Alkali rare earth sulfate crystal and use thereof

Abstract

This invention provides an alkali metal rare earth sulfate crystal and its application. The chemical formula of the alkali metal rare earth sulfate crystal is ARE(SO4)2, belonging to the non-centrosymmetric monoclinic crystal system, wherein A is an alkali metal element selected from any one of K, Rb, and Cs; RE is a rare earth element selected from any one of La, Ce, and Y. In the embodiments of this invention, the obtained RbLa(SO4)2 crystal exhibits excellent high-temperature resistance and long-term stability; at a wavelength of 880 nm, the RbLa(SO4)2 crystal... p The birefringence value can reach up to approximately 85%, exhibiting strong non-centrosymmetric polarization characteristics; simultaneously, it maintains stable birefringence properties across a broad spectral range of 400-2000 nm; compared to existing similar crystal materials, this crystal material exhibits excellent second harmonic generation characteristics. These characteristics indicate that the crystal material of this invention has extremely broad application prospects in key nonlinear optical devices such as laser frequency conversion, and is expected to provide strong support for breakthroughs and innovations in related technologies.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of nonlinear optical materials, in particular to an alkali rare earth sulfate crystal and application thereof. BACKGROUND

[0002] Deep ultraviolet laser has important application value in many high-tech fields due to its high photon energy. As a core component of deep ultraviolet laser light source, solid-state laser plays an indispensable role. Among them, the second-order nonlinear optical crystal material occupies a crucial position in the key technology of deep ultraviolet solid-state laser. The practical application of the second-order nonlinear optical crystal applied in the field of deep ultraviolet solid-state laser needs to meet a series of strict requirements:

[0003] (1) Noncentrosymmetric structure: Noncentrosymmetric structure is a prerequisite for realizing the second-order nonlinear optical effect, because the central symmetric structure will result in zero polarizability, so that the second-order nonlinear optical effect cannot be generated;

[0004] (2) Wide optical transparent window: In order to ensure that the crystal can effectively transmit deep ultraviolet laser and avoid energy loss in the nonlinear conversion process, in general, the absorption cutoff edge of the crystal λ ≤200nm and the band gap energy E g >6.2eV;

[0005] (3) Large second harmonic generation efficiency (SHG): The effective nonlinear optical coefficient (d d eff ) of the crystal must be significantly larger than that of the reference material KDP (potassium dihydrogen phosphate) (0.39pm / V@1064nm);

[0006] (4) Moderate birefringence: Proper birefringence can realize phase matching in the deep ultraviolet region, and in general, the birefringence Δ n ≥0.06;

[0007] (5) Good physical and chemical stability: The crystal needs to have good physical and chemical stability such as anti-hydration, thermal stability, mechanical strength, and chemical corrosion resistance;

[0008] (6) Scale crystal growth: Easy to grow large size and high quality single crystal (>5mm) to meet the needs of industrial application;

[0009] (7) High laser damage threshold: The crystal should have a laser damage threshold not less than 1GW / cm²;

[0010] (8) Environmentally friendly performance: In the selection of materials, the green chemistry principle should be followed to avoid the use of toxic and harmful substances.

[0011] However, there are often mutual constraints between these requirements, making the development of new deep ultraviolet second-order nonlinear optical crystal materials a great challenge. Currently, although the reported KTiOPO4(KTP) crystal has a large frequency doubling coefficient, its ultraviolet cutoff edge is 400 nm, which cannot be transmitted in the deep ultraviolet band; β -BaB2O4(BBO) and LiB3O5(LBO) can be transmitted in the deep ultraviolet band, but cannot achieve phase matching. Chen Chuangtian et al. created KBe2BO3F2(KBBF) which can generate deep ultraviolet coherent light by direct frequency doubling, but KBBF crystal still has some defects. First, KBBF crystal has strong layered growth habit, and it is difficult to obtain single crystal with large thickness; second, beryllium oxide, a highly toxic substance, is used in the synthesis and growth process of KBBF crystal, which poses potential threat to human health and environment, which is not consistent with the current environmental protection development concept. Therefore, it is a major challenge to develop deep ultraviolet second-order nonlinear optical crystal materials that have superior performance and meet environmental protection requirements.

[0012] Rare earth metals have a series of unique physical and chemical properties due to their unique 4 f electronic structure, and have become a hot spot in the research of functional materials. Due to its unique electronic structure and large ionic radius, when the rare earth cation coordinates with the anion, it usually forms a highly distorted coordination geometry with a large dipole moment, which enables the structure motif centered on the rare earth cation to optimize the control of the second harmonic effect of the material. In addition, rare earth cations have the advantages of non-toxicity and environmental protection, which makes them an ideal material choice with great potential in the field of second-order nonlinear optical crystals.

[0013] Among many rare earth compounds, rare earth sulfate is of great concern due to its rich and diverse structures. As a non-pi conjugated anion, sulfate has excellent optical transparency, and as a polydentate ligand, sulfate can form various coordination modes with rare earth cations, building a variety of crystal structures. These structures endow rare earth sulfates with excellent performance, making them have broad application prospects in the field of nonlinear optical crystal materials. However, as the core device of deep ultraviolet solid-state laser, second-order nonlinear optical crystal materials usually need to withstand high temperature. Therefore, how to withstand high temperature without changing the crystal structure and maintaining good second-order nonlinear optical performance in practical application is still a scientific problem to be solved in the field of rare earth materials. SUMMARY

[0014] The purpose of the present application is to overcome at least one of the deficiencies of the prior art and provide an alkali rare earth sulfate crystal and its application.

[0015] The technical solution adopted by the present application is:

[0016] In a first aspect, the present application provides an alkali rare earth sulfate crystal, the chemical formula of the alkali rare earth sulfate crystal is ARE(SO4)2, and the alkali rare earth sulfate crystal belongs to a non-centrosymmetric monoclinic system.

[0017] A is an alkali metal element, and is any one of K, Rb, and Cs;

[0018] RE is a rare earth element, and is any one of La, Ce, and Y.

[0019] Optionally, the chemical formula of the alkali rare earth sulfate crystal is RbLa(SO4)2, and the unit cell parameters of the crystal are as follows: a = 5.4035(3) Å, b = 8.9554(5) Å, c = 6.9173(4) Å, α = 90°, β = 91.383(2)°, γ = 90°, Z = 2, and the unit cell volume is V = 334.63(3) Å 3 .

[0020] Optionally, the chemical formula of the alkali rare earth sulfate crystal is KLa(SO4)2, and the unit cell parameters of the crystal are as follows: a = 5.3776(3) Å, b = 8.7492(5) Å, c = 6.8744(4) Å, α = 90°, β = 91.415(2)°, γ = 90°, Z = 2, and the unit cell volume is V = 323.34(3) Å 3 .

[0021] Optionally, the preparation method of the alkali rare earth sulfate crystal comprises the following steps:

[0022] 1) mixing an alkali metal raw material, a rare earth raw material, and water to form an initial mixed raw material;

[0023] 2) synthesizing under hydrothermal conditions at 190-230°C for 96-120 h, and cooling to obtain the alkali rare earth sulfate crystal.

[0024] Optionally, the molar ratio of the alkali metal and the rare earth element in the alkali metal raw material and the rare earth raw material is 0.95-1.05.

[0025] Optionally, the alkali metal raw material is an alkali metal sulfate.

[0026] Optionally, the rare earth raw material is a rare earth sulfate.

[0027] Optionally, the cooling rate in step 2) is 0.09-0.11 ℃ / min.

[0028] In a second aspect, the alkali metal rare earth sulfate crystal provided by the first aspect of the present application is applied in a second-order nonlinear optical crystal material.

[0029] Optionally, the second-order nonlinear optical crystal material is a deep ultraviolet solid laser.

[0030] The present application has the following beneficial effects:

[0031] The present application ingeniously couples the optical properties of rare earth cations and alkali metal cations with the chemical properties of non-π conjugated sulfate groups, thereby optimizing the second-order nonlinear optical performance. Rare earth elements have unique 4 f electron configurations, large ionic radii, and rich coordination geometries, making them easy to form high-polarizability lattices with anion groups, which can significantly improve the macroscopic second-order nonlinear polarizability. The non-π conjugated sulfate group has a wide HOMO-LUMO energy gap and a large hyperpolarizability, giving the crystal excellent transmittance in the deep ultraviolet region. When the sulfate tetrahedron is coordinated with the rare earth cation, it is easy to produce a highly distorted and asymmetric local coordination environment, inducing a strong local dipole moment, thereby amplifying the second-order nonlinear optical response by several times. In addition, the alkali metal cation does not exist d-d and f-f transition, and is transparent in the ultraviolet region, thereby significantly improving the optical transparency of the material. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 is a schematic diagram of the crystal structure of the alkali metal rare earth sulfate of the present application, where RE = La, Ce, or Y; A = K, Rb, or Cs;

[0033] Figure 2 is a photo of the RbLa(SO4)2 crystal synthesized in Example 1;

[0034] Figure 3 a in is the ultraviolet spectrum of the RbLa(SO4)2 crystal, Figure 3 b in is the band gap calculation spectrum of the RbLa(SO4)2 crystal;

[0035] Figure 4 a in is the thermogravimetric spectrum of the RbLa(SO4)2 crystal, Figure 4 b in is the powder XRD spectrum of the RbLa(SO4)2 crystal;

[0036] Figure 5 This is a schematic diagram of a femtosecond laser-pumped nonlinear optical measurement system.

[0037] Figure 6 To be in the near-infrared range of 800-940nm, Figure 6 In this context, 'a' represents the wavelength-dependent SHG response of RbLa(SO4)2 pumped by a femtosecond laser. Figure 6 In the diagram, b represents the logarithmic plot of SHG intensity as a function of power. Figure 6 In this context, 'c' represents the SHG intensity of RbLa(SO4)2 and Y-cut quartz under the same conditions. Figure 6 In the figure, d represents the curve of the polarity SHG intensity of RbLa(SO4)2 as a function of the rotation angle of the linear polarizer;

[0038] Figure 7 The refractive index of RbLa(SO4)2 crystal is shown below. n x , n y , n z These are the three mutually perpendicular principal axes of the crystal;

[0039] Figure 8 For RbLa(SO4)2 and KDP crystal in λ Comparison of SHG signal spectrum at 1064nm. Detailed Implementation

[0040] The present invention will be further described below with reference to specific embodiments, but the present invention is not limited thereto.

[0041] This invention synthesizes a second-order nonlinear optical crystal by integrating rare-earth cations, alkali metal cations, and non-π-conjugated sulfate groups. The general molecular formula of the crystal is ARE(SO4)2 (RE = La, Ce, Y; A = K, Rb, Cs), and it exists in a non-centrosymmetric monoclinic crystal system. Pn Crystallization within a space group, structural diagram as shown below. Figure 1 As shown.

[0042] 1) Preparation of second-order nonlinear optical crystal materials

[0043] This invention provides a method for preparing second-order nonlinear optical crystal materials of alkali metal rare earth sulfates, comprising the following steps:

[0044] (1) Place a mixture of La2(SO4)3 (2.5 mmol), A2SO4 (A=K or Rb; 2.5 mmol) and deionized water (2-5 mL) in a 10 mL stainless steel high-pressure reactor lined with polytetrafluoroethylene;

[0045] (2) rapidly heated to 190-230℃, and kept at this temperature for 96-120h;

[0046] (3) after the reaction is completed, then slowly cooled to 30℃ at a rate of 0.1℃ / min, to obtain colorless transparent crystals.

[0047] Finally, the crystal structure information of the obtained RbLa(SO4)2 second-order nonlinear optical crystal is a = 5.4035(3) Å, b = 8.9554(5) Å, c = 6.9173(4) Å, α = 90°, β = 91.383(2) °, γ = 90°, Z = 2, and the unit cell volume is V = 334.63(3) Å 3 . The photo of the crystal is shown in Figure 2 , and is applied to subsequent characterization and performance test.

[0048] The crystal structure information of the KLa(SO4)2 second-order nonlinear optical crystal is a = 5.3776(3) Å, b = 8.7492(5) Å, c = 6.8744(4) Å, α = 90°, β = 91.415(2) °, γ = 90°, Z = 2, and the unit cell volume is V = 323.34(3) Å 3 .

[0049] In the above crystal, each lanthanum ion (La3+) is coordinated with ten oxygen atoms of seven sulfate groups to form a distorted [LaO 10 ] polyhedron. The [LaO 10 ] polyhedrons are connected by sharing edges to construct a two-dimensional [LaO 10 ] ∞ layer, and the alkali metal cations are located between the layers. This coordination mode significantly improves the macroscopic second-order nonlinear susceptibility.

[0050] The solution method is selected to synthesize the second-order nonlinear optical crystal. The solution method avoids the melt method which may grow non-same-component molten compounds that will decompose into other substances after heating. Meanwhile, the crystal can be obtained without high-temperature sintering, which can effectively avoid high-temperature calcination.

[0051] 2) Optical transparency test

[0052] The RbLa(SO4)2 crystal sample prepared in step 1) is tested on a Shimadzu UV3600i spectrophotometer ultraviolet-visible-near infrared spectrophotometer.

[0053] The RbLa(SO4)2 crystal obtained by the present application is found by ultraviolet-visible spectrophotometer test that when the incident light wavelength reaches 190 nm, the crystal sample still maintains a high transmittance of 90.5%, which is close to the theoretical limit value (a in Figure 3 The direct band gap width calculated by further band gap is 7.22 eV, which is highly consistent with the transmittance test result (b in Figure 3 The above data fully prove that the crystal has excellent deep ultraviolet transmittance performance, which has important application potential in deep ultraviolet optical devices, laser systems and ultraviolet lithography fields.

[0054] 3) Stability test

[0055] The RbLa(SO4)2 crystal sample prepared in step 1) is tested on a Mettler Toledo TGA / SDTA851 thermal analyzer thermogravimetric analyzer.

[0056] The RbLa(SO4)2 crystal obtained by the present application has no crystal lattice distortion of its crystal structure under a high temperature environment below 1000°C (a in Figure 4 At the same time, after the crystal is placed at room temperature for continuous storage for 12 months, X-ray diffraction analysis shows that its crystal structure has no phase change or crystal lattice distortion (b in Figure 4 The above experimental results show that the crystal material obtained by the present application has excellent high temperature resistance and long term stability, which provides a reliable basis for its application in high temperature environment and long term storage.

[0057] 4) Second-order nonlinear optical performance test

[0058] The crystal material RbLa(SO4)2 obtained by the present application uses a set of customized femtosecond laser pumped nonlinear optical measurement system to deeply evaluate the second-order nonlinear optical characteristics of RbLa(SO4)2. Under the premise of keeping the incident power unchanged, the excitation wavelength is gradually adjusted from 800 nm to 920 nm, and wavelength-dependent second harmonic generation measurement experiments are carried out.

[0059] The femtosecond laser pumped nonlinear optical measurement system is as shown in Figure 5As shown, the brief introduction is as follows: Spectra-Physics Solstice Maitai HP laser (80 MHz, 100 fs) is used as excitation source. The laser is modulated by a ultrafast variable attenuator (Newport VA-BB) to control the incident energy accurately, while keeping the pulse temporal characteristics unchanged; then the polarization direction is finely adjusted by a / 2 wave plate, and the femtosecond spot with a diameter of about 3 µm is obtained by focusing the sample through an Olympus BX43 microscope. The second harmonic (SHG) signal generated by the sample is first filtered by a band-pass filter to effectively shield ambient stray light, then collected by a 20× objective lens (NA = 0.45) and coupled to a SpectraPro HRS-300 high-resolution CCD spectrometer (spectral resolution < 0.1 nm) to realize rapid and high-fidelity detection of weak nonlinear signals. λ

[0060] As shown in the figure, the RbLa(SO4)2 crystal exhibits significant second harmonic generation response capability in the whole excitation wavelength range. At a wavelength of 880 nm, the intensity of the second harmonic shows a significant quadratic relationship with the input power, which powerfully proves that the second harmonic generation process of the crystal material obtained by the application has two-photon absorption characteristics. Under the excitation of 880 nm linearly polarized light, the second harmonic intensity of the RbLa(SO4)2 crystal is two orders of magnitude higher than that of Y-cut quartz. Due to the extremely strong energy of the second harmonic generation, which exceeds the measurement range of the current experimental device, the laser-induced damage threshold of RbLa(SO4)2 cannot be determined. Figure 6

[0061] Under the optimized linearly polarized excitation condition of 880 nm, the second harmonic intensity of the RbLa(SO4)2 crystal is two orders of magnitude higher than that of Y-cut quartz. d eff d eff The effective second-order nonlinear optical coefficient (d33) of RbLa(SO4)2 is estimated to be about 3.28 pm / V, which is significantly higher than the coefficient values of benchmark deep ultraviolet nonlinear optical crystals such as KBBF and KDP, fully indicating that the RbLa(SO4)2 crystal material exhibits excellent performance in nonlinear optical performance and has extremely high application potential. The polarization degree of the crystal material obtained by the application is calculated as follows: ρ I max -I min ) / (I max +I min ) I max I ​​​​​​min The results show that the RbLa(SO4)2 crystal material has a SHG efficiency of about 85% at 880 nm, which indicates that the SHG of the RbLa(SO4)2 crystal material not only has a significant anisotropy, but also exhibits a strong non-centrosymmetric polarization characteristic. ρ

[0062] As shown in Figure 7 , the nonlinear optical crystal prepared by the present application has a calculated birefringence Δ n =0.023 at a wavelength of 1064 nm. Further analysis shows that the crystal has stable birefringence performance in a wide spectral range of 400-2000 nm. This excellent birefringence characteristic is derived from the special non-centrosymmetric structure and electronic band characteristics of the crystal, which has important application value in the fields of laser modulation and polarized optical devices.

[0063] As shown in Figure 8 , the crystal obtained by the present application has excellent SHG performance through nonlinear optical performance test. The SHG efficiency of the crystal reaches 1.5 times that of the standard KDP crystal by using the Kurtz-Perry powder method under the irradiation of 1064 nm fundamental light. Compared with the existing alkali sulfate crystals, as shown in Table 1 below, the crystal of the present application has more outstanding SHG performance.

[0064] Table 1

[0065] Crystal chemical formula SHG efficiency Li8NaRb3(SO4)6 [1] ]]> 0.5 times KDP crystal (NH4)2Na3Li9(SO4)7 [2] ]]> 1.1 times KDP crystal NH4NaLi2(SO4)2 [2] ]]> 0.5 times KDP crystal [RbLa(SO4)2] 1.5 times KDP crystal

[0066] This outstanding nonlinear optical performance is derived from the special non-centrosymmetric structure (space group Pn ) and the electronic cloud distribution characteristics of the non-π conjugated system of the crystal. The excellent nonlinear optical performance of the crystal material obtained by the present application makes it have a very broad application prospect in the field of key nonlinear optical devices such as laser frequency conversion, and is expected to provide strong support for the breakthrough and innovation of related technologies.

[0067] In summary, the present application has the following beneficial effects:

[0068] (1) The integrated cation strategy in the present application is used to obtain a second-order nonlinear optical crystal material by a hydrothermal synthesis method.

[0069] (2) The obtained second-order nonlinear optical crystal material has high crystallinity, and the crystal structure can be analyzed by single crystal X-ray diffraction.

[0070] ​(3) The obtained second-order nonlinear optical crystal material is nontoxic and meets the current green and sustainable development concept.

[0071] (4) The alkali rare earth sulfate crystal material obtained through the synthesis strategy has good stability.

[0072] (5) The obtained second-order nonlinear optical crystal material has moderate birefringence and strong frequency doubling output performance.

[0073] (6) The obtained second-order nonlinear optical crystal material has a large band gap.

[0074] The application provides a novel second-order nonlinear optical crystal material synthesized based on rare earth sulfate and alkali metal through a solution method, and has the characteristics of nontoxicity, environmental protection, stable crystal structure, easy preparation and the like, and has large practical application potential.

[0075] The above is a further detailed description of the application, and cannot be regarded as a limitation on the specific implementation of the application. For ordinary skilled persons in the technical field to which the application belongs, simple deduction or replacement without departing from the concept of the application is within the protection scope of the application.

[0076] Reference:

[0077] [1]Yanqiang Li, Sangen Zhao, Pai Shan, etal .Li8NaRb3(SO4)6·2H2O as a new sulfated deep-ultraviolet nonlinear optical material[J]. Journal of Materials Chemistry C , 2018, 6(45): 12240-12244. (Li Yanqiang, Zhao Sangen, Shan Pai, et al. Li8NaRb3(SO4)6·2H2O as a new sulfated deep-ultraviolet nonlinear optical material[J]. Journal of Materials Chemistry C, 2018, 6(45): 12240-12244)

[0078] [2]Yanqiang Li, Fei Liang, Sangen Zhao, etal .Twonon-π-conjugated deep-UV nonlinear optical sulfates[J]. Journal of the American Chemical Society , 2019, 141(9): 3833-3837. (Li Yanqiang, Liang Fei, Zhao Sangen, et al. Two non-π-conjugated deep-UV nonlinear optical sulfates[J]. Journal of the American Chemical Society, 2019 141(9): 3833-3837.)

Claims

1. An alkali rare earth sulfate crystal, characterized by, Applied in deep ultraviolet solid laser, the chemical formula of the alkali rare earth sulfate crystal is RbLa(SO4)2, which belongs to non-centrosymmetric monoclinic system, and the space group is Pn The cell parameters of the crystal are as follows: a = 5.4035(3) Å, b = 8.9554(5) Å, c = 6.9173(4) Å, α = 90˚, β = 91.383(2)˚, γ = 90˚, Z = 2, and the cell volume is V = 334.63(3) Å 3 .

2. The alkali rare earth sulfate crystal according to claim 1, characterized by, The preparation method of the alkali metal rare earth sulfate crystal comprises the following steps: 1) mixing alkali metal raw material, rare earth raw material and water to form initial mixed raw material; 2) synthesizing under hydrothermal condition at 190-230 ℃ for 96-120 h, and cooling to obtain the alkali metal rare earth sulfate crystal.

3. The alkali rare earth sulfate crystal of claim 2, wherein The molar ratio of alkali metal and rare earth element in the alkali metal raw material and the rare earth raw material is 0.95-1.

05.

4. The alkali rare earth sulfate crystal of claim 2, wherein The alkali metal raw material is alkali metal sulfate.

5. The alkali rare earth sulfate crystal of claim 2, wherein The rare earth raw material is rare earth sulfate.

6. The alkali rare earth sulfate crystal of claim 2, wherein The cooling rate in the step 2) is 0.09-0.11 ℃ / min.

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