Characterization of angularly resolved EUV emission from 2-μm-wavelength laser-driven Sn plasmas using preformed liquid disk targets

Abstract

The emission properties of tin plasmas, produced by the irradiation of preformed liquid tin targets by several-ns-long 2 μm-wavelength laser pulses, are studied in the extreme ultraviolet (EUV) regime. In a two-pulse scheme, a pre-pulse laser is first used to deform tin microdroplets into thin, extended disks before the main (2 μm) pulse creates the EUV-emitting plasma. Irradiating 30- to 300 μm-diameter targets with 2 μm laser pulses, we find that the efficiency in creating EUV light around 13.5 nm follows the fraction of laser light that overlaps with the target. Next, the effects of a change in 2 μm drive laser intensity (0.6-1.8×1011 W cm-2) and pulse duration (3.7-7.4 ns) are studied. It is found that the angular dependence of the emission of light within a 2% bandwidth around 13.5 nm and within the backward 2π hemisphere around the incoming laser beam is almost independent of intensity and duration of the 2 μm drive laser. With increasing target diameter, the emission in this 2% bandwidth becomes increasingly anisotropic, with a greater fraction of light being emitted into the hemisphere of the incoming laser beam. For direct comparison, a similar set of experiments is performed with a 1 μm-wavelength drive laser. Emission spectra, recorded in a 5.5-25.5 nm wavelength range, show significant self-absorption of light around 13.5 nm in the 1 μm case, while in the 2 μm case only an opacity-related broadening of the spectral feature at 13.5 nm is observed. This work demonstrates the enhanced capabilities and performance of 2 μm-driven plasmas produced from disk targets when compared to 1 μm-driven plasmas, providing strong motivation for the use of 2 μm lasers as drive lasers in future high-power sources of EUV light.