中国激光, 2024, 51 (7): 0701004, 网络出版: 2024-03-29  

激光烧蚀低密度等离子体产生高次谐波的研究进展

Advances in High-Order Harmonic Generation from Laser-Produced Low-Density Plasmas
高健 1,2,*吴健 1,2
作者单位
1 华东师范大学精密光谱科学与技术国家重点实验室,上海 200241
2 华东师范大学重庆研究院精密光学重庆市重点实验室,重庆 401121
摘要
超短强激光脉冲与物质相互作用产生的高次谐波辐射是一种相干的极紫外或软X射线光源,并且在时间上还是阿秒脉冲串。在不同介质中探寻更有效的高次谐波产生方案一直是研究热点。利用飞秒激光烧蚀低密度等离子体羽可将高次谐波扩展到几乎任何固体材料,极大地丰富了媒介的选择性。由于某些材料的等离子体内低电离态离子共振跃迁频率与谐波波长存在匹配,使得在极紫外波段特定阶次谐波表现出明显的共振增强效应,从而能够获得强单色的高次谐波辐射。结合纳米颗粒的近场增强效应和较大的电子回撞截面,极紫外波段的高次谐波转换效率可以进一步得到提高。激光等离子体高次谐波有望产生高脉冲能量、增强阶次可调和高重复频率的相干极紫外辐射。综述单阶谐波共振增强效应的产生原理和研究进展,分析各种优化方法和光场调控手段,并对未来的发展趋势进行展望。
Abstract
Significance

High-order harmonics generated by ultrashort intense laser pulses interacting with matter represent a significant advance in the production of coherent extreme ultraviolet (EUV) or soft X-ray sources, and they also lead to the generation of attosecond pulse trains in the time domain. A single isolated attosecond burst could be obtained by various gating techniques. Such a promising source can be widely used in applications such as diagnosis of ultrafast dynamical processes, coherent diffractive imaging (CDI) with high resolution, probing of magnetic materials, and mask defect inspection. The exploration of more efficient harmonic generation schemes in different media has been a fascinating research topic. The employment of low-density plasma plumes subject to intense femtosecond laser pulses has allowed the extension of high-order harmonics to arbitrary solids, greatly enriching the selectivity of the materials. Due to the resonance of a specific harmonic with the ionic transition possessing strong oscillator strength, intensely monochromatic high-order harmonics can be produced in the EUV region. In addition, the resonance-enhanced harmonic can be tuned by laser wavelength, chirp, two-color field and target structure. By taking advantage of the near-field enhancement and the large recombination cross section of the nanoparticles, the conversion efficiency of the EUV harmonics can be further improved.

Progress

The first observation of resonance-induced enhancement of the single harmonic generated in the plasma was made in 2006, in which a very strong intensity of the H13 harmonic from the In plasmas was two orders of magnitude higher than neighboring orders. In the same year, the enhancement of the H17 harmonic in Sn plasmas was also demonstrated. The typical resonance-enhanced harmonic spectra are shown in Figs. 3, 7 and 9. The other plasmas produced by Mn, Cr, Sb and Zn are enhanced at H31, H29, H21 and H9, respectively. The "four-step" model can be used to interpret the peculiarly resonant harmonic, as illustrated in Fig. 5. The last step of the electrons in the "three-step" model of gas harmonics is replaced by the radiationless transition to the autoionizing state and relaxation to the ground state with EUV emission. The highest cut-off order H101 harmonic (7.9 nm) was obtained in the Mn plasmas. The conversion efficiency of the resonance-induced enhancement of the monochromatic harmonic has approached 10-4. In order to improve the conversion efficiency of the harmonics in the EUV region, the nanoparticle-containing plasmas are utilized, as shown in Figs. 3 and 7. Since the generation process is dominated by neutral atoms, it is difficult to enhance the higher harmonic orders. By adopting different laser chirps, two-color field, mixed targets and multi-jet plasmas, the resonant wavelength, harmonic cut-off order and harmonic intensity can be tuned. In addition, high-order harmonics can be controlled using structured vortex and vector beams, as presented in Figs. 8?13. The calculated attosecond pulse trains with a duration of 240 as in the laser-produced In plasmas and the experimentally measured attosecond pulse trains with a duration of 300 as in the Cr plasma plume were obtained. The metallic cylindrical rod was mounted on a rotating holder, which can achieve stable harmonic radiation in more than 106 shots under 1 kHz laser pulses (Fig. 14).

Conclusions and Prospects

In this article, we review the mechanism and research progress in the resonance-induced enhancement of the single harmonic from a laser-produced plasma. Many efforts have been made to tune the resonance wavelength, extend the cut-off order, increase the conversion efficiency and improve the harmonic stability. To be an applicable EUV source, the plasma harmonics must be generated with high photon flux and high repetition rate. This should be greatly improved if the nanoparticles interact with the laser pulses at high repetition rate through some techniques. We also expect that the gating techniques can be applied to produce isolated single attosecond pulse.

高健, 吴健. 激光烧蚀低密度等离子体产生高次谐波的研究进展[J]. 中国激光, 2024, 51(7): 0701004. Jian Gao, Jian Wu. Advances in High-Order Harmonic Generation from Laser-Produced Low-Density Plasmas[J]. Chinese Journal of Lasers, 2024, 51(7): 0701004.

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