JP2000264787A - Production of nonlinear optical crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a crystal having a large thickness in the c-axis direction by providing a temperature gradient to a crucible containing a mixture of Sr2Be2B2O7 and a flux and growing a crystal by the solution Bridgman method. SOLUTION: A single crystal having more than 4-5 mm thickness in the c-axis direction is obtained and the size is sufficient for use as a nonlinear optical material for UV. A mixture of Sr2Be2B2O7 and a flux is put in a crucible, this crucible is moved in an electric furnace having a temperature gradient and a single crystal is grown from the top of the crucible by the Bridgman method. A strontium borate compound or this compound and an alkali fluoride are preferably used as the flux. The crucible 1 is moved downward in the furnace and cooling and crystal growth proceed from the bottom. The top of the crucible 1 is pointed in such a way that the amount of nuclei formed is reduced and one crystal is formed in the early stage. A powdery mixture is melted at 1,100 deg.C in a platinum crucible, poured in the crucible 1 for the Bridgman method and cooled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a crystal for modulating the wavelength of incident laser light by utilizing a nonlinear optical effect, and a method for manufacturing a nonlinear optical crystal.

[0002]

_2. Description of the Related Art Nonlinear optical crystal Sr ₂ Be ₂ B ₂ O ₇ (SBBO)
an Institute of Research on the Structure of Matte
r, a nonlinear optical material for UV (UVNLO) invented by the research group of Professor CTChen of the Chinese Academy of Science. Other nonlinear optical materials for ultraviolet light include B
There are materials such as aB ₂ O ₄ (BBO) and LiB ₃ O ₅ (LBO). However, since the absorption edge of BBO is at 189 nm, the generation of a wavelength of about 200 nm or less causes a problem of absorption, and the dispersion in the ultraviolet region is large, so that SHG (second harmonic generation: Secondary)
The shortest wavelength that can be phase-matched by Harmonic Generation) is 20
The limit is about 5 nm. LBO has an excellent absorption edge of 160 nm, but the shortest wavelength that can be phase-matched by SHG is limited to about 277 nm because of its small birefringence. BBO and LB
O is not suitable for generating light having a wavelength of 200 nm or less.

On the other hand, SBBO has an absorption edge as low as about 155 nm, and the shortest wavelength that can be phase-matched by SHG is 177 nm or less. Therefore, SBBO is extremely effective for generating a wavelength of 200 nm or less. For the same reason, there is KBe ₂ BO ₃ F ₂ (KBBF) as a material suitable for generating light with a wavelength of 200 nm or less.
This has extremely high cleavage properties due to the problem of the crystal structure, and it is extremely difficult to obtain a crystal having a sufficient size as a nonlinear optical material. Therefore, it is considered that SBBO is the only material best suited as a nonlinear optical material suitable for generating light having a wavelength of substantially 200 nm or less.

[0004]

However, although this SBBO is extremely excellent in physical constants, it is not as easy as KBBF to obtain a crystal of a sufficient size as a nonlinear optical material. There is a problem. The primary cause is the crystal structure of SBBO. In the SBBO crystal structure, BO ₃ groups are bound infinitely in the plane direction by a tetrahedron of BeO ₄ ,
The structure is such that the macro-anion sheets made of Be-BO are layered. However, the binding between the macroanion sheets is due to the tetrahedron of BeO ₄
Sr ions alternate with each other, so that the crystal growth rate in this direction is much slower than in the plane direction, and the crystal shape tends to be plate-like. The second cause is that SBBO is a material of a relative harmonic melting system, and does not melt with the same composition, but is decomposed into a material with a different composition. Therefore, the SBBO crystal cannot be melted and recrystallized by the melt growth method.
Therefore, the SBBO composition can be grown only by a solution growth method in which the SBBO composition is dissolved in a solvent composition and precipitated from the solution. In this case, the growth rate of the SBBO crystal is extremely slow. In particular, the growth rate of the plate-like crystal in the thickness direction (c-axis direction) is extremely slow.

When this SBBO is grown by the flux method, which is the most common method of the solution growth method, it is difficult to obtain a large single crystal by depositing a large number of thin plate-like crystals.
Although the inventor, chen et al., Reported that the SBBO was grown by the flux method, SBBO is currently not industrially available as a nonlinear optical material for ultraviolet applications.

[0006]

In order to solve such a problem, according to the present invention, a nonlinear optical crystal Sr ₂ Be ₂ B ₂ O ₇ (SBB
O) is prepared by the solution Bridgman method, and a method for producing a nonlinear optical crystal is provided.

[0007]

In the solution Bridgman method attempted in this example, a mixture of a target substance for crystal growth and a solvent substance for dissolving it at a high temperature is placed in a crucible, and the crucible itself is exposed to a temperature gradient. Moving through the electric furnace,
A single crystal is grown from the crucible tip.

In this case, the crucible moves along the temperature gradient in the furnace from the high temperature section to the low temperature section, and the moving crucible is gradually cooled from the tip direction. Therefore, when the tip of the crucible reaches the liquidus temperature of the crucible content or lower, nucleation starts at the tip, and the crucible moves to further cool and crystal growth occurs.
Further, by the movement of the crucible, the crystal growth proceeds toward the rear of the crucible as the line having the temperature corresponding to the liquidus temperature advances toward the rear of the crucible.
Here, the crucible to be used usually has a pointed shape, so that the amount of generated nuclei is reduced as much as possible, and an attempt is made to unite the crystals in the initial stage.

FIG. 1 shows an electric furnace (a conceptual diagram of a crystal synthesis furnace) used in this embodiment, and FIG. 2 shows a conceptual diagram of a crucible. The electric furnace is of a vertical type, and a crucible 1 is suspended in a cylindrical furnace as shown in the figure. The crucible moves downward in the furnace, whereby cooling and crystal growth progress from the lowermost part. The crucible 1 is made of platinum, and has a shape in which a cylindrical pipe is attached to a cylindrical tip pointed like a pencil. In the tip part, even if a seed crystal is put, or even if there is no seed crystal, even if polynucleus formation occurs at a stretch in the region reaching the liquidus temperature from the crucible when the first nucleation occurs from the supersaturated state, It is designed so that it can be sufficiently absorbed.

The synthesis of SBBO and solution composition (SBBO dissolved in solvent composition) was performed according to the following procedure. For SBBO, powder synthesis by solid-state reaction is performed, and 2SrCO ₃ + 2BeO + 2H ₃ BO ₃ → Sr ₂ Be ₂ B ₂ O ₇ + 2CO ₂ ↑ + 3H
Utilizes the chemical reaction of ₂ O ↑. The raw material powder used was SrCO3: 295.26 g (2 mol), 2BeO: 50.02 g (2 mol), 2H3BO3: 123.68 g (2 mol). It was placed in a platinum crucible of × 150 mm and heat-treated at 950 ° C. for 48 hours. The heat-treated powder is crushed and mixed using a mortar and then again at 950 ° C.
For 48 hours. The heat-treated powder was pulverized and mixed again using a pot, and then heat-treated at 950 ° C. for 24 hours.

SrB ₂ O ₄ and Na are used as solvent materials for crystal growth.
F was used. Here, SrB ₂ O ₄ performs powder synthesis by a solid phase reaction, and utilizes a chemical reaction of SrCO ₃ + 2H ₃ BO ₃ → SrB ₂ O ₄ + CO ₂ ↑ + 3H ₂ O ↑. The raw material powder used was SrCO ₃ : 147.63 g (1 mol) 2H ₃ BO ₃ : 123.68 g (2 mol). After weighing these, they were thoroughly mixed using a cross rolling mixer, and then Φ100 × 150 mm It was put in a platinum crucible and heat-treated at 800 ° C. for 5 hours. The heat-treated powder is pulverized and mixed using a mortar and then again at 800 ° C for 5 minutes.
Time heat treatment was applied.

The mixing ratio of SBBO and SrB ₂ O ₄ as a solvent is
Judgment is made from the state diagram of -SrB ₂ O ₄ system. This two-component system is a eutectic system, and since there is a eutectic point in the composition of SBBO: SrB ₂ O ₄ = 22: 78, the first precipitation of SBBO from the liquid phase is richer than this. The composition was selected in the region where the phase becomes SBBO. Further, NaF was also used as a solvent to lower the deposition temperature and the viscosity of the solution. The composition ratio of this example is as follows. SBBO: SrB ₂ O ₄ : NaF = 44: 56: 56 × 55/45 Each sample of SBBO, SrB ₂ O ₄ and NaF is SBBO: 24.52 g (0.075 mol) SrB ₂ O ₄ : 16.54 g (0.095 mol) NaF: A mortar was mixed in the state of a powder sample of 4.96 g (0.177 mol). The mixed powder was melted in a 60 cc platinum crucible at 1100 ° C. for 30 minutes, poured into a platinum crucible for Bridgman in a state of a sufficiently low-viscosity liquid phase, and cooled.

The crucible filled with the sample was suspended in a furnace using a platinum rhodium wire. After setting the crucible, raise the furnace temperature at 3.3 ℃ / min, hold the crucible tip at 1100 ℃ or more for 6 hours, until the crucible tip reaches 1030 ℃
The temperature was lowered at a rate of ° C./min, and the furnace temperature was maintained in that state. 6 in this state
After waiting time, start driving the crucible, the driving speed of the crucible is 20mm
/ Went by month. The crystal growth ends when the crucible has moved a total of 90 mm. As a result, a single crystal having a thickness of 6 mm in the c-axis direction was obtained. As a result of analyzing this single crystal by X-ray diffraction,
It was confirmed that it was the required SBBO single crystal.

Further, the SBBO crystal was processed and measured for SHG characteristics. An example of a wavelength conversion experiment of 1064 nm will be described below. The experimental method is shown in FIG. A Q-switched YAG laser manufactured by continuum was used as an input light source, and an SBBO crystal was placed near the focal point of the beam focused by the lens. The parameters of the laser light are as follows. Wavelength: 1064 nm Repetition: 10 Hz Average power: 80 nW Pulse width: about 10 ns Pulse energy: 8 mJ Peak power: 800 kW Lens focal length: 450 mm Beam diameter at crystal position: about 0.1 mm (diameter) Peak power density at crystal: about 10 GW / cm ² Further, generation of the second harmonic by the SHG, and the phase velocity in the input optical crystal has been limited in the direction in which the phase velocity becomes equal at the second harmonic in the crystal caused by the SHG crystal That is, the so-called phase matching condition must be satisfied. This is a condition determined by the wavelength of the incident light. In order to search for a direction that satisfies the phase matching condition (called the phase matching angle by taking an angle from the c axis), the sample crystal can be arbitrarily selected with respect to the incident beam. It is attached to the goniometer. As a result of the experiment, when the angle θ of the incident light with respect to the c-axis of the crystal was θ = 21 °, 532 nm light, which is the 1064 nm second harmonic of the incident beam, was observed in the transmitted light.
This coincides with the phase matching angle of the SBBO with respect to the incident light of 1064 nm.

As a comparative experiment, a crystallization experiment by a normal flux method was also performed, and the results were compared. The synthesis of the sample was the same as that of the solution Bridgman method. SBBO: SrB ₂ O ₄ : NaF = 44: 56: 56 × 55/45 Each sample of SBBO, SrB ₂ O ₄ , and NaF was SBBO: 24.52 g ( _{0.075mol) SrB 2 O 4: 16.54g} (0.095mol) NaF: using the powder sample of 4.96 g (0.177 mol). The powder was mixed in a mortar and then melted at 1100 ° C. for 30 minutes in a 60 cc platinum crucible. After setting the crucible in the muffle furnace, raise the furnace temperature at 3.3 ° C / min.
The temperature was maintained at 1100 ° C. for 6 hours, the temperature was lowered to 1030 ° C. at 3.3 ° C./min, and after the temperature was maintained for 6 hours, the temperature was lowered to 800 ° C. at 2 ° C./day. At this time,
The cooling rate is considered to be substantially equal to the cooling rate at one point in the crucible of the Bridgman method. Result, c
Although a large number of SBBO plate crystals with a maximum thickness of about 2 mm in the axial direction were produced, none of the sizes obtained by the solution Bridgman was included.

[0016]

As described above, according to the present invention, by growing an SBBO crystal by the solution Bridgman method, it is difficult to obtain by a normal flux method, and has a sufficient size as a nonlinear optical material for ultraviolet rays of 200 nm or less. Axial thickness 4-5
A single crystal of mm or more can be obtained more effectively.

[Brief description of the drawings]

FIG. 1 Schematic of the growth furnace

FIG. 2 Schematic of crucible

[Figure 3] Diagram showing SHG evaluation method

[Explanation of symbols]

1 crucible 2 Scale 3 crucible drive motor 4 protective tube (Al ₂ O ₃₎ 5 refractories 6 heating elements (SiC) 7 R thermocouple 8 glass wool 31 Q-switched YAG laser 32 SBBO crystal 33 power meter

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2K002 AB12 CA02 FA12 HA20 4G077 AA02 BD06 BD08 CD02 EC08 MB04

Claims (5)

[Claims] 1. A nonlinear optical crystal by a solution Bridgman method, wherein a crystal is grown by generating a temperature gradient in a crucible containing a mixture of Sr ₂ Be ₂ B ₂ O ₇ (SBBO) and a solvent composition. Manufacturing method. 2. A method for producing a nonlinear optical crystal according to claim 1, wherein a strontium borate compound or a strontium borate compound and an alkali fluoride are used as a solvent composition. 3. The method for producing a nonlinear optical crystal according to claim 2, wherein the strontium borate compound is SrB ₂ O _4.
A method for producing a nonlinear optical crystal, characterized by using: 4. The method for producing a nonlinear optical crystal according to claim 3, wherein the composition ratio of SrB ₂ O ₄ is 28: 72 ≦ Sr ₂ Be ₂ B ₂ O ₇ :
A method for producing a nonlinear optical crystal, characterized in that it is used in the range of SrB ₂ O ₄ ≦ 48: 52. 5. The method for producing a nonlinear optical crystal according to claim 1, wherein a platinum crucible is used as the crucible, and a crucible whose inner surface is protected by platinum is used.