Optically pumped metastable rare gas lasers (OPRGL) have garnered significant attention as a possible candidate for high-energy laser systems. However, despite the chemically stable inert gas used as the gain medium, laser output power exhibits considerable decay in a sealed chamber during operation. To investigate the underlying mechanism responsible for the attenuation of power, the plasma gas temperature was measured using optical emission spectroscopy method, along with time-resolved number density of Ar metastable atom, laser waveforms, output power and intensity of OH radical emission lines. The results indicate that the increase in impurity concentration driven by gas temperature rise causes laser performance degradation through collisional energy transfer, while the associated enhancement in collisional rate provides a limited compensating effect for this degradation. These findings suggest that suppressing the gas temperature rise is beneficial for stable laser operation. Using a high-thermal-conductivity CuW50 electrodes, a temperature reduction of approximately 50 K was achieved under high-power pumping. The stability of output power was improved by about 40%. This work may shed some light on the future research and development of the OPRGL, particularly under prospective highpower operating conditions.

High-throughput and compact volume phase holographic (VPH) grating transmission spectrometers are widely employed in scientific research, agriculture, and industrial applications. Conventional transmission spectrometers generally adopt a fixed configuration and therefore have limitations in simultaneously achieving high spectral resolution and broad wavelength coverage. To address the limited tunability of transmission spectrometers, this work presents the theoretical analysis and experimental validation of a transmission spectrometer incorporating a novel catadioptric grating assembly, which consists of a transmitting VPH and a planar reflector. A catadioptric system is a combination of reflective (catoptric) and refractive (dioptric) elements. In the proposed configuration, a VPH grating and a plane mirror arranged at a fixed 90° angle form the catadioptric dispersion module. Synchronous rotation of this assembly enables wavelength scanning. The structure ensures that the diffracted ray along the optical axis of the imaging lens maintains the Bragg condition across the scanning range, thereby preserving maximum diffraction efficiency. The optical configuration and structural parameters of the spectrometer were theoretically derived, and a prototype spectrometer with an f-number of 1.8 employing a 2400 g/mm grating was constructed. Measurements demonstrate that, when the rotation angle is tuned from 30.5° to 50.5°, the accessible spectral range covers from 410 nm to 650 nm. Spectral response measurements using a tungsten–halogen light source confirm that the spectrometer maintains an acceptable diffraction efficiency across the entire tuning range. The measured spectral resolution is 0.1 nm at 626 nm with a 2400 g/mm grating and 0.18 nm with a 1500 g/mm grating. The spectrometer was further applied to fiber-enhanced gas Raman spectroscopy, where it successfully resolved the closely spaced Raman peaks of CH4 and C2H6 that are difficult to distinguish using conventional compact spectrometers. These results demonstrate that the proposed tunable catadioptric spectrometer simultaneously provides excellent wavelength tunability and high spectral resolution.

Optically pumped metastable rare gas lasers have attracted considerable interest as promising candidates for high energy laser systems. At present, producing a large-volume gain medium with high metastable atom density remains a critical challenge. In this work, a temporally synthesized segmented discharge based on repetitively fast pulsed DC discharge is employed to produce a gain medium with a volume of 0.96 cm 3 and a peak number density approaching 10 14 cm −3 at atmospheric pressure. Under high-power pumping, an average output power of 51.1 W is achieved in the quasi-CW regime at 50 kHz, with approximately 40% conversion efficiency and a power density of 53.2 W/cm 3 . Further improvements are expected to enhance output power at higher repetition rates.

We report results from the optical interferometric diagnosis of laser-produced tin droplet plasma for the extreme ultraviolet (EUV) light sources. Nomarski interferometry was utilized to study the early stages of the laserproduced tin droplet plasma formation for the first time and obtain its spatial distribution of electron density. To mitigate the plasma's heating and suppress background radiation of plasma, we employed the low-power CW probe laser and narrow-band filter in our experiment. The time-resolved interference patterns were recorded by ICCD. The electron density distribution within the plasma was ascertained by analyzing the phase differences observed in the interference patterns, employing the method of inverse Abel transform. Our experimental results indicate that the plasma electron density varies between 3 x 1018 and 8 x 1018 cm-3.

An external power build-up cavity of a line-narrowed 407-nm laser diode for Raman gas analysis was demonstrated to possess good gas detection capabilities. By employing an ordinary laser diode without anti-reflection coating or and a bandpass interference filter in an external cavity resonance, the laser linewidth was narrowed by resonant optical feedback, and tens of watts of external cavity power were built up. The coupling mechanism between the semiconductor laser and the external cavity are discussed, as well as the noise background in the experimental results. The Raman spectrum of ambient air was analyzed, achieving a methane detection limit of 1 ppm.

In this study, xenon plasma was sustained using continuous fiber lasers with varying incident directions, and the stability of plasma was analyzed through both experimental and theoretical approaches. High-speed cameras were employed to analyze the plasma's shape and size, while photodetectors recorded the plasma's brightness fluctuations. It was found that plasma sustained by lasers incident from below exhibited greater stability, as indicated by smaller standard deviations of the centroid and axial diameter, as well as reduced brightness fluctuation amplitudes. Schlieren method revealed an upward circulating hot bubble surrounding the plasma. Fluid dynamic analysis of gas flows surrounding the high-temperature plasma demonstrated turbulent flow in the region above the plasma and laminar flow below it, as determined by Reynolds number evaluation. This observation provides a fundamental explanation for the superior stability observed in plasmas sustained by laser irradiation from below. A direct relationship between Kelvin-Helmholtz instability and the formation of toroidal vortex structures was confirmed. Based on combined experimental and simulation results, the optical configuration for plasma maintenance was optimized. By employing bottom laser irradiation with F=0.8, a highly stable, small-diameter plasma with nearly perfect circular symmetry was achieved.

The paper employs various optical analysis methods to study the stability of continuous fiber laser sustained Xe plasma under different laser power and focusing systems. High-speed cameras were used to capture plasma images from two directions, allowing a quantitative description of the fluctuation amplitudes of the plasma centroid and diameter. Standard deviation analysis revealed that a tightly focused system and increased laser power are more favorable for improving plasma stability. High-speed photodiodes and piezoelectric film sensors were simultaneously employed to monitor changes in plasma luminous intensity and internal gas pressure, showing that the fluctuation signals of both were consistent in shape and variation. The Schlieren method revealed the presence of periodically rising thermal bubbles around the plasma. Fourier transform spectra identified a 30 Hz low frequency and several hundred Hz high frequencies in the fluctuations of the plasma centroid, diameter, and brightness, while the piezoelectric signal contained only the low frequency. Further analysis concluded that the high frequencies originate from laser power fluctuations, while the low frequencies are caused by gas convection. As the laser power increases, the frequency of the low frequency gradually decreases to a gentle level.

Laser-sustained plasma (LSP), a unique method of obtaining high brightness broadband light sources, has been used for rapid optical inspection of wafer defects and has thus attracted great interest. Then Xe plasma sustained under high pressure by a continuous fiber laser was investigated using optical emission spectroscopy (OES). Emission spectra of spatial integration diagnosis have shown that with the increase of laser power and the decrease of Gaussian beam focusing number F, the electron temperature fitted by Boltzmann plot method increased from 0.45 eV to 0.85 eV, the corresponding electron density was roughly an order of magnitude of 10(16)-10(17) cm(-3), and the calculated absorption coefficient of plasma was distributed in 0.9-1.5 cm(-1). To further analyze the spatial distribution of plasma parameters, the spatial resolved OES of LSP was acquired, and the twodimensional electron temperature and electron density were determined. The non-uniform distribution of the plasma parameters was analyzed due to the existence of thermal gravity convection and forced convection. The serious self-reversal of emission lines of 881.94 nm and 823.16 nm was observed and analyzed.

Objective With the continuous decrease in feature size in the semiconductor industry, extreme ultraviolet lithography (EUVL) is becoming increasingly crucial in ultrahigh integrated circuit manufacturing. The emission characteristics of tin (Sn) in terms of its high conversion efficiency (CE) and spectral purity (SP) make it the best choice for current EUVL systems. Laser - produced plasma (LPP) and laser - induced discharge plasma (LDP) are the most important technical methods for generating extreme ultraviolet rays. LDP has the low cost, simple structure, high operation rate, and high energy injection, and it is promising for mask inspection, microscopic imaging, and spectral metrology. Unlike the LPP source, many parameters, such as the electrode structure, discharge gap, laser wavelength, and current rise rate, make it difficult to design an LDP source; therefore, it is necessary to investigate the influence of these parameters on the performance of the LDP EUV source. Methods An experimental setup is designed to investigate the laser - induced discharge of tin plasma. A pulsed carbon dioxide laser is used to ablate a tin plate cathode and produce an expanding pre - ionized plasma as the discharge medium, decreasing the threshold for the breakdown voltage. A vacuum arc gradually formes between the cathode and the stainless - steel hemisphere anode, and the current bombards the electrodes and generates more plasma. The time - domain waveform of the current is recorded, and the extreme ultraviolet spectra of the LPP and LDP are analyzed. A radiative magneto - hydrodynamic program Z* is used to simulate the laser plasma and discharge plasma. Results and Discussions The experimental LPP - EUV spectrum shows a peak at 13.7 nm when the laser energy is 145 mJ, whereas the peaks of the LDP spectra show a significant red shift compared to those of the LPP spectrum (Fig. 3). When the voltage is 7 kV, the in - band (bandwidth of 2% at 13.5 nm wavelength) spectral intensity of the LDP does not significantly increase compared with that under the LPP condition. At a voltage of 15 kV, the in - band spectral intensity increases significantly. The simulation results show that the time - domain signals of the LDP - EUV have multiple peaks, and when the voltage reaches 15 kV, the second peak of the EUV radiation is higher than the first peak (Fig. 6). When the current intensity is sufficiently high, the Joule heat generated by the current is sufficient to compensate for the energy transmitted via plasma diffusion and thermal radiation, causing the plasma temperature to increase and the EUV radiation area to extend. Subsequently, the plasma between the electrodes rapidly collapses as the current intensity oscillates and decays. The total and EUV radiation powers reach their maximum values near the current peak. When the discharge voltage increases from 7 kV to 15 kV, the total and EUV radiation powers both increase, and the highest EUV radiation power reaches 0.025 MW. The total radiation energy increases from 842.00 mJ to 3.85 J, and the total EUV radiation energy increases from 3.5 mJ to 65.0 mJ. The CE increases from 0.054% to 0.23%, and the SP increases from 0.42% to 1.69%. At a voltage of 7 kV, the maximum EUV radiation power density is 0.09 MW/cm(3) , and the EUV radiation is mainly concentrated near the anode (Fig. 7). When the voltage is 15 kV, the maximum EUV radiation power density can reach 0.3 MW/cm(3), and the EUV radiation is mainly concentrated on both sides of the electrodes (Fig. 8). The average ionization degree of the plasma in the main area of EUV radiation is 10-12. Conclusions In this study, the EUV radiation emitted by the LDP and LPP is conducted experimentally and theoretically. Compared to that of the LPP source, the temperature of the LDP source is significantly higher, and more Sn10+, Sn11+, and Sn12+ ions are present. Transitions between multiple excited states gradually replace those between single excited and ground states. However, the plasma size of the LDP source is very large, resulting in a low radiation power density. The light source for mask inspection requires strong brightness; therefore, further research on the Z - pinch mechanism is required to reduce the plasma size and improve brightness. The discharge voltage significantly influences the in - band EUV radiation of the LDP source. This phenomenon demonstrates the major advantage of the LDP light source: the extreme ultraviolet output power can be increased by increasing the injection of electrical energy. However, CE and SP still need to be improved by increasing the current rise rate. Shortening the current rise time and reducing the inductance of the discharge circuit can be good approaches.

Droplet-based laser-produced plasma source shows enormous significance in extreme ultraviolet lithography, which places high demands on the stability of tin droplets. This paper presents a tin droplet target system including the tin droplet generator (DG), droplet diagnosis, and spatiotemporal synchronization of tin droplets and laser. Shadowgraph technology is used to determine the stability of tin droplets. The characteristics of the DG were analyzed, and the operation parameter maps are provided. By varying operating frequencies from 18.4 kHz to 49.3 kHz, the diameter and spacing of droplets can be adjusted in the ranges of 120-200 mu m and 200-1100 mu m respectively. Both theoretical calculations and experimental results show that tin droplets keep high stability when the operation parameters locate at the optimal range. The long-term lateral stability is also proven under a high degree of vacuum. Additionally, the application feasibility of the DG system is verified by the experiments of laser impact tin droplets.

Extreme ultraviolet (EUV) light source is an important part of EUV lithography system in semiconductor manufacturing. The EUV light source requires that the 4p64dn-4p54d(n+1) + 4dn-14f transitions of Sn8+similar to 13+ ions emit thousands of lines which form unresolved transition arrays near 13.5 nm. Laser -induced discharge plasma is one of the important technical means to excite target into an appropriate plasma condition. Laser -induced discharge plasma has a simple structure and a low cost. It also has important applications in mask inspection, microscopic imaging, and spectral metrology. In the design and production process, there are many factors that can influence the conversion efficiency, such as current, electrode shape, and laser power density. The simulation method is a convenient way to provide guidance for optimizing the parameters. In this paper, a completed radiation magneto -hydrodynamic model is used to explore the dynamic characteristics of laserinduced discharge plasma and its EUV radiation characteristics. To improve the accuracy, a more detailed global equation of state model, an atomic structure calculation model including relativistic effect and a collision radiation model are proposed simultaneously. The simulation reconstructs the discharge process effectively, which is divided into five stages in the first half cycle of current, including expansion of laser plasma, column formation of discharge plasma, diffusion of discharge plasma, contraction of discharge plasma, and re -diffusion of discharge plasma. It is revealed that the pinch effect during the current rising time exerts a significant influence on the generation of EUV radiation. The conversion efficiency of EUV radiation is still low under our existing conditions, and hopefully a higher rising rate of current can improve the conversion efficiency in the future work.

The 13.5 nm extreme ultraviolet (EUV) light radiated by laser-produced tin droplet plasma is successfully used in manufacturing integrated circuits with critical dimensions below 7 nm. As a vital component in EUV source, the tin droplet generators (DG) are comprehensively investigated. We focus on the principle, mechanical structure, and performance parameters of tin DG from aspects of industrial manufacturing and scientific research. Moreover, we analyzed the existing droplet control schemes and droplet synchronization technology for long-term laser-droplets alignment. Finally, the potential development direction on tin droplet targets is summarized.

Objective With the continuous decrease in feature size in the semiconductor industry, extreme ultraviolet lithography (EUVL) is becoming increasingly crucial in ultrahigh integrated circuit manufacturing. The emission characteristics of tin (Sn) in terms of its high conversion efficiency (CE) and spectral purity (SP) make it the best choice for current EUVL systems. Laser - produced plasma (LPP) and laser - induced discharge plasma (LDP) are the most important technical methods for generating extreme ultraviolet rays. LDP has the low cost, simple structure, high operation rate, and high energy injection, and it is promising for mask inspection, microscopic imaging, and spectral metrology. Unlike the LPP source, many parameters, such as the electrode structure, discharge gap, laser wavelength, and current rise rate, make it difficult to design an LDP source; therefore, it is necessary to investigate the influence of these parameters on the performance of the LDP EUV source. Methods An experimental setup is designed to investigate the laser - induced discharge of tin plasma. A pulsed carbon dioxide laser is used to ablate a tin plate cathode and produce an expanding pre - ionized plasma as the discharge medium, decreasing the threshold for the breakdown voltage. A vacuum arc gradually formes between the cathode and the stainless - steel hemisphere anode, and the current bombards the electrodes and generates more plasma. The time - domain waveform of the current is recorded, and the extreme ultraviolet spectra of the LPP and LDP are analyzed. A radiative magneto - hydrodynamic program Z* is used to simulate the laser plasma and discharge plasma. Results and Discussions The experimental LPP - EUV spectrum shows a peak at 13.7 nm when the laser energy is 145 mJ, whereas the peaks of the LDP spectra show a significant red shift compared to those of the LPP spectrum (Fig. 3). When the voltage is 7 kV, the in - band (bandwidth of 2% at 13.5 nm wavelength) spectral intensity of the LDP does not significantly increase compared with that under the LPP condition. At a voltage of 15 kV, the in - band spectral intensity increases significantly. The simulation results show that the time - domain signals of the LDP - EUV have multiple peaks, and when the voltage reaches 15 kV, the second peak of the EUV radiation is higher than the first peak (Fig. 6). When the current intensity is sufficiently high, the Joule heat generated by the current is sufficient to compensate for the energy transmitted via plasma diffusion and thermal radiation, causing the plasma temperature to increase and the EUV radiation area to extend. Subsequently, the plasma between the electrodes rapidly collapses as the current intensity oscillates and decays. The total and EUV radiation powers reach their maximum values near the current peak. When the discharge voltage increases from 7 kV to 15 kV, the total and EUV radiation powers both increase, and the highest EUV radiation power reaches 0.025 MW. The total radiation energy increases from 842.00 mJ to 3.85 J, and the total EUV radiation energy increases from 3.5 mJ to 65.0 mJ. The CE increases from 0.054% to 0.23%, and the SP increases from 0.42% to 1.69%. At a voltage of 7 kV, the maximum EUV radiation power density is 0.09 MW/cm(3) , and the EUV radiation is mainly concentrated near the anode (Fig. 7). When the voltage is 15 kV, the maximum EUV radiation power density can reach 0.3 MW/cm(3), and the EUV radiation is mainly concentrated on both sides of the electrodes (Fig. 8). The average ionization degree of the plasma in the main area of EUV radiation is 10-12. Conclusions In this study, the EUV radiation emitted by the LDP and LPP is conducted experimentally and theoretically. Compared to that of the LPP source, the temperature of the LDP source is significantly higher, and more Sn10+, Sn11+, and Sn12+ ions are present. Transitions between multiple excited states gradually replace those between single excited and ground states. However, the plasma size of the LDP source is very large, resulting in a low radiation power density. The light source for mask inspection requires strong brightness; therefore, further research on the Z - pinch mechanism is required to reduce the plasma size and improve brightness. The discharge voltage significantly influences the in - band EUV radiation of the LDP source. This phenomenon demonstrates the major advantage of the LDP light source: the extreme ultraviolet output power can be increased by increasing the injection of electrical energy. However, CE and SP still need to be improved by increasing the current rise rate. Shortening the current rise time and reducing the inductance of the discharge circuit can be good approaches.

The advanced research on bright 13.5 nm extreme ultraviolet (EUV) light sources with low debris is of crucial importance for the semiconductor industry. The scheme of laser produced colliding plasma holds the potential to significantly improve the conversion efficiency (CE) from the laser energy to the required EUV light sources. A pulsed Nd:YAG laser beam was split into two beams using a polarizing cube and focused on a planar Sn target surface at a distance of 2.5 mm to generate two colliding plasmas. It was found that the optical radiation intensity of the stagnation layer generated in the colliding plasmas region was much higher than that of a single-pulse plasma under the same laser energy. Furthermore, the stagnation layer could exist for a longer time with a higher optical radiation intensity, which meant that the formation of the stagnation layer effectively converted the ion kinetic energy into optical radiation energy. The time-of-flight (TOF) ion signal of the colliding plasma exhibited a much narrower distribution compared to the seed plasma. At the same laser energy, the ion kinetic energy of the colliding plasma was lower than that of the single-pulse plasma. The TOF peak voltage and total charge showed a stronger angular dependence for the colliding plasma. At high laser energy, the colliding plasma could significantly reduce the ion's kinetic energy without changing the CE of the EUV.

Laser produced plasma extreme ultraviolet light source, as the exposure light source for next-generation lithography, faces urgent challenges due to its low conversion efficiency and significant debris contamination. Using a triple Langmuir probe, we conducted a study on the parameters and kinetic characteristics of tin droplet-based laser produced plasma that serves as extreme ultraviolet light source. In this paper, we report a design of triple Langmuir probe circuit based on the BWL (Bulk Wirewound Low-value) sampling resistor, which ensures that high-frequency signals remain undistorted during acquisition. Utilizing the “shadowgraph” method, we calculated the average deflection angles of droplet debris jets under different alignment conditions. Combining the diagnostic results from the probe, we found that the alignment accuracy between the laser and droplet directly influences the parameters of plasma. The parameters and kinetic characteristics of plasma were also diagnosed and analyzed at different angles and distances by moving the probe. Employing the triple Langmuir probe, measurements and analyses of laser produced plasma parameters and kinetic characteristics were achieved, providing a convenient method for diagnosing extreme ultraviolet lithography light sources. This work also offers an effective basis for optimizing the light sources.

Optically pumped metastable rare gas lasers have been extensively investigated as promising high-energy lasers. These systems employ discharge-excited metastable inert gases as the lasing medium. Following optical pumping, a buffer gas, typically helium, is introduced to facilitate a collisional population transfer to the p(2)[1/2](1) level, thereby establishing population inversion. To date, laser outputs in the watt level have been demonstrated. However, further power scaling crucially depends on the ability to stably generate high metastable densities at elevated pressures approaching atmospheric conditions. In this Letter, we report a pulsed discharge technique based on a peaking capacitor rapid discharge circuit, which is capable of producing metastable particle densities exceeding 10(14)cm(-3) at pressures up to 900 mbar. By employing this discharge approach in conjunction with transverse optical pumping, we have realized a maximum output power of 12.5Wfrom a semiconductor-pumped metastable argon laser system. (c) 2024 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.

Low-density foam targets are expected to be good candidates for efficient laser produced plasma (LPP) and soft Xray (SXR) sources due to the less energetic debris compared with solid targets. Three Ti targets with different densities were irradiated by a pulsed Nd:YAG laser in vacuum. The average ion energy and the time-of-flight (TOF) peak voltage decreased as the Ti target density decreased. Furthermore, the differences among the three Ti targets increased with increasing laser intensity. The average ion energy of low-density foam Ti targets had a slightly weaker angular dependence compared with solid Ti targets. The plasma plume optical images illustrated that solid Ti plasma expanded faster in the direction of laser incidence with stronger anisotropy compared to foam Ti plasma. In addition, the optical radiation intensity integrated from a wavelength region of 200-850 nm of plasma plume increased with decreasing initial target density, which indicated that more absorbed laser energy was converted to plasma optical radiation energy (internal energy) instead of ion kinetic energy for targets with lower initial densities. The SXR radiation had no significant difference among the three Ti targets. These results demonstrate that appropriately reducing the initial target density can significantly reduce kinetic energy of ion debris without affecting the SXR radiation output.

Optically pumped metastable rare gas laser (OPRGL), as a potential high-energy laser has attracted much attention. During extensive research on OPRGL, the realization of volume discharge with high-metastable density has always been a priority and is essential to the efficient lasing of OPRGL. A large-volume plasma of He/Ar with high metastable density at atmospheric pressure can be generated by applying peaking capacitors near the electrodes. In this article, 0.8 cm3 of He/Ar plasma at a pressure of 900 mBar, with a peak value of metastable argon density higher than 1014 cm-3 was realized. The metastable density was measured by absorption spectroscopy based on the absorption bandwidths acquired at a pressure range of 400-900 mbar where a probe beam at a weak line with wavelength 772.38 nm was applied. Furthermore, the high metastable density was verified by the laterally-observed laser-induced fluorescence, as well as the laser oscillation of 100 mW under a longitudinal pumping with a short gain length of 8 mm.

This study aimed to develop and evaluate an in-season nitrogen (N) management strategy for maize (Zea mays L.) by combining crop growth modeling, active canopy sensing and machine learning. At V8 stage, the CERES-Maize model was first used with weather data fusion to predict in-season economic optimal side-dress N rate (EOSN). Active canopy sensing data was fused with other related data using machine learning regression to diagnose maize N status. The predicted EOSN was adjusted based on estimated maize N status. Compared to the predicted EOSN based on modeling only, the recommended EOSN based on the new strategy showed higher correlation (R2=0.70) and lower difference (10.36 kg/ha) with the measured values. It is concluded that the combination of crop growth modeling, active canopy sensing and machine learning is a promising strategy for in-season site-specific N management of maize.

By utilizing the combination of multiple dichroic mirrors, droplet detection, synchronization of laser with droplets, and droplet deformation observation in two directions can be realized in a very compact design. A shadowgraph technology is used to investigate the laser-induced tin droplet two-dimensional deformation. Particularly, compressed and propelled targets in side-view and expensed disk-like targets in front-view shadowgraphs are observed at the same time. In addition, the propulsion and expansion processes of the droplet are presented, keeping consistent with the prediction of the theoretical model. All these results highlight the ability of our compact design for droplet dynamic evolution monitoring.