The Influence of In³⁺ on the Crystal Growth and Visible Band Photorefraction of Uranium-Doped Lithium Niobate Single Crystals

A series of lithium niobate crystals co-doped with uranium and indium was successfully grown by the modified vertical Bridgman method for the first time. With increasing In3+ ion doping concentration, the segregation coefficient of uranium and indium progressively deviated from 1. The structural refinement indicated that uranium ions with high valence preferred to occupy the Nb sites in LN: In, U crystals. LN: In2.0, U0.6 achieved multi-wavelength holographic writing with diffraction efficiency comparable to commercial crystals LN:Fe0.3, demonstrating a response time that was four times shorter than LN:Fe0.3. XPS analysis was employed to investigate the valence states of In3+ ions in LN: In2.0, U0.6, in which uranium ions presented three valences of +4, +5 and +6. Furthermore, the ‘real threshold concentration’ of In3+ ions in LN: In, U was calculated using the Li-vacancy model, which is consistent with the results obtained from the experimental study of the OH− absorption spectrum. Discussions on the photorefractive centers in LN: In, U are also provided. This study not only demonstrates the impact of doping In3+ ions on the growth of LN:U crystals, but also offers new insights into the photorefractive properties of LN in the visible band.