Terbium / gallium garnet single crystal, method for manufacturing the same, and faraday rotator

Abstract

To provide a TGG single crystal without a linear scatterer and achieve a high-performance optical isolator.SOLUTION: A TGG single crystal is grown into a crystal from the melt of a congruent composition by the Czochralski method. The oxide powder of a Tb and Ga compound as a single crystal raw material is used for obtaining the melt of the congruent composition; in the TGG single crystal represented by a chemical-formula Tb(3+x) Ga(5-x) O12, 0.00<X<0.08 is preferable; the crystal growth from the melt of the congruent composition can reduce excess vacancies causing a linear scatterer and suppress the occurrence of the linear scatterer in the TGG single crystal; and the lattice constant measurement of the grown TGG single crystal can verify the melt of the congruent composition.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] This invention relates to terbium gallium garnet single crystals (Tb 3 Ga 5 O 12 This paper relates to a TGG single crystal, its manufacturing method, and a Faraday rotator. [Background technology]

[0002] An optical isolator is an optical element that rotates linearly polarized light that passes through a polarizing plate and enters a magnetic field using a Faraday rotator that has the Faraday effect. By attaching this element to the emission end of a laser light-emitting element, it is possible to block the return light to the laser light-emitting element, and so it is widely used to stabilize the laser light emitted by laser light-emitting elements. In optical isolators for fiber lasers with a wavelength range of around 1 μm, terbium gallium garnet single crystals (hereinafter referred to as "TGG single crystals") that have no absorption in the near-infrared to visible range are used as the Faraday rotation material. (Non-Patent Documents 1, 2)

[0003] TGG single crystals are grown from a melt with a stoichiometric ratio (stoichiometric composition) by the floating zone method (Non-Patent Document 4), the edge-defined film-fed growth method (Non-Patent Document 3), the Czochralski method, etc. (Patent Documents 1 to 4). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2013-056785 A [Patent Document 2] JP 2015-057365 A [Patent Document 3] JP 2015-083523 A [Patent Document 4] Japanese Unexamined Patent Publication No. 1-152605 [Non-patent literature]

[0005] [Non-Patent Document 1] Weizhao Jin, Jingxin Ding, Li Guo, et al., Growth and performance research of Tb3Ga5O12 magneto-optical crystal, J. Cryst. Growth 484 (2018) 17-20. [Non-Patent Document 2] D. J. Dentz, R. C. Puttbach, R.F. Belt, Terbium Gallium garnet for Faraday Effect Devices, AIP Conference Proceedings 18 (1974) 954. [Non-Patent Document 3] Naifeng Zhuang, Caigen Song, Liwei Guo, et al., Growth of terbium gallium garnet (TGG) magneto-optic crystals by edge-defined film-fed growth method, J. Cryst. Growth 381 (2013) 27-32. [Non-Patent Document 4] Zhe Chen, Leiyang, Xiangyong Wang, et al., Highly transparent terbium gallium garnet crystal fabricated by the floating zone method for visible infrared optical isolators, Opt. Mater. 46 (2015) 12-15. [Non-Patent Document 5] WTStacy,Dislocations, facet regions and growth striations in garnet substrates andlayers, J. Cryst. Growth, 24-25 (1974) 137-143. Summary of the Invention [Problem to be solved by the invention]

[0006] As a method for producing single crystals with a garnet structure such as TGG single crystals, powdered oxides of Tb 4 O 7 (99.99%) and Ga 2 O 3 (99.99%) raw material is melted and crystal orientation is adjusted by the Czochralski method. <111> The crystal growth method in which the crystal is pulled up in the direction of the crystal is generally used (Patent Documents 1 to 4).

[0007] The TGG single crystal thus prepared was irradiated with white light. <111> When observing the inside of a crystal from the inside direction, linear scattering bodies extending straight and radially in the crystal growth direction can be seen. The scatterer affects the extinction ratio and reduces the ability to block return light. This is one of the factors that deteriorate the performance of the optical isolator. The object of the present invention is to provide a TGG single crystal without linear scatterers and to realize a high-performance optical isolator. [Means for solving the problem]

[0008] Generally, in terms of single crystal properties, a stoichiometric composition is preferable, but in terms of crystal growth, a congruent composition is preferable because it is possible to easily obtain a crystal with a uniform composition without compositional deviation. The present invention is characterized in that a TGG single crystal is grown from a melt with a congruent composition by the Czochralski method. The inventors have found that by growing a TGG single crystal in this manner, the generation of linear scatterers can be suppressed. In addition, they have found that a melt with a congruent composition can be obtained by measuring the lattice constant of the grown TGG single crystal.

[0009] The cause of the line-shaped scatterers is believed to be the distortion of helical dislocations that occurs when screw dislocations absorb excess vacancies around them and move upward, or impurities around them. The excess vacancies are also believed to occur when defects are introduced from the growth interface because the stoichiometric composition of the melt is different from the congruent composition.

[0010] In the growth of TGG single crystals, it has been common to use a melt of raw materials weighed and mixed according to the stoichiometric composition, because the stoichiometric composition was considered to be congruent (Patent Documents 1 to 4). In the growth by the Czochralski method, the use of a melt with a congruent composition makes it possible to grow a crystal with a uniform composition and reduces excess vacancies in the crystal. As a result of intensive research, the inventors have found that when a TGG single crystal is grown by the Czochralski method, it is possible to grow a crystal without linear scatterers by growing the crystal from a melt having a congruent composition, thereby improving the quality of the crystal. They also found that a melt having a congruent composition can be obtained by measuring the lattice constant of the grown TGG single crystal. Effect of the Invention

[0011] According to the present invention, instead of the conventional stoichiometric composition, the Tb / Ga composition ratio in the melt is optimized and a TGG single crystal is grown using a melt with a congruent composition, thereby making it possible to suppress the generation of linear scatterers, and thus to provide a TGG single crystal free of linear scatterers, thereby realizing a high-performance optical isolator. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram of a TGG single crystal growth apparatus according to the present invention. [Diagram 2] 1 is a graph showing the lattice constant of a TGG single crystal according to the present invention. [Diagram 3] 1 is a graph showing the extinction ratio of TGG single crystals having different densities of linear scatterers according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] An embodiment of the present invention will be described below with reference to the drawings. The single crystal growing method (manufacturing method) of the present invention is a method for growing a terbium gallium garnet (TGG) single crystal having a garnet structure. A TGG single crystal having a garnet structure can be grown by a melt pulling method such as the Czochralski method.

[0014] <Growing device> In the crystal growth method of the present invention, crystal growth is performed using a growth apparatus as shown in Fig. 1. The main parts of this growth apparatus 1 are a crucible 4 that is covered on the outside with a heat-insulating material 2 and into which raw material 3 is placed, a high-frequency heating coil 5 that is disposed on the outside of the heat-insulating material 2 and heats the raw material in the crucible 4, and a pulling shaft 7 that is provided above the crucible 4 so as to be able to rise and fall, a seed crystal 6 is held at its tip, and the pulling shaft 7 rotates in the direction of the arrow, and these components are incorporated in a sealed pressure vessel not shown.

[0015] To grow a single crystal, first, after charging and filling the crucible 4 of the above growth apparatus with the raw material 3, a high-frequency current is passed through the high-frequency heating coil 5 to generate heat in the crucible 4 by high-frequency induction heating, and the raw material 3 in the crucible 4 is heated to a temperature above its melting point and melted. Next, the lifting shaft 7 is lowered to bring the seed crystal 6 into contact with the central part of the melted raw material melt. Then, while adjusting the high-frequency power input to the crucible 4 to gradually solidify the raw material melt around the seed crystal 6, the operation of raising the seed crystal 6 while rotating it is continuously performed to grow a single crystal 8.

[0016] <Single crystal raw material> In the present invention, as the raw material for the single crystal, a compound of Tb and Ga elements can be mixed and used. As the Tb compound, Tb of Tb oxide 4 O 7 or oxides containing Tb 2 O 3 can be used. As the Ga compound, Ga of Ga oxide 2 O 3 containing oxides can be used. The oxide powder preferably has a purity of 99.99% or more, which is generally used for optical crystal materials. The raw materials weighed according to Tb and Ga in Tb (3+x) Ga (5-x) O 12 (0.00 < x < 0.08) are thoroughly mixed. The mixed powder is formed into a compact by cold isostatic pressing (CIP). The formed raw material is preferably reacted at 1200°C in the atmosphere.

[0017] <Growth atmosphere> In the growth of the TGG single crystal of the present invention, it is preferably carried out in an inert gas atmosphere such as nitrogen or argon with a slight addition of oxygen (1 - 2 vol%) to prevent the evaporation of gallium oxide.

[0018] <Growth method> In the method for growing a TGG single crystal having a garnet structure of the present invention, the crystal orientation of the grown single crystal is <111> Since the lifting direction is <111> At the beginning of pulling, the rotation speed and pulling speed of the seed crystal 6 are controlled so that the outer diameter of the grown single crystal 8 gradually expands, forming a shoulder directly below the seed crystal 6. When the outer diameter of the grown single crystal 8 reaches the desired outer shape, the rotation speed and pulling speed of the seed crystal are controlled to form a body. When the crystal has grown to a desired size, it is separated from the melt and slowly cooled to room temperature. The obtained TGG single crystal is preferably annealed in air at 1450°C for 24 hours to remove thermal distortion.

[0019] <Evaluation method> The TGG single crystal of the present invention is evaluated by removing the shoulder at the start of growth and the tail at the end of growth from the grown crystal, and using 1 mm thick wafers cut from the start and end of the cylindrical single crystal straight body. Both end faces of the remaining 8 cm cylindrical single crystal straight body are mirror polished and irradiated with white light to measure the crystal structure. <111> The wafer was observed from the direction to check for the presence or absence of linear scatterers. In addition, one side of the 1 mm thick wafer was mirror polished, and the lattice constant was measured using the bond method. Methods for analyzing minute differences in composition include ICP optical emission spectroscopy (inductively coupled plasma optical emission spectroscopy, ICP-OES / ICP-AES) and measurement of lattice constants using the bond method. EXAMPLES

[0020] (Example 1) to (Example 5) 99.99% pure Tb 4 O 7 Powder and 99.99% pure Ga 2 O 3 The powder was treated with TGG, (3+x) Ga (5-x) O 12 The powders were weighed out so that x = 0.0245 to 0.0255, and mixed in a shaker for 6 hours. After that, a pressure of 100 MPa was applied using a CIP to form the powder into a cylindrical shape. The molded body was placed in a platinum container and sintered at a temperature of 1200°C for 12 hours. The iridium crucible filled with this fired body was placed in a Czochralski furnace, the surrounding area was kept warm with high melting point refractory material, and an induction coil was placed around it. A high frequency current was passed through the coil to induction heat the crucible. After the raw material was melted, a seed crystal was attached and a single crystal was grown. After being pulled up a specified distance, the crystal was separated from the melt and cooled to room temperature. The obtained crystal was processed to check for the presence or absence of linear defects and to measure the lattice constant.

[0021] In order to reduce manufacturing costs, when growing crystals using the Czochralski method, it is common to charge the crucible with the raw materials for the pulled crystal, rather than replacing the raw materials in the crucible each time the crystal is grown, and this series of operations was repeated five times. The TGG single crystals obtained by the first to fifth pulling operations are designated as (Example 1) to (Example 5) corresponding to the number of pulling operations.

[0022] (Comparative Example 1) to (Comparative Example 5) 99.99% pure Tb 4 O 7 Powder and 99.99% pure Ga 2 O 3 The powder was treated with TGG, (3+x) Ga (5-x) O 12 The powders were weighed out so that x = 0.00 at 100°C, and mixed in a shaker for 6 hours. After that, a pressure of 100 MPa was applied using a CIP to form the powder into a cylindrical shape. The molded body was placed in a platinum container and sintered at a temperature of 1200°C for 12 hours. The iridium crucible filled with this fired body was placed in a Czochralski furnace, the surrounding area was kept warm with high melting point refractory material, and an induction coil was placed around it. A high frequency current was passed through the coil to induction heat the crucible. After melting the raw material, a seed crystal was attached and a single crystal was grown. After pulling up a specified distance, the crystal was separated from the melt and cooled to room temperature. The obtained crystal was processed to check for the presence or absence of linear scatterers and to measure the lattice constant.

[0023] As in the example, the growth was carried out five times by adding the melt having the stoichiometric composition, and the results are named (Comparative Example 1) to (Comparative Example 5) according to the number of times the crystal was pulled up.

[0024] (Observation of linear scatterers) From the TGG single crystals grown in the examples and comparative examples, the shoulder at the start of growth and the tail at the end of growth were removed, and 1 mm thick wafers were cut out from the start and end of the cylindrical single crystal straight body. Both end faces of the remaining 8 cm cylindrical single crystal straight body were mirror-polished. The polished end faces were irradiated with white light to obtain a mirror-finished wafer. <111> The presence or absence of linear scatterers was confirmed by observing from the direction. The results are shown in Table 1. The criteria for evaluation ranks (a, B, C) are as follows: Linear scatterer: diameter 100μm~300μm, total length 1mm~50mm 100cm 3 Number of linear scattering sites in a crystal A: 6 or less B: 7~19 pieces C: 20 or more pieces Linear scatterers were hardly observed in Examples 1 to 5. On the other hand, many linear scatterers were observed in Comparative Examples 1 to 5, and it is considered that the performance is deteriorated compared to Examples 1 to 5 when used as a crystal material for an optical isolator.

[0025] (Measurement of lattice constant) From the TGG single crystals grown in the examples and comparative examples, the shoulder at the start of growth and the tail at the end of growth were removed, and 1 mm thick wafers were cut out from the start and end of the cylindrical single crystal body. The surfaces of the wafers were polished to measure the lattice constants of the samples. The lattice constants were measured by the bond method. The bond method is a method of calculating the lattice constant by measuring the angle at which characteristic X-rays are incident on a sample and satisfying the Bragg diffraction condition. The bond method measures the rotation angle of the sample, so it is less susceptible to the effects of X-ray angular divergence and wavelength spread, and is said to be able to measure the lattice constant with high accuracy by performing various corrections such as temperature correction.

[0026] The lattice constant is a physical property that is sensitive to the Tb / Ga composition ratio, and by measuring the lattice constant at two locations, the start and end of the straight body of the TGG crystal, it is possible to observe the movement of the composition ratio in the melt. The measurement results are shown in Figure 2 and Table 1. Note that the lattice constant difference in Table 1 is the difference in lattice constant between the start and end of the straight body of a cylindrical single crystal with a length of 8 cm, and is expressed in units of "Å / 8 cm". Stoichiometric composition (Tb 3 Ga 5 O 12 The difference in lattice constant between the start and end of the straight body of the single crystal grown from the molten Tb is a maximum of 0.0008 Å / 8 cm, and the difference increases with the number of charges (Comparative Examples 1 to 5). On the other hand, the congruent composition (Tb 3+x Ga 5-x O 12 ) (x=0.0245-0.0255) The difference in lattice constant between the start and end of the straight body of a single crystal grown from a melt was about 0.0001 Å / 8 cm, and although the lattice constant increased slightly with each increase in the number of charges, the difference remained almost unchanged (Examples 1-5). A composition in which the lattice constant is almost constant at the beginning and end of the straight body is considered to be a congruent composition. It was found that growing a crystal from a melt of this composition resulted in a TGG single crystal without linear scatterers.

[0027] (Extinction ratio measurement) FIG. 3 is a graph showing the results of measuring the extinction ratio of TGG single crystals with different densities of radial scatterers in one embodiment of the present invention. In the figure, A, B, and C are the measurement results of representative samples corresponding to evaluation ranks A, B, and C in the observation of linear scatterers. The extinction ratio was measured using a polarizer and a HeNe laser. The TGG single crystal was placed on the light beam from the HeNe laser light source so that it was sandwiched between two polarizers. In this case, the amount of transmitted light is maximum when the orientations of the two polarizers are the same, and is minimum when the orientations of the two polarizers are perpendicular. The maximum amount of transmitted light at this time is called P max , the minimum transmitted light amount is P min Then, the polarization extinction ratio Re (dB) is Re = 10 log (P max / P minThe higher the density of the linear scatterers, the lower the extinction ratio.

[0028] As described above, the crystal structure of TGG is Tb (3+x) Ga (5-x) O 12 It is concluded that a composition where x = 0.0245 to 0.0255 is a congruent composition because the difference in lattice constant between the start and end parts of the straight body of the TGG single crystal is small. When grown from this composition, it is possible to provide a TGG single crystal without linear scatterers, making it possible to realize a high-performance optical isolator.

[0029] In addition, the lattice number of the TGG single crystal grown from the melt of the congruent composition is almost constant between 12.3497 and 12.3499 Å with respect to the number of growth cycles, whereas the lattice number of the TGG single crystal grown from the melt of the stoichiometric composition varies greatly between 12.3476 and 12.3489 Å with respect to the number of growth cycles. This shows that by growing the crystal with a melt composition that gives a lattice number of 12.3497 to 12.3499 Å, it is possible to obtain a high-quality TGG single crystal that can suppress the generation of linear scatterers.

[0030] [Table 1]

[0031] (Comparative Example 6) (Comparative Example 7) 99.99% pure Tb 4 O 7 Powder and 99.99% pure Ga 2 O 3 The powder was treated with TGG, (3+x) Ga (5-x) O 12 The powders were weighed out so that x=0.08 (Comparative Example 6) and x=0.15 (Comparative Example 7) were obtained, and mixed in a shaker for 6 hours. After that, a pressure of 100 MPa was applied using a CIP to form the powder into a cylindrical shape. The molded product was placed in a platinum container and sintered at a temperature of 1200°C for 12 hours. The iridium crucible filled with this fired body was placed in a Czochralski furnace, the surroundings were heat-insulated with a high-melting-point refractory, and an induction coil was installed around it. A high-frequency current was passed through the coil to inductively heat the crucible. After the raw material was melted, a seed crystal was attached and a single crystal was grown. After pulling up a predetermined distance, the crystal was separated from the melt and cooled to room temperature. The obtained crystal was processed to confirm the presence or absence of linear scatterers and measure the lattice constant. The measurement results are shown in Table 2. The lattice constant difference in Table 2 is the difference in lattice constant between the starting end and the ending end in the straight cylindrical part of the 8-cm-long columnar single crystal, and its unit is expressed as "Å / 8 cm".

[0032] Since the results of the lattice constant measurement and the lattice constant difference between the starting end and the ending end of the straight cylindrical part are large even in the first growth, it can be said that Comparative Example 6 and Comparative Example 7 deviate greatly from the congruent composition. Although the evaluation of linear scatterers has not been performed for Comparative Example 6 and Comparative Example 7, there is a high probability that a large number of linear scatterers are generated due to the deviation from the congruent composition. Therefore, Tb (3+x) Ga (5-x) O 12 In order to obtain a high-quality crystal in a TGG single crystal having the crystal structure of, it can be said that 0.00 < x < 0.08 is preferable.

[0033]

Table 2

[0034] It should be noted that the crystal growth method of the present invention is not limited to the Czochralski method described in the examples, and it goes without saying that it can be applied to methods for growing crystals using a melt in general, such as the Bridgman method, the FZ method, and the EFG method.

Industrial Applicability

[0035] As described above, according to the present invention, when a TGG single crystal is grown by the Czochralski method, a melt having a congruent composition is used instead of a melt having a stoichiometric composition as has been conventionally used, thereby making it possible to suppress the generation of linear scatterers, and thus to provide a TGG single crystal free of radial scatterers, thereby realizing a high-performance optical isolator. [Explanation of symbols]

[0036] 1. Cultivation device 2 Heat insulation material 3 Raw materials 4 Crucible 5 Coil 6 seed crystals 7 Lifting shaft 8. Single Crystal

Claims

1. Chemical formula Tb （３＋ｘ） G （５－ｘ） O １２ A terbium gallium garnet (TGG) single crystal represented by the formula: wherein x is greater than 0.00 and less than 0.

08.

2. Chemical formula Tb （３＋ｘ） G （５－ｘ） O １２ 1. A terbium gallium garnet (TGG) single crystal represented by the formula:

3. 3. The terbium gallium garnet (TGG) single crystal according to claim 1, characterized in that the lattice constant is greater than 12.3494 Å and less than 12.3502 Å.

4. Tb as a raw material for crystal growth ２ O ３ or Tb ４ O ７ Tb oxide powder selected from the group consisting of Tb oxide powder and Ga ２ O ３ The terbium gallium garnet (TGG) single crystal according to any one of claims 1 to 3, characterized in that the terbium gallium garnet (TGG) single crystal is grown by the Czochralski method from a melt of a Ga oxide powder containing the compound, and that after completion of crystal growth, a cylindrical single crystal obtained by removing a shoulder at the growth start end and a tail at the growth end has a lattice constant difference between a start end and a end end of a straight body portion of 0.0001 Å / 8 cm or less.

5. Tb as a raw material for crystal growth ２ O ３ or Tb ４ O ７ Tb oxide powder selected from the group consisting of Tb oxide powder and Ga ２ O ３ A method for producing a terbium gallium garnet single crystal, which is grown from a melt of a Ga oxide powder containing the chemical formula Tb （３＋ｘ） G （５－ｘ） O １２ The method for producing a terbium gallium garnet (TGG) single crystal according to any one of claims 1 to 4, characterized in that raw materials are weighed out so that X of the terbium gallium garnet (TGG) single crystal represented by the formula (I) is greater than 0.00 and less than 0.08, and the raw materials are heated to melt and then cooled to crystallize.

6. A Faraday rotator using the terbium gallium garnet (TGG) single crystal according to any one of claims 1 to 4.

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