*3.3. Optical Damage Resistance Ability*

We used the light spot distortion method to measure the optical-damage-resistance ability of crystals. As shown in Figure 6, at the maximum laser power in our lab, we can observe that the transmitted spots of the LN:Mo,Zn7.2 crystal remained circular, under the effect of 671 nm laser, when the light intensity was maintained at an intensity of 7.8 <sup>×</sup> 104 W/cm<sup>2</sup> for 5 min. A similar phenomenon occurred when under the action of the 532 nm and 488 nm lasers, the light intensities were 2.6 <sup>×</sup> <sup>10</sup><sup>5</sup> <sup>W</sup>/cm2 and 3.2 <sup>×</sup> 105 <sup>W</sup>/cm2, respectively. However, the transmitted spot of the LN:Mo,Zn5.4 crystal was significantly stretched in the c-axis direction compared to the original incident one. This performance broadens the range of applications for the LN:Mo,Zn7.2 crystal, allowing it to operate at higher light levels. The above experiment shows that the LN:Mo,Zn7.2 crystal not only has realized itself as an excellent photorefractive material, but also a high-intensity-application crystal.

**Figure 6.** The incident spots of (**a**) 671nm, (**d**) 532nm, and (**h**) 488nm lasers; (**b**,**e**,**i**) transmitted spots of LN:Mo,Zn5.4 crystals under the action of different color lasers; (**c**,**f**,**j**) transmitted spots of LN:Mo,Zn7.2 crystal under the action of different color lasers.

## **4. Discussion**

In the absorption difference spectrum of Figure 4a, we can observe that the absorption near 488 nm is higher than other spectra in the visible band, which also confirms that it has higher diffraction efficiency than other bands. Also, the apparent absorption peak near 330 nm indicates that there may be a deep defect level here. Because of the low power of our laboratory 325 nm laser and the absence of working laser of suitable wavelength, further research on deep defect level is needed.

The XPS studies reveal that the valence change of Mo ions occurs before and after entering the LN:Mo,Zn crystal. In the process of crystal growth, the convertible Mo ions enter the crystal and are isolated from oxygen, so they are not easily oxidized and present three valence states. On the contrary, the Mo ions in the residue can come in contact with sufficient oxygen and be easily oxidized, so the Mo ions in the residue only exist in the +6 valence state. According to previous reports [17], the Mo4<sup>+</sup> and Mo5<sup>+</sup> ions may occupy the Li sites (MoLi<sup>3</sup>+/4<sup>+</sup>), and Mo6<sup>+</sup> ions may occupy the Nb sites (MoNb<sup>+</sup>) severed as UV photorefractive centers. On the base of the lithium vacancy model, a large number of intrinsic defects (such as NbLi<sup>4</sup>+, small polaron, and bipolaron, etc.) limit the response time of the crystal. At low concentrations, the molybdenum and zinc ions tend to occupy the Li position, which could push the NbLi<sup>4</sup><sup>+</sup> out and shorten the response time. When the zinc ion concentration exceeds the threshold, all the NbLi<sup>4</sup><sup>+</sup> were replaced and the MoLi<sup>3</sup>+/4<sup>+</sup> were also substituted due to their higher valence than +2 valence zinc ions and transferred to the MoNb<sup>+</sup> ions. As the UV photorefractive center, the MoNb<sup>+</sup> ions may attract an electron from O2-, which could further speed up the photorefraction process [26]. However, for the +3 and +4 valence photorefractive resistant elements, the substitution for the MoLi<sup>3</sup>+/4<sup>+</sup> defect is weak. Thus, the valence state of the optical damage resistant elements may be the critical factor determining the crystal properties.

As the main obstacle of the optical storage materials, the storage speed has been limiting the commercial application of LN crystals. The response time and sensitivity of LN:Mo,Zn crystal was optimized by the zinc codoping, compared with LN:Mo,Zr crystals, which is similar to the LN:Mo,Mg crystals. These results confirm that the valence state of the optical damage resistant elements may be the critical factor determining the crystal properties. Compared to the LN:Mo,Mg6.5 crystals, the response time in LN:Mo,Zn7.2 crystals was several times longer than that of LN:Mo,Mg6.5 crystals at the same wavelength. Especially with the shortening of wavelength, this gap was further widened. We think this due to the individuality of the elements, such as the ionic radius, electronegativity, outer electron configuration, etc. As it is well known, the conduction band is generally provided by d electrons. For Zn2<sup>+</sup> and Mg2<sup>+</sup>, Zn2<sup>+</sup> has d electrons, while Mg ions have no d electrons, which results in a significant difference in the effect of the two on the crystal. Overall, the valence state of the optical-damage-resistant elements and the individuality of the elements may be the main factor determining the crystal properties.
