## 3 Results and Discussion
### Two-Wavelength Coupling Experiment
Consequently, this promotes the photorefractive effect of the crystal. The complete replacement of Nb\({}_{\rm Li}^{4+}\) by Zn\({}^{2+}\) ions occurs when the concentration of Zn\({}^{2+}\) ions reaches 7 mol %. The Zn\({}^{2+}\) ions begin to occupy the regular Li site and form a new defect group, denoted as Zn\({}_{\rm Li}^{+}\). Consequently, there is continuous decreases in the concentrations of Nb\({}_{\rm Li}^{4+}\) and V\({}_{\rm Li}^{-}\), while the photoconductance \(\sigma_{ph}\) exhibits a consistent increase. As a result, the writing time \(\tau_{w}\) continues to decrease. The infrared absorption spectra results indicate that when the concentration of Zn\({}^{2+}\) ions increases to 7 mol%, Fe\({}^{3+}\) and Cu\({}^{2+}\) ions start to occupy the Nb sites in the normal lattice, thereby forming defect structures Fe\({}_{\rm Nb}^{2-}\) and Cu\({}_{\rm Nb}^{3-}\). In contrast, Zn\({}^{2+}\) ions solely occupy the Li sites, leading to the formation of the defect structure Zn\({}_{\rm Li}^{+}\). The same result was also observed in the light-induced scattering experiment. When the concentration of Zn\({}^{2+}\) ions was increased to 7 mol%, self-compensating defect clusters of Fe\({}_{\rm Nb}^{2-}\) - Cu\({}_{\rm Nb}^{3-}\) - 5 Zn\({}_{\rm Li}^{+}\) were formed, leading to a significant enhancement in the photoconductance and ultimately resulting in a higher exposure energy. This suggests that elevating the concentration of Zn\({}^{2+}\) ions facilitates the incorporation of Fe\({}^{3+}\) and Cu\({}^{2+}\) ions into the lattice structure. Consequently, as the concentration of Zn\({}^{2+}\) ions increases, the concentrations of Fe\({}^{3+}\) and Cu\({}^{2+}\) ions acting as photorefractive centers also rise, thereby enhancing the saturation diffraction efficiency \(\eta_{s}\). The saturation diffraction efficiency \(\eta_{s}\) of the Zn:Cu:Fe:LiNbO\({}_{3}\) crystal can be achieved more rapidly with an increasing influx of Fe\({}^{3+/2+}\) and Cu\({}^{2+/+}\) ions into the lattice, thereby minimizing the time required for reaching \(\eta_{s}\) when the concentration of Zn\({}^{2+}\) ions reaches 7 mol%. The dynamic range \(M/\#\) and sensitivity \(S\) of the crystal are directly proportional to the saturation diffraction rate \(\eta_{s}\) and inversely proportional to the writing time \(\tau_{w}\); thus, enhancing the concentration of doped ions can optimize both the dynamic range \(M/\#\) and sensitivity \(S\). The saturation refractive index modulation \(\Delta n_{s}\) is proportional to the concentration of the electron acceptor. The concentrations of Fe\({}^{3+}\) and Cu\({}^{2+}\) ions into the crystal gradually increase, the cross-section of the captured electrons becomes larger with the increase in the concentration of electron acceptors, and the saturation refractive index modulation \(\Delta n_{s}\) gradually increases. The photoconductivity \(\sigma_{ph}\) is positively correlated with the concentration of Zn\({}^{2+}\) ions, so the photoconductivity \(\sigma_{ph}\) also steadily increases. The results revealed a significant enhancement in the holographic storage parameters with increasing Zn\({}^{2+}\) ion doping concentration. Notably, the writing time constant (\(\tau_{w}\)) for sample Zn7 decreased to 196.4 s, a substantial reduction compared to the 603.9 s observed for sample Zn1. Furthermore, sample Zn7 exhibited a maximum sensitivity (\(S\)) of 1.04, approximately four times higher than that of sample Zn1 (0.29). The dynamic range (\(M/\#\)) of sample Zn7 also increased significantly, reaching 9.81, which was markedly