Thermal-mechanical modeling of nodular defect embedded within multilayer coatings

Abstract

The initiation of laser damage within optical coatings can be better understood by thermal-mechanical modeling of coating defects. The result of this modeling shows that a high-temperature rise and thermal stress can be seen just inside the nodular defect compared to surrounding coating layers. The temperature rise and thermal stress tend to increase with seed diameter. Shallower seed tend to cause higher temperature rise and greater thermal stress. There is a critical seed depth at which thermal stress is largest. The composition of the seed resulting from different coating-material emission during evaporation can affect the temperature rise and thermal stress distribution.

Further investigation of the characteristics of nodular defects

“…The height of this nodular defect is 2:3 μm, and the diameter is 1:04 μm. Therefore, previous experiments and theories that identify them as nearly equal [5][6][7][8] are one sided or idealized and are valid just for the first kind of nodule.…”

“…A simple parabolic model similar to the nodule geometry is widely used by researchers [5][6][7][8][9][10]. Electric field enhancements induced by nodular defects with different sizes, depths, and shapes have been studied based on this simple model [5][6][7][8]. Nodular defect-induced electric field enhancement can provide insight into laserinduced stress.…”

“…Nodular defects in multilayer dielectric films have been widely investigated since the 1990s [1][2][3][4][5][6][7][8][9][10][11][12][13]. Nodules are initiated by seed particles present on the substrate or deposited in the film during deposition and form a parabolic structure with the subsequent deposition of materials [1,2].…”

Comprehensive studies of IR to UV light intensification by nodular defects in HfO2/SiO2multilayer mirrors

“…Multiple studies have been performed on properties of the nodular defects, 2, 7-14 their growth mechanism 8,[15][16][17][18][19] and interaction with applied laser radiation. 5,6,12,[20][21][22][23] Light intensification by nodules has been shown to be sensitive to various defect characteristics (such as size, geometry, depth, etc.) as well as laser irradiation conditions (wavelength, polarization, angle of incidence, etc.)…”

“…as well as laser irradiation conditions (wavelength, polarization, angle of incidence, etc.) 5,6,12,[20][21][22][23] Up to now most of the parametric studies were carried out for the infrared (IR) spectral range. There is still a lack of knowledge on wavelength dependence of light intensification.…”