KrF laser-induced breakdown of gases

Measurements have been made of the threshold intensities for gas breakdown at the KrF 248 nm laser wavelength. The threshold intensities were measured as a function of pressure for air, H₂, CH₄ and the rare gases.     

Analysis of sodium aerosol using transversely excited atmospheric CO2 laser-induced gas plasma spectroscopy

Taking advantages of the special characteristics of a transversely excited atmospheric (TEA) CO₂ laser, the analysis of sodium aerosol has been successfully conducted by using laser-induced gas plasma spectroscopy (LIGPS) method. In this study, the sodium aerosol was deposited on a nickel metal plate; the metal plate functions as a subtarget to initiate a gas plasma. When a pulsed TEA CO₂ laser was focused on the metal surface, a large-volume and high-temperature gas plasma was induced. The fine particles of sodium then entered into the gas plasma region to be dissociated and excited. By using this technique, a semi quantitative analysis of sodium aerosol was made. The detection limit of sodium was approximately 200 ppb.     

Effect of impurities on laser-induced air breakdown at 1.06 μ

Experiments have been performed to determine the reduction of the laser-induced breakdown threshold of 1 atmosphere air due to the presence of particulate matter. SiO₂ and carbon fibers with diameters ranging from 6.6 μ to 68 μ were placed in the focal volume and irradiated by 30 nsec, 1.06 μ laser pulses. The reduction in breakdown threshold is found to be comparable to that previously reported for 10.6 μ radiation. Within the experimental accuracy, the breakdown threshold scales as λ^-2.     

Gas-phase thermometry in a high-pressure cell using BaMgAl<Subscript>10</Subscript>O<Subscript>17</Subscript>:Eu as a thermographic phosphor

We demonstrate the feasibility of laser-induced phosphorescence thermography for gas-phase temperature field measurement in a high-pressure cell. BaMgAl₁₀O₁₇:Eu (BAM) was used as a thermographic phosphor; it shows a blue-shifted laser-induced emission spectrum with increasing temperature. Local temperature was determined from the intensity ratio of two disjunctive emission bands. A new seeding device was developed to suspend the solid thermographic phosphor particles in a gas environment. The particle suspension time was modeled and validated by experiments. The influence of multiple scattering and other aspects of quantitative measurement were examined. The technique is currently capable of measuring up to 650 K, limited by signal intensity. The 2D temperature distributions were measured with a precision better than 60 K at 650 K. Multiple scattering limits the spatial resolution to only about 10 mm along the line of sight.     

Acoustic emission monitoring during laser shock cleaning of silicon wafers

A laser shock cleaning is a new dry cleaning methodology for the effective removal of submicron sized particles from solid surfaces. This technique uses a plasma shock wave produced by laser-induced air breakdown, which has applied to remove nano-scale silica particles from silicon wafer surfaces in this work. In order to characterize the laser shock cleaning process, acoustic waves generated during the shock process are measured in real time by a wide-band microphone and analyzed in the change of process parameters such as laser power density and gas species. It was found that the acoustic intensity is closely correlated with the shock wave intensity. From acoustic analysis, it is seen that acoustic intensity became stronger as incident laser power density increased. In addition, Ar gas has been found to be more effective to enhance the acoustic intensity, which allows higher cleaning performance compared with air or N₂ gas.     

Review of Laser-Induced Plasma,Its Mechanism,and Application to Quantitative Analysis of Hydrogen and Deuterium

Abstract

A comprehensive review of important progress achieved over the last 30 years regarding knowledge of laser-induced plasmas generated by CO₂ and Nd:YAG lasers in a variety of ambient gases is presented in this article, as well as research results on the extension of laser-induced breakdown spectroscopy (LIBS) for quantitative analysis of light elements, especially hydrogen and deuterium. First, the formation of shock wave–induced expanding secondary plasma in low-pressure ambient gases is discussed along with the dynamic characteristics of the secondary plasma expansion process. The unique advantages of low-pressure gas plasma are explained in relation to the successful detection of the sharp H and D emission lines. The experimental results using helium ambient gas are presented with emphasis on the role of He gas plasma in introducing an additional delayed excitation mechanism involving the helium metastable excited state, which resulted in the complete resolution of H and D emission lines, separated by only 0.18 nm. The development of a laser precleaning treatment and special double-pulse techniques further produced a linear calibration line with zero intercept applicable to quantitative H and D analyses of zircaloy sample, with either low- or high-pressure ambient He gas. More recent use of a transversely excited atmospheric (TEA) CO₂ laser in place of an Nd:YAG laser has demonstrated the much desired larger excited helium plasma and thereby resulted in significant emission enhancement and improved detection sensitivity.     

脉冲激光作用下气溶胶导致大气击穿研究

 利用锁模Nd:YAG激光及其倍频激光照射漂浮在大气中的Al₂O₂、ZnO、ZrO₂、18号玻璃粉等气溶胶粒子，在不同光学参数下对光致大气击穿阈值进行了测量。测量结果表明：空气中含较大粒子(d >30 μm)的光致击穿阈值比含较小粒子(d <1μm)的空气击穿阈值低2~3个数量级；含杂空气的击穿阈值随入射激光束直径的增大而下降，随气溶胶粒子直径的增大而减小，且随气溶胶成份的不同而不同；光致击穿阈值随波长的减小而增大。     

Spatial and Temporal Characteristics of an Excimer Laser-Induced Lead Plasma Emission

A laser-induced plasma was generated from a lead target using an ArF excimer laser (λ = 193 nm) and characterized by time-resolved and time-integrated spatially resolved spectrometry. The ambient atmosphere (gas composition and pressure) influenced the emission intensity for both atomic and ionic lines. The emission of laser-induced lead plasma varied with time as well as the location in the plasma. Lead ion emission decayed more rapidly than lead atomic line emission. High excitation temperatures and nonlinear optical phenomena were observed in the laser-induced lead plasma. Gas breakdown and the subsequent shielding effect of the incident laser beam in different ambient gas compositions and pressures were discussed to explain the different efficiency of metal ion for matioa in the plasma. The experimental results show that spatial discrimination of the laser-induced plasma emission is desirable for direct spectrochemical analysis.     

Laser ablation of aqueous solutions with spatially homogeneous and heterogeneous absorption

The ablation efficiency of aqueous solutions with different concentrations and spatially homogeneous (CuCl₂ solution) and heterogeneous (ink solution) absorption was studied as a function of the pulse-energy fluence (Nd:YAG laser, λ=1064 nm, τ_p = 20 ns). The latter was varied over a wide range from 0.15 J/cm² to 8.00 J/cm². The ablation threshold of solutions with heterogeneous absorption was found to be much lower (3 to 4 times) than the ablation threshold of solutions with homogeneous absorption and with the same average absorption coefficient. The ablation efficiency of heterogeneous solutions was higher by more than an order of magnitude. It was found that the ablation efficiency increases drastically for both types of solutions as the pulse energy fluence was raised to exceed the ablation threshold by 2 or 3 times. At such energy fluences, along with small droplets, larger droplets (1.5–2 mm cross section) could be ejected. This points to the ablation of solutions being affected by a hydrodynamic shock formed as a result of the pulsed recoil pressure excerted by the ablation products. The differences between the ablation processes for solutions with homogeneous and heterogeneous absorption as well as the hydrodynamic destruction at high energy fluences are discussed.     

A novel double-pulse laser plasma spectroscopic technique for H analysis in metal samples utilizing transversely excited atmospheric-pressure CO2 laser-induced metastable He atoms

The analysis of hydrogen in a metal sample (zircaloy-4), which is usually difficult to perform using conventional laser-induced breakdown spectroscopy (LIBS) techniques, has been achieved using a double-pulse technique under He gas at atmospheric pressure. In this technique, a transversely excited atmospheric-pressure (TEA) CO₂ laser (1.5 J, 200 ns) was focused onto the metal surface to induce a strong He gas plasma whilst simultaneously focussing a Nd-doped yttrium aluminum garnet (Nd:YAG) laser (120 mJ, 8 ns), synchronized with the TEA CO2 laser, onto the metal to ablate atoms into the resulting He gas plasma. The emission spectrum obtained shows a narrow H linewidth with low background intensity and long lifetime emission, thereby indicating that excitation takes place via metastable He atoms. The H emission from H₂O can be suppressed by a careful pretreatment involving heating the sample in a vacuum chamber.     

Shielding effect of laser-induced plasma in glass: pulse-to-pulse evolution of nitrogen and analyte emission lines

A characteristic of the plasma shielding effect was investigated through simultaneous measurement of time evolution of nitrogen emission in ambient air and analyte-specific emission in a glass matrix with varying lens-to-sample distance (LTSD), laser pulse energy, and repetition rate. Even under the threshold energy of air breakdown, strong nitrogen-emission lines could be measured by laser-induced breakdown spectroscopy (LIBS) for the glass samples in air at atmospheric pressure. The time evolution of the nitrogen emission was correlated reversely with a variation of various analyte emissions in the glass samples. Based on the reverse relation between the intensities of nitrogen and calcium emission intensities, the corrected values of the calcium emission line were calculated. This methodology shows consistent results independent of experimental conditions such as different LTSDs, laser energies, and repetition rates.     

典型放电气体的击穿场强阈值研究

为了研究等离子体产生时的气体击穿特性,利用低气压条件下气体击穿场强阈值模型,分析了He、Ne、Ar、Kr、Xe和Hg蒸汽等6种典型放电气体的击穿阈值随入射波频率、电子温度、气体压强以及气体温度的变化规律。结果表明:气体击穿阈值随气体压强的增大而减小,随气体温度、电子温度和入射脉冲频率的增大而增大。气体压强和入射频率对击穿阈值的影响大于气体温度和电子温度,在所考虑的范围内,气体压强对击穿场强的影响约为100 V/m,入射脉冲频率对击穿场强的影响为50~300 V/m,气体温度和电子温度对击穿场强的影响为20~30 V/m。当考虑气体压强、气体温度以及电子温度等因素的影响时,各种气体的击穿场强阈值产生的变化规律相类似;但考虑入射频率的影响时,不同气体的击穿场强阈值差异很大。在所考虑的典型放电气体中,Xe具有最低的击穿场强阈值,He的击穿阈值最大。     

用激光诱导击穿光谱测量铝合金的激光烧蚀阈值

采用正交几何配置的双波长双脉冲激光烧蚀-激光诱导击穿光谱技术准确测量了铝合金样品的激光烧蚀阈值。在烧蚀激光波长为532 nm、脉宽为12 ns并采用焦距为2 cm的非球面透镜强聚焦的条件下,得到铝合金的激光能量烧蚀阈值为48 J,等效的能量密度烧蚀阈值为9.8 J/cm2。该技术是一种新的激光烧蚀阈值的光谱测量手段,与传统的测量技术相比,具有高灵敏、准确、快捷和便利的特点,可以用于不同材料的激光烧蚀阈值的准确测量。     

Characteristics of emission from laser-induced plasma of metallic compounds in gaseous condition: The effects of gas pressure and laser pulse energy

Low-pressure laser-induced breakdown spectroscopy (LIBS) was employed to measure mercury, strontium, and cesium under different conditions. Mercury was measured in detail to discuss the effects of pressure, laser pulse energy, delay time, and buffer gas. The continuum emission from plasma reduced dramatically to enhance signal-to-background ratio due to the change of influence of electron impact ionization process when reducing pressure and decreasing laser pulse energy to some extent. The comparison of mercuric chloride and mercury, as well as strontium and cesium measurements, demonstrates the enhancement of detection ability for trace species measurement using low-pressure LIBS.     

波导等离子体限幅器中气体的选择与触发条件计算

 为保护电子设备不受高功率微波损坏,在矩形波导中嵌入等离子体限幅器。计算了不同气体的微波击穿场强随气体压强以及微波频率的变化规律。在高气压条件下（1 333~133 320 Pa）,气体击穿场强随气压增大而增大,在计算的4种气体中Ne的击穿场强最小;低气压条件下（1.333 2~133.32 Pa）,气体击穿场强随气压增大而减小,且Xe具有最小击穿场强。高气压条件下气体的击穿场强明显高于低气压下的击穿场强。计算结果表明：当填充133.32 Pa的Xe时,限幅器能够在约30 km范围内,有效地防护10 GW级高功率微波对电子设备的损坏。     

Scavenging dissolved oxygen via acoustic droplet vaporization

Acoustic droplet vaporization (ADV) of perfluorocarbon emulsions has been explored for diagnostic and therapeutic applications. Previous studies have demonstrated that vaporization of a liquid droplet results in a gas microbubble with a diameter 5–6 times larger than the initial droplet diameter. The expansion factor can increase to a factor of 10 in gassy fluids as a result of air diffusing from the surrounding fluid into the microbubble. This study investigates the potential of this process to serve as an ultrasound-mediated gas scavenging technology. Perfluoropentane droplets diluted in phosphate-buffered saline (PBS) were insonified by a 2 MHz transducer at peak rarefactional pressures lower than and greater than the ADV pressure amplitude threshold in an in vitro flow phantom. The change in dissolved oxygen (DO) of the PBS before and after ADV was measured. A numerical model of gas scavenging, based on conservation of mass and equal partial pressures of gases at equilibrium, was developed. At insonation pressures exceeding the ADV threshold, the DO of air-saturated PBS decreased with increasing insonation pressures, dropping as low as 25% of air saturation within 20 s. The decrease in DO of the PBS during ADV was dependent on the volumetric size distribution of the droplets and the fraction of droplets transitioned during ultrasound exposure. Numerically predicted changes in DO from the model agreed with the experimentally measured DO, indicating that concentration gradients can explain this phenomenon. Using computationally modified droplet size distributions that would be suitable for in vivo applications, the DO of the PBS was found to decrease with increasing concentrations. This study demonstrates that ADV can significantly decrease the DO in an aqueous fluid, which may have direct therapeutic applications and should be considered for ADV-based diagnostic or therapeutic applications.     

Laser-induced ionization and intrinsic breakdown of wide band-gap solids

A simple scaling of bulk laser-induced breakdown threshold for wide band-gap solids is derived on the basis of a recent modification of the Keldysh photo-ionization model [43, 46]. Contrary to most traditional models, the modification is based on rigorous energy dependence of reduced effective electron–hole mass. The dependence leads to a specific ionization regime with an extremely high ionization rate resulting in intensive generation of conduction-band electrons. The regime is characterized by a well-determined threshold intensity that is proposed to be associated with the threshold of bulk intrinsic laser-induced breakdown (LIB) by visible and near-infra-red laser radiation. That allows deriving dependence of LIB threshold on laser and material parameters. The presented model provides explanation for the experimental results on LIB thresholds that have not received theoretical interpretation. In particular, it reproduces empirical dependence of breakdown threshold on the average inter-atomic spacing derived from the experimental data. The LIB threshold evaluated from the presented model is very close to experimental data on bulk LIB by tightly focused beams in wide band-gap solids. PACS 78.47.+p; 42.50.Hz; 42.50.Ct     

Ps four-wave mixing measurements of electric field in a ns pulse discharge in a hydrogen diffusion flame

Time-resolved electric field in ns pulse discharge plasmas generated in room air and in an atmospheric pressure hydrogen diffusion flame has been measured by ps four-wave mixing, for plane-to-plane electrode geometry. Electric field is put on the absolute scale using the Laplacian field measured before breakdown. The results show that peak electric field during breakdown in the flame, approximately 40?kV/cm, is significantly lower compared to that in room air, 75?kV/cm, due to higher temperature of combustion products. In both cases, peak electric field is higher compared to DC breakdown field. Both in air and in the flame, the electric field follows the applied voltage before breakdown and decreases rapidly after breakdown, due to charge separation and plasma self-shielding. The electric field in air is compared with the predictions of an analytic model of ns pulse breakdown, showing good agreement between the predicted and the measured breakdown field. The model also predicts earlier breakdown as well as breakdown voltage reduction as the temperature is increased, in qualitative agreement with the experimental data. The use of the present ps four-wave mixing diagnostics for measurements of electric fields below ～20?kV/cm in atmospheric pressure flames is challenging, due to low signal-to-noise. The sensitivity of the present diagnostics is controlled by the high temperature and low N₂ fraction in the combustion product mixture, as well as by the limited bandwidth of the Stokes beam generated by the stimulated Raman cell, which provides access only to several rotational levels of nitrogen molecules. The present diagnostics will have much better sensitivity in high-pressure flames, since the four-wave mixing signal scales as the squared number density of nitrogen.     

预处理对355nm激光作用下熔石英损伤增长的影响

测试了经化学蚀刻、紫外激光预处理及其共同处理后的熔石英355 nm激光损伤阈值;研究了处理前后其损伤斑面积随激光辐照脉冲数的增长情况。结果表明,处理后熔石英355 nm激光损伤阈值得到了提高,且损伤斑面积增长变慢。利用CO2激光对熔石英表面损伤点进行了修复处理,修复后的损伤点R-on-1抗损伤阈值和基底阈值相当,损伤增长得到有效抑制。