Improvements in the laser damage behavior of Ta2O5/SiO2interference coatings by modification of the top layer design

We have investigated the properties and laser damage behavior of Ta2O5/SiO2 quarter wave stacks designed for λ=1 µm operation by substituting the Ta2O5 layer by either Y2O3 or HfO2 and the SiO2 by Al2O3 in the top 3 pairs of the multilayer stack. The high reflectors were deposited by dual ion beam sputtering. Laser damage at 1 µm using 350 ps showed enhanced performance when the Ta2O5/SiO2 stack had HfO2 or Y2O3 in its top few pairs. targets were used. For the present study fused silica substrates, with surface roughness of ~ 6 A were used. The total thickness of each HR stack was ~ 5 µm. The surface quality of the IBS HR was determined using an atomic force microscopy (AFM) using a NovaScan ESPM 3D operated in tapping mode. Root mean square (RMS) surface roughness was calculated using the included software for AFM. Glancing angle x-ray diffraction scans of the single layers showed the films are amorphous. Optical properties of the transparent films were investigated using a spectrophotometer in the wavelength range 190 to 1100 nm. Scattering measurements were performed using total integrating sphere with laser light illumination at λ=1.064 µm (details to be published elsewhere). The optical absorption loss of the coatings deposited on fused silica, was measured at λ = 1.064 µm with the photothermal common-path interferometry (PCI) 7 . Laser damage measurements were performed using 100-on-1 test at λ= 1.03