Nd,Y:CaF2 (NYCF) crystals are exceptional gain materials for high-power laser drivers; however, laser-induced damage remains a substantial challenge that restricts their broader application. In this study, by establishing an in situ testing system for photothermal weak absorption and the laser-induced damage threshold (LIDT), the relationship between the photothermal weak absorption characteristics of NYCF and its LIDTs was analyzed. A fully connected neural network was employed to facilitate deep learning of these relationships, thereby enabling non-destructive evaluation of NYCF via photothermal weak absorption. Moreover, this study examined both the effect of spot size during testing and the influence of crystal orientation on the evaluation outcomes. The underlying mechanisms were further elucidated by investigating NYCF's thermal mechanical properties and damage characteristics. This work not only offers a rapid, non-destructive method for evaluating the laser damage resistance of NYCF using artificial intelligence but also enhances the understanding of its damage mechanisms.

The mechanistic origins underlying the anisotropic brittleness and incipient plasticity of fluorite-structured alkaline earth fluorides (MF2, M = Ca, Sr, Ba) are revealed by a unified first-principles framework. Our computations revealed that the notorious {111} cleavage propensity originates from an exceptionally low tensile strength, a critically limited strain threshold governed by the onset of phonon instability, and the minimum cleavage energy among low-index planes. In contrast, incipient plastic flow is dominated by slip along the <110>{001} system, which possesses not only the lowest shear strength but also, decisively, the lowest generalized stacking fault energy (GSFE) barrier, a feature enabled by significant anionic relaxation. The competition between these deformation modes is rigorously quantified by the ratio of the cleavage energy to the slip energy barrier E-C/Gamma. The resulting E-C/Gamma landscape exhibits profound anisotropy. A near-unity ratio (similar to 1.0) on {110} and {111} planes signifies virtually identical energetic costs for cleavage and slip nucleation, thereby rationalizing the extreme brittleness. Conversely, the conspicuously high E-C/Gamma ratio (>3.8) on {001} planes affirms that dislocation-mediated plasticity is energetically strongly favored over fracture, consistent with experimental evidence. We reveal that this fundamental deformation anisotropy stems from the pronounced directionality of M-F bonds coupled with intense electrostatic repulsion between like-charged ions during shear, which collectively dictate a unique energetic landscape. This work establishes the E-C/Gamma criterion as a predictive framework, providing a fundamental basis for understanding deformation mode selection in fluorite-structured crystals and directly bridging atomic-scale bonding to macroscopic mechanical response.

The degradation of calcium fluoride (CaF2) crystals in deep ultraviolet (DUV) lithography systems poses a critical challenge to the safe and stable operation of such systems. Understanding the degradation dynamics and underlying physical mechanisms induced by high-energy photon irradiation is essential for improving the radiation resistance of CaF2 crystals. This work proposes a rapid assessment method for evaluating the radiation-resistant lifetime of CaF2 crystals based on X-ray exposure. By characterizing the dynamic degradation behavior of CaF2 crystals under various X-ray doses and integrating multidimensional defect characterization techniques, the physical mechanism of X-ray-induced degradation was systematically elucidated, with dislocation density identified as the dominant factor governing the radiation resistance of CaF2 crystals. This study not only elucidates the dynamic evolution and critical factors of degradation processes in CaF2 crystals under X-ray irradiation but also lays a solid foundation for the future optimization of their radiation-resistant properties in high-energy radiation environments.

Nd,Y:CaF2 (NYCF) and Nd,Y:SrF2 (NYSF) crystals are excellent types of laser gain media owing to their enhanced absorption-emission cross sections and fluorescence lifetimes, and high thermal conductivity. In this study, the laser damage behaviors of crystals with different orientations under a 1064 nm nanosecond laser are analyzed through investigation of the thermo-mechanical properties with different orientations. The results demonstrate that the superior thermal conductivity and flexural strength of NYCF crystals contribute to a higher thermal shock resistance parameter and laser-induced damage threshold (LIDT) compared to NYSF crystals. The {110} plane exhibits the highest LIDT due to optimal thermo-mechanical properties, while the {111} plane shows the lowest LIDT due to its minimal flexural strength and weakest thermal shock resistance. A strong linear correlation between radial flexural strength and LIDT further confirms the critical role of intrinsic thermo-mechanical properties in determining the damage resistance for nanosecond laser pulses. This work could provide insights for designing high-power laser gain media and optimizing crystallographic orientations to enhance performance under extreme laser conditions.

Calcium fluoride (CaF2) crystals are widely utilized in deep-ultraviolet (DUV) lithography due to their excellent optical properties. The laser-induced degradation and damage of CaF2 crystals is a critical concern that restricts its extended application. Impurities of CaF2 crystal are considered a key factor affecting its laser resistance. Establishing the quantitative relationship and mechanism of impurity content impacting the degradation and damage characteristics of CaF2 crystal is essential. This study investigated the characteristics of different impurity contents affecting the degradation and laser-induced damage thresholds (LIDTs) of CaF2 crystals under X-ray and 193 nm pulsed laser irradiations, and quantitatively analyzed the degradation process and mechanism. Our findings demonstrate that impurities at ppm levels significantly diminish the transmittance of CaF2 crystals across various wavelengths following X-ray irradiation. In contrast, these impurities have a negligible effect on the LIDT test results, suggesting distinct damage mechanisms between X-ray and laser irradiation. This study provides valuable insights for optimizing the CaF2 crystal fabrication process and enhancing irradiation resistance.

Through the investigation of spectral characteristics, the evolution of cluster proportions and the cross-relaxation process in Tm:CaF2 crystals as a function of Tm doping concentration has been clarified. A quantitative model has been established to describe the relationship between these factors. At low concentrations (0.01-0.5 at.%), Tm ions rapidly form clusters with negligible cross-relaxation. As the concentration increases from 0.5 at.% to 2 at.%, the cluster ratio surpasses 90%, and its growth rate decelerates, while both the cross-relaxation rate and efficiency significantly increase. When the Tm concentration exceeds 2 at.%, the cluster ratio and cross-relaxation efficiency nearly reach 100% and remain stable. These findings provide valuable insights for the application of Tm:CaF2 in high-energy pulsed laser systems.

This study aims to investigate the various responses of the optical characteristics to He-ion irradiation at different fluences and energies in Nd,Y:CaF2 crystals. Through optical absorption and photoluminescence spectra, we observe that electrons excited by He-ion irradiation are trapped by anionic vacancies, He ions and Nd3+ to form F-centers, He atoms and Nd2+, respectively. At an energy of 5 MeV, F-centers decrease with increasing fluence, whereas at a fluence of 1 x 1013 ions/cm2, increasing the energy promotes Nd2+ formation, and a further increase in the fluence based on an energy of 16 MeV can significantly promote the production of Nd2+. After the irradiation, the electrons from F-centers are also captured by Nd3+, leading to the decay of F-centers and an increase in Nd2+ with time. In addition, the growth of CaF2 dendrites with multiple morphologies on the surface by high-energy irradiation is observed. It is explained by the surface recrystallization between the escape of F and surface metallization due to high-energy irradiation. This research provides significant insights into the irradiation conditions modulating defect generation and evolution, as well as laying the groundwork for future applications of radiation-exposed crystals as radiation dosimeters and lasers.

High gain greater than 106 is crucial for the preamplifiers of joule-class high-energy lasers. In this work, we present a specially designed compact amplifier using 0.5%Nd,5%Gd:SrF2 and 0.5%Nd,5%Y:SrF2 crystals. The irregular crystal shape enhances the gain length of the laser beam and helps suppress parasitic oscillations. The amplified spontaneous emission (ASE) induced by the high gain is analyzed through ray tracing. The balance between gain and ASE is estimated via numerical simulation. The gain spectral characteristics of the two-stage two-pass amplifier are examined, demonstrating the advantages of using different crystals, with bandwidths up to 8 nm and gains over 106. In addition, the temperature and stress distributions in the Nd,Gd:SrF2 crystal are simulated. This work is expected to contribute to the development of high-peak-power ( $\ge$ terawatt-class) high-energy (joule-class) laser devices.

The comprehensive investigation into the distribution characteristics of internal defects across various growth areas of Nd, Gd: SrF2 (NGSF) crystals is presented in this paper by photothermal weak absorption, stress-induced birefringence, and laser confocal microscopy. The stress-gathering and absorption bands resulting from the accumulation of internal defects predominantly consist of fractures, empty tubes, and inclusions with impurities, as observed through SEM. Furthermore, laser-induced damage characteristics were examined to explore the mechanisms of internal defect-induced damage by a 1064 nm pulsed laser, revealing the exponential-Chapman relationship between weak absorption and laser damage threshold fluence. Based on the distribution profile of weak absorption, stress birefringence, and damage points, the damage mechanism is analyzed and attributed to the absorption of laser energy by the internal defects, further emphasizing that these internal defects are precursors to laser-induced damage.

Er:CaF2 crystal, characterized by low doping and high efficiency, is a suitable material for single-crystal fiber (SCF). 3 at.% Er:CaF2 SCFs were grown using the laser heated pedestal growth (LHPG) method. Significant oxidation-induced whitening phenomena were observed during growth. Increasing the growth rate helped mitigate further deterioration due to oxidation. This is likely because higher growth speeds allow the SCF to quickly move away from temperature ranges conducive to oxidation. As oxidation progressed, the phonon energy of 3 at.% Er:CaF2 SCF increased, causing the emission intensity at - 1 mu m to decrease from 77 % in the initial source rod to 6 % in the fully whitened state. Additionally, the lifetime of the 4I11/2 level decreased by approximately 10 times, from 8.988 ms to 0.822 ms. Continuous laser output at - 2.8 mu m was achieved using the transparent portion of 3 at.% Er:CaF2 SCF. With an output mirror transmission of 2 %, a maximum output power of 251 mW and a slope efficiency of 15.9 % were obtained. The laser experiment demonstrated the potential application of LHPG-grown Er:CaF2 SCF in - 2.8 mu m lasers, but further performance enhancement requires additional strategies to address oxidation issues. This work provides valuable insights into the growth of Er:CaF2 SCF using the LHPG method.

In this work, we report on a comparison of continuously pumped laser system based on 0.5% Nd, 5% Y:SrF2 or 0.5% Nd, 5% Gd:SrF2 generating around the wavelength of 1.06 mu m passively mode-locked by a SESAM designed for low pump power operation. For the dispersion compensated cavity with a GTI mirror, the pulse duration of 386 and 415 fs at the repetition rate of 125 MHz has been achieved for 0.5% Nd, 5% Gd:SrF2 and 0.5% Nd, 5% Y:SrF2, respectively. The spectral width of generated radiation points to non-transform-limited pulse and the possibility of further pulse shortening for both cases.

The concentration dependence of the fluorescence and lifetime evolutions of states relevant to three nearinfrared emission bands were measured and analyzed for Tm:SrF2 crystals. The fast quenching of -1.2 mu m fluorescence (3H5) was observed as the Tm concentration increased. The cross-relaxation and multi-phonon relaxation processes between Tm3+ ions cannot explain this phenomenon. Through spectral and rate-equation analysis, two possible energy transfer mechanisms, cross-relaxation from the 3H5 level and cooperative energy transfer within Tm3+ clusters, are proposed. And based on the new modified model, the quantum efficiency of the Tm -2 mu m lasers could surpass the traditional 200 % limit. This new energy transfer has the potential for exceptionally high efficiency excitation of Tm-doped laser materials.

Laser crystals have been developed by combination paradigm for more than sixty years, and the methodology is difficult to be continued in uncovering new laser materials. Recently, the local structure design has been proposed and advances have been obtained. This review systematically summarizes the development history of rare earth clusters, cluster structures, evolution characteristics, design principles and the utilization for regulating spectral properties and laser performance of rare earth doped fluorite crystals. We also highlight the future opportunities for development of new laser materials. It is believed that this review will provide valuable insights into rational design principles and new paradigm for development of laser materials.

Employing electron paramagnetic resonance (EPR) and excitation and photoluminescence (PL) spectra, changes of the local structure of Gd3+ ions were investigated for the CaF2 crystals containing 0.00015, 0.17, 1.22, 5.75 at% Gd ions, respectively. The obtained spin Hamiltonian parameters of the cubic configuration Gd3+ monomer are g = 1.9862, B 4 = -2.3153 ± 0.015 MHz and B 6 = -0.0005 ± 0.001 MHz. The peak-to-peak width of the EPR lines of the cubic configuration Gd3+ is significantly broadened and partially quenched with the increase of the Gd3+ concentration, which indicates that the exchange and dipole interactions between Gd3+-Gd3+ are enhanced. From theoretical calculations, combined with experimentally measured angular dependence, we found that for the 1.22 at% Gd:CaF2 crystals, Gd3+ remains essentially cubic, and the majority of the Gd3+ ions are distributed as a dimer conformation along the [110] direction and at a distance of about 7.7 Å.

Dislocations are ubiquitous in crystalline solids and play a critical role in both material properties and device performance. Due to the numerous variables influencing crystal growth, such as temperature, pressure, and impurity levels, the synthesis of high-quality crystals with minimal defects remains a challenging and enduring goal in crystal research. Calcium fluoride (CaF2) which is widely used in optical devices typically contains dislocations on the order of 10-5 cm-2. The sensitivity of the weak CaF2 lattice to electron beams makes at atomic-level imaging using traditional scanning transmission electron microscopy difficult. Here, we utilize a combination of characterization methods to analyze the microstructure of dislocations in CaF2, and the existence of both stored statistical dislocations (SSDs) and geometrically necessary dislocations (GNDs) are identified. SSDs mainly consist of perfect dislocations with a small amount of Frank partial dislocations, while GNDs are predominantly composed of Frank partial dislocations and stacking faults. Based on the calculated formation energies of dislocations and fluorite vacancies, the formation mechanisms of SSDs and GNDs are proposed. The findings in the present study deepen our understanding of the dislocations in CaF2 and offer insights into strategies to further reduce the defect concentration in CaF2 crystals.

Significance As solid-state lasers develop toward higher power, better beam quality, miniaturization, and lower cost, they face such challenges as degraded optical performance caused by thermal effects (e.g., thermal lensing and thermal stress birefringence) induced by high-power pumping. Traditional gain media can no longer meet the demands for efficient energy conversion and specific wavelength output. Single-crystal fiber (SCF), a new type of laser gain medium typically with a diameter of tens to hundreds of micrometers, combines the advantages of crystalline materials (high thermal conductivity, high damage threshold, and excellent mechanical properties) and traditional optical fibers (high aspect ratio and large surface area). It significantly enhances the thermal management performance of laser gain elements, offering a promising solution to the aforementioned issues. SCFs can be prepared through mechanical processing, which is only suitable for SCFs with a diameter of approximately 1 mm-with sharply increasing processing costs and technical difficulties as the diameter decreases. They can also be directly grown from melts, with the main methods including micro-pulling-down (mu -PD) and laser-heated pedestal growth (LHPG). Among these, LHPG, featuring ultrahigh heating temperature (>3000 degrees C), large temperature gradient (>4000 K/cm), and crucible-free growth, stands out as the optimal method for growing flexible SCFs with a core diameter of less than 100 mu m. Progress In recent years, rare-earth-doped SCFs have achieved remarkable progress in laser oscillators and amplifiers. In the field of laser oscillators, in 2012, D & eacute;len et al. demonstrated the high-power output capability of 1 degrees o Yb:YAG SCF. Using a 600 W, 940 nm laser diode (LD) for end pumping, they achieved a continuous laser output of 251 W with an optical - optical conversion efficiency of 44 degrees o (Fig. 8), setting a record for SCF laser output power. In 2020, the Liu team from Shandong University increased the 1064 nm continuous laser power of Nd:YAG SCF to 72.3 W, with an efficiency of 47.3 degrees o. In 2025, Tang et al. from Harbin Institute of Technology used a 788 nm fiber-coupled LD to end-pump a composite-structured Tm:YAP SCF, achieving maximum continuous laser outputs of 11.9 and 20.6 W under single-end and double-end pumping, with corresponding slope efficiencies of 53.2 degrees o and 40.7 degrees o (Fig. 11). In 2024, the Zhao team from Jiangsu Normal University used two serially connected 0.5 degrees o Ho:YAG SCFs as the gain medium, pumped by a 1907 nm Tm-doped fiber laser (153 W pump power), achieving over 100 W output at 2.1 mu m with an optical - optical conversion efficiency of 67.6 degrees o (Fig. 14). Constrained by the special energy level structure of Er ions, traditional Er-doped oxide crystals require high Er3+doping concentrations to ensure laser efficiency, but strong absorption from high doping hinders the high aspect ratio advantage of SCFs, limiting their application in mid-infrared lasers. Our team grew Er:CaF2 SCFs using the multi-microporous crucible method, realizing continuous laser operation of Er-doped SCFs at approximately 2.8 mu m. The 3 degrees o Er:CaF2 SCF achieved a maximum output of 0.939 W at 2756.9 nm, with a slope efficiency of nearly 35 degrees o, reaching the Stokes limit (Fig. 16). In laser amplifiers, Nd:YAG and Yb:YAG SCFs show potential in amplifying the power and energy of ultrashort pulses, with further pulse energy amplification achieved using such technologies as chirped pulse amplification (CPA), division pulse amplification (DPA), and coherent beam synthesis (CBC). In 2025, Cao et al. from Xi'an Institute of Optics and Mechanics realized high-peak-power ultrafast lasers based on a three-stage end-pumped Yb:YAG SCF single-pass amplified CPA system, obtaining a near-transform-limited pulse width of 323 fs and a peak power of up to 2.6 GW (Fig. 25). In addition, progress has been made in SCF structural design. In 2018, Dubinskii et al. from the U.S. Army Laboratory grew an approximately 120-mu m-thick pure YAG single-crystal film on a 100 mu m 1 degrees oYb:YAG SCF using liquid phase epitaxy (LPE). This cladded SCF had a transmission loss of only 0.011 dB/cm at 632 nm, achieving a 68.7 degrees o optical-optical conversion efficiency and approximately 50 W quasi-continuous output at 1030 nm (Fig. 30). In 2023, our team achieved axial gradient doping of Nd:YAG SCFs through source rod concentration distribution design based on LHPG, obtaining a maximum output of 6.46 W at approximately 1.06 mu m with a slope efficiency of 44 degrees o (Fig. 31). Conclusions and Prospects SCFs, with excellent thermal/mechanical properties, weak nonlinear effects, and a wide transmission band, hold great promise in high-power ultrafast and mid-infrared lasers. Progress in growing low-phonon-energy sesquioxide and fluoride SCFs facilitates mid-infrared laser power breakthroughs; with matured preparation processes and optimized laser designs/water cooling, mid-infrared outputs exceeding 100 W are expected. Future directions include improving cladding quality and core-cladding matching for long-distance waveguide gain amplification and integrating doping concentration design during growth to suppress thermal effects, thereby advancing laser power and efficiency.

Nd,Y:SrF2 crystal is considered as a promising laser gain material in high-energy laser systems due to the advantages of long emission lifetime, low nonlinear refractive index and high thermal conductivity....

Er:CaF2 crystals are crucial gain media for producing 3 mu m mid-infrared (MIR) lasers pumped by 976 nm continuous-wave (CW) lasers owing to their low phonon energy and high conversion efficiency. This study investigated the damage characteristics and mechanism of Er:CaF2 crystals irradiated with a 976 nm CW laser. The laser-induced damage threshold of Er:CaF2 crystals with different Er3+ doping levels was tested; the damage morphology consists of a series of regular 70 degrees cracks related to the angle of the crystal slip system on the surface. A finite-element model was used to calculate the temperature and stress fields of the crystals. The results indicated that the damage can be attributed to surface tensile stresses caused by the temperature gradient, and crystals with higher doping concentrations were more susceptible to damage owing to stronger light absorption. These findings provide valuable insights into the development of high-power MIR lasers.

The study presents Pr,La:CaF 2 single crystals grown using the temperature gradient technique, showing distinct Pr 3+ –La 3+ cluster types, with two types of centers showing cubic symmetry.