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Thermal ablation of copper films by a single Ti:Sapphire femtosecond laser pulse of wavelength 800 nm and duration 100 fs was investigated experimentally and theoretically. The laser experiments were performed; the ablation depth and crater profiles were measured for laser fluences up to 1037 J/cm2. A comprehensive axisymmetric model, including a two-temperature model, phase change models for rapid melting and evaporation, and a phase explosion model for ejecting metastable liquid and vapor, was developed to simulate the laser material ablation process. The simulated ablation depths and crater profiles agree well with the experimental measurements. [ABSTRACT FROM AUTHOR]

Effects of film thickness on ultrafast phase change during femtosecond laser interaction with gold films were studied numerically. The laser pulses were generated in the form of single pulse, multiple pulses, and pulse train. The results show that the melting depth will increase significantly when film thickness is smaller than 600 nm, at the expense of a mild rise of the maximum temperature. When the thickness increases above 1100 nm, the effect of thickness on the laser-materials interaction is very limited. For multiple pulse irradiations, the dependence of melting depth on pulse number and pulse interval is decided by film thickness. When the film thickness is less than 600 nm, more pulses and larger intervals cause deeper melting, while for the thicker film the effects of pulse number and interval are opposite. For pulse train irradiation, the pulse number per train and repetition rate have very limited influence on temperature and melting depth. The pulse interval causes some change in temperature, but not in the melting depth. [ABSTRACT FROM AUTHOR]

Melting, vaporization and resolidification processes of thin gold film irradiated by a femtosecond pulse laser are studied numerically. The nonequilibrium heat transfer in electrons and lattice is described using a two-temperature model. The solid–liquid interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, is obtained by considering the interfacial energy balance and nucleation dynamics. An iterative procedure based on energy balance and gas kinetics law to track the location of liquid–vapor interface is utilized to obtain the material removal by vaporization. The effect of surface heat loss by thermal radiation was discussed. The influences of laser fluence and duration on the evaporation process are studied. Results show that higher laser fluence and shorter laser pulse width lead to higher interfacial temperature, deeper melting and ablation depths. [ABSTRACT FROM AUTHOR]

Ultrafast deformation is caused by intense heating of femtosecond lasers that involve ultrafast energy exchange between electrons and metal lattices in the femto- to picosecond domain. This work outlines unique features including interactions between T-waves and stress waves in the femtosecond domain and the hot-electron blast that can destroy cold lattices before they become thermally excited. Dominating groups are identified to characterize the ultrafast thermomechanical response. [ABSTRACT FROM AUTHOR]

Four optical models of reflectivity and absorption coefficient are investigated in this letter. After compared with existing experimental data, the extended Drude model is incorporated into a two-temperature model to simulate laser energy deposition and thermal response, including solid-liquid and liquid-vapor phase change, in a gold film irradiated by a femtosecond laser pulse. Dynamic reflectivity and absorption coefficient should be employed in modeling ultrafast laser heating except for very low laser fluencies. [ABSTRACT FROM AUTHOR]

A sample-based stochastic model is presented to investigate the effects of uncertainties of various input parameters, including laser fluence, laser pulse duration, thermal conductivity constants for electron, and electron-lattice coupling factor, on solid-liquid phase change of gold film under nano- to femtosecond laser irradiation. Rapid melting and resolidification of a free-standing gold film subject to nano- to femtosecond laser are simulated using a two-temperature model incorporated with the interfacial tracking method. The inteifacial velocity and temperature are obtained by solving the energy equation in terms of volumetric enthalpy for control volume (CV). The convergence of variance (COV) is used to characterize the variability of the input parameters, and the interquartile range (IQR) is used to calculate the uncertainty of the output parameters. The IQR analysis shows that the laser fluence and the electron-lattice coupling factor have the strongest influences on the interfacial location, velocity, and temperatures. [ABSTRACT FROM AUTHOR]

Ultrashort laser pulses can be generated in the form of a pulse train. In this paper, the ultrafast phase change processes of a 1 μm free-standing gold film irradiated by femtosecond laser pulse trains are simulated numerically. A two-temperature model coupled with interface tracking method is developed to describe the ultrafast melting, vaporization, and resolidification processes. To deal with the large span in time scale, variable time steps are adopted. A laser pulse train consists of several pulse bursts with a repetition rate of 0.5-1 MHz. Each pulse burst contains 3-10 pulses with an interval of 50 ps-10 ns. The simulation results show that with such configuration, to achieve the same melting depth, the maximum temperature in the film decreases significantly in comparison to that of a single pulse. Although the total energy depositing on the film will be lifted, more energy will be transferred into the deeper part, instead of accumulating in the subsurface layer. This leads to lower temperature and temperature gradient, which is favorable in laser sintering and laser machining. [ABSTRACT FROM AUTHOR]