Effects of the electric field and optical bandgap on femtosecond laser-induced damage of composite low-dispersion mirrors

The continual development of petawatt (PW) laser systems has led to increasingly high requirements for the damage threshold of broadband low-dispersion mirrors (BLDMs). The femtosecond laser-induced damage threshold (LIDT) is closely related to the intrinsic properties of the coating materials. Given the limitation of the intrinsic optical bandgap of materials, it is difficult to improve the LIDT using traditional Bragg structures. This study analyzed the effects of the electric field and the optical bandgap on the femtosecond laser-induced damage threshold of BLDMs. On this basis, a (HfO 2 -SiO 2 )/Ta 2 O 5 /SiO 2 BLDM was proposed, which can achieve high reflectance and low group delay dispersion in the spectral range of 800 nm ± 48 nm for p-polarization. This BLDM uses HfO 2 -SiO 2 mixture materials with different mixing ratios as high-refractive-index materials in the upper layers and simultaneously optimizes the electric-field distribution. Compared with the traditional HfO 2 /Ta 2 O 5 /SiO 2 BLDM with Bragg structure, the LIDT of this novel BLDM is elevated from 0.31 J/cm 2 to 0.49 J/cm 2 , irradiated by a p-polarized 40 fs laser pulse at an incident angle of 45°. This means that the LIDT improved by 58%. The damage morphology and calculated equivalent integral electric field distribution results indicate that damage occurs at the location with the maximum electric field intensity in high-refractive-index material. This study demonstrates that the femtosecond LIDT of BLDMs can be enhanced not only by electric field optimization but also by utilizing high-optical-bandgap mixture coatings. By combining these two approaches, the damage threshold of the BLDM developed in this study has been significantly improved. This study presents a new approach for enhancing the LIDT of BLDMs in the femtosecond regime.