Nonlinear optical crystal and preparation method and application thereof

Abstract

The invention belongs to the technical field of nonlinear optical crystal material preparation, and particularly relates to a nonlinear optical crystal and a preparation method and application thereof. The chemical formula of the nonlinear optical crystal is Pb3O2 (IO3) 2, the nonlinear optical crystal belongs to an orthorhombic crystal system, and the space group is Pna21; and the cell parameters are as follows: # imgabs0 # # imgabs1 # alpha = beta = gamma = 90 degrees. The band gap value of the crystal is about 2.88 eV, the high transmittance of the crystal in the broadband range of 0.8-13 microns exceeds 90%, and the thermal stability of the crystal can reach 360 DEG C; a powder frequency doubling effect test shows that the frequency doubling coefficient of the crystal is 0.1 * KDP, and the birefringence of the crystal at the wavelength of 546 nm is up to 0.22, so that the crystal has important application potential in optical polarization devices.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of nonlinear optical crystal materials, and particularly relates to a nonlinear optical crystal, a preparation method thereof, and an application thereof. Background Art

[0002] Nonlinear optical crystals are core materials in modern optical technologies and have important applications in fields such as laser frequency conversion, optical modulation, and optical polarization devices. With the rapid development of laser technologies and optical devices, the performance requirements for nonlinear optical crystals are increasing day by day, especially for higher demands in aspects such as wide-band transmittance, thermal stability, mechanical strength, and optical properties (such as second harmonic generation effect and birefringence). The performance of nonlinear optical crystals directly determines the efficiency, stability, and application scope of optical devices. Therefore, the development of high-performance nonlinear optical crystals has become a research hotspot in the field of optical materials.

[0003] Currently, common nonlinear optical crystals include potassium dihydrogen phosphate (KDP), β-barium borate (BBO), and lithium niobate (LiNbO3), etc. These crystals exhibit excellent nonlinear optical properties in specific wavelength bands, but there are still many limitations in practical applications. For example, although the KDP crystal has a high transmittance in the visible light band, its transmittance in the ultraviolet and infrared bands is relatively low, which limits its application in wide-band optical devices. In addition, the mechanical strength of the KDP crystal is relatively low, it is easily deliquescent in air, and surface damage is likely to occur during long-term use, affecting its optical properties and service life. The β-barium borate (BBO) crystal has a high transmittance and a large nonlinear optical coefficient in the ultraviolet band, but its thermal stability is poor, and it is prone to phase change or decomposition at high temperatures, resulting in a significant decline in performance and making it difficult to adapt to complex working environments. In addition, the growth process of the BBO crystal is complex and the cost is high, which limits its large-scale application. The lithium niobate (LiNbO3) crystal performs well in electro-optic modulation and acousto-optic devices, but its birefringence is relatively low, making it difficult to meet the requirements of high-precision optical polarization devices and limiting its application potential in polarization devices.

[0004] In addition to the above crystals, some other nonlinear optical crystals such as potassium titanyl arsenate (KTA), gallium selenide (GaSe), etc. are also applied in specific fields, but these crystals also have their own limitations. For example, the KTA crystal has a high transmittance in the mid-infrared band, but its mechanical strength is relatively low, it is easy to break, and it is prone to deliquescence in a humid environment. The GaSe crystal performs well in the far-infrared band, but its thermal stability is poor and it is difficult to work stably at high temperatures.

[0005] In summary, the existing nonlinear optical crystals still have obvious deficiencies in aspects such as wide-band transmittance, thermal stability, mechanical strength, and optical properties, and it is difficult to meet the urgent needs of modern optical technologies for high-performance materials. Summary of the Invention

[0006] In order to solve the problems in the prior art, the object of the present invention is to provide a nonlinear optical crystal, a preparation method thereof and an application thereof. The crystal of the present invention has good optical properties and thermal stability.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] In the first aspect, the present invention provides a nonlinear optical crystal. The chemical formula of the nonlinear optical crystal is Pb3O2(IO3)2, which belongs to the orthorhombic system and the space group is Pna21; its unit cell parameters are ɑ = β = γ = 90°.

[0009] In the second aspect, the present invention provides a preparation method of a nonlinear optical crystal, which includes the following steps: mixing a raw material containing lead element, a raw material containing iodine element and a raw material containing oxygen element to obtain a mixture A, mixing the mixture A and water and stirring to obtain a mixture B, crystallizing the mixture B and cooling it to room temperature, and then filtering, washing and drying to obtain the nonlinear optical crystal; wherein, the pH value of the mixture B is not less than 10.

[0010] Preferably, the molar ratio of lead element, iodine element and oxygen element in the mixture A is 3:(2 - 7):(11 - 42).

[0011] Preferably, the raw material containing lead element is lead nitrate or lead oxide.

[0012] Preferably, the raw material containing iodine element is iodic acid or diiodine pentoxide.

[0013] Preferably, the raw material containing oxygen element is sodium hydroxide.

[0014] Preferably, the crystallization conditions are: crystallizing at 180°C - 200°C for not less than 120 hours.

[0015] Preferably, the cooling rate for cooling to room temperature is 5°C / h - 10°C / h.

[0016] In the third aspect, the present invention provides an application of a nonlinear optical crystal in the fields of preparing nonlinear optical devices and optical polarization devices.

[0017] Preferably, the nonlinear optical device is a laser frequency doubling device; the optical polarization device is a polarizing element.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The crystal of the present invention exhibits excellent properties in terms of optical performance and stability. Specifically, the bandgap value of the crystal of the present invention is approximately 2.88 eV, and it has a high transmittance of over 90% in the wide wavelength range from 0.8 μm to 13 μm; the crystal of the present invention is not easily fragmented and deliquescent in air, and its thermal stability can reach 360 °C, enabling it to adapt to complex environmental conditions and can be used for the preparation of non-linear optical devices and polarization devices. Through the powder second harmonic generation effect test, the second harmonic generation coefficient of the crystal of the present invention is measured to be 0.1 × KDP, and the birefringence of the crystal of the present invention at a wavelength of 546 nm is as high as 0.22, making it have important application potential in optical polarization devices.

[0020] Furthermore, the preparation method of the present invention adopts the hydrothermal synthesis method, which is simple to operate and suitable for large-scale preparation; at the same time, the raw materials used in the present invention are all chemicals that are easily obtainable and relatively inexpensive, reducing the production cost. Brief Description of the Drawings

[0021] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and thus should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0022] Figure 1 Schematic diagram of the Pb-O framework in the non-linear optical crystal of the present invention;

[0023] Figure 2 is Figure 1 Local enlarged view of point A in

[0024] Figure 3 Schematic diagram of the structure of the non-linear optical crystal of the present invention;

[0025] Figure 4 Ultraviolet diffuse reflection spectrum of the non-linear optical crystal of the present invention;

[0026] Figure 5 Infrared transmission spectrum of the non-linear optical crystal of the present invention;

[0027] Figure 6 Schematic diagram of the thermogravimetric decomposition of the non-linear optical crystal of the present invention;

[0028] Figure 7 Schematic diagram of the SHG signal of the non-linear optical crystal of the present invention;

[0029] Figure 8 Birefringence of the non-linear optical crystal of the present invention. Detailed Description of the Embodiments

[0030] To enable those skilled in the art to understand the features and effects of the present invention, the following provides a general description and definition of the terms and expressions mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used herein shall have the ordinary meaning understood by those skilled in the art with respect to the present invention. In case of conflict, the definition in this specification shall prevail.

[0031] The theories or mechanisms described and disclosed herein, whether correct or incorrect, shall not limit the scope of the present invention in any way, that is, the content of the present invention can be implemented without being limited by any specific theory or mechanism.

[0032] In this text, all features defined in the form of numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are only for the sake of brevity and convenience. Accordingly, the description of a numerical range or percentage range should be regarded as having covered and specifically disclosed all possible sub-ranges and individual numerical values within the range (including integers and fractions).

[0033] In this text, unless otherwise specified, "comprising", "including", "containing", "having" or similar terms cover the meanings of "consisting of" and "consisting essentially of". For example, "A comprises a" covers the meanings of "A comprises a and others" and "A consists only of a".

[0034] In this text, for the sake of concise description, all possible combinations of all technical features in each embodiment or example are not described. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each embodiment or example can be combined arbitrarily, and all possible combinations should be considered as being within the scope described in this specification.

[0035] The following further describes the present invention in detail with reference to the accompanying drawings:

[0036] The first object of the present invention is to provide a nonlinear optical crystal. The chemical formula of the crystal is Pb3O2(IO3)2, which belongs to the orthorhombic crystal system and the space group is Pna21; its unit cell parameters are ɑ = β = γ = 90°. As Figure 1 and Figure 2 shown, there are three coordination modes of Pb atoms. Pb1 coordinates with four oxygen atoms, and the bond length of Pb1 - O is between Among them. Pb2 coordinates with six oxygen atoms, and four of the oxygen atoms come from the IO3 group. The bond lengths of Pb2 - O1 and Pb2 - O2 are The bond length of Pb2 - O in Pb2 - O - I is Pb3 coordinates with three oxygen atoms, O3 comes from the IO3 group, and the bond lengths of Pb3 - O1 and Pb3 - 02 are The bond length of Pb3-O3 is It can be seen that Pb-O-I is much larger than the Pb-O bond length. Three kinds of lead coordinations form a "basket" - shaped basic unit with O3 bridging atoms as the "handle" and Pb1 as the bottom. The basic units are shared by the Pb1-O1-O2-Pb1 plane and form a Pb-O framework chain along the c-axis direction. The framework chains are interconnected along the a-axis with the IO3 group where O3 is located as the bridging unit, and finally form a layered structure in the ac plane.

[0037] The crystal of the present invention exhibits excellent properties in terms of optical performance and stability. Specifically, as Figure 4 shown, the band gap value of the crystal is about 2.88 eV, indicating its high optical transparency in the visible to near-infrared band. Secondly, as Figure 5 shown, the crystal has a high transmittance of more than 90% in the wide wavelength range of 0.8 μm to 13 μm, showing good light transmission performance.

[0038] In terms of stability, the crystal prepared by the present invention exhibits good physical and chemical stability. As Figure 6 shown, the crystal is not easily fragmented and deliquescent in air, and its thermal stability can reach 360 °C, capable of adapting to complex environmental conditions. It can be used for the preparation of nonlinear optical devices and polarization devices, and its high stability provides a reliable guarantee for its long-term use in practical applications.

[0039] In terms of nonlinear optical performance, through the powder second harmonic generation effect test, the second harmonic generation coefficient of the crystal is 0.1×KDP, indicating its moderate nonlinear optical effect. At the same time, as Figure 7 and Figure 8 shown, the birefringence of the crystal at a wavelength of 546 nm is as high as 0.22, making it have important application potential in optical polarization devices.

[0040] The second object of the present invention is to provide a preparation method of a nonlinear optical crystal, comprising the following steps:

[0041] Mix the raw material containing lead element, the raw material containing iodine element and the raw material containing oxygen element to obtain mixture A, mix mixture A and water and stir to obtain mixture B. After crystallization at 180 °C - 200 °C for not less than 120 hours and then cooling to room temperature, and then through filtration, washing and drying, the nonlinear optical crystal is obtained; wherein, the pH value of mixture B is not less than 10; the molar ratio of lead element, iodine element and oxygen element in mixture A is 3:(2 - 7):(11 - 42); the cooling rate for cooling to room temperature is 5 °C / h - 10 °C / h.

[0042] In the present invention, the lead element (Pb2+ ) As a cation in the crystal, it can form a stable coordination structure with iodate (IO3 - ) and oxygen ions (O 2- ) to construct the basic framework of the crystal; lead ions have a relatively large ionic radius and high polarizability, which can enhance the nonlinear optical effect of the crystal, making it have excellent performance in fields such as laser frequency conversion and optical modulation. Iodine exists in the crystal in the form of iodate (IO3 - ). Iodate has a non-centrosymmetric structure, which is the key factor for the crystal to generate nonlinear optical effects. Oxygen participates in the formation of the crystal structure in the form of oxygen ions (O 2- ), and together with lead ions and iodate, it constructs a stable crystal framework. Among them, oxygen is introduced in the form of sodium hydroxide (NaOH), which can adjust the pH value of the mixture to ensure that the reaction environment is alkaline (pH≥10), which is beneficial to the growth and stability of the crystal.

[0043] The raw material containing lead element is lead nitrate (Pb(NO3)2) or lead oxide (PbO). Lead nitrate and lead oxide have high solubility in water, which is convenient for uniform mixing. Both can react quickly with iodic acid or iodine pentoxide to produce the target crystal. In addition, lead nitrate and lead oxide are common lead salts with low prices, which are suitable for industrial production.

[0044] The raw material containing iodine element is iodic acid (HIO3) or iodine pentoxide (I2O5). Iodic acid and iodine pentoxide can provide iodate (IO3 - ), and its non-centrosymmetric structure is the key for the crystal to generate nonlinear optical effects. Iodic acid and iodine pentoxide can react quickly with lead salts to produce the target crystal, and iodic acid and iodine pentoxide are stable in an alkaline environment, which is suitable for the reaction conditions with a high pH value.

[0045] The raw material containing oxygen element is sodium hydroxide (NaOH). Sodium hydroxide can effectively adjust the pH value of the mixture to ensure that the reaction environment is alkaline (pH≥10), which is beneficial to the growth and stability of the crystal. Sodium hydroxide decomposes into Na + and OH - in aqueous solution, and OH - can jointly construct the crystal structure with lead ions and iodate. In addition, sodium hydroxide is a common alkaline substance with a low price, which is suitable for industrial production.

[0046] The present invention prepares a single-crystal pure-phase nonlinear optical crystal by the hydrothermal synthesis method, adopting the methods of solution mixing and crystallization, which is simple to operate and suitable for large-scale preparation; at the same time, the raw materials used in the present invention are all chemicals that are easy to obtain and relatively low in price, reducing the production cost.

[0047] The third object of the present invention is to provide an application of a nonlinear optical crystal in the field of preparing nonlinear optical devices; wherein, the nonlinear optical device is a laser frequency doubling device. The crystal of the present invention has the characteristics of high transmittance in a wide wavelength band, can efficiently transmit laser beams, reduce energy loss, and thus improve the conversion efficiency of the laser frequency doubling device. Compared with the problem of low transmittance of traditional crystals (such as KDP) in the ultraviolet or infrared wavelength bands, the excellent light transmission performance of the crystal of the present invention in a wide wavelength band provides an important guarantee for its application in laser frequency doubling devices. Through the powder frequency doubling effect test, the frequency doubling coefficient of the crystal of the present invention is 0.1×KDP, enabling it to effectively achieve the conversion of laser frequency, while avoiding energy loss or device damage caused by too strong nonlinear effects, and showing good balance and practicability in laser frequency doubling devices. The crystal of the present invention can still remain stable at a temperature as high as 360°C, enabling it to adapt to the heat generated by the laser frequency doubling device under high-power laser irradiation, and avoiding performance degradation or structural damage caused by temperature rise. Compared with the problem that traditional crystals are prone to phase change or decomposition in a high-temperature environment, the high thermal stability of the crystal of the present invention provides a reliable guarantee for its long-term stable operation in a complex working environment. The crystal of the present invention is not easily broken or deliquescent in air, showing excellent physical and chemical stability. This characteristic not only extends the service life of the laser frequency doubling device, but also reduces the maintenance cost, making it more economical and reliable in practical applications.

[0048] The fourth object of the present invention is to provide an application of a birefringent crystal in the field of preparing optical polarization devices; wherein, the polarization device is a polarizer. The birefringence of the crystal of the present invention is as high as 0.22 at a wavelength of 546 nm, enabling it to effectively separate and control the polarization state of light waves, and thus improve the performance of the polarization device. Compared with the problem of low birefringence of traditional crystals (such as LiNbO3), the crystal of the present invention shows stronger competitiveness in high-precision optical polarization devices. The crystal of the present invention has the characteristics of high transmittance in a wide wavelength band, enabling it to efficiently transmit polarized light in a wide wavelength band, reduce energy loss, and thus improve the working efficiency and application range of the polarization device. The crystal of the present invention is not easily broken or deliquescent in air, and its thermal stability can reach 360°C, enabling it to work stably for a long time under complex environmental conditions, and avoiding performance degradation or device damage caused by environmental changes. Compared with the problem that traditional crystals are prone to failure in a humid or high-temperature environment, the crystal of the present invention provides an important guarantee for the reliability and durability of the polarization device.

[0049] The following further elaborates the present invention in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0050] Conventional instruments and equipment in the art are used in the following examples. For the experimental methods without specific conditions noted in the following examples, they are generally carried out under conventional conditions or according to the conditions recommended by the manufacturer. Various raw materials are used in the following examples. Unless otherwise stated, commercially available products are used, and their specifications are the conventional specifications in the art. In the specification of the present invention and the following examples, unless otherwise specified, "%" represents weight percentage, "parts" represents weight parts, and the ratios represent weight ratios.

[0051] Example 1

[0052] 3 moL of Pb(NO3)2, 7 moL of HIO3 and 3 moL of NaOH were mixed to obtain mixture A. Mixture A was mixed with H2O to obtain mixture B, and the pH value of mixture B was 10. Mixture B was sealed in a hydrothermal reaction kettle and placed in a programmable box furnace. It was crystallized at 200 °C for 120 h, then cooled to room temperature at a rate of 5 °C / h, and then obtained a colorless transparent nonlinear optical crystal after filtration, washing and drying.

[0053] Example 2

[0054] 3 moL of Pb(NO3)2, 1 moL of I2O5 and 5 moL of NaOH were mixed to obtain mixture A. Mixture A was mixed with H2O to obtain mixture B, and the pH value of mixture B was 10. Mixture B was sealed in a hydrothermal reaction kettle and placed in a programmable box furnace. It was crystallized at 200 °C for 120 h, then cooled to room temperature at a rate of 6 °C / h, and then obtained a colorless transparent nonlinear optical crystal after filtration, washing and drying.

[0055] Example 3

[0056] 3 moL of PbO, 1 moL of I2O5 and 3 moL of NaOH were mixed to obtain mixture A. Mixture A was mixed with H2O to obtain mixture B, and the pH value of mixture B was 10. Mixture B was sealed in a hydrothermal reaction kettle and placed in a programmable box furnace. It was crystallized at 190 °C for 130 h, then cooled to room temperature at a rate of 8 °C / h, and then obtained a colorless transparent nonlinear optical crystal after filtration, washing and drying.

[0057] Example 4

[0058] 3 moL of PbO, 4 moL of HIO3 and 2 moL of NaOH were mixed to obtain mixture A. Mixture A was mixed with H2O to obtain mixture B, and the pH value of mixture B was 11. Mixture B was sealed in a hydrothermal reaction kettle and placed in a programmable box furnace. It was crystallized at 180 °C for 140 h, then cooled to room temperature at a rate of 9 °C / h, and then obtained a colorless transparent nonlinear optical crystal after filtration, washing and drying.

[0059] Example 5

[0060] Mix 3 mol PbO2, 6 mol HIO3 and 6 mol NaOH to obtain mixture A, mix mixture A and H2O to obtain mixture B, and the pH value of mixture B is 11; seal mixture B in a hydrothermal reaction kettle, put it into a programmable box furnace, crystallize at 180 °C for 140 h, then cool it to room temperature at a rate of 10 °C / h, and then obtain a colorless and transparent nonlinear optical crystal after filtration, washing and drying.

[0061] Example 6

[0062] Mix 3 mol PbO2, 5 mol HIO3 and 8 mol NaOH to obtain mixture A, mix mixture A and H2O to obtain mixture B, and the pH value of mixture B is 11; seal mixture B in a hydrothermal reaction kettle, put it into a programmable box furnace, crystallize at 180 °C for 150 h, then cool it to room temperature at a rate of 10 °C / h, and then obtain a colorless and transparent nonlinear optical crystal after filtration, washing and drying.

[0063] Crystal structure analysis of nonlinear optical crystal Pb3O2(IO3)2:

[0064] Use single-crystal X-ray diffraction and powder X-ray diffraction methods to analyze the structures of the crystals prepared in Examples 1-6.

[0065] Among them, single-crystal X-ray diffraction is carried out on an Agilent SuperNova Dual Wavelength CCD X-ray single-crystal diffractometer. The data collection temperature is 100 K, and the diffraction light source is monochromatized Mo-Kα rays. The data is processed by the Multi-Scan method for absorption correction. The structure analysis is completed using the SHELXTL-2014 program package; the positions of heavy atoms are determined by the direct method, and the coordinates of the remaining atoms are obtained by the difference Fourier synthesis method; all atomic coordinates and anisotropic thermal parameters are refined by the full-matrix least-squares method based on F 2 2.

[0066] Among them, the single-crystal X-ray diffraction results show that the crystal prepared in Example x has the chemical formula Pb3O2(IO3)2, belongs to the orthorhombic system, and the space group is Pna21; its unit cell parameters are ɑ = β = γ = 90°, and its crystal structure is as Figure 3 shown.

[0067] Diffuse reflectance absorption spectrum and infrared transmission spectrum test:

[0068] The diffuse reflectance absorption spectrum test using the crystal prepared in Example 1 was carried out on a Lambda-750 ultraviolet-visible-near-infrared spectrophotometer produced by Perkin-Elmer Corporation in the United States. The crystal sample was ground into powder, and BaSO4 was used as the reference substrate. The test results are as Figure 4 shown. The band gap is about 2.88 eV, and the ultraviolet cut-off edge is about 430 nm.

[0069] The infrared transmission spectrum test using the crystal prepared in Example 1 was carried out on a Fisher Nicolet S50 FT-IR spectrometer, and the wave number range was 4000 - 400 cm -1 . The test results are as Figure 5 shown. The crystal has a high transmittance of over 90% in the wide wavelength range from 0.8 μm to 13 μm, showing good light transmission performance.

[0070] Second harmonic generation test experiment and results:

[0071] The second harmonic generation test experiment using the crystal prepared in Example 1 is specifically as follows: The laser with a wavelength of 1064 nm generated by a Q-switched Nd:YAG solid laser with a frequency converter was used as the fundamental light, irradiated the crystal powder to be tested, and a photomultiplier tube was used to detect the generated second harmonic, and an oscilloscope was used to display the harmonic intensity. Under the same test conditions, the intensity of the second harmonic generated by the sample to be tested was compared with the intensity of the second harmonic generated by the reference crystal KH2PO4 (KDP), so as to obtain the relative magnitude of the second harmonic generation effect of the sample.

[0072] The test results show that the second harmonic generation coefficient of the compound Pb3O2(IO3)2 is about 0.1 times that of the KH2PO4 (KDP) crystal.

[0073] Birefringence theoretical calculation:

[0074] The CASTEP (Cambridge Serial Total Energy Package) module of Materials Studio software was adopted. The CASTEP software is an ab initio quantum mechanics program based on the method, which can predict various properties through the number and type of atoms, including lattice parameters, band structures, structural properties, molecular symmetries, solid state densities, wave functions, charge densities, and optical properties. The CASTEP software provides an ab initio method for molecular dynamics, which can be used to simulate the solid, interface, and surface properties of various material systems such as metals, semiconductors, and ceramics. Using the cif file obtained by X-ray diffraction analysis, the birefringence property of the crystal of the present invention was calculated using the CASTEP software, and the calculation results are as Figure 8 shown. The birefringence of the crystal of the present invention at a wavelength of 546 nm is as high as 0.22.

[0075] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A nonlinear optical crystal, characterized in that: The chemical formula of the nonlinear optical crystal is Pb3O2(IO3)2, which belongs to the orthorhombic system and has a space group of Pna21. Its unit cell parameters are ɑ=β=γ=90°.

2. The method for preparing a nonlinear optical crystal according to claim 1, characterized in that: The following steps are involved: A raw material containing lead element, a raw material containing iodine element and a raw material containing oxygen element are mixed to obtain a mixture A, the mixture A and water are mixed and stirred to obtain a mixture B, the mixture B is crystallized and cooled to room temperature, and then filtered, washed and dried to obtain the nonlinear optical crystal; wherein the pH value of the mixture B is not less than 10.

3. The method for preparing a nonlinear optical crystal according to claim 2, characterized in that: The molar ratio of lead element, iodine element and oxygen element in the mixture A is 3:(2-7):(11-42).

4. The method for preparing a nonlinear optical crystal according to claim 2, characterized in that: The raw material containing lead element is lead nitrate or lead oxide.

5. The method for preparing a nonlinear optical crystal according to claim 2, characterized in that: The raw material containing iodine element is iodic acid or iodine pentoxide.

6. The method for preparing a nonlinear optical crystal according to claim 2, characterized in that: The raw material containing oxygen element is sodium hydroxide.

7. The method for preparing a nonlinear optical crystal according to claim 2, characterized in that: The crystallization conditions are: crystallization at 180° C. to 200° C. for no less than 120 hours.

8. The method for preparing a nonlinear optical crystal according to claim 2, characterized in that: The cooling rate to room temperature is 5°C / h to 10°C / h.

9. Use of the nonlinear optical crystal as claimed in claim 1 in the field of preparing nonlinear optical devices and optical polarization devices.

10. The use according to claim 9, characterized in that: The nonlinear optical device is a laser frequency doubling device; the optical polarization device is a polarizer element.

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