Laser damage induced by microscopic defects in optical components

According to the above numerical analysis results, it can be seen that cracks may be generated around the nodule seed and propagate along the radial direction. However, considering that the boundaries of nodules are usually relatively weak, especially those close to the seed; in addition, the interface of the film layer HfO₂/SiO₂ is also relatively weak, so cracks are easily generated at these two parts and develop along this direction, forming damage shown in the figure below.

We used FIB etching technology to observe the boundary structure characteristics of nodule defects, and found that the closer to the seed, the worse the nodule boundary, and as the number of deposited film layers increased, the boundary gradually became continuous. The instability of the nodule boundary is not good for nodule damage, and if the film is under external thermal stress conditions, cracks will first occur at the nodule. In essence, the combination of the nodule defect and the film stack determines its response characteristics under different laser flux. According to the combination of nodular defects and film stacks, nodular defects can be divided into two types. In the first type, with the deposition of the subsequent film layers, the boundary of the cone becomes continuous with the surrounding film layers. In the second type, the boundary of the cone is not continuous with the surrounding film layers. The nodular defect shown in the figure (a) below was irradiated by laser pulses with power density of 79.8 J/cm² (1064 nm, ~10 ns), and some nodular defects were not destroyed, as shown by the arrow in the figure (b). Experiments have proved that the cone boundaries of these unchanged nodular defects are continuously deposited with the subsequent film layers, and finally the cone boundaries are continuous with the surrounding film layers, as shown in the red box in figure (d). After treating by laser flux of 79.8 J/cm², the second type of defect does not exist. It can be seen that the first type of the nodular defect has a higher damage threshold than that of the second type.

Through the characterization of nodule defects, combined with the analysis of coating deposition technology, it can be proved that the nodule defects originate from the pollution particles on the substrate and the splashed particles when the film material evaporates. The research of the Livermore Laboratory (LLNL) in the United States shows that the phase transition temperature of HfO₂ is 1700 ℃. During the electron beam evaporation process, the phase transition process causes serious spattering, and the spattering particles become nodular seeds attached onto the substrate, making it easy to generate nodule defects on the film. Therefore, the researchers used metal hafnium as the coating material to avoid the sputtering problem induced by the phase change, thereby reducing the nodular defect density of the film by an order of magnitude and greatly improving the laser damage resistance of the coating.

From the above analysis, it can be seen that if we can obtain the defect characteristics of induced laser damage, then the corresponding damage mechanism and control technology can be targeted. However, with the development of lasers with shorter wavelengths and shorter pulse widths, the scale of defects that induce laser damage has shrunk to the nanometer scale, which cannot be directly characterized from the morphology, so it is difficult to trace the source and establish a relationship with laser damage. This kind of problem is still puzzling researchers, and it is also a hot issue in the field of laser damage research that needs to be solved urgently.