Quantitative assessment and suppression of defect-induced scattering in low-loss mirrors

In this Letter, defect-induced scattering in 1064 nm high-reflection coatings prepared by dual ion beam sputtering and its suppression were investigated by artificial nodules, finite-difference time-domain simulations, angular resolved scattering (ARS) measurements, and planarization technology. After establishing the geometric model of the nodules grown from ϕ 1 μm SiO₂ microspheres, the far-field scattering of the multiple nodules was determined by intensity superposition. For a nodule density of 100  mm^-2, there is good agreement between the simulated and measured ARS. The total scattering is ∼500  ppm for the multilayer coating with artificial nodules, which is more than 10 times that for the coating without nodules. Next, an iterative deposition-etching process was used to planarize the defects, which reduced scattering by almost one order of magnitude. Moreover, detailed characterization of the planarized seeds reveals that the planarization technology is a complex process, and it still does not produce a perfect flat surface. The results showed that there is a pit over each planarized seed in the coating surface, which leads to additional scattering. The possible reasons for the presence of these pits are briefly discussed, and the directions for further research are provided at the end of this Letter.