Huaiyu Cui

Extreme ultraviolet (EUV) lasers based on capillary discharge offer a compact and efficient short-wavelength source; yet, compared with the 46.9 nm laser well-established in practical applications, achieving high gain at 69.8 nm in neon-like argon ions (Ar8+) remains challenging. Instead of relying on capillary length extension, saturation was achieved by optimizing plasma conditions through enhanced energy injection in a 35 cm capillary. Increasing the discharge current to 19 kA yielded a record gain coefficient of 0.47 cm−1, the highest value reported to date for a 69.8 nm laser. This enabled saturation at an active length of 32 cm with a gain-length product of 15, reaching a maximum of 16.5 at 35 cm. Further increasing the current to 24 kA causes the laser intensity and gain to decrease markedly, with the gain coefficient reduced to 0.25 cm−1 and no saturation observed. In addition, by spectrally isolating the 69.8 nm laser with a self-made monochromator, this work achieves the measurement of the 69.8 nm laser pulse width under high main current. One-dimensional magnetohydrodynamic (MHD) simulations show that the higher main-pulse current produces intensified Joule heating and a faster pinch, leading to excessive electron density and overionization near the implosion point. This suppresses the Ar8+ density required for population inversion and limits the achievable gain. These combined experimental and computational results clarify the role of main-pulse current in affecting plasma state for 69.8 nm lasing and guide for optimizing discharge-waveform parameters in capillary-based EUV laser sources.

Structured beams carrying orbital angular momentum (OAM) provide powerful capabilities for applications in optical tweezers, super‐resolution imaging, quantum optics, and ad‐vanced microparticle manipulation. However, it is challenging for generate and control the OAM beams at the extreme ultraviolet (EUV) region due to the lack of suitable wave front shaping optics arise from being limited to the strong absorption of most materials. Here, we use a modified Fermat‐spiral photon‐sieve splitter to simultaneously generate two focused doughnut beams with opposite helical phase. Our technique enables us to produce splitting focused vortex beams with different rotation directions at EUV wavelengths. Additionally, we provide experimental evidence showcasing the capabilities of our method and further detect the helical phase by self‐reference interferometry. This work not only opens a route for OAM‐driven applications in EUV radiation, but also paves the way to studies of holographic technique by EUV splitter.

Extreme Ultraviolet (EUV) radiation is a short-wavelength light source that has important applications in many fields, such as optical communication, particle manipulation, and ultrahigh resolution imaging. However, the highly absorptive nature of EUV light makes it challenging to design suitable focusing optics, such as focal splitters, to properly manipulate the energetic light. Here, we propose modified spiral photon sieves to transform EUV laser light into azimuthally splitting focusing. A genetic algorithm was used to design and optimize the azimuthally focal splitters. A capillary discharge EUV laser at 46.9 nm was used to verify the effectiveness of our proposed method, and PMMA targets were used to record the focused laser spot. The profile of the recorded patterns measured by atomic force microscopy shows that the focal spots in the experiment are diffraction-limited and agreed with the theoretical analysis. The proposed technique provides a new way for manipulating EUV light and further extends the applications ranging from EUV to soft x rays.

A capillary discharge extreme ultraviolet laser is focused and wavefront split at 46.9 nm by a tubular optical element. The reflectivity at 46.9 nm is both simulated and measured to be higher than 90% with a slight optical aberration. The operating principle of the tubular element for focusing and wavefront splitting is discussed. Dense and intense grating-like fringes with a period of ~150 nm are achieved. The method used in this work allows nano-scale processing with extreme ultraviolet laser at single-shot exposure mode.

Using a plane–plane resonator composed of silicon carbide mirrors, we achieve for the first time multi-pass amplification of a 46.9 nm laser pumped by capillary discharge. In terms of the temporal characteristics, for an initial argon pressure of 17 Pa, triple-pass amplification of the laser is obtained at a delay time between the pre-pulse and the main pulse currents of 40 µs, and quadruple-pass amplification is obtained at a delay time of 50 µs. The experimental results show that the gain duration of the plasma column is more than 6 ns. In terms of spatial characteristics, the spot of the output laser has a reduced full width at half maximum divergence compared with that from a laser without a resonator.

Monocrystalline barium fluoride (BaF₂) slab targets were irradiated by focused 46.9-nm laser radiation at various fluence levels above the ablation threshold. Well-developed ablation patterns with sharp edges were studied by AFM (atomic force microscopy). Their inner surfaces were uniformly covered by periodic structures. The spatial period of the ripples depends on the laser fluence. When the sample is rotated by 45°, the orientation of the grating-like structure changes accordingly. Thus, the grating vector of the periodic structure seems to be coupled to the crystallographic planes of the single crystal. This means that the XUV-laser induced ripples reported here differ from LIPSS (laser-induced periodic surface structures) associated with interference phenomena occurring on illuminated surfaces. Therefore, other mechanisms are discussed to explain the formation of the periodic nanostructures reported in this article.

In this work, we performed computer modeling of high electrical discharge in an Ar filled alumina capillary in order to investigate the parameters of the discharge system that may lead to achieve an intense laser beam. One-dimensional magnetohydrodynamics (MHD) simulations were used to study the parameters of discharge produced Ar plasma. The radial evolution of plasma parameters such as electron temperature, electron density, and Ne-like argon density was estimated from MHD simulations. Additionally, the influence of the main discharge current on the pinching time and plasma radius was analyzed with the one-dimensional MHD model. The active medium for lasing is created by a high amplitude main pulse and pre-pulse. Specifically, we investigated the optimum main discharge current and Ar filling pressure. The main discharge current of 30 kA was declared as the best current to obtain the 46.9 nm laser with a 4 mm alumina capillary. The influence of the main discharge current and lasing pressure on laser beam intensity and beam characteristics has been studied theoretically and experimentally. The saturated laser intensity was obtained with a main discharge current of 30 kA. A severe reduction in laser intensity was observed above the main discharge current of 30 kA due to capillary wall ablation. The laser beam divergence was observed to be less than 3.5 mrad. The theoretical results obtained from MHD simulations are in good agreement with the experimental results of laser intensity and laser beam characteristics.

In the experiment of an Ne-like Ar 46.9 nm laser produced by capillary discharge, the effects of the addition of He into Ar on an Ne-like Ar 46.9 nm laser pulse and laser spot were studied. It was found that the laser pulse amplitude increased first and then decreased with pressure increase of He. In comparison with the laser pulse amplitude with pure Ar, the maximum amplitude with Ar–He mixture increased by approximately 34%. Under the condition of gain saturation of a 46.9 nm laser, the laser spatial distribution changed gradually with the addition of He into pure Ar. To analyze the influence of the addition of He into Ar on the laser spot, a theoretical simulation was performed.

Interaction of Extreme Ultraviolet (EUV) laser with matters is an attractive subject since novel phenomena always occur under the effect of high energy photons. In this paper, the thermal effect involved in the interaction of a capillary discharge 46.9 nm laser with copper was studied theoretically and experimentally. The temperature variation of the laser-irradiated region of copper was calculated. According to the results, the copper surface was ablated obviously and presented the trace of melting, evaporation, and resolidification, which suggested the thermal effect occurred on the surface during the laser irradiation.