Electron-lattice relaxation time dynamics and separation time dynamic of multiple pulse femtosecond laser ablation process on gold.

A fundamental study of the interaction of ultrashort pulses and metal will be useful for predicting the ablation morphology and optimizing the process parameters. To study the ultrashort laser pulse interaction on gold, a set of coupled partial differential equations of the two-temperature model was solved in the spatial and time domains with dynamic optical properties and phase explosion mechanism. In an extended Drude model which also takes into account inter-band transitions, the reflectivity and absorption coefficient are contemplated based on the electron relaxation time. The laser energy deposition and phase explosion ablation mechanism are analyzed in the case of succession of laser pulses on the gold with experimental results for fundamental wavelength 1030 nm and fluence ranging from 3 J/cm² to 18 J/cm². Electron-lattice thermal relaxation time and separation time are important factors for multi-pulse laser ablation and have been studied. The simulation results demonstrate that by increasing the number of pulses with a shorter separation time compared to electron-lattice thermal relaxation time, lattice temperature can be considerably increased without a noticeable increase in ablation depth. In the study of multiple pulses femtosecond laser ablation, the computational model indicates that succession of laser pulses with a pulse separation time of 50 ps or longer can significantly boost the ablation rate at the same laser fluence. Thus, the deviation from experimental and simulation results gives rise to the conclusion that temporal pulse manipulation with separation time greater than the electron-lattice relaxation time is a useful technique for increasing ablation rate in industrial fast femtosecond laser processing.