Ultrashort Laser Pulses Induced Material Surface

Auditorium B. Touschek (Laboratori Nazionali di Frascati, Via Enrico Fermi 40, Frascati)

Auditorium B. Touschek

Laboratori Nazionali di Frascati, Via Enrico Fermi 40, Frascati

Via Enrico Fermi 40 00044 Frascati

Modification of structure and phase properties of the metal and alloy surface layers under superfast cooling velocities are widely used to produce superfine crystalline and amorphous structures by means of laser pulses of various durations. Rapid development of ultrashort lasers opens up the possibilities for new applications of material precision treatment as compared to conventional lasers. Nanostructuring the metal surface layers makes use of the femtosecond laser pulses with the near-threshold intensities when the metal melting takes place in a small volume without any significant ablation. After the end of laser pulse irradiation the melt is rapidly crystallized due to the heat propagation into the specimen depth. Due to the superfast cooling (109 К/s and higher) the size of produced crystallites might be compared with an interatomic space. In this case, if the cooling velocity exceeds the frozen one, an amorphous layer can be formed. In literature the forming of a superfine crystalline as well as an amorphous surface structure under the laser pulse irradiation is called “laser glazing”. In case of a femtosecond laser pulse the both heating and melting processes take place under essentially nonlinear and non-equilibrium conditions. However, the velocity of heat propagation into the material depth finally defines the kinetics of the melt cooling after the action of a supershort laser pulse; for metals this velocity is about 1013 - 1015 К/s. Thus, the time of cooling becomes more than 10-12 s. It means that a heat model can be applied for classical consideration of crystallization kinetics. The crystallization kinetics for melted metals has been studied under superfast cooling velocities realized in treatment of the materials by ultrashort laser pulses. An explicit solution to kinetic equation for the size distribution function of crystalline phase nuclei at the fast cooling of the melt was analyzed both analytically and numerically. The presented expressions for the crystal nuclei size and the new phase volume have been used to describe experiments on micro- and nanostructuring of the metal surface layers under the action of femtosecond laser pulses.

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