Effect of Zn²⁺ Ion Concentration on the Light-Induced Scattering and Holographic Storage Properties of Zn:Cu:Fe:LiNbO₃ Crystals

Abstract

Lithium niobate (LiNbO₃), a multifunctional crystalline material, has critical importance in advancing holographic storage systems. However, persistent challenges such as optical damage, limited diffraction efficiency, and slow response kinetics hinder its practical implementation. This work systematically examines the correlation between the Zn²⁺ dopant concentration and the defect architecture, photodamage resistance, and holographic storage properties of Zn:Cu:Fe:LiNbO₃ crystals, employing advanced characterization techniques to elucidate structure–property relationships and optimize performance metrics. The experimental data reveal a pronounced Zn²⁺ doping concentration dependence in both photodamage resistance and holographic storage capabilities. Notably, Zn:Cu:Fe:LiNbO₃ crystals doped with 7 mol% Zn²⁺ achieve a substantial 416-fold improvement in photodamage resistance (786.55 J/cm²) relative to the 1 mol% doped variant. Concurrently, these optimally doped crystals demonstrate superior holographic storage performance, characterized by a response time of 196.4 s, a dynamic range of 9.81, a diffraction efficiency of 66.7%, and a sensitivity of 1.04. The observed performance enhancement is fundamentally attributed to Zn²⁺ doping, which concomitantly suppresses intrinsic defect formation and tailors the spatial distribution of Fe³⁺/Cu²⁺ photorefractive centers within the crystal lattice. These mechanistic insights establish critical guidelines for the rational design of next-generation holographic storage materials with optimized photorefractive response and defect engineering capabilities.

1. Introduction

Lithium niobate (LiNbO₃, LN) crystals have established themselves as pre-eminent candidates for holographic volume storage applications owing to their exceptional electro-optic coefficients and nonlinear optical properties [1,2]. Nevertheless, the practical deployment of undoped LiNbO₃ in advanced storage systems faces substantial barriers stemming from inherent recombination centers, ionic mobility restrictions, volatile charge retention, and pronounced optical susceptibility [3]. To circumvent these limitations, recent advances have focused on the strategic incorporation of transition metal dopants (e.g., Fe³⁺, Cu²⁺, Mn²⁺, Ni²⁺), which engineer defect-mediated bandgap states to regulate carrier dynamics and amplify the photorefractive response through controlled electron-hole trapping mechanisms [4,5,6].

Among transition metal-doped LiNbO₃ systems, iron-doped crystals (Fe:LiNbO₃) have emerged as benchmark materials, achieving saturation diffraction efficiencies exceeding 70% at 488 nm illumination while demonstrating exceptional hologram stability, with grating persistence exceeding several months under dark storage conditions [7]. This material has enabled groundbreaking progress in volumetric data storage, notably demonstrated by the successful encoding of 10,000 high-resolution holograms within a 5.0 mm-thick crystal, achieving a record volumetric density of 6.7 Gbits/cm³ [8] and facilitating correlation recognition systems with 95% operational accuracy [9]. Despite these advances, Fe:LiNbO₃ crystals remain constrained by two critical shortcomings: inherent fan-out scattering noise arising from photovoltaic field anisotropies and intrinsically slow response kinetics, which collectively impede their deployment in real-time storage architectures [10].

Recent advances in multi-element co-doping have yielded notable progress in non-volatile holographic storage, with Mn:Fe:LiNbO₃, Tb:Fe:LiNbO₃, and Ce:Cu:LiNbO₃ crystals demonstrating enhanced charge retention and spatial phase stability under prolonged illumination [11,12,13,14,15]. Among these systems, dual doping with Fe³⁺ and Cu²⁺ has proven particularly effective: Fe:Cu:LiNbO₃ crystals exhibit a 60-fold acceleration in writing speed and a 67-fold increase in sensitivity compared to single-doped analogues, attributed to synergistic electron-hole trapping at multivalent defect sites [16,17]. Despite these improvements, persistent limitations in secondary response kinetics underscore the need for advanced defect engineering approaches.

Building on these developments, the strategic integration of optical-damage-resistant ions (e.g., Mg²⁺, Sc³⁺, In³⁺, Hf⁴⁺, Zr⁴⁺) with photorefractive species has emerged as a promising strategy for suppressing intrinsic lattice defects and optimizing charge carrier dynamics [18,19,20,21,22]. Particularly noteworthy is Zn²⁺ doping, which exhibits exceptional multifunctionality: At concentrations above 6.2 mol%, it simultaneously enhances the photorefractive response while achieving an unprecedented ~200-fold improvement in photodamage resistance compared to undoped counterparts [23,24,25,26]. This unique dual-action mechanism establishes Zn²⁺ as a paradigm-shifting dopant for engineering high-performance holographic storage media.

In this study, we present an innovative Zn:Fe:Cu:LiNbO₃ crystal system that synergistically integrates Zn²⁺ ion doping with dual photorefractive centers (Fe³⁺ and Cu²⁺). Building upon prior investigations by our group, high-concentration Zn²⁺ doping in Cu:Fe:LiNbO₃ crystals significantly improves structural homogeneity, a critical factor in minimizing holographic distortion and noise during recording/readout processes [27]. Here, we systematically explore the impact of high-concentration Zn²⁺ doping on two underexamined yet technologically vital properties: photodamage resistance and holographic storage performance. By combining defect structure analysis with holographic parameter characterization, this work establishes a materials optimization framework to advance next-generation holographic storage technologies.

2. Materials and Methods

2.1. Material Preparation

The Zn:Cu:Fe:LiNbO₃ crystals, which contained 1 mol% Fe₂O₃ and 1 mol% CuO, were grown in air using the Czochralski technique with varying doping concentrations of ZnO (1, 3, 5, and 7 mol%). The optical quality of the crystal is highly dependent on the purity level of its raw materials, namely, Nb₂O₅, LiCO₃, ZnO, CuO, and Fe₂O₃. Therefore, these raw materials used for crystal growth must have a minimum purity level of 99.99%. The Nb₂O₅, LiCO₃, ZnO, CuO, and Fe₂O₃ materials were thoroughly mixed for over 24 h before being placed into a platinum crucible. They were then heated to 750 °C for 2 h to eliminate CO₂ and subsequently further heated to 1150 °C for another 2 h to facilitate the formation of a solid reaction polycrystalline material suitable for crystal growth.

The growth conditions were selected as follows: the temperature gradient was set at 2 °C/mm, while the polling rate and rotation rate were controlled within the range of 0.7 to 1.7 mm/h and 15–28 rpm, respectively. After the completion of crystal growth, it was gradually cooled to room temperature at a controlled rate of 35–40 °C/h. Simultaneously, to mitigate the impact of crystal spontaneous polarization on optical properties, all crystals were subjected to an artificial polarization process for 8 h in an intermediate-frequency furnace with a temperature of 1100 °C with a nearly zero temperature gradient, and a current density of 5 mA/cm², resulting in the attainment of single-domain crystals. The wafers with dimensions of 5 mm × 8 mm × 2 mm (x × y × z) were obtained by cutting from the central region of the Zn:Cu:Fe:LiNbO₃ crystals along the y-axis. The samples, denoted as Zn1, Zn3, Zn5, and Zn7, had varying concentrations of Zn²⁺ ions.

2.2. Characterization of Infrared Absorption Spectrum

The defect structures of Zn:Cu:Fe:LiNbO₃ crystals were systematically characterized using Fourier-transform infrared (FTIR) spectroscopy. Infrared absorption spectra were acquired using an AVATAR370 spectrometer(Thermo Fisher Scientific, USA) under consistent experimental conditions, with a spectral range of 3400–3600 cm⁻¹, corresponding to the characteristic OH⁻ stretching vibration region. This analytical approach is particularly effective for defect structure analysis, as identical molecular groups and bond types exhibit characteristic and reproducible absorption peaks [28].

During crystal growth, the H⁺ ions from both the raw materials and growth atmosphere incorporate into the LiNbO₃ lattice, predominantly existing as OH⁻ groups [29]. The OH⁻ stretching vibration demonstrates exceptional sensitivity to local ionic environments, making infrared spectroscopy an invaluable tool for investigating ion occupancy and defect configurations in LiNbO₃ crystals. The precise analysis of OH⁻ absorption bands provides critical insights into the distribution and coordination of dopant ions within the crystal lattice.

2.3. Characterization of Light-Induced Scattering

The optical damage resistance of Zn:Cu:Fe:LiNbO₃ crystals with varying Zn²⁺ ion concentrations was quantitatively evaluated using the light scattering energy flux threshold method. This phenomenon, characterized by the onset of pronounced internal scattering when incident laser energy exceeds a critical threshold, is a well-established indicator of optical damage in photorefractive materials. The scattering intensity exhibits saturation behavior, reaching a maximum value that remains constant despite further increases in incident energy, a characteristic known as the light-induced scattering threshold effect.

The experimental setup, illustrated in Figure 1, employed a semiconductor laser source with an adjustable attenuator for precise power control. When incident light (I₀) interacts with the crystal, it partitions into three components according to the following energy balance equation:

$$I_0=I_T+I_A+I_R$$

(1)

where I_T, I_A, and I_R represent transmitted, absorbed, and reflected light intensities, respectively. The scattering phenomenon is quantitatively described by the relationship between scattered light intensity (I_S) and transmitted light [30]:

$$I_S=I_{T0}-I_{Tt}$$

(2)

Here, I_T0 and I_Tt denote the transmitted light intensities at the initial time and time t, respectively. The scattering ratio (R_s) is defined as

$$R_s=I_S/I_{T0}$$

(3)

The scattering ratio serves as a quantitative measure of scattering intensity. The effective incident light intensity (I_eff) is determined by

$$I_{eff}+I_R=I_0$$

(4)

where R represents the surface reflectivity. The exposure energy (E_r), a critical parameter for optical damage resistance, is calculated through temporal integration of the effective intensity,

$$E_r=I_{eff}\tau =(1-R)\times I_0\tau$$

(5)

where τ represents the scattering time constant, determined by fitting the dynamic scattering curve to the following equation:

$$\sqrt{R_s}=\sqrt{R_{s,sat}}\left[1-exp\left({\displaystyle \frac{-t}{\mathsf{\tau }}}\right)\right]$$

(6)

The saturation scattering ratio (R_s,_sat) and time constant (τ) provide quantitative measures of the crystal’s optical damage resistance. Longer saturation times and higher exposure energies correlate with enhanced resistance to optical damage, offering crucial insights into the material’s performance characteristics.

2.4. Characterization of Two-Wavelength Coupling Experiment

The holographic storage properties of Zn:Cu:Fe:LiNbO₃ crystals were investigated using a two-wavelength coupling experiment [31,32]. This technique is employed to study the interaction between two light waves in nonlinear optical materials, enabling the observation and analysis of phenomena such as energy transfer and phase modulation between light waves. Consequently, it provides insights into the nonlinear optical properties of the materials. The experimental setup is illustrated in Figure 2. A Kr⁺ laser operating at a wavelength of 476 nm and a He-Ne laser operating at a wavelength of 633 nm were used to generate the recording and readout beams, respectively. The recording beam was split into two beams, I_s and I_r, of equal intensity (120 mW/cm²) using a continuously adjustable beam splitter. These beams were polarized in the incidence plane and directed toward the crystal at the corresponding Bragg angle of 16°, intersecting symmetrically inside the crystal to form a grating vector along the c-axis.

The beams were precisely controlled by electronic shutters placed in the object light, reference light, and detection light paths, separating the holographic storage into different storage processes. During grating recording, the shutters in the object light and reference light paths were simultaneously opened to record the refractive index grating, while the probe light was activated to detect the diffraction grating in real time. During grating reading, only the shutter in the probe light path was opened, allowing the shallow-level grating to be erased while the deep-level grating was fixed, enabling nonvolatile reading. This process, known as the erasure process, involves eliminating the grating recorded in the shallow layer and fixing the grating recorded in the deep center. The variation in the diffracted light intensity over time was measured using a precision power meter.

In the two-wavelength coupling experiment, several parameters, including diffraction efficiency (η), sensitivity (S), and dynamic range (M/#), were used to evaluate the holographic storage performance of the materials. Diffraction efficiency is a critical indicator for assessing the performance of holographic storage systems, as it directly reflects the efficiency of information recording and reading. Holographic storage records information in materials through interference patterns, and the diffraction efficiency indicates the material’s response capability to these patterns. High diffraction efficiency implies that the material can efficiently record interference patterns, thereby storing more information.

The diffraction efficiency is defined as the ratio of the intensity of the diffracted light (I_d) to that of the transmitted light (I_t):

$$\eta ={\displaystyle \frac{I_d}{I_t}}\times 100\%$$

(7)

The writing time, or response time, is a crucial parameter in holographic storage, reflecting the dynamic formation of gratings. The formation of a refractive index grating through a dynamic time process can be described by

$$\sqrt{\eta }=\sqrt{{\eta }_{sat}}(1-\exp (-t/{\tau }_w\left)\right)$$

(8)

Here, τ_w represents the writing time, which determines the speed of recording, and η_sat is the saturation diffraction efficiency during recording, a fixed parameter determined by the material’s properties.

The process of eliminating the refractive index grating can be described by

$$\sqrt{\eta }=\sqrt{{\eta }_{sat}}(1-\exp (-t/{\tau }_e\left)\right)$$

(9)

In this equation, τ_e represents the time constant for erasing, which characterizes the rate at which the grating is eliminated [33]. The saturation diffraction efficiency during recording, η_sat, is determined by

$${\eta }_s={sin}^2\left({\displaystyle \frac{\pi d∆{\eta }_{sat}}{\lambda cos{\theta }_{cry}}}\right)$$

(10)

Here, η_s is the maximum diffraction efficiency, determined by the sample thickness (d), the recording beam wavelength (λ), and the refraction angle (θ_cry) of the incident light within the crystal.

Photorefractive sensitivity is another critical parameter for evaluating the performance of photorefractive materials. It reflects the efficiency of the material in generating refractive index changes under light irradiation. Higher photorefractive sensitivity indicates a faster response to light intensity and more significant refractive index changes, which are beneficial for applications such as holographic storage and optical information processing. The photorefractive sensitivity S is defined as [34]

$$S={\left[{\displaystyle \frac{d\sqrt{\eta }}{dt}}\right]}_{t=0}/IL$$

(11)

This equation represents the relationship between the total optical intensity (I) and the thickness of the crystal plate (L).

The dynamic range M/# is another essential parameter for evaluating photorefractive materials. It describes the maximum range of light intensity changes that the material can effectively record and read. A larger dynamic range indicates a wider range of light intensity changes that the material can handle, enhancing its performance in applications such as holographic storage and optical information processing. The dynamic range M/# is defined as [35]

$${\eta }_{\omega }={\left({\displaystyle \frac{M/\#}{M}}\right)}^2$$

(12)

Here, η_ω represents the diffraction efficiency when the M-th hologram is stored, and M indicates the number of holograms.

In the two-wave coupling experiment, only one recording is performed, the erasing process of the phase grating is recorded, and the writing time is short. Therefore, the sensitivity (S) and dynamic range M/# can be approximated as

$$S={\left[{\displaystyle \frac{d\sqrt{\eta }}{dt}}\right]}_{t=0}/IL\approx \sqrt{{\eta }_s}/({\tau }_w\times IL)$$

(13)

$$M/\#={\tau }_e{\left[{\displaystyle \frac{d\sqrt{\eta }}{dt}}\right]}_{t=0}\approx {\displaystyle \frac{{\tau }_e}{{\tau }_w}}\sqrt{{\eta }_s}$$

(14)

These equations provide a simplified yet effective way to estimate the sensitivity and dynamic range in the context of the two-wavelength coupling experiment.

3. Results and Discussion

3.1. Infrared Absorption Spectrum

The OH⁻ absorption peak of pure LiNbO₃ crystal is 3482 cm⁻¹. ${\mathrm{V}}_{\mathrm{L}\mathrm{i}}^{-}$ possesses a negative charge and exhibits an attractive force toward H⁺ ions, thereby resulting in the formation of a complex defect group ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+}-3{\mathrm{V}}_{\mathrm{L}\mathrm{i}}^{-}-\mathrm{O}{\mathrm{H}}_{\mathrm{L}\mathrm{i}}^{-}$, corresponding to the absorption peak at 3482 cm⁻¹ [36,37].

The absorption peaks of samples Zn1, Zn3, and Zn5 in Figure 3 exhibit slight deviations from those observed in pure LiNbO₃ crystal, occurring at 3483 cm⁻¹, 3484 cm⁻¹, and 3486 cm⁻¹, respectively, while sample Zn7 displays two distinct OH⁻ absorption peaks at 3502 cm⁻¹ and 3528 cm⁻¹. The shift in the absorption peak can be elucidated by the underlying mechanism, as follows: The [Li]/[Nb] ratio of the congruent LiNbO₃ crystals is 0.946, leading to the generation of intrinsic defects such as Li vacancy and anti-site Nb during the crystal growth process due to the absence of Li. The presence of a significant number of Li vacancies in the crystal leads to the incorporation of Nb into the Li site, resulting in the formation of anti-site Nb. In this study, the concentrations of doped Fe³⁺ and Cu²⁺ ions were fixed at a low level. When Fe³⁺ and Cu²⁺ ions enter the LiNbO₃ crystal, they first occupy ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+}$ and form defects ${\mathrm{F}\mathrm{e}}_{\mathrm{L}\mathrm{i}}^{2+}$ and ${\mathrm{C}\mathrm{u}}_{\mathrm{L}\mathrm{i}}^{+}$, which are positively charged and therefore do not attract H⁺. The doping of Fe³⁺ and Cu²⁺ ions has little impact on the absorption peak.

Therefore, the primary factor influencing the absorption peak is the variable Zn²⁺ ions, and it has been reported that the threshold concentration of Zn²⁺ ions is approximately 6 mol%. Additionally, when Zn²⁺ ions enter LiNbO₃ crystals, they exhibit a preference for occupying ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+}$ to form ${\mathrm{Z}\mathrm{n}}_{\mathrm{L}\mathrm{i}}^{+}$. Before the doping concentration of Zn²⁺ reached the threshold, the main components in the crystal were still ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+}$ and ${\mathrm{V}}_{\mathrm{L}\mathrm{i}}^{-}$, so the absorption peaks of samples Zn1, Zn3, and Zn5 were still ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+}-3{\mathrm{V}}_{\mathrm{L}\mathrm{i}}^{-}-\mathrm{O}{\mathrm{H}}_{\mathrm{L}\mathrm{i}}^{-}$. The OH⁻ absorption bands of samples Zn1, Zn3, and Zn5 were all located at about 3482 cm⁻¹. With the increase in the Zn²⁺ ion concentration, the decrease in the concentrations of ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+}$ and ${\mathrm{V}}_{\mathrm{L}\mathrm{i}}^{-}$ in the crystal had a certain effect on H⁺, and the position of the absorption peak moved within a certain range.

When the concentration of Zn²⁺ ions exceeds the threshold concentration of 7 mol%, Zn²⁺ ions continue to occupy the conventional Li sites owing to their ionic radius and polarization capabilities, while pushing Fe³⁺ ions and Cu²⁺ ions to the Nb site, resulting in defect groups of ${\mathrm{F}\mathrm{e}}_{\mathrm{N}\mathrm{b}}^{2-}$ and ${\mathrm{C}\mathrm{u}}_{\mathrm{N}\mathrm{b}}^{3-}$. Subsequently, some H⁺ ions are attracted toward the vicinity of ${\mathrm{F}\mathrm{e}}_{\mathrm{N}\mathrm{b}}^{2-}$, resulting in the formation of a defect group consisting of ${\mathrm{F}\mathrm{e}}_{\mathrm{N}\mathrm{b}}^{2-}-\mathrm{O}{\mathrm{H}}_{\mathrm{L}\mathrm{i}}^{-}-{\mathrm{Z}\mathrm{n}}_{\mathrm{L}\mathrm{i}}^{+}$, leading to an absorption peak near 3502 cm⁻¹. The H⁺ ion exhibits a higher affinity toward ${\mathrm{C}\mathrm{u}}_{\mathrm{N}\mathrm{b}}^{3-}$ compared to ${\mathrm{V}}_{\mathrm{L}\mathrm{i}}^{-}$ and ${\mathrm{F}\mathrm{e}}_{\mathrm{N}\mathrm{b}}^{2-}$, leading to the aggregation of H⁺ ions near ${\mathrm{C}\mathrm{u}}_{\mathrm{N}\mathrm{b}}^{3-}$ and the formation of a defect group of ${\mathrm{C}\mathrm{u}}_{\mathrm{N}\mathrm{b}}^{3-}-\mathrm{O}{\mathrm{H}}_{\mathrm{L}\mathrm{i}}^{-}-{2\mathrm{Z}\mathrm{n}}_{\mathrm{L}\mathrm{i}}^{+}$, thereby resulting in an additional absorption peak at 3528 cm⁻¹. From the above analysis, we can draw the conclusion that when the doping concentration of Zn²⁺ ions reaches 7 mol%, Fe³⁺ and Cu²⁺ ions occupy the Nb sites, and Zn²⁺ ions continue to occupy the Li sites to complete the charge compensation.

3.2. Light-Induced Scattering

The change curve in Figure 4 demonstrates a negative correlation between the transmitted light intensity and the duration of illumination. As the exposure time increases, there is an initial rapid decline in transmitted light intensity, followed by a gradual leveling off. The increase in Zn²⁺ doping concentration can be observed to significantly prolong the stabilization time for transmitted light intensity, particularly when the doping concentration reaches 7 mol%. At this level of doping, the time required for the curve slope to approach zero is extended by an order of magnitude. According to the analysis of the crystal scattering ratio in Figure 5, it can be observed that the square root of the crystal scattering ratio exhibits a positive correlation with exposure time and demonstrates a gradual and steady change following a significant increase as exposure time increases. To facilitate analysis, the calculated data for light-induced scattering of the Zn:Cu:Fe:LiNbO₃ crystals are listed in

Table 1. Light-induced scattering parameters of Zn:Cu:Fe:LiNbO₃ crystals, illumination light intensity (I), effective incident intensity (I_eff), exposure time (τ), and total exposure energy (E_r).

Crystals	t (s)	I (mW/cm2)	Ieff (mW/cm2)	Er (J/cm2)
Zn1	22.84	96.76	82.7	1.89
Zn3	498.06	670.41	573	285.37
Zn5	785.37	699.78	598.1	469.71
Zn7	931.69	987.71	844.2	786.55

.

According to the Li-site vacancy model, the intrinsic defects ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+}$ and ${\mathrm{V}}_{\mathrm{L}\mathrm{i}}^{-}$ in LiNbO₃ crystals serve as photorefractive sensitive centers, thereby compromising the resistance ability of light-induced scattering [38,39,40]. The enhanced resistance ability to light-induced scattering of Zn²⁺ ions is mainly formed into ${\mathrm{Z}\mathrm{n}}_{\mathrm{L}\mathrm{i}}^{+}$ by substituting ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+}$, which reduces ${\mathrm{V}}_{\mathrm{L}\mathrm{i}}^{-}$ in order to maintain the charge balance. The non-radiative compound effect of the intrinsic defect on the electron is thus significantly attenuated, thereby enhancing the resistance ability to light-induced scattering.

With the increase in the Zn²⁺ ion concentration, ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+}$ is almost completely replaced, the exposure energy of sample Zn5 reaches 469.71 J/cm², and the resistance ability of light-induced scattering was greatly improved. When the concentration of Zn²⁺ ions increases to 7 mol%, Zn²⁺ ions continue to occupy the normal Li position, and part of Fe³⁺ and Cu²⁺ ions are crowded out from the Li position to the Nb position. The defective structures of ${\mathrm{F}\mathrm{e}}_{\mathrm{L}\mathrm{i}}^{2+}$ and ${\mathrm{C}\mathrm{u}}_{\mathrm{L}\mathrm{i}}^{+}$ transform into ${\mathrm{F}\mathrm{e}}_{\mathrm{N}\mathrm{b}}^{2-}$ and ${\mathrm{C}\mathrm{u}}_{\mathrm{N}\mathrm{b}}^{3-}$, which have weak electron trapping ability; the corresponding photoconductance in the crystal increases sharply, thereby significantly increasing the exposure energy. Due to the high concentration of Zn²⁺ ions within the crystal, self-compensating defect clusters of ${\mathrm{F}\mathrm{e}}_{\mathrm{N}\mathrm{b}}^{2-}-{\mathrm{C}\mathrm{u}}_{\mathrm{N}\mathrm{b}}^{3-}-5{\mathrm{Z}\mathrm{n}}_{\mathrm{L}\mathrm{i}}^{+}$ are formed within the crystal, thereby significantly enhancing the resistance ability to the light-induced scattering of Zn:Cu:Fe:LiNbO₃ crystal. Sample Zn7 is the preferred material for mitigating the light-induced scattering effect, having an Er value of 786.55 J/cm², which is approximately 416 times higher than that of Zn1.

3.3. Two-Wavelength Coupling Experiment

In this experiment, variable-valence Fe³⁺ and Cu²⁺ ions were incorporated into the crystal lattice as photorefractive centers. Specifically, Fe²⁺ and Cu⁺ acted as donor levels, while Fe³⁺ and Cu²⁺ ions served as acceptor levels. These two ions play a crucial role as the sensitive centers for photorefraction and significantly influence the photorefractive properties of the crystal, thereby having a substantial impact on its holographic storage performance.

The Zn²⁺ ion remains in a stable univalent state within the crystal, thereby abstaining from participating in both the charge transport and grating formation processes. The carrier transport model of Zn:Cu:Fe:LiNbO₃ crystal is illustrated in Figure 6, where the conduction band edge is represented by the 4d orbital of the Nb⁵⁺ ion, and the valence band edge is represented by the 2p orbital of the O²⁻ ion. The iron exists as Fe³⁺ and Fe²⁺ ions, serving as shallow central levels, while the copper exists as Cu²⁺ and Cu⁺ ions, acting as deep central levels. The deep-level carriers of the Cu⁺ and Fe²⁺ ions are excited through energy absorption under illumination conditions, thereby complementing each other by facilitating energy transfer and modifying the ionization cross-section as well as capturing the cross-section. The enhancement in the photorefractive properties can be attributed to the utilization of double photorefractive ions in LiNbO₃ crystals. The Fe³⁺ and Cu²⁺ ions undergo electron acquisition to form Fe²⁺ and Cu⁺ ions, thereby initiating a continuous release of electrons. Subsequently, these released electrons are captured by migrating Fe³⁺ and Cu²⁺ ions, thus completing the transportation process.

The process can be represented as follows:

Fe²⁺ + hv → Fe³⁺ + e

(15)

Fe³⁺ + e → Fe²⁺

(16)

Cu⁺ + hv → Cu²⁺ + e

(17)

Cu²⁺ + e → Cu⁺

(18)

The presence of doped Zn²⁺ ions directly influences the occupancy of ions and the types of defects in Zn:Cu:Fe:LiNbO₃ crystals, subsequently leading to changes in their internal structure and defect type, which in turn affect the photoconductivity of the crystal and consequently its photorefractive effect.

Figure 7 illustrates the readout curves of the nonvolatile two-wavelength holographic recording.

Table 2. Experiment results of holographic storage properties of Zn:Cu:Fe:LiNbO₃ crystal.

Sample	τw (s)	τe (s)	ηs (%)	S (cm/J)	M/#	Δns (×10−5)	σph (×10−12 cm/ΩW)
Zn1	603.9	782.4	50.2	0.29	0.92	4.60	0.41
Zn3	528.1	1108.6	57.3	0.36	1.59	4.92	0.47
Zn5	483.5	1390.9	62.8	0.41	2.28	5.15	0.51
Zn7	196.4	2358.2	66.7	1.04	9.81	5.30	1.26

presents the holographic storage parameters of Zn:Cu:Fe:LiNbO₃ crystals, which were determined through a two-wavelength coupling experiment. It is evident that the diffraction efficiency of the crystal exhibits a gradual increase as the concentration of Zn²⁺ ions rises.

When the doping concentration of Zn²⁺ ions is below 7 mol%, they preferentially replace the ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+}$ in the lattice, which reduces the intrinsic defect concentration and then increases the crystal conductivity. The writing time τ_w can be expressed as follows [41]:

$${\tau }_w\approx {\displaystyle \frac{\epsilon {\epsilon }_0}{4\pi {\sigma }_{ph}}}$$

(19)

where ε represents the dielectric constant; its specific value is 2.8 × 10⁻¹¹ and does not change due to doping ions. ε₀ is the dielectric constant in free space, and its specific value is 8.9 × 10⁻¹².

The writing time τ_w exhibits a negative correlation with the photoconductance σ_ph. As the concentration of Zn²⁺ ions increases, the defect concentration of ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+}$ decreases, thereby enhancing the photoconductivity σ_ph of the crystal and reducing the writing time τ_w. Consequently, this promotes the photorefractive effect of the crystal. The complete replacement of ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+}$ by Zn²⁺ ions occurs when the concentration of Zn²⁺ ions reaches 7 mol %. The Zn²⁺ ions begin to occupy the regular Li site and form a new defect group, denoted as ${\mathrm{Z}\mathrm{n}}_{\mathrm{L}\mathrm{i}}^{+}$. Consequently, there is continuous decreases in the concentrations of ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+}$ and ${\mathrm{V}}_{\mathrm{L}\mathrm{i}}^{-}$, while the photoconductance σ_ph exhibits a consistent increase. As a result, the writing time τ_w continues to decrease.

The infrared absorption spectra results indicate that when the concentration of Zn²⁺ ions increases to 7 mol%, Fe³⁺ and Cu²⁺ ions start to occupy the Nb sites in the normal lattice, thereby forming defect structures ${\mathrm{F}\mathrm{e}}_{\mathrm{N}\mathrm{b}}^{2-}$ and ${\mathrm{C}\mathrm{u}}_{\mathrm{N}\mathrm{b}}^{3-}$. In contrast, Zn²⁺ ions solely occupy the Li sites, leading to the formation of the defect structure ${\mathrm{Z}\mathrm{n}}_{\mathrm{L}\mathrm{i}}^{+}$. The same result was also observed in the light-induced scattering experiment. When the concentration of Zn²⁺ ions was increased to 7 mol%, self-compensating defect clusters of ${\mathrm{F}\mathrm{e}}_{\mathrm{N}\mathrm{b}}^{2-}-{\mathrm{C}\mathrm{u}}_{\mathrm{N}\mathrm{b}}^{3-}-5{\mathrm{Z}\mathrm{n}}_{\mathrm{L}\mathrm{i}}^{+}$ 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 Zn²⁺ ions facilitates the incorporation of Fe³⁺ and Cu²⁺ ions into the lattice structure. Consequently, as the concentration of Zn²⁺ ions increases, the concentrations of Fe³⁺ and Cu²⁺ ions acting as photorefractive centers also rise, thereby enhancing the saturation diffraction efficiency η_s.

The saturation diffraction efficiency η_s of the Zn:Cu:Fe:LiNbO₃ 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 η_s when the concentration of Zn²⁺ ions reaches 7 mol%. The dynamic range M/# and sensitivity S of the crystal are directly proportional to the saturation diffraction rate η_s and inversely proportional to the writing time τ_w; thus, enhancing the concentration of doped ions can optimize both the dynamic range M/# and sensitivity S. The saturation refractive index modulation Δn_s is proportional to the concentration of the electron acceptor. The concentrations of Fe³⁺ and Cu²⁺ 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 Δn_s gradually increases. The photoconductivity σ_ph is positively correlated with the concentration of Zn²⁺ ions, so the photoconductivity σ_ph also steadily increases.

The results revealed a significant enhancement in the holographic storage parameters with increasing Zn²⁺ ion doping concentration. Notably, the writing time constant (τ_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 higher than the 0.92 recorded for sample Zn1. Additionally, the saturation diffraction efficiency (η_s) of Zn7 reached 66.7%, representing a 16.5% improvement over the 50.2% achieved by Zn1. In summary, the holographic storage performance of the crystal improved significantly with higher Zn²⁺ ion doping concentrations. Specifically, when the Zn²⁺ ion doping concentration reached 7 mol%, the Zn:Cu:Fe:LiNbO₃ crystal demonstrated exceptional holographic storage performance, making it a highly promising material for such applications.

4. Conclusions

In summary, the Czochralski method was employed to grow Cu:Fe:LiNbO₃ crystals doped with varying concentrations of Zn²⁺ ions. Infrared spectroscopy revealed that when the Zn²⁺ ion doping concentration reached 7 mol%, Fe³⁺ and Cu²⁺ ions occupied Nb sites in the lattice, leading to the formation of defect structures ${\mathrm{F}\mathrm{e}}_{\mathrm{N}\mathrm{b}}^{2-}$ and ${\mathrm{C}\mathrm{u}}_{\mathrm{N}\mathrm{b}}^{3-}$. The incorporation of Zn²⁺ ions significantly enhanced the exposure energy of the crystals. Notably, sample Zn7 emerged as the optimal material for mitigating light-induced scattering effects, exhibiting an Er value of 786.55 J/cm², approximately 416 times higher than that of Zn1. The two-wavelength coupling experiments demonstrated that the holographic storage properties improved with increasing Zn²⁺ ion concentration. Among the tested samples, Zn7 exhibited the shortest response time (196.4 s) and the highest values for key performance metrics: a dynamic range (M/#) of 9.81, a saturation diffraction efficiency (η_s) of 66.7%, and a sensitivity (S) of 1.04. These parameters are critical for advancing volume holographic data storage technologies. Overall, these findings underscore the potential of Zn:Cu:Fe:LiNbO₃ crystal, particularly at a Zn²⁺ ion doping concentration of 7 mol%, as a high-performance material for holographic storage applications. These advancements could pave the way for more efficient, reliable, and high-capacity holographic storage systems, addressing the growing demand for advanced data storage solutions in the era of big data and beyond.