1.3 Doping of Lithium Tantalate Crystal

Different fields have different requirements for the properties of lithium tantalate crystals. When being used to prepare high-density and large-capacity holographic information storage devices, LiTaO₃ crystals need to have excellent photorefractive properties. Due to the particularity of the crystal structure of LiTaO₃, its physical properties can be adjusted through doping, for example, the widely used photorefractive doping. The photorefractive sensitive ions doping into LiTaO₃ crystals act as photorefractive centers, and ionize free electrons or holes under light to participate in the construction of the space electric field, and thus to enhance the photorefractive effect of the crystal. This doping method requires that the absorption wavelength of the dopant ions in the crystal match the actual required wavelength. The dopant ions need to exist in two oxidation states at the same time in the crystal, acting as a carrier donor and a trap respectively. When the two oxidation states transform into each other, electrons or holes are generated. At present iron, manganese, copper, etc. are commonly used for photorefractive doping, and iron has the best performance. Follow-up studies showed that chromium, cobalt, and nickel had little effect on the storage properties of the LiTaO₃ crystal.

When LiTaO₃ crystals are used in other optical fields, it is necessary to do anti-photorefractive doping which needs the dopant to be stable in valence to reduce the photorefractive center of the crystal. Zhang Guoquan found through the study of transition group elements that a single valence state and a full-shell structure are necessary conditions for anti-photorefractive dopant ions. Table 4 shows the outer electron shell structures of several ions.

Table 4. Possible valences and outer shell structures of photodamage-resistant ions