Lifting the efficiency of ultrashort pulse laser ablation of FeNi in water towards the physical limit by spatio-temporally distributing intensity

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Abstract
Ultrashort pulsed laser ablation in liquids allows the synthesis of surfactant-free nanoparticles. However, redeposition of ablated material and nonlinear attenuation in the liquid reduce process efficiency on sub-nanosecond timescales. On longer timescales, shielding by transient and persistent microbubbles can reduce process efficiency. Here, we investigate the ablation of FeNi in water by spatio-temporal distributing the laser intensity. We apply double-pulses with an inter-pulse delay of 600 ps and apply scanning to avoid bubble shielding and employ incubation by irradiating the same sample position multiple times. This isolates the contributions of redeposition and nonlinear losses. In total, we reach a power specific productivity of 75 mg/(W×h) comparable to a specific energy for ablation of about 400 J/mm³ with a pulse duration of 10 ps and a total peak fluence of 10 J/cm². This is a significant improvement to the single-pulse value of 670 J/mm³ and the highest reported value of 2000 J/mm³. The results do not indicate suppression of redeposition by the second pulse at 600 ps inter-pulse delay. However, employing double-pulses produces 30 % smaller nanoparticles. These results demonstrate that spatio-temporally distributing intensity is an effective way to increase the productivity of laser ablation in liquids towards the physical limit.

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