Optimal pulse duration for femtosecond laser ablation

Femtosecond laser ablation occurs on timescales faster than the thermalization of the excited electrons and the lattice in solid materials. The ultrafast deposition of energy competes with the slower electron-phonon energy redistribution, raising the question of what is the optimal pulse duration for efficient deposition of energy while minimizing peripheral damage, and whether the shortest pulse is always the most efficient. We studied femtosecond laser ablation of silicon and several metals, varied the pulse duration while keeping all other parameters equal, and looked for optimal conditions. The main findings in our study are that at low fluences, not too high above the ablation threshold, the shortest pulses are the most efficient, whereas under high fluence conditions, well above the ablation threshold, longer pulses ablate more efficiently. In order to facilitate eventual direct, real time optimization, we developed a diagnostics tool for the monitoring of the ablation efficiency over a wide range of pulse durations. The intensity of the emission at atomic lines (i.e. the 289 nm line in Silicon, calibrated by plasma emission at other wavelengths) provides such information, while optical and AFM microscopy provide reliable information about the quality of ablated structures.