JPH0524998A - Production of mtioxo4 group single crystal and ktiopo4 single crystal - Google Patents

Abstract

PURPOSE:To provide the method for producing KTP single crystals having a excellent properties as nonlinear optical crystals. CONSTITUTION:This method features that KPO3 and TiO2 materials discharging no moisture by the reaction at the time of melting are used as raw material substance used for the growth of KTP single crystals, they are melted in a crusible 11 by a flux method to prepare a raw material molten soln. 20 and KTiOPO4 single crystals are produced from the above raw material molten soln. 20 by a pulling up method using seed crystals 22.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a MTiOXO _{4 group} single crystal used in a laser light wavelength conversion optical element and a method for producing a KTiOPO ₄ single crystal belonging to this group.

[0002]

2. Description of the Related Art Conventionally, when a certain kind of crystal is irradiated with light, a phenomenon in which a harmonic wave of the irradiated light is obtained is a so-called S
HG (Second Harmonic Genera
known as This type of crystal is called a nonlinear optical crystal. For example, 1.
When this nonlinear optical crystal is irradiated with 06 μm infrared light,
Green (0.53 .mu.m) light having a half wavelength is obtained.
Attention has been paid to an optical wavelength conversion technique that uses such a nonlinear optical crystal to generate light that cannot be obtained by a normal laser.

As guidelines for showing the performance of SHG, (a) high conversion efficiency, (b) the phase matching angle is in a range that is easy to use and wide, (c) transparent and stable, Etc.

Conventionally known SHG crystals include KDP, LiNbO ₃ , β-BBO and the like. However, these crystals have advantages and disadvantages. For example, KDP is water-soluble and unstable. Although good quality crystals of LiNbO ₃ have been obtained, the conversion efficiency is low when a device is formed as a bulk crystal. β-BBO does not have hygroscopicity, but has a phase transition temperature near its melting point, which makes it difficult to form crystals.

Recently, as an alternative to these SHG crystals, KT which is stable, has high conversion efficiency, and is easy to grow crystals.
An iOPO ₄ single crystal (hereinafter referred to as a KTP single crystal) is drawing attention. KTP single crystal is not hygroscopic and has a fracture threshold of about 200 W / cm ² And the wavelength is 0.35 μm
In the range of up to 4.0 μm, it is transparent and exhibits excellent conversion efficiency.

However, when producing this KTP single crystal,
When the starting raw material is dissolved, a bumping reaction of the solution occurs and the raw material and the like are scattered from the platinum crucible. This is for the following reasons. Conventionally, KH ₂ PO ₄ and TiO ₂ are generally used as starting materials for producing KTP single crystals. This releases H ₂ O as shown in the following equation. KH ₂ PO ₄ + TiO ₂ → KTiOPO ₄ + H ₂ O ↑

The H ₂ O produced by this reaction rapidly evaporates during the temperature rise, which causes bumping. For this reason,
The phenomenon that the starting material and the flux material overflow from the platinum crucible occurs, the composition of the melt inevitably changes, and the obtained crystal becomes different from the original KTP crystal. Therefore, the KTP single crystal obtained by the conventional method does not serve as a non-linear optical crystal and has a practical problem.

[0008]

As described above, the conventionally known nonlinear optical crystal has advantages and disadvantages, which is a practical problem. KTP, which is also called an excellent nonlinear optical crystal
In the conventional manufacturing method, the composition of the melt is changed by the bumping reaction, and a desired single crystal cannot be obtained.

The present invention has been made in view of the above points, and provides a method for producing an MTiOXPO _{4 group} single crystal and a KTP single crystal, which can obtain a single crystal having a desired composition by preventing a bumping reaction of a melt. The purpose is to

[0010]

SUMMARY OF THE INVENTION The present invention is directed to MTiOXO.
_When producing a Group ₄ single crystal, a raw material that does not release water in the reaction during melting is used as a starting raw material. In the case of KTP single crystals, examples of such starting materials are KPO ₃ and TiO ₂ . This starting material is used, for example, as a phosphoric acid-based flux (K ₄ P ₂ O ₇ or 2KP
O ₃ ) is melted by a flux method to form a raw material melt, and crystals are pulled from the raw material melt. A KTP crystal, which is a crystal to be grown, can also be used as a starting material.

[0011]

According to the present invention, by using a starting material for KTP crystal growth that does not release any water due to the reaction, the bumping reaction at the time of dissolution can be prevented and a high quality KTP single crystal can be obtained. it can.

In particular, KPO ₃ and TiO _{2 are used} as starting materials.
When used, the molar ratio of KPO ₃ / TiO ₂ is 0.5 to
The flux was set to 3.0 and K ₄ P ₂ O ₇ ,
Using a phosphoric acid-based flux such as 2KPO ₃ , the melting of the starting material is set to a temperature of 950 ° C. or lower, and the cooling rate is 1 ° C./h (particularly preferably 0.5 ° C./h
By setting the temperature to 0.1 ° C / h), a large KT with good quality can be obtained.
It becomes possible to obtain a P single crystal, which is sufficiently practical as a nonlinear optical crystal.

[0013]

The details of the present invention will be described below with reference to the illustrated embodiments.

FIG. 1 is a sectional view showing a schematic structure of a crystal pulling apparatus used in one embodiment of the present invention. In the figure, 10 is a container, and a platinum crucible 11 filled with a raw material melt 20 is accommodated in the container 10. The crucible 11 is installed on a support base 12, and a support shaft 13 perpendicular to the support base 12 is attached to the lower surface of the support base 12. The support shaft 13 is provided so as to penetrate the bottom of the container 10, and can be rotated and moved up and down by being driven from the outside. A thermocouple 14 for measuring the temperature of the raw material melt 20 is attached to the support shaft 13.

An alumina shield member 15 for heat insulation is arranged on the top of the crucible 11, and an alumina shield member 16 for heat insulation is also arranged on the bottom and side portions of the crucible 11. By these shield members 15 and 16, the crucible 11
Is surrounded. A heater 17 for heating the crucible is inserted inside the shield member 16.

Above the crucible 11, a support rod (crystal pulling rod) 21 capable of rotating and moving up and down is installed, and a seed crystal 22 is attached to the lower end of the support rod 21. After the seed crystal 22 is brought into contact with the raw material melt 20, the crystal is pulled and grown by pulling it under predetermined conditions. In the figure, 25 is an observation window and 26 is an atmosphere adjusting port. An example of producing a KTP single crystal using the pulling device configured as described above will be specifically described.

In this embodiment, KPO is used as the KTP raw material.
₃ (potassium metaphosphate) and TiO ₂ (titanium dioxide)
Was used. As the flux, K ₄ P ₂ O ₇ (potassium pyrophosphate) and 2KPO ₃ were mixed and used, and the ratio KTP / flux KTP / flux was set to 0.55 by weight. At this time, the raw material composition was KPO ₃ 2 mol, TiO ₂ 2
Molar, flux composition is K ₄ P ₂ O ₇ 1 mol, 2KPO
₃₁ mol. These spilling raw material and flux are housed in the platinum crucible 11 of the pulling apparatus shown in FIG.

First, the heater 17 was heated to melt the raw materials and the like. At this time, the raw material powder melted from the lower part of the crucible and gradually became a liquid as the temperature increased, but the phenomenon such as bumping did not occur. After the temperature has reached 950 ° C., the temperature is maintained for 10 hours to completely dissolve the raw materials in the crucible 11 to cause a reaction and degassing.

Next, the seed crystal 22 is brought into contact with the raw material melt 20 so that the seed crystal 22 stains the raw material melt 20.
After that, it is gradually cooled at a rate of 1 ° C. for 1 hour to grow crystals.
During these series of experiments, the crucible 11 and the seed crystal 22 are each rotated at 5,10 rpm. The pulling is performed by pulling the seed crystal 22 at a speed of 1 mm / h, and the grown crystal is pulled out from the melt when the melt temperature reaches 750 ° C., and the growth is stopped.

The KTP single crystal thus obtained is
With a diameter of 1 cm and a length of 1.5 cm, microscopic observation revealed no microscopic holes inside, and the crystals were completely transparent without clouding.

Next, as a comparative example, K is changed to KTP.
A starting material synthesized with H ₂ PO ₄ (potassium dihydrogen phosphate) and TiO ₂ was used, and the flux was subjected to crystal pulling using the same phosphoric acid-based flux as in the example. KTP
/ Flux was set to 0.55 as in the example, and these were mixed well and housed in the platinum crucible 11 of the pulling apparatus shown in FIG. When the heater 17 was heated to dissolve the raw materials and the like, the raw material powder moved up and down around the part where the raw material powder had melted, and the movement became vigorous as the temperature rose, and the gelled raw material repeatedly bumped. While rising, the phenomenon of overflowing from the crucible 11 occurred. After that, as the temperature rises, the bumping also subsides, and the raw material remaining in the crucible melts and gradually becomes a liquid. The subsequent procedure was the same as in the above-mentioned example, and the crystal was pulled up. The KTP single crystal thus obtained has a diameter of 1 cm and a length of 1 cm.
It was a crystal in which white turbidity was partially observed, which is visually observable.

K obtained by these examples and comparative examples
A TP single crystal having a thickness of 1.0 mm was cut out from the center of the crystal in the direction perpendicular to the pulling axis. Then, the results of measuring the light transmittance after finishing both surfaces with optical mirror surfaces are shown in FIG. The solid line in the figure is the case of the KTP crystal according to the example, and the broken line is the comparative example. In the crystal according to the comparative example, 2.
Although absorptions at 8 μm and 3.4 μm are recognized, it is clear that the crystals of the Examples do not have such absorption and are transparent, and the rising at the absorption edge is steep.

In the above examples, KPO ₃ was set to 2 moles (KPO ₃ / TiO ₂ molar ratio was 1) among the starting materials, but 4 moles (K / Ti) of K excess side, that is, KPO ₃ was set.
Similarly, when the molar ratio was set to 2), completely transparent crystals having a diameter of 1 cm and a length of 1.5 cm were obtained. K / in the K excess region
When the Ti molar ratio was set to 3.5, the diameter was still 1c
Transparent crystals of m and 1.5 cm in length were obtained. In the Ti excess region. Even when the K / Ti molar ratio was set to 0.5, transparent crystals having a diameter of 1 cm and a length of 1.5 cm were obtained.

FIG. 3 shows the transmittance using light absorption for the crystals obtained when the above-mentioned raw materials having different K molar ratios were used. The solid line A is a crystal obtained with a K / Ti molar ratio of 1 and 2, the broken line B is a crystal made with a K / Ti molar ratio of 0.5, and the chain line C is obtained with a K / Ti molar ratio of 3.5. The characteristics of the obtained crystals are shown respectively.

From FIG. 3, the obtained KTP crystal is transparent to a shorter wavelength because the K / Ti molar ratio is 1, 2, 0.5.
In the case of 3.5, there was a shift of about 10 nm to the wavelength side. However, even if the K / Ti molar ratio is 0.5, it is 400 to 500.
There was an absorption shoulder near nm, and the absorption shoulder was large at a K / Ti molar ratio of 3.5.

Experiments were carried out in the above Examples with various K / Ti molar ratios. When the K / Ti molar ratio was 0.1 or less, "su" was introduced into the obtained crystals to cause cloudiness. Was remarkable. Further, when the molar ratio is 3.0 or more, the decrease in absorption is large near the blue region of 400 to 500 nm, and the light extraction efficiency is significantly deteriorated. Therefore, K
The / Ti molar ratio is preferably set in the range of 0.5 to 3.0. Further, in the K / Ti molar ratio range of 1.0 to 2.0, completely transparent crystals were obtained, and further, no reduction in absorption in the blue region was observed, and as a result, the light extraction efficiency was improved.

Next, when experiments were carried out by changing the melting temperatures of the raw materials of the examples, the largest and most transparent crystals were obtained at the melting temperatures of 850 ° C. and 950 ° C., but the melting temperature was 850 ° C.
Below, only small crystals grew in consideration of the solubility. Further, when the melting temperature is 950 ° C. or higher, the solubility curve also becomes steep, resulting in lack of reproducibility, cell growth, and a lot of cloudiness.

Further, the melting temperatures of the examples are 850 ° C. and 95.
The experiment was conducted by fixing the temperature at 0 ° C. and changing the cooling rate variously.
If the cooling rate was 1 ° C./h or less, a transparent large-sized good crystal was obtained, and particularly, the highest quality crystal was obtained in the cooling rate range of 0.5 ° C. and 0.1 ° C.

In each of the crystals obtained by the above experiment,
Q-switched YAG laser with an average output of 10 W (wavelength 1.0
6 μm), green SHG emission with a wavelength of 0.53 μm was observed. The relationship between the conversion efficiency of the light emitted in this manner and the melting temperature is shown in FIG. 4, and the relationship with the cooling rate is shown in FIG.

As shown in FIGS. 4 and 5, the temperatures at which the raw materials are completely heated and melted are 850 ° C. and 950 ° C.
It was clarified that the SHG conversion efficiency was improved by setting the temperature in the above range and cooling the crystal growth thereafter at 1 ° C./h or less. This is due to the phenomenon of increasing the refractive index of crystals that do not contain water. The higher the refractive index, the better the extraction of light to the outside, and the amount of light absorbed inside is suppressed as much as possible.
The conversion efficiency can be improved. Next, an example using another starting material will be described.

In this example, polycrystal or single crystal (white turbid or small crystal) KTP already obtained is used as a KTP raw material. As the flux, K ₄ P ₂ O ₇ and KPO ₃ were mixed and used as in the previous example, and the ratio KTP / flux KTP / flux was set to 0.55 in terms of weight ratio. Each raw material was melted in the platinum crucible 11 of the pulling apparatus shown in FIG. 1 to form a raw material melt, as in the previous example. Even in this example, the phenomenon such as bumping did not occur. After the temperature reaches 950 ℃ 1
It is held for 0 hour to completely dissolve the raw materials in the crucible 11 to cause a reaction, and degassing is performed. Next, the seed crystal 22 is brought into contact with the raw material melt 20 and gradually cooled at a rate of 1 ° C. for 1 hour to perform crystal growth. The pulling is performed by pulling the seed crystal 22 at a speed of 1 mm / h, and the grown crystal is pulled out from the melt when the melt temperature reaches 750 ° C., and the growth is stopped.

The KTP single crystal thus obtained is
With a diameter of 1 cm and a length of 1.5 cm, microscopic observation revealed no microscopic holes inside, and the crystals were completely transparent without clouding. Table 1 shows the K obtained according to this example.
The result of the impurity analysis by the ICP method for the TP single crystal is shown together with the crystal according to the comparative example.

[0033]                             Table 1   Impurities Fe Ca Al Zr Cu Ni Na   Example 1 1 10 30 <10 <10 10   Comparative Example 10 2 30 50 20 20 <10 40                                                     (Ppm wt)

As is clear from Table 1, the KTP crystals obtained in the examples have smaller amounts of each impurity element mixed in than the crystals of the comparative example. Especially Na is 1 /
4, Fe (transition metal), which is generally regarded as harmful as an optical material, is reduced by about one digit. From the above, it is clear that the KTP crystals obtained in this example have been highly purified and the absorption by impurities has been reduced.

When the crystals according to this example and the crystals according to the comparative example were processed into optical elements for wavelength conversion and irradiated with a Q-switch YAG laser (wavelength 1.06 μm), SHG emission of wavelength 0.53 μm green was observed. Was done.

FIG. 6 shows the relationship between the output power and the incident power. It can be seen that the wavelength conversion optical element using the crystal obtained in the example has a laser output power about three times stronger than that of the wavelength conversion optical element of the comparative example.

FIG. 7 shows the results of measuring the change over time in output power for the purpose of investigating damages due to laser light irradiation using the same sample. As shown in the figure, the ratio of the optical output to the incident time hardly deteriorated in the wavelength conversion optical element of the present example, but decreased with time in the wavelength conversion optical element of the comparative example. That is, it can be seen that the wavelength conversion optical element of the present example is less damaged by laser light irradiation than the wavelength conversion optical element of the comparative example.

It is considered that the difference in the characteristics shown in FIGS. 6 and 7 is due to the change in the refractive index due to the water content and impurities contained in the crystal. Since the crystal according to the present embodiment contains neither water nor impurities, the refractive index It is due to the increase phenomenon of, the greater the refractive index, the better the light extraction to the outside,
This is because the amount of the substance absorbed inside is suppressed as much as possible and the deterioration of the crystal itself hardly occurs. Next, an example in which the KTP crystal obtained by the example using the KTP crystal as a starting material is applied will be described.

In recent years, short-wavelength coherent light sources have been developed for application to high-density optical disc systems, measurement and display systems, and the like. Particularly in an optical disc system, since the spot diameter of the light focused on the disc surface is proportional to the wavelength of the light source, a short wavelength light source is essential to realize high density. As a short-wavelength light source, semiconductor lasers have the advantages of small size, light weight, and low power consumption, so development of shorter-wavelength lasers using new materials is in progress, and oscillation wavelengths in the 0.6 μm band (red) have already been developed. The InGaAlP-based semiconductor laser having the above has reached the level of practical use.

However, although researches have been conducted on green or blue semiconductor lasers having shorter wavelengths, lasers that continuously oscillate at room temperature have not been obtained, and there is no prospect of commercialization.

On the other hand, as another means for realizing a light source with a short wavelength, a wavelength conversion optical element using a nonlinear optical crystal has been developed. This utilizes the second harmonic generation (SHG) of a nonlinear optical crystal, and is formed by forming an ion exchange KTP waveguide on a KTP crystal substrate, for example.

Here, the KTP crystal is an S for YAG laser.
At present, it is considered to be the most excellent crystal as an HG material.
Moreover, the laser damage threshold is high, it has no hygroscopicity, is thermally stable, has a small temperature change in the refractive index, and has good temperature characteristics. On the other hand, as a drawback, the generation of 0.53 μm light of SHG is the limit on the short wavelength side due to the phase matching relationship, and a semiconductor laser as a fundamental wave light source cannot be used. That is, the conversion efficiency is remarkably reduced on the shorter wavelength side than 0.53 μm, and if a semiconductor laser having a small optical output is used as the input light, the optical output after wavelength conversion is extremely small and it cannot be put to practical use.

Therefore, in this embodiment, a wavelength conversion optical element which can increase the conversion efficiency on the short wavelength side and can obtain a practically sufficient output blue laser light in combination with a semiconductor laser was prepared. .

FIG. 8 is a sectional view showing a schematic structure of the wavelength conversion optical element according to this embodiment. In the figure, reference numeral 41 denotes a waveguide section in which K and Rb are replaced by an ion exchange method on a nonlinear optical crystal plate, and this waveguide section 41 is a nonlinear optical crystal K.
The TP single crystal 42 is coated on three sides. The waveguide 41 and the KTP crystal 42 have a rectangular cross section.

Since the waveguide 41 contains Rb in consideration of the refractive index and laser damage, it is irradiated with the semiconductor laser light 43 having a wavelength of about 0.9 μm, which is the external excitation light.
Harmonics (blue laser with wavelength of about 0.45 μm) are generated. The laser light 43 is output as a fundamental wave 45 from the end surface of the waveguide portion 41 that has been mirror-processed, and the optical second harmonic is K
The short-wavelength light 46 is output from the end face of the TP crystal 42. Next, a specific method of manufacturing the wavelength conversion optical element of FIG. 8 will be described with reference to FIG.

First, as shown in FIG. 9 (a), a rubidium chloride aqueous solution 5 containing Rb is formed on the upper surface of the flat KTP single crystal 42.
1 is applied, and then a glass mask 52 is placed on this solution as shown in FIG. 9 (b). Then, by irradiating an electromagnetic wave having a wavelength range shorter than the absorption edge of the KTP single crystal, for example, an excimer laser beam 53 through the glass mask 52, a portion of the single crystal 42 other than the mask has a refractive index as shown in FIG. 9 (c). The waveguide section 42 which is a different layer is formed.
Finally, the aqueous solution 51 is washed away by washing with water or the like to obtain the device shown in FIG. 9 (d).

As a result, the structure shown in FIG. 8 is realized. After that, the end face of the waveguide 42 is mirror-processed if necessary. When the wavelength conversion optical element thus obtained is irradiated with a semiconductor laser beam, an optical second harmonic wave is generated via the waveguide section 41 containing Rb, and this optical second harmonic wave is generated.
Harmonics are emitted from the end face of the crystal flat plate 42. As a result, a wavelength of about 0.45 μm from the end of the optical element
It is possible to obtain the blue laser light.

As described above, according to the present embodiment, the conversion efficiency is obtained by using the KTP single crystal obtained by the present invention which does not contain water at all and has few Fe impurities which are harmful to light. It has become possible to realize a blue laser with high efficiency. That is, the refractive index of the KTP single crystal used in this example is larger than that of the conventional KTP single crystal by 2 to 3 × 10 ⁻³ , and is due to the difference depending on the starting material used for the growth. This also seems to correlate with transmittance. That is, by using a KTP crystal as a starting material, there is no bumping of the material when the material is melted, moreover, impurities can be extracted due to remelting, impurities are suppressed during crystal growth, and as a result there is no optical deterioration. A completely transparent KTP single crystal is obtained. Then, using this KTP single crystal, a nonlinear optical single crystal with high conversion efficiency can be realized.

In the above examples, the production of KTP single crystals was explained exclusively, but in general, the present invention is based on MTiOXO ₄
It can be applied to a genus single crystal. Where MT
The iOXO _{4 group} single crystal is a single crystal having the same properties as the KTP single crystal and has a structure in which M is a metal element such as K, Pb, Sc or Tl and X is a group V element such as P or As. In addition to KTP, KTiOAsO ₄ , PbTiOAs
O ₄ etc. are widely known.

[0050]

As described in detail above, according to the present invention, K
By using a material containing no water as a starting material for the growth of the TP single crystal, the transparency is high and the impurities are small.
A KTP single crystal useful as a wavelength conversion optical element can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view showing a schematic configuration of a crystal growth pulling apparatus used in an embodiment of the present invention.

FIG. 2 is a view showing the transmittance characteristics of KTP single crystals obtained in Examples together with Comparative Examples.

FIG. 3 is a diagram showing the transmittance characteristics of KTP single crystals in Examples with different raw material compositions.

FIG. 4 is a diagram showing a raw material melting temperature dependency of SHG light output of a KTP single crystal according to an example.

FIG. 5 is a graph showing the cooling rate dependence of the SHG light output of KTP single crystals according to an example.

FIG. 6 is a diagram showing light conversion efficiency characteristics of a KTP single crystal according to another example together with a comparative example.

FIG. 7 is a view showing a time-dependent change of the KTP single crystal of the same example together with a comparative example.

FIG. 8 is a perspective view showing a schematic configuration of a wavelength conversion optical element.

FIG. 9 is a view showing a manufacturing process of the wavelength conversion element using the KTP single crystal according to the example.

[Explanation of symbols]

10 ... container, 11 ... crucible, 12 ... support base, 13 ... Support rod, 14 ... thermocouple, 15, 16 ... Alumina member, 17 ... heater, 20 ... Raw material melt, 21 ... Support rod, 22 ... Seed crystal, 25 ... Observation window, 26 ... atmosphere adjustment port, 41 ... Waveguide part, 42 ... KTP crystal, 43 ... Semiconductor laser light, 46 ... Optical second harmonic (blue light), 45 ... Basic wave.

Claims (4)

[Claims] 1. When pulling a MTiOXO _{4 group} single crystal from a raw material melt to produce it, crystal growth is performed by using a material containing no hydrogen (H) as a starting material.
Method for producing single crystal of TiOXO _{4 group} . 2. KPO ₃ and TiO ₂ are used as starting raw materials, and these are melted in a crucible by a flux method to form a raw material melt, and K is pulled from this raw material melt by a pulling method.
KTi characterized by producing TiOPO ₄ single crystal
Method for producing OPO ₄ single crystal. 3. KPO ₃ and TiO ₂ are used as starting materials, and these are melted in a crucible by a flux method to form a raw material melt, the temperature of the raw material melt is kept at 950 ° C. or lower, and the cooling rate is 1 ° C. A method for producing a KTiOPO ₄ single crystal, characterized in that the crystal pulling is carried out at a rate not higher than / h. 4. A KTiOPO ₄ crystal is used as a starting material, which is melted in a crucible by a flux method to form a material melt, and KT is pulled from this material melt by a pulling method.
KTiO characterized by producing iOPO ₄ single crystal
Method for producing PO ₄ single crystal.