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.

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.

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.