In this work,we investigated the influence of air gas pressures on the expansion features of nanosecond laser ablated aluminum plasma in the absence and presence of a nonuniform magnetic field using fast photography.A particular emphasis was given to the plume dynamics（shape,size） with the combined effects of ambient gas pressures and an external magnetic field.Free expansion,sharpening effect,and hemi-spherical structures of the aluminum plasma were observed without a magnetic field under different gas pressures.Analysis of the resulting plume images with the combined effects of air gas pressures and a magnetic field show significant changes,such as plume splitting,elliptical geometry changes,radial expansion,and plume confinement.Furthermore,the total size of the plasma plume with a magnetic field was measured to be smaller than the plasma plume without a magnetic field at several background pressures.
The propagation of a plasma shock wave generated from an Al target surface ablated by a nanosecond Nd：YAG laser operating at 355 nm in air is investigated at the different focusing positions of the laser beam by using a time-resolved shadowgraph imaging technique. The results show that in the case of a target surface set at the off-focus position, the condition of the focal point behind or in front of the target surface greatly influences the evolution of an Al plasma shock wave, and an ionization channel forms in the case of the focal point set in front of the target surface. Moreover, it is found that the shadowgraph with the evolution time around 100 ns shows that a protrusion appears at the front tip of the shock wave if the focal point is at the target surface. In addition, the calculated results of the expanding velocity of the shock wave front, the mass density, and pressure just behind the shock wave front are presented based on the shadowgraphs.
For the next-generation beyond extreme ultraviolet lithography （EUVL） sources, gadolinium （Gd） plasma with emis- sion wavelength at 6.7 nm seems to be the leading candidate. Similar to the Sn target 13.5 nm light source, ion debris mitigation is one of the most important tasks in the laser-produced Gd plasma EUV source development. In this paper, a dual-laser-pulse scheme, which uses a low energy pulse to produce a pre-plasma and a main pulse after a time delay to shoot the pre-plasma, is employed to mitigate the energetic ion generation from the source. Optimal conditions （such as pre-pulse energy and wavelength, and the time delay between the pre-pulse and the main pulse for mitigating the ion energy） are experimentally obtained, and with the optimal conditions, the peak of the ion energy is found to be reduced to 1/18 of that of a single laser pulse case. Moreover, the combined effect by applying ambient gas to the dual-pulse scheme for ion debris mitigation is demonstrated, and the result shows that the yield of Gd ions is further reduced to around 1/9 of the value for the case with dual laser pulses.
Metals in nature exhibit a mediocre wettability and a high optical reflectance from the visible region to the infrared. This Letter reports that, by formation of nano- and microscale structures via a simple raster sca.nning of a focused femtosecond laser pulse without any further treatment, structured aluminum and nickel surfaces exhibit combined features of superhydrophobicity with a contact angle of 155.5°, and a high optical absorption with a rcflectivity of several percent over a broad spectral range （0.2-2.5μm）. Thus, a multifunctional structured metal surface that integrates superhydrophobicity and a high broadband absorptivity has been easily realized by one-step femtosecond laser processing.
基于聚偏二氟乙烯压电传感器,对铜靶材中纳秒激光脉冲诱导的冲击波传播过程进行了实验研究,给出了铜靶材内冲击压强随激光脉冲能量和靶材厚度的变化规律.实验结果表明:500 m J激光脉冲能量作用到2 mm厚的铜靶材产生的冲击压强达到2.1 MPa;激光脉冲能量从200 m J增加到500 m J,在铜靶材厚度为2和4 mm条件下,冲击压强分别增加了162%和231%;而当铜靶材厚度从2 mm增加到6 mm时,在400和500 m J激光脉冲能量作用下,铜靶材内冲击压强分别降低了32%和49%.
以合金钢为样品,利用Cr I 520.8nm和Fe I 522.7nm谱线强度变化分析预烧蚀激光参数对纳秒532nm/1064nm正交预烧蚀双脉冲激光诱导击穿光谱(DP-LIBS)增强的影响,对实验条件进行了优化。结果表明在预烧蚀脉冲能量为70mJ,主烧蚀脉冲能量为100mJ,预烧蚀脉冲焦点距样品表面1mm,距主烧蚀脉冲光路1mm时,DP-LIBS具有最大光谱增强。Cr I 520.8nm谱线在30ms时光谱增强达到12.5,然后快速下降。最后初步分析了正交预烧蚀DP-LIBS光谱增强机理。
