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

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 (2μm) 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 13.5nm follows the fraction of laser light that overlaps with the target. Next, the effects of a change in 2-μm drive laser intensity (0.6–1.8×1011W/cm2) and pulse duration (3.7–7.4ns) are studied. It is found that the angular dependence of the emission of light within a 2% bandwidth around 13.5nm 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 5.5–25.5nm wavelength range, show significant self-absorption of light around 13.5nm in the 1-μm case, while in the 2-μm case only an opacity-related broadening of the spectral feature at 13.5nm 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.

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