使用预成型液体盘靶从2- $μ$ m波长laser驱动的Sn等离子体中角分辨EUV发射的表征
Characterization of angularly resolved EUV emission from 2-$μ$m-wavelength laser-driven Sn plasmas using preformed liquid disk targets
摘要：
研究了几ns长的2- $μ$ m波长激光脉冲辐照预成型液态锡靶产生的锡等离子体在极紫外区的发射特性。在双脉冲方案中，在主脉冲（2 $μ$ m）产生EUV发射等离子体之前，首先使用预脉冲激光将锡微滴变形成薄的扩展盘。用2- $μ$ m的激光脉冲照射30- 300- $μ$ m直径的靶，我们发现在产生13.5nm左右的EUV光的效率遵循与靶重叠的激光的分数。接下来，研究了2- $μ$ m驱动激光强度（0.6-1.8 $\times 10^{11}$ W/cm $^2$）和脉冲持续时间（3.7-7.4ns）的变化的影响。发现在13.5nm附近的2%带宽内和入射激光束周围的后向2-$μ$半球内的光发射的角度依赖性几乎与2- $μ$ m驱动激光的强度和持续时间无关。随着目标直径的增加，在该2%带宽中的发射变得越来越各向异性，更大部分的光被发射到入射激光束的半球中。为了直接比较，用1- $μ$ m波长驱动激光器进行一组类似的实验。在5.5-25.5nm波长范围内记录的发射光谱显示，在1- $μ$ m的情况下，在13.5nm附近的光的显著自吸收，而在2- $μ$ m的情况下，仅观察到在13.5nm处的光谱特征的不透明度相关的加宽。这项工作证明了增强的能力和性能的2- $μ$ m驱动的等离子体产生的磁盘目标相比，1- $μ$ m驱动的等离子体，提供了强大的动机使用2- $μ$ m激光器作为驱动激光器在未来的高功率源的EUV光。
英文摘要：
The emission properties of tin plasmas, produced by the irradiation of preformed liquid tin targets by several-ns-long 2-$μ$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$μ$m) pulse creates the EUV-emitting plasma. Irradiating 30- to 300-$μ$m-diameter targets with 2-$μ$m 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-$μ$m drive laser intensity (0.6-1.8$\times 10^{11}$W/cm$^2$) 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 2$π$ hemisphere around the incoming laser beam is almost independent of intensity and duration of the 2-$μ$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-$μ$m-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-$μ$m case, while in the 2-$μ$m 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-$μ$m-driven plasmas produced from disk targets when compared to 1-$μ$m-driven plasmas, providing strong motivation for the use of 2-$μ$m lasers as drive lasers in future high-power sources of EUV light.
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