Nodular-defect induced degradation of laser damage resistance of 532 nm high reflectivity coating after exposure to gamma rays

• Published in: Optics express 2024-12, Vol.32 (27), p.49021
• Main Authors: Peng, Yunqi, Xia, Xusheng, Xia, Zhilin, Liu, Xuyi, Zhang, Weili, Cui, Yun, Nasibli, Humbet, Qin, Yuyang, Shao, Jianda, Zhao, Yuan'an
• Format: Article
• Language: English
• ISSN: 1094-4087; 1094-4087
• DOI: 10.1364/OE.543814

The coupling effect of gamma-ray radiation and 532 nm nanosecond laser radiation on optical coatings and substrates was investigated. Fused silica and S-BSL7 glass with 532 nm high reflectivity (HR) coatings were irradiated using Co gamma-ray source at a dose rate of 1 Gy/s for a total dose of 1-500 kGy. After irradiation, the samples were subjected to raster scan testing using a laser with a pulse width of approximately 8.6 nanosecond and wavelength of 532 nm to measure their laser-induced damage threshold (LIDT). The results showed that S-BSL7 glass was significantly darkened after gamma-ray irradiation, whereas fused silica did not change color, but E' color centers appeared at a dose of 500 kGy. All the coated samples maintained a high level of reflectivity. No significant changes are observed in the material structures of the samples. However, the LIDT of the high reflectivity film decreased significantly after 500 kGy irradiation, whereas the LIDT of all other samples showed no obvious degradation. Scanning electron microscopy of the damage pit cross-sections revealed that all the damage was initiated by nodular defects, suggesting that gamma rays affected the nodular structure. We used Monte Carlo simulations to compare the energy deposition and electron excitation conditions of the film with a nodular structure and an intact multilayer structure under gamma-ray irradiation. We found that the nodular structure slightly enhanced the effect of gamma rays on the film. Considering the small size of the nodules, this small enhancement was only apparent under high-dose gamma-ray irradiation. In general, we confirmed that gamma-ray irradiation influenced the LIDT of the 532 nm HR coating through nodular defects other than the coating stack.