Thermomechanical analysis of nodule damage in HfO2/SiO2 multilayer coatings

Abstract

Samples with nodular defects grown from gold nanoparticles are prepared, and laser-induced damage tests are conducted on them. Nodular defects, which are in critical state of damage, are cross-sectioned by focusing on the ion beam and by imaging using a field emission scanning electron microscope. The crosssectional profile shows that cracks are generated and propagated along the nodular boundaries and the HfO2/SiO2 interface, or are even melted. The thermomechanical process induced by the heated seed region is analyzed based on the calculations of temperature increase and thermal stress. The numerical results give the critical temperature of the seed region and the thermal stress for crack generation, irradiated with threshold fluence. The numerical results are in good agreement with the experimental ones.OCIS Nodular defect, a typical defect in multilayer coatings, largely limits the improvement of the laser-induced damage threshold (LIDT) in the nanosecond regime and thus has been widely investigated. Electric field enhancement in the nodule, induced by the microlens effect of the dome structure of nodular defects, is an important factor in reducing the LIDT [1−5] . Recently, Liu et al. has examined the relationship between the structure feature of the nodular defect and the resistance of laserinduced damage (LID) [6−8] . Some simple thermal stress simulations and a simple thermo-mechanical model had been developed to understand the damage mechanism of the nodular defect [9−11] . The critical condition of crack generation regarded as a damage criterion and the corresponding LIDT were given based on the average temperature of the seed zone. However, the critical temperature of crack generation, the distribution of thermal stress, and the position of crack generation remain to be elucidated.Our previous work focused on investigating the characteristics and the electric field enhancement of nodular defects [12] . The geometric characteristics of nodular defect were determined by the seed. The height of the nodular dome and the continuity of the nodular boundary mainly determine the LIDT. The strong absorptive seed and microlens effect of the nodule play important roles in the LID of the nodule.In this letter, samples with nodular defects grown from gold nanoparticles were prepared, LID tests were performed. The thermomechanical process induced by the heated seed zone was analyzed. The morphology of a critically damaged nodule was analyzed with a double-beam microscope (Auriga, Zeiss, Germang) which combines the functions of the focused ion beam and the field emission scanning electron microscope (FESEM).The preparation of the samples, which are multilayer coatings with gold nanoparticles deposited on the surface of the substrate, and the details of the LID tests have been described in Ref. [12]. The total thickness of the coatings is about 4.0 µm, and the films are deposited by electron beam evaporation. The reflection coefficient of these samples is about 99% at 1 064 nm. The morphologies of nodule ejection ...

Effect of subsurface impurity defects on laser damage resistance of beam splitter coatings

“…One morphological feature is the noduleejected pit, and the nodule geometry extracted from the cross-sectional image shown in Figure 6(e) indicates that the nodule seed is located on the substrate surface with a diameter of approximately 183 nm. The other morphological feature is a micro-crack surrounding a nodule, as shown in Figures 6(f) and 6(g), which is consistent with reports pertaining to un-ejected nodule damage [30] . The diameters of the nodule seeds in Figures 6(f) and 6(g) can be estimated using Equation (1) [31,32] :…”

“…As shown in Figures 10(a The simulation results indicate that for absorption particles with a diameter of approximately 30 nm, a significant temperature rise and thus high thermal stress will occur under laser irradiation. High thermal stress may contribute to the morphological feature of the micro-crack around nodule [30] . With increasing seed diameter, nodule defects will induce significant local E-field enhancement, thus resulting in a nodule-ejection damage morphology [38] .…”