Spatiotemporal pulse-shaping effects on 4-μm laser-driven Sn microdroplet plasmas for extreme ultraviolet emission

Drive lasers near the 4-μm wavelength offer a fundamental thermodynamic advantage for extreme ultraviolet (EUV) lithography by optimally balancing laser absorption and in-band EUV opacity. Using radiation-hydrodynamics simulations, we investigate spatiotemporal pulse-shaping effects on 4-μm-driven Sn microdroplet plasmas under an industrially relevant overfill geometry. An energy-conserved full-factorial strategy evaluates the independent influences of pulse duration, temporal envelope, and transverse spatial profile. Results reveal that temporal and spatial shaping govern distinct physical processes. Temporally, box-shaped profiles establish a quasi-steady-state hydrodynamic regime that sustains optimal ionization, preventing the severe over-ionization of high-peak Gaussian pulses and the under-heating of extended low-power pulses. Spatially, although transverse intensity variations negligibly impact macroscopic energy absorption, angular emission analyses demonstrate that flat-top beams uniformly ablate the target periphery. This suppresses the optically thick peripheral plasma shroud inherent to Gaussian beams, thereby minimizing angle-dependent self-absorption and enhancing isotropic EUV photon escape. Ultimately, combining a 15-ns box-shaped temporal envelope with a spatial flat-top profile achieves a maximum conversion efficiency of 3.35%. Thus, optimizing EUV emission requires utilizing temporal shaping to sustain intrinsic emissivity and spatial flattening to minimize radiation transport losses.