Nanosecond laser ablation threshold of liquid tin microdroplets

The laser ablation threshold is an important parameter that governs the response of materials to intense laser irradiation. Here we study the ablation threshold of liquid tin, by irradiating tin microdroplets with nanosecond laser pulses having finely controlled temporal shape and duration. We use the time-dependent reflection from the droplet as the main observable, which exhibits a sharp decrease in magnitude at a given time instance that depends on the laser intensity. This moment marks the generation of a plasma that strongly absorbs the following incident laser light, rapidly expands, and thereby sets in motion the remainder of the liquid droplet. We find an inverse-square dependence of this plasma-onset time on laser intensity and attribute this scaling to the presence of one-dimensional heat diffusion during irradiation. This scaling and its one-dimensional thermal origin is strongly established in literature and follows from a square-root scaling of the thermal diffusion depth with time. Our experiment unambiguously shows that this scaling law holds for our specific case of nanosecond laser impact on tin microdroplets. The results presented in this work are of particular interest to target preparation and metrology in extreme-ultraviolet sources utilizing tin microdroplet targets.