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85LM-PS-2
Ultrashort laser heating and ablation by one and two pulses of donut-like spatial form
A. Fedotov1,3, Y. Okrut2, Y. Tsitavets3,I. Gnilitskyi4
1HiLASE Centre, Institute of Physics of the Czech Academy of Sciences, Dolni Brezany, Czech Republic
2Belarusian State University, Energy Physics, Minsk, Belarus 3Belarusian State University, Computer Modelling, Minsk, Belarus
4University of Modena and Reggio Emilia, Department of Engineering Sciences and Methods, Modena, Italy
We have investigated the process of metal melting and ablation caused by femtosecond laser pulses of high intensity by means of numerical experiment. We have used finite element model implemented in COMSOL Multiphysics for the simulation in the framework of two-temperature model (separate electronic and lattice temperatures and heat conduction). We have considered temperature dependent transport coefficients and phase transitions possibility as well. During the study we developed COMSOL 3-dimensional transient model for gold that allows us to model the laser heating and melting in metals, induced by ultrafast laser pulses both of gaussian and non-gaussian spatial form. We have studied the temperature dynamics and ablation characteristics from the number of pulses (single or pair) and the form (gaussian or donut-like) (Figure).
It was revealed that the heat energy deposited from a single or double pulsed beam of a donut form (pulse duration Tp = 100 fs, beam radius R = 10 p,m) preserve the volume right under beam center of symmetry unheated for the time comparable to 10Tp. The temperature under the center of symmetry of the incident pulse varies slightly, and it has been established that at a depth of 20 nm below the surface there is a collapsing thermal region. This feature of the propagation of the temperature field is of interest for the study of nanostructured surfaces and other relevant applications.
We achieve the highest ablation depth for the pair of pulses irradiation when the second pulses is launched while the melting front reaches a maximum. If the second pulse is launched when temperature increase cause by the first pulse is highest, the ablation depth associated with evaporation will be lower. This is due to a change in the absorption coefficient k, which decreases with increasing temperature.
Maximal lattice temperature, K
Fig. 1. Maximal ablation depth achieved in system vs. maximal lattice temperature for different intensity of
incident pulses
References
[1] Venkatakrishnan, K. Femtosecond pulsed laser ablation of thin gold film / K. Venkatakrishnan, B. Tan, B.K.A. Ngoi // Opt. Laser Technol. - 2002. - Vol. 34, № 3. - P. 199-202.
[2] Hopkins, P.E. Effects of electron scattering at metal-nonmetal interfaces on electron-phonon equilibration in gold films / P.E. Hopkins, J.L. Kassebaum, P.M. Norris // J. Appl. Phys. - 2009. -Vol. 105, № 2. - P. 023710.
[3] Rethfeld B. Ultrafast dynamics of nonequilibrium electrons in metals under femtosecond laser irradiation / B. Rethfeld [et al.] // Phys. Rev. B. - 2002. - Vol. 65, № 21.
