Horizontal directional crystallization growth method of Cr < 4 + >: LuAG crystal

Abstract

The invention discloses a horizontal directional crystallization growth method of a Cr < 4 + >: LuAG crystal, which comprises the following steps: in a single crystal growth process, adopting a zone melting mode of three-phase coexistence of a single crystal, a melt and a pre-crystallized polycrystal raw material, controlling a melting zone to be 20-40mm, and enabling the pre-crystallized polycrystal raw material to form a narrow melting zone so as to obtain a solute non-conservative system, so that components are controlled; the uniform doping of Cr < 4 + > in a LuAG matrix is realized, and the Cr < 4 + >: LuAG crystal with uniformly doped Cr < 4 + > is obtained by reasonably controlling the growth rate. And the crystal prepared by the method is relatively high in quality and has no defects such as a core and a side core, and the problem that the high-quality Cr < 4 + >: LuAG crystal with uniform and controllable components is difficult to prepare by the existing crystal growth method is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of crystal growth, and particularly relates to a horizontal directional crystallization growth method for Cr 4+ :LuAG crystals. Background Art

[0002] With the rapid development of technology, lasers and nonlinear optical devices play an increasingly important role in many high-tech fields, such as communication, medical treatment, material processing, etc. Against this background, Cr 4+ :LuAG (chromium-doped lutetium aluminum garnet) crystals, as a new type of optical material, have received extensive attention and research in recent years due to their unique physical and chemical properties.

[0003] Cr 4+ :LuAG crystals have a broad absorption bandwidth and high saturated absorption characteristics, which enable them to exhibit excellent performance in the formation and modulation of laser pulses. Their saturated absorption effect can regulate the output power of the laser, thereby effectively controlling the pulse duration and laser intensity to meet the requirements of high-precision and high-efficiency laser applications. In addition, compared with traditional saturable absorbers prepared from two-dimensional materials such as graphene WS2 (WS2 refers to transition metal sulfides) and black phosphorus, Cr 4+ :LuAG crystals not only have a good optical damage threshold, but also have characteristics such as good uniformity and high thermal stability, which make them have important application values in fields such as short-pulse laser systems, laser frequency locking, and laser amplification.

[0004] However, although Cr 4+ :LuAG crystals have many advantages, their preparation process faces a series of challenges. The LuAG matrix belongs to the cubic crystal system and has a melting point as high as 2010°C, which makes its growth process require high temperatures and special process conditions. Among various growth methods, the melt method is widely used because it can grow crystals with larger sizes and higher quality.

[0005] In the melt method, the Czochralski method is a common and effective crystal growth technique. However, when applied to the production of LuAG substrates, the Czochralski method has some significant limitations. On the one hand, the Czochralski method requires a crucible made of precious metal iridium, which not only increases the production cost, but also causes losses during the production process, further increasing the cost. On the other hand, during the growth of single crystals by the Czochralski method, stirring and pulling operations are required, and these operations are prone to causing irregular convective motions in the melt, thereby affecting the growth quality of the crystals. The grown single crystals often have large residual stresses and dislocation densities, which will affect their optical properties and application effects. More importantly, the single crystals grown by the Czochralski method often have the problem of uneven ion doping, seriously affecting the performance of the crystal materials. In addition, the single crystals grown by the Czochralski method also have core and side core defects, and these defects need to be removed during subsequent processing and use, thereby reducing the material utilization rate. Therefore, finding a preparation method that can not only ensure the excellent performance of Cr 4+ :LuAG crystals, but also reduce costs and improve material utilization rate is of great significance for promoting the development of high-tech application fields such as lasers and nonlinear optical devices. Summary of the Invention

[0006] Aiming at the above deficiencies existing in the prior art, the purpose of the present invention is to provide a horizontal directional crystallization growth method for Cr 4+ :LuAG crystals, which solves the problem that it is difficult to prepare high-quality Cr 4+ -ion uniformly doped Cr 4+ :LuAG crystals, as well as the problems of relatively high preparation cost and low material utilization rate of Cr 4+ :LuAG crystals.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is as follows: A horizontal directional crystallization growth method for Cr 4+ :LuAG crystals, comprising the following steps: (1) Spread the Cr 4+ :LuAG pre-crystallization material evenly in a boat-shaped crucible, then insert the seed crystal into the seed crystal groove at one end of the boat-shaped crucible near the shoulder angle, and then place the boat-shaped crucible containing the pre-crystallization material and the seed crystal into the working furnace, and make the equal-width area of the boat-shaped crucible located in the heating area of the working furnace, and close the furnace chamber; (2) Turn on the water cooling cycle and evacuate the working furnace. When the vacuum degree is less than 1×10 -3At Pa, turn on the heating power supply of the working furnace to heat the pre-crystallized material in the boat-shaped crucible in the heating area. By controlling the heating power, the pre-crystallized material in the heating area is melted to form a narrow melting zone with a melting zone length of 20 - 40 mm, and keep it warm for 1 - 2 h to ensure the stability of the melt state; (3) After the melt state is stable, keep the heating power unchanged, horizontally move the boat-shaped crucible to align the heating area with the end of the boat-shaped crucible equipped with the seed crystal for heating, and at the same time ensure that the seed crystal is located at the edge of the heating area, keep it warm for 20 - 60 min. When the melting length of the seed crystal is 1 / 4 - 1 / 3 of the total length of the seed crystal, move the boat-shaped crucible slowly towards the end of the boat-shaped crucible equipped with the seed crystal at a growth rate of 0.5 - 1 mm / h, and carry out seeding and shoulder growth in sequence. After the shoulder growth is completed, move the boat-shaped crucible slowly towards the end of the boat-shaped crucible equipped with the seed crystal at a growth rate of 1 - 2 mm / h until all the pre-crystallized material is completely melted and crystallization is completed. After cooling and annealing, Cr 4+ :LuAG crystal is obtained; during the moving process, the melt in the part of the boat-shaped crucible gradually away from the heating area generates supercooling degree, which serves as the crystallization driving force to promote the generation of single crystal.

[0008] Further, in step (3), the cooling and annealing procedure is as follows: first cool at a cooling rate of 15 - 35 °C / h to 1700 - 1800 °C, and keep it warm at this temperature for 20 - 30 h, then cool to room temperature at a cooling rate of 30 - 70 °C / h, open the furnace chamber, and take out the crystal. During the cooling and annealing process, although too fast cooling rate or too short holding time can reduce the crystal growth cycle, too fast cooling rate or too short holding time will weaken the elimination degree of stress during annealing and increase the cracking risk. Therefore, the present invention adopts a segmented cooling method for annealing, first cooling at a smaller rate and keeping it warm for 20 - 30 h to achieve in-situ annealing, reducing the internal thermal stress of the crystal, and then cooling to room temperature at a faster speed to improve the crystal preparation efficiency.

[0009] Further, place the Cr 4+ :LuAG crystal obtained in step (3) in a high-temperature annealing furnace and carry out high-temperature annealing in an air environment. The annealing temperature is 1250 - 1450 °C, and the annealing time is 24 - 72 h to promote the residual Cr 3+ to Cr 4+ transformation, thereby increasing the concentration of Cr 4+ ; after the annealing is completed, the crystal is cooled with the furnace and taken out for use.

[0010] Further, in step (1), the preparation method of the Cr 4+ :LuAG pre-crystallized material is as follows: According to Lu3Al （5-x） Cr x O 12Weigh the powder raw materials Cr2O3, Lu2O3, and Al2O3 according to their stoichiometric ratios, and simultaneously weigh the CaCO3 powder raw material. The weight of CaCO3 is 1 to 7 times the weight of Cr2O3. Subsequently, mix the powder raw materials evenly, melt and cool to obtain a solid solution, and then crush the obtained solid solution into small pieces to be used as the Cr 4+ :LuAG pre-crystallization material; where x is the doping concentration of Cr 3+ , and the value range of x is 0.1 to 3 at%. The crystals grown in this experiment are formulated according to the cation vacancy compensation electrovalence balance method, that is, (5 - x)Al2O3 + 3Lu2O3 + xCr2O3 = 2Lu3Al （5-x） Cr x O 12 Cr 4+ is obtained by adding Cr 3+ powder with a doping concentration of 0.1 to 3 at% of Cr2O3 to the raw materials, and adding CaCO3 powder with a mass of 1 to 7 times the weight of Cr2O3 to generate Ca 2+ as a charge compensation ion, thereby promoting the conversion of Cr 3+ to Cr 4+ to obtain a higher concentration of tetrahedrally coordinated Cr 4+ ions. When selecting co-doped ions, factors such as volume matching, charge balance, the stability of co-doped ions, and their influence on the crystal spectral characteristics need to be comprehensively considered. Among them, Ca 2+ is a more suitable choice. The reason why the addition amount of CaCO3 is 1 to 7 times that of Cr2O3 is that too little is not conducive to the conversion of Cr 4+ , and too much will result in Cr,Ca:LuAG crystals being prepared. The doping concentration of Cr 3+ is 0.1 to 3 at%, which can meet the commercial concentration requirements of Cr 4+ :LuAG. It should be noted that since very little calcium carbonate is added, the calcium oxide formed after the decomposition of calcium carbonate later remains in the crystal as a trace impurity and will not affect the preparation of Cr 4+ :LuAG crystals.

[0011] Furthermore, in step (1), the seed crystal adopts the <111>, <100>, or <110> direction. LuAG belongs to a crystal of the cubic crystal system, and generally these three directions are used. In the present invention, when conducting crystal growth with a seed crystal, the crystal orientation of the seed crystal preferably adopts the <111> direction. This is because, generally, the crystal based on LuAG has the highest symmetry along the <111> direction and produces fewer defects during the growth process. Therefore, when conducting crystal growth with a seed crystal, without specific requirements, the seed crystal preferably adopts the <111> direction, and it is easier to obtain high-quality crystals.

[0012] Furthermore, the shoulder release angle in the shoulder release stage is 80~120º. The shoulder release angle should not be too large or too small. When the shoulder release angle is too small, the part of the crystal before the shoulder release cannot be reasonably utilized, and the crystal utilization rate is low; if it is too large, it will cause difficulty in shoulder collection. When the shoulder release angle is 90º, the entire crystal ingot can be used most reasonably, which well solves the problem of crystal ingot utilization rate. At the same time, shoulder collection is also easier. Therefore, the preferred shoulder release angle is 90º.

[0013] Furthermore, the boat-shaped crucible is a boat-shaped molybdenum crucible. The molybdenum crucible used in the present invention has a melting point of 2630°C and is easy to grow high-melting-point crystals. During the growth process, the crucible has high stability, is not easy to leak, and has low cost, thereby increasing production efficiency.

[0014] The present invention adopts a horizontal directional crystallization method to prepare Cr 4+ :LuAG crystal, during the single crystal growth process, by controlling the melting zone length to 20~40mm, the growth rate of seeding and shoulder release to 0.5~1mm / h, and the growth rate of the equal width growth stage to 1~2mm / h, Cr 4+ The reason why the melting zone length is controlled at 20~40mm is to obtain a narrow melting zone, because for the zone melting method of the solute non-conservative system, the shorter the melting zone length, the more uniform the distribution of the grown crystals, so as to reduce the Cr 4+ The concentration of doping ions caused by the segregation effect gradually increases with the increase of crystal growth time, thereby achieving the purpose of controllable components. However, the length of the melting zone should not be too short, because if the melting zone is too short, it is easy to produce an undercooled melt or a partially undercooled melt, which is not conducive to the stability of the growth interface. If the interface is unstable, it is easy to produce dendrites or cellular structures, which is not conducive to obtaining high-quality crystals. At the same time, during the crystal growth process, a certain temperature is also required to maintain the fluidity of the melt. Too low a temperature (or too small a melting zone) will lead to poor melt fluidity, thereby affecting the growth of the crystal.

[0015] The inventors of the present invention have found through research that Cr ions can appear in different valence states in the LuAG lattice, among which Cr 3+ The most stable, but due to Cr 4+ is the luminescence center, so in order to promote Cr 3+ To Cr 4+ Transformation, compared to the preparation of Cr 3+ :LuAG crystal, the present invention is used in the preparation of Cr 4+ :LuAG crystals, CaCO3 is added to the melt to produce Ca 2＋ As charge compensation ions, annealing in an oxidizing atmosphere is performed after the crystal growth is completed, thereby promoting Cr 3+ To Cr 4+ Transformation, improve Cr 4+concentration. In addition, during the crystal growth process, by increasing the length of the molten zone, the fluidity of the melt is improved, thereby promoting the uniform distribution of doped ions. At the same time, the increase in the length of the molten zone can also increase the temperature gradient of the melt, avoid constitutional supercooling, and improve the quality of the crystal. However, an excessively large temperature gradient is likely to increase the thermal stress, thereby increasing the risk of crystal cracking, and it will also affect the uniform distribution of doped ions. Therefore, the length of the molten zone should not exceed 40 mm. At the same time, the growth rate is reduced, so that Cr ions have enough time to convert to the tetravalent state and enter the tetrahedral lattice sites, thereby realizing the doping of Cr 4+ doping, and it can also provide enough time for the melt to uniformly release the latent heat and reduce the local temperature and concentration fluctuations caused by too fast growth rate, thereby reducing the risk of constitutional supercooling. At the same time, avoid too low growth rate leading to a decrease in production efficiency and an increase in cost. Therefore, the present invention realizes the uniform doping of Cr 4+ in the LuAG matrix by reasonably controlling the length of the molten zone and the growth rate, and obtains Cr 4+ uniformly doped high-quality Cr 4+ :LuAG crystal. For the growth rate, at the beginning, it is grown at a relatively low growth rate of 0.5 - 1 mm / h in order to seed and shoulder as stably as possible and ensure the smooth growth of the crystal at the beginning. When entering the equal-width zone and the crystal grows relatively stably, the growth rate is appropriately increased to 1 - 2 mm / h, which can improve the growth efficiency of the crystal.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. The horizontal directional crystallization growth method of the Cr 4+ :LuAG crystal of the present invention adopts a zone melting method in which a single crystal, a melt, and a pre-crystallized polycrystalline raw material coexist during the single crystal growth process, and the molten zone is controlled within a specific size range to form a narrow molten zone with the pre-crystallized polycrystalline raw material, so as to obtain a solute non-conservative system, thereby controlling the composition (distribution of doped ions) and realizing the uniform doping of Cr 4+ in the LuAG matrix, and by reasonably controlling the growth rate, a Cr 4+ uniformly doped high-quality Cr 4+ :LuAG crystal is obtained. The crystal prepared by this method has high quality, no defects such as core and side core, and solves the problem that it is difficult to prepare high-quality Cr 4+ :LuAG crystal with controllable and uniform composition by the existing crystal growth methods. And the method is simple and easy to implement, and the preparation efficiency is relatively high.

[0017] 2. The boat-shaped molybdenum crucible adopted in the present invention has a relatively low cost, a large free upper surface, accounting for 35%-40% of the total melt contact area. The grown crystal has a relatively low dislocation density, and the height of the boat-shaped crucible is much smaller than its length, which creates favorable conditions for the evaporation of impurity components in the melt, facilitating the effective reduction of crystal scattering and improving the transparency and optical properties of the crystal. Moreover, there is no rotation operation in the present invention, and combined with the relatively low melt height, there is almost no convective movement in the melt, increasing the stability of the growth front, and the grown crystal has no core or side core, making it easy to obtain high-quality crystals. At the same time, the machining allowance is reduced, the difficulty of subsequent processing is lowered, and the production efficiency is significantly improved. In addition, during crystal preparation, in-situ annealing is adopted to reduce the internal stress in the crystal and reduce the possibility of crystal cracking.

[0018] 3. Before the seed crystal growth in the present invention, the equal-width area of the boat-shaped crucible is first heated to melt the polycrystalline material to obtain a narrow melt zone, and then the end close to the seed crystal is heated at the same power, and then the seed crystal is slowly grown. This is because the shape of the equal-width area is a cuboid, which is convenient for observing the temperature situation and judging whether the solid-liquid interface is stable. The front end of the crucible where the seed crystal is located is triangular, and the solid-liquid interface is very short, making it difficult to judge whether the solid-liquid interface is stable and more difficult to control the power, which easily increases the risk of excessive melting of the seed crystal. By the above method, the risk of excessive melting of the seed crystal is avoided, the operation difficulty is lower, the risk is smaller, and it is convenient for popularization and application.

[0019] 4. The large-size plate-like Cr 4+ :LuAG laser crystal is grown by the method of the present invention. The size can be 130mm×85mm×20mm, and it has a low internal defect density and excellent physical properties, meeting the requirements of the high-end laser application field. During actual production, the crucible size can also be adjusted according to needs to change the crystal size. Detailed implementation mode

[0020] The following combines specific embodiments to further describe the detailed implementation mode of the present invention in detail.

[0021] Example 1 This example provides a horizontal directional crystallization growth method for Cr 4+ :LuAG crystal, including the following steps: (1) Furnace loading: Weigh the powder raw materials Cr2O3, Lu2O3, and Al2O3 according to the stoichiometric ratio of Lu3Al （5-x） Cr x O 12 , and at the same time weigh the CaCO3 powder raw material. The weight of CaCO3 is 2 times the weight of Cr2O3. Then, after mixing the powder raw materials evenly, melt and cool to obtain a solid solution, and then crush the obtained solid solution into small pieces as Cr4+ : LuAG pre-crystallized material; where x is the doping concentration of Cr 3+ , x = 1.0 at%, and the total weight of the raw materials is 2 kg. Subsequently, 2 kg of Cr 3+ with a doping concentration of 1.0 at% of Cr 4+ : The LuAG pre-crystallized material was laid flat in a boat-shaped molybdenum crucible after alcohol washing (the crucible size is 200 mm × 100 mm × 30 mm), and a pure LuAG seed crystal was inserted into the seed crystal groove at one end of the boat-shaped molybdenum crucible near the shoulder angle, and the seed crystal orientation was

[111] ; then the boat-shaped molybdenum crucible containing the pre-crystallized material and the seed crystal was placed in a single crystal furnace, and the equal-width area of the boat-shaped molybdenum crucible was located in the heating area of the single crystal furnace, and then the furnace chamber was closed; (2) Melting the pre-crystallized material: Turn on the vacuum pump and water-cooling circulation, evacuate the vacuum. When the vacuum degree reaches 9×10 -4 Pa, turn on the heating power supply of the single crystal furnace, and perform resistance heating and temperature rise on the heating area. When the power reaches 25 kW, observe the melt flow. It is found that the liquid level is unstable, the length of the melting zone is 17 mm, and the curvature of the solid-liquid interface shape is relatively large. It is judged that the melt state is poor. Subsequently, the temperature is increased to 25.9 kW at a rate of 0.3 kW / h until the length of the melting zone is 35 mm, and the solid-liquid interface state tends to be linear, and the melt is kept stable for 1.5 h; (3) Crystal seeding, shoulder opening, and equal-width growth: Move the boat-shaped molybdenum crucible so that the pre-crystallized material near the seed crystal end is located in the heating area, and the seed crystal is just at the edge of the heating area. When the melting length of the seed crystal reaches 1 / 3 of the total length of the seed crystal, keep the temperature for 30 min, keep the power unchanged, turn on the automatic crucible movement program, and move the boat-shaped molybdenum crucible towards the end with the seed crystal at a growth rate of 0.8 mm / h, and perform the crystal seeding and shoulder opening growth processes in sequence. After the shoulder opening is completed, move the boat-shaped molybdenum crucible towards the end with the seed crystal at a growth rate of 1.2 mm / h to perform equal-width growth until the crystallization process ends; During the movement, the melt in the part of the boat-shaped molybdenum crucible that gradually moves away from the heating area generates supercooling, which serves as the crystallization driving force to promote the generation of single crystals, and the shoulder opening angle is 90º.

[0022] (4) Cooling and annealing: After crystallization is completed, enter the cooling and annealing stage. First, cool down at a rate of 25 °C / h to the annealing temperature of 1730 °C and keep the temperature for 30 h; then cool down to room temperature at a rate of 50 °C / h, and finally open the furnace and take out the crystal.

[0023] (5) Annealing in air atmosphere: Place the grown crystal in a high-temperature annealing furnace and perform high-temperature annealing in an air environment. The annealing temperature is set at 1300 °C, and the annealing time is 36 h to promote the residual Cr 3+ to transform into Cr 4+ , thereby improving Cr 4+Concentration; after the annealing is completed, the crystal is cooled with the furnace and taken out for use.

[0024] After detection, the Cr 4+ :LuAG crystal grown by this process has good quality and the size is 120mm×70 mm×18mm.

[0025] Example 2 This example provides a horizontal directional crystallization growth method for Cr 4+ :LuAG crystal, which includes the following steps: (1) Loading the furnace: According to the stoichiometric ratio of Lu3Al （5-x） Cr x O 12 , weigh the powder raw materials Cr2O3, Lu2O3, and Al2O3, and at the same time weigh the CaCO3 powder raw material. The weight of CaCO3 is 4 times the weight of Cr2O3. Subsequently, after mixing the powder raw materials evenly, melt and cool to obtain a solid solution, and then break the obtained solid solution into small pieces as the Cr 4+ :LuAG pre-crystallization material; where x is the doping concentration of Cr 3+ , x = 0.5 at%, and the total weight of the raw materials is 1.5 kg. Subsequently, lay 1.5 kg of Cr 3+ Cr with a doping concentration of 0.5 at% 4+ :LuAG pre-crystallization material flat in a boat-shaped molybdenum crucible after alcohol washing (the crucible size is 200mm×100mm×30mm), and insert a pure LuAG seed crystal into the seed crystal groove at one end of the boat-shaped molybdenum crucible near the shoulder angle. The seed crystal direction is

[100] ; then place the boat-shaped molybdenum crucible containing the pre-crystallization material and the seed crystal into the single crystal furnace, and make the equal-width area of the boat-shaped molybdenum crucible located in the heating area of the single crystal furnace, and then close the furnace chamber; (2) Melting the pre-crystallization material: Turn on the vacuum pump and water cooling circulation, extract the vacuum. When the vacuum degree reaches 8.5×10 -4 Pa, turn on the heating power supply of the single crystal furnace, and perform resistance heating to raise the temperature of the heating area. When the power reaches 30 kW, observe the melt flow state, and find that the length of the melting zone is too long and the convex degree of the solid-liquid interface is relatively large. Judge that the heating power is too high, and then fine-tune the power at a rate of 0.2kW / h until the solid-liquid interface tends to be flat. At this time, the length of the melting zone is 30mm, and keep the melt stable for 1h; (3) Crystal seeding, shoulder broadening, and equal-width growth: Move the boat-shaped molybdenum crucible so that the pre-crystallized material near the seed crystal end is located in the heating zone, and the seed crystal is just at the edge of the heating zone. When the melted length of the seed crystal reaches 1 / 4 of the total length of the seed crystal, keep it at a constant temperature for 45 minutes with the power unchanged. Then turn on the automatic crucible movement program and move the boat-shaped molybdenum crucible towards the end with the seed crystal at a growth rate of 1 mm / h for 50 hours, successively performing the crystal seeding and shoulder broadening growth processes. After the shoulder broadening is completed, move the boat-shaped molybdenum crucible towards the end with the seed crystal at a growth rate of 1.5 mm / h for equal-width growth until the crystallization process ends. During the movement, the melt in the part of the boat-shaped molybdenum crucible gradually away from the heating zone generates supercooling, which serves as the crystallization driving force to promote the generation of single crystals. The shoulder broadening angle is 90°.

[0026] (4) Cooling and annealing: After crystallization is completed, enter the cooling and annealing stage. First, cool it at a cooling rate of 30 °C / h to the annealing temperature of 1700 °C and keep it at a constant temperature for 20 hours. Then cool it to room temperature at a rate of 60 °C / h. Finally, open the furnace and take out the crystal.

[0027] (5) High-temperature annealing in air environment: Place the grown crystal in a high-temperature annealing furnace and perform high-temperature annealing in an air environment. The annealing temperature is set at 1350 °C and the annealing time is 48 hours to promote the transformation of residual Cr3+ to Cr4+, thereby increasing the concentration of Cr4+. After annealing, the crystal is cooled with the furnace and taken out for use.

[0028] After testing, the quality of the Cr 4+ :LuAG crystal grown by this process is good, and its size is 130 mm × 85 mm × 20 mm.

[0029] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and do not limit the technical solutions. Those of ordinary skill in the art should understand that any modifications or equivalent replacements made to the technical solutions of the present invention without departing from the purpose and scope of the present technical solution shall be covered by the scope of the claims of the present invention.

Claims

1. A Cr 4+ : A method for horizontal directional crystal growth of LuAG crystals, characterized in that: The following steps are involved: (1) Cr 4+ : LuAG pre-crystallization material is spread flat in a boat-shaped crucible, and then a seed crystal is inserted into a seed crystal groove near one end of the boat-shaped crucible near the shoulder angle, and the boat-shaped crucible containing the pre-crystallization material and the seed crystal is placed in a working furnace, and the equal width area of ​​the boat-shaped crucible is located in the heating area of ​​the working furnace, and the furnace is closed; (2) Turn on the water cooling cycle and evacuate the working furnace. When the vacuum degree is less than 1×10 -3 Pa, turn on the heating power of the working furnace to heat the pre-crystallized material in the boat-shaped crucible in the heating area, and control the heating power to melt the pre-crystallized material in the heating area to form a narrow melting zone with a length of 20-40 mm, and keep the temperature for 1-2 hours to ensure the stability of the melt state; (3) After the melt state is stable, keep the heating power unchanged, move the boat crucible horizontally so that the heating area is aligned with the end of the boat crucible containing the seed crystal for heating, and ensure that the seed crystal is located at the edge of the heating area. Keep warm for 20-60 minutes. When the melting length of the seed crystal is 1 / 4-1 / 3 of the total length of the seed crystal, move the boat crucible slowly toward the end of the boat crucible containing the seed crystal at a growth rate of 0.5-1mm / h, and perform seeding and shoulder release growth in turn. After the shoulder release is completed, continue to move the boat crucible slowly toward the end of the boat crucible containing the seed crystal at a growth rate of 1-2mm / h until all the pre-crystallization materials are completely melted and crystallization is completed. After cooling and annealing, Cr 4+ :LuAG crystal; During the movement, the melt in the boat-shaped crucible that gradually moves away from the heating area produces supercooling, which serves as a driving force for crystallization and promotes the production of single crystals.

2. According to claim 1, Cr 4+ : A method for horizontal directional crystal growth of LuAG crystals, characterized in that: In step (3), the cooling annealing procedure is as follows: first cool to 1700-1800°C at a cooling rate of 15-35°C / h, and keep at this temperature for 20-30h, then cool to room temperature at a cooling rate of 30-70°C / h, open the furnace, and take out the crystal.

3. According to claim 2, Cr 4+ : A method for horizontal directional crystal growth of LuAG crystals, characterized in that: The Cr obtained in step (3) 4+ The LuAG crystal is placed in a high temperature annealing furnace and annealed in air at a temperature of 1250-1450°C for 24-72 hours to promote the removal of residual Cr. 3+ To Cr 4+ Transformation, thereby increasing Cr 4+ concentration; after annealing, the crystal is cooled with the furnace and taken out for use.

4. The Cr according to claim 1 4+ : A method for horizontal directional crystal growth of LuAG crystals, characterized in that: In step (1), the Cr 4+ : The preparation method of LuAG pre-crystallization material is as follows: （5-x） Cr x O 12 The powder raw materials Cr2O3, Lu2O3, Al2O3 and CaCO3 are weighed at the same time. The weight of CaCO3 is 1 to 7 times the weight of Cr2O3. The powder raw materials are then mixed evenly, melted and cooled to obtain a solid solution, and the obtained solid solution is broken into small pieces as Cr 4+ :LuAG pre-crystallized material; wherein x is Cr 3+ The doping concentration of x ranges from 0.1 to 3 at%.

5. The Cr according to claim 1 4+ : A method for horizontal directional crystal growth of LuAG crystals, characterized in that: In step (1), the seed crystal is <111> , <100> or <110> direction.

6. The Cr according to claim 1 4+ : A method for horizontal directional crystal growth of LuAG crystals, characterized in that: The shoulder release angle during the shoulder release phase is 80~120º.

7. The Cr according to claim 1 4+ : A method for horizontal directional crystal growth of LuAG crystals, characterized in that: The boat-shaped crucible is a boat-shaped molybdenum crucible.