Lithium Niobate Crystal Preparation, Properties, and Its Application in Electro-Optical Devices

Abstract

Lithium Niobate (LiNbO₃, LN) crystals are multifunctional optical materials with excellent electro-optical, acousto-optical, and nonlinear optical properties, and their broad spectral transparency makes them widely used in electro-optical modulators, tunable filters, and beam deflectors. Near Stoichiometric Lithium Niobate (NSLN) crystals have a lithium to niobium ratio ([Li]/[Nb]) close to 1:1,demonstrate superior performance characteristics compared to composition lithium niobate (Congruent Lithium Niobate (CLN), [Li]/[Nb] = 48.5:51.5) crystals. NSLN crystals have a lower coercive field (~4 kV/mm), higher electro-optic coefficient (${\gamma }_{33}$ = 38.3 pm/V), and better nonlinear optical properties. This paper systematically reviews the research progress on preparation methods, the physical properties of LN and NSLN crystals, and their applications in devices such as electro-optical modulators, optical micro-ring resonators, and holographic storage. Finally, the future development direction of NSLN crystals in the preparation process (large-size single-crystal growth and defect control) and new electro-optical devices (low voltage deflectors based on domain engineering) is envisioned.

1. Introduction

LN crystals are versatile optical materials with great application value, and their broad spectral transparency (UV-to-mid-infrared wavelength bands), high electro-optic coefficient (${\gamma }_{33}$ = 30.8 pm/V) [1], and excellent chemical stability and mechanical strength have made them the core materials for integrated photonics and electro-optical devices [2,3,4,5]. Especially in the 400–500 nm visible wavelength band, their transmittance is as high as 95%, and combined with low optical loss (<0.1 dB/cm) and fast response characteristics (ps scale), LN crystals are widely used in high-efficiency electro-optic modulators [6,7,8], tunable filters [9,10,11], and nonlinear optical devices [12,13,14,15].

It has been shown that the physical properties of LN crystals strongly depend on the lithium-niobium ratio ([Li]/[Nb]) [16,17,18,19,20]. Commercial homocomponent lithium niobate (congruent lithium niobate, CLN) crystals with a [Li]/[Nb] ratio of 48.5:51.5 have a large number of intrinsic defects in their lattice, resulting in a coercive field as high as 21.1 kV/mm, which severely limits their application in domain engineering. On the other hand, Near Stoichiometric Lithium Niobate (NSLN) crystals have a [Li]/[Nb] ratio close to 1:1, and the intrinsic defects are reduced by increasing the Li content, which significantly optimizes crystal performance. For NSLN crystals, the nonlinear coefficient (d₃₃ = 44.3 pm/V) is improved [8], indicating that the electro-optical performance is significantly improved with Li content; its coercive field is reduced to 4 kV/mm [21,22], which enables low-voltage-driven domain inversion, i.e., domain modulation is more readily achieved with increasing Li content, and its absorption edge is blueshifted [23], and the transmittance is improved in UV wavelengths with better optical quality. These properties make NSLN an ideal material for the preparation of low-power electro-optical deflectors [24], high-density holographic storage, and other devices [25]. However, the industrialization of NSLN crystals still faces challenges, such as the complexity of the large-size single-crystal growth process and the difficulty in controlling the homogeneity of the components [26].

Based on this, this paper presents a systematic review of the research progress of LN and NSLN crystals in terms of crystal structure properties and defect mechanisms, growth methods (e.g., direct-drawing method, flux method, vapor transport equilibrium method, etc.), and performance optimization strategies and their applications in electro-optical devices (modulators, deflectors, etc.). Some problems are targeted, and proposed solutions are presented in the Summary section. Finally, the future development trends of NSLN crystals in the directions of domain engineering, thin film integration, and quantum light source are discussed, and these research results will provide an important theoretical foundation and technological support for the development of core photonic devices in the fields of next-generation high-speed optical communication and quantum information processing.

2. Physical Properties

2.1. Crystal Structure

At room temperature, lithium niobate belongs to the ferroelectric phase with a trigonal crystal system and space group R3c. The Nb and Li atoms undergo cooperative displacement relative to the center of the oxygen octahedra, causing a separation between the positive and negative charge centers, while the oxygen octahedra (NbO₆) tilt along the c-axis, and Li⁺ is located in the interstitial sites of the octahedra, resulting in the loss of centrosymmetry in the ferroelectric phase [27]. At high temperatures, it transforms into the paraelectric phase, also belonging to the trigonal crystal system with space group $R\overline{3}\mathrm{c}$. The displacements of Nb and Li atoms disappear, returning to the centrosymmetric positions within the oxygen octahedra, and the oxygen octahedra (NbO₆) adopt a more regular arrangement [28]. The structural differences between the phases are primarily reflected in atomic displacements and symmetry, and the positions of lithium and niobium atoms in the crystal structures of the ferroelectric and paraelectric phases are illustrated in Figure 1 [29].

2.2. Defects

A large number of defects occur due to the absence of Li within CLN crystals. For example, niobium antisite defects (${Nb}_{Li}^{4+}$), where niobium atoms occupy lithium sites [30] are possible because the ionic radius of Nb⁵⁺ of 64 pm is smaller than the ionic radius of Li⁺ of 74 pm (this value is based on a six-coordination environment due to the coordination of six oxygen atoms around the lithium sites [31]). The occupation of Li sites by Nb atoms leads to a local charge excess (+4), and the mechanism of its charge compensation can be found in two main theoretical models: the niobium vacancy model ($5N\mathrm{b}{}_{L\mathrm{i}}{}^{4+}+4V_{N\mathrm{b}}^{5-}$) [32,33] or the lithium vacancy model ($N{\mathrm{b}}_{L\mathrm{i}}^{4+}+4V_{L\mathrm{i}}^{-}$) [34,35,36,37]. A third defect model, in which lithium vacancies coexist with oxygen vacancies ($2V_{L\mathrm{i}}^{-}+V_O^{2+}$), has also been proposed historically [38], but structural tests [39] and electronic structure calculations [40] have shown that the probability of oxygen vacancies being present in lithium niobate is extremely low.

Determining the defect structure is challenging both experimentally and theoretically, leading to conflicting reports in the literature. Nevertheless, substantial evidence now supports the lithium vacancy model. Specifically, X-ray and neutron diffraction structural refinement analyses reveal that approximately 1% of Li sites are occupied by Nb atoms, with another 4% remaining vacant [35,39]. We also found data on defect quantities per cubic centimeter: in congruent lithium niobate (CLN), the defect densities per cubic centimeter are 4 × 10¹⁹ cm⁻³ for niobium antisites (${Nb}_{Li}^{4+}$) and 16 × 10¹⁹ cm⁻³ for lithium vacancies (${\mathrm{V}}_{\mathrm{L}\mathrm{i}}^{-}$). For near-stoichiometric lithium niobate (NSLN), the increased lithium content reduces these defect concentrations by one order of magnitude, resulting in 4 × 10¹⁸ cm⁻³ ${Nb}_{Li}^{4+}$ and 16 × 10¹⁸ cm⁻³ ${\mathrm{V}}_{\mathrm{L}\mathrm{i}}^{-}$ [18]. The scaling relationship between (${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+}$) and (${\mathrm{V}}_{\mathrm{L}\mathrm{i}}^{-}$) is in perfect agreement with the chemical formulae expected from the lithium vacancy model: $\left[L{\mathrm{i}}_{0.95}{V}_{L\mathrm{i}0.04}^{-}N{\mathrm{b}}_{L\mathrm{i}0.01}^{4+}\right]N\mathrm{b}{O}_3$, where the lithium site occupancy is shown in square brackets. Recent studies based on density-functional theory (DFT) have all pointed out that the $N{\mathrm{b}}_{L\mathrm{i}}^{4+}+4V_{L\mathrm{i}}^{-}$ combination of defects is indeed the most energetically stable configuration [41,42,43]. In contrast, NSLN crystals significantly reduce such defects due to the close stoichiometric ratio of lithium content ([Li]/[Nb] ≈ 1). The filling of vacancies by Li+ leads to a more complete lattice and a weakening of the octahedral distortion [44].

Since each niobium antisite(Nb_Li) requires four lithium vacancies(V_Li) in the neighborhood, possible chains such as Li-Nb_Li-□-Li, Li-□-Nb_Li-□, and Li-□-□-Nb_Li, etc. have been proposed as potential combinations. Figure 2 illustrates a possible schematic of the equilibrium state of Nb_Li and V_Li defect complexes in the upper and lower domains [45]. During domain reversal, it can be observed that lithium ions must rearrange to transition from the metastable state (a) to the stable defect equilibrium state (b). At room temperature, the lack of lithium mobility may lead to a frustrated state of defect complexes in (a), which can be released into state (b) after high-temperature annealing.

LN possesses an indirect bandgap of 3.65 eV, which is generally consistent with our experimental results; the valence and conduction bands of LN are primarily composed of contributions from the 2p orbitals of oxygen atoms and the 4d orbitals of niobium atoms, with the density of states peaks at −54 eV and −30 eV mainly contributed by the 4s and 4p orbital electrons of Nb atoms, while the peak at −15 eV is predominantly attributed to the 2s orbital electrons of O atoms [46]. In CLN crystals, the Fermi level position is primarily influenced by defect concentration. When defect concentration is low, the Fermi level resides near the valence band edge within the bandgap, where the most prevalent and stable intrinsic defects are niobium antisites (${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+}$) and lithium vacancies (${\mathrm{V}}_{\mathrm{L}\mathrm{i}}^{-}$), while niobium vacancies only become stable when the Fermi level approaches the conduction band minimum. As the Fermi level increases, ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+}$ point defects can capture one electron ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{3+}$ to form small polarons or simultaneously capture two electrons ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{2+}$ to form bipolarons, both of which introduce new optical absorption peaks in lithium niobate crystals. The bipolarons can serve as ${\mathrm{N}\mathrm{b}}_{\mathrm{L}\mathrm{i}}^{4+/2+}$ intrinsic photorefractive centers, representing the primary reason for the intrinsic photorefractive properties of CLN crystals. The fundamental origin of abundant intrinsic defects in CLN crystals lies in the variable valence state of niobium (Nb); therefore, controlling Nb valence variation will help reduce crystal defect density and regulate its optoelectronic characteristics [47].

2.3. Physical Properties

A comparison of the key properties of CLN and NSLN is shown in

Table 1. Comparison of CLN and NSLN performance [22].

	CLN	SLN
Tc (°C)	~1140	~1190
A	5.1499	5.1482
C	13.864	13.857
d31	6.1	6.3
d33	34.1	44.3
Ec (KV/mm)	~22	<4
${\mathsf{\gamma }}_{33}$ (pm/V)	31.5	38.3
Thermal expansion coefficient (×10−6$)$	$\mathsf{\alpha }$c = 2.7 $\mathsf{\alpha }$a = 19.2 [48]	$\mathsf{\alpha }$c = 2.8769 $\mathsf{\alpha }$a = 17.1807 [49]

. From the data in Table 1, it can be seen that compared with CLN crystals, NSLN crystals exhibit significant advantages in Curie temperature (T_c), lattice constants a and c, nonlinear electro-optic coefficient (d₃₃), coercive field (E_c), and electro-optic coefficient (${\gamma }_{33}$).

The Curie temperature (T_c) of LN crystals is closely related to the Li content, and the Curie temperature and crystal growth temperature are plotted with Li content, as shown in Figure 3 [50]. As the Li content increases, the Curie temperature becomes higher, and the Curie temperature reaches 1200 °C when the Li content approaches the stoichiometric ratio. With the increase in Li content, the crystal growth temperature increases first, and when the Li content reaches 48.5%, the crystal gradually approaches the stoichiometric ratio; at this time, the number of defects in the crystal structure decreases, and the lattice structure tends to be stable, and this stable structure reduces the energy required for crystal growth, and therefore, the growth temperature decreases, and when the Li content continues to increase, the crystal structure gradually tends to the ideal stoichiometric ratio, at this time. When the Li content continues to increase, the crystal structure gradually converges to the ideal stoichiometric ratio, at which time the ferroelectricity of the crystals increases and the growth temperature gradually converges to the Curie temperature. This is because the Curie temperature is a critical point where the stability and ferroelectricity of the crystal structure reach equilibrium, and near this temperature, the energy required for crystal growth is similar to the energy required to maintain its ferroelectricity. In summary, the concentration of internal defects in LN crystals decrease with the increase in Li content.

The variation of the magnitude of the coercive field with Li concentration is shown in Figure 4 [51]. According to Figure 4, the coercivity field shows a monotonically decreasing trend when the Li concentration content in the crystal increases from 48.5 mol% (CLN) to 49.9 mol% (NSLN). This change is closely related to the domain dynamics behavior, i.e., the Li-rich environment decreases the domain wall migration energy barriers [52,53,54], and the inversion process is more easily triggered; the electric domain boundaries of NSLN show higher geometrical regularity, and the domain walls are smoother [55]. Conventional domain engineering uses uniform planar electrodes to apply an electric field perpendicular to the Z-cut LN wafers, and the full-domain inversion of the bulk material can be triggered when the external field exceeds the coercive field (CLN~21.1 kV/mm, NSLN~4 kV/mm) [56]. However, this method makes it difficult to achieve precise manipulation of micro- and nanoscale domain patterns. To break through this limitation, Zhang X et al. [57] developed a scanning force microscopy (SFM)-based nano-domain writing technique, which is based on the principle of applying a bias voltage slightly higher than the coercive field of the NSLN crystals between the conductive SFM tip and the metal substrate and inducing the nano-scale domain inversion on the surface of the wafer by point-by-point scanning; the spatiotemporal nucleation–growth pattern of domains was derived by modulating the duration of the electric field. The temporal and spatial domain nucleation-growth patterns were derived by modulating the duration of the electric field; The hexagonal domain structure observed in NSLN and the triangular domain structure found in CLN are illustrated in Figure 5a–c [29]. Notably, compared to CLN crystals, NSLN exhibits a lower intrinsic defect concentration [58], which contributes to its unique properties. Under a strong external electric field applied along the +c direction, Li and Nb atoms drift toward the -c direction, with Li atoms traversing the oxygen plane to reach the opposite side, resulting in the reversal of the spontaneous polarization field direction and the domain orientation within the lattice.

NSLN crystals exhibit significantly better optoelectronic properties than CLN due to their composition closer to the ideal chemical ratios. The enhanced electro-optic performance is evidenced by an increase in the electro-optic coefficient (${\mathsf{\gamma }}_{33}$) by about 22% (from 31.5 pm/V for CLN to 38.3 pm/V for NSLN); And the enhanced nonlinearity is evidenced by an increase in the second-order nonlinear coefficient (d₃₃) by about 30% (from 34.1 to 44.3 pm/V) [22], as well as shorter absorption edges [59]. The absorption spectra near the absorption edges of the NSLN crystal and CLN crystal absorption spectra near the absorption edge are shown in Figure 6. According to Figure 6, the absorption spectra of NSLN crystals show a shift in the absorption edge to shorter wavelengths [60].

3. Crystal Preparation Methods

3.1. Lithium Niobate Crystal Preparation Method

The phase diagram of LN in the Li₂O-Nb₂O₅ binary system is shown in Figure 7 [45]. Currently, the main preparation methods for LN crystals include the direct drawing method, the Bridgman method, and so on.

3.1.1. Czochralski Method

The (Czochralski, CZ) method, as a classical method for growing LN crystals, due to its mature processing, uniform composition, and superior optical characteristics, has been extensively utilized [60]. Since the first successful growth of high-quality LN crystals by Ballman et al. [61] in 1965, the method has continued to evolve. Fukuda et al. [62] grew a lithium niobate crystal with dimensions of 20 × 1.5 × 40 mm using the Czochralski (Cz) method, and the growth schematic is shown in Figure 8. Chow et al. [63] achieved the growth of large-size crystals up to 2.25 inches in diameter; Shigematsu et al. [64] effectively suppressed the refractive index fluctuations induced by subgranular boundaries using a platinum crucible radio-frequency heating process; Thirumavalavan’s team [65], on the other hand, prepared 2-inch crack-free single-domain crystals to meet surface acoustic wave (SAW) device requirements. In terms of crystal quality optimization, Bhagavannarayana et al. [66] prepared 1-inch-diameter in situ-polarized LN crystals with the CZ method using a growth system developed in-house by the National Physical Laboratory (NPL) [67] in the United States, confirming that the post-growth annealing and polarization treatments can significantly improve the nonlinear optical and photovoltaic properties. Notably, the Li/Nb ratio of LN crystals has a decisive influence on their physical properties [51,68]. For this reason, Kan et al. [69] developed the continuous charging straight drawing method (Continuous Charging–Czochralski, CC-CZ), and they successfully grew 20 mm diameter crystals with uniformly controllable Li/Nb ratios (55.0–60.0 mol% Li₂O) from lithium-rich melts by accurately controlling the charging rate and the temperature field, and their lattice. The lattice parameters and unusual refractive index both show excellent consistency. In recent years, large-sized crystals have turned into a major development direction. Wang et al. [70] achieved the stable growth of 6-inch high-quality LN crystals with an isotropic ratio of 48.5 mol% Li₂O by using a low-cost automated diameter control system of their design, marking a new stage of industrial production of the technology, (Sanshou Ceramics Ltd., Japan). 8-inch lithium niobate crystals and wafers 8-inch lithium niobate crystals and wafers have been successfully industrialized. Domestically, Tiantong Holding Company Limited (Tiantong for short) and CLP Technology Deqing Huaying Electronics Company Limited (Deqing Huaying for short) prepared 8-inch lithium niobate crystals and wafers in 2000 and 2019, respectively, but they have not been industrialized for mass production [71].

3.1.2. The Bridgman Method

Although the CZ method is currently the dominant technology for the commercialization of LN and its doped single crystals [72], the process is still facing critical issues such as component ion polarization, crystal cracking, and coloration. To break through these limitations, researchers are working to improve the conventional technique and develop novel growth methods. For the optimization of the conventional CZ technique, Chen H et al. [73] proposed an innovative solution of using a sealed crucible combined with an alumina powder insulating layer design, which effectively suppressed melt volatilization and temperature fluctuations. This improvement significantly enhanced the stability of crystal growth. At the same time, alternative growth techniques have shown unique advantages. The modified Bridgman method has attracted attention because of its high efficiency and potential for large-scale production. This technique allows for the simultaneous growth of 6–8 pieces of 3-inch crystal rods in a single furnace, which dramatically increases the output efficiency [73]. The device diagram of the vertical Bridgman furnace is shown in Figure 9 [74], and the special design of the vertical Bridgman furnace enables it to have a small solid-phase temperature gradient, optimized liquid-solid interface control, and significantly reduced raw material cost, etc. Xia H et al. [75] successfully applied the Bridgman method to grow Cr³⁺-doped LN crystals (doping 0.1–0.5 mol%) and systematically investigated their luminescence characteristics. Compared with the CZ method, the Bridgman method is not only a relatively simple process; in addition to maintaining crystal quality, this approach greatly reduces manufacturing expenses [76], indicating promising industrial applicability.

In summary, as the most mature commercial production technology, the CZ method has achieved three breakthroughs through continuous innovation, from the early development of 1-inch to 8-inch large-size crystals, the use of the CC-CZ technique to achieve precise regulation of the Li/Nb ratio (55.0–60.0 mol% Li₂O), RF heating, in situ polarization, and other processes to effectively suppress defects. As an emerging alternative technique, the Bridgman method shows its unique value of high productivity, reduced raw material consumption, and simplified process. Together, the two methods have promoted the leap of LN crystals from laboratory research to industrial application, providing key material support for the development of optoelectronic devices.

3.2. Preparation of Lithium Niobate Crystals with a Close Stoichiometric Ratio

3.2.1. Czochralski Method

Double Crucible Czochralski (DCCZ) Method

In recent years, significant progress has been made in the growth technology of NSLN crystals, among which the dual crucible direct drawing method (DCCZ) has demonstrated unique advantages. Nalwa et al. [45] innovatively developed a DCCZ device equipped with an automatic powder supply system, and the schematic diagram of the DCCZ method device is shown in Figure 10. This system effectively solved the problem of component fluctuation in the conventional growth process by precisely controlling the raw material supply. Yao S.’s team [49] successfully applied this technique to grow high-quality SLN crystals, which verified the reliability of the DCCZ method. In terms of process optimization, Wang H et al. [77] prepared NSLN crystals from lithium-rich melts using the improved CZ method, and the X-ray rocking curve analysis confirmed the excellent crystalline quality. More strikingly, Wang F. et al. [7] successfully grew 3-inch NSLN single crystals using a large Pt crucible by optimizing the temperature gradient distribution through thermal field simulation. The test data show that the Li content is stable in the range of 49.65–49.78 mol%, the axial and radial component uniformity is good, and the crystal quality meets the demand of high-end optical applications. These technological breakthroughs have not only solved the problem of the large-size growth of NSLN crystals but also laid a solid foundation for their industrial production. By precisely controlling the thermal field parameters and raw material supply system, the researchers have successfully achieved the synergistic optimization of component homogeneity, crystalline quality, and growth efficiency. The resulting crystals, as shown in Figure 11, are transparent and free of macroscopic defects such as inclusions, bubbles, cracks, and twin boundaries. The maximum diameter of the NSLN crystals is about 80 mm, and the height of the isodiameter portion is 25 mm. The complexity and high cost of the double crucible direct-drawing method have not been reported for NSLN crystals grown over 4 inches [7,60].

Melt Supply Dual Crucible Method

Sun J et al. [60] innovatively developed the melt supply dual crucible technique, a schematic of the dual crucible technology setup for an automated melt supply system, as illustrated in Figure 12. This technique achieves large-size growth of NSLN crystals by precisely controlling the melt components. The core design of the system includes a lower crucible with lithium-rich melt [Li₂O]/[Nb₂O₅] = 58/42 as the crystal growth feedstock, and an upper crucible configured with a deviating stoichiometric ratio of compensating melt [Li₂O]/[Nb₂O₅] = 36/64, with the melt replenishment process accurately controlled by a computer. This melt supply system has the following significant advantages over conventional powder supply technologies [11,69]. The material replenishment accuracy is improved by about 30%, the melt diffusion rate is increased by 2–3 times, and the crucible cleaning process is simplified by 50%. The research team successfully prepared 3-inch high-quality NSLN crystals using this technique, as shown in Figure 13. The diameter of the crystal is 3 inches, the height is 2 inches, the radial composition deviation is 0.02 mol%, and the axial composition deviation is less than 0.03 mol%/cm. This breakthrough not only solves the technical problems of large-size growth of NSLN crystals but also provides a reliable technological route for their industrial production. The successful development of melt supply technology marks an important shift from empirical control to precise regulation of the LN crystal growth process.

Hanging Crucible Czochralski (HCCZ) Method

Yan T et al. [78] proposed an innovative Hanging Crucible Czochralski (HCCZ) method of direct pulling, which is based on a unique sluice-type powder feeding system. High-quality growth of SLN crystals was achieved. This technique adopts a suspended platinum sleeve design to replace the inner crucible structure of the conventional dual crucible method (DCCZ), combined with numerical simulations to optimize the growth parameters and the development of an accurately controllable sluice gate-type powder feeding mechanism. Compared with the conventional DCCZ method, the HCCZ technique demonstrates the advantages of the suspension design to reduce the thermal convection disturbance, single-shot growth to achieve a larger crystal yield, and the sluice gate feeding system to ensure precise control of the components. After optimizing the growth conditions, both SLN and MgOSLN crystals grown using the HCCZ method have single ferroelectric domains. Both types of crystals are transparent and free of cracks and inclusions. Using the proposed HCCZ system, the maximum diameter of the SLN crystals is about 70 mm. The high crystal quality produced provides the basis for better physical and optical properties, which are important for applications in the laser industry.

Zone-Leveling Czochralski (ZLCZ) Method

Tsai C B et al. [79] used the zone leveling Czochralski (ZLCZ) technique for the first time to grow MgO-doped SLN crystals. The schematic diagram of the combined technological setup of zone melting and the straight drawing method is shown in Figure 14a. During the crystal pulling process, the stoichiometric solid material is continuously supplied from below by lifting the crucible. In addition, the composition of the lithium-rich solution is the same as that used to grow SLN crystals in the DCCZ. However, bubble formation was found to be unavoidable in the case of solid feed [80]. To avoid bubble mixing, an inner crucible with small holes was used, which is shown in Figure 14b. This technological breakthrough not only expands the growth method of doped SLN crystals but also provides an innovative solution to the problem of bubble defects in functional crystal growth.

3.2.2. K₂O Cosolvent Method

The flux method is a method to adjust the melting point by adding a certain amount of flux (typically 6wt% K₂O) to the melt, thus obtaining SLN crystals. Malovichko G I et al. [59] attempted to grow LN single crystals from a melt with a Li/Nb ratio of 0.946. The stoichiometric specimen was successfully obtained after the addition of 6wt% K₂O to the melt. Remarkably, the specimen was essentially free of potassium, and the Li₂O content in the crystals was stable at (50 ± 0.15)mol%. Yatsenko A et al. [81], on the other hand, focused on the study of NSLN crystals (NSLN₁ and NSLN₂). The crystals they grew were from homogeneous melts containing excess alkali component K₂O (5wt% and 5.5wt%, respectively), and the NSLN crystals were grown from melts to which 5.0wt% and 5.5wt% K₂O were added, as shown in Figure 15. The growth process of such crystals belongs to the typical solution melting process, exactly the top seed crystal solution growth mode. The grown crystals have a high optical homogeneity, which is slightly less homogeneous than that of CLN crystals, but retains high-quality crystalline properties, and potassium does not enter the crystal structure during the entire growth process. The top seed crystal co-solvent method faces challenges such as many crystal defects, due to unstable components and other growth challenges, with the crystallization rate remaining low, making large-size crystal production difficult [82].

3.2.3. Vapor Transfer Equilibrium (VTE) Method

In addition to the aforementioned crystal growth methods, another effective way to obtain NSLN crystals is to employ the vapor transport equilibrium (VTE) technique. This method was first proposed by Holman et al. [83], which is based on the principle of placing CLN wafers in a lithium-rich atmosphere for high-temperature heat treatment, which enhances the Li₂O content in the crystals by Li⁺ ion diffusion into the crystal lattice [84]. During the development of VTE technology, Bordui et al. [85] systematically investigated the phase boundary relationship between lithium-poor, lithium-rich, and single-phase feedstocks, laying the foundation for subsequent studies. Subsequently, several researchers successfully prepared SLN or NSLN crystals using the VTE method. For example, Liang et al. [86] obtained NSLN crystals with Li₂O content >49.9 mol% and homogeneous and crack-free crystals by applying VTE to CLN crystals with different orientations (X-, Y-, and Z-cut) and thicknesses (1–3.2 mm) at 1100 °C in a Li-rich environment. The problem of the X- and Y-cut samples, which are prone to cracking, was effectively solved by optimizing the cooling rate. Bhatt et al. [87] similarly employed the VTE technique to convert CLN wafers into NSLN crystals through lithium-rich annealing [85,88], though currently limited to thin wafers. Dong et al. [89] subsequently replicated this approach for successful NSLN wafer fabrication. The diffusion device diagram is shown in Figure 16, where Figure 16a demonstrates a schematic diagram of the preparation of NSLN wafers, and Figure 16b shows the physical picture after assembly. The absence of twins and cracks on the surface of NSLN wafers is ensured by controlling the process parameters. The diffusion method, on the other hand, is a simple process but prone to wafer bending due to the non-uniform contraction of the lattice constant with the increase in lithium content, as shown in Figure 17. The VTE process is essentially a dynamic equilibrium between Li⁺ diffusion and the migration of the antisite defects of Nb⁵⁺, a mechanism that affects the structural integrity of the crystals. However, even if NSLN crystals are obtained, intrinsic point defects will still exist within them.

In summary, NSLN crystal growth methods are rich and diverse, each with its own features and advantages. As a traditional crystal growth method, the direct pulling (CZ) method, through continuous technological improvements, such as the dual crucible direct pulling (DCCZ) method, the melt-supplied dual crucible method, the hanging crucible direct pulling (HCCZ) method, the zonal leveling direct pulling (ZLCZ) method, and the DC direct pulling method, has already significantly improved the growth efficiency and quality of crystals. These techniques have successfully achieved synergistic optimization of component homogeneity, crystalline quality, and growth efficiency through precise control of the thermal field parameters and raw material replenishment system, laying a solid foundation for the large-size growth and industrial production of NSLN crystals. The double crucible direct drawing method is complex and costly. The K₂O co-solvent method adjusts the melting point by adding flux to obtain SLN crystals, which cannot only effectively regulate the melt properties but also does not pollute the crystal components. The top-seeded crystal co-solvent method faces the challenges of many crystal defects and unstable components, such as the slow growth rate, which makes it difficult to grow large-sized crystals. The vapor transport equilibrium (VTE) method provides another effective way to obtain NSLN crystals by boosting the Li₂O content in the crystals through high-temperature heat treatment, and although this method is currently only applicable to thin wafers, it avoids problems such as streakiness or local compositional inhomogeneity due to fluctuations in temperature at the growth interface. However, because of the non-uniform contraction of the lattice constant with the increase in Li content, which can easily lead to wafer bending. As technological advancements continue to evolve, these methods are anticipated to drive further innovations and breakthroughs in NSLN crystal growth and applications, with optimal growth routes being selected based on specific device requirements. Future breakthroughs will focus on the defect control of large-size crystals, improvement of doping uniformity, and optimization of the cost–benefit ratio.

4. Applications of Electro-Optical Devices

4.1. Electro-Optical Modulators

An electro-optical modulator (EOM), as a key device in optoelectronic systems, plays an irreplaceable role in the fields of optical communication, data center, 5G network, and quantum technology. Its core function is to achieve efficient conversion of electrical to optical signals, and its performance directly determines the transmission quality of the entire optical communication system [90].

Superior performance metrics compared to conventional bulk-material modulation devices. Oikawa et al. [91] innovatively designed Z-cut Ti: LiNbO₃ modulators with PR structure and branching electrodes. Its 4-cm electrode device achieved a low driving voltage of 4.1V at a rate of 10Gb/s, and the optical response fully meets the high-speed communication requirements. Ohmae et al. [92] developed an EOM using MgO-doped SLN crystals, which successfully realized the stable phase modulation of a 100W high-power laser. Experiments confirmed that the material can effectively suppress nonlinear optical effects, providing an ideal modulation solution for high-power laser systems.

Integrated modulator technology is again innovative. The silicon-LN hybrid ring modulator pioneered by Chen et al. [93] is groundbreaking. The device is designed with a 15 μm radius micro-ring and heterogeneous integration by BCB bonding to achieve 5 GHz bandwidth and 9 Gb/s high-speed modulation, with a voltage-length product down to 1.8 V cm [94], and the modulation frequency has been enhanced to 100 GHz by optimizing the structure of the traveling-wave electrodes [95] while the driving voltage can be reduced to less than 1 V [96], which is an electric field that interacts with the LiNbO₃ cladding in the portion of the optical modes, modifying the effective refractive index of the modes through a linear electro-optical effect. Fully compatible with complementary metal oxide semiconductor(CMOS) driver circuits.

Table 2. Performance parameters of different types of electro-optical modulators.

Typology	Bandwidths	Drive Voltage	Scenario
Traditional LN	10 GHz	4.1 V	General communications
MgO:SLN			High power systems
Si-LN hybrid	100 GHz	<1V	High-speed integrated systems

summarises the performance parameters of various types of EOMs in comparison.

In summary, although it is known that microstructured LN devices are limited by the difficulty of etching despite their superior performance [92], Ti diffusion/proton exchange waveguides are still predominantly used in most commercial devices [97]. It is particularly noteworthy that MgO-doped NSLN crystals have successfully expanded the boundary of EOM applications in high-power lasers by increasing the threshold of resistance to optical damage. Due to the exclusive polarization alignment of LNO along the z-axis, PPLN optimizes the performance of light polarized parallel to this axis, maximizing the utilization of the high ${\mathsf{\gamma }}_{33}$ coefficient. A schematic diagram of the periodically poled lithium niobate (PPLN) waveguide is shown in Figure 18 [29]. Positioning the waveguide on the surface of the PPLN wafer confines the pump beam throughout the entire interaction length, thereby significantly enhancing the conversion efficiency. Hu et al. [98] demonstrated a folded Mach-Zehnder interferometer modulator based on X-cut LNO thin film, accompanied by an effective polarization procedure for device activation. The proposed modulator design innovatively achieves a shorter device length without compromising its performance. A schematic diagram of a typical folded Mach-Zehnder interferometer modulator is shown in Figure 19 [29]. In the future, with the maturity of the thin-film LN preparation process and the optimization of the novel electrode structure, EOM will play an even more critical role in optical interconnections and quantum communications.

4.2. Optical Micro-Ring Resonators

LN-based micro-ring resonators may become the main choice for realizing highly compact electro-optical filter devices for LAN applications [99].

Regarding technological breakthroughs and performance enhancements, Guarino et al. [100] achieved the first optical micro-ring resonator in a sub-micron LN film with the innovation of preparing a high refractive index contrast film using an improved crystal ion-slice bonding technique to achieve an R = 100 µm micro-ring resonator demonstrating an electro-optic tuning rate of 0.14 GHz/V, which opens up a new avenue for the chip-scale integration of optical devices. Wang et al. [101] made an important breakthrough through a semiconductor standard process. The LN microdisk resonator with a thickness of 0.5 μm and a radius of 39.6 μm was prepared with a quality factor (Q) of 1.19 × 10⁶, which is 2.5 times higher than the previous record (4.84 × 10⁵ [102]), and achieves a high tuning rate of 3.0 GHz/V, which highlights the excellent electro-optical characteristics of LNs.

Regarding process optimization and performance limit, Zhang et al. [103] prepared sub-wavelength LN waveguides with a propagation loss of only 2.7 ± 0.3 dB/m through etching process innovation, achieving intrinsic Q (Q_L) ~10⁷ and loaded Q (Q_L) up to 5 × 10⁶. Demonstrating the ultra-high Q of near-critical coupling.

In summary, submicron LN thin-film resonators have achieved Q values exceeding 10⁶ orders of magnitude; tuning rates up to 3.0 GHz/V; and semiconductor process-compatible bulk preparation capabilities. These advances provide an ideal platform for applications such as tunable filters, nonlinear optical microcavities, and integrated quantum light sources. Future work should target lower propagation losses, improved process reproducibility, and integrated multifunctional platforms.

4.3. Electro-Optical Deflector

As a voltage-controlled beam-scanning device, the electro-optical deflector, with its GHz-class high-speed response, low optical loss, and easy integration [104,105,106,107,108], has shown important application prospects in the fields of optical communication switching and laser radar. In recent years, the technology has mainly achieved the following breakthroughs.

For breakthroughs in domain engineering technology, Bo et al. [109] used scanning force microscopy to achieve nanoscale domain engineering of SLN single crystals, domain inversion technology can flexibly construct domain structures of arbitrary shapes [110], which provides a new idea for device design [63], and the use of isosceles triangular arrays of electrodes can optimize the electric field distribution and reduce the operating voltage [111].

For advances in thin film devices, the crystal slicing technique developed by Djukic et al. [112], where single crystal films are prepared by He⁺ ion injection and wet etching [113], the LN-injected surface by RF sputtering, followed by slicing and post-stripping annealing. The sliced device was placed face down on a metallized LN support sample such that the electrode-covered film surface was in contact with the metallized substrate. Then, a SiO₂ buffer layer was deposited on the negative z-plane of the film by applying a non-conducting layer and then another LN block was bonded to the CIS film/bottom substrate structure with silver epoxy to complete the encapsulation of the CIS scanning device, which maintains the original domain structure while realizing the miniaturization of the device [112,114].

For optimization of waveguide-type devices, Wang et al. [115] designed an APE waveguide with a transverse refractive index contrast Δn≈0.001 and beam deflection control using sawtooth electrodes. A 1064 nm laser coupled to the input of the waveguide through a single-mode fiber with a spot size of 10 μm and a beam waist size of 4.5 μm. The input fiber is located in the middle of the waveguide and the output light is focused into an infrared CMOS camera. The integrated scheme of Cai et al. [116] supports electro-optical/thermo-optical dual-mode operation with a real-world dynamic deflection speed of 2.5 GHz.

However, the main challenges that are also currently faced are the high operating electric field requirement, limiting practicality [117], and the balance between the deflection angle and the driving voltage. Novel electrode structure design, heterogeneous integration technology, and low-power driving schemes are the future directions to promote its practical application in next-generation optical switching networks and lidar systems.

4.4. Holographic Storage

The international academic and industrial communities are actively promoting research on body holographic storage methods and materials [118,119]. Currently, commercial applications mainly use iron-doped lithium niobate (Fe:LN) crystals, which have high diffraction efficiency and sensitivity but still suffer from two key defects, such as long response time (milliseconds) and low resistance to optical damage. These limitations mainly stem from intrinsic defects (${Nb}_{Li}^{4+/5+}$) in the crystal. Near stoichiometric ratio iron-doped lithium niobate (Fe:NSLN) shows significant advantages: the response speed is improved by two orders of magnitude (microseconds) compared to Fe:CLN, making it an excellent red light holographic storage material, but the iron doping concentration needs to be controlled below 0.10% mass fraction to avoid the increase in photodamage [120]. Novel doping systems were also developed. Kong et al. [121] systematically investigated a variety of doping schemes. Single doping systems, tetravalent ion (Hf, Zr, and Sn) doping, and V and Mo doping have fast response and multi-wavelength storage properties. Composite doping systems, Hf/Fe, Zr/Fe double doping, Zr/Cu/Ce triple doping, and Zr/Fe/Mn triple doping show excellent non-volatile storage performance.

The main issues currently faced are Fe:LN response speed limiting commercial applications and optimization of the balance between doping concentration and performance. Therefore, the development of faster response multi-element doping systems, the optimization of the preparation process of near stoichiometric ratio materials, and the exploration of novel non-volatile storage mechanisms are the future development directions.

4.5. Lasers

Ti: Er: LiNbO₃ waveguide lasers have shown great potential in the fields of high-speed optical communications, precision sensing, and integrated photonics due to their excellent performance. A systematic review of recent advances in the field has been carried out by Becker C et al. [122], the coupling of up to 135 mW of pump radiation with a central wavelength of 1480 nm from a laser diode into a Ti: Er: LiNbO₃ waveguide resonator using fiber WDM.

Current research indicates that the output power and stability of such lasers can still be significantly improved by further optimizing waveguide loss and pump coupling efficiency, thus promoting their applications in emerging fields such as quantum optics or on-chip sensing.

For high-speed fiber communication applications, harmonic mode-locked Ti:Er:LiNbO₃ waveguide lasers have achieved repetition-rate pulse outputs of up to 10 GHz by adopting an integrated traveling-wave phase modulator as the mode-locker, which can directly satisfy the demand of the future 40 Gb/s communication systems without the need for additional optical time-division multiplexing [123,124,125]. These lasers have high peak pulse power and can excite optical solitons directly in the fiber, eliminating the need for amplification. Notably, harmonic mode-locked lasers make full use of the excellent electro-optical properties of LN crystals [126], but attention should be paid to the problem of high-frequency beat noise due to their possible supermodel oscillations.

In the field of sensing and measurement, switching lasers have been commercially used in applications such as distance measurement and collision avoidance sensing. Thanks to the excellent properties of LN crystals, such as high Er³⁺ doping concentration (up to the solid solubility limit without significant fluorescence quenching) and long fluorescence lifetime, these lasers have excellent energy storage capacity and power conversion efficiency. Theory predicts that their peak power can reach the order of kilowatts [127]. In addition, the excellent electro-optical characteristics of LNs allow for monolithic integration of intracavity switches [128], which facilitates compact and reliable laser designs.

For wavelength division multiplexing (WDM) systems, narrow linewidth DBR lasers exhibit unique advantages. An integrated optical Ti:Er:LiNbO₃ DBR laser with single-frequency operation has been realized by etching the surface grating with a holographically defined ion beam [129]. Its emission wavelength is determined by the grating period and it is easy to integrate monolithically with components such as external modulators, providing an ideal solution for WDM systems.

Self-doubling lasers [130] are expected to be pump sources for low-noise wavelength converters within the 1.55 μm communication window. In addition, intracavity integration of lasers with quasi-phase-matched parametric devices (e.g., self-pumped integrated optical parametric oscillators) will also be an important research direction.

Overall, Ti:Er:LiNbO₃ waveguide lasers are bound to play a more important role in the field of optical communication and photonic integration due to their excellent performance and integration advantages. In the future, by further optimizing their mode-locking stability, increasing their output power, and enhancing device integration, these lasers are expected to play an important role in more critical fields.

5. Summary and Outlook

LN crystals have shown significant application value in electro-optical modulators, optical micro-ring resonators, electro-optical deflectors, holographic storage, and lasers due to their excellent optoelectronic properties and chemical stability. NSLN crystals significantly reduce the intrinsic defect concentration by optimizing the lithium-to-niobium ratio, which improves electro-optical coefficients, nonlinear coefficients, and other key properties and at the same time lowers the coercive field, which makes them more advantageous in domain inversion and integrated photonics devices. In recent years, the direct drawing method, fluxing method, and fluxing method have been used. In addition, in recent years, advances in crystal growth techniques such as the direct drawing method, flux method, and vapor transport equilibrium method have promoted the preparation of large-size and high-quality NSLN crystals, laying the foundation for the development of high-performance electro-optical devices. However, the practical application of LN/NSLN crystals still faces key challenges.

The component homogeneity of large-size NSLN crystals is still difficult to precisely regulate, especially since the gradient distribution of the lithium-niobium ratio has not been completely solved, leading to spatial inhomogeneity of electro-optical performance. After many theoretical and experimental studies, our group proposes to grow NSLN crystals by means of a combination of lithium-rich direct-drawing and diffusion methods, which can solve the problems of component segregation and the small size of the grown crystals, etc. The growth of NSLN crystals by means of lithium-rich direct-drawing and diffusion methods is undergoing further research.

Existing periodic polarization techniques, such as electric field polarization, in NSLN reduce the coercive field, but the stability of the domain wall is insufficient and is prone to degradation under a high-frequency electric field, which affects the reliability of the device. Using femtosecond laser-induced domain inversion technology, submicron periodic domain structures are prepared in NSLN, combined with in situ electric field modulation to achieve high-precision domain wall positioning and solve the problem of domain diffusion in conventional electric field polarization.

Heterogeneous integration of NSLN thin films with silicon-based platforms faces the problems of thermal expansion coefficient mismatch and interface loss, which limits their application in high-density photonic integrated circuits. Wafer bonding and selective etching techniques are used to insert a stress buffer layer between the NSLN film and the silicon waveguide to reduce the interface loss.

To give full play to the potential of LN/NSLN crystals in high-speed optical communications, quantum technology, high-power laser systems, and other fields, future research can focus on the following directions:

(1)

Domain structure design: Explore the influence of domain patterns, such as periodic polarization, on electro-optical performance and develop efficient and low-power beam deflectors and modulators.

(2)

Heterogeneous integration technology: Combine NSLN thin films with silicon-based photonic platforms to develop hybrid integrated electro-optical devices and promote the development of compact photonic chips.

(3)

Quantum light source: Tap the potential of NSLN crystals in quantum light sources, such as the generation of entangled photon pairs and holographic storage, to meet the needs of next-generation optoelectronic integration.

In the future, with the further development of thin-film preparation processes and domain engineering technology, NSLN crystals are expected to play a more important role in high-speed optical communications, quantum technology, high-power laser systems, and other fields to meet the needs of next-generation optoelectronic integration devices and provide key solutions for integrated photonics, quantum information technology, and other fields.