## 1 Introduction  
In order to achieve clean and sustainable energy resources, high-power laser systems have been developed worldwide for pursuing inertial confinement fusion (ICF), such as the National Ignition Facility (NIF) in the United States [1; 2], the Laser MegaJoule (LMJ) in France [3], the High Power laser Energy Research facility (HiPER) in Europe [4] and the ShenGuang (SG)-III laser facility in China [5]. To obtain the extremely high pressure and temperature required for ICF ignition, a huge amount of large-aperture optics with high precision surfaces are required to temporally, spatially and spectrally control the laser beams. The laser beams are finally coupled together, focusing simultaneously on a very tiny target filled with fusion fuels. Among these optics, fused silica is an amorphous state of silicon dioxide, which has extremely low thermal conductivity, super strong thermal-shock resistance, low dielectric loss, and high deformation (1370 K) and softening (2000 K) temperatures. Besides, the fused silica optics possess such broad optical transmission spectra that over 1000 pieces of silica optics have been widely used in the sub-laser systems with fundamental (1\(\omega\)) and triple (3\(\omega\)) frequencies of the ICF facilities. The fused silica optics serve as switch and vacuum windows, wedged focus lens, diffraction grating, debris shields and so on [6; 7]. Since the fused silica optics are exposed to high-power lasers in their actual applications, the well prepared engineering silica components should have very high laser damage resistance. However, due to the weak mechanical properties of fused silica, the introduction of undesirable by-products (viewed as defects) on the surface or subsurface of finished brittle silica parts in the actual cutting, grinding, polishing, coating and cleaning processes is inevitable. The surface and subsurface defects can extend downward several to tens of microns beneath the finished surface, which would greatly lower the energy output capacity of the ICF laser facilities. Currently, thanks to the development of various new advanced processing techniques (e.g., laser conditioning [8], thermal annealing [9; 10]), the bulk laser-induced damage threshold (LIDT) of fused silica can reach up to 475 \(\pm\) 25 GW/cm\({}^{2}\) (1064 nm, 8 ns), which is very close to the theoretically intrinsic LIDT and almost one order of magnitude higher than the surface LIDT [11]. This means that the laser-induced damage on surface and subsurface of fused silica plays a dominant role in restricting the promotion of energy output capacity of ICF facilities. Similar to the fused silica optics applied in high power laser systems, many other optical materials (e.g., potassium dihydrogen phosphate KDP as well as its deuterated analog DKDP, fluoride and selenide crystals) are also expected to possess high optical qualities, like optical transparency and laser damage threshold [12; 13; 14; 15]. For these crystal materials, rapid growth of large-size boules with desired optical properties has been being a great challenge, and the growth conditions are susceptible to affect the crystal structures, which is closely associated with the bulk laser damage threshold. As a result, the efforts to promote the optical properties of these optical materials are mainly focused on exploring the mechanisms of bulk laser-induced damage and optimizing the growth parameters to improve the crystal structures for increasing the bulk LIDT [14; 15; 16]. For fused silica optics, though the internal structure of silica is stable and robust, its hard and brittle characteristics would make the ultra-precision manufacturing of defect-free silica surfaces a great challenge. Therefore, the study on laser damage of fused silica optics should primarily aim at the defects on a finished surface or subsurface.