Laser ablation of tin droplets with a 2 μ m wavelength laser

We report on the mass ablation of tin microdroplets irradiated by a long-pulse 2 μm wavelength laser, a wavelength of particular interest for next-generation extreme ultraviolet (EUV) light sources for nanolithography. The spherical droplet serves as a well-defined platform for studying quasi-stationary laser ablation fronts and benchmarking theoretical studies and radiation-hydrodynamic modeling results. We find that the mass ablation rate ṁ scales as ṁ∼D02I0.39(9), with D0 the initial droplet diameter and I the laser intensity. The measured power exponent closely matches the value of 0.45 predicted by detailed simulations from de Lange et al. [Plasma Sources Sci. Technol. 33, 105003 (2024)]. However, we observe a significantly higher absolute ablation rate. Differences in beam geometry and pulse duration may contribute to this discrepancy, indicating the need for further refined modeling. The mass efficiency of generating EUV light (in terms of mJ/m3) increases with decreasing droplet diameter, separating into mass-limited and mass-saturated regimes. Additionally, the mass efficiency increases with increasing laser intensity; this suggests that an industrial light source might benefit from a laser intensity higher than what would be optimal strictly for EUV generation on small tin droplets to optimize mass efficiency.