【摘要】 基于拉曼光谱成像技术，揭示了应力抑制对提升薄膜抗激光损伤性能的关键作用，进而开发了一种阶梯降温退火方法，实现了对氧化铪（HfO2）薄膜微观结构的精准调控。实验结果表明，该方法在高效修复氧空位的同时，能够有效抑制残余应力，从而显著减弱薄膜的光学吸收。经该方法处理的HfO2薄膜，其纳秒脉冲激光损伤阈值较传统退火样品最高提升了15.8%，且在与多种其他后处理方法的对比中表现出更优异的抗激光损伤性能。本研究针对HfO2基脉冲激光薄膜，提出一种应力调控策略，该成果为我国高性能强光光学元件的自主研制提供了有价值的科学基础与探索方向。更多还原

【Abstract】 Objective HfO₂（hafnium dioxide） serves as the high-refractive-index material of choice for high-performance pulsed laser devices operating across the visible to near-infrared spectrum. Nevertheless, under intense laser irradiation, HfO₂ thin films represent the most vulnerable component within optical systems, and their damage resistance directly determines the overall laser power-handling capacity of these components. Post-treatment techniques can effectively mitigate structural defects introduced during film deposition and improve the mechanical properties of the coatings, leading to a marked increase in the laser-induced damage threshold（LIDT） of the components. As a result, post-treatment emerges as an indispensable and crucial step in the fabrication of HfO₂ thin films. Among various post-treatment approaches, thermal annealing stands out for its comprehensive and fundamental enhancement of film performance, offering broad applicability. It fulfills an irreplaceable role, particularly in restoring stoichiometric balance, controlling residual stress, and optimizing the optical and mechanical stability of HfO₂ films. Based on Raman spectroscopic imaging, this work clarifies the pivotal role of residual stress suppression in improving the laser damage resistance of HfO₂ films. A novel stepwise cooling annealing protocol is devised, which not only efficiently repairs oxygen vacancies but also effectively suppresses the onset of residual stress, thereby significantly enhancing the LIDT of HfO₂ films. The outcomes of this study provide valuable scientific insights and point to promising research avenues for the development of high-performance laser optical components.Methods In this work, HfO₂ thin films are deposited via ion-beam sputtering under various oxygen flow rates, followed by conventional annealing and stepwise cooling annealing treatments, respectively. Based on differences in annealing processes, the samples are divided into three groups for comparative analysis. The first two groups of samples are subjected to a linear heating ramp from 27 ℃ to 400 ℃ over 2 h, followed by a 2 h dwell at 400 ℃. After that, the first group of samples（conventional annealing） undergoes direct natural cooling to 27 ℃, while the second group（stepwise cooling annealing） first undergoes natural cooling to 200 ℃, held for 0.5 h, and then continues cooling to room temperature（27 ℃）. To further verify the unique advantages of stepwise cooling annealing in improving the optical performance of HfO₂ films, a third group of samples is established, which undergoes the same heating process and a 2.5 h dwell at 400 ℃ before being directly cooled naturally to 27 ℃（i.e., with a 0.5 h longer dwell time at high temperature compared to the first group）. The films are systematically characterized using a spectrophotometer, spectroscopic ellipsometer, atomic force microscope, laser interferometer, X-ray photoelectron spectrometer, photothermal weak absorption system, and micro-Raman spectrometer. Following the ISO 21254-1 standard, the LIDTs of all film samples are measured using the “1-on-1” method.Results and Discussions In this study, the as-deposited HfO₂ films exhibit a leading as-deposited LIDT. Furthermore, after undergoing stepwise cooling annealing, the films demonstrate a significant increase in the damage threshold increment（10.52 J/cm²）, surpassing conventional annealing processes and rivaling other types of post-treatment methods（Fig. 5, Tables 1 and 2）. The 12 mL/min HfO₂ film exhibits a compressive stress state in the as-deposited condition（-2085 MPa）. Conventional annealing slightly reduces the stress, while stepwise cooling annealing significantly reduces the residual stress to-362 MPa（Fig. 6）. For the 15 mL/min HfO₂ film, the as-deposited compressive stress is-330 MPa, which increases to-533 MPa after conventional annealing. In contrast, stepwise cooling annealing transforms the stress nature from compressive to tensile, with the absolute value substantially reduces to 102 MPa（Fig. 6）. Additionally, using micro-Raman spectroscopy, it is observed that after the stepwise cooling annealing process, the phase transition region expands significantly, while the delamination area visibly contracts（Fig. 9）.Conclusions To meet the urgent demand for high laser-damage resistance in optical thin-film components for high-energy laser systems, this study develops a post-treatment strategy for HfO₂ films based on a stepwise cooling annealing process. This approach introduces a 200 ℃ holding step during the cooling phase of conventional annealing, which not only effectively alleviates thermal stress caused by the mismatch in thermal expansion coefficients between the film and the substrate but also provides the necessary energy and time for repairing oxygen vacancy defects in the film, thereby significantly enhancing the nanosecond laser-induced damage threshold of HfO₂ films. Experimental results show that for HfO₂ films deposited under high oxygen flow conditions, stepwise cooling annealing can fully replenish oxygen vacancies, significantly suppress residual film stress, and notably reduce optical absorption at the 1064 nm wavelength. To further elucidate the mechanism by which the annealing process regulates stress evolution, this study innovatively combines Raman spectroscopic imaging with atomic force microscopy to achieve micro-area phase analysis of damage morphologies. It is observed that the effective suppression of residual stress by the stepwise cooling treatment is specifically manifested as a significant reduction in the area of delamination regions within the damage morphology caused by stress release. Laser damage threshold tests reveal that, compared to conventional annealing processes, the stepwise cooling treatment increases the damage threshold of HfO₂ films by up to 15.8% and demonstrates superior laser damage resistance when compared to multiple existing post-treatment methods. Furthermore, this strategy also exhibits excellent applicability to HfO₂/SiO₂ multilayer reflective coatings, raising the nanosecond laser damage threshold at 1064 nm from 34.32 J/cm² to 69.38 J/cm²—an improvement of 102.2%. This study proposes an innovative technical strategy for enhancing the radiation tolerance of oxide-based pulsed-laser thin films under intense light, and the findings may provide preliminary theoretical references and practical insights for the independent development of high-performance, high-power optical components.更多还原