Modeling of Ultrashort Pulse-Train Laser Heating of Metal Films

Recently, a new laser micromachining technique using multiple ultrashort pulses with a picosecond-to-nanosecond separation as a train group has demonstrated some promising application potential, which also has high scientific merits in micro-/nano-scale ultrafast energy transport. However, the underlying physics of this technique remain poorly understood. In this study, the improved two-temperature model recently developed by the authors is used to analyze the pulse train technology, in which the full-run quantum treatments are used to calculate the significantly varying properties, including the electron heat capacity, electron relaxation time, electron conductivity, reflectivity, and absorption coefficient. This paper reports the effects of the pulse number per train, pulse separation, and repetition rate. Results from the present study have demonstrated that 1) it can increase the photon efficiency and further reduces the heat-affected zone; 2) the number of bursts required for melting decreases with the repetition rate, but at tens of MHz or higher, the repetition rate has a negligible impact; 3) the lattice temperature right after the thermolization time first increases to a peak and then decreases as the pulse separation increases.