JP2007269626A - Single crystal of lithium niobate, its optical element and method for manufacturing the crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a single crystal of lithium niobate for optical use excellent in polarity control characteristics, nonlinear optical characteristics and electro-optic characteristics, and to provide an optical element using the single crystal, and a manufacturing method for stably growing a lithium niobate single crystal. <P>SOLUTION: A single crystal of lithium niobate is obtained by growing from a melt of a composition having an excessive Li over its stoichiometric composition and having a molar fraction of Li<SB>2</SB>O/(Nb<SB>2</SB>O<SB>5</SB>+Li<SB>2</SB>O) within a range from 0.56 to 0.60, and is characterized in that: the melt contains a Mg element; the single crystal of lithium niobate contains a Mg element by 0.1 to 3.0 mol% with respect to the single crystal of lithium niobate crystal; the molar ratio Li<SB>2</SB>O/(Nb<SB>2</SB>O<SB>5</SB>+Li<SB>2</SB>O) in the single crystal of lithium niobate is in a range from 0.490 to less than 0.500; a voltage to be applied necessary for polarization inversion at room temperature is less than 3.7 kV/mm; and the single crystal of lithium niobate is to be used for an optical element using a polarity inversion structure. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a lithium niobate single crystal for optical use having excellent polarization control characteristics, nonlinear optical characteristics, and electro-optical characteristics, and a wavelength conversion element, an optical modulator, a switch, a deflector optical element using the single crystal, and the like. The present invention relates to an optical element and a production method for stably growing the lithium niobate single crystal.

Functional optical single crystals that can control optical properties by external information signals such as electricity, light, and stress are indispensable materials in various optoelectronic fields such as optical communication, display recording, measurement, and light-light control. It has become. In particular, certain oxide single crystals have a particularly large interaction between optical properties and external factors. Therefore, wavelength converters using nonlinear optical effects, optical modulators, switches, and deflections using electro-optical effects are used. It is used as an optical element such as a container.

These crystals are often used as devices in their grown state, but some ferroelectric crystals can reverse the direction of dielectric polarization without breaking the crystal when a voltage is applied. Therefore, the functionality is also improved by periodically inverting the polarization.

For example, in a wavelength conversion element, wavelength conversion by a quasi phase matching method (Quasi-Phase-Matching: QPM) can be performed by periodically inverting the domain structure of ferroelectric polarization. This method is an effective means in that high-efficiency conversion is possible in a wide wavelength range. Therefore, this method is strongly required in the fields of optical communication, display recording, measurement, medical care, etc., from ultraviolet, visible to red. It is expected as a wavelength conversion element for realizing laser light sources having various wavelengths in a wide wavelength range extending to the outside.

Further, in the electro-optic element, for example, according to a known document (Non-Patent Document 1), a lens or prism-like domain-inverted structure is formed in the ferroelectric crystal, and the laser light that has passed through this structure has an electro-optic effect. Optical elements that deflect using light, cylindrical lenses, beam scanners, switches, etc. are attracting attention as new optical elements.

LiNbO ₃ single crystal (hereinafter abbreviated as LN single crystal) is a ferroelectric material mainly used as a substrate for surface acoustic wave devices and optical modulators, but is transparent in a wide wavelength range from visible to infrared. Yes,
A periodic polarization structure can be created by applying a voltage, which has practical optical nonlinearity and electro-optical characteristics to some extent, and can supply a single crystal with a large diameter and high composition uniformity at a relatively low cost. Therefore, in recent years, it has been attracting attention as a substrate for wavelength conversion elements (hereinafter abbreviated as QPM elements) by QPM and electro-optical elements as described above.

The LN single crystals that have been available so far, including the substrate of the surface acoustic wave device, contain about several percent of non-stoichiometric defects, and the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.00. It was limited to 485 matched melt composition. This is because the phase diagram of an LN single crystal has been known for a long time, and conventionally, in order to produce an LN single crystal with high compositional homogeneity, the crystal and the melt have the same composition and are in a coexisting molten composition. It is because it was thought that it was good to grow from a melt with a molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) of 0.485 by the rotational pulling method. Also, known examples (Non-Patent Document 2
As shown in (4), for the purpose of enhancing the light damage resistance, 4.5 mol% or more of Mg is added to the LN crystal having the coincidence melting composition.

What is important in realizing a QPM element is to produce a small and highly efficient element. Although miniaturization and high efficiency of an element greatly depend on the element structure, there are very large elements that are limited to the material characteristics used, that is, the material characteristics inherent in the crystal. For example, the conversion efficiency of a QPM element is proportional to the nonlinear optical constant and the square of the interaction length, and is proportional to the fundamental wave power density. The interaction length and fundamental power density are determined by the accuracy of the device design and fabrication process, and are likely to be improved by technological improvements, while nonlinear optical constants are inherently possessed by materials. Material properties.

Since LN is one of the most popular nonlinear optical materials, many measurements of nonlinear optical constants have been performed since ancient times. The LN crystal having a coincidence melt composition that has been reported so far has a nonlinear optical constant d ₃₃ of 1.064 microns, and is generally about 27 to 34 pm / V. , Up to twice as much. These values are obtained by relative measurement for determining the ratio of the nonlinear optical constant with the reference substance. However, since the absolute value of the reference substance itself has not been established and the values used by people vary, the variation has become so large.

In the conventional measurement method, the absolute value of the reference substance is based on a value obtained by absolute measurement in which the absolute value of the nonlinear optical constant is directly measured. However, the second is the typical measurement method.
There was a large difference in the values obtained between the harmonic generation (SHG) method and the parametric fluorescence (PF) method. For example, quartz d ₁₁ has a fundamental wavelength of 1.064 microns and is 0.3 pm / V in the absolute value scale based on the SHG method, whereas it is 0.5 pm / V in the PF method.

Although the absolute value of the nonlinear optical constant was inaccurate, for example, according to known literature (Non-patent Document 3), as a result of careful measurement of both the SHG method and the PF method, the reported value of the past PF method is It was overestimated because the influence of stray light at the time of measurement could not be excluded, and it was clarified that a consistent value was obtained by both methods. Recently, it has finally been possible to measure absolute values with high accuracy, and it has been corrected that LN crystals with coincident melting compositions, including those with Mg added, have a nonlinear optical constant ₃₃ of 24.9 to 25.2 pm / V. Has been.

In addition, when an LN single crystal is used for an electro-optic element, a large electro-optic constant is desired. L
Although the electro-optic constant of the N single crystal is not necessarily large among the ferroelectric single crystals, a high-quality, large-diameter single crystal can be manufactured inexpensively and stably, so that the substrate of an optical element utilizing various electro-optic effects It has been used as a material. The electro-optic constant of LN single crystals has generally been measured using Mach-Zehnder interferometry. In an LN single crystal having a congruent melt composition that has been used conventionally, the electro-optic constants r ₁₃ and r ₃₃ are reported to be about 8.0 pm / V and about 32.2 pm / V, respectively. For this reason, an element structure using a large electro-optic constant r ₃₃ constant has a great merit in reducing the size and efficiency of the element.

In recent years, research to reduce the existence of non-stoichiometric defects in LN single crystals with a congruent melting composition, that is, research to bring the crystal composition ratio close to the stoichiometric ratio, the existence of this non-stoichiometric defect has the nonlinear optical constant inherent to LN crystals. It has been clarified that the applied voltage required to create a periodic polarization structure is increased. For example, according to a known document (Non-patent Document 4), the polarization inversion voltage is set to 5 kV / mm or less by being close to the stoichiometric composition.

In recent years, studies have been carried out to reduce the existence of non-stoichiometric defects in LN single crystals having a congruent melting composition with the aim of greatly improving material performance. In order to make the LN crystal having a stoichiometric composition practical, research on its growth method has been actively conducted. For example, according to known literature (Non-Patent Document 5), by fostering congruent or stoichiometric crystals from the melt with the addition of 6 mol% or more K ₂ O in the composition, to reduce the defect density, stoichiometric It is said that the thing of a composition close | similar to is obtained. Further, in the publicly known document (Non-patent Document 6), Mg, Zn, In, and Sc
Comparison results of light damage resistance when any of these elements is added have been reported. The known document (Patent Document 1), a single crystal precipitated from Li ₂ O concentration from 53 to 61 mol% of lithium niobate melt by the CZ method, comprising the addition of 0.1 to 3 mol% magnesium oxide A method for producing light-damaged LN crystals is disclosed.

As shown in FIG. 2, from the phase diagram of Li ₂ O and Nb ₂ O ₅ , Li ₂ O / (Nb ₂ O ₅ + L
By setting the molar fraction of i ₂ O) to 0.58 to 0.60, Li ₂ O / (Nb ₂ O ₅ + Li ₂ O)
It can be seen that crystals having a mole fraction of about 0.500 can be grown. However, as can be seen from the phase diagram, this melt composition ratio is very close to the eutectic point, and when a crystal having a composition close to the constant ratio is grown from a melt having a composition in which the Li concentration is higher than the constant ratio, As a result of precipitation, excess Li component remains in the crucible, and the composition ratio of Li and Nb in the melt gradually changes, so that the melt composition ratio reaches the eutectic point immediately after the start of growth. It will come. Therefore, when using the Czochralski method (hereinafter abbreviated as CZ method), which has been used as a means for industrially mass-producing large-diameter LN crystals, solidification of crystals having a composition close to the stoichiometric ratio. The rate is only about 10%.

In the publicly known literature (Non-patent Document 7 and Patent Document 2), the present inventors have used a method of growing while continuously supplying raw materials as means for increasing this low solidification rate (hereinafter abbreviated as a continuous supply method). )
Has proposed. Specifically, the mole fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) in the growth melt is 0.585 to 0.595, the crucible is doubled, and the crystal is pulled up from the inner crucible. The growth rate is obtained by measuring the weight of the crystal being pulled up as needed, and the powder having the same component as the crystal is continuously supplied at the rate between the outer crucible and the inner crucible. By using this method, a long crystal can be grown, and a crystal solidification rate of 100% can be realized with respect to the raw material supply amount. The inventors have disclosed a specific composition L in the known literature (Patent Document 3).
It discloses that excellent photorefractive sensitivity can be obtained by adding 18 ppm or more of cerium to an N crystal and a quasi-stoichiometric composition LN crystal, and that the continuous supply method is suitable for the production of this cerium-added LN crystal.

The LN single crystal is often used as a QPM element. As an important process technology for realizing high efficiency, there is a technology for generating a periodic domain-inverted domain with high accuracy. That is, in order to maximize nonlinear optical characteristics, the ratio of the width of polarization inversion (hereinafter abbreviated as polarization inversion width) is set to 1: 1. The polarization inversion width varies depending on the phase matching wavelength of the target wavelength conversion element. For example, in the long wavelength phase matching such as the infrared region, the polarization inversion width is several tens of microns. The polarization inversion voltage of the LN single crystal having the coincidence melting composition is set to 21 kV / mm or more.
M. Yamada et al., Appl.Phys.Lett., 69, p3659,1996 DABryan et al. Appl. Phys. Lett. 44,847,1984 I. Shoji et al., J. Opt. Soc. Am. B, 14, p2268, 1997 V. Gopalan et al. Appl. Phys. Lett. 72, p1981, 1998 GIMolovichiko et al. Appl. Phys. A, 56, p103, 1993 T. Volk et al., Ferrelectr. Lett., Vol. 20, pp. 97-103, 1995 Applied physics, 65,931,1996 Japanese Patent Laid-Open No. 5-270992 Japanese Patent Application No. 10-274047 (Japanese Patent Laid-Open No. 2000-103697) Japanese Patent Application Laid-Open No. 11-199391

The LN single crystal having a coincidence melting composition belongs to a class of crystals that exhibit large nonlinearity among existing nonlinear optical crystals, but is still insufficient when an element is actually fabricated. In recent years, as the degree of completeness of device design and the accuracy of the manufacturing process have improved, there is a limit to the drastic improvement in device characteristics only by improving the process, so the d constant itself is set to a larger value. It is hoped to do.

However, it is gradually clear that the crystal growth method that pulls up from a melt having a higher Li concentration than the coincidence melt composition using the continuous supply method has a serious problem in terms of yield from an industrial viewpoint. It has become. That is, when a high Li concentration melt is used,
Our study showed that the composition of the growing crystal strongly depends on the composition ratio of the melt, unlike the case where the crystal is grown at the coincident melt composition ratio.

This means that in order to grow crystals with uniform optical properties and good optical homogeneity with high reproducibility, it is necessary to grow crystals from a melt always kept at the same composition ratio. In the case of the LN crystal, the voltage necessary for forming the nonlinear optical constant and the periodic inversion structure, and the electro-optic constant are sensitive to the crystal composition ratio. Therefore, in order to extract the maximum characteristics, the Li ₂ O / (Nb ₂ O ₅ +
It will have to be fixed to the state close to the mole fraction of li ₂ O) extremely 0.500.

For example, the continuous supply method is characterized by excellent composition controllability from the start to the end of growth, but it is very important to determine the melt composition ratio at the start of growth, and the initial setting is the desired melt If the composition is deviated from the composition, the entire grown crystal does not satisfy the nonlinear optical constant d ₃₃ or the inversion voltage required. In order to prevent this, it is possible to pull up a small crystal before growth, check the composition ratio of the melt from the composition ratio of the crystal, and add the missing component to correct the deviation. A minimum of several days is required to grow small crystals and confirm the component ratio, which greatly reduces productivity.

In addition, the continuous supply method is an extremely effective method for controlling the composition. However, when the growth time is as long as several days to one week, a small amount of raw material from the melt surface maintained at a high temperature is used. Evaporation can occur. The variation over time of the crystal composition due to this cannot be ignored when it is necessary to grow a crystal with a stoichiometric composition in which the composition is completely uniformly controlled. Due to this variation in crystal composition, it is very difficult to grow crystals with the same characteristics at a high yield, and the growth of a complete stoichiometric composition LN single crystal without defects from a melt with a high Li concentration. The technology has not been put into practical use industrially.

In addition, it has been very difficult to form a complete reversal width ratio of 1: 1 with high reproducibility in the LN crystal having the coincidence melting composition. That is, in the voltage application method, a periodic electrode is provided on one side of an LN single crystal having a z-cut conforming melt composition, and a uniform electrode is provided on the opposite side, and a pulse voltage is applied through this electrode, so that the portion immediately below the periodic electrode is z-axis oriented. However, the inversion polarization width does not always match the electrode width, and the manufacturing error is large. In addition, there is a problem that the inversion is interrupted in the middle of the z-axis direction of the opposite surface and the inversion is interrupted or the polarization inversion width is different on both sides of the z-cut crystal, so the ideal polarization inversion width ratio is realized. There wasn't.

In the case of a short wavelength application such as the visible region to the ultraviolet region, the polarization inversion width necessary for phase matching is about 3 microns, which makes it more difficult to create compared to the long wavelength use. However, even a long wavelength QPM element that is relatively easy has not achieved an ideal element. One of the causes is the height of the applied voltage (hereinafter abbreviated as polarization inversion voltage) necessary for polarization inversion of the LN single crystal having the coincidence melt composition. When the polarization inversion voltage is as high as 21 kV / mm or higher and the substrate thickness is less than 0.5 mm, a polarization inversion grating can be formed on the entire substrate. When the thickness is 5 mm or more, complete polarization inversion formation becomes difficult, and the thickness is 1.0 m.
Above m, the formation of an accurate polarization capable of realizing an element has not been achieved.

Moreover, even if the substrate thickness is less than 0.5 mm, a polarization inversion period of several microns as for short wavelengths is not realized. In particular, a coincident melt composition L to which 5 mol% or more of MgO is added
In the case of N, since the internal electric field is large, the symmetry of the ferroelectric hysteresis curve (PE curve) is poor and the rise of the PE curve in the vicinity of the coercive electric field is gentle and not steep, so the direction opposite to the spontaneous polarization from the outside There is a problem that the control of the reversal of the spontaneous polarization when the electric field is applied is poor.

Furthermore, in the case of the coincidence melt composition LN to which 5 mol% or more of MgO is added, the electrical resistance is reduced by about 3 to 4 orders of magnitude or more compared to the case of no addition, so that delicate control of the applied voltage is difficult, It is more difficult to create a polarization inversion width ratio of 1: 1. Although it is said that this problem can be solved by using the corona discharge method for polarization reversal, even in this case, the problem of the thickness of the polarization reversal sample has not been solved.

Laser light that passes through a wavelength conversion element that utilizes the nonlinear optical effect of an LN single crystal, a light modulation element that utilizes an electro-optic effect, and a lens or prism-like domain-inverted structure formed on the LN single crystal. In order to realize new optical elements such as optical elements, cylindrical lenses, beam scanners, switches, etc. that deflect the light using the electro-optic effect, it is important to stably produce small and highly efficient elements .

Even in elements using these electro-optic effects, the miniaturization and high efficiency of the elements depend on the fabrication accuracy of the element structure, but there are also many factors that are limited by the material characteristics used. For example, the performance of an optical element using the electro-optic effect of an LN single crystal in which the reversal of the refractive index is formed by the polarization reversal structure, the design of the lens or prism-shaped polarization reversal structure, the accuracy of the fabrication process of the polarization reversal structure, and It is determined by the size of the electro-optic constant of the material.

In a conventional LN crystal having a congruent melting composition, it is difficult to control the domain inversion structure because a large applied voltage is required for domain inversion. Furthermore, the electro-optic constant is a characteristic inherent in the material, and it has been considered difficult to improve it with the same crystal. Further, depending on the wavelength and intensity of light used, the occurrence of optical damage may be a major difficulty. In such a case, a crystal obtained by adding 5 mol% or more of MgO to the coincidence melt composition LN crystal is resistant to light damage. Although it was expected from the superiority of the material, the fabrication of a highly accurate lens and prismatic domain-inverted structure was realized due to the problem of material characteristics that the control of the reversal of spontaneous polarization was poor, similar to the fabrication of a QPM device. It wasn't.

As the degree of completeness of device design and the accuracy of manufacturing processes have improved as in recent years, the improvement of the performance of the material itself is desired because the improvement of optical element characteristics has become limited by process improvement alone. ing. For this reason, a technique for growing LN single crystals having a stoichiometric composition with reduced non-stoichiometric defects has been developed. The crystal growth method for pulling up the stoichiometric composition LN single crystal from the melt having a higher Li concentration than the coincidence melt composition using the continuous supply method has a problem in terms of yield from an industrial viewpoint. It has become increasingly clear.

That is, the continuous supply method is an extremely effective method for controlling the composition. However, when the growth time is as long as several days to about one week, a small amount of raw material from the melt surface maintained at a high temperature is used. Evaporation can occur. The variation over time of the crystal composition due to this cannot be ignored when it is necessary to grow a crystal with a stoichiometric composition in which the composition is completely uniformly controlled. Due to this variation in crystal composition, it is very difficult to grow crystals with the same characteristics at a high yield, and the growth of a complete stoichiometric composition LN single crystal without defects from a melt with a high Li concentration. The technology has not been put into practical use industrially.

This means that in order to grow crystals with uniform optical properties and good optical homogeneity with high reproducibility, it is necessary to grow crystals from a melt always kept at the same composition ratio. In the case of the LN crystal, the voltage necessary for forming the nonlinear optical constant and the periodic inversion structure, and the electro-optic constant are sensitive to the crystal composition ratio. Therefore, in order to extract the maximum characteristics, the Li ₂ O / (Nb ₂ O ₅ +
The molar fraction of Li ₂ O) has to be fixed in a state very close to 0.500.

The performance of an optical element using an LN single crystal includes the periodic polarization reversal structure, the design of the lens or prism-shaped polarization reversal structure, the accuracy of the fabrication process of the polarization reversal structure, and the nonlinear optical constant of the material,
It is determined by the electro-optic constant and the light damage resistance. In a conventional LN crystal having a congruent melting composition, it is difficult to control the domain inversion structure because a large applied voltage is required for domain inversion. Furthermore, the electro-optic constant is a characteristic inherent in the material, and it has been considered difficult to improve it with the same crystal. Also, depending on the wavelength and intensity of the light used, the occurrence of photodamage may be a major difficulty. In such a case, 5 mol is added to the coincidence melt composition LN crystal.
% Of MgO added crystals are expected to have excellent light damage resistance, but because of the problem of material properties that control of spontaneous polarization inversion is poor, it is possible to fabricate highly accurate lenses and prismatic domain-inverted structures. Was not.

As a result of earnest research, the present inventors have achieved a lithium niobate single crystal having non-stoichiometric defects but maintaining the same characteristics as the lithium niobate single crystal having a perfect stoichiometric composition,
By adding Mg element having substantially no absorption in the visible light range in the range of 0.1 to 3 mol%, the polarization inversion voltage is small without deteriorating the nonlinear optical constant d ₃₃ and the electro-optical characteristic r _33. Even if it is a lithium niobate single crystal having a certain amount of non-stoichiometric defects, although the defect portion of Li can be filled with the third element and close to the stoichiometric composition,
Nonlinear optical constants equal to the size of a perfect LN single crystal having a molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) of 0.500, an applied voltage necessary for creating a periodically polarized structure, and electro-optics It has been found that a constant can be realized, and further, this means can be applied to a wide range of lithium niobate single crystals having a molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) of 0.490 or more and less than 0.500. It is found that the present invention is effective, and the present invention is made here.

The effect of adding Mg to d ₃₃ can be explained as follows. Since the nonlinear optical characteristics of the LN crystal are generated by the combination of the Li element and the O element, the nonlinearity decreases as the Li defect increases, and the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is increased. L00 crystal of 0.500 contains Li
Since there is no defect, it exhibits maximum nonlinearity. In the case of crystals that do not have a stoichiometric composition, excess N
Although the b element enters the Li defect portion, the nonlinearity as a whole is reduced because the nonlinearity hardly occurs in the bond between the Nb element and the O element.

On the other hand, in the case of Mg addition, Mg enters the Li defect portion and nonlinearity due to the combination of Mg element and O element occurs. The bond nonlinearity between the Mg element and the O element is as follows.
Even if the molar fraction of the Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) of the crystal changes due to a change in the composition ratio of the growth melt, it is similar to the non-linearity generated by the bonding of elements. Since the Mg element present in the liquid fills the Li defect, the maximum nonlinear optical property is maintained even if the molar ratio of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) of the crystal varies somewhat. it is conceivable that.

The effect of Mg addition on the polarization inversion voltage can be explained as follows. The reason why the polarization reversal voltage of the stoichiometric crystal is significantly reduced as compared with the conventional coincidence melt composition LN single crystal can be explained by the fact that the number of Li defects pinning the polarization reversal is reduced. On the other hand, when Mg is added, L
Although the molar fraction of i ₂ O / (Nb ₂ O ₅ + Li ₂ O) varies within the range of 0.490 or more and less than 0.500, the minimum voltage value is indicated when Mg is present at the Li site. It is considered that the pinning effect in the substituted state is smaller than that of the Li defect.

However, since the pinning effect in the state where Mg is substituted at the Li site is larger than that of the defect-free portion, this effect can be obtained because of the crystal Li ₂ O / (Nb ₂ O ₅ + Li ₂ O
) Appears only in a narrow range of not less than 0.490 and less than 0.500. For example, when Mg is added to a crystal having a congruent melting composition, a decrease in polarization reversal voltage is also seen, but on the other hand, since the electrical resistance is about 4 orders of magnitude or less compared to the case without addition, Polarization reversal was not possible with a normal voltage application method, and a special method called a corona discharge method was required.
_{Li 2 O / (Nb 2 O} 5 + Li 2 O) molar fraction of sharply since the addition amount of the required Mg is small and 0.1~3.0mol of electric resistance in a range of less than 0.490 or more 0.500 There is no significant decline.

Further, the effect of adding Mg to r ₃₃ has not been clarified at the present time, but is considered to be almost the same as the effect on d ₃₃ . That is, if the bond between the Li element and the O element of the LN crystal is the main manifestation factor of the electro-optic characteristics, the electro-optic constant decreases with the increase of Li defects, and Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) LN crystals with a mole fraction of 0.500 contain L
Since there is no i defect, it can be expected to show the maximum electro-optic constant.

In the case of a crystal having a non-stoichiometric composition, excess Nb element enters the Li defect portion, but since the electro-optic characteristic is hardly generated in the combination of Nb element and O element, the overall electro-optic constant is reduced. On the other hand, in the case of adding Mg, Mg enters the Li defect portion, and electro-optical characteristics are generated by the combination of the Mg element and the O element. If the combined electro-optical characteristics of the Mg element and the O element are similar to the electro-optical characteristics generated by the combination of the Li element and the O element, the crystal Li ₂ O / Even if the molar fraction of (Nb ₂ O ₅ + Li ₂ O) changes, the Mg element present in the melt fills the Li defects, so that the crystalline Li ₂ O
It can be explained that the maximum electro-optic constant is maintained even if the molar ratio of / (Nb ₂ O ₅ + Li ₂ O) varies somewhat.

In the present invention, for example, in the continuous supply method, even if the setting of the melt composition ratio at the start of growth is deviated from the desired melt composition by adding 0.1 mol or more of Mg, Li ₂ O / (
A single crystal having the same nonlinear optical constant, polarization structure creation voltage, and electro-optic constant as that of an LN single crystal having a molar fraction of (Nb ₂ O ₅ + Li ₂ O) of 0.500 can be grown, resulting in the yield. Can be greatly improved.

Furthermore, the melt temperature at the interface between the crystal and the melt due to the fluctuation of the melt composition ratio during the growth due to the evaporation of the melt and the inhomogeneity of the composition ratio in the growth melt and the temperature distribution in the crucible fluctuations, but _{Li 2 O / (Nb 2 O} 5 + Li 2 O) heterogeneous mole fraction of is generated in the crystal, nonlinear optical constant and polarization structures create a voltage by the present invention, and an electro-optical constant is Li ₂ O / (Nb ₂
O ₅ + Li ₂ O) is not dependent on the molar fraction, so that these characteristics do not occur inhomogeneous, and as a result, there is a growing condition for stably producing LN crystals having both high homogeneity and excellent performance. It becomes extremely gentle.

Here, the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) was set to 0.490 or more and less than 0.500 because the polarization inversion voltage was not lowered in the crystal having a composition smaller than 0.490. This is because it was sufficient. Furthermore, the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.490 or more and 0.5
In crystals with a composition less than 00, there is almost no internal electric field, and the hysteresis curve (PE
Curve) and the rise of the PE curve near the coercive electric field makes it very easy to control the reversal of the spontaneous polarization when an electric field in the opposite direction to the spontaneous polarization is applied from the outside. This is a great merit.

Further, in the case of a crystal having a composition with a molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) of 0.490 or more and less than 0.500, the necessary Mg addition concentration is less than 3 mol%. It is possible to prevent a sudden drop in electrical resistance as seen in crystals with 5.0 mol% Mg added to crystals with a consistent melting composition, and a very high polarization inversion width ratio of 1: 1. An efficient QPM element can be manufactured.

With the above configuration, in the optical element that converts the wavelength of the laser light incident on the single crystal, the wavelength 1
. A non-linear optical constant d _{33 at} 064 microns is 26 pm / V or more, and an applied voltage required for polarization reversal at room temperature is less than 3.7 kV / mm, and an LN single crystal can be manufactured. It is. The thickness in the z-axis direction is 1.0 mm or more and the period of polarization inversion is 3
A QPM element of 0 micron or less is realized for the first time by the LN crystal of the present invention, and a QPM element having a polarization reversal period of 5 microns or less is also realized for the first time by the present invention.

Furthermore, in the optical element that controls the laser light incident on the single crystal using the electro-optic effect of the single crystal, the electro-optic constant r ₃₃ is 36 at a wavelength of 0.633 microns.
It is possible to produce a lithium niobate single crystal characterized by being pm / V or more. An optical element characterized in that light deflection, focusing, and switching are performed with high efficiency and stability by utilizing a large refractive index change of a structure obtained by inverting the ferroelectric polarization of a lithium niobate single crystal. This is the first realization with crystals.
(Example)

Examples of the present invention are shown below. In the case of the LN single crystal, the LN single crystal obtained by the ordinary pulling method from the coincidence melt composition melt has an excess of the Nb component, but the composition of the melt has an excessive Li component (for example, Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) close to the stoichiometric composition when crystals are grown from a melt with a molar fraction of 0.56 to 0.60). Is 0
. 500, that is, a single crystal in which the non-stoichiometric defect concentration is suppressed as much as possible can be obtained. In this example, an LN single crystal close to a stoichiometric composition was grown from a melt with an excessive Li component composition by using a double crucible method for continuously supplying raw materials.

Commercially available high-purity Li ₂ O and Nb ₂ O ₅ raw material powders were prepared, and the ratio of Li ₂ O: Nb ₂ O ₅ was 0.56.
0.60: and Li components excess material of _{0.44~0.40, Li 2 O: Nb 2} O 5 = 0.50: 0.
50 stoichiometric composition raw materials were mixed. Next, rubber press molding was performed at a hydrostatic pressure of 1 ton / cm ² , and each was sintered in an atmosphere of about 1050 ° C. to prepare a raw material bar. Further, the mixed stoichiometric composition raw material is sintered in the atmosphere at about 1150 ° C. as a raw material for continuous supply, pulverized, and the size is 50
Classification was performed in the size range of micron to 500 microns.

Next, when the single crystal was grown by the double crucible method, the prepared raw material rod made of the Li component excess raw material was filled in the inner and outer crucibles in advance, and then the crucible was heated to prepare a Li component excess melt. In experiments confirming the effect of Mg addition, a commercially available high purity MgCO ₃ was used during the filling.
Were pre-filled into the inner and outer crucibles. As for the weight of MgCO _{3 to be} filled, five kinds of experiments were conducted in which the Mg concentration in the melt was 0.1, 0.2, 0.5, 1.0, and 3.0 mol% with respect to Nb in the melt. went. For comparison, experiments were conducted with Mg concentrations of 0, 0.05, and 5.0 mol%.

Here, the principle of the double crucible method for growing the stoichiometric composition LN crystal will be briefly described with reference to FIGS. FIG. 2 shows the phase diagram of LN. As can be seen from the phase diagram, the LN single crystal obtained by the ordinary pulling method from the coincidence melt composition melt of the LN single crystal becomes excessive in the Nb component, but the composition of the melt is significantly increased in the Li component (for example, Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) molar fraction is 0
. 56 to 0.60) Li ₂ O / (Nb ₂ O ₅₎ close to the stoichiometric composition
+ Li ₂ O), which is 0.500, that is, a single crystal in which the non-stoichiometric defect concentration is suppressed as much as possible can be obtained.

FIG. 1 shows a growth furnace 1 used in the present invention. The structure of the double crucible used in this example is a structure in which a cylinder 36 (referred to as an inner crucible) whose height is 7.5 mm higher than that of the outer crucible is installed inside the outer crucible 35, and at the bottom of the inner crucible. A hole leading from the outer crucible to the inner crucible was provided. The holes were approximately 20 mm × 30 mm in a substantially quadrangular shape, and three holes were provided in the inner crucible. Here, the crucible material used for the growth was made of platinum, and the periphery was covered with a growth furnace body 47 to prevent the inflow of the external atmosphere.

The shape of the double crucible used was such that the height / diameter ratio of the outer crucible 35 was 0.45, and the diameter ratio of the inner crucible / outer crucible was 0.8. The outer crucible 35 is 150mm in diameter and 6mm in height.
The inner crucible 36 had a diameter of 120 mm and a height of 75 mm. There is a space 34 of about 15 mm on one side between the inner crucible 36 and the outer crucible 35, and the raw material supply pipe 37 is stably installed so that the raw material 45 can fall smoothly. The state of the melt surface was observed with a video camera (not shown). If the crucible is not rotated, almost no convection on the surface of the melt can be seen, but if the crucible rotation speed is gradually increased, the forced melt convection in the direction of rotation becomes stronger. An effect was observed.

Crystals were grown from the inner crucible melt 41 with an excess of Li component. After the temperature of the melt is stabilized at a predetermined temperature by the input power to the high-frequency oscillator 48 and the high-frequency induction coil 43, the single LN in a single polarization state of 5 mm × 5 mm × length 70 mm cut out in the Z-axis direction Seed crystal 40
The LN single crystal 42 was grown by connecting to the lower part of the rotation support bar 38, attaching to the melt 41, rotating the crystal while controlling the melt temperature, and pulling it upward. The growing atmosphere was in the air. The rotation speed of the LN single crystal 42 was constant within a range of 5 to 20 rpm, and the pulling speed was changed within a range of 0.5 to 3.0 mm / h.

Automatic diameter control was performed on the straight body of the crystal so that a 2-inch wafer could be made from the grown crystal. The growth weight of the grown crystal is measured by the load cell 52, and the raw material 45 having a constant ratio with a molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) in an amount commensurate with the amount of crystallized growth is 0.500. The crucible 35 was supplied. Here, since the change in the growth amount of the LN single crystal 42 is obtained by the computer 49, the supply of the raw material 45 is started when the growth of the single crystal 42 starts from the LN seed crystal 40 and the diameter control is stabilized.

The raw material 45 was supplied through the supply pipe 37 made of ceramics or noble metal, and the raw material 45 stored in a sealed container 46 that also had a weight measuring sensor installed in advance on the growth furnace body 47 was supplied. Gas 51 was introduced into the supply pipe 37 and the sealed container 46 at a rate of 50 to 500 cc per minute through the gas pipe 33 provided with a valve. The flow rate of the gas 51 was optimized by the amount and particle size of the raw material 45 to be supplied per unit time. Thereby, smooth raw material supply without scattering and clogging in the supply pipe 37 was performed. By rotating the noble metal double crucible during growth, the melt convection is forced to be constant and at the same time the crystal growth interface is flat or convex with respect to the liquid surface. Controlled. With about 1.5 weeks of growth in each composition,
A colorless and transparent LN crystal having a diameter of 60 mm and a length of 110 mm and having no cracks was obtained.

For all the crystals obtained, the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) was determined by chemical analysis. The measurement position of the sample is set to the measurement position a at the axial center of the crystal 15 mm away from the seed crystal, and at three points every 10 mm in the direction away from the seed crystal from the measurement position a along the axial center. It was set as b, c, d. The measurement sample was cut out as a 7 mm square cube around the measurement position.

Table 1-1 shows the measurement results of the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O). In chemical analysis, it is difficult to accurately determine the absolute value of the composition ratio. In the case of an LN crystal, Li ₂ O / (Nb ₂ O ₅
+ Li ₂ O) includes an error of about 0.001 to 0.005. Therefore, the composition of LN crystals having a composition close to a constant ratio was analyzed very carefully. The results in Table 1 show the average values of the results of evaluating the same sample using several different analyzers. As a result, in the case of an LN single crystal, Li ₂ O / ((
The value of the molar fraction of (Nb ₂ O ₅ + Li ₂ O) did not exceed 0.005. Moreover, the measurement regarding Mg content of these samples was also performed, and it was confirmed that the content of crystals was almost the same as the Mg concentration added to the melt.

(Table 1) No addition of mole fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) 0.05 mol% 0.10 mol% 0.20 mol% 0.50 mol% 1.00 mol% 3.00 mol% 5.00 mol% a 0.492
494 0.496 0.498 0.496 0.494 0.492 0.489b 0.493 0.493 0.495 0.499 0.498 0.495 0.
492 0.490c 0.494 0.494 0.496 0.498 0.497 0.495 0.491 0.491d 0.494 0.496 0.494
0.497 0.495 0.495 0.492 0.490

Next, the nonlinear optical constants of these samples were measured. We performed absolute measurement using the wedge method, and determined the absolute value of the nonlinear optical constant accurately by analyzing the measurement data in consideration of the effect of multiple reflection. As a result, a material having a high refractive index such as an LN single crystal (n> 2
Revealed that had most conventional values are overestimated relative) was measured d ₃₃ of LN crystal melting congruent composition, 25.1P which agrees well with the results sought in the literature
A value of m / V was obtained. The laser beam used for measurement has a single longitudinal mode continuous oscillation wavelength of 1.
0.064 microns. Table 2 shows the measurement results.

When the addition amount of Mg is 0.1 mol% or more, the crystalline Li ₂ O / (Nb ₂ O ₅ + Li
_Although the molar fraction of ₂ O) varies widely between 0.489 and 0.499,
All of the values are 30.0 pm / V or more, whereas when they are less than 0.1 mol%, they tend to be slightly inferior. The absolute measurement method using a wedge method, unlike the absolute measurement method by the conventional phase matching method, the diagonal components such as d ₃₃ can also be measured. In addition, it is extremely difficult to perform rigorous analysis considering multiple reflections with the rotating Maker fringe method, and in order to accurately determine nonlinear optical constants, measurement is performed under conditions where non-reflective coating is applied and multiple reflections do not occur. There is no choice but to do.
From the above, it can be said that the absolute measurement by the wedge method is a very effective measurement method.

(Table 2) Nonlinear optical constant d ₃₃ (unit: pm / V)
No addition 0.05mol% 0.10mol% 0.20mol% 0.50mol% 1.00mol% 3.00mol% 5.00mol% a 27.9 29.
5 30.1 30.2 30.1 30.1 30.3 30.3b 28.8 29.5 30.0 30.3 30.0 30.4 30.2 30.1c 29.0
29.6 30.0 30.2 30.1 30.3 30.2 30.0d 29.1 29.6 30.2 30.1 30.3 30.1 30.1 30.1

Next, for each single crystal obtained in the same manner as described above, from each location of the measurement positions a to d,
A z-plate sample having a cross section of 10 mm × 10 mm and a thickness of 1.0 mm was cut out. After electrodes were formed on both z-axis surfaces, a voltage was applied to measure the voltage at which the crystal causes polarization reversal. Table 3 shows the measurement results.

When the addition amount of Mg is 0.1 mol% or more, all are 3.7 kV / mm or less,
If it is 0.2 mol% or more, a constant value near a smaller value of 3.1 kV / mm can be obtained. These crystals have almost no internal electric field, excellent symmetry of ferroelectric hysteresis curve (PE curve), and good rise of PE curve in the vicinity of coercive electric field. it is conceivable that.

On the other hand, when the amount is less than 0.1 mol%, the polarization inversion voltage tends to be slightly higher than that of the added crystal of a larger amount. On the other hand, when Mg was added in an amount of 5 mol% or more, the polarization inversion was reduced, but the variation from sample to sample tended to increase. This is because the rise of the PE curve in the vicinity of the coercive field of the ferroelectric hysteresis curve (PE curve) is gentle and difficult to measure the absolute value of the polarization inversion voltage, and also due to the electrical resistance of the material. It was thought to be the cause. When the reversal voltage of the melt coincidence composition crystal was measured under the same sample shape and measurement conditions, measurement was difficult in some cases. Measurement was possible with a thin sample having a thickness of about 0.2 to 0.5 mm, which was a very high value of 21.0 kV / mm.

(Table 3) Inversion voltage (unit: kV / mm)
No addition 0.05mol% 0.10mol% 0.20mol% 0.50mol% 1.00mol% 3.00mol% 5.00mol% a 5.2 3.8
3.3 3.1 3.0 3.1 3.0 2.3b 5.0 3.9 3.4 2.8 3.0 2.9 3.1 2.9c 4.9 3.9 3.3 2.9 3.0
3.1 3.1 2.1d 4.8 3.8 3.3 3.0 3.1 3.1 3.1 2.5

Next, with respect to each single crystal obtained in the same manner as described above, from each location of the measurement positions a to d, x
Samples of 5 mm × 3 mm × 2 mm were cut out in the, y and z directions. After forming electrodes on both z-axis surfaces, the electro-optic constant of the sample was measured using Mach-Zehnder interferometry. Table 4 shows the measurement results.

As shown in Table 4, it was revealed that some of these constants were very sensitive to the crystal composition. That is, the molar fraction of crystalline Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.4.
The LN single crystal of 90 or more and less than 0.500 does not increase the electro-optic constant r _{13 as} compared with the conventional matched melt composition LN single crystal, but r ₃₃ increases by about 20% or more and becomes about 36 pm / V or more. It was clarified that it was very large compared to the value of about 31.5 pm / V of the melt composition LN single crystal.

In particular, the electro-optic constant tended to increase as it approached the stoichiometric composition. Also,
In the crystal added with Mg, when the addition amount was 0.1 mol% or more, a further increase was seen to 38 pm / V or more, and the maximum of 39.5 pm / V was obtained especially in the crystal added with about 1 mol%. . On the other hand, when the added amount of Mg is more than 1 mol%, the electro-optic constant tends to gradually decrease.

(Table 4) Electro-optic constant r ₃₃ (unit: pm / V)
No addition 0.05mol% 0.10mol% 0.20mol% 0.50mol% 1.00mol% 3.00mol% 5.00mol% a 36.0 36.
7 38.1 38.5 38.8 39.2 38.4 36.9b 36.2 37.0 38.0 38.4 38.6 39.5 38.6 37.1c 37.1
37.8 38.1 38.4 38.8 39.3 38.3 36.5d 37.8 37.6 38.1 38.3 39.0 39.2 38.2 36.4

A commercially available raw powder of high purity Li ₂ O, Nb ₂ O ₅ is prepared, and the ratio of Li ₂ O: Nb ₂ O ₅ is 0.56.
~ 0.60: 0.44 ~ 0.40 Li component excess raw material was mixed. Next, a rubber press was formed at a hydrostatic pressure of 1 ton / cm ² and sintered in the atmosphere at about 1050 ° C. to prepare a raw material bar. next,
At the time of single crystal growth by the single crucible method, that is, the conventional CZ method, the raw material rod made of the Li component excess raw material was filled in advance, and then the crucible was heated to prepare a Li component excess melt. In an experiment for confirming the effect of Mg addition, a commercially available high-purity MgCO ₃ was previously filled in a crucible at the time of filling. The weight of MgCO _{3 to be} filled is such that the Mg concentration in the melt is Nb in the melt.
And five types of experiments of 0.1, 0.2, 0.5, 1.0, and 3.0 mol% were performed.

For comparison, the same experiment was conducted except that no addition, 0.05, and 0.5 mol% were added. The crucible used for the growth was made of platinum. The shape of the crucible used was a cylindrical shape, and the size was 150 mm in diameter and 100 mm in height. Throughout the growth, the surface of the melt was observed with a video camera. Unlike Example 1, strong melt convection was observed even without crucible rotation.

Crystals were grown from near the center of the crucible on the melt surface. After stabilizing the temperature of the melt at a predetermined temperature, an LN single crystal in a single polarization state of 5 mm × 5 mm × length 70 mm cut in the Z-axis direction is attached to the melt as a seed crystal 60, and the melt temperature A single crystal was grown by rotating the crystal while pulling it up and pulling it upward. The crucible was fixed without rotating. The growing atmosphere was in the air. The rotation speed of the crystal is constant at 2 rpm, and the pulling degree is 0.5 to 3.0.
It was changed in the range of mm / h. Automatic diameter from immediately after seeding so that the diameter of the straight body of the crystal is about 60 mm while measuring the growth weight of the grown crystal with a load cell throughout the growth so that a 2 inch wafer can be made from the grown crystal. Control was performed. In the growth of this example, the raw material during the growth as in the case of using the double crucible of Example 1 was not supplied.
FIG. 3 shows a schematic diagram of the obtained LN single crystal.

When Mg was not added or when various concentrations of Mg were added, the transparent single crystal part 61 was grown up to about 30 mm immediately after the start of the growth when the diameter was raised to 60 mm. The portion pulled up after reaching the eutectic point is not an LN single crystal,
It was a ceramic layer 62.

For each of the obtained crystals, the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) was determined by chemical analysis. The measurement position is the center position of the crystal 5 mm away from the seed crystal 60 as the measurement position g, and two points are taken at 10 mm intervals in the direction away from the seed crystal 60 along the axis center from the measurement position g. It was set as the parts h and i. The measurement sample was cut out as a 7 mm square cube around the measurement position. Table 5 shows the measurement results of the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O).
We also measured the Mg content of these samples, and the content of crystals was Mg added to the melt.
It was confirmed that the concentration was almost the same.

(Table 5) No addition of mole fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) 0.05 mol% 0.10 mol% 0.20 mol% 0.50 mol% 1.00 mol% 3.00 mol% 5.00 mol% g 0.489
489 0.491 0.495 0.495 0.491 0.491 0.490h 0.491 0.494 0.495 0.498 0.496 0.492 0.
492 0.494i 0.498 0.496 0.497 0.499 0.498 0.494 0.496 0.496

Next, the nonlinear optical constants of these samples were measured. The wedge method was used for the measurement. Table 6 shows the measurement results. From the table, it can be seen that when the added amount of Mg is less than 0.1 mol%, the nonlinear optical constant d ₃₃ gradually increases as it approaches the eutectic point from the seed crystal. This increase is considered to have occurred as a result of fluctuations in the melt composition ratio over time because the raw material was not supplied during the growth. On the other hand, when the added amount of Mg is less than 0.1 mol%,
There is no increase as seen in the case of 0.1 mol% or more. Even within the distance of 10 mm from the measurement position g to i, the non-linear optical constant d ₃₃ is almost constant and is 0.2 mol%.
The above shows the maximum value of 30 pm / V or more almost uniformly throughout the crystal.

(Table 6) Nonlinear optical constant d ₃₃ (unit: pm / V)
No addition 0.05mol% 0.10mol% 0.20mol% 0.50mol% 1.00mol% 3.00mol% 5.00mol% g 25.9 27.
0 29.5 30.0 30.1 29.9 30.1 29.5h 27.5 28.2 30.1 30.2 30.0 30.2 30.0 29.9i 29.6
29.5 30.0 30.1 30.1 30.1 30.1 30.1

Next, each single crystal obtained in the same manner as described above is manufactured, and from each of the measurement positions g to i,
A z-plate sample having a cross section of 10 mm × 10 mm and a thickness of 1.0 mm was cut out. After electrodes were formed on both z-axis surfaces, a voltage was applied to measure the voltage at which the crystal causes polarization reversal. Table 7 shows the measurement results. From the table, it can be seen that when the addition amount of Mg is less than 0.1 mol%, the polarization inversion voltage gradually decreases as the eutectic point approaches from the seed crystal. This decrease is considered to have occurred as a result of fluctuations in the melt composition ratio over time because the raw material was not supplied during the growth. On the other hand, when the added amount of Mg is 0.1 mol% or more, the decrease as seen in the case of less than 0.1 mol% is not observed, and the inversion voltage is also observed at a distance of 10 mm from the measurement position g to i. Is within 0.5 kV / mm, and at 0.2 mol% or more, the entire crystal shows a minimum value of 3.1 kV / mm almost uniformly.

(Table 7) Inversion voltage (unit: kV / mm)
No addition 0.05mol% 0.10mol% 0.20mol% 0.50mol% 1.00mol% 3.00mol% 5.00mol% g 7.0 5.0
3.6 3.1 3.1 3.1 3.1 2.9h 5.2 3.8 3.3 3.1 3.1 3.1 3.1 3.0i 3.1 3.1 3.1 3.1 3.1
3.1 3.1 2.6

Next, various optical functional elements were manufactured by periodically inverting the polarity of the LN single crystal produced in the same manner as in Example 1. The production of a QPM element that generates blue or green light with respect to a fundamental wave of near-infrared light of 840 nm or 1064 nm will be described. Regarding the crystals obtained in Example 1, one wafer was cut out from the crystals to which Mg was added at each concentration. The cut wafers had a diameter of 2 inches and a thickness of 0.3 mm, 0.5 mm, 1.0 mm, 2.0 mm,
3.0 mm was prepared.

Cut into z-axis orientation with both sides polished, use lithograph on + z plane, thickness 500nm
A comb-shaped pattern was formed using the Cr film as an electrode. In order to generate blue and green light harmonics with high efficiency, the electrode period is 3.0 microns and 6.
It was 8 microns. Next, an insulating film with a thickness of 0.5 microns is overcoated on the + z plane.
The storage process was performed at 50 degreeC for 8 hours. Next, a high voltage pulse was applied between both z planes of the crystal, sandwiched between electrodes via a lithium chloride aqueous solution. The current flowing through the LN single crystal was monitored through a 1 kilohm resistor.

After the domain-inverted lattice was formed, the y-plane of the crystal serving as the side surface was polished and etched with a mixed solution of hydrofluoric acid and nitric acid, and the state of the domain-inverted lattice was observed. By repeating this observation and polarization inversion for each sample, the pulse width and current of the applied voltage are optimized, and the polarization inversion lattice width ratio and the shape of the polarization inversion are ideally 1: 1 over the entire sample. It was made to approach (1: 0.95-1).

As a result of the experiment, even when the sample thickness is 0.3 mm, 0.5 mm, 1.0 mm, 2.0 mm, or 3.0 mm, a polarization inversion grating width ratio of about 1: 1 is obtained for most samples. However, it could not be obtained with crystals having a Mg concentration higher than 3 mol%. Specifically, the straightness of polarization inversion was poor, and there was a tendency that inversion lattices in which adjacent neighbors were connected were formed in many places. This is because the Mg concentration becomes too high, and it becomes difficult to apply a fine periodic voltage because the electric resistance is reduced, so that crystal inhomogeneity occurs, and the place where a large amount of Mg is contained, in particular, goes straight through the polarization inversion. It seems that it was hindered. That is, when considering the creation of an element for polarization inversion, the Mg addition concentration is desirably 3 mol% or less.

An optical functional element having a polarization inversion grating was prepared in the same manner as in Example 3 except that the diameter was 2 inches and the thickness was 1.0 mm. The target of domain-inverted lattice width is every 5 microns, ideally 1 each
: 1 (1: 0.95-1). As a result of the experiment, almost 1 for most samples
: 1 (1: 0.95 to 1) domain-inverted lattice width ratio was obtained, but the Mg concentration was 3 mo.
Crystals higher than 1% could not be obtained.

Next, a lens or prism-like domain-inverted structure was produced on the LN single crystal produced in Example 1, and optical elements such as a deflection element, a cylindrical lens, a beam scanner, and a switch using the electro-optic effect were produced. A z-cut LN single crystal having a diameter of 2 inches and a thickness of 0.2 to 2.0 mm and polished on both sides was prepared, and an Al electrode having a thickness of about 200 microns was formed on both z faces by sputtering. Lenses and prismatic patterns were formed. Then +
A pulsed voltage was applied to the z plane at about 3.5 KV / mm to reverse the polarization. Further, heat treatment was performed in air at 500 ° C. for about 5 hours. As a result, the non-uniformity of the refractive index introduced upon polarization reversal was eliminated. Further, the end face of the crystal was mirror-polished to form a laser light incident / exit surface.

The performance of the optical element using the electro-optic effect of the LN single crystal formed with the reversal of the refractive index by the prototype domain-inverted structure is the design of the lens- and prism-like domain-inverted structure, the accuracy of the process for producing the domain-inverted structure, and It was determined by the size of the electro-optic constant of the material. In the polarization inversion structure of the lens or prism pattern produced here as a prototype, it should be noted that the control of the polarization inversion property is very easy, so that good element characteristics were obtained.

In a conventional LN crystal having a congruent melting composition, it is difficult to control the domain inversion structure because a large applied voltage is required for domain inversion. In addition, the conventional LN crystal of the coincidence melt composition with 5 mol of MgO
In the case of LN single crystal added in an amount of at least%, the control of spontaneous polarization inversion is poor, and it was difficult to produce a highly accurate lens or prism-like domain-inverted structure.

This is the case when a lens or prism-like domain-inverted structure is produced on the LN single crystal produced in Example 1 and an optical element such as a deflection element, a cylindrical lens, a beam scanner, or a switch using the electro-optic effect is produced. There was no problem. Since further the crystals have a large electro-optical constant r ₃₃ than the congruent crystals, better device performance was obtained with a smaller operating voltages. For example, in the case of a deflection element, a voltage of about 600 V / mm is about 6 °.
A large deflection angle was obtained. In addition, a lens that operates in the vicinity of about 100 V / mm,
Switching operation at V / mm was also obtained.

In this embodiment, the voltage application method is described in detail as an embodiment in which the polarization is inverted at a temperature equal to or lower than the Curie temperature. According to the present invention, 1) Ti diffusion method. 2) SiO ₂ loading heat treatment method. 3)
Proton exchange heat treatment method. 4) Electron beam scanning irradiation method. Even when other methods such as the above are used, it is possible to realize an optical element in which a periodically poled lattice is formed with high accuracy by using a stoichiometric composition LN single crystal excellent in crystal perfection and controllability. It is.

In addition, although an embodiment in which a QPM element that generates blue or green light with respect to a fundamental wave of near-infrared light of 840 nm or 1064 nm is created is described in detail here, the fundamental wave is generated according to the present invention. The present invention is not limited to two wavelengths, and can be applied to a wavelength region where the LN single crystal is transparent and phase matching is possible. Further, the optical functional element that periodically inverts the polarization structure of the lithium niobate single crystal of the present invention and shortens or lengthens the wavelength of an incident laser having a wavelength in the visible to near-infrared region is a second harmonic. The present invention can be applied to various application fields such as remote sensing and gas detection such as optical parametric oscillator elements as well as wave generating elements.

Next, a lens or prism-like domain-inverted structure was produced on the LN single crystal produced in Example 1, and optical elements such as a deflection element, a cylindrical lens, a beam scanner, and a switch using the electro-optic effect were produced. A 2 inch diameter, 0.2-2.0 mm thick, double-side polished z-cut LN single crystal was prepared. Al electrodes with a thickness of about 200 microns were formed on both z faces by sputtering, and using a lithograph, Lenses and prismatic patterns were formed. Then +
A pulsed voltage was applied to the z plane at about 3.5 KV / mm to reverse the polarization. Further, heat treatment was performed in air at 500 ° C. for about 5 hours. As a result, the non-uniformity of the refractive index introduced at the time of polarization reversal was eliminated. Further, the end face of the crystal was mirror-polished to form a laser light incident / exit surface. The performance of the optical element using the electro-optic effect of the LN single crystal formed with the reversal of the refractive index by the prototyped domain-inverted structure is the design of the lens- and prism-like domain-inverted structure, the accuracy of the process for producing the domain-inverted structure, and It was determined by the size of the electro-optic constant of the material.

In the polarization reversal structure of the lens or prismatic pattern produced here as a prototype, it should be noted that excellent device characteristics were obtained because the polarization reversal property was very easy to control. Further, since this crystal has a larger electro-optic constant r ₃₃ than a crystal having a coincidence melt composition, better device performance can be obtained with a smaller operating voltage. For example, in the case of a deflection element, about 600V
A large deflection angle of about 6 ° was obtained at a voltage of / mm. In addition, a lens operating in the vicinity of about 100 V / mm and a switching operation at about 500 V / mm were obtained.

In this embodiment, the voltage application method is described in detail as an embodiment in which the polarization is inverted at a temperature equal to or lower than the Curie temperature. According to the present invention, 1) Ti diffusion method. 2) SiO ₂ loading heat treatment method. 3)
Proton exchange heat treatment method. 4) Electron beam scanning irradiation method. Even when other methods such as the above are used, it is possible to realize an optical element in which a periodically poled lattice is formed with high accuracy by using a stoichiometric composition LN single crystal excellent in crystal perfection and controllability. It is.

In addition, although an embodiment in which a QPM element that generates blue or green light with respect to a fundamental wave of near-infrared light of 840 nm or 1064 nm is created is described in detail here, the fundamental wave is generated according to the present invention. The present invention is not limited to two wavelengths, and can be applied to a wavelength region where the LN single crystal is transparent and phase matching is possible. Further, the optical functional element that periodically inverts the polarization structure of the lithium niobate single crystal of the present invention and shortens or lengthens the wavelength of an incident laser having a wavelength in the visible to near-infrared region is a second harmonic. The present invention can be applied to various application fields such as remote sensing and gas detection such as optical parametric oscillator elements as well as wave generating elements.

As described in detail above, according to the present invention, L ₂ crystal Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is obtained.
Without adding the third element Mg to make the molar fraction of Li ₂
O / (Nb ₂ O ₅ + Li ₂ O)) The LN crystal having the same nonlinear optical constant, polarization reversal voltage, and electro-optic constant as that of a perfect LN crystal having a molar fraction of 0.500 is given with high efficiency. be able to. By utilizing this, the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) having the highest wavelength conversion characteristics and electro-optical characteristics in the entire crystal is 0.490 or more and 0.
. LN crystals close to the stoichiometric composition that are less than 500 can be grown.

It is an example which shows the growth furnace of the LN single crystal used for this invention. It is a figure which shows the phase diagram of Li and Nb. It is a schematic diagram which shows the aspect of the LN single crystal at the time of using a single crucible.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Growing furnace 35 Outer crucible 36 Inner crucible 37 Raw material supply pipe 40 Seed crystal 41 Melt 42 LN single crystal 43 High frequency induction coil 45 Raw material 47 Growing furnace body 51 Gas 52 Load cell 61 Single crystal part 62 Ceramic layer

Claims (10)

Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) molar fraction of 0.56 to 0.60, Li niobate single crystal grown from a melt having a composition in which Li is in excess of the stoichiometric composition Because
The melt contains Mg element,
The lithium niobate single crystal contains 0.1 to 3.0 mol% of the Mg element with respect to the lithium niobate single crystal,
The molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) in the lithium niobate single crystal is between 0.490 and less than 0.500,
The applied voltage required to reverse the polarization at room temperature is less than 3.7 kV / mm,
The lithium niobate single crystal is for an optical element utilizing a domain-inverted structure. In lithium niobate single crystal according to claim 1, lithium niobate single crystal, wherein the nonlinear optical constant d ₃₃ is 26 Pm/V or more at a wavelength of 1.064 microns. In lithium niobate single crystal according to claim 1, lithium niobate single crystal, characterized in that in the wavelength 0.633 microns is electro-optic constant r ₃₃ is 36 Pm/V more. An optical element for converting the wavelength of a laser beam incident on a single crystal, wherein the ferroelectric polarization of the lithium niobate single crystal according to any one of claims 1 to 3 is inverted and quasi-phase-matched An optical element characterized by performing: 5. The optical element according to claim 4, wherein the thickness of the element in the z-axis direction is 1.0 mm or more and the period of polarization inversion is 30 microns or less. 6. The optical element according to claim 4, wherein the period of polarization inversion is 5 microns or less. An optical element for controlling laser light incident on a single crystal by utilizing an electro-optic effect of the single crystal, wherein the ferroelectric polarization of the lithium niobate single crystal according to claim 1 is inverted. An optical element characterized in that light is deflected, focused, and switched using a large refractive index change of the structure. A method for producing a lithium niobate single crystal according to claim 1,
In the crucible, a Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) molar fraction of 0.56 to 0.60 is an excess of the Li component in excess of the stoichiometric composition, and 0.1 to Filling and heating with Mg element satisfying the range of 3.0 mol% to make a melt,
Dipping a seed crystal of lithium niobate single crystal in the melt, pulling it up while rotating,
A method in which an amount of a stoichiometric composition raw material commensurate with the pulled lithium niobate single crystal is supplied to the crucible. The method according to claim 8, wherein the lithium niobate single crystal is for an optical element using a domain-inverted structure. The crucible is a double crucible consisting of an inner crucible and an outer crucible,
The Li component excess raw material and the Mg element are filled in the inner crucible and the outer crucible,
The method according to claim 8, wherein the stoichiometric composition raw material is supplied to the outer crucible by continuous supply.

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