Laser-radiation-induced damage to the surface of lithium niobate and tantalate single crystals

An investigation was made of the surface damage caused by the single-mode radiation of a neodymium-glass laser operating under free-oscillation (λ = 1.06 μ) and giant-pulse (λ = 1.06 and 0.53 μ) conditions. The damage thresholds were identical at λ = 1.06 and 0.53 μ and were 120 MW/cm² for LiNbO₃ and 240 MW/cm² for LiTaO₃. The surface damage was observed after several flashes. It was established that the damage threshold under free-oscillation conditions increased with decreasing diameter of the light spot, whereas under giant-pulse conditions the threshold was independent of the size of the light spot. The surface damage mechanisms were considered and it was concluded that the Nb⁵⁺→Nb⁴⁺ reduction process was responsible for the damage to the surface of LiNbO₃.