Currently, the functional laser-induced damage threshold (LIDT) of high reflective (HR) coating for high power laser system is limited by a kind of damage called "growth-type" damage, of which the lateral size and longitudinal depth will increase under multiple laser irradiation pulses, and eventually lead to failure of the component. The "growth-type" damage problem of the dielectric coating is one of the research hotspots in recent years. Previous investigation results indicate that the pit-type defect is one of the main sources that cause the "growth-type" damage, and the typical initial damage morphology of the “growth-type” damage is delamination surrounded by large area plasma ablation. However, there is still no quantitative study on the influence of pit-type defect of substrate surface on the LIDT of HR coating, and how to suppress the damage of HR coating caused by structural defect is not clear yet.

Recently, a research group led by SHAO Jianda and ZHU Meiping from Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences (CAS), proposed to fabricate pits of different size (Diameter: 2-9 um, depth: 0.2-0.6 um) using femtosecond laser, and quantitative experiments have been carried out to study the effect of pits with different size on LIDT of HR coating at fundamental wavelength of Nd: YAG laser. This work has been published on Scientific Reports [6, 27076, 2016, http://www.nature.com/articles/srep27076] titled “Laser-resistance sensitivity to substrate pit size of multilayer coatings”. In the work published on Optics Letters [41, 15, 3403-3016, 2016, http://dx.doi.org/10.1364/OL.41.003403] titled “Multilayer deformation planarization by substrate pit suturing”, the group proposed a "suturing" technique to suppress the micro-cracks, as well as e-field and stress enhancement caused by the pit-type defect on substrate surface. The LIDT of HR coating on pit is enhanced from ~20 J/cm2 to ~80 J/cm2, as shown in the figure below. In addition, a “non-interfacial” multilayer technique based on e-beam co-evaporation has been proposed to solve the delamination problem caused by the discrete interface in traditional multilayer coating. The intrinsic damage threshold of HR coating at third harmonic wavelength is increased more than twice, as published in Optics Letter [41, 6, 1253-1256, 2016, http://dx.doi.org/10.1364/OL.41.001253] titled “Improving laser damage resistance of 355 nm high-reflective coatings by co-evaporated interfaces”.

Figure. The schematic diagram of the "suturing" technique for pit-type defects. The multilayer coating on pits with (a) and without (b) "suturing" layer. (c) LIDT of HR coating with and without "suturing" layer. (Image by ZHU Meiping)

The "suturing" technique for pit-type defect, as well as the “non-interfacial” multilayer technique based on e-beam co-evaporation proposed by researchers from SIOM, has brought forth new methods to suppress the “growth” damage and further enhance the LIDT of multilayer coating.