大口径 KDP/DKDP 晶体超精密制造技术研究进展（特邀）

Significance Large-aperture potassium dihydrogen phosphate (KDP) and deuterated potassium dihydrogen phosphate (DKDP) crystals are the only nonlinear optical materials suitable for serving as frequency conversion elements and optical switches in high-power laser facility. However, their anisotropy, soft-brittleness, hygroscopicity, thermal sensitivity, and propensity for cracking impose significant challenges to ultra-precision manufacturing. Conventional grinding and polishing processes are prone to leaving abrasive particles embedded on the surface. These particles serve as precursors to laser-induced damage, significantly diminishing the laser damage resistance of the crystals. Consequently, the simultaneous attainment of full spatial-frequency bandwidth precision and a high laser-induced damage threshold (LIDT) constitutes a pivotal challenge in the advancement of high-power laser facility. Progress To address these challenges, an integrated technical route of“single-point diamond turning (SPDT) + sub-nanosecond laser conditioning + sol-gel coating”has been established. Significant progress has been made in the following areas. Ultra-precision cutting technology and equipment: an anisotropic constitutive model for soft-brittle crystals was developed to reveal the brittle-ductile transition (BDT) behavior during cutting (Fig. 1). Simulations identified the optimal cutting direction along 45° within the (001) plane and a BDT depth of approximately 150 nm. Through process optimization and the innovative design of an integrated vacuum chuck with variable hole density and active temperature control (Fig. 2), surface figure accuracy better than 4λ (λ= 632.8 nm) and sub-nanometer roughness [root mean square (RMS)=0.59 nm] were achieved on large-aperture KDP crystals. Surface defects induced by fly-cutting, such as brittle indentations, cracks, protrusion pressure points, ballast, and plastic scratches, were systematically characterized (Fig. 4). Fluorescence microscopy (405 nm) revealed that defects like brittle indentations, cracks, protrusions, and ballast exhibit higher fluorescence intensity than defect-free regions, indicating stronger laser energy absorption and lower LIDT (Fig. 5, Table 1). An explosion simulation model was innovatively proposed to quantify the damage thresholds for different defect types and to elucidate the underlying damage mechanisms (Fig. 8). This model simplifies the complex multi-field coupling problem into a quantifiable explosive process, revealing that local mechanical strength and absorption capability are key factors affecting LIDT. Sub-nanosecond laser conditioning: the mechanisms underlying laser conditioning for the elimination or passivation of both point defects and structural defects were elucidated. A pulse width of 500 ps was identified as the optimal parameter within the 300‒800 ps range, as it provides sufficient peak power for electronic excitation while exceeding the lattice heat transfer time necessary for thermal effects. After applying this offline conditioning process to 400 mm aperture DKDP crystals, under ultraviolet laser irradiation, the surface damage density was reduced from 5.02 to 0.55 pp/cm², and the bulk damage density decreased from 2‒3 to 0.3‒0.8 pp/mm³ (Fig. 10, Table 2), marking a critical step toward engineering application. Sol-gel coatings: to enhance environmental stability and optical performance, multifunctional coatings were developed via sol-gel methods. Moisture barrier coating: a novel network-ball embedded structure was created by embedding hexamethyldisilane (HDMS)-modified SiO₂ nanoparticles into a siloxane polymer matrix (Fig. 11). This structure yields a tunable refractive index (1.21‒1.44), high hydrophobicity (contact angle increased to 109.4°), and exceptional moisture resistance (less than 0.1% transmission loss after 27 weeks at 80% relatively humidity). Antireflective (AR) coating: using methyltriethoxysilane (MTES) to seal surface pores after HMDS modification, an AR coating with low residual reflectance (less than 0.5%@355 nm), high LIDT (more than 20 J/cm²), and excellent oil contamination resistance (only 0.097% transmission drop after 20 weeks) was achieved (Fig. 12). Bilayer coating system: a precisely designed bilayer system for dual-wavelength (527 nm & 351 nm) antireflection was realized. It combines a high-refractive-index moisture barrier layer and a low-refractive-index AR layer, exhibiting outstanding optical uniformity and environmental stability (0.7% transmission drop after 19 weeks in high humidity) (Fig. 13). Conclusions and Prospects This review comprehensively summarizes recent breakthroughs in ultra-precision manufacturing technology for large-aperture KDP/DKDP crystals achieved through an integrated process route. Significant advances in fly-cutting theory and equipment, defect characterization and suppression, laser conditioning, and functional coating design have collectively and notably enhanced the surface accuracy, laser damage resistance, and environmental stability of these critical optical components. Looking forward, future research should prioritize several key directions: 1) exploring novel processes such as ultra-precision polishing to further suppress mid-spatial-frequency ripples induced during machining; 2) developing multifunctional composite coatings that exhibit lower curing temperatures, higher LIDT, and extended operational lifetimes; and 3) establishing a full-process database that correlates manufacturing defects with damage performance, along with developing efficient, non-destructive online evaluation techniques for comprehensive performance assessment of large-aperture crystal components.