Nanosecond laser-induced damage of nodular defects in dielectric multilayer mirrors [Invited]

Abstract

Our recent studies on nodular damage in dielectric multilayer mirrors were first reviewed, and the main findings are taken as a foundation to further investigate the influence of seed absorptivity and asymmetrical boundary on the laser-induced damage of nodules. Experimental results showed that the seed absorptivity had a big influence on laser-induced damage thresholds (LIDTs) of the prepared nodules. A direct link between the cross-sectional |E|2 distributions and damage morphologies of nodules was found, which can perfectly explain the observed dependence of LIDTs on seed absorptivity. Another series of asymmetrical nodules were also studied in this work. The measured LIDTs of asymmetrical nodules were about 40%-70% lower than the LIDTs of the symmetrical nodules initiating from the same-sized SiO2 seeds. The weaker mechanical stability and the nonuniform |E|2 distributions are two main reasons for the lower laser damage resistance of the asymmetrical nodules.

Laser-induced damage of intrinsic and extrinsic defects by picosecond pulses on multilayer dielectric coatings for petawatt-class lasers

“…PE defects change slightly but remain stable up to 50 to 100 shots at fluences at or below 5 J∕cm 2 ; similar changes were observed during raster scanning (with 1 to 2 higher fluence shots), see Fig. 5(b) for Type I damage (most probably similar to a gentle nodular ejection, as observed for the case of subgrowth threshold, ns-pulse damage of artificial nodular defects 29,30 ). ii.…”

Origins of light scattering from thin film coatings

“…Considering that the beam diameter in the actual application is even larger (about 10 mm), the contributions of the isolated defects on the total scatter can be neglected because (i) their scatter level is only slightly higher than the roughnessinduced background, in particular for the nodule-type of defect present on the samples of this study and (ii) the cumulative defect area is small compared to the total area. Although the present study reveals that defects are of minor relevance for the scattering properties of our coatings, defects can play a critical role for the laser stability [36]. However, this does not seem to be the case for our application since no damage was observed when illuminating the samples at wavelengths from 450 nm to 1750 nm with fluences of up to 0.7 J/cm 2 .…”

“…However, dark field microscopy revealed numerous small defects randomly distributed over the sample with an average size of about 6 μm and an average density of about 1 per mm 2 . The main reason for these defects are nanometer sized seed defects on the substrate, which form nodules in the film structure that increase in diameter during thin film growth and can have a critical impact with respect to laser-induced damage [36].…”

Substrate and coating defect planarization strategies for high-laser-fluence multilayer mirrors

“…The final defect dimensions were to within less than 40% of the as-designed ones. Although highly absorbing inclusions in nodular defects generally have very low laser resistance [16,23], the desire for a well-defined defect shape and location to facilitate cross sectioning necessitated selection of a metal such as platinum.…”

“…These defects originate as inclusions that are then over coated with multilayer thin films. Electric-field modeling has clearly shown light intensification due to the geometric and interference nature of these defects [7][8][9][10][11][12][13][14][15][16]. The inclusions are caused by contaminates on the surface or from particulates generated during the coating process that become imbedded inclusions within the multilayer stack.…”