The effect of an electric field on the thermomechanical damage of nodular defects in dielectric multilayer coatings irradiated by nanosecond laser pulses

Abstract

Thermomechanical damage of nodules in dielectric multilayer coatings that are irradiated by nanosecond laser pulses has been interpreted with respect to mechanical properties and electric-field enhancement. However, the effect of electric-field enhancement in nodular damage, especially the influence of electric-field distributions, has never been directly demonstrated through experimental results, which prevents the achievement of a clear understanding of the damage process of nodular defects. Here, a systematic and comparative study was designed to reveal how electric-field distributions affect the damage behavior of nodules. To obtain reliable results, two series of artificial nodules with different geometries and film absorption characteristics were prepared from monodisperse silica microspheres. After establishing simplified geometrical models of the nodules, the electric-field enhancement was simulated using a three-dimensional finite-difference time-domain code. Then, the damage morphologies of the artificial nodules were directly compared with the simulated electric-field intensity profiles. For both series of nodules, the damage morphologies reproduced our simulated electric-field intensity distributions very well. These results indicated that the electric-field distribution was actually a bridge that connected the nodular mechanical properties to the final thermomechanical damage. Understanding of the damage mechanism of nodules was deepened by obtaining data on the influence of electric-field distributions on the damage behavior of nodules. Researchers in China have studied how the electric-field distribution of light affects the laser-induced damage of coatings. Xinbin Cheng and co-workers from Tongji University in Shanghai fired 10-ns-duration laser pulses from a 1,064 nm Nd:YAG laser onto a multilayer dielectric coating on BK7 glass substrate. To simulate the role of nodular defects, which act as precursors for damage, the team introduced 0.3–1.9 μm silica microspheres to the glass substrate prior to application of the coating. They then modelled the electric-field distribution at different nodule sites using finite-difference time-domain simulations, and experimentally analysed the laser-induced damage using microscopy and focused ion-beam technology. The results are expected to aid the development of coatings to provide superior resistance to laser damage.