Ion doping induced photoconductivity enhancement and fast photorefractive response in LiNbO3 and LiTaO3 crystals: defect structure evolution and mechanism

Abstract

Lithium niobate (LiNbO₃, LN) and lithium tantalate (LiTaO₃, LT) crystals are two of the most important photorefractive materials with extensive applications in holographic storage and optical information processing. However, their long photorefractive response time and relatively low sensitivity have significantly restricted their practical applications. In this work, a systematic study on the ion doping effects on the photorefractive properties of LN and LT crystals was performed. Pr:Fe:LiNbO₃ and Zn:LiTaO₃ crystals with different doping concentrations were successfully grown by the conventional Czochralski method. X-ray diffraction (XRD) and infrared (IR) spectroscopy were employed to investigate the defect structure evolution and ion occupation mechanism in Zn:LiTaO₃ crystals. The results revealed that Zn²⁺ ions preferentially replace the antisite tantalum (Ta_Li⁴⁺) defects at low doping concentrations, while occupying both Li and Ta sites simultaneously above the threshold concentration, forming a Zn_Ta³⁻–3Zn_Li⁺ self-compensation structure. Two-wave coupling experiments demonstrate that both doping strategies significantly enhance the photorefractive response speed. In Pr:Fe:LiNbO₃, the introduction of Pr ions reduces the response time from 142 s to 35 s while maintaining a high exponential gain coefficient. In Zn:LiTaO₃, increasing Zn concentration leads to a monotonic decrease in response time, attributed to the alteration of the dominant charge transport mechanism. Theoretical analysis confirms that the enhancement in photoconductivity, driven by the modification of defect structures, is the primary mechanism responsible for the fast response in both crystal systems. These results provide a comprehensive guide for optimizing ferroelectric crystals for real-time optical information processing.