Characterization Of Angularly Resolved Euv Emission From 2-Mu M-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 mu m-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 mu m) pulse creates the EUV-emitting plasma. Irradiating 30- to 300 mu m-diameter targets with 2 mu m 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 mu m drive laser intensity (0.6-1.8 x 10(11) 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 2 pi hemisphere around the incoming laser beam is almost independent of intensity and duration of the 2 mu m 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 mu m-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 mu m case, while in the 2 mu m 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 mu m-driven plasmas produced from disk targets when compared to 1 mu m-driven plasmas, providing strong motivation for the use of 2 mu m lasers as drive lasers in future high-power sources of EUV light.