JPH06316498A - Production of potassium niobate single crystal - Google Patents

Abstract

PURPOSE:To reduce the optical loss of a KNbO3 single crystal by heat-treating a potassium niobate single crystal converted to a single domain at a specified temp. CONSTITUTION:A potassium niobate single crystal converted to a single domain is heat-treated below the orthochromic-to-tetragonal phase transition temp. In this case, a conductive film 2 is formed on the faces 3 and 4 of a potassium niobate single crystal 1 to be heat-treated on which positive and negative surface charges are generated, and the films 2 are electrically short-circuited by a lead wire 15.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to potassium niobate (hereinafter simply referred to as KNb) used for wavelength conversion or optical modulation.
Called O ₃ ) More specifically, it relates to a method for producing a single crystal, and more specifically, KNbO capable of reducing optical loss.
₃ A method for producing a single crystal body.

[0002]

BACKGROUND OF THE INVENTION KNbO ₃ single crystals have been noted as a non-linear optical material because they have the property of converting semiconductor laser light, which is monochromatic light in the vicinity of 860 nm or 980 nm, into harmonics of ½ wavelength, respectively. ing.

Conventionally, this KNbO₃The single crystal is manufactured by T.W. FUK
UDA, Y. UEMATU J. J. A. P. 11 (1
973) 163, K₂CO₃When
Nb ₂O_FiveAdd a powder mixture of and at temperatures above 1050 ° C.
Heat-melt, immerse the seed crystal in the melt, and adjust the melt temperature.
It is manufactured by gradually lowering and growing.

The KNbO ₃ single crystal thus obtained is obtained because it involves a cubic-tetragonal phase transition and a tetragonal-orthorhombic phase transition in the cooling process from the melting point to room temperature. The KNbO ₃ single crystal is in a multi-domain state including dipoles having various directions.

KNbO in such a multi-domain state
_{When a} single domain portion is cut out from ₃ single crystals and used as a nonlinear optical material, there is a problem that it is difficult to obtain a large number of optical elements having a desired optical aperture diameter and optical path length.

Therefore, conventionally, a KNbO ₃ single crystal in a multi-domain state as described above is heated to a temperature of about 200 ° C. and an electric field of 1 KV / cm or more is applied in the c-axis direction of the KNbO ₃ single crystal to form a single domain. A KNbO ₃ single crystal is produced by performing a chemical treatment. The single domain-processed KNbO ₃ single crystal thus obtained is used as an optical element.

However, when the KNbO ₃ single crystal subjected to the single domain treatment as described above is used as an optical element, the optical loss when light passes through the crystal is large, and therefore this optical loss is caused. However, there is a problem in that the temperature inside the crystal rises and the wavelength conversion efficiency decreases.

Further, there is a possibility that the KNbO ₃ single crystal may return to the multi-domain state again due to the rise of the temperature inside the crystal, and the optical loss may further increase.

[0009]

THE INVENTION An object of the present invention has been made to try to solve the problems in the prior art as described above, KNbO _3, such as to be able to reduce an optical loss of a single domain of treated KNbO ₃ single crystal It is intended to provide a method for producing a single crystal.

[0010]

SUMMARY OF THE INVENTION The method for producing a KNbO ₃ single crystal according to the present invention is characterized in that a single domain-processed KNbO ₃ single crystal is heat-treated at a temperature not higher than a phase transition temperature between an orthorhombic crystal and a tetragonal crystal. I am trying. Especially heat treated KNbO
_{The 3} single crystal has a conductive film provided on both surfaces of the KNbO ₃ single crystal where positive and negative surface charges are generated, and it is preferable that the conductive film is electrically short-circuited.

According to the present invention, the optical loss of the KNbO ₃ single crystal can be reduced.

[0012]

DETAILED DESCRIPTION OF THE INVENTION The method for producing a KNbO ₃ single crystal according to the present invention will be specifically described below.

In the present invention, the single domain KNbO ₃ single crystal is heat-treated at a temperature not higher than the phase transition temperature between the orthorhombic and tetragonal KNbO ₃ single crystals. The KNbO ₃ single crystal having a single domain state can be obtained by a conventionally known method. For example, a KNbO ₃ single crystal obtained by a conventionally known method is subjected to a single domain conversion treatment (poling treatment). You can Such a single domain treatment can be performed, for example, by applying an electric field of 1 KV / cm or more to a KNbO ₃ single crystal in a multi-domain state at a temperature of about 200 ° C. The single domain treatment as described above is preferably performed by applying an electric field in the c-axis direction of the KNbO ₃ single crystal.

FIG. 1 shows a single domain KNbO ₃
The outline of a single crystal is shown. Reference numeral 1 is a single domain-processed KNbO ₃ single crystal. The planes 5 and 6 perpendicular to the a-axis of the single crystal are a-planes, the planes 7 and 8 perpendicular to the b-axis are b-planes, and the planes 3 and 4 perpendicular to the c-axis are c-planes. When a predetermined electric field in the c-axis direction of the KNbO ₃ single crystal multi-domain state is applied, KNbO ₃ single crystal multi-domain state is the single domain state aligned in a substantially constant direction dipoles in the crystal.

As shown in FIGS. 2 (a) and 2 (b), the heat-treated KNbO ₃ single crystal is provided with conductive films on both sides where positive and negative surface charges are generated. It is preferable that these conductive films are electrically short-circuited. It is considered that the surface charge of the KNbO ₃ single crystal as described above occurs due to the temperature change of the entire crystal or the deviation of the temperature distribution, the strain distribution, etc. in the crystal. These conductive films have a plane (that is, c
It is preferably provided on the surface).

The conductive film 2 is not particularly limited as long as it has conductivity sufficient to remove surface charges, and examples thereof include inorganic conductive materials such as silver, carbon, gold and silver-palladium, and polyaniline. The organic conductive material can be exemplified.

As a method of forming such a material as a conductive film, for example, a method of applying a conductive paste, a method of depositing a conductive substance, and a method of forming by sputtering are listed. You can

The short-circuit means is not particularly limited as long as the conductive film has conductivity to the extent that it can be electrically short-circuited. As such a short-circuit means, the conductor 1 shown in FIG.
5 can be illustrated.

The structures and arrangements of the above-mentioned conductive film and short-circuit means are not particularly limited, and in short, it is sufficient to remove charges generated on the crystal surface. Further, these conducting wires and the like are provided so as to ensure conduction on the surface where positive and negative charges are generated without obstructing the optical path, and are provided corresponding to the cut-out angle, shape, etc. of the crystal.

In the present invention, the single domain KNbO ₃ single crystal as described above is heat-treated at a temperature not higher than the phase transition temperature between the orthorhombic system and the tetragonal system. From the analysis results, it is known that the orthorhombic-tetragonal phase transition temperature of the KNbO ₃ single crystal is 215 ± 5 ° C.

Therefore, in the present invention, the heat treatment of the KNbO ₃ single crystal is 220 ° C. or lower, preferably 180 to 220.
C., more preferably 190 to 210.degree. C.

The heat treatment of such KNbO ₃ single crystal is
It is desirable to perform the treatment for 60 hours or longer, preferably 120 hours or longer, and more preferably 240 hours or longer. Generally, increasing the heat treatment temperature can shorten the heat treatment time.

However, if the heat treatment is performed above the phase transition temperature, the KNbO ₃ single crystal is highly likely to be multi-domained. Further, a conductive film is provided on the surface of the KNbO ₃ single crystal where charges are generated. If this conductive film is electrically short-circuited, the surface charges generated during the heat treatment are removed, and KNbO by this heat treatment is removed. ₃ It is possible to suppress multi-domain formation of a single crystal. This makes it possible to obtain a KNbO ₃ single crystal with little optical loss while maintaining the single domain state of the crystal.

Further, when the KNbO ₃ single crystal is heat-treated, it is preferable not to apply mechanical shock such as vibration and shock, or thermal shock such as rapid heating and rapid cooling. Due to such mechanical and thermal shocks, the single domain KNbO ₃ single crystal may be multi-domained, and cracks may occur in the single crystal.

In order to reduce the thermal shock, it is desirable to raise and lower the temperature of the KNbO ₃ single crystal at 500 ° C./hour or less, preferably 200 ° C./hour or less.

The KN according to the present invention thus obtained
The bO ₃ single crystal is used as an optical element such as a wavelength conversion element.

[0027]

According to the present invention, the KNbO ₃ single crystal subjected to the single domain treatment is heat-treated at a temperature not higher than the phase transition temperature between the orthorhombic crystal and the tetragonal crystal.
The optical loss of the bO ₃ single crystal can be reduced.

In particular, when a conductive film is provided on both surfaces of the KNbO ₃ single crystal where surface charges are generated, and the heat treatment is performed while the conductive film is electrically short-circuited, the single domain state is maintained. KNb with minimal optical loss
An O ₃ single crystal is obtained. Since such a KNbO ₃ single crystal body has little optical loss, it is useful as a high-quality optical element, particularly as an optical modulation and nonlinear optical element.

[0029]

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following examples.

[0030]

Example 1 Single domain treatment K ₂ CO ₃ and Nb ₂ O ₅ mixture was heated and melted at a temperature of 1050 ° C. or higher, a seed crystal was immersed in the obtained melt, and the temperature of the melt was gradually increased. A KNbO ₃ single crystal was prepared by lowering. A sample (hereinafter referred to as sample 1) was cut out from the multi-domain KNbO ₃ single crystal thus obtained, and an electric field of 1 KV / cm was applied at 200 ° C. in the c-axis direction to form a single domain.

Production of Single Crystal FIG. 3 shows an outline of a single domain single crystal.
Surfaces 5 and 6 are planes (a
Surface). This a-face was mirror-polished for use as a light-incident and light-exiting face in light transmittance measurement.

Reference numeral 10 in FIG. 3 denotes an optical axis when measuring the light transmittance of the present crystal, which is set substantially parallel to the a-axis of the present crystal.
The thickness of the crystal body along the optical axis 10 was 0.5 cm. Surfaces 7 and 8 are surfaces (b surfaces) substantially perpendicular to the b-axis of the sample, and these surfaces have already become mirror surfaces immediately after the growth of the single crystal from the melt, and the light incidence in the light transmittance measurement is as it is. And used as a light emitting surface. Reference numeral 9 denotes an optical axis at the time of measuring the light transmittance, which is set substantially parallel to the b axis of the sample. The thickness of the crystal body along the optical axis 9 was set to about 2.8 cm.

By using desired light parallel to the optical axis 9 and the optical axis 10, the light transmittance for a-axis, b-axis and c-axis polarized light was measured, and the result was used to evaluate the optical loss. went.

3 and 4 in FIG. 3 are planes (c-plane) substantially perpendicular to the c-axis of the sample, and positive and negative surface charges are generated on this plane. Therefore, carbon paste was applied on the surfaces 3 and 4 to form the conductive film 2.

Further, the conductive films formed on the surfaces 3 and 4 were electrically short-circuited by the conductive wire 15. Heat treatment Sample 1 produced as described above was heated to 20 for 1 hour.
The process of raising the temperature to 0 ° C., maintaining at this temperature for 60 hours, and then cooling to room temperature over 1 hour was repeated 5 times to perform heat treatment.

Evaluation of Optical Element A light transmittance spectrum (c-axis polarized light) before heat treatment and a light transmittance spectrum (c-axis polarized light) after heat treatment (total heat treatment time 300 hr) are shown in FIG. 4 (a) and FIG. Figure 4
It shows in (b). Further, FIG. 4C plots the calculated values of the light transmittance obtained by considering only the reflection on the sample surface.

When there is no optical loss such as light absorption or light scattering, it is considered that the light transmittance of the KNbO ₃ crystal is in agreement with FIG. 4 (c). Therefore, the difference between (c) and (a) and the difference between (c) and (b) in FIG. 4 indicate the optical loss of Sample 1 before and after the heat treatment, respectively.

Comparing the two, sample 1 was subjected to heat treatment.
It can be seen that the optical loss of 1 was reduced. The optical loss per 1 cm of the sample length at wavelengths of 430 nm, 473 nm and 532 nm was calculated from the measurement result of the light transmittance of the c-axis polarized light and plotted against the total heat treatment time.

The results are shown in FIG. 5A and 5B
And (c) show wavelengths of 430 nm and 473 nm, respectively.
And optical loss at 532 nm. With respect to each wavelength, the optical loss significantly decreases by the total heat treatment time of about 120 hours, and becomes a substantially constant value at the total heat treatment time of about 240 hours.

Next, the optical loss per a sample length of 1 cm at a wavelength of 473 nm of a-axis polarized light, b-axis polarized light and c-axis polarized light was plotted against the total heat treatment time. Results are shown in FIG.

It was found that the optical loss was reduced by the heat treatment in all the polarization directions of a, b and c.

[0042]

[Example 2] Single domain conversion treatment A sample was cut out from the same KNbO ₃ single crystal as the sample 1 of Example 1 was cut out, and 1 K at 200 ° C in the c-axis direction.
An electric field of V / cm was applied to form a single domain. This sample was cut into pieces to obtain samples 2, 3 and 4.

Preparation of Single Crystals Samples 2, 3 and 4 are schematically shown in FIG. These surfaces 7 and 8 are surfaces (b surfaces) substantially perpendicular to the b-axis of the sample, and since they are mirror surfaces, they were used as light emitting / incident surfaces in light transmittance measurement. Reference numeral 9 is an optical axis at the time of measuring the light transmittance, which was substantially parallel to the b axis of the sample. Surfaces 3 and 4 are planes (c-plane) substantially perpendicular to the c-axis of the sample, and positive or negative surface charges are generated. Therefore, carbon paste was applied on the surfaces 3 and 4 to form the conductive film 2. In addition, the conductive films formed on the surfaces 3 and 4 were electrically short-circuited by the copper wire 15. The thickness of the crystal body along the optical axis 9 was about 2.8 cm for each of Samples 2, 3, and 4.

Heat Treatment Sample 2 produced as described above was heated to 20 for 1 hour.
The process of raising the temperature to 0 ° C., maintaining at this temperature for 60 hours, and then cooling to room temperature over 1 hour was repeated 5 times to perform heat treatment. Further, the samples 3 and 4 were treated in the same manner as the heat treatment of the sample 2 except that the temperature rising temperatures were 180 ° C. and 150 ° C., respectively.

With respect to all of Samples 2, 3 and 4, the light transmittance of the c-axis polarized light was measured every one step. The optical loss per 1 cm of sample length at a wavelength of 473 nm was calculated from the obtained light transmittance of each sample and plotted against the total heat treatment time.

The results are shown in FIG. In FIG. 7, (a),
(B) and (c) show the optical loss of samples 2, 3 and 4, respectively. From these results, it was found that the higher the heat treatment temperature, the smaller the optical loss.

[0047]

Example 3 Single Domain Processing Single crystal was prepared in the same manner as in Example 1. A part of the sample was cut out, subjected to the single domain treatment as described above, and then cut again to prepare a total of 6 samples. These were designated as Samples 5-10.

Preparation of Single Crystals Samples 5, 6 and 7 are schematically shown in FIG. In FIG. 1, 7 and 8 are planes (b-planes) substantially perpendicular to the b-axis of the sample. Since these planes are already mirror surfaces immediately after the growth of the single crystal from the melt, the light transmittance is measured directly. It was used as a light incident surface and a light emitting surface. Reference numeral 9 in FIG. 1 denotes an optical axis at the time of measuring the light transmittance, which is substantially parallel to the b axis of the sample.
The thickness of the crystal along the optical axis 9 was about 2.8 cm in all of Samples 5, 6 and 7. No conductive film was formed on Samples 5, 6 and 7. On the other hand, Samples 8, 9 and 10 were prepared in the same manner as Samples 2, 3 and 4, and the outline thereof is shown in FIG.

Heat Treatment Samples 5 and 8 thus produced were placed on the same setter, heated to 200 ° C. over 1 hour, held at this temperature for 100 hours, and then cooled down to room temperature over 1 hour. did.

As a result of this heat treatment, sample 5 was made multi-domain, but sample 8 maintained the single domain state. In addition, the light transmittance was measured using a part of the single domain state that remained in the sample 5.

As compared with the results of measuring the light transmittance of Sample 8, the effect of reducing the optical loss by the heat treatment was almost the same. The results are shown in Table 1.

Similarly, Samples 6 and 9 were placed on the same setter, heated to 180 ° C. over 1 hour, held at this temperature for 100 hours, and then cooled down to room temperature over 1 hour.

As a result of this heat treatment, sample 6 was multi-domained but sample 9 maintained the single domain state. When the two samples were compared, the effect of reducing the optical loss due to the heat treatment was about the same.

The results are shown in Table 1. Further, Samples 7 and 10 were placed on the same setter, heated to 150 ° C. in 1 hour, held at this temperature for 100 hours, and then cooled to room temperature over 1 hour.

As a result of this heat treatment, the sample 7 was multi-domained, but the sample 10 maintained the single domain state. When the two samples were compared, the effect of reducing the optical loss due to the heat treatment was about the same.

The results are shown in Table 1. In Table 1, ◯ indicates that the single domain state is maintained after the heat treatment,
X indicates that the sample was multi-domained by the heat treatment.

It was found that by forming a conductive film on both surfaces of the sample surface where positive and negative surface charges are generated and short-circuiting them, it is possible to perform processing at a higher temperature while suppressing multi-domain formation.

The presence of the conductive film did not have any adverse effect on the effect of reducing the optical loss. Even when the conductive films were short-circuited by using an Al foil instead of the conductive wire, the similar effect of suppressing the multi-domain was recognized.

[0059]

[Table 1]

[Brief description of drawings]

FIG. 1 is a schematic view of a KNbO ₃ single crystal after a single domain treatment.

2 (a) is a schematic view of a KNbO ₃ single crystal after forming a conductive film, and FIG. 2 (b) is a-a in FIG. 2 (a).
It is a schematic sectional drawing cut | disconnected by the surface.

3 is a schematic diagram of Sample 1 used in Example 1. FIG.

FIG. 4 is data of light transmittance spectrum of KNbO ₃ single crystal.

FIG. 5 is a graph showing the relationship between the optical loss of KNbO ₃ single crystal and the heat treatment time for each wavelength.

FIG. 6 is a diagram showing the relationship between the optical loss of KNbO ₃ single crystal at a wavelength of 473 nm and the heat treatment time for each polarization direction.

FIG. 7 is a diagram showing the relationship between the optical loss of KNbO ₃ single crystal at a wavelength of 473 nm and the heat treatment time for each heat treatment temperature.

[Explanation of symbols] 1 ... KNbO ₃ single crystal 2 ... Conductive film 3, 4 ... Single crystal c-plane 15 ... Conductor wire

Claims (2)

[Claims] 1. A method for producing a potassium niobate single crystal, which comprises subjecting a single domain-processed potassium niobate single crystal to a heat treatment at a temperature not higher than a phase transition temperature between an orthorhombic system and a tetragonal system. 2. A conductive film is provided on the surface of the potassium niobate single crystal to be heat-treated, where positive and negative surface charges are generated, and the conductive film is electrically short-circuited. The method for producing a potassium niobate single crystal according to claim 1.