Nanosecond laser-induced surface damage and its mechanism of CaF2 optical window at 248 nm KrF excimer laser

Abstract

Calcium fluoride (CaF 2) crystals is a kind of important optical material for ultraviolet (UV) and deepultraviolet (DUV) lithography and high-power laser-related applications. However, its laser-induced damage threshold (LIDT) directly affects the laser power, so that the above-mentioned applications could be limited. Therefore, the research on the damage characteristics and laser damage resistance of caf 2 crystals is urgent. A 3D Finite-Difference Time-Domain (FDTD) method with Maxwell spinor equation is used, and the results show that the electric field intensity of rear surface is larger than that of front surface, which causes a lower threshold and is consistent with the experimental observations. And a thermo-mechanical coupled finite element model (FEM) of CaF 2 with ce 2 o 3 impurities, which are introduced by polishing process, has semiquantitatively described the damage mechanism of CaF 2 by 248 nm-excimer laser. Due to the large band gap energy (12.1 eV), high transmission at UV range, very low value of nonlinear refractive index, good optical isotropy and excellent chemical stability 1-5 , CaF 2 optical window materials have been widely used. The results of surface damage of different types of CaF 2 materials induced by different laser parameters showed that the damage behavior was closely dependent on the materials characteristics as well as laser parameters 6-13. Among the works of ultraviolet excimer laser damage of ultraviolet windows, fused silica has been reported mostly 14-24. In contrast, the reports on fluoride windows are relatively few, and most of the studies were just focused on the damage of fluoride films. Only a few reports are limited to the laser damage of single fluoride windows at 1064 nm wavelength 25 , and most of them were concerned with the damage caused by the fundamental 1064 nm laser and triple frequency 355 nm ultraviolet laser. There are few publications about the damage caused by shorter wavelength excimer laser, such as 248 nm laser. Because ultraviolet excimer lasers present favorable characteristics such as high photon energy, high coupling efficiency as well as high peak power 26-30 , they are expected to be widely used in precision laser machining and military area. This research will provide great significance in deeply understanding the interaction between ultraviolet laser and material, offer important reference in laser precision machining and laser attack-defense field, and also provide theoretical basis for the anti-laser damage ability of optical components. Based on the self-developed PLD-50 excimer laser, a set of LIDT testing system was established. The CaF 2 samples were divided into 2 groups, one group was highly polished, and the other one was roughly polished. First, the structural defects from polishing the CaF 2 crystal were characterized with fluorescence spectroscopy. Second, the samples were irradiated in a 1-on-1 mode with increasing energy density. Afterwards, the surface fragmentation and damage characteristics of CaF 2 crystals were analyzed vi...

Laser-induced damage threshold based on thermal effects in high-purity silica optical fiber

“…Several methods have been used for LIDT simulation, such as the finite element method (FEM) and the finite difference time domain (FDTD). [30][31][32] The order of LIDT magnitude in simulation can be consistent with that in experiments currently. However, the mechanisms and performance of laser-induced damage in materials have not been clearly investigated yet.…”

“… – 29 The numerical method is the main method to study the light field distribution and LIDT. Several methods have been used for LIDT simulation, such as the finite element method (FEM) and the finite difference time domain (FDTD) 30 – 32 The order of LIDT magnitude in simulation can be consistent with that in experiments currently.…”

Laser-induced damage of dielectric-enhanced surface-modified single-point-diamond-turned Al-6061 multiband mirrors

“…The results suggest that the FDTD simulation is a powerful tool to predicat defect-initated laser-induced damage. In addition, the FDTD simulation technique has been applied to dielctric mirrors with random-shaped seeds [11], defects on the femtosecond laser damage resistance of multilayer dielectric mirrors and gratings [12,13], and nanosecond laser-induced surface damage of CaF2 optics at 248 nm [14], precipitates for the mechanical strengthening of the alloy. In addition, there are other inclusions.…”