging from 5.5 mJ to 11.5 mJ. Emission lines of Si (I), Si (II), and H (I) were observed. The H alpha line was attributable to a small amount of water vapor derived from natural humidity in the surrounding air. The H alpha line was widely used to measure electron density in laser-induced plasma. The results show that an increase in the sample temperature improved the spectral intensity of the Si line, whereas the emission intensity of the H alpha line declined. The reduction in the H alpha line was due to lower air density near the heated target surface. This investigation enhances the understanding of laser-induced breakdown spectroscopy applications at higher sample temperatures.

Effect of distances between lens and sample surface on laser-induced breakdown spectroscopy with spatial confinement

Spatial confinement can significantly enhance the spectral intensity of laser-induced plasma in air. It is attributed to the compression of plasma plume by the reflected shockwave. In addition, optical emission spectroscopy of laser-induced plasma can also be affected by the distance between lens and sample surface. In order to obtain the optimized spectral intensity, the distance must be considered. In this work, spatially confined laser-induced silicon plasma by using a Nd:YAG nanosecond laser at different distances between lens and sample surface was investigated. The laser energies were 12 mJ, 16 mJ, 20 mJ, and 24 mJ. All experiments were carried out in an atmospheric environment. The results indicated that the intensity of Si (I) 390.55 nm line firstly rose and then dropped with the increase of lens-to-sample distance. Moreover, the spectral peak intensity with spatial confinement was higher than that without spatial confinement. The enhancement ratio was approximately 2 when laser energy was 24 mJ.

Shielding and diagnostics of laser-induced air plasmas generated in collinear double pulse configuration

Plasma shielding in collinear double-pulse laser-induced breakdown spectroscopy was characterized in terms of its time-resolved absorptivity, photographic images, and emission spectra. In experiments performed with the double pulse beam configuration, one to generate plasma and another a probe beam, the transmission of the probe beam was measured at various inter-pulse delay times up to 1500 ns as the plasma opacity evolves rapidly with time. Photographic images obtained at the same time delay indicate that the plasma absorptivity becomes weaker as plasma evolves. At times up to 500 ns after plasma initiation, plasma absorptivity rose above 70% but later decreased markedly. When viewing the transmission and photographic images, the beam-plasma interaction realized with the double-pulse configuration is explainable as laser-supported absorption waves. To clarify the phenomenon and plasma properties when the probe beam and plasma are coupled, Stark broadening was used to obtain the electron number density, which is of the order of 1018 cm−3. Also, emission spectra under single- and double-pulse experimental configuration were obtained and analyzed using the Boltzmann-plot method to provide the plasma temperatures. The intensities from the double-pulse experiments were slightly stronger, in agreement with the laser energy absorption data. Moreover, from emission signal enhancements obtained from the double-pulse experiments, the increase in the intensity of the ionic emission was more than that obtained from the atomic emission for which a larger fraction of the probe beam energy was absorbed.

Damage detection in pipes based on acoustic excitations using laser-induced plasma

Abstract A health-monitoring system is proposed to detect holes drilled in a pipe based on laser plasma acoustic excitations and acoustic measurements. In this system, an acoustic excitation is applied to a pipe via a laser-induced plasma in air generated by a high-power Nd: YAG pulse laser. Laser-induced plasmas can realize non-contact acoustic impulse excitations. A microphone is used to measure the time response of the acoustic pressure. In this study, we focus on the detection of a hole in the pipe. The reflection of the acoustic wave due to a hole drilled in the pipe induces a change in the time response of the acoustic pressure. Applying a continuous wavelet transform to the measured time response data with/without the hole can locate the position of the hole. This study demonstrates the effectiveness of the present damage detection method based on an acoustic excitation using a laser-induced plasma.

Terahertz photonics of microplasma and beyond

THz air photonics using laser-induced air plasma is one of the leading frontiers in the THz community. Ambient air, when excited with an intense femtosecond laser beam, exhibits the remarkable ability to generate and detect pulsed THz waves through a nonlinear optical process. Significant advances in the use of air plasma for emitting, controlling, enhancing and measuring broadband THz waves have opened up a range of research opportunities. However, one of the major challenges for the research community and in real world applications is that plasma formation requires the use of an intense laser (mJ pulse energy), but most femtosecond laser oscillators only have pulse energies in the range of pJ to tens of nJ. The investigation of THz photonics, specifically the exploration of laserinduced plasmas at the micro-nano scale and beyond, is a frontier. Microplasmas generated by tightly focused optical excitation beams with controlled polarization serve as a new THz source with its unique radiation pattern and easy operation. The laser energy threshold for THz wave generation, the power scaling and the generation efficiency from microplasmas are significantly different from those of elongated plasmas. Our estimation indicates that the micronano plasma approach could reduce the necessary optical pulse energy by five orders of magnitude, while still obtaining a comparable or better signal-to-noise ratio for THz time-domain spectroscopy. This would be made possible by the high electron density (1019 cm–3 or more) achievable with a tight focus laser excitation, which correlates with the THz generation efficiency, and the use of laser oscillators with a much higher pulse repetition rate, as compared to the currently employed amplified laser systems (100 MHz vs 1 kHz). The THz micro-nano plasma is expected to lead to key technologies that will enable further interdisciplinary research and continued advancements of numerous THz wave sensing and spectroscopy developments. It serves as a vehicle for studying the extreme THz science.

On the improvement of signal repeatability in laser-induced air plasmas

The relatively low repeatability of laser-induced breakdown spectroscopy (LIBS) severely hinders its wide commercialization. In the present work, we investigate the optimization of LIBS system for repeatability improvement for both signal generation (plasma evolution) and signal collection. Timeintegrated spectra and images were obtained under different laser energies and focal lengths to investigate the optimum configuration for stable plasmas and repeatable signals. Using our experimental setup, the optimum conditions were found to be a laser energy of 250 mJ and a focus length of 100 mm. A stable and homogeneous plasma with the largest hot core area in the optimum condition yielded the most stable LIBS signal. Time-resolved images showed that the rebounding processes through the air plasma evolution caused the relative standard deviation (RSD) to increase with laser energies of > 250 mJ. In addition, the emission collection was improved by using a concave spherical mirror. The line intensities doubled as their RSDs decreased by approximately 25%. When the signal generation and collection were optimized simultaneously, the pulse-to-pulse RSDs were reduced to approximately 3% for O(I), N(I), and H(I) lines, which are better than the RSDs reported for solid samples and showed great potential for LIBS quantitative analysis by gasifying the solid or liquid samples.

Influence of distance between sample surface and focal point on spectral intensity of nanosecond laser-induced silicon plasma in air

The influence of distance between sample surface and focal point on optical emission spectroscopy of laser-induced silicon plasma by a nanosecond Nd:YAG laser operating at the wavelength of 1064 nm was investigated in air. Our results show that the emission intensity of Si (I) 390.6 nm line and N (II) 399.5 nm line depends strongly on the distance between sample surface and focal point. When the surface of ablated sample is away from the focal point of focusing lens, the neutral atomic line (Si(I) signal to be measured) is much higher than the ionic line (interference signal N (II)). Therefore, we can improve the intensity of Si (I) signal to be measured, and reduce the intensity of interference signal N (II). The presented result is mainly based on the reduction of interaction between the plasma plume and the ambient air, leading to much weaker collisions.

Numerical modeling for investigating the optical breakdown threshold of laser-induced air plasmas at different laser characteristics

In this work, we report a numerical investigation of two sets of experimental measurements that were previously carried out to study the breakdown threshold dependence on laser characteristics (wavelength, pulse width, and spot size) in the breakdown of laboratory air at different pressures. The study aimed to inspect the significance of the physical mechanisms in air breakdown as related to the applied experimental conditions. In doing so, we adopted a simple theoretical formulation relying on the numerical solution of a rate equation that describes the growth of electron density due to the joined effect of multi-photon and avalanche ionization processes given in our earlier work [Gaabour et al., J. Mod. Phys. 3, 1683–1691 (2012)]. Here, the rate equation is adapted to include the effect of electron loss due to attachment processes. This equation is then solved numerically using the Runge–Kutta fourth order technique. The influence of electron gain and loss processes on the breakdown threshold is studied...

Spectral Characteristics of Laser-Induced Graphite Plasma in Ambient Air**supported by National Natural Science Foundation of China (No. 61205149), Scientific Research Foundation for the Returned Overseas Chinese Scholars of State Education Ministry, Science Research Funds of Chongqing Municipal Education Commission (KJ1500436), Scientific and Technological Talents Training Project of Chongqing (CSTC2013kjrc-qnrc40002), Key Project of Foundation and Advanced Technology Research Project of Chongqing

An experimental setup of laser-induced graphite plasma was built and the spectral characteristics and properties of graphite plasma were studied. From the temporal behavior of graphite plasma, the duration of CN partials (B2 Σ+ → X2 Σ+) emission was two times longer than that of atomic carbon, and all intensities reached the maximum during the early stage from 0.2 μs to 0.8 μs. The electron temperature decreased from 11807 K to 8755 K, the vibration temperature decreased from 8973 K to 6472 K, and the rotational temperature decreased from 7288 K to 4491 K with the delay time, respectively. The effect of the laser energy was also studied, and it was found that the thresholds and spectral characteristics of CN molecular and C atomic spectroscopy presented great differences. At lower laser energies, the electron excited temperature, the electron density, the vibrational temperature and rotational temperature of CN partials increased rapidly. At higher laser energies, the increasing of electron excited temperature and electron density slow down, and the vibrational temperature and rotational temperature even trend to saturation due to plasma shielding and dissociation of CN molecules. The relationship among the three kinds of temperatures was Telec>Tvib>Trot at the same time. The electron density of the graphite plasma was in the order of 1017 cm−3 and 1018 cm−3.

Investigation of polarization state of terahertz radiation from compact laser-induced plasma in air

A comprehensive theoretical study is presented on the polarization state of THz wave radiation from a two-colour laser-induced plasma, and its dependency on optical pulse polarizations and phase difference. We extend the so-called photocurrent model to calculate three-dimensional distribution of photocurrent from which the polarization state of the THz radiation is calculated. It is shown that the calculation results from the photocurrent model, is in agreement with the previously reported experiments. Moreover, we show the possibility of creating any desired polarization state (i.e. elliptical, circular or linear) for the THz radiation through such generation mechanism by adjusting proper parameters.

Laser-induced plasmas in air studied using two-color interferometry

Temporally and spatially resolved density profiles of Cu atoms, electrons, and compressed air, from laser-induced copper plasmas in air, are measured using fast spectral imaging and two-color interferometry. From the intensified CCD images filtered by a narrow-band-pass filter centered at 515.32 nm, the Cu atoms expansion route is estimated and used to determine the position of the fracture surface between the Cu atoms and the air. Results indicate that the Cu atoms density at distances closer to the target (0–0.4 mm) is quite low, with the maximum density appearing at the edge of the plasma's core being ∼4.6 × 1024 m−3 at 304 ns. The free electrons are mainly located in the internal region of the plume, which is supposed to have a higher temperature. The density of the shock wave is (4–6) × 1025 m−3, corresponding to air compression of a factor of 1.7–2.5.

Tomography of homogenized laser-induced plasma by Radon transform technique

Tomography of a laser-induced plasma in air is performed by inverse Radon transform of angle-resolved plasma images. Plasmas were induced by single laser pulses (SP), double pulses (DP) in collinear geometry, and by a combination of single laser pulses with pulsed arc discharges (SP-AD). Images of plasmas on metallurgical steel slags were taken at delay times suitable for calibration-free laser-induced breakdown spectroscopy (CF-LIBS). Delays ranged from few microseconds for SP and DP up to tens of microseconds for SP-AD excitation. The white-light and the spectrally resolved emissivity ε(x,y,z) was reconstructed for the three plasma excitation schemes. The electron number density Ne(x,y,z) and plasma temperature Te(x,y,z) were determined from Mg and Mn emission lines in reconstructed spectra employing the Saha-Boltzmann plot method. The SP plasma revealed strongly inhomogeneous emissivity and plasma temperature. Re-excitation of plasma by a second laser pulse (DP) and by an arc discharge (SP-AD) homogenized the plasma and reduced the spatial variation of ε and Te. The homogenization of a plasma is a promising approach to increase the accuracy of calibration-free LIBS analysis of complex materials.

Terahertz wave absorption via preformed air plasma

Terahertz wave generation from laser-induced air plasma has continued to be an exciting field of research over the course of the past decade. In this paper, we report on an investigation concerning terahertz wave absorption with preformed plasma created by another laser pulse. We examine terahertz absorption behavior by varying the pump power and then analyze the polarization effect of the preplasma beam on terahertz wave absorption. The results of experiments conducted in which a type-I beta barium borate (BBO) crystal is placed before the preformed air plasma indicate that the fundamental (ω) and second harmonic (2ω) pulses can also influence terahertz absorption.