Study of the subsurface photorefractive effect in lithium niobate

A new phenomenon of a strong irreversible variation of the refractive index (Δn∼−0.2) in a skin layer (thickness ∼300 A) has been discovered. The effect arises under the strong absorption of high-power uv radiation (249 nm, 5 ns, 20 MW/cm2). The characteristics of the subsurface photorefractive effect (SPRE) were identified by using the data on measurements of the reflection coefficient of a crystal and of the diffraction efficiency of a recorded phase grating.