Comparative study of laser-induced damage on deep ultraviolet high-reflection coatings with protective layers.

Deep ultraviolet (DUV) lasers, particularly at 193 and 248 nm, are critical for applications such as photolithography and inertial confinement fusion. However, the high-power operation required in these systems induces damage to high-reflection (HR) coatings, a challenge that remains not fully understood in the DUV regime due to limitations in laser sources and deposition technologies. This work presents a systematic and comparative study on the laser-induced damage mechanisms of HR coatings with protective layer under nanosecond DUV laser pulses. Theoretical models incorporating standing-wave electric field distribution, thermomechanical stress, and the effects of defects are developed. Comparative experiments are conducted on dielectric HR coatings exposed to 193 nm (25 ns) and 248 nm (13 ns) excimer lasers, evaluating damage thresholds, morphologies, and the protective role of capping layers. Results show that damage progresses incrementally with increasing laser pulse fluence and count. Among the coatings studied, the LaF3/AlF3 HR coating exhibits better damage resistance compared to the Al2O3/MgF2 coating. The application of a MgF2 protective layer markably enhances the damage threshold by shifting the peak standing-wave electric field forward, thereby reducing absorption in the uppermost high-index layer. These findings provide critical insights into DUV laser damage mechanisms, offer practical guidance for HR coating selection, and establish a reference for future studies on DUV laser-induced damage.