Department of Physics, University of York, York YO10 5DD, UK.
Opt Lett. 2010 Nov 15;35(22):3820-2. doi: 10.1364/OL.35.003820.
We demonstrate that interferometric probing with extreme ultraviolet (EUV) laser light enables determination of the degree of ionization of the "warm dense matter" produced between the critical and ablation surfaces of laser plasmas. Interferometry has been utilized to measure both transmission and phase information for an EUV laser beam at the photon energy of 58.5 eV, probing longitudinally through laser-irradiated plastic (parylene-N) targets (thickness 350 nm) irradiated by a 300 ps duration pulse of wavelength 438 nm and peak irradiance 10(12) W cm(-2). The transmission of the EUV probe beam provides a measure of the rate of target ablation, as ablated plasma becomes close to transparent when the photon energy is less than the ionization energy of the predominant ion species. We show that refractive indices η below the solid parylene N (η(solid) = 0.946) and expected plasma values are produced in the warm dense plasma created by laser irradiation due to bound-free absorption in C(+).
我们证明，利用极紫外（EUV）激光的干涉探测能够确定激光等离子体的临界和烧蚀表面之间产生的“热密物质”的电离程度。干涉法已被用于测量在光子能量为 58.5 eV 的 EUV 激光束的透射和相位信息，该激光束纵向探测激光辐照的塑料（聚对二甲苯-N）靶（厚度 350nm），该靶由持续时间为 300ps、波长为 438nm 和峰值辐照度为 10(12)W/cm(-2)的激光脉冲辐照。EUV 探针束的透射提供了目标烧蚀率的测量，因为当光子能量小于主要离子种类的电离能时，被烧蚀的等离子体变得接近透明。我们表明，由于 C(+) 的束缚-自由吸收，在激光辐照产生的热密等离子体中产生了低于固体聚对二甲苯 N 的折射率 η（η(solid) = 0.946）和预期的等离子体值。
