JP2011121840A - Terbium oxide crystal for magneto-optic element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a terbium oxide crystal for a magneto-optic element, the crystal having high transmittance and containing trivalent Tb ion at high concentration. <P>SOLUTION: There is provided a translucent terbium oxide crystal for the magneto-optic element, wherein a crystal system represented by a composition formula of (Tb<SB>1-a</SB>M<SB>a</SB>)<SB>2</SB>O<SB>3</SB>(where M represents at least one element selected from Er, Tm, Yb, Lu, Sc, Mg, Zr and Hf, and (a) satisfies 0.01≤a<0.3), is the cubic crystal system crystalline body, and straight line transmittance is 70% or more per 3 mm length at both of 1.06 μm and 532 nm wavelengths. A scull-melt method is preferable for producing the crystal, the method including filling a water-cooled vessel 1 with a raw material 2 for growing a crystal, heating and melting the center part of the raw material to a high temperature while not melting an outer skin 2a in the outer part of the raw material 2 in contact with the water-cooled vessel, sintering and densifying into a scull state so as to function as a crucible, and sufficiently melting the raw material 2, then reducing the high frequency power, lowering the vessel 1 to cool from the bottom to crystallize. Alternatively, a floating-zone method can be used. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to a terbium oxide crystal for a magneto-optical element and a method for producing the same. The present invention also relates to a magneto-optical device using the terbium oxide crystal as a magneto-optical element.

An optical isolator using the magneto-optical effect is a magneto-optical element used in a laser system. The optical isolator includes a polarizer, a Faraday rotator, an analyzer, and a magnet. The phenomenon that the plane of polarized light rotates when polarized light passes through a material placed in a magnetic field is known as the Faraday effect, and its rotation angle Θ depends on the strength H of the magnetic field and the length L of the substance.
Θ = VHL
It is represented by The proportional coefficient V is called the Verde constant and is a characteristic value depending on the material. When a material having a large V is used for the Faraday rotator, the same isolation performance can be obtained even if the Faraday rotator and the permanent magnet are small, so that the element can be miniaturized. The field of use of optical isolators includes lasers for microfabrication of semiconductors, semiconductor lasers for optical fiber communication, lasers for cutting and heat treatment of steel and ceramics, laser scalpels for medical use, etc. Recently, SHG (second harmonic) elements Incorporated into a visible green laser or blue laser that has been wavelength-converted using a laser is also being used.

As an optical isolator for visible light to near infrared light, a complex oxide TGG (terbium gallium garnet, chemical formula Tb ₃ Ga ₅ O ₁₂ ) single crystal containing trivalent terbium ions is used. A TGG single crystal having a melting point of 1725 ° C. can be prepared by melting a raw material using a crucible that does not react with the raw material melt, for example, a noble metal iridium crucible, and producing a large crystal by the Czochralski method. The Verde constant of a TGG crystal is 40 rad / (T · m), which is considered to be relatively large, but usually a crystal with a length of 2 to 3 cm is required for a Faraday rotator. Further, in order to reduce the Faraday rotator length, a single crystal material having a large Verde constant is desired.

In general, Non-Patent Document 1 describes that in order to increase the Verde constant, the content per unit volume of trivalent terbium ions should be increased. For example, TAG (terbium aluminum garnet, chemical formula Tb ₃ Al ₅ O ₁₂ ) contains the same number of terbium ions as TGG in a small crystal unit cell, resulting in a higher number of terbium per unit volume than TGG. Since they do not melt together, large single crystals cannot be grown by the Czochralski method.

On the other hand, Tb ₂ O ₃ crystal, which is a simple oxide of terbium that does not contain gallium or aluminum, has a melting point as high as about 2400 ° C., so that a iridium crucible made of noble metal cannot be used like a TGG single crystal. The Tb ₂ O ₃ crystal has a monoclinic B-type rare earth structure as a stable phase at a high temperature, in addition to a cubic C-type rare earth structure. Therefore, when a crystal having a B-type rare earth structure is first crystallized from the melt, a phase transition occurs in the C-type rare earth structure during the cooling process, resulting in numerous cracks in the crystal. Cracks due to phase transitions are unsuitable for magneto-optical elements because they scatter laser light and significantly reduce the transmittance. For the above reasons, it has been difficult to industrially produce large Tb ₂ O ₃ single crystals.

Furthermore, terbium ions are more stable in the atmosphere at room temperature than oxides containing tetravalent terbium ions in a highly oxidized state together with trivalent terbium ions than Tb ₂ O ₃ composed only of trivalent terbium ions. . Such oxides have many compositions such as Tb ₄ O ₇ with 50% Tb ^{3+ and} 50% Tb ⁴⁺ , Tb ₁₁ O ₂₀ , Tb ₂₄ O ₄₄ , Tb ₁₆ O _{30 and the} like. When the melt is simply solidified, black-brown Tb ₄ O ₇ is obtained. However, Tb ₄ O ₇ does not transmit light having a visible wavelength from the near infrared, and is not suitable for a magneto-optical element for the wavelength application.

Optical isolators require a high extinction ratio. When the extinction ratio is low, the controllability of the polarization of the laser light is poor, and as a result, the separation ability (isolation performance) of the return light is impaired. A high melting point crystal material has a problem that the extinction ratio decreases due to strain birefringence due to thermal stress during cooling unless an appropriate growth method and growth conditions are selected.
Journal of Applied Physics, Volume 35, Number 8, 2338

The problem to be solved by the present invention is not a crystal for a magneto-optical element typified by a conventional TGG single crystal, but there is no crack due to a phase transition by adding a subcomponent that stabilizes a cubic crystal structure, By applying a crystal mainly composed of Tb ₂ O ₃ whose valence of Tb ions is substantially only trivalent to this field, the transmittance is high and trivalent Tb ions are contained at a high concentration. The object is to provide a terbium oxide crystal for a magneto-optical element.

The present invention
(1) Composition formula (Tb _1-a M _a ) ₂ O ₃ (wherein M is one or more elements selected from Er, Tm, Yb, Lu, Sc, Mg, Zr, Hf, 0.01 ≦ The crystal system represented by a <0.3) is a cubic crystal body, and the linear transmittance per 3 mm length at 1.06 μm and 532 nm is 70% or more, Translucent terbium oxide crystal for magneto-optical element.
(2) A magneto-optical device using the terbium oxide crystal as a magneto-optical element. (3) The raw material is heated and melted in a reducing atmosphere without using a noble metal crucible. The present invention relates to a method for producing a terbium oxide crystal for a magneto-optical element, characterized by crystallizing a cubic crystal body and cooling to room temperature without causing a phase transition.

According to the present invention, it is possible to obtain a crystal body containing substantially only trivalent Tb ions as a main component without containing a higher oxide of terbium that causes coloring. When oxygen is contained even in a trace amount in the atmosphere for crystal growth, a wide absorption derived from higher-order oxidation state terbium ions occurs in the wavelength region of visible light to near infrared light, so that the color is blackish brown. It is not suitable as an optical isolator for lasers in the wavelength range. When a crystal is grown in a reducing atmosphere, no absorption occurs in the wavelength range of 400 nm to 1100 nm except for a narrow absorption near 500 nm caused by trivalent terbium ions, and a crystal having excellent transparency can be obtained. .

Furthermore, by growing a crystal having a cubic C-type rare earth structure from the melt from the beginning, it is possible to obtain a large single crystal free from cracks without causing a phase transition in the cooling process.

By adding an oxide of at least one element of the group consisting of Er, Tm, Yb, Lu, Sc, Mg, Zr, and Hf as a minor component that suppresses the phase transition at a molar ratio of 1% or more and less than 30%, It is possible to directly grow a crystal having a cubic C-type rare earth structure from a melt instead of an oblique B-type rare earth crystal. Since this crystal has no phase transition in the cooling process, a large single crystal can be obtained without causing cracks due to the phase transition. When the addition amount is 30% or more, the Verde coefficient becomes small because the concentration of terbium ions becomes small. If the addition amount is less than 1%, a phase transition occurs, so that the grown crystal has many cracks, and the device cannot be manufactured.

It is also effective to employ a crystal manufacturing method with a large temperature gradient immediately above the melt. Although details of the mechanism are unknown, if the temperature gradient is steep, it becomes possible to increase the supercooling of the melt, and a crystal having a cubic C-type rare earth structure which is a low temperature phase as a metastable phase can be obtained. It is thought that it becomes easy to crystallize. When the skull melt method or the floating zone method is selected as the crystal production method, a large temperature gradient can be easily realized.

The terbium oxide crystal of the present invention has a higher Verde constant than terbium gallium garnet (Tb ₃ Ga ₅ O ₁₂ ) by containing trivalent terbium ions at a higher concentration than conventional. Therefore, since a large Faraday rotation angle can be obtained even with a crystal size smaller than that of the conventional material, it is possible to reduce the size of the isolator element. A small optical isolator is suitable for mounting on a fiber laser. Furthermore, since the magnet for applying the magnetic field can be made small, it can be prevented to the minimum to the magnetic field to the surrounding electronic components, and can contribute to the stabilization of the fiber laser system and the workpiece.

In the present invention, expensive noble metal crucibles are not used for growing terbium oxide crystals. Furthermore, since a single crystal can be grown from a melt with a large temperature gradient, the crystal growth rate can be increased, and there are advantages in cost, mass production, and economy.

A method for manufacturing a crystal is described below.

A terbium oxide having a purity of 99.9% or more, such as Tb ₄ O ₇ or Tb ₂ O ₃ , can be used as a raw material for the crystal. Terbium salts that decompose into oxides and volatile materials by the melting temperature can also be used.

A compound of at least one element selected from the group consisting of Er, Tm, Yb, Lu, Sc, Mg, Zr, and Hf can be added to the Tb raw material in a molar ratio of 1% or more and less than 30%. These elements have the effect of crystallizing crystals having a cubic C-type rare earth structure directly from the melt. As an addition method, an oxide of the element or a salt of the element may be used. Each raw material is weighed and mixed so as to have a target composition. This powder is formed by a uniaxial press or CIP (cold isostatic press) or the like, and used as a raw material for crystal growth.

In order to obtain the crystal of the present invention from the raw material for crystal growth, the melting point of the crystal is as high as about 2400 ° C. Therefore, the melt is held without using a noble metal crucible, and a large temperature gradient is generated. Adopt a configurable single crystal manufacturing method.

Although there are a plurality of methods for producing the single crystal, it is necessary that a crystal having a size required for the magneto-optical element can be grown and that the residual strain inside the crystal is small. For example, the skull melt method is suitable.

In the skull melt method, as shown in FIG. 1, a water-cooled container 1 is filled with a raw material 2 for crystal growth, and the central portion of the raw material is heated and melted to a high temperature. The outer skin 2a of the outer portion of the raw material 2 that is in contact with the water-cooled container is not melted, but is sintered and densified in a skull shape to serve as a crucible. An electron beam, arc discharge, or laser can be used as a heating source for melting the raw material, but high-frequency induction heating is suitable for forming a large crystal. In this figure, a high-frequency induction heating coil 3 is shown. Yes. After the raw material 2 is sufficiently melted, the high frequency power is reduced, and the container 1 is lowered and cooled from the bottom to be crystallized. In this way, a columnar crystal lump can be obtained. Alternatively, the crystal can be pulled up from the melt. The reason for flowing the cooling water is that the outer skin 2a of the raw material 2 is not melted. The water-cooled container and the melt are close to each other, and a large temperature gradient can be easily realized.

The skull melt method is used for crystal growth of cubic zirconia that can be melted in an air atmosphere. In order to apply the Skull Melt method to terbium oxide crystal growth, it is necessary to place each element in a sealed container and strictly control the oxygen partial pressure of the atmosphere, and keep it in a reducing atmosphere so as not to generate highly oxidized terbium ions. There is.

As another crystal growth method, a floating zone (FZ) method can also be adopted. The FZ method is a crystal growth method in which a raw material to be grown is formed into a rod shape, a portion of the rod is heated to partially melt, and the melt is slowly moved while holding the melt zone with surface tension to obtain a single crystal. It is. Although there are methods using a high-frequency induction current or laser as a heating source, the FZ method by focused heating is suitable for high melting point oxide crystals.

FIG. 2 is a conceptual diagram of the FZ method using an image furnace by condensing heating. The lamp 5 is placed as a heat source at one focal point of the spheroid mirror 4, and the raw material rod 6 at the other focal point is condensed and heated. The raw material bar 6 is fixed to the upper shaft 10. The seed crystal 7 is fixed to the lower shaft 11, the lower end of the raw material rod 6 is heated and melted, the upper and lower shafts are moved to join the seed crystal 7, and a melt zone 8 having an appropriate length is formed. The single crystal 9 is obtained by moving downward while rotating the shaft in opposite directions. The seed crystal 7 may be a Tb ₂ O ₃ crystal or a sintered raw material rod. Further, in growing, the rotation of both shafts may be stopped when necessary. The melting zone is isolated by a transparent quartz tube so that the crystal growth atmosphere can be freely controlled. In order to heat the focus intensively, a large temperature gradient can be obtained.

When using a crucible, a crucible made of tungsten metal or rhenium metal, or a tungsten rhenium alloy, which is a high melting point metal that can withstand a reducing atmosphere, can be used. As a crystal growth method in this case, a general pulling method, Bridgman method, slow cooling method, heat exchange method, or the like may be employed.

By using the above single crystal manufacturing method and strictly controlling the crystal growth atmosphere so as not to generate higher-order oxidized terbium ions, the valence of Tb ions is substantially composed of only three valences. _A crystal having a cubic C-type rare earth structure containing ₂ O ₃ as a main component, free from cracks due to phase transition, and linear transmission per 3 mm length at 1.06 μm and 532 nm is 70% or more. Thus, a terbium oxide crystal for a magneto-optical element can be obtained.

Example 1
131.71 g of Tb ₂ O ₃ powder with a purity of 99.9% and 5.51 g of Sc ₂ O ₃ powder with a purity of 99.9% were weighed (molar ratio 90:10) and mixed. Thereafter, CIP molding was performed at a pressure of 100 MPa.

The molded body was filled in a water-cooled container 1. Furthermore, a small piece of metal terbium with a purity of 99.9% was placed on the upper part of the compact, and high frequency induction heating was performed. After induction heating to 2500 ° C. or higher, the container 1 is lowered and cooled from the bottom to be solidified. After solidification, the high frequency coil 3 is turned off and cooled to room temperature while flowing cooling water. Thereafter, the single crystal was taken out from the container 1 and the outer skin 2a was peeled off to obtain a crystal lump. X-ray diffraction analysis revealed that the grown crystal had a cubic C-type rare earth structure. A 2 mm × 2 mm × 3 mm prism was cut out from the obtained crystal lump, and a sample was prepared by mirror-polishing two opposing 2 mm × 2 mm surfaces. When a transmission spectrum in the wavelength range of 1.1 μm to 400 nm of a 3 mm thick crystal was measured using a spectrophotometer Hitachi U-4100, only a sharp absorption band due to trivalent terbium near 500 nm was observed, and tetravalent terbium was observed. The absorption band due to was not observed. The linear transmittance at wavelengths of near infrared light of 1.06 μm and visible light of 532 nm was 70% or more. Next, when the sample was placed in a magnetic field of 0.5 T and sandwiched between Glan-Thompson prisms and the Verde coefficient at 1.06 μm was measured, it was found that the sample was larger than a TGG single crystal of the same size. The extinction ratio measured with the same measurement system was 30 dB.
Examples 2-5

A total amount of 50 g of the Tb ₂ O ₃ powder having a purity of 99.9% and the oxide powder of the additive element M having a purity of 99.9% were weighed. Each powder and 50 cc of ethyl alcohol were put into an alumina mortar and mixed using a pestle for 2 to 3 hours until the ethyl alcohol evaporated and the mixture was not liquid. The mixed powder was further air-dried. Table 1 shows the molar ratio of the kind of additive element M and Tb.

This powder was packed in a rubber tube, shaped into a rod shape having a diameter of 5 mm and a length of 100 mm, and then molded using CIP at a pressure of 100 MPa.

The molded body is attached as a raw material rod 6 and a seed crystal 7, the output of the lamp 5 is adjusted to melt the lower end of the raw material rod 6, and after joining the seed crystal 7, both shafts are moved downward by 10-30 mm / It was moved by hr to obtain a (Tb _1-a M _a ) ₂ O ₃ single crystal. The obtained single crystal was a transparent body free from cracks due to phase transition.

A sample having a thickness of 3 mm was cut from the obtained single crystal. The cut surface was mirror polished. When the adjacent portion of the collected sample was analyzed by powder X-ray diffraction analysis, all the grown crystals had a cubic C-type rare earth structure. Using a spectrophotometer Hitachi U-4100, a transmission spectrum of a 3 mm thick crystal in a wavelength range of 1.1 μm to 400 nm was measured. In Examples 2, 3, and 5, only a sharp absorption band due to trivalent terbium near 500 nm was observed. In Example 4, in addition to this, intrinsic absorption by Yb was present from 900 nm to 1000 nm. In any sample, an absorption band due to tetravalent terbium was not observed. As shown in Table 1, the linear transmittance at a wavelength of near infrared light of 1.06 μm and visible light of 532 nm was 70% or more. When these samples were placed in a 0.5 T magnetic field and sandwiched by Glan-Thompson prisms and the Verde coefficient at 1.06 μm was measured, they were larger than the TGG single crystal of the same size. Table 1 shows the extinction ratio measured with the same measurement system.

As shown in the above test results, it can be seen that a crystal suitable for a magneto-optical element having a high Verde constant, extinction ratio, and linear transmittance is obtained.

Schematic diagram of the skull melt method Schematic diagram of the floating zone method

DESCRIPTION OF SYMBOLS 1 Container 2 Raw material 3 High frequency coil 4 Spherical mirror 5 Lamp 6 Raw material rod 7 Seed crystal 8 Melt zone 9 Single crystal 10 Upper shaft 11 Lower shaft

Claims (6)

Composition formula (Tb _1-a M _a ) ₂ O ₃ (wherein M is one or more elements selected from Er, Tm, Yb, Lu, Sc, Mg, Zr, Hf, 0.01 ≦ a <0) .3) is a cubic crystal, and the linear transmittance per 3 mm length at 1.06 μm and 532 nm is 70% or more, Translucent terbium oxide crystals. 2. A magneto-optical device using the terbium oxide crystal of claim 1 as a magneto-optical element. 2. The method for producing a terbium oxide crystal for a magneto-optical element according to claim 1, wherein the raw material is melted in a reducing atmosphere without using a noble metal crucible, and a crystal system is obtained from this melt. A method for producing a terbium oxide crystal, characterized in that crystallization is performed and cooling to room temperature without causing a phase transition. 4. The crystal production method according to claim 3, wherein the production method is a skull melt method. 4. The crystal production method according to claim 3, wherein the production method is a floating zone method. 4. The crystal manufacturing method according to claim 3, wherein the crucible material is tungsten metal, rhenium metal or tungsten rhenium alloy.