Optical absorption properties of doped lithium niobate crystals

Abstract

The optical transmittance of pure lithium niobate (LiNbO3) single crystals and of LiNbO3 single crystals heavily doped with MgO, and double doped with MgO and TiO2 were measured from the ultraviolet to the visible range with the incident light being perpendicular and parallel, respectively, to the Z axis of the crystals. The wavelength dependence of the absorption coefficient alpha and its root alpha 1/2 ( alpha versus hv and alpha 1/2 versus hv, respectively) were calculated and the characteristics of the absorption edges were discussed. The absorption edges below 3.8 eV of all samples are attributable to indirect transition. The energy gaps Eg and E'g of the crystals, which correspond to the direct transition and the indirect transition, respectively, and the energy of phonons taking part in the indirect transition were calculated. It was found that Eg and E'g are related to the type and amount of doped ions, and doping with MgO and with TiO2 makes the energy gap E'g increase and decrease, respectively, causing the indirect transition absorption edges to move towards the ultraviolet and infrared, respectively.