Thermal and long-term stability of fast-ion-irradiation-induced refractive index changes in lithium niobate crystals

Irradiation of congruently melting, nominally undoped lithium niobate crystals (LiNbO3) with high-energy, low-mass He-3 ions, which are transmitted through the crystal, causes large and stable changes of the refractive index. In the irradiated regions, the extraordinary index is increased whereas the ordinary index is diminished. The decay of these changes upon annealing treatments up to 400A degrees C is investigated, exhibiting a strongly non-monoexponential decay behaviour. Long-term measurements of the refractive index changes yield no pattern erasure on a timescale of four years.