Ultrafast thermoelasticity modeling of microbump formation irradiated by femtosecond laser

Finite element method and ultrafast thermoelasticity model are combined to simulate the microbump formation irradiated by a femtosecond laser. It has been shown that the effect of microbump formation is related to the characteristic of incident femtosecond laser and the thermoelasticity properties of the film. The numerical results exhibit good agreements with the experimental results in both the shape and height of the conical microbump structure, which verify the effectiveness of the ultrafast thermoelasticity model in experiments. It should be helpful for selecting appropriate materials for nanotexturing of thin films by ultrafast lasers.     

Numerical modeling of generation of few-cycle pulses in a mode-locked laser

We have performed numerical simulation of a laser generating few-cycle pulses in the regime of coherent mode locking. It is shown that such a laser can support generation of pulses with a duration close to the reciprocal of the main laser transition frequency of the gain medium in a wide range of laser parameters.     

固体中激光热弹超声的光穿透效应

本文从热弹性基本方程出发,理论上研究了非金属固体材料受激光热弹激励超声时,光穿透效应对所产生声波的影响。在一维模型中,通过求解热扩散方程和热弹性位移方程,得到弹性位移的一般表达式,分析了材料的光叩收性质与声波位移振幅的关系。在二维模型中,对于轴对称分布的情况,忽略热扩散的影响,采用积分变换法求解热弹性运动方程,推导出远场区中的位移解析表达式。给出了纵波和横波的指向性图案,讨论了光穿透效应对声源指向性的影响。     

Effect of optical penetration on laser generatedthermoelastic ultrasound in solids

I.IntroductionTbegenerationofu1trasoundbytheirradiationofpulsedlaseratso1idsurfacehasbeenwidelystlldied,boththeorctica1lyandexpcrimentallyI1-8].Intheprocessoflaserthermoelasticg6nerationofu1trasound,temperatureriseinduccdbytheabsorptionoflaserenergyproducestherma1expansion,andthcnanultrasonicsourceiscreated.Sofar,thestudyof1asergenera-honofultrasoundinso1idsconcentratesmain1yonmeta1s,anduntilrecent1y,1ittleattentionhasbenpaidtonon-mctals.Inametal,1aserenergyisabsorbedonlyatthesurfaceofthesamp…     

Effect of laser beam incidence angle on the thermoelastic generation in semi-transparent materials

When a laser beam is absorbed in a semi-transparent material, a volume acoustic source is created owing to penetration of the laser beam inside the material and to thermoelastic transduction. Many experimental and theoretical studies have been conducted to better understand this ultrasound generation process with normal laser light incidence on the material surface. The purpose of this paper is to analyze the effects of the asymmetry caused by oblique incidence of a laser line source on the generation of acoustic waves in semi-transparent isotropic materials. Experiments on a glass plate demonstrate that such an obliquely incident laser light strongly affects bulk acoustic waves generation. Compressional and shear waves are enhanced and the loss of symmetry of the acoustic source causes asymmetrical behavior of the acoustic waves. Surprisingly, compressional-wave amplitude decreases whereas shear-wave amplitude increases in the region where the electromagnetic energy is refracted. This feature is explained by semi-analytical calculations.     

Simulation on thermoelastic stress field and laser ultrasound waveform in non-metallic materials by using FEM

The thermoelastically generated stress and ultrasound fields in non-metallic materials by laser illumination are presented by using finite element method (FEM). The model accounts for the effects of thermal diffusion and optical penetration, as well as the finite width and duration of the laser source. The numerical results show that the strength and feature of the force source can be related to the heat input and certain material properties. The relationships between the stress waves and the ultrasound waveforms, particularly the precursor, are analyzed. The typical surface acoustic waves (SAWs) and the bulk waveforms at the epicenter are presented to illustrate the generated field and provide insight to the relevance of different mechanism taken into account in the model. PACS 02.70.Dh; 43.35.+d; 42.62.-b     

Numerical modeling and experimental investigation of TiC formation on titanium surface pre-coated by graphite under pulsed laser irradiation

A model for carbonization of titanium surface by pulsed Nd:YAG laser was developed. The Ti substrate was covered with a relatively thick graphite layer prior to be processed under the laser beam. The experiments were performed at 15 J pulse energy with various pulse durations and overlapping factor to validate the results obtained from the numerical calculations. The model results such as temperature gradient, surface temperature, and the cooling rate were correlated with the micro-hardness of the alloyed layer. Higher pulse durations and overlapping factors which lead to the heat input increasing will result in significant rising in the micro-hardness values. The hardness values of the processed layer partially containing TiC, increased up to 10 times of the Ti substrate.     

Theory of the generation of mechanical vibrations by laser radiation in solids containing internal stresses on the basis of the thermoelastic effect

The behavior of the nonsteady deformations in solids containing internal stresses under irradiation by temporally modulated laser radiation is analyzed. In the framework of the nonlinear theory of thermoelasticity a model is proposed for the excitation of mechanical vibrations with allowance for the dependence of the thermoelastic coupling parameter on the initial deformation. For the case of a piezoelectric method of detecting the mechanical vibrations in a uniformly deformed sample, an analytical expression is obtained for the electrical signal taken off from the piezoelement. The behavior of the piezoelectric signal under various conditions is investigated, and the results are compared with the available experimental data and found to be in qualitative agreement. Zh. Tekh. Fiz. 69, 59–63 (July 1999)     

Numerical modeling of a XeCl laser excited by microwave discharge

12 , 169 (1987). Reasonably good agreements in the peak output power and laser efficiency have been achieved. Model calculations also predict that an efficiency as high as 2.7% can be obtained once the conditions of the above-mentioned experiments have been optimized. From the consideration that the skin depth effectively limits the absorption length of the microwave pumping and hence the excitable volume, it is concluded that high input power densities (>2 MW/cm³) and higher gas pressures (between 3 and 10 atm) are the preferable conditions to achieve higher efficiency. Preliminary calculations on CCl₄ containing XeCl gas mixtures show that improvement in laser efficiency by several folds may be achieved as a result of the higher intrinsic efficiency of excimer formation. Received: 23 September 1996 / Revised version: 25 March 1997     

Combined continuum-atomistic modeling of ultrashort-pulsed laser irradiation of silicon

Ultrafast thermomechanical responses of silicon thin films due to ultrashort-pulsed laser irradiation were investigated using an atomic-level hybrid method coupling the molecular dynamics and the ultrafast two-step energy transport model. The dynamic reflectivity and absorption were considered, and the effects of laser fluence and pulse duration on the thermomechanical response were studied. It was found that both the carrier temperature and number density rapidly increase to their maximum while the lattice temperature rises at a much slower rate. The ultrafast laser heating could induce a strong stress wave in the film, with the maximum compressive and tensile stress occurring near the front and back surfaces, respectively. For laser pulses of the same duration, the higher the laser fluence is, the higher the carrier temperature and density and lattice temperature are induced. For the same laser fluence, a longer pulse generally produces lower carrier density and temperatures and weaker stress shock strength. However, for the fluence of 0.2 J/cm², the lowest lattice temperature was simulated for a 100-fs pulse compared to the 1-ps and 5-ps pulses, due to the increase of reflectivity by high carrier density. It is also shown that the optical properties as functions of lattice temperature usually employed are not suited for modeling ultrafast laser interactions with silicon materials.     

Excitations of thermoelastic waves in plates by a pulsed laser

The method of the eigenfunction expansion, also known as the expansion in normal modes, is employed to study numerically the axisymmetric excitation of the thermoelastic waves in plates by a pulsed laser. This method gives a systematic treatment and allows one to investigate not only the quasistatic and dynamic thermoelastic responses of pulsed photothermal deformation on the time scale of 1 s, but also the thermoelastic generation of longitudinal, transverse, and surface acoustic waves in thick materials, as well as the excitations of the Rayleigh-Lamb wave modes in thin plates. The formalism is particularly suitable for waveform analyses of the excitations of transient Lamb waves in thin plates because one needs only to calculate the contributions of several lower eigenmodes. The numerical technique provides a quantitative tool for the experimental determination of material properties, especially the mechanical and elastic properties of free-standing films and thicker sheet materials by thermoelastic detection.     

Numerical modeling of the thermal lensing effect in a grazing-incidence laser

The numerical modeling of thermal lensing effect is investigated in a grazing-incidence laser. The deformation of the bounce face is introduced into the modeling for the first time, and the Gaussian distribution of the pump light and the anisotropic heat conduction are considered. The results indicate that the proportion of the deformation on the bounce face to the thermal lensing effect is as high as 80% for small grazing-incident angle of 5°. The thermal lensing effect sensitively depends on the pump power, grazing-incident angle and the pump distribution in a grazing-incidence bounce geometry laser.     

Numerical simulation of the generation of gain switched laser pulses

Gain switched laser pulses with FWHM of 32 ps having an optical peak power exceeding 400 W are generated using low cost single heterostructure (SH) Fabry–Perot laser diodes. To examine and to optimize this effect in detail, a thorough physical device simulation is done. Maxwell's equations together with a simplified version of the Boltzmann transport equation (BTE) and various parameters like carrier recombination and material gain, form a coupled set of nonlinear, inhomogeneous partial differential equations, that are discretized using the finite difference scheme and solved selfconsistently.     

Numerical modeling of ultrashort-pulse laser ablation of silicon

Silicon ablation by a single ultrashort laser pulse is simulated through a computer model. The agreement between results obtained through the model and experimental data found in the literature supports the hypothesis made by the authors in considering thermal evaporation as the dominant ablation mechanism in silicon. Two distinctive thresholds are defined for the ablation procedure leading to a better interpretation of experimental data. The dependence of ablation fluence thresholds on both wavelength and pulse width is discussed. An approximate analytical model describing the crater formation process is proposed and indicative results are presented.     

高速气流中激光加热平板数值模拟与分析

 采用流固耦合方法,数值模拟了高速流场中激光作用下来流速度对平板温度分布的影响。结果表明:无激光辐照时,高速气流中平板有较高的气动生热平衡温度,且平板-气流之间的换热系数随来流速度增大而增大;在平板前沿换热系数增长最快,沿平板长度方向增速趋于平缓。分析了激光辐照时高速气流中激光加热平板的温度分布情况,考察了来流速度不同时,气动生热、散热和激光辐照对平板温度的影响,给出了激光辐照后的温升情况和温度分布,分析了在不同速度来流下,对流散热、摩擦生热和激光加热之间的竞争关系,结果表明,平板温度具体分布主要是加热过程竞争的结果。     

Numerical modeling of a four-wave-mixing-assisted Raman fiber laser

We present a new numerical model of cascaded Raman fiber lasers that takes into account the chromatic dispersion of the fiber and includes the full spectrum of the intracavity field. This model explains and describes remarkably well a new operating regime found experimentally and reveals that chromatic dispersion is truly a new degree of freedom in the design of cascaded Raman lasers.     

Numerical modeling of an integrated erbium-doped glass laser

An effective method for determining the laser wavelength, pump-power threshold, and slope efficiency of an integrated erbium-doped glass laser with step index profile is presented. Calculations based on this method show good agreement with experimental results published in the literature. Optimization of the laser wave-guide was also performed.     

TEA-CO2 laser generation of ultrasound in non-metals

An industrial TEA-CO₂ laser, operating at a wavelength of 10.6 μm, has been used to produced broadband ultrasonic pulses in polymers. The generation mechanism falls into three categories. At low power densities ≤ 10⁷ W cm⁻² a thermoelastic regime predominates. As the power density is increased in the range (1–5) × 10⁷ W cm⁻² ablation of the material surface plays an increasingly important role in the acoustic generation. Thirdly, at greater power densities, plasma breakdown just above the material surface serves as the means of generation. This paper describes the acoustic sources for these types of generation mechanism and presents theoretically calculated acoustic waveforms to match those recorded experimentally.     

Numerical modeling of the local relaxation kinetics of the KrF laser

A multilevel mathematical model of the local kinetics of a laser medium of initial composition He-Kr-F₂ is constructed by analyzing the relaxation mechanisms of a plasma produced when an electron beam enters a dense gas. A working program is written for solving with a BÉSM-6 computer the corresponding system of balance equations for the populations and temperatures. Some results of demonstration computations are cited. Ways and prospects of improving the model are discussed.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva AN SSSR, Vol. 145, pp. 131–159, 1984.