JPH06305894A - Thick film of lithium niobate single crystal and its production - Google Patents

Abstract

PURPOSE:To provide a new material for reducing photorefractive effects causing problems in using a lithium niobate bulk single crystal as an optical element for the second harmonic generation and a method for preparing the material. CONSTITUTION:The single crystal is a liquid-phase epitaxial single crystal film obtained by regulating the thickness of a lithium niobate single crystal to >=50mum and prepared from a flux containing Pb0-B2O3 or GeO2 as one component thereof by using the Z face or the Y or X face tilted from the Z face by an angle within the range of 0.2-2.0 deg. as a substrate face orientation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a promising lithium niobate single crystal film thickness as a material for a second harmonic generating optical element and a method for producing the same.

[0002]

2. Description of the Related Art It is required to improve the recording density of an optical recording disk using a laser beam as a light source. 1
The area of the medium required for each bit depends on the wavelength of the laser light used. Therefore, the shorter the light source wavelength, the higher the recording density. At present, a red semiconductor laser is used as a light source of an optical disk, but if laser light having a wavelength of blue to green can be used, improvement in recording density can be expected.

For this reason, studies have been made on materials for the second harmonic generating optical element capable of halving the wavelength of incident laser light. A lithium niobate single crystal has been proposed as a promising material. In this case, the thickness is required to be 50 μm or more because light is coupled to the end face of the crystal for use. Lithium niobate crystals have hitherto been at their melting point of 126
It has been prepared as a bulk crystal at 0 ° C. by the pulling method.

On the other hand, studies using a lithium niobate crystal as a waveguide element have been made, for example, (Japanese Examined Patent Publication No. Sho 56).
-47160), Li ₂ O-
A technique of epitaxially growing a lithium niobate on a Z-plane substrate from a V ₂ O _{5 type} flux has also been proposed.

[0005]

When a bulk crystal of lithium niobate prepared by the pulling method is used, the composition of the obtained crystal is not always stoichiometric because it is solidified from molten lithium niobate to form a single crystal. Is not
Lot of niobium composition _{Li 0. 9 5 Nb 1.} 0 1 O 3,
It has a so-called harmonic melt composition. Therefore, when light is incident on the crystal, there is a problem that the "photorefractive effect" occurs in which the light is absorbed and the refractive index of the crystal changes, and the efficiency of second harmonic generation decreases. The first cause of the photorefractive effect in bulk crystals is
The crystal composition is a harmonic melt composition that deviates from the stoichiometric composition as described above, and the second cause is that there are many point defects that exist in thermal equilibrium because they are grown at high temperatures. is there. That is, the number of constituent ions occupying vacancies and interstitial spaces in the crystal is large.

On the other hand, Li ₂ O-- which has been conventionally used
When a liquid phase epitaxial single crystal film of lithium niobate is used as the substrate surface of a lithium niobate crystal using a V ₂ O _{5 type} flux (Japanese Patent Publication No. 56-47160), the film thickness is 2-3 μm. In this case, it was revealed from the experiments by the present inventors that although a single crystal film having a flat surface can be obtained, the hillock composed of {012} grows and develops when the thickness becomes 10 μm or more. When the film thickness was further increased over time, compositional supercooling occurred, and a flux was incorporated into the single crystal film, which revealed that it cannot be used as an optical material. This is because the mechanism of crystal growth is the rate-determining surface kinetics in hillrock composed of {012} facets.

[0007]

As described above, in the crystal for the second harmonic generation optical element using the lithium niobate single crystal, it was prepared by the liquid phase epitaxial method in order to reduce the photorefractive effect. Thickness is 50
It is proposed to use a single crystal film having a thickness of μm or more.

For this reason, when the Z plane is the substrate plane orientation, the Z plane, which is originally a kink plane crystallographically, is solid-liquid without growing hillocks composed of {012} facets. It was found that it is an essential requirement to grow a crystal while maintaining it as an interface, and for this reason, a means for continuing the growth while maintaining the crystal growth mechanism at the transport-controlled rate was obtained. That, PbO-B ₂ O as a flux
It is to use a flux based on ₃ series or GeO ₂ as one of the components. Note that the Y or X plane may be used as the substrate plane orientation.

[0009]

By using a single crystal film having a film thickness of 50 μm or more, it has sufficient mechanical strength as well as being cut out from a bulk crystal prepared by the pulling method, and can be connected to an external optical system. A lithium niobate crystal is obtained, which serves as a material for an optical element for second harmonic generation.

[0010] greater PbO-B ₂ O ₃ -based viscosity than Li ₂ O-V ₂ O ₅ system as a flux, or by using a flux for the GeO ₂ as one of its components, the generation of hillocks It becomes possible to grow while maintaining a solid-liquid interface parallel to the used substrate surface, and it is possible to obtain a single crystal film having a film thickness of 50 μm or more. The same effect is obtained not only for the Z plane but also for the substrate tilted from the Z plane by 0.2-2 ° in the a-axis direction. The same effect can be obtained by using the Y and X planes.

Further, since it can be formed at a temperature lower than that of a bulk crystal, the number of vacancies causing the photorefractive effect and the number of constituent ions occupying the interstitial positions are reduced, so that the photorefractive effect is significantly reduced.

[0012]

【Example】

(Example 1) As shown in Table 1, Li ₂ CO ₃ and Nb ₂
O ₅ and O ₅ were precisely weighed so that the molar ratio was 1: 1 and mixed and fired to obtain lithium niobate (LiNbO ₃ ). LiNbO ₃ is 8 with respect to the composition of the total melt containing the flux.
86.59 mol% of PbO and 5.41 mol% of B ₂ O ₃ were precisely weighed and mixed so as to have a mol%, and the mixture was stirred and held at a temperature of 1000 ° C. for 5 hours in a platinum crucible. Then, the temperature of the melt was lowered to 830 ° C., the lithium niobate single crystal Z-plane substrate was immersed in the melt for 8 hours, and a lithium niobate single crystal film was formed on the substrate. Was always flat during the period. When the substrate was pulled up again from the melt, an epitaxial single crystal film having a thickness of 50 μm and a mirror-finished surface was obtained. A crystal piece of 5 mm square was cut out from this thick film crystal, and the end face was optically polished. From this end face, the wavelength 541
irradiating a 25 W / cm ² argon green laser of nm,
The photorefractive effect was investigated using a helium-neon laser. Since this epitaxial single crystal thick film has an optical stoichiometric composition and is grown at a low temperature unlike a bulk single crystal, it occupies vacancies and interstitial position-constituting ions that cause the photorefractive effect. since there is no change in refractive index even when the argon laser was irradiated 10 minutes 2 × 10 ^- greeted ^6. The composition of this single crystal film was analyzed. As a result, Pb had an excessively large ionic radius, so that it did not occupy any lattice position of lithium or niobium of lithium niobate, and was excellent in crystallinity and optical characteristics. On the other hand, when examining the photorefractive effect of lithium niobate created in pulling method from the congruent melt, 10 minutes of argon laser irradiation 10 × 10 ^- change in refractive index ⁵ was observed.

(Comparative Example 1) Under the same conditions as in Example 1, when the time for immersing the substrate was 4.5 hours,
An epitaxial single crystal film having a thickness of 45 μm was obtained. A crystal piece of 5 mm square was cut out from this thick film crystal, and the end face was optically polished. We tried to generate a second harmonic by coupling light of a desired wavelength parallel to the crystal growth surface from this end face, but because the crystal was thin, it was impossible to couple with an external optical system, and the second harmonic was generated. It was impossible to use it as a wave generating optical element. Also, the mechanical strength was not sufficient.

Comparative Example 2 A plate-like sample having a thickness of 50 μm was cut out from a lithium niobate bulk crystal grown by the pulling method, and the end face was optically polished. The photorefractive effect was measured from the end face under the same conditions as in Example 1 by injecting an argon laser, and the result was 10 × 10.
A refractive index change of- ⁵ occurred.

(Comparative Example 3) Further, as an epitaxial crystal growth flux, Li ₂ O-V ₂ conventionally proposed.
When an epitaxial film is formed using the O ₅ system,
Since vanadium was incorporated into the crystal, it was colored green and the optical absorption coefficient was increased. In addition, as the thickness of the epitaxial single crystal film increased, the crystal growth interface became uneven, and eventually the flux was incorporated into the crystal. Therefore, it cannot be used as an optical element for generating a second harmonic.

(Example 2) As shown in Table 1, the stoichiometric composition of lithium niobate (LiNbO ₃ ) was 11.26 mol% with respect to the composition of the total melt containing the flux. 83.20 mol% PbO and 5.54 mol
% B ₂ O ₃ was precisely weighed and mixed, and the mixture was stirred and held at a temperature rise of 1050 ° C. for 7 hours in a platinum crucible. After that, the temperature of the melt is lowered to 903 ° C., and the substrate whose surface is tilted 0.2 ° from the Z-plane of the lithium niobate single crystal in the a-axis direction is immersed in the melt for 14 hours and melted again. When the substrate was pulled up from the solution, an epitaxial single crystal film having a thickness of 100 μm was obtained. A crystal piece of 5 mm square was cut out from this thick film crystal, and the end face was optically polished. A 25 W / cm ² argon green laser having a wavelength of 541 nm was irradiated from this end face in parallel with the crystal growth face, and the photorefractive effect was examined using a helium-neon laser. As a result, even if the argon laser is irradiated for 10 minutes, the change in the refractive index is 8 × 10.
^-It was ⁶ . The composition of this single crystal film was analyzed. As a result, Pb had an excessively large ionic radius, so that it did not occupy any lattice position of lithium or niobium of lithium niobate, and was excellent in crystallinity and optical characteristics. Since the film thickness was as thick as 100 μm, the incidence of light from the end face was easier and more reliable.

Example 3 As shown in Table 1, the stoichiometric composition of lithium niobate (LiNbO ₃ ) was adjusted to 7 mol% with respect to the composition of the total melt containing the flux.
5.00 mol% PbO and 7.00 mol% B
₂ O ₃ is precisely weighed and mixed, and the temperature is raised in a platinum crucible 12
The mixture was held at 00 ° C for 5 hours with stirring. Then, the melt temperature is set to 9
When the temperature of the substrate was lowered to 80 ° C. and the substrate surface was tilted 2.0 ° from the Z-plane in the direction between the two equivalent a-axes, the substrate was immersed in the melt for 12 hours, and the substrate was pulled up again from the melt. Thickness 6
A 0 μm epitaxial single crystal film was obtained. A crystal piece of 5 mm square was cut out from this thick film crystal, and the end face was optically polished. From this end face, the wavelength of 541n is parallel to the crystal growth face.
The photorefractive effect was examined using a helium-neon laser by irradiating a 25 W / cm ² argon green laser of m. As a result, argon LES - refractive index change with a The irradiated 10 minutes 10 ^- greeted ⁵ below. Also,
Analysis of the composition of this single crystal film revealed that Pb has an excessively large ionic radius, so that it does not occupy either lattice position of lithium or niobium of lithium niobate.
It was excellent in crystallinity and optical properties.

(Comparative Example 4) In Example 3, when the angle of inclination of the substrate surface from the Z plane was 2.3 °, hillocks started to be generated when the film thickness became 50 μm or more, and the solid-liquid interface shape was formed. However, it was not flat and the incorporation of the flux into the epitaxial crystal was observed, which was not suitable for use as an optical element.

Example 4 As shown in Table 1, the stoichiometric composition of lithium niobate (LiNbO ₃ ) was 3.12 mol% with respect to the composition of the total melt containing the flux. 93.00 mol% PbO and 3.78 mol
% B ₂ O ₃ was precisely weighed and mixed, and the mixture was stirred and held at a temperature rise of 750 ° C. for 6 hours in a platinum crucible. Then, the temperature of the melt was lowered to 650 ° C., the lithium niobate single crystal Y-plane substrate was dipped in the melt for 18 hours, and the substrate was pulled up from the melt again to obtain an epitaxial single crystal film with a thickness of 60 μm. Was given. A crystal piece of 5 mm square was cut out from this thick film crystal, and the end face was optically polished. A 25 W / cm ² argon green laser having a wavelength of 541 nm was irradiated from this end face in parallel with the crystal growth face, and the photorefractive effect was examined using a helium-neon laser. As a result, the change in the refractive index is 9 × 10 even if the argon laser is irradiated for 10 minutes.
^-It was ⁷ .

Example 5 As shown in Table 1, the stoichiometric composition of lithium niobate (LiNbO ₃ ) was 6.24 mol% with respect to the composition of the total melt containing the flux. 88.78 mol% PbO and 4.98 mol
% B ₂ O ₃ was precisely weighed and mixed, and the mixture was stirred and held at a temperature rise of 1010 ° C. for 6 hours in a platinum crucible. Then, the temperature of the melt is lowered to 880 ° C., and the lithium niobate single crystal Z
When the plane substrate was immersed in the melt for 6 hours and the substrate was pulled up from the melt again, an epitaxial single crystal film having a thickness of 53 μm was obtained. A crystal piece of 5 mm square was cut out from this thick film crystal, and the end face was optically polished. A 25 W / cm ² argon green laser having a wavelength of 541 nm was irradiated from this end face in parallel with the crystal growth face, and the photorefractive effect was examined using a helium-neon laser. As a result, the change in the refractive index is 7.5 × even if the argon laser is irradiated for 10 minutes.
10 ^- was ^7.

Example 6 As shown in Table 2, the stoichiometric composition of lithium niobate (LiNbO ₃ ) was adjusted to 5.40 mol% with respect to the composition of the total melt containing the flux. 83.20 mol% PbO and 11.40 mo
1% of GeO ₂ was precisely weighed and mixed, and the mixture was kept in a platinum crucible with stirring at a temperature rise of 980 ° C. for 5 hours. After that, the temperature of the melt was lowered to 912 ° C., the Y-plane of the lithium niobate single crystal was used as a substrate, and the substrate was immersed in the melt for 14 hours, and the substrate was pulled up from the melt again. was gotten. A crystal piece of 5 mm square was cut out from this thick film crystal, and the end face was optically polished. A 25 W / cm ² argon green laser having a wavelength of 541 nm was irradiated from this end face in parallel with the crystal growth face, and the photorefractive effect was examined using a helium-neon laser. As a result, the refractive index change with an argon laser was irradiated 10 minutes 4 × 10 ^- was ^6.

Example 7 As shown in Table 2, the stoichiometric composition of lithium niobate (LiNbO ₃ ) was adjusted to 8.65 mol% with respect to the composition of the total melt containing the flux. 74.31 mol% PbO, 11.34 mol%
GeO ₂ and 5.70 mol% B ₂ O ₃ were precisely weighed and mixed, and the mixture was stirred and held at a temperature rise of 1070 ° C. for 8 hours in a platinum crucible. After that, the temperature of the melt was lowered to 887 ° C., the Z-plane of the lithium niobate single crystal was used as a substrate, the substrate was immersed in the melt for 22 hours, and the substrate was pulled up again from the melt. was gotten. A crystal piece of 5 mm square was cut out from this thick film crystal, and the end face was optically polished. From this end face, parallel to the crystal growth plane,
The photorefractive effect was examined using a helium-neon laser by irradiating a 25 W / cm ² argon green laser with a wavelength of 541 nm. As a result, the refractive index change with an argon laser was irradiated 10 minutes 4 × 10 ^- was ^6. Since the film thickness was as thick as 110 μm, the incidence of light from the end face was easier and more reliable.

(Example 8) As shown in Table 2, the stoichiometric composition of lithium niobate (LiNbO ₃ ) was 3.12 mol% with respect to the composition of the total melt containing the flux. 65.28 mol% GeO ₂ and 31.60 m
O2% of B ₂ O ₃ was precisely weighed and mixed, and the temperature was raised in a platinum crucible and the mixture was kept at 1005 ° C. for 5 hours with stirring. Then, the temperature of the melt was lowered to 869 ° C., the surface of the lithium niobate single crystal was immersed in the melt for 11 hours, and the substrate was pulled up from the melt again. As a result, an epitaxial single crystal film having a thickness of 55 μm was formed. was gotten. A crystal piece of 5 mm square was cut out from this thick film crystal, and the end face was optically polished. 25 W / cm ² at a wavelength of 541 nm, parallel to the crystal growth surface from this end face.
Was irradiated with argon green laser, and the photorefractive effect was examined using a helium-neon laser. As a result, the refractive index change with an argon laser was irradiated 10 minutes 9.5 × 10 ^- was ^7.

Other Examples Na, Zn, Mn, Cr, F as impurities added to the lithium niobate crystal.
Similar results were obtained when using e, Mg, Nd, Ti, Ni, Co and Rh.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

According to the present invention, a lithium niobate single crystal thick film with reduced photorefractive effect can be obtained. This makes it possible to provide an effective lithium niobate single crystal thick film as a material for a second harmonic generation optical element.

Claims (4)

[Claims] 1. A lithium niobate single crystal thick film, which is formed on a lithium niobate single crystal substrate by a liquid phase epitaxial method and has a thickness of 50 μm or more. 2. PbO from 75.00 mol% to 93.0
In the range of 0 mol%, B ₂ O ₃ is added from 3.7 mol% to 7.0
A method for producing a lithium niobate single crystal thick film, which comprises forming a lithium niobate single crystal thick film on a lithium niobate single crystal substrate by a liquid phase epitaxial method from a flux containing a mol% range. _{3. At} least one of PbO and B ₂ O ₃ and G
A flux containing eO ₂ and having a GeO ₂ content of 1
A lithium niobate single crystal thick film is formed on a lithium niobate single crystal substrate by a liquid phase epitaxial method from a flux of 1.0 mol% to 65.3 mol%. Thick film manufacturing method. 4. The substrate plane orientation is the Z plane or 0.2.
The method for producing a lithium niobate single crystal thick film according to claim 2 or 3, wherein a surface tilted from the Z-plane within a range of ~ 2 ° is used.