JPH10259096A - Growing of lithium tetraborate single crystal for optics - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a single crystal ingot used without waste part when cutting out an optical substance such as a wavelength conversion element by growing lithium tetraborate single crystal for optics by Czochralski method or Bridgman method by using a seed crystal obtained by cutting out the seed crystal in the phase matching direction tilted at a specific angle θm to the c-axis of the lithium tetraborate single crystal. SOLUTION: A seed crystal is cut out in the direction tilted at 32±3 deg. to the c-axis of a single crystal. A growing device 20 has a platinum crucible 21 storing molten liquid 21a of polycrystal of lithium tetraborate. A heater 24 such as a resistance-heating heater for melting the polycrystal of the lithium tetraborate in the crucible 21 through insulators 22 and 23. The temperature of the molten liquid 21a in the crucible 21 is measured by a thermocouple 29. Insulating walls 25 and 26 are doubly installed in the upper part of the crucible, and a rotating and pulling up structure 27 is formed so as to penetrate the insulating walls 25 and 26. The seed crystal is arranged at the tip of the structure 27, and the objective single crystal 28 grows therefrom.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for growing a lithium tetraborate single crystal. More specifically, the present invention relates to a method for growing an optical lithium tetraborate single crystal using a seed crystal cut in a phase matching orientation.

[0002]

_2. Description of the Related Art Lithium tetraborate (Li ₂ B ₄ O ₇ ) single crystal was discovered by Wattmore in 1981 and has attracted attention as a substrate material for new surface acoustic wave (SAW) devices. On the other hand, attempts have been made to use this lithium tetraborate single crystal as an optical material such as a wavelength conversion element, but the conversion efficiency was low, so that it could not be put to practical use. Conventionally, these lithium tetraborate single crystals are prepared by bringing a seed crystal cut in the <110> direction of a lithium tetraborate single crystal into contact with a lithium tetraborate melt from above by the Czochralski method (CZ method). The seed crystal is grown by pulling up the seed crystal or bringing the seed crystal into contact with the lithium tetraborate melt from below by the Bridgman method and then lowering the melt.

[0003]

The inventors of the present invention have made intensive studies on the optical characteristics of lithium tetraborate single crystal, and have found that
It has been found that when a laser beam having a predetermined wavelength is irradiated at a predetermined angle with respect to the optical axis (c-axis) of the single crystal, the wavelength is converted with high conversion efficiency. However, when a wavelength conversion element is cut into a predetermined size from a single crystal ingot grown by the above-described conventional method, a large number of single crystal portions that cannot be used effectively occur, resulting in waste. An object of the present invention is to provide a method for growing a lithium tetraborate single crystal for optics that can use a single crystal ingot without waste when cutting an optical material such as a wavelength conversion element from the grown single crystal ingot.

[0004]

The invention according to claim 1 is
As shown in FIG. 1, a lithium tetraborate single crystal for optics 28 was formed by a Czochralski method using a seed crystal 10 cut out from a c-axis of a lithium tetraborate single crystal into a phase matching direction inclined by a predetermined angle θm. It is a method of nurturing. By this growing, a lithium tetraborate single crystal 28 whose longitudinal direction is the pulling direction of the seed crystal is obtained. Therefore, a wavelength conversion element having a phase matching angle θm in the longitudinal direction of this single crystal is cut out from this single crystal 28, By irradiating the conversion element with laser light parallel or perpendicular to the cutting direction,
The wavelength can be efficiently converted, and the single crystal 28 can be used with good yield.

According to a sixth aspect of the present invention, as shown in FIG. 2, a predetermined angle θm from the c-axis of a lithium tetraborate single crystal.
In this method, an optical lithium tetraborate single crystal 33 is grown by a Bridgman method using a seed crystal 30 cut out at a phase matching orientation that is only inclined. By this growth, a lithium tetraborate single crystal 33 whose longitudinal direction is the moving direction of the melt is obtained. Therefore, a wavelength conversion element having a phase matching angle θm in the longitudinal direction of this single crystal is cut out from this single crystal 33, By irradiating the conversion element with laser light parallel or perpendicular to the cutting direction, wavelength conversion can be performed efficiently and the single crystal 33 can be used with good yield.

According to a second aspect of the present invention, an optical tetraborate is formed by a Czochralski method using a seed crystal cut at an angle of 32 ± 3 degrees from the c-axis of a lithium tetraborate single crystal, not shown. This is a method of growing a lithium single crystal. Although not shown, the invention according to claim 7 uses a lithium tetraborate single crystal for optics by the Bridgman method using a seed crystal cut at an angle of 32 ± 3 degrees from the c-axis of the lithium tetraborate single crystal. It is a method of nurturing. θm
From the lithium tetraborate single crystal obtained by growing at a temperature of 32 ± 3 degrees in the same manner as described above, and Nd: parallel or perpendicular to the cutting direction with respect to this wavelength conversion element.
By irradiating the laser beam of the fundamental wave from the YAG laser, it is possible to generate light of the wavelength of the second harmonic having high coherence.

According to a third aspect of the present invention, there is provided an optical tetraborate for optical lithography using a Czochralski method using a seed crystal cut at an angle of 40 ± 3 degrees from the c-axis of a lithium tetraborate single crystal, not shown. This is a method of growing a lithium single crystal. Although not shown, the invention according to claim 8 uses a lithium tetraborate single crystal for optics by the Bridgman method using a seed crystal cut at an angle of 40 ± 3 degrees from the c-axis of the lithium tetraborate single crystal. It is a method of nurturing. θm
Is cut out from the lithium tetraborate single crystal obtained by growing at a temperature of 40 ± 3 degrees in the same manner as described above, and Nd is parallel or perpendicular to the cutting direction with respect to this wavelength conversion element.
By irradiating a laser beam of a fundamental wave and its second harmonic from a YAG laser, light of a third harmonic wavelength with high coherence can be produced.

According to a fourth aspect of the present invention, an optical tetraborate is formed by a Czochralski method using a seed crystal, which is not illustrated, cut in an orientation inclined by 66 ± 3 degrees from the c-axis of a lithium tetraborate single crystal. This is a method of growing a lithium single crystal. Although not shown, the invention according to claim 9 uses a lithium tetraborate single crystal for optics by the Bridgman method using a seed crystal cut at an angle of 66 ± 3 degrees from the c-axis of the lithium tetraborate single crystal. It is a method of nurturing. θm
From a single crystal of lithium tetraborate obtained by growing at a temperature of 66 ± 3 ° in the same manner as described above, and Nd is parallel or perpendicular to the cutting direction with respect to this wavelength conversion element.
When a laser beam of a second harmonic of the fundamental wave is irradiated from the YAG laser, light of a fourth harmonic wavelength having high coherence can be generated.

According to a fifth aspect of the present invention, there is provided an optical tetraborate for optical lithography using a Czochralski method using a seed crystal, which is not shown, but which is cut at an angle of 74 ± 3 degrees from the c-axis of lithium tetraborate single crystal. This is a method of growing a lithium single crystal. Further, although not shown, the invention according to claim 10 uses a lithium tetraborate single crystal for optics by the Bridgman method using a seed crystal cut in an orientation inclined by 74 ± 3 degrees from the c-axis of the lithium tetraborate single crystal. It is a method of nurturing. θ
A wavelength conversion element is cut out from the lithium tetraborate single crystal obtained by growing at a m of 74 ± 3 degrees in the same manner as described above, and N
By irradiating the fundamental wave and its fourth harmonic laser light from the d: YAG laser, it is possible to produce light of the fifth harmonic having high coherence.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The lithium tetraborate single crystal of the present invention is produced by the Czochralski method or the Bridgman method. That is, as shown in FIG. 1, in the Czochralski method, the lithium tetraborate single crystal growing apparatus 20 has a platinum crucible 21 in which a melt 21a of lithium tetraborate polycrystal is stored. Since lithium tetraborate has a low melting point among oxides, it can be grown in a platinum crucible. A heating device 24 such as a resistance heater 29 for melting the lithium tetraborate polycrystal in the crucible 21 is provided around the platinum crucible 21 through heat insulating materials 22 and 23. The temperature of the melt 21 a in the crucible 21 is detected by a thermocouple 29. Insulation wall 2 on top of crucible 21
5, 26 are provided in duplicate, and these insulating walls 2
A rotation / pulling-up mechanism 27 is provided to penetrate through 5 and 26. The seed crystal 10 is disposed at the tip of the mechanism 27.
In this method, after the lithium tetraborate polycrystal in the crucible 21 is melted by the heating device 24, the seed crystal 10 is brought into contact with the melt 21 a, and the seed crystal 10 is pulled up while rotating by the rotation / pulling mechanism 27. As a result, a lithium tetraborate single crystal 28 as shown is grown.

As shown in FIG. 2, in the Bridgman method, a crucible 31 made of platinum or graphite is suspended from above by a suspending device (not shown) inside a Bridgman furnace 32. The crucible 31 has a conical bottom end. A seed crystal 30 is arranged at this lower end. The Bridgman furnace 32 has an electric heater 32a for heating the internal crucible 31 to a temperature equal to or higher than the melting point of lithium tetraborate. In this method, a polycrystal of lithium tetraborate is put in a crucible 31, and a crucible 31 is driven into a furnace heated by an electric heater 32a by driving a hanging device (not shown). Near the highest point of the temperature distribution, the lithium tetraborate polycrystal melts. This melting brings the melt 31 a into contact with the upper end of the seed crystal 30. When the crucible 31 is further lowered to a position where the heater 32a is not disposed in this state, the melt 31a crystallizes from the lower end of the crucible, and as the descent lowers, all of the melt finally crystallizes, and the lithium tetraborate single crystal 33 grows. Is done.

The feature of the present invention is that the seed crystal 10
Alternatively, the 30 cutout directions are phase matching directions inclined at a predetermined angle θm from the c-axis of the lithium tetraborate single crystal. Here, the phase matching direction θm is the angle between the laser beam and the optical axis (c-axis) when the wavelength conversion element generates the second harmonic. Since the lithium tetraborate single crystal is a negative uniaxial crystal, the phase matching condition is a type I crystal. Therefore, as shown in FIG. 3, only the phase matching angle θm needs to be considered as the phase matching condition, and there is no need to consider the rotation angle Φ on the c-plane. FIG. 4 is a characteristic diagram showing the wavelength of the laser beam incident on the lithium tetraborate single crystal on the horizontal axis and the phase matching angle on the vertical axis. From FIG. 4, the phase matching angle of the wavelength can be easily obtained from the wavelength of the laser beam incident on the lithium tetraborate single crystal. For example, N
This wavelength is 1.0 when a d: YAG laser is used as a light source.
Since it is 6 μm (1064 nm), an auxiliary line a is drawn,
If an auxiliary line b is drawn from a point p on the characteristic curve, a wavelength of 10
The phase matching angle θm at 64 nm is determined to be 32 degrees. Similarly, when a laser light source other than the Nd: YAG laser is used, the phase matching angle θm can be obtained from the wavelength based on FIG. N which is a typical laser
d: Each phase matching angle θm of the second harmonic, the third harmonic, the fourth harmonic, and the fifth harmonic of the YAG laser is 3
2, 40, 66, and 74 degrees. However, the third harmonic and the fifth harmonic are obtained by sum frequency (SFG).

[0013]

Next, examples of the present invention will be described together with comparative examples. <Example 1> 1300 g of lithium tetraborate polycrystalline raw material powder having a predetermined molar ratio and a purity of 99.99% was charged into a platinum crucible 21 having a diameter of 90 mm and a height of 100 mm shown in FIG. Was melted and then pulled up to a predetermined pulling direction by the CZ method. That is, in this example, the seed crystal 10 cut at 32 degrees from the c-axis was used. A temperature gradient (temperature drop gradient) of 10 mm directly above the melt surface and the melt is 90
° C / cm, and the temperature gradient above it is 30 ° C / cm
I made it. Furthermore, when growing the straight body of the single crystal 28, 0.4 mm
The single crystal was pulled up at a rate of / hour, and a lithium tetraborate single crystal 28 having a diameter of 2 inches and a length of a straight body of 65 mm was grown. Example 2 A lithium tetraborate single crystal having a diameter of 2 inches and a length of a straight body of 65 mm was grown in the same manner as in Example 1, except that a seed crystal cut at 40 degrees from the c-axis was used. .

Example 3 1300 g of the same lithium tetraborate polycrystalline raw material powder as in Example 1 was charged into a platinum crucible 31 having a diameter of 2 inches and a height of 100 mm shown in FIG.
The crucible 31 was lowered into the Bridgman furnace 32 in which the electric heater 32a was energized. The raw material powder was melted at the highest point of the temperature distribution in the furnace. The seed crystal 30 was cut at 66 degrees from the c-axis. The temperature gradient from interface A to 2 cm in FIG. 2 was set at 40 ° C./cm, and the temperature gradient below it was set at 20 ° C./cm. Further, at the time of growing the straight body of the crystal, the crucible 31 was lowered at a speed of 5 mm / day to grow a lithium tetraborate single crystal having a diameter of 2 inches and a length of the straight body of 65 mm. <Example 4> A lithium tetraborate single crystal having a diameter of 2 inches and a length of a straight body of 65 mm was grown in the same manner as in Example 3 except that a seed crystal cut at 74 degrees from the c-axis was used. .

<Comparative Evaluation 1> Examples 1 to 4
In order to evaluate the characteristics of the second harmonic of the lithium tetraborate single crystal, these four single crystals were pulled 10 mm × 10 mm × 30 mm in the descending direction of the melt, respectively.
After cutting into a rectangular parallelepiped of the same shape and size, opposing end surfaces perpendicular to the cutting direction were optically polished to obtain four samples. These polished surfaces were used as an incident surface and an outgoing surface of the laser beam, and the samples were irradiated with the wavelengths shown in Table 1 for each sample using a Nd: YAG laser or a combination of this laser and a lithium tetraborate single crystal. As a result, the laser light of the obtained wavelength is shown in Table 1.

[0016]

[Table 1]

As is clear from Table 1, green light having a wavelength of 532 nm, which is a second harmonic, is emitted from the first embodiment, ultraviolet light having a wavelength of 355 nm, which is a third harmonic, is emitted from the second embodiment, and green light having a wavelength of 355 nm is emitted from the third embodiment. Ultraviolet light having a wavelength of 266 nm, which is a fourth harmonic, and ultraviolet light having a wavelength of 213 nm, which is a fifth harmonic, were confirmed from Example 4.

Comparative Example 1 A lithium tetraborate single crystal having a diameter of 2 inches and a length of a straight body of 65 mm was grown in the same manner as in Example 1 except that a seed crystal cut along the c-axis was used. . <Comparative Example 2> A lithium tetraborate single crystal having a diameter of 2 inches and a length of a straight body of 65 mm was grown in the same manner as in Example 1 except that a seed crystal cut in the <110> direction was used as a seed crystal. . <Comparative Example 3> A lithium tetraborate single crystal having a diameter of 2 inches and a length of a straight body of 65 mm was grown in the same manner as in Example 1 except that a seed crystal cut along the x-axis was used.

<Comparative Evaluation 2> Comparative Examples 1 to 3
Phase matching angle θm = 40 from the single crystal obtained in
, 66 ° and 74 ° were cut out. The sizes of these samples were as in Examples 1 to 4, respectively.
10mm x 10mm x 30mm of the same shape and size as the sample
Cut into rectangular parallelepiped. Table 2 shows the number of samples obtained from the single crystals of Examples 1 to 4 and the number of samples obtained from the single crystals of Comparative Examples 1 to 3.

[0020]

[Table 2]

As is apparent from Table 2, the number of samples obtained from the single crystals of Examples 1 to 4 is about 1.3 times the number of samples obtained from the single crystals of Comparative Examples 1 to 3. Double to about 2.4 times more. Thus, it was found that when growing a lithium tetraborate single crystal in the phase matching orientation, the grown single crystal can be used without waste.

[0022]

As described above, according to the present invention, the Czochralski method or the Czochralski method is used by using a seed crystal cut out from a c-axis of a lithium tetraborate single crystal into a phase matching direction inclined by a predetermined angle θm. By growing a lithium tetraborate single crystal for optics by the Bridgman method, a single crystal ingot can be used at a high yield without wasting when cutting an optical material such as a wavelength conversion element from the grown single crystal ingot. it can.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an apparatus for growing a lithium tetraborate single crystal by the Czochralski method of the present invention.

FIG. 2 is a configuration diagram of an apparatus for growing a lithium tetraborate single crystal by the Bridgman method of the present invention.

FIG. 3 is a schematic diagram showing a definition of a phase matching angle in a crystal.

FIG. 4 is a diagram showing a relationship between an incident wavelength of a laser beam on a lithium tetraborate single crystal and a phase matching angle.

[Explanation of symbols]

 10,30 Seed crystal 21a, 31a Lithium tetraborate melt 28,33 Lithium tetraborate single crystal

Claims (10)

[Claims] 1. A seed crystal cut out from a c-axis of a lithium tetraborate single crystal in a phase matching direction inclined by a predetermined angle (θm).
A method of growing an optical lithium tetraborate single crystal by the Czochralski method using (10). 2. From the c-axis of the lithium tetraborate single crystal, 32
A method of growing an optical lithium tetraborate single crystal by the Czochralski method using a seed crystal cut in an orientation inclined by ± 3 degrees. 3. From the c-axis of lithium tetraborate single crystal, 40
A method of growing an optical lithium tetraborate single crystal by the Czochralski method using a seed crystal cut in an orientation inclined by ± 3 degrees. 4. From the c-axis of the lithium tetraborate single crystal, 66
A method of growing an optical lithium tetraborate single crystal by the Czochralski method using a seed crystal cut in an orientation inclined by ± 3 degrees. 5. The lithium tetraborate single crystal has a value of 74 from the c-axis.
A method of growing an optical lithium tetraborate single crystal by the Czochralski method using a seed crystal cut in an orientation inclined by ± 3 degrees. 6. A seed crystal cut out from a c-axis of a lithium tetraborate single crystal in a phase matching direction inclined by a predetermined angle (θm).
A method for growing a lithium tetraborate single crystal for optics by the Bridgman method using (30). 7. From the c-axis of the lithium tetraborate single crystal, 32
A method for growing a lithium tetraborate single crystal for optics by the Bridgman method using a seed crystal cut in an orientation inclined by ± 3 degrees. 8. The c-axis of the lithium tetraborate single crystal is 40
A method for growing a lithium tetraborate single crystal for optics by the Bridgman method using a seed crystal cut in an orientation inclined by ± 3 degrees. 9. From the c-axis of the lithium tetraborate single crystal, 66
A method for growing a lithium tetraborate single crystal for optics by the Bridgman method using a seed crystal cut in an orientation inclined by ± 3 degrees. 10. From the c-axis of the lithium tetraborate single crystal, 7
A method of growing a lithium tetraborate single crystal for optics by the Bridgman method using a seed crystal cut in an orientation inclined by 4 ± 3 degrees.