Enhanced extreme ultraviolet conversion efficiency of a 2 μ m laser-driven preformed tin-droplet target using short picosecond pre-pulses

In this study, an extreme ultraviolet (EUV) conversion efficiency (CE) of 6.9% was obtained in simulation by controlling the delay between a picosecond pre-pulse and a main-pulse with a wavelength of 2 μm; this value is about 7.5% higher than the ns pre-pulse scheme of the EUV emission produced by a 2 μm laser-driven tin-droplet target. In the simulation, the tin droplet expanded into hollow spherical structures after irradiation with a picosecond pre-pulse laser. Notably, compared with a nanosecond pre-pulse, in this case, the density of the tin plasma was lower, and the expansion range was wider for the same delay. Therefore, ps pre-pulse can make the tin plasma reach the optimal state of EUV emission in a shorter delay. In both pre-pulse schemes, the CE value maintains a high value (>5.7%) within a certain delay range (800 ns) between pre-pulse and main-pulse. In this study, the FLASH radiation hydrodynamic code and FLYCHK atomic code were used to investigate the energy conversion and spectra. The results obtained can be potentially useful for EUV lithography under a two-pulse scheme.