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.
The polarization-resolved laser-induced breakdown spectroscopy （PRLIBS） technique, which can significantly reduce the polarized emission from laser plasma by placing a polarizer in front of the detector, is a powerful tool to improve the line-to-continuum ratio in LIBS applications. It is shown that the continuum emission from the plasma produced through ablating an Al sample by nanosecond laser pulses is much more polarized than the discrete line emission with the singlepulse PRLIBS technique. The effects of laser fluence and detection angle on the Al polarization spectrum are systematically explored experimentally. The calculated result of the polarization spectrum as a function of laser fluence shows that it is in agreement with the experimental observations.
An intense supercontinuum（SC） in the near-ultraviolet range is generated from filamentation by focusing a 400-nm laser into fused silica with a microlens array（MLA）. The spectrum of the SC is shown to be sensitive to the distance between the MLA and fused silica. In our optimal conditions, the near-ultraviolet SC can cover a range of 350-600 nm,where a bandwidth of approximately 55 nm above the 1μJ/nm spectral energy density and 20 nm bandwidth with tens ofμJ/nm are achieved. In addition, the energy conversion efficiency of the 400 nm laser for SC generation is further analyzed.A maximum conversion efficiency of 66% is obtained when the entrance face of fused silica is set around the focus of the MLA.
A comparative investigation of the resistance and ability to trigger high voltage（HV） discharge for a single filament（SF） and multiple filaments（MFs） has been carried out.The experimental results show that the trend of the breakdown threshold of the SF exactly follows that of its resistance,but this is not the case for the MF.The MF＇s resistance is much smaller than the SF＇s.However,the MF shows a slightly higher HV breakdown threshold than the SF.The underlying physics is that the measured resistance of the MF is collectively contributed by every filament in the MF while the HV breakdown threshold is determined by only one single discharging path.
Aluminum samples have been analyzed by femtosecond polarization-resolved laser-induced breakdown spectroscopy （fs-PRLIBS）. We compare the obtained spectra with those obtained from nanosecond PRLIBS （ns-PRLIBS）. The main specific features of fs-PRLIBS are that a lower plasma temperature leads to a low level of continuum and no species are detected from the ambient gas. Furthermore, signals obtained by fs-PRLIBS show a higher stability than those of ns-PRLIBS. However, more elements are detected in the ns-PRLIBS spectra.
