**1. Introduction**

Volume holographic storage is a technology that can store information at high density inside a crystal, which exploits the "inside" of the storage material, rather than only using its surface, resulting in a massive increase in the capacity of the data storage. Lithium niobate (LN) crystal is an essential material in many applications [1–5], such as surface acoustic wave, waveguides, volume holographic storage, piezoelectric, pyroelectric, and integrated optics [6–9]. The performance of volume holographic storage is particularly prominent [10–18]. However, there are still some problems in applying it to real life. One of the critical issues is that the holographic response of existing bulk holographic materials is not fast enough. The photorefractive performance of the LN crystal can be improved by using the doping technique.

In 2012, the photorefraction of Mo-doped LN crystals was reported. T. Tian et al. found that Mo6<sup>+</sup> doping promotes the photorefractive properties of LN crystals [19]. Then in 2013, they studied fast UV-Vis photorefractive response of Zr and Mg codoped LiNbO3:Mo and found that their experimental phenomena were very different [20]. To explore the effect of the valence state of optical-damage-resistant ions on the properties of codoped crystals, in 2018, L. Zhu et al. investigated into In and Hf codoped LiNbO3:Mo crystals [21,22]. They proposed that these crystals, likewise, can improve the photorefractive properties of codoped LN crystals. Although Zr and Mg, In, Hf are

elements with optical damage resistance in LN crystal, their effects on the photorefraction of LN:Mo crystal are different. To further study the effect of the optical damage resistant on the photorefractive properties of the codoped LN crystals, and explore the specific defect structure inside the crystal, we chose Zn and Mo codoped LN crystals.

## **2. Materials and Methods**

We grew crystals by using the Czochralski method. The raw materials used were Li2CO3, Nb2O5, MoO3, and ZnO with a purity of 99.99%. The Li/Nb ratio in the initial melt was 48.38/51.62. According to the previous experimental results, it was found that 0.5 mol% Mo was superior in the monodoped lithium niobate crystal, and the doping threshold of Zn was 7 mol%. So, the concentration we chose of MoO3 was 0.5 mol%, and the concentrations of ZnO were 5.4 and 7.2 mol%, respectively. For comparison, the congruent lithium niobate (CLN) and monodoped with Mo lithium niobate crystals were also grown. The crystals were labeled as CLN, LN:Mo0.5, LN:Mo,Zn5.4, and LN:Mo,Zn7.2. Before preparing polycrystals, these materials were thoroughly mixed and sintered. The conditions for crystal growth were a pulling rate of 0.6 mm/h and a rotation speed of 14 r/min. The boules were about 4.0 cm long and had a diameter of about 3.5 cm. The subsequent polarization was applied in the furnace, with a polarization current of 30 mA for 15 min. The crystals were then cut into 3 mm and 1 mm-thick plates after the annealing treatment, and optically polished in the y-face for optical measurement.

We measured the diffraction efficiency and response time of the crystals by the two wave coupling holographic experiment as shown in Figure 1. Two writing beams of extraordinary light with equal light intensity were employed to write the holograms, and their crossing angle was designed to be 30◦ in air. The instruments used in the experiments were helium cadmium laser (442 nm), argon ion laser (488 nm), and semiconductor solid state laser (532 nm and 671 nm). The total light intensities used here were 250 mW/cm<sup>2</sup> for 442 nm, 400 mW/cm<sup>2</sup> for 488 nm and 532 nm, and 1200 mW/cm2 for 671 nm. The measured diffraction efficiency was defined as η = *Id*/(*Id* + *It*), where *Id* and *It* is the diffracted and transmitted light intensities of the readout beam, respectively. The response time constant τ*<sup>r</sup>* and the saturation diffraction efficiency η*<sup>s</sup>* were deduced by fitting the function η(*t*) = η*s*[1 − exp(−*t*/τ*r*)] 2 to the data. The photorefractive sensitivity [23] was defined as *<sup>S</sup>* = (*<sup>d</sup>* <sup>√</sup>η/*dt*)*t*=0/(*IL*), where *<sup>I</sup>* is the recording intensity and *L* is the grating thickness. The UV-visible transmission spectrum of the crystals was measured using a UV-4100 spectrophotometer (Hitachi Science and Technology, Tokyo, Japan) with a range of 300–800 nm and a resolution of 1 nm/step. The measurement of the infrared spectrum was carried out by A MAGNA-560 FT-IR spectrometer (Thermo Nicolet Corporation, Madison, America) with a range of 400–4000 cm−<sup>1</sup> and a resolution of 2 cm−1/step.

**Figure 1.** The schematic diagram of the experimental two-wave holographic setup.

## **3. Results**

## *3.1. Photorefractive Properties*

Figure 2a–d show the typical curves of diffraction efficiencies for the LN:Mo,Zn7.2 crystal as a function of time at 442 nm, 488 nm, 532 nm, and 671 nm lasers, respectively. The saturation diffraction efficiencies of the LN:Mo,Zn7.2 crystal to each wavelength were 4.50%, 17.72%, 2.32%, and 0.19%, respectively. From the figures, we can see that the diffraction efficiencies of the LN:Mo,Zn7.2 crystal

increased with the shortening of the wavelength, but the mutation occurred at 488 nm, where the diffraction efficiency is four times that of 442 nm. The reason for this is probably due to the lower power of the 442 nm laser, which was the maximum laser power in our lab. However, the crystal had different responsiveness to each band. Among the lasers used in our experiments, especially in the 671 nm band, the required intensity was the highest.

The response times of the LN:Mo,Zn7.2 crystal were 0.65 s, 3.57 s, 1.71 s, and 6.61 s under the action of 442 nm, 488 nm, 532 nm, and 671 nm lasers, respectively. As the laser wavelength decreased, the response time of the LN:Mo,Zn7.2 crystal gradually decreased, but the response time at 488 nm was about two times longer than the response time taken at 532 nm. The reason for this phenomenon is likely to be related to its higher diffraction efficiency, so we used photorefractive sensitivity to reflect the comprehensive photorefractive ability of the LN:Mo,Zn7.2 crystal.

**Figure 2.** The diffraction efficiency of the 7.2 mol% Zn and 0.5 mol% Mo codoped with LiNbO3 (LN:Mo,Zn7.2) crystal versus time at (**a**) 442 nm, (**b**) 488 nm, (**c**) 532 nm, and (**d**) 671 nm laser, respectively.

Figure 3 shows the photorefractive properties of LN:Mo,Zn crystals at 442 nm, 488 nm, 532 nm, and 671 nm laser wavelengths. For comparison, experimental data of CLN crystals is also included. The saturation diffraction efficiency is depicted in Figure 3a, the response time in Figure 3b, and the photorefractive sensitivity in Figure 3c. It can be seen from Figure 3a that the saturation diffraction efficiency of the LN:Mo,Zn crystal was improved due to the addition of zinc. In particular, LN:Mo, Zn7.2 had better performance in each band than other crystals and was more special at 488 nm. LN:Mo0.5 crystal had the highest saturation diffraction efficiency. Moreover, for all doped samples, the saturation diffraction efficiencies at 488 nm were higher than that at 442 nm. As shown in Figure 3b, the response time of the codoped crystal was greatly shortened due to the incorporation of Zn. In order to obtain a comparison of the crystals synthesis effects, we used the photorefractive sensitivity of the crystals to analyze that. Figure 3c shows that the photorefractive sensitivity of all our crystals increased with the shortening of the wavelength. When the zinc element exceeded the threshold, the photorefractive sensitivity of the LN:Mo,Zn crystal in different bands was improved. The photorefractive sensitivity of the LN:Mo,Zn7.2 crystal was significantly better than other crystals in each wavelength band. The photorefractive sensitivity of the LN:Mo,Zn7.2 crystal was 4.35 cm/J, 0.98 cm/J, 0.74 cm/J, and 0.02 cm/J at 442 nm, 488 nm, 532 nm, and 671 nm lasers, respectively. Compared with the data of LN:Mo,Zr2.5 and LN:Mo,Mg6.5 in Reference [20] in the same wavelength, the zinc doping enhanced the photorefractive properties of LN:Mo crystal, and its variation phenomena was the same as that of LN:Mo,Mg crystals, and was opposite to LN:Mo,Zr crystals. This indicates that the valence state of optical damage resistant ions plays a key role.

**Figure 3.** (**a**) The diffraction efficiency, (**b**) response time, and (**c**) photorefractive sensitivity of CLN, LN:Mo0.5, LN:Mo,Zn5.4, and LN:Mo,Zn7.2 crystals at 442 nm, 488 nm, 532 nm, and 671 nm lasers, respectively.

#### *3.2. Spectral Analysis*

The spectral characteristics of the crystals in different wavelengths can reveal the defect structure of the crystals. We recorded the wavelength of the absorption coefficient at 20 cm−<sup>1</sup> as the absorption edge of the crystal. From the inset of Figure 4a, the absorption edge of the LN:Mo,Zn5.4 crystal had a blue-shift owing to the ZnO doping. The LN:Mo,Zn7.2 crystal occurred a red-shift for its doping concentration exceeding the threshold, but this move was very tiny. It can be seen from the UV-Vis absorption difference spectrum of Figure 4a, which indicates the difference in absorption between the test crystals and the CLN crystal, that the LN:Mo,Zn crystals have an absorption valley near 319 nm, and their values were very close to the absorption edge of the LN:Mo,Zn crystal. According to X. Li et al. research, the absorption edge of pure LN crystal is caused by the defect of lithium vacancy (VLi<sup>−</sup>) [24]. Therefore, we speculate that the absorption valley was caused by VLi<sup>−</sup>. There is a distinct absorption peak near 330 nm and 480 nm, which is similar to the LN:Mo,Mg and LN:Mo,Zr crystals. This is most likely due to the deep impurity level introduced by Mo ions. According to the existing energy band information of the ABO3 ferroelectric crystal, we know that for LN crystals, the valence band top is formed by the 2p orbital of oxygen, and the lowest layer of the empty conduction band is provided by the 4d orbit of the transition metal Nb5<sup>+</sup> ions. Molybdenum ions and niobium ions belong to transition metal ions and have a 4d orbital. When molybdenum ions are incorporated into the LN crystal, its orbits will affect the bonding strength of the Nb-O bond, resulting in a change in the band gap width of the LN crystal. The absorption band of the visible band indicates that there are multiple energy levels in the band gap which contribute to the photorefractive processing.

**Figure 4.** (**a**) UV-visible absorption difference spectra of LN:Mo, LN:Mo,Zn relative to CLN crystals, and the inset shows absorption edges at 20 cm<sup>−</sup>1. (**b**) FT-IR transmission spectra of CLN, LN:Mo and LN:Mo,Zn crystals.

From the FT-IR transmission spectrum which is shown in Figure 4b, we can observe that in the LN:Mo,Zn crystals, the main absorption peak of LN:Mo,Zn7.2 crystal appeared at 3527 cm−1, the minor peak appeared at 3504 cm−1, and the peak appeared at 3483 cm−<sup>1</sup> in CLN, LN:Mo0.5 and LN:Mo,Zn5.4. The absorption peak transferred from 3483 cm<sup>−</sup><sup>1</sup> to 3527 cm<sup>−</sup>1, which implies that the Zn2<sup>+</sup> concentration in LN:Mo,Zn7.2 crystal exceeded the threshold, the (NbLi) <sup>4</sup><sup>+</sup> defects disappeared, and the (ZnNb) <sup>3</sup><sup>−</sup> appeared in the crystal.

To detect the defect structures inside the crystal, we analyzed the valence of Mo ions of LN:Mo,Zn7.2 crystal by the X-ray photoelectron spectroscopy (XPS) [25]. As shown in Figure 5a, molybdenum ions exhibited three valence states in the LN:Mo,Zn7.2 crystal, in which the combined peaks of 233.5 eV and 236.7 eV represent +6 valence, the peaks of 231 eV and 234.5 eV represent +5 valence, and the peaks of 229.3 eV and 232.5 eV represent +4 valence. As a comparison, the molybdenum element in the residue of the crucible only showed a +6 valence state formed by two combined peaks of 232.6 eV and 235.7 eV shown in Figure 5b. These results show that the valence states of Mo ions changed when they enter the crystals, which lead to the generation of new defects.

**Figure 5.** X-ray photoelectron spectroscopy of Mo in LN:Mo,Zn7.2 crystal (**a**) in the crystal and (**b**) in the residue of the crucible.
