CN110578173B - A kind of nonlinear optical crystal strontium lithium silicon sulfur and its preparation method and application - Google Patents

Abstract

The invention provides a nonlinear optical crystal strontium lithium silicon sulfur and a preparation method and application thereof. The chemical formula of the nonlinear optical crystal strontium lithium silicon sulfur is SrLi ₂ SiS ₄ , the molecular weight is 257.83, and it is colorless and transparent non-centrosymmetric. Structural single crystal, with [LiS ₄ ], [SiS ₄ ] and [SrS ₈ ] groups as structural units. The present invention adopts a high-temperature solid-phase synthesis method to synthesize nonlinear optical crystal strontium lithium silicon sulfur, which has excellent optical properties, long infrared absorption cut-off edge, wide band gap, high laser damage threshold, and large nonlinear optical coefficient. Infrared nonlinear optical crystals have a wide range of potential applications in high-power infrared laser systems.

Description

A kind of nonlinear optical crystal strontium lithium silicon sulfur and its preparation method and application

technical field

The invention relates to a laser frequency doubling crystal in the mid-far infrared band, in particular to a nonlinear optical crystal strontium lithium silicon sulfur and a preparation method and application thereof.

Background technique

With the advent of lasers, a series of novel optical phenomena have been discovered one after another, one of which is nonlinear optical effects. In 1961, Franken irradiated a beam of ruby laser on a quartz crystal and discovered the frequency doubling effect of nonlinear optics for the first time, which opened the prelude to the research of nonlinear optical materials.

The nonlinear optical effect originates from the interaction between the laser and the medium. When a laser beam with a certain polarization direction passes through a nonlinear optical crystal (such as AgGaS ₂ ) in a certain incident direction, the frequency of the beam will change. When the laser propagates in a medium with non-zero second-order polarizability, nonlinear optical effects such as frequency doubling, sum frequency, difference frequency, and optical parametric amplification will occur. As a coherent monochromatic light source with high intensity and good directionality, laser is widely used in scientific research, industry, transportation, national defense, medical and health and other related fields. To find a practical laser medium, the use of nonlinear optical crystals for frequency conversion to obtain various laser light sources with wide tuning has become a frontier topic in the development of laser technology.

Nonlinear optical crystal materials can be divided into three categories according to the range of their transmission bands: mid- and far-infrared nonlinear optical materials, visible light and near-infrared band nonlinear optical materials, and ultraviolet and deep ultraviolet band nonlinear optical materials. At present, the main nonlinear optical materials mainly include KDP (KH ₂ PO ₄ ), BBO (β-BaB ₂ O ₄ ), LBO (LiB ₃ O ₅ ) and AGS (AgGaS ₂ ). The output of infrared light sources, especially mid- and far-infrared light sources, requires higher-energy pump sources. Increasing the laser damage threshold can overcome the problem of crystal damage under high-power lasers and greatly improve the output power. Generally, the size of the band gap of the crystal is an important factor in determining the laser damage threshold. The larger the band gap, the higher the laser damage threshold. Most of the practical infrared nonlinear optical materials today are ABC ₂ chalcopyrites, such as commercial crystals such as AgGaS ₂ and ZnGeP ₂ , but these crystals have some serious deficiencies, such as low laser damage threshold, low laser damage The two-photon absorption problem of near-infrared lasers (such as Nd:YAG 1064 nm) and the severe anisotropic thermal expansion make it difficult to obtain large-sized, high-quality single crystals, etc. Therefore, how to prepare mid-to-far-infrared nonlinearity with high laser damage threshold Optical crystals are one of the current research hotspots.

SUMMARY OF THE INVENTION

One of the objects of the present invention is to provide a strontium lithium silicon sulfur compound.

The second purpose of the present invention is to provide a preparation method of a strontium lithium silicon sulfur compound.

The third object of the present invention is to provide a nonlinear optical crystal strontium lithium silicon sulfur.

The fourth object of the present invention is to provide a method for preparing a nonlinear optical crystal strontium lithium silicon sulfur.

The fifth object of the present invention is to provide the application of a nonlinear optical crystal strontium lithium silicon sulfur.

One of the objects of the present invention is achieved in this way:

A strontium lithium silicon sulfur compound, the chemical formula is SrLi ₂ SiS ₄ , the molecular weight is 257.83, and it is a colorless and transparent pure strontium lithium silicon sulfur compound.

Further, the strontium-lithium-silicon-sulfur compound is a strontium-lithium-silicon-sulfur crystal, which is a non-centrosymmetric single crystal, belongs to the tetragonal crystal system, the space group is I -42 m , and the unit cell parameter is a = b = 6.469(3) Å , c = 7.689(7) Å, α = β = γ = 90°, Z = 2, unit cell volume V = 321.8(4) Å ³ .

Specifically, the valences of Sr atom, Li atom, Si atom and S atom in the strontium lithium silicon sulfur crystal structure are +2, +1, +4 and -2, respectively; Li atom and four adjacent S atoms form [LiS ₄ ] Tetrahedral structure; Si atom also forms [SiS ₄ ] tetrahedron with its _four adjacent S atoms _; The tetrahedrons are connected to form annular channels; Sr atoms are embedded in the annular channels formed by [LiS ₄ ] and [SiS ₄ ] tetrahedrons to form 8-coordinated [SrS ₈ ] dodecahedrons.

Furthermore, the strontium-lithium-silicon-sulfur crystal is a mid-to-far-infrared nonlinear optical crystal with a band gap of 3.94 eV, an infrared cutoff edge of ~20 μm, a laser damage threshold of about 21 times that of AgGaS ₂ , and a strong nonlinear optical effect: in the Under the irradiation of 2.09 μm fundamental frequency light, in the particle size range of 200-250 μm, the frequency doubling ability of the crystal powder sample is 0.4 times that of commercial crystal AgGaS ₂ and about 13 times that of KH ₂ PO ₄ (KDP).

The second purpose of the present invention is achieved in this way:

A preparation method of strontium lithium silicon sulfur compound, comprising the following steps:

(a) Mix the raw materials of SrS, Li ₂ S and SiS ₂ with a molar ratio of 1:1~5:1 and put them into a graphite crucible, then put them into a quartz glass tube, and put them in a vacuum degree of 10 ^-5 ~ Under the condition of 10 ^-1 Pa, vacuumize and seal;

(b) Put the sealed quartz glass tube in step (a) into a high-temperature sintering furnace, heat it up to 400~700 °C, and keep it for 7~15 h; then heat it up to 750~950 °C, and keep it for 70~110 h; then , cooled to room temperature to obtain strontium lithium silicon sulfur compound.

In step (a), the raw materials of SrS, Li ₂ S and SiS ₂ are weighed in an airtight container with a water content and an oxygen content of 0.01 to 0.2 ppm; preferably, the airtight container is filled with an inert gas, preferably argon. glove box.

In step (b), preferably, the temperature is raised to 400-700°C at a heating rate of 20-40°C/h; preferably, the temperature is raised to 750-950°C at a heating rate of 25-40°C/h; Cool down to room temperature at a rate of 2 to 7 °C/h.

The third purpose of the present invention is achieved in this way:

A nonlinear optical crystal strontium lithium silicon sulfur, the chemical formula is SrLi ₂ SiS ₄ , the molecular weight is 257.83, it is a colorless and transparent non-centrosymmetric single crystal, the space group is I -42 m , and the unit cell parameter is a = b = 6.469(3) Å, c = 7.689(7) Å, α = β = γ = 90°, Z = 2, unit cell volume V = 321.8(4) Å ³ .

Further, the nonlinear optical crystal strontium lithium silicon sulfur belongs to the tetragonal crystal system, and the valences of Sr atom, Li atom, Si atom and S atom in the crystal structure are +2, +1, +4 and -2 respectively; [LiS ₄ ], [SiS ₄ ] and [SrS ₈ ] groups constitute structural elements, in which the Li atom and the adjacent four S atoms form a [LiS ₄ ] tetrahedral structure; the Si atom also forms a [LiS 4 ] tetrahedral structure with its adjacent four S atoms [SiS ₄ ] tetrahedron is formed; [LiS ₄ ] tetrahedron is connected to form a layered structure with a common vertex, and the layers are connected by [SiS ₄ ] tetrahedron to form a ring channel; Sr atoms are embedded in [LiS ₄ ] ] and [SiS ₄ ] tetrahedron linked to form a ring channel, the formation of 8-coordinated [SrS ₈ ] dodecahedron.

Furthermore, the nonlinear optical crystal strontium lithium silicon sulfur is a mid-to-far infrared nonlinear optical crystal with a band gap of 3.94 eV, an infrared cutoff edge of ~20 μm, a laser damage threshold of about 21 times that of AgGaS ₂ , and a nonlinear optical effect. Strong: Under the irradiation of 2.09 μm fundamental frequency light, the frequency doubling ability of the crystal powder sample is 0.4 times that of commercial crystal AgGaS ₂ in the particle size range of 200~250 μm, about 13 times that of KH ₂ PO ₄ (KDP) times.

The fourth purpose of the present invention is achieved in this way:

The preparation method of the aforementioned nonlinear optical crystal strontium lithium silicon sulfur, comprising the following steps:

(a) Mix the raw materials of SrS, Li ₂ S and SiS ₂ with a molar ratio of 1:1~5:1 and put them into a graphite crucible, then put them into a quartz glass tube, and put them in a vacuum degree of 10 ^-5 ~ Under the condition of 10 ^-1 Pa, vacuumize and seal;

(b) Put the sealed quartz glass tube in step (a) into a high-temperature sintering furnace, heat it up to 400~700 °C, and keep it for 7~15 h; then heat it up to 750~950 °C, and keep it for 70~110 h; then , and cooled to room temperature to obtain nonlinear optical crystal strontium lithium silicon sulfur.

In step (a), the raw materials of SrS, Li ₂ S and SiS ₂ are weighed in an airtight container with a water content and an oxygen content of 0.01 to 0.2 ppm; preferably, the airtight container is filled with an inert gas, preferably argon. glove box.

In step (b), preferably, the temperature is raised to 400-700°C at a heating rate of 20-40°C/h; preferably, the temperature is raised to 750-950°C at a heating rate of 25-40°C/h; Cool down to room temperature at a rate of 2 to 7 °C/h.

The fifth purpose of the present invention is achieved in this way:

The application of the aforementioned nonlinear optical crystal strontium lithium silicon sulfur in the preparation of mid- and far-infrared band laser frequency doubling crystals, infrared communication devices and infrared laser guidance devices.

The invention adopts a high-temperature solid-phase synthesis method to successfully prepare a new compound strontium lithium silicon sulfur and a mid-far infrared nonlinear optical crystal strontium lithium silicon sulfur, the crystal is composed of [LiS ₄ ], [SiS ₄ ] and [SrS ₈ ] groups The constituent structural element has excellent optical properties, long infrared absorption cut-off edge, wide band gap, high laser damage threshold, and large nonlinear optical coefficient. As a new type of mid- and far-infrared nonlinear optical crystals, it has a wide range of potential applications in high-power infrared laser systems.

Description of drawings

FIG. 1 is the powder XRD pattern of the nonlinear optical crystal strontium lithium silicon sulfur, wherein the curve a is the experimental value, and the curve b is the theoretical value.

FIG. 2 is a schematic diagram of the structure of the nonlinear optical crystal strontium lithium silicon sulfur.

Figure 3 is a band gap diagram of the nonlinear optical crystal strontium lithium silicon sulfur.

FIG. 4 is an infrared spectrum of the nonlinear optical crystal strontium lithium silicon sulfur.

Figure 5 is a schematic diagram of the nonlinear optical system when the nonlinear optical crystal strontium lithium silicon sulfur is used as a frequency doubling crystal, wherein, 1, laser, 2, all-converging lens, 3, strontium lithium silicon sulfur crystal powder, 4, outgoing beam, 5. Filters.

Fig. 6 is a graph showing the frequency doubling intensity of the nonlinear optical crystal strontium lithium silicon sulfur and the particle size of the sample.

7 is the powder XRD pattern of the nonlinear optical crystal strontium lithium silicon sulfur prepared in Comparative Example 1, wherein the curve a is the experimental value, and the curve b is the theoretical value.

Detailed ways

The present invention will be further described below in conjunction with the examples, and the following examples are only for illustration and do not limit the protection scope of the present invention in any way.

The processes and methods not described in detail in the following examples are conventional methods well known in the art. The reagents used in the examples are all analytically pure or chemically pure, and can be purchased commercially or prepared by methods well known to those of ordinary skill in the art. The following embodiments all achieve the purpose of the present invention.

Example 1

The starting materials 0.119 g SrS, 0.046 g Li ₂ S and 0.092 g SiS ₂ were weighed in a glove box with 0.01 ppm water and oxygen content and filled with inert gas argon in a molar ratio of 1:1:1 , grind the weighed raw material evenly in a mortar, then put it into a clean graphite crucible, and then put it into a quartz glass tube with a length of 20 cm and a diameter of 10 mm. The sealed quartz glass tube was raised from room temperature to 400 °C at a heating rate of 30 °C/h, kept for 10 ^h , and then heated at a temperature of 30 °C/h. The temperature was raised to 800 °C, and the temperature was kept for 80 h. The various components in the raw material composition reacted to obtain the compound; finally, the temperature was cooled to room temperature at a rate of 5 °C/h, and the sample was taken out and ground in a mortar to obtain a colorless and transparent small Granular SrLi ₂ SiS ₄ powder.

The obtained strontium lithium silicon sulfur powder is subjected to powder X-ray diffraction analysis, and the results of the obtained X-ray diffraction spectrum and the SrLi ₂ SiS ₄ theoretical X-ray spectrum analyzed by the single crystal structure are shown in FIG. 1 . It can be seen from Fig. 1 that the experimental values agree with the theoretical values of the theoretical X-ray spectrum of SrLi ₂ SiS ₄ analyzed by the single crystal structure, indicating that the crystal is pure phase strontium lithium silicon sulfur.

The structural schematic diagram of the crystal is shown in Figure 2. The valences of Sr atom, Li atom, Si atom and S atom in the crystal structure are +2, +1, ₊₄ and -2 respectively; ₄ ] and [SrS ₈ ] groups form structural units: Li atom forms [LiS ₄ ] tetrahedral structure with four adjacent S atoms; Si atom also forms [SiS ₄ ] tetrahedron with its adjacent four S atoms; [ LiS ₄ ] tetrahedrons form a layered structure in a common vertex connection, and the layers are connected by [SiS ₄ ] tetrahedrons to form annular channels; Sr atoms are embedded in [LiS ₄ ] and [SiS ₄ ] tetrahedral links In the formed annular channel, an 8-coordinated [SrS ₈ ]dodecahedron is formed.

The band gap of the obtained strontium lithium silicon sulfur crystal was measured by ultraviolet-visible-near-infrared diffuse reflectance spectrometer, and the results are shown in Fig. 3. It can be concluded from the figure that the band gap of the nonlinear optical crystal strontium lithium silicon sulfur is 3.94 eV, which is wider than that of the silver sulfur gallium (AgGaS ₂ ) crystal, which is 2.64 eV.

The obtained strontium-lithium-silicon-sulfur crystals were characterized by infrared spectroscopy, and the results obtained are shown in FIG. 4 . It can be seen from the figure that the infrared absorption cut-off edge of the nonlinear optical crystal strontium lithium silicon sulfur is longer.

Example 2

The starting material SrS: _Li2S : _SiS2 was weighed in a glove box with a water and oxygen content of 0.01 ppm and filled with an inert gas argon in a molar ratio of 1:2:1 and ground in a mortar After uniform, put it into a clean graphite crucible, put it into a quartz glass tube with a length of 20 cm and a diameter of 10 mm, and seal the quartz glass tube containing the raw materials under the condition of vacuum degree of 10 ^-3 Pa. ; then the sealed quartz glass tube was raised from room temperature to 400 °C at a heating rate of 20 °C/h, held for 7 h, then heated to 750 °C at a temperature of 30 °C/h, and held for 70 h; finally, the temperature was 2 The rate of ℃/h was cooled to room temperature, the graphite crucible was taken out, and a colorless and transparent small particle SrLi ₂ SiS ₄ single crystal was obtained. The single crystal X-ray diffraction analysis showed that the crystal was a strontium lithium silicon sulfur crystal.

Example 3

The starting material SrS: _Li2S : _SiS2 molar ratio was weighed in a glove box with water content and oxygen content of 0.05 ppm and filled with inert gas argon to be 1:3:1, and ground in a mortar and uniform, and then put Put it into a clean graphite crucible, put it into a quartz glass tube with a length of 20 cm and a diameter of 10 mm, and vacuumize the quartz glass tube containing the raw materials under the condition of a vacuum degree of 10 ^-3 Pa and seal it; The good quartz glass tube was heated from room temperature to 600 °C at a heating rate of 40 °C/h, held for 10 h, then heated to 900 °C at a temperature of 30 °C/h, and held for 80 h; finally, the temperature was 4 °C/h. The rate was cooled down to room temperature, and the graphite crucible was taken out to obtain a colorless and transparent small particle SrLi ₂ SiS ₄ single crystal.

Example 4

The starting material SrS: _Li2S : _SiS2 molar ratio was weighed in a glove box with water content and oxygen content of 0.15 ppm and filled with inert gas argon to be 1:4:1, ground in a mortar and evenly placed Put it into a clean graphite crucible, put it into a quartz glass tube with a length of 20 cm and a diameter of 10 mm, and vacuumize the quartz glass tube containing the raw materials under the condition of a vacuum degree of 10 ^-2 Pa and seal it; The good quartz glass tube was heated from room temperature to 600 °C at a heating rate of 38 °C/h, kept for 15 h, and then heated to 900 °C at a temperature of 40 °C/h for 100 h; finally, the temperature was 6.5 °C/h. Cool down to room temperature, take out the graphite crucible, and obtain a colorless and transparent small particle SrLi ₂ SiS ₄ single crystal.

Example 5

The starting material SrS: _Li2S : _SiS2 molar ratio was weighed in a glove box with water content and oxygen content of 0.2 ppm and filled with inert gas argon to be 1:5:1, ground in a mortar and evenly placed Put it into a clean graphite crucible, put it into a quartz glass tube with a length of 20 cm and a diameter of 10 mm, vacuumize the quartz glass tube containing the raw materials under the condition of vacuum degree of 10 ^-1 Pa, and seal it; The good quartz glass tube was heated from room temperature to 650 °C at a heating rate of 35 °C/h, held for 15 h, then heated to 950 °C at a temperature of 35 °C/h, and held for 110 h; finally, the temperature was 7 °C/h. The rate was cooled down to room temperature, and the graphite crucible was taken out to obtain a colorless and transparent small particle SrLi ₂ SiS ₄ single crystal.

Example 6

Figure 5 is a typical schematic diagram of a nonlinear optical system when the nonlinear optical crystal strontium lithium silicon sulfur is used as a frequency doubling crystal. According to the optical system shown in Fig. 5, any kind of strontium lithium silicon sulfur mid-far infrared nonlinear optical crystal obtained in Examples 1 to 5 is placed on the position of the strontium lithium silicon sulfur crystal powder 3, and at room temperature, Q-switched The (Ho:Tm:Cr:YAG) laser 1 emits an infrared beam with a wavelength of 2090 nm as a light source, and is injected into a strontium lithium silicon sulfur nonlinear optical crystal through a condensing lens 2 to generate a frequency-doubled light with a wavelength of 1045 nm, which is emitted The beam 4 contains infrared light with a wavelength of 2090 nm and light with a wavelength of 1045 nm, which is filtered by a filter 5 to obtain a frequency-doubled light with a wavelength of 1045 nm.

The relationship between the frequency doubling intensity of the nonlinear optical crystal strontium lithium silicon sulfur and the particle size of the sample was measured, and the AgGaS ₂ crystal with the same particle size (particle size range of 200-250 μm) was used as the control, and the obtained results are shown in Figure 6. It can be seen from Figure 6 that under the same particle size, the output intensity of the nonlinear optical crystal strontium lithium silicon sulfur is 0.4 times that of the _AgGaS crystal, and about 13 times that of the KDP crystal.

At the same time, a 1064 nm laser was used to irradiate the strontium lithium silicon sulfur and AgGaS ₂ microcrystalline samples, and the color changes on the surface of the samples were observed to determine whether the crystals were damaged, and the laser damage thresholds of the nonlinear optical crystals strontium lithium silicon sulfur and AgGaS ₂ crystals were measured. shown in Table 1.

Table 1 Comparison of laser damage thresholds between SrLi ₂ SiS ₄ and AgGaS ₂ (as a reference)

*AGS = AgGaS ₂

It can be seen from Table 1 that under the irradiation of 1064 nm laser, the nonlinear optical crystal strontium-lithium-silicon-sulfur powder sample exhibits a high laser damage threshold, which is about 21 times that of the _AgGaS crystal. Therefore, the strontium-lithium-silicon-sulfur crystal has The excellent optical properties make it have wider potential application value in high-power laser systems.

Comparative Example 1

The starting materials 0.119 g SrS, 0.321 g Li ₂ S and 0.092 g SiS ₂ were weighed in a glove box with 0.01 ppm water and oxygen content and filled with inert gas argon in a molar ratio of 1:7:1 , grind the weighed raw material evenly in a mortar, then put it into a clean graphite crucible, and then put it into a quartz glass tube with a length of 20 cm and a diameter of 10 mm. The sealed quartz glass tube was raised from room temperature to 400 °C at a heating rate of 30 °C/h, kept for 10 ^h , and then heated at a temperature of 30 °C/h. The temperature was raised to 800 °C, and the temperature was kept for 80 h. The various components in the raw material composition reacted to obtain the compound; finally, the temperature was cooled to room temperature at a rate of 5 °C/h, and the sample was taken out and ground in a mortar to obtain SrLi ₂ SiS ₄ powder.

The strontium lithium silicon sulfur prepared in Comparative Example 1 was characterized by powder XRD, and the obtained results are shown in FIG. 7 . It can be seen from FIG. 7 that Li ₄ SiS ₄ impurities are present in the strontium lithium silicon sulfur crystal obtained in Comparative Example 1.

The optical properties of the strontium-lithium-silicon-sulfur crystal prepared in Comparative Example 1 were tested, and the results obtained are shown in Table 2.

Table 2

It can be seen from Table 2 that the frequency doubling effect of the strontium lithium silicon sulfur crystal prepared in Comparative Example 1 is reduced by about half, and the introduced impurities affect the nonlinear optical properties of the crystal.

Claims (10)

1. a strontium lithium silicon-sulfur compound, it is characterized in that, chemical formula is SrLi ₂ SiS ₄ , molecular weight is 257.83, it is colorless and transparent strontium lithium silicon-sulfur pure sample, and this strontium lithium silicon-sulfur compound is strontium lithium silicon-sulfur crystal , is a non-centrosymmetric single crystal, belongs to the tetragonal crystal system, the space group is I -42 m , the unit cell parameters are a = b = 6.469(3) Å, c = 7.689(7) Å, α = β = γ = 90°, Z = 2, unit cell volume V = 321.8(4) Å ³ . 2. the preparation method of strontium lithium silicon-sulfur compound according to claim 1, is characterized in that, comprises the steps: (a) Mix the raw materials of SrS, Li ₂ S and SiS ₂ with a molar ratio of 1:1 to 5:1 uniformly, put them into a reaction vessel, and pump them under the condition of a vacuum of 10 ^-5 to 10 ^-1 Pa Seal after vacuum; (b) Put the sealed reaction vessel in step (a) into a high-temperature sintering furnace, heat up to 400-700 °C, and keep the temperature for 7-15 h; then heat up to 750-950 °C, and keep the temperature for 70-110 h; after that, cool down Drop to room temperature to obtain strontium lithium silicon sulfur compound. 3 . The method for preparing a strontium lithium silicon sulfur compound according to claim 2 , wherein in step (a), weigh SrS and Li in an airtight container with a water content and an oxygen content of 0.01 to 0.2 ppm. ₄ . S and SiS ₂ raw materials. 4. A nonlinear optical crystal strontium lithium silicon sulfur, characterized in that the chemical formula is SrLi ₂ SiS ₄ , the molecular weight is 257.83 , it is a colorless and transparent non-centrosymmetric single crystal, the space group is 1-42 m , and the crystal The cell parameters are a = b = 6.469(3) Å, c = 7.689(7) Å, α = β = γ = 90°, Z = 2, and the unit cell volume V = 321.8(4) Å ³ . 5 . The nonlinear optical crystal strontium lithium silicon sulfur according to claim 4 , wherein the crystal is a tetragonal system, and the structural base is composed of [LiS ₄ ], [SiS ₄ ] and [SrS ₈ ] groups. 6 . Yuan. 6 . The nonlinear optical crystal strontium lithium silicon sulfur according to claim 4 , wherein the nonlinear optical crystal strontium lithium silicon sulfur is a mid-far infrared nonlinear optical crystal, and its band gap is 3.94 eV. 7 . 7. the preparation method of nonlinear optical crystal strontium lithium silicon sulfur according to claim 4, is characterized in that, comprises the steps: (a) Mix the raw materials of SrS, Li ₂ S and SiS ₂ with a molar ratio of 1:1 to 5:1 uniformly, put them into a reaction vessel, and pump them under the condition of a vacuum of 10 ^-5 to 10 ^-1 Pa Seal after vacuum; (b) Put the sealed reaction vessel in step (a) into a high-temperature sintering furnace, heat up to 400-700 °C, and keep the temperature for 7-15 h; then heat up to 750-950 °C, and keep the temperature for 70-110 h; after that, cool down Drop to room temperature to obtain nonlinear optical crystal strontium lithium silicon sulfur. 8. the preparation method of nonlinear optical crystal strontium lithium silicon sulfur according to claim 7, is characterized in that, weighing SrS, Li ₂ S and SiS in the airtight container that water content, oxygen content are 0.01～0.2 ppm ₂ raw materials. 9 . The method for preparing a nonlinear optical crystal strontium lithium silicon sulfur according to claim 7 , wherein in step (b), the temperature is raised to 400 to 700° C. at a heating rate of 20 to 40° C./h; The heating rate is ~40 °C/h to 750~950 °C. 10. The application of the nonlinear optical crystal strontium lithium silicon sulfur according to claim 4 in the preparation of mid- and far-infrared band laser frequency-doubling crystals, infrared communication devices or infrared laser guidance devices.

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