Potassium dihydrogen phosphate (KDP) crystals have excellent optical properties such as high nonlinear conversion efficiency, superior photoelectric and piezoelectric properties, transparency in a wide range of optical spectra, and good optical homogeneity. Therefore, KDP crystals have been used in many advanced fields, such as in the laser ignition facilities as large aperture crystals for optical switching and frequency conversion. However, extensive trials in the past decades showed that a KDP crystal is an extremely difficult-to-handle material for its high sensitivity to external stresses and environmental conditions, such as contact stresses, temperature variation and moisture invasion. It is also a soft material yet easy to fracture. A critical challenge is that it is unclear about the underlying physics of the damages which have been frequently observed after the ultra-precision surfacing of an optical KDP lens. As a result, when it is used in a Pockels photoelectric switch in a high-density laser system, for instance, its optical performance (e.g., its laser-induced damage threshold) is unexpectedly low. These have hindered significantly the development of high performance laser systems. This paper aims to provide a comprehensive review on the critical integrity issues of KDP components and their characterization methods, from the points of view of microstructure, micro/nano-mechanics, deformation mechanisms and laser-induced damage associated with the most commonly used surfacing technology––ultra-precision machining.

中文翻译：

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无损光学KDP组件的超精密加工：机理和问题

磷酸二氢钾（KDP）晶体具有出色的光学特性，例如高非线性转换效率，优异的光电和压电特性，在宽光谱范围内具有透明性以及良好的光学均匀性。因此，KDP晶体已经在许多先进领域中使用，例如在激光点火设备中，作为用于光开关和频率转换的大孔径晶体。但是，过去几十年的大量试验表明，KDP晶体由于对外部应力和环境条件（如接触应力，温度变化和水分侵入）的高度敏感性，是一种极难处理的材料。它也是一种柔软的材料，但容易断裂。一个关键的挑战是，不清楚光学KDP透镜的超精密堆焊后经常观察到的损坏的潜在物理原理。结果，例如，当将其用于高密度激光系统中的普克尔斯光电开关中时，其光学性能（例如，其激光引起的损伤阈值）出乎意料地低。这些严重阻碍了高性能激光系统的开发。本文旨在从微观结构，微观/纳米力学，变形机制以及与最常用堆焊技术相关的激光引起的损害的角度，对KDP组件的关键完整性问题及其表征方法进行全面的综述。 –超精密加工。