Nanosecond laser-induced surface damage and its mechanism of CaF2 optical window at 248\u2009nm KrF excimer laser

Xin Li,Xian-an Dou,Yue Hu,Hong Zhu,Xi Wang

doi:10.1038/s41598-020-62469-y

Abstract

Calcium fluoride (CaF2) crystals is a kind of important optical material for ultraviolet (UV) and deep-ultraviolet (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 CaF2 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 CaF2 with Ce2O3 impurities, which are introduced by polishing process, has semiquantitatively described the damage mechanism of CaF2 by 248 nm-excimer laser.

Highlights

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[1,2,3,4,5], CaF2 optical window materials have been widely used
The lower damage threshold of the rear surface compared to the incident surface is consistent with the Finite-Difference Time-Domain (FDTD) simulation and the approximate value of the damage threshold could be reproduced with a thermo-mechanical FEA simulation
The effect that the damage threshold for the rear surface is lower for a roughly polished compared to a highly polished incident surface could not be replicated with the presented simulation schemes

Summary

Introduction

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[1,2,3,4,5], CaF2 optical window materials have been widely used. Because ultraviolet excimer lasers present favorable characteristics such as high photon energy, high coupling efficiency as well as high peak power[26,27,28,29,30], they are expected to be widely used in precision laser machining and military area. 2 phenomena were found: (1) the damage threshold of the rear surface of the optical window material is lower than that of the front surface, and (2) the LIDT of highly polished samples is higher than that of roughly polished samples regardless of incident surface or rear surface. The effect that the damage threshold for the rear surface is lower for a roughly polished compared to a highly polished incident surface could not be replicated with the presented simulation schemes

Methods

Findings

Conclusion