Effect of electric field distribution on the morphologies of laser-induced damage in hafnia-silica multilayer polarizers

Hafnia-silica multilayer polarizers were deposited by e-beam evaporation onto BK7 glass substrates. The polarizers were designed to operate at a wavelength of 1064 nm at Brewster's angle (56 degrees). The polarizers were tested with a 3-ns laser pulse at 45 degrees, 56 degrees, and 65 degrees incidence angle in order to vary the electric field distribution in the multilayer, study their effects on the damage morphology, and investigate the possible advantages of off-use angle laser conditioning. The morphology of the laser-induced damage was characterized by optical and scanning electron microscopy. Four distinct damage morphologies (pit, flat bottom pit, scald, and outer layer delamination) were observed. These damage morphologies were found to depend strongly on the angle of incidence of the laser beam. In particular, massive delamination observed at 45 degrees and 56 degrees incidence, did not occur at 65 degrees. Instead, large and deep pits were found at 65 degrees. The electric field distribution, the temperature rise and the change in stress in the multilayer were calculated to attempt to better understand the relationship between damage morphology, electric field peal; locations, and maximum thermal stress gradients. The calculations showed a two-fold increase in stress change in the hafnia top layers depending on the incidence angle. The stress gradient in the first hafnia-silica interface was found to be highest for 45 degrees, 56 degrees, and 65 degrees, respectively. Finally the maximum stress was deeper in the multilayer at 65 degrees. Although the limitations of such simple thermal mechanical model are obvious, the results can explain that outer layer delamination is more likely at 45 degrees and 56 degrees than 65 degrees and that damage sites are expected to be deeper at 65 degrees.