中国激光, 2024, 51 (7): 0701009, 网络出版: 2024-04-02  

毛细管放电46.9 nm极紫外激光应用研究进展

Advanced Applications for Capillary Discharge 46.9 nm Extreme Ultraviolet Laser
作者单位
1 哈尔滨工业大学光电子信息科学与技术系,黑龙江 哈尔滨 150080
2 哈尔滨工业大学可调谐激光技术国家级重点实验室,黑龙江 哈尔滨 150080
摘要
毛细管放电极紫外激光是一种小型化的纳秒极紫外激光光源。相比自由电子激光和同步辐射等短波长光源,该光源具有运行成本低、单脉冲能量高和机时充足等显著优势。随着毛细管放电极紫外激光光源的发展,其输出已提高至深度饱和区,并且实现了重复频率输出、多波长输出等多样化输出方式。小型化的灵活性和优质的输出参数使其逐渐成为进行极紫外激光应用研究的理想光源。本文介绍了自1994年毛细管放电极紫外激光成功输出至今,该光源在微纳结构加工、物质成分检测、生物科学以及高分辨成像等领域的前沿应用。在微纳加工方面,极短的波长和极小的能量衰减深度使得该光源能够在纳米量程内进行材料的刻蚀。同时,较长的激光脉宽增加了极紫外激光诱导自组织微纳结构的可能性。在物质成分检测方面,极紫外激光的高能量光子能够以单光子电离材料表面,结合飞行时间质谱仪测量纳米尺度范围内的材料成分,便可实现超高分辨的物质组成分布检测。在生物科学领域,极紫外激光能够实现对微观生物样本的三维成分扫描,获得更多的表征信息。在高分辨成像方面,基于极紫外激光的短波长和良好的相干性,以Gabor同轴等方法进行高分辨成像能达到接近照明光水平的成像分辨率。已有的应用成果表明,毛细管放电极紫外激光是探索微观世界、制造微观结构的有力工具。在人类对短波长光源需求日益增长的今天,毛细管放电极紫外激光将有更多的机会展现它的应用价值和优势。
Abstract
Significance

Laser development represents a significant leap forward in the history of human science. The aggregation of billions (or potentially more) of photons in the same mode makes the laser the “brightest light”, “fastest knife” and “most accurate ruler”. Rapid laser source development and related technologies have promoted numerous breakthrough advances in the military, civilian and fundamental scientific fields. The lasers application areas are related to the laser source output parameters. When the laser output wavelength is shortened to extreme ultraviolet (EUV) and X-ray bands, the high photon energy produced by the light source and the extremely small diffraction limit make these short wavelength radiation sources favorable tools for exploring the microscopic world through cutting-edge scientific research. This includes micro-nano scale imaging and measurement, high-temperature, high-density plasma diagnostics, and high-resolution nanostructure generation.

Wavelength shortening has introduced many laser generation difficulties. To achieve short-wavelength laser output, scientists worldwide have invested significant effort into constructing large-scale short-wavelength laser sources, such as LCLS, LCLS-II in the USA and SXFEL, DCLS in China. Current research on such laser sources represents humanity’s exploration of the material essence forefront and the deepest understanding of nature. Large-scale short-wavelength laser sources generate high-quality laser outputs, leading to significant scientific research achievements. However, some drawbacks exist including high operating costs and complex operation processes, thus, making it difficult to address the high demand for laser utilization in fundamental scientific research. In this situation, how to miniaturize short-wavelength laser has gained attention. Capillary discharge pumping is a proven mechanism for achieving miniaturized EUV laser output. International research groups have been studying this field since Rocca demonstrated this type of laser output in 1994. In 2004, the Zhao group from the Harbin Institute of Technology self-developed this laser, which remains the only EUV laser source in operation that uses capillary discharge pumping in China. Subsequently, capillary discharge 46.9 nm wavelength EUV lasers have made significant improvements in output energy, coherence and multi-wavelength output, which have already become an ideal light source for EUV laser application research. Meanwhile, capillary discharge EUV lasers have been applied in micro/nano-structure processing, material composition detection and high-resolution imaging fields.

Progress

In the micro/nano-processing field, the 46.9 nm laser is capable of creating ablation patterns of PMMA photoresist of 82 nm diameter by the third order diffraction focusing of a freestanding Fresnel zone plate (Fig.4). The ablation pattern walls are extremely clean. The results demonstrate the feasibility of utilizing focused EUV lasers for nanoscale direct writing processes. The EUV laser interference effect is another approach for creating micro/nano-structures. With a tubular optical element, the 46.9 nm laser is focused and split to trigger light interference simultaneously, and focused interference fringes are formed and recorded on the PMMA with the period of ~150 nm (Fig.8). Nano-structures self-formation is also particular surface behavior which is triggered by 46.9 nm laser irradiation. With given material, the ablation process could be modulated and create novel phenomena. With single-layer graphene assistance, self-formed nanoparticles could be created all over the ablation area using single laser pulse exposure (Fig.14). The advantage of these self-formed nanostructures is that the scale of the structures is not dependent on the radiation source diffraction limitation, which increases the flexibility of nano-processing technology. This suggests significant potential for the 46.9 nm laser in this particular field. Recently, the 46.9 nm laser has been predominantly utilized in spectrochemistry. Because of high photon energy, this laser is capable of ionizing atoms or molecules using single photon-ionization. By coupling the time-of-flight mass spectrometer, the 46.9 nm laser analyzes the target surface composition. Furthermore, high-resolution imaging with the composition contrast can be achieved by scanning the surface (Fig.21). Therefore, this research connects EUV lasers with biology, chemistry and physics applications at the atomic and molecular scale.

Conclusions and Prospects

The capillary discharge EUV laser is a miniaturized laser source. Compared with short-wavelength light sources, such as free-electron lasers and synchrotron radiation sources, capillary discharge lasers have the advantages of low operating cost, high single-pulse energy and sufficient user time. Superior laser characteristics and the flexibility of miniaturization make it a suitable radiation source for EUV laser applications. This study presents cutting-edge applications for this laser in the micro/nano-structure processing, material composition detection, biological disciplines and high-resolution imaging fields, to date. Hence, it can be confirmed that the capillary discharge EUV laser is a powerful tool for probing and processing micro/nano-structures. Currently, the demand for short-wavelength light sources is increasing, thus indicating, that in accordance with its potential application value, multiple future advantageous development opportunities are expected to emerge.

崔怀愈, 申玉杰, 赵东迪, 安博, 赵永蓬. 毛细管放电46.9 nm极紫外激光应用研究进展[J]. 中国激光, 2024, 51(7): 0701009. Huaiyu Cui, Yujie Shen, Dongdi Zhao, Bo An, Yongpeng Zhao. Advanced Applications for Capillary Discharge 46.9 nm Extreme Ultraviolet Laser[J]. Chinese Journal of Lasers, 2024, 51(7): 0701009.

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