JP2003267798A - Lithium niobate single crystal, its optical element, and its manufacturing method - Google Patents

Abstract

[PROBLEMS] A lithium niobate single crystal is expected to be used as a material for an optical element because a single crystal having excellent optical properties and a large diameter and high composition homogeneity can be supplied at a relatively low cost. It had been. However, conventional commercially available lithium niobate single crystals are:
Because of the congruent composition containing several percent of non-stoichiometric defects, there was a limit in improving device performance. SOLUTION: In a lithium niobate single crystal grown from a melt having a composition in which Li is more than a stoichiometric composition, Mg,
Zn, Sc, the mole fraction of Li ₂ O /, characterized in that it contains _{0.1~3mol% (Nb 2 O 5 +} Li 2 O) at least one element of the lithium niobate single crystal of In Is between 0.490 and less than 0.500, a single crystal of lithium niobate, an optical device thereof, and a method for producing the same.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium niobate single crystal having a stoichiometric composition which is excellent in polarization control characteristics, nonlinear optical characteristics and electro-optical characteristics, and a wavelength conversion element and an optical element using the single crystal. The present invention relates to an optical element such as a modulator, a switch, and a deflector optical element, and a manufacturing method for stably growing the lithium niobate single crystal.

[0002]

2. Description of the Related Art So-called functional optical single crystals, whose optical properties can be controlled by external information signals such as electricity, light, and stress, are used in various optoelectronic fields such as optical communication, display recording, measurement, and light-light control. Has become an indispensable material. In particular, a certain type of oxide single crystal has a particularly large interaction between optical properties and external factors, so a wavelength conversion element using a nonlinear optical effect or an electro-optical effect is used.
It is used as an optical element such as an optical modulator, a switch, and a deflector.

In many cases, such a crystal is used as an element in the as-grown state, but some ferroelectric crystals have a direction of dielectric polarization without destruction of the crystal when a voltage is applied. Since it can be inverted, the functionality is also enhanced by periodically reversing the polarization.

For example, in a wavelength conversion element, it is possible to perform wavelength conversion by a quasi-phase matching method (Quasi-Phase-Matching: QPM) by periodically inverting the domain structure of ferroelectric polarization. Since this method is an effective means in that it is possible to perform highly efficient conversion in a wide wavelength range, it is strongly demanded in the fields of optical communication, display recording, measurement, medical treatment, etc., from ultraviolet, visible to red. It is expected as a wavelength conversion element for realizing laser light sources of various wavelengths in a wide wavelength range extending to the outside.

Further, in the electro-optical element, for example,
Known literature (M. Yamada et al., Appl.Phys.Lett., 69, p
3659, 1996), an optical element, a cylindrical lens, a beam scanner, which forms a lens or prism-shaped polarization inversion structure in a ferroelectric crystal and deflects the laser light passing through it by using the electro-optic effect, Switches are attracting attention as new optical elements.

LiNbO ₃ single crystal (hereinafter abbreviated as LN single crystal) is a ferroelectric substance that is mainly used as a substrate for surface acoustic wave devices and optical modulators, but it has a wide wavelength range from visible to infrared. , Which is transparent, can create a periodic polarization structure by applying a voltage, has practically practical optical nonlinearity and electro-optical characteristics, and compared with a single crystal with a large diameter and high composition homogeneity. Since it can be supplied at a cheap price,
It is also attracting attention as a substrate for a wavelength conversion element based on QPM (hereinafter abbreviated as QPM element) and an electro-optical element as described above.

Up to now, the available LN single crystal, including the substrate of the surface acoustic wave device, contains a non-stoichiometric defect of about several%, and has a molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O). Is 0.
It was limited to 485 coincident melt compositions. The reason for this is L
The phase diagram of N single crystal has been known for a long time, and conventionally, in order to produce an LN single crystal having a high composition homogeneity, Li ₂ which is a congruent melting composition in which the crystal and the melt coexist in equilibrium
This is because it has been considered that it is preferable to grow the melt from the melt having an O / (Nb ₂ O ₅ + Li ₂ O) mole fraction of 0.485 by the spin-up method. In addition, known examples (DA Bryan et al. A
ppl. Phys. Lett. 44, p847, 1984), for the purpose of enhancing light damage resistance,
It is also practiced to add Mg in an amount of 4.5 mol% or more to LN crystals having an identical melting composition.

What is important in realizing a QPM device is:
It is to manufacture a small and highly efficient device. Although the miniaturization and high efficiency of an element largely depend on the element structure, the material properties used, that is, the material properties essentially possessed by the crystal are very important factors. For example, QPM
The conversion efficiency of the element is proportional to the square of the nonlinear optical constant and the interaction length, and is proportional to the fundamental wave power density. The interaction length and the fundamental wave power density are determined by the accuracy of the element design and fabrication process, and there is a great possibility that they will be improved by improving technology, whereas the nonlinear optical constant is inherent in the material. It is a material characteristic.

Since LN is one of the most popular non-linear optical materials, many measurements of non-linear optical constants have been performed since long ago. The LN crystal of the congruent melting composition that has been reported so far has a nonlinear optical constant d ₃₃ of wavelength 1.06.
At 4 microns, typically about 27-34 pm / V
However, the variation among the reported values is surprisingly large, reaching a maximum of 2 times. These values were obtained by relative measurements to determine the ratio of nonlinear optical constants with the reference material. However, the absolute value of the reference substance itself has not been determined, and the values used by different people were different, so the variation was so large.

In the conventional measuring method, the absolute value of the reference substance is based on the value obtained by the absolute measurement in which the absolute value of the nonlinear optical constant is directly measured. However, there is a large difference in the obtained values between the second harmonic generation (SHG) method and the parametric fluorescence (PF) method, which are typical measurement methods. For example, quartz d ₁₁ is the fundamental wavelength 1.06
At 4 microns, the absolute value scale based on the SHG method is 0.3 pm / V, whereas it is 0.5 pm / V with the PF method.
It becomes V.

Although the absolute value of the nonlinear optical constant was inaccurate, for example, in the known literature (I. Shoji et al., J. Opt. Soc.
m. B, 14, p2268, 1997), as a result of careful absolute measurement of both the SHG method and the PF method, the past reported values of the PF method could not exclude the effects of stray light during measurement. Was overestimated, and it was clarified that essentially both measurement methods gave consistent values. Recently, it became possible to measure the absolute value with high accuracy, and it was reported that the nonlinear optical constant ₃₃ of the LN crystal of the congruent composition, including the one with added Mg, was 24.9 to 25.2 pm / V. Has been done.

Further, when an LN single crystal is used for an electro-optical element, a large electro-optical constant is desired. Although the electro-optical constant itself of LN single crystal is not necessarily large among ferroelectric single crystals, it is a substrate for optical elements that utilizes various electro-optical effects because a high-quality, large-diameter single crystal can be manufactured inexpensively and stably. It has been used as a material. The electro-optic constants of LN single crystals have generally been measured using Mach-Zehnder interferometry. It has been reported that electro-optic constants r ₁₃ and r ₃₃ of the conventionally used LN single crystal having the congruent melting composition are about 8.0 pm / V and about 32.2 pm / V, respectively. Therefore, an element structure using a large electro-optic constant r ₃₃ has a great advantage in reducing the size and increasing the efficiency of the element.

In recent years, a study for reducing the existence of non-stoichiometric defects in LN single crystals having a congruent melting composition, that is, a study for bringing the crystal composition ratio closer to the stoichiometric ratio, has revealed that the existence of these non-stoichiometric defects is
It has been clarified that the non-linear optical constant originally possessed by the N crystal is lowered, and further the applied voltage required to create a periodic polarization structure is increased. For example, known literature (V. Gopalan et al. Appl. Phys. Lett. 72, p1981, 1
998), the polarization inversion voltage becomes 5 kV / mm or less by approaching the stoichiometric composition.

Further, in recent years, studies have been conducted to reduce the presence of non-stoichiometric defects in LN single crystals having a congruent composition with the aim of significantly improving the material performance. In order to make LN crystals having a stoichiometric composition practical, researches on their growing method are also actively conducted. For example, publicly known literature (GI Molovichiko et
al. Appl. Phys. A, 56, p103, 1993), this defect density can be reduced by growing a crystal from a melt obtained by adding 6 mol% or more of K ₂ O to a congruent melting composition or a stoichiometric composition. However, it is said that a composition having a composition close to a stoichiometric ratio can be obtained.

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 + Li ₂ O) of the growth melt was obtained.
By setting the mole fraction of 0.58 to 0.60, Li ₂ O
It can be seen that a crystal having a mole fraction of / (Nb ₂ O ₅ + Li ₂ O) close to 0.500 can be grown. However, as can be seen from the phase diagram, this melt composition ratio is extremely close to the eutectic point, and
When a crystal having a composition close to the stoichiometry is grown from a melt having a composition higher than the stoichiometry, an excess of Li component is left in the crucible with the precipitation of the crystals, Li
Since the composition ratio of Nb and Nb gradually changes, the composition ratio of the melt reaches the eutectic point immediately after the start of growth. Therefore, when the Czochralski method (hereinafter abbreviated as CZ method), which has been used as a means for industrially mass-producing large-diameter LN crystals, is used, solidification of crystals having a composition close to a stoichiometric ratio is performed. The rate is only about 10%.

The inventors of the present invention have filed Japanese Patent Application No. 10-274047.
Japanese Patent Laid-Open No. 2000-103697 proposes a method of growing a raw material while continuously supplying it (hereinafter abbreviated as continuous supply method) as a means for increasing the low solidification rate. Specifically, the Li ₂ O / (N
b ₂ O ₅ + Li ₂ O) has a molar fraction of 0.585 to 0.59.
5, the crucible has a double structure, and the crystal is pulled up from the inner crucible, and the weight of the pulled crystal is measured at any time to obtain the growth rate. At that rate, the powder of the same component as the crystal is removed from the inner crucible and the inner crucible. It is a method of supplying continuously during. By using this method, it is possible to grow a long crystal and achieve a crystal solidification rate of 100% with respect to the amount of raw material supplied.

The LN single crystal is often used as a QPM element. As an important process technology for achieving high efficiency, there is a technology for accurately generating a periodically domain-inverted domain. That is, in order to maximize the nonlinear optical characteristics, the ratio of the polarization inversion width (hereinafter abbreviated as the polarization inversion width) is made to be 1: 1. The polarization inversion width differs depending on the phase matching wavelength of the target wavelength conversion element. For example, in the case of long-wavelength phase matching in the infrared region, the polarization inversion width is ten and several microns. The polarization reversal voltage of the LN single crystal having the coincident melting composition is set to 21 kV / mm or more.

[0018]

The LN single crystal having the congruent melting composition belongs to the category of crystals exhibiting a large non-linearity among the existing non-linear optical crystals, but when the device is actually manufactured, Is still insufficient. As in recent years, as the degree of perfection of element design and the accuracy of manufacturing process have improved, the limit of significant improvement in element characteristics has become apparent only by improving the process. Is desired.

However, the crystal growth method of pulling up from a melt having a Li concentration higher than the congruent melting composition using the continuous supply method has a big problem in terms of yield from an industrial viewpoint. Gradually became apparent. That is, when a melt with a high Li concentration is used, unlike the case of growing a crystal with a consistent melting composition ratio,
Our research has revealed that the composition of the growing crystal strongly depends on the composition ratio of the melt.

This means that in order to grow a crystal having uniform optical characteristics and good optical homogeneity with high reproducibility, it is necessary to grow a crystal from a melt always kept at the same composition ratio. and, if the LN crystal, the voltage necessary for the formation of the non-linear optical constant and periodic inversion structure, and electro-optic coefficient is sensitive to the crystal composition ratio, the draws its greatest characteristics, crystals of Li ₂ O This means that the mole fraction of / (Nb ₂ O ₅ + Li ₂ O) must be fixed to a state extremely close to 0.500.

For example, the continuous feeding 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 desired. If it deviates from the melt composition, the entire grown crystal does not satisfy the nonlinear optical constant d ₃₃ and the reversal 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. It takes at least a few days to grow a small crystal and confirm the composition ratio, which significantly reduces productivity.

Further, the continuous feeding method is a very effective method for controlling the composition, but when the growing time is as long as several days to one week, it is possible to obtain a small amount from the melt surface kept at a high temperature. Evaporation of quantities of raw materials can occur. The change with time in the crystal composition due to this cannot be ignored if it is necessary to grow a crystal with a stoichiometric composition in which the composition is completely and uniformly controlled. This variation in crystal composition makes it very difficult to grow crystals with the same characteristics at a high yield, and to grow a perfect stoichiometric LN single crystal without defects from a melt with a high Li concentration. The technology has not been industrially put into practical use.

Further, in the LN crystal having the coincident melting composition, it was very difficult to form the polarization reversal width ratio to a perfect 1: 1 with high reproducibility. That is, in the voltage application method, a periodic electrode is provided on one surface of a LN single crystal of z-cut coincident melting composition and a uniform electrode is provided on the opposite surface, and a pulse voltage is applied through this electrode so that the portion immediately below the periodic electrode is z-axis oriented. Although the polarization is inverted toward, the inversion polarization width and the electrode width do not always match, and the manufacturing error is large. Also, z on the opposite side
The ideal polarization inversion width ratio has not been realized due to problems such as the inversion being interrupted during the formation of the polarization inversion in the axial direction and the polarization inversion width being different on both surfaces of the z-cut crystal.

In the case of short wavelength use such as visible to ultraviolet range, the polarization inversion width required for phase matching is about 3 μm, which is more difficult to prepare than that for long wavelength use. However, even a long-wavelength QPM element, which is relatively easy, has not been realized as an ideal element. One of the causes is
There is a high applied voltage (hereinafter abbreviated as a polarization reversal voltage) necessary for the polarization reversal of the LN single crystal having the coincident melting composition. The polarization inversion voltage is as high as 21 kV / mm or more. Due to this high inversion voltage, it is possible to form the polarization inversion grating on the entire substrate when the substrate thickness is less than 0.5 mm, but the thickness is 0. If the thickness is 0.5 mm or more, it is difficult to completely form the polarization inversion, and if the thickness is 1.0 mm or more, the accurate polarization formation capable of realizing the device has not been achieved.

Further, even if the substrate thickness is thinner than 0.5 mm, the polarization inversion period of a few microns as in the case of short wavelength is not realized. In particular, in the case of the coincident melting composition LN in which MgO is added at 5 mol% or more, the internal electric field is large, so that the hysteresis curve (PE curve) of the ferroelectric has poor symmetry, and the PE curve rises in the vicinity of the coercive field. Since it is gentle and not steep, there is a problem that the control of the reversal of the spontaneous polarization when an electric field is applied from the outside in the direction opposite to the spontaneous polarization is poor.

Further, in the case of the congruent melting composition LN in which MgO is added by 5 mol% or more, the electric resistance is reduced by about 3 to 4 digits or more as compared with the case of no addition, so that the applied voltage is finely controlled. And it is more difficult to create a polarization inversion width ratio of 1: 1. It is said that this problem can be solved by using the corona discharge method for polarization inversion, but even in this case, the problem of the thickness of the polarization inversion sample has not been solved yet.

A wavelength conversion element utilizing the non-linear optical effect of LN single crystal, a light modulating element utilizing the electro-optical effect, and a lens and prism-shaped polarization inversion structure formed on the LN single crystal are produced and passed through. In order to realize new optical elements such as optical elements that deflect the laser light using the electro-optical effect, cylindrical lenses, beam scanners, switches, etc., it is important to stably manufacture small and highly efficient elements. That is.

Also in the devices utilizing these electro-optical effects, the miniaturization and the high efficiency of the devices depend on the manufacturing precision of the device structure, but these are also largely limited by the material properties to be used. For example, the performance of an optical element utilizing the electro-optic effect of LN single crystal in which the inversion of the refractive index is formed by the polarization inversion structure depends on the accuracy of the design of the lens and prismatic polarization inversion structure and the manufacturing process of the polarization inversion structure, and It is determined by the magnitude of the electro-optic constant of the material.

In the conventional LN crystal having the congruent melting composition, it is difficult to control the polarization inversion structure because a large applied voltage is required for polarization inversion. Further, the electro-optical constant is a characteristic that a material has essentially, and it was considered difficult to improve it with the same crystal. In addition, depending on the wavelength and intensity of the light used, the occurrence of optical damage may be a major difficulty. In such a case, the coincident melt composition LN
It was expected that a crystal obtained by adding MgO of 5 mol% or more to the crystal is excellent in the light damage resistance. However, the accuracy of the control is not good because of the problem of the material property that the control of the reversal of the spontaneous polarization is poor as in the case of manufacturing the QPM element. The production of good lenses and prism-shaped polarization inversion structures has not been realized.

As the degree of completion of the element design and the accuracy of the manufacturing process have been improved as in recent years, there is a limit to the significant improvement of the optical element characteristics only by improving the process. Therefore, the performance of the material itself is improved. Is desired. Therefore, a technique for growing an LN single crystal having a stoichiometric composition with reduced non-stoichiometric defects has been developed. The crystal growth method of pulling a stoichiometric LN single crystal from a melt having a Li concentration higher than the congruent melting composition using a continuous supply method has a problem in yield from an industrial viewpoint. Things have gradually become clear.

That is, the continuous feeding method is an extremely effective method for controlling the composition, but when the growing time is as long as several days to one week, it is only a little from the melt surface kept at high temperature. Evaporation of quantities of raw materials can occur. The change with time in the crystal composition due to this cannot be ignored if it is necessary to grow a crystal with a stoichiometric composition in which the composition is completely and uniformly controlled. This variation in crystal composition makes it very difficult to grow crystals with the same characteristics at a high yield, and to grow a perfect stoichiometric LN single crystal without defects from a melt with a high Li concentration. The technology has not been industrially put into practical use.

This means that in order to grow a crystal having uniform optical properties and good optical homogeneity with high reproducibility, it is necessary to grow the crystal from the melt always kept at the same composition ratio. and, if the LN crystal, the voltage necessary for the formation of the non-linear optical constant and periodic inversion structure, and electro-optic coefficient is sensitive to the crystal composition ratio, the draws its greatest characteristics, crystals of Li ₂ O This means that the mole fraction of / (Nb ₂ O ₅ + Li ₂ O) must be fixed to a state extremely close to 0.500.

The performance of the optical element using the LN single crystal is determined by the precision of the periodic polarization inversion structure, the design of the lens and prism-like polarization inversion structure, the manufacturing process of the polarization inversion structure, and the nonlinear optical constants and electrical properties of the material. It is determined by the optical constant and the degree of light damage resistance. In the conventional LN crystal having the congruent melting composition, it is difficult to control the polarization inversion structure because a large applied voltage is required for polarization inversion. Further, the electro-optical constant is a characteristic that a material has essentially, and it was considered difficult to improve it with the same crystal. In addition, depending on the wavelength and intensity of the light used, the occurrence of photodamage may be a major difficulty. In such a case, a crystal obtained by adding 5 mol% or more of MgO to the LN crystal having the same melting composition is resistant to light damage. However, due to the problem of the material characteristics that the control of the reversal of spontaneous polarization is poor, it has not been possible to fabricate a highly accurate lens or prism-shaped polarization reversal structure.

[0034]

Means for Solving the Problems As a result of intensive research aimed at achieving the above-mentioned object, the present inventors have found that any element of Mg, Zn, Sc and In which has substantially no absorption in the visible light region is selected. By adding to the melt in a total amount of 0.1 pm to 3 mol%, a small polarization reversal voltage can be obtained without decreasing the nonlinear optical constant d ₃₃ and the electro-optical characteristic r ₃₃ .
The defect portion of Li can be filled with the third element, and even if it is a lithium niobate single crystal having a non-stoichiometric defect to some extent, although it is close to the stoichiometric composition, Li ₂ O /
Achieves the same nonlinear optical constants as the size of a perfect LN single crystal with a molar fraction of (Nb ₂ O ₅ + Li ₂ O) of 0.500 and the applied voltage and electro-optical constants necessary for creating a periodically polarized structure. It was discovered that this means Li ₂ O /
It has been found that the present invention is effective in a wide range of lithium niobate single crystals having a molar fraction of (Nb ₂ O ₅ + Li ₂ O) of 0.490 or more and less than 0.500. is there.

Mg, Zn, Sc, In for this d ₃₃
The effect of adding any of the elements, for example, Mg can be explained as follows. The nonlinear optical characteristics of LN crystal are Li element and O
Since it is generated by the combination of elements, the non-linearity decreases with the increase of Li defects, and Li ₂ O / (Nb ₂ O ₅ + Li ₂ O)
The LN crystal having a molar fraction of 0.500 exhibits the maximum non-linearity due to the absence of contained Li defects. In the case of a crystal that does not have a stoichiometric composition, an excessive Nb element enters the Li defect portion, but since the nonlinearity hardly occurs in the bond between the Nb element and the O element, the overall nonlinearity becomes small.

On the other hand, in the case of adding Mg, Mg enters the Li defect portion and causes nonlinearity due to the bond between the Mg element and the O element. The bond nonlinearity between the Mg element and the O element is similar to the nonlinearity caused by the bond between the Li element and the O element, and further, the crystal Li ₂ O / (Nb ₂ O) is caused by the change in the composition ratio of the grown melt. _{Even if} the mole fraction of ( ₅ + Li ₂ O) changes, the Mg element existing in the melt fills the Li defects, so that the crystal Li ₂ O / (Nb ₂ O ₅ + Li
It is considered that the maximum nonlinear optical property is maintained even if the molar ratio of ( ₂ O) varies to some extent.

The effect of adding Mg on the polarization inversion voltage can be explained as follows. The drastic reduction in the polarization reversal voltage of the stoichiometric crystal compared to the conventional congruent melting composition LN single crystal can be explained by the decrease in the number of Li defects pinning the polarization reversal. On the other hand, when Mg is added,
The molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.49.
The reason why the minimum voltage value is shown despite the variation in the range of 0 or more and less than 0.500 is that the pinning effect in the state where Mg is substituted in the Li site is smaller than that of Li defects. Conceivable.

However, since the pinning effect in the state where Mg is substituted in the Li site is larger than that in the defect-free portion, this effect can be obtained in crystalline Li ₂
The O / (Nb ₂ O ₅ + Li ₂ O) mole fraction is conspicuous only in a narrow range of 0.490 or more and less than 0.500. For example, when Mg is added to a crystal having a congruent melting composition, a decrease in the polarization reversal voltage is also seen, but on the other hand, the electrical resistance is reduced by about 4 digits or more as compared with the case where no addition is made. The polarization cannot be reversed by the usual voltage application method, and a special method called the corona discharge method was required. L
The molar fraction of i ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.490.
Required Mg, Zn, S in the range of 0.500 or more and less than 0.500
Since the added amounts of c and In are as small as 0.1 to 3.0 mol, there is no sudden decrease in electric resistance.

The effect of Mg addition on r ₃₃ has not been clarified at this point, but it 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 expression factor of the electro-optical characteristics, the electro-optical constant decreases as the Li defects increase, and Li ₂ O / (Nb ₂ O ₅ + Li The LN crystal having a molar fraction of ₂ O) of 0.500 can be expected to show the maximum electro-optical constant because it contains no Li defects.

In the case of a crystal not having a stoichiometric composition, excess Nb
Although the element enters into the Li defect portion, the electro-optical constant is small as a whole because the electro-optical characteristic is hardly generated by the combination of the Nb element and the O element. On the other hand, M
In the case of adding g, Mg enters the Li defect portion,
Electro-optical characteristics are generated by the combination of the element and the O element. If the bond electro-optical characteristic of the Mg element and the O element is similar to the electro-optical characteristic generated by the bond of the Li element and the O element, the crystal Li caused by the change in the composition ratio of the grown melt is further increased.
_{Even if} the mole fraction of ₂ O / (Nb ₂ O ₅ + Li ₂ O) changes, the Mg element present in the melt fills the Li defects, so that the crystal Li ₂ O / (Nb ₂ It can be explained that the maximum electro-optical constant is maintained even if the molar ratio of (O ₅ + Li ₂ O) varies to some extent.

In the present invention, for example, in the continuous feeding method, the setting of the melt composition ratio at the start of growth is temporarily deviated from the desired melt composition by adding 0.1 mol or more of Mg, Zn, Sc and In. Even if it is present, Li ₂ O / (Nb ₂ O ₅
+ Li ₂ O) has a non-linear optical constant, polarization structure creation voltage, and electro-optical constant that have the same size as the LN single crystal having a 0.500 mole fraction, and as a result, the yield is greatly improved. It is possible.

Further, at the interface between the crystal and the melt due to the change in the melt composition ratio caused during the growth due to the evaporation of the melt and the non-uniformity of the composition ratio in the grown melt, and the temperature distribution in the crucible. Due to fluctuations in melt temperature, Li ₂ O /
Inhomogeneity of the mole fraction of (Nb ₂ O ₅ + Li ₂ O) occurs, but according to the present invention, a nonlinear optical constant, a polarization structure forming voltage,
And the electro-optical constant is Li ₂ O / (Nb ₂ O ₅ + Li ₂ O)
Since these properties do not depend on the mole fraction of these, the inhomogeneity of these properties does not occur, and as a result, the growth conditions for stable production of LN crystals having both high homogeneity and excellent performance become extremely gentle. Is.

Here, Li ₂ O / (Nb ₂ O ₅ + Li ₂ O)
The reason why the mole fraction of 0.490 or more and less than 0.500 is set is that the crystal having a composition smaller than 0.490 did not sufficiently reduce the polarization inversion voltage. Furthermore, Li ₂ O
In the crystal having a composition in which the molar fraction of / (Nb ₂ O ₅ + Li ₂ O) is 0.490 or more and less than 0.500, almost no internal electric field is observed, and the hysteresis curve (PE curve) of the ferroelectric substance has symmetry. It is a great merit that the control of the reversal of the spontaneous polarization when an electric field in the opposite direction to the spontaneous polarization is applied from the outside is extremely excellent and the PE curve rises well in the vicinity of the coercive field.

Further, in the case of a crystal having a composition in which the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.490 or more and less than 0.500, the required Mg addition concentration is less than 3 mol%. Therefore, it is possible to prevent a sharp decrease in electric resistance as seen in a crystal in which 5.0 mol% of Mg is added to a crystal having an identical melting composition, and the polarization inversion width ratio is approximately 1: 1. A very high efficiency QPM device can be manufactured.

In the optical device having the above-mentioned structure for converting the wavelength of the laser light incident on the single crystal, the nonlinear optical constant d ₃₃ is 26 pm / V or more, and the applied voltage required for polarization reversal at room temperature is 3 It is possible to produce LN single crystals characterized by a voltage of less than 0.7 kV / mm. A QPM element having a thickness of 1.0 mm or more in the z-axis direction and a polarization inversion period of 30 μm or less is an LN of the present invention.
The present invention was first realized by using a crystal, and was further realized by the present invention for a QPM element having a polarization inversion period of 5 μm or less.

Further, with the above structure, in the optical element for controlling the laser light incident into the single crystal by utilizing the electro-optical effect of the single crystal, the electro-optical constant r ₃₃ is 36 pm / V at the wavelength of 0.633 μm. It is possible to produce a lithium niobate single crystal characterized by the above. An optical element characterized by highly efficiently and stably deflecting, focusing, and switching light by utilizing a large change in the refractive index of a structure obtained by inverting the ferroelectric polarization of a lithium niobate single crystal is an LN of the present invention. It was first realized by crystals.

[0047]

EXAMPLES Examples of the present invention will be shown below. LN single crystal
If it is obtained from the congruent melt composition melt by the usual pulling method
The LN single crystal has an excessive Nb component, but the composition of the melt
Excessive Li component excess (eg Li₂O / (Nb₂O_Five+ Li
₂O) with a mole fraction of 0.56 to 0.60)
When a crystal is grown, Li having a close stoichiometric composition₂O / (Nb ₂O
_Five+ Li₂0.500 which is the mole fraction of O), that is,
It is possible to obtain a single crystal with the non-stoichiometric defect concentration suppressed as much as possible.
It In this embodiment, the double crucible method with continuous feed of raw materials is used.
By using the melt of Li component excess composition, L close to stoichiometric composition
N single crystal was grown.

Example 1 Commercially available high-purity Li₂O, Nb₂
O_FiveThe raw material powder of₂O: Nb₂O_FiveRatio of 0.
56-0.60: 0.44-0.40 Li component excess raw material
And Li₂O: Nb ₂O_Five= 0.50: 0.50 stoichiometric composition
The raw materials were mixed. Next, 1 ton / cm²By the hydrostatic pressure of
Bar press molding, each in the atmosphere of about 1050 ℃
A raw material rod was prepared by sintering. In addition, a mixed stoichiometric composition source
Used as a raw material for continuous supply in the atmosphere at about 1150 ° C
Knotted, crushed, size 50 micron or more 500 micro
It was classified in the range of size.

Next, when growing a single crystal by the double crucible method, the raw material rods made of the Li-rich material are filled in the inner and outer crucibles in advance, and then the crucible is heated to remove the Li-rich melt. Created. In an experiment for confirming the effect of adding Mg, a commercially available high-purity MgCO
₃ was pre-filled in the inner and outer crucibles. M to fill
The weight of gCO ₃ is such that the Mg concentration in the melt is 0.1, 0.2, 0.5, 1.0, 3.0 mol with respect to Nb in the melt, respectively.
% Of 5 experiments were performed. For comparison, an experiment was conducted with the Mg concentration set to 0, 0.05 and 5.0 mol%.

Here, a double layer for growing a stoichiometric LN crystal is used.
Brief explanation of the principle of the crucible method with reference to FIGS. 1 and 2.
To do. FIG. 1 shows a phase diagram of LN. As seen in the phase diagram
In addition, normal pulling from LN single crystal congruent composition melt
Although the LN single crystal obtained by the method has an excess of Nb component,
The composition of Li has a significant excess of Li component (for example, Li₂O / (Nb ₂
O_Five+ Li₂O) molar fraction was 0.56 to 0.60)
When a crystal is grown from the melt, Li having a close stoichiometric composition is obtained.₂O /
(Nb₂O_Five+ Li₂0.500, which is the molar fraction of O),
That is, to obtain a single crystal with a non-stoichiometric defect concentration suppressed as much as possible.
You can

FIG. 2 shows the growth furnace 1 used in the present invention. The structure of the double crucible used in this embodiment is a structure in which a cylinder 36 (called an inner crucible) having a height of 7.5 mm higher than that of the outer crucible is installed inside the outer crucible 35,
The bottom of the inner crucible was provided with a hole leading from the outer crucible to the inner crucible. These holes were approximately rectangular in shape of about 20 mm × 30 mm and provided in three places in the inner crucible. Here, the material of the crucible used for the growth was made of platinum, and the surrounding was covered with the growth furnace body 47 to prevent the inflow of the external atmosphere.

The shape of the double crucible used is the outer crucible 35.
The height / diameter ratio was 0.45, and the inner crucible / outer crucible diameter ratio was 0.8. Its size is a crucible 3
5 had a diameter of 150 mm and a height of 67.5 mm, and 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 a raw material supply pipe 37 was stably installed in the space 34 so that the raw material 45 could drop 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 is observed, but as the rotation speed of the crucible is gradually increased, it can be seen that the forced convection of the melt in the rotating direction becomes stronger. The effect was observed.

Crystals were grown from the melt 41 of the inner crucible having an excess of Li component. After the temperature of the melt was stabilized at a predetermined temperature by the power supplied to the high-frequency oscillator 48 and the high-frequency induction coil 43, the LN unit in a single polarization state of 5 mm × 5 mm × length 70 mm cut out in the Z-axis direction was cut. Seed crystal 4
The LN single crystal 42 was grown by connecting it to the lower part of the rotary support rod 38 as 0 and attaching it to the melt 41, and rotating the crystal while pulling it upward while controlling the melt temperature. The growing atmosphere was the atmosphere. The rotation speed of the LN single crystal 42 is 5 to
It is constant within the range of 20 rpm and the pulling speed is 0.5.
It was changed in the range of up to 3.0 mm / h.

Automatic diameter control was performed on the straight body of the crystal so that a wafer having a diameter of 2 inches could be produced from the grown crystal. The grown crystal growth weight was measured by the load cell 52, and an amount of Li ₂ O / (Nb ₂
O ₅ + Li ₂ O) raw material 4 with a stoichiometric ratio of 0.500
5 was fed to the outer crucible 35. Here, LN single crystal 4
Since the change in the growth amount of 2 is obtained by the computer 49, the raw material 45 is supplied from the LN seed crystal 40 to the single crystal 4
It started from the time when the growth of No. 2 started and the diameter control became stable.

The raw material 45 is supplied by a hermetically sealed container 46 which is also installed in advance on the upper part of the growth furnace 47 and which also functions as a weight measuring sensor.
The raw material 45 stored inside was supplied through the supply pipe 37 made of ceramics or noble metal. Gas 51 was flowed 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 equipped with a valve. The flow rate of the gas 51 was optimized depending on the amount of the raw material 45 supplied per unit time and the particle size. As a result, the raw material was smoothly supplied without scattering and clogging in the supply pipe 37. By rotating the precious metal double crucible during the growth, at the same time as homogenizing the supplied powder raw material with the melt, the convection of the melt is forced to make the crystal growth interface flat or convex with respect to the liquid surface. Controlled. By growing each composition for about 1.5 weeks, a colorless and transparent LN crystal body having a diameter of 60 mm and a length of 110 mm and having no crack was obtained.

For all the crystals obtained, Li ₂ O /
The mole fraction of (Nb ₂ O ₅ + Li ₂ O) was determined by chemical analysis. The measurement position of the sample is the axial center of the crystal that is 15 mm away from the seed crystal as the measurement position a, and the measurement position a is 10 mm apart in the direction away from the seed crystal along the axial center.
The points were taken, and measurement parts b, c, and d were set in order. The measurement sample was cut into a 7 mm square cube centered on the measurement position.

Table 1-1 shows Li ₂ O / (Nb ₂ O ₅ + Li ₂
The measurement result of the mole fraction of O) is shown. In chemical analysis, it is difficult to accurately determine the absolute value of the composition ratio, and in the case of LN crystal, the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is about 0.
It contains an error of about 001 to 0.005. Therefore,
The composition was analyzed very carefully for LN crystals having a composition close to the stoichiometry. 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 the LN single crystal, even if the composition is close to the stoichiometric ratio, in the crystal to which Mg or the like is added, Li ₂ O / (Nb ₂ O ₅
The value of the mole fraction of + Li ₂ O) did not exceed 0.005. In addition, the Mg content of these samples was also measured, and it was confirmed that the crystal content was almost the same as the Mg concentration added to the melt.

(Table 1) Li ₂ O / (Nb ₂ O ₅ + Li ₂ O)
No molar fraction of 0.05mol% 0.10mol% 0.20mol% 0.50mol% 1.00mol
% 3.00mol% 5.00mol% a 0.492 0.494 0.496 0.498 0.496 0.494 0.492 0.489 b 0.493 0.493 0.495 0.499 0.498 0.495 0.492 0.490 c 0.494 0.494 0.496 0.498 0.497 0.495 0.491 0.491 d 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 have determined the absolute value of the nonlinear optical constants accurately by performing the absolute measurement using the wedge method and analyzing the measurement data considering the effect of multiple reflection. As a result, a material with a high refractive index (n
It was revealed that most of the conventional values for> 2) were overestimated, and the d ₃₃ of LN crystals with a melt-matched composition was measured and found to be 25.1 pm / V, which is in good agreement with the result found in the literature. The value was obtained. The laser light used for measurement has a wavelength of 1.06 continuous oscillation in the single longitudinal mode.
It is 4 microns. Table 2 shows the measurement results.

When the amount of Mg added is 0.1 mol% or more,
If the crystal is Li₂O / (Nb ₂O_Five+ Li₂O)
The mole fraction varies widely between 0.489 and 0.499.
Despite that, everything is over 30.0pm / V
While the value is less than 0.1 mol%,
It tends to be slightly inferior. By the absolute measurement method using the wedge method
Is different from the conventional absolute measurement method by the phase matching method, d₃₃
Diagonal components such as can be measured. Also, a rotating Maker
In the fringe method, rigorous analysis considering multiple reflections is performed.
Is extremely difficult, and it is necessary to accurately obtain the nonlinear optical constants.
For anti-reflection coating, multiple reflection does not occur
There is no choice but to measure under the conditions. From the above, the wedge
That absolute measurement by the method is a very effective measurement method
be able to.

(Table 2) Nonlinear optical constant d ₃₃ (unit: pm
/ V) No additive 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 30.3 b 28.8 29.5 30.0 30.3 30.0 30.4 30.2 30.1 c 29.0 29.6 30.0 30.2 30.1 30.3 30.2 30.0 d 29.1 29.6 30.2 30.1 30.3 30.1 30.1 30.1

Next, regarding each single crystal obtained in the same manner as above, a cross section of 1 is obtained from each of the measurement positions a to d.
A 0 mm × 10 mm z-plate sample having a thickness of 1.0 mm was cut out. After forming electrodes on both z-axis surfaces, voltage is applied,
The voltage at which the crystal causes polarization reversal was measured. Table 3 shows the measurement results.

When the added amount of Mg was 0.1 mol% or more, all were 3.7 kV / mm or less and 0.2 m
When it is ol% or more, a smaller value, a constant value near 3.1 kV / mm, is obtained. In these crystals, the internal electric field is hardly seen, the hysteresis curve (PE curve) of the ferroelectric substance is excellent, and the PE curve rises in the vicinity of the coercive field, so there is little variation in measured values. it is conceivable that.

On the other hand, when the content is less than 0.1 mol%, the polarization reversal voltage tends to be slightly higher than that of the added crystals in a larger amount. On the other hand, the amount of Mg added is 5 mol%
When the above additions were made, the polarization reversal was reduced, but there was a tendency for the variation to be increased for each sample. This is the P in the vicinity of the coercive field of the hysteresis curve (PE curve) of the ferroelectric substance.
It is considered that the cause is that it is difficult to measure the absolute value of the polarization reversal voltage because the rise of the -E curve is smooth and bad, and also due to the electrical resistance of the material. Incidentally, when the reversal voltage of the melt coincidence composition crystal was measured under the same sample shape and measurement conditions, the measurement was difficult in some cases. Measurement is possible with a thin sample with a sample thickness of 0.2 to 0.5 mm. 2
It was a very high value of 1.0 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 2.3 b 5.0 3.9 3.4 2.8 3.0 2.9 3.1 2.9 c 4.9 3.9 3.3 2.9 3.0 3.1 3.1 2.1 d 4.8 3.8 3.3 3.0 3.1 3.1 3.1 2.5

Next, regarding each single crystal obtained in the same manner as described above, x, y, z are measured from the respective measurement positions a to d.
A sample of 5 mm × 3 mm × 2 mm was cut out in the azimuth direction. After forming electrodes on both z-axis surfaces, the electro-optical constant of the sample was measured by using the 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 crystal composition. That is, crystalline Li ₂ O / (Nb ₂ O ₅ + Li ₂
In the LN single crystal having a mole fraction of O) of 0.490 or more and less than 0.500, the electro-optic constant r ₁₃ does not increase as compared with the conventional congruent melting composition LN single crystal, but r ₃₃ increases by about 20% or more. It was found that the value was about 36 pm / V or more, which was much larger than the value of the congruent melting composition LN single crystal of about 31.5 pm / V.

In particular, the electro-optical constant tended to increase as the composition became closer to the stoichiometric composition. In addition, when the amount of Mg added was 0.1 mol% or more, a further increase of 38 pm / V or more was observed, and a maximum of 39.5 pm / V was obtained with the crystal added approximately 1 mol%. Was given. On the other hand, there was also a tendency that the electro-optical constant gradually decreased when the added amount of Mg exceeded 1 mol%.

(Table 4) Electro-optical constant r ₃₃ (unit: pm /
V) No additive 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 36.9 b 36.2 37.0 38.0 38.4 38.6 39.5 38.6 37.1 c 37.1 37.8 38.1 38.4 38.8 39.3 38.3 36.5 d 37.8 37.6 38.1 38.3 39.0 39.2 38.2 36.4

Example 2 Commercially available high-purity Li ₂ O and Nb ₂
A raw material powder of O ₅ was prepared, and the ratio of Li ₂ O: Nb ₂ O ₅ was 0.1.
56-0.60: 0.44-0.40 Li component excess raw materials were mixed. Next, rubber press molding was performed at a hydrostatic pressure of 1 ton / cm ² and sintering was performed in the atmosphere at about 1050 ° C. to prepare a raw material rod. Next, single crucible method, that is, conventional CZ
When growing a single crystal by the method, a raw material rod made of a Li-rich material was prepared in advance, and then the crucible was heated to produce a Li-rich melt. In the experiment for confirming the effect of adding Mg, a commercially available high-purity MgC was used during this filling.
The crucible was pre-filled with O ₃ . The weight of MgCO _{3 to be} filled is determined by the Mg concentration in the melt with respect to Nb in the melt,
Five kinds of experiments of 0.1, 0.2, 0.5, 1.0 and 3.0 mol% were conducted.

In addition, as a comparison, no addition, 0.05, 0.5
The experiment was performed in the same manner except that mol% was added. The crucible used for the growth was made of platinum. The crucible used has a cylindrical shape with a diameter of 150 m.
The height was 100 mm. The state of the melt surface was observed with a video camera throughout the growing process. Unlike Example 1,
Strong melt convection was observed even in the absence of 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 out in the Z-axis direction was attached to the melt as a seed crystal 60, and the melt temperature The single crystal was grown by rotating the crystal while pulling up and pulling it upward. The crucible was fixed without rotating. The growing atmosphere was the atmosphere. The rotation speed of the crystal was constant at 2 rpm, and the pulling rate was changed in the range of 0.5 to 3.0 mm / h. The weight of the grown crystal was measured with a load cell throughout the growth so that a wafer with a diameter of 2 inches could be created from the grown crystal, and the diameter of the straight body part of the crystal was about 60 mm. Control was performed. In the growth of this example, the raw materials were not supplied during the growth as in the case of using the double crucible of Example 1. L obtained in FIG.
The schematic diagram of N single crystal is shown.

When Mg was not added and when various concentrations of Mg were added, the transparent single crystal part 61 could be grown from immediately after the start of growth to about 30 mm when pulled up with a diameter of 60 mm. After that, the portion reaching the eutectic point and pulled up after reaching the eutectic point was not the LN single crystal but the ceramic layer 62.

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

(Table 5) Li ₂ O / (Nb ₂ O ₅ + Li ₂ O)
No molar fraction of 0.05mol% 0.10mol% 0.20mol% 0.50mol% 1.00mol
% 3.00mol% 5.00mol% g 0.489 0.489 0.491 0.495 0.495 0.491 0.491 0.490 h 0.491 0.494 0.495 0.498 0.496 0.492 0.492 0.494 i 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 the seed crystal approaches the eutectic point.
It is considered that this increase was caused as a result of the composition ratio of the melt varying with time because the raw material was not supplied during the growth. On the other hand, when the addition amount of Mg is less than 0.1 mol%, the increase as seen in the case of 0.1 mol% or more is not observed. Distance from measurement position g to i 1
The non-linear optical constant d ₃₃ stays within a substantially constant value even during 0 mm, and shows a maximum value of 30 pm / V or more almost uniformly over the entire crystal at 0.2 mol% or more.

(Table 6) Nonlinear optical constant d ₃₃ (unit: pm
/ V) No additive 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 29.5 h 27.5 28.2 30.1 30.2 30.0 30.2 30.0 29.9 i 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 above was manufactured, and a cross section of 1 was obtained from each of the measurement positions g to i.
A 0 mm × 10 mm z-plate sample having a thickness of 1.0 mm was cut out. After forming electrodes on both z-axis surfaces, voltage is applied,
The voltage at which the crystal causes polarization reversal was measured. Table 7 shows the measurement results. From the table, when the added amount of Mg is less than 0.1 mol%, as the seed crystal approaches the eutectic point,
It can be seen that the polarization inversion voltage is gradually decreasing. It is considered that this decrease was caused as a result of the melt composition ratio varying with time because the raw material was not supplied during the growth. On the other hand, when the amount of addition of Mg is 0.1 mol% or more, the decrease as seen in the case of less than 0.1 mol% is not seen, and the reversal voltage is maintained even within the distance of 10 mm from the measurement position g to i. Is within 0.5 kV / mm,
And, if it is 0.2 mol% or more, it is almost uniform throughout the crystal 3.1.
It shows the minimum value of kV / mm.

(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 2.9 h 5.2 3.8 3.3 3.1 3.1 3.1 3.1 3.0 i 3.1 3.1 3.1 3.1 3.1 3.1 3.1 2.6

Example 3 Next, various optical functional devices were manufactured by periodically reversing the polarization of the LN single crystal produced in the same manner as in Example 1. The production of a QPM element that emits blue or green light with respect to the fundamental wave of near infrared light of 840 nm or 1064 nm will be described. Regarding the crystals obtained in Example 1, wafers were cut out one by one from the crystals to which Mg was added at each concentration. The cut wafer has a diameter of 2 inches and a thickness of 0.3 mm and 0.5 m, respectively.
m, 1.0 mm, 2.0 mm, and 3.0 mm were prepared.

Cut out in the z-axis direction with both sides polished,
Lithography was used on the + z surface to form a comb-shaped pattern using a 500 nm-thick Cr film as an electrode. First-order QP for highly efficient generation of blue and green light harmonics
The electrode periods were 3.0 and 6.8 microns to provide the M structure. Next, a 0.5-micron-thick insulating film was overcoated on the + z surface and a storage treatment was performed at 350 ° C. for 8 hours. Next, a high voltage pulse was applied to both z-planes of the crystal by sandwiching them between electrodes via a lithium chloride aqueous electrolytic solution.
The current flowing through the LN single crystal was monitored through a 1 kilohm resistance.

After forming the domain-inverted lattice, the y-plane of the crystal to be the side surface was polished and etched with a mixed solution of hydrofluoric acid / nitric acid to observe the state of the domain-inverted lattice. For each sample,
By repeating this observation and the polarization inversion, the pulse width of the applied voltage and the current are optimized, and the polarization inversion lattice width ratio and the shape of the polarization inversion are 1: 1 (1: 0), respectively, over the entire sample. .95-1).

As a result of the experiment, the thickness of the sample was 0.3 mm and 0.3 mm.
In all cases of 5 mm, 1.0 mm, 2.0 mm and 3.0 mm, a polarization inversion lattice width ratio of almost 1: 1 could be obtained for most samples, but the Mg concentration was 3 mol%.
It could not be obtained with higher concentration of crystals. Specifically, the linearity of polarization reversal was poor, and there was a tendency that inversion lattices with adjacent neighbors were formed in many places. This is because the Mg concentration became too high, and the electrical resistance decreased, making it difficult to apply a fine periodic voltage, resulting in crystal inhomogeneity, and in particular, where Mg was contained in large amounts, the polarization reversal progressed straight. Probably because it interfered. In other words, when considering the production of an element for reversing polarization, the Mg addition concentration is 3 mol.
% Or less is desirable.

Example 4 The diameter is 2 inches and the thickness is 1.
An optical functional element having a polarization inversion grating was prepared in the same manner as in Example 3 except that the thickness was 0 mm. The target of the polarization inversion lattice width is every 5 microns, and ideal 1: 1 (1: 0.9)
I tried to get closer to 5-1). As a result of the experiment, it was possible to obtain a polarization inversion lattice width ratio of almost 1: 1 (1: 0.95 to 1) for most of the samples, but it was not possible to obtain it in a crystal having a Mg concentration higher than 3 mol%. .

(Embodiment 5) Next, L prepared in Embodiment 1 is used.
Optical elements such as a deflection element, a cylindrical lens, a beam scanner, and a switch, which utilize the electro-optical effect, are produced by producing a polarization inverting structure of a lens or prism on N single crystal. 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. Al electrodes having a thickness of about 200 μm were formed on both z faces by sputtering, and a lithographic method was used. , Lens and prismatic patterns were formed. After that, a pulsed voltage of about 3.5 KV / mm was applied to the + z plane to reverse the polarization. 50 more
Heat treatment was performed in air at 0 ° C. for about 5 hours. As a result, the nonuniformity of the refractive index introduced at the time of polarization reversal was eliminated. Furthermore, the end face of the crystal was mirror-polished to form a laser light input / output surface.

The performance of the optical element utilizing the electro-optic effect of the LN single crystal in which the refractive index inversion is formed by the prototyped polarization inversion structure depends on the design of the lens or prism-like polarization inversion structure and the manufacturing process of the polarization inversion structure. It was determined by the accuracy and the magnitude of the electro-optic constant of the material. It should be noted that the polarization inversion structure of the lens or the prismatic pattern manufactured here was obtained, and good element characteristics were obtained because the control of the polarization inversion property was very easy.

In the conventional LN crystal having the congruent melting composition, it was difficult to control the polarization inversion structure because a large applied voltage is required for polarization inversion. Further, in the case of the conventional LN crystal having the congruent melting composition, in which LN single crystal in which 5 mol% or more of MgO is added, the control of the reversal of the spontaneous polarization is poor, so that it is difficult to manufacture a lens or a prismatic polarization reversal structure with high precision.

In the case where a lens or prism-shaped polarization inversion structure is formed on the LN single crystal prepared in Example 1 and an optical element such as a deflecting element, a cylindrical lens, a beam scanner or a switch utilizing the electro-optical effect is produced. , No such problem was seen. Further, since the present crystal has a larger electro-optic constant r ₃₃ than that of the crystal having the congruent melting composition, superior device performance was obtained at a smaller operating voltage. For example, in the case of the deflection element, a large deflection angle of about 6 ° was obtained at a voltage of about 600 V / mm. Also, about 10
Lens that operates near 0 V / mm, or about 500 V / mm
The switching operation at was also obtained.

In this embodiment, the voltage application method was described in detail as an embodiment in which the polarization is inverted at a temperature below the Curie temperature. According to the present invention, 1) Ti diffusion method. 2) Si
O ₂ loading heat treatment method. 3) Proton exchange heat treatment method. 4) Electron beam scanning irradiation method. Even when using other methods such as
Stoichiometry composition L with excellent crystal perfection and controllability
By using the N single crystal, it is possible to realize an optical element in which the periodically poled grating is formed with high accuracy.

Further, here, 840 nm or 106
An example in which a QPM element that generates blue or green light with respect to the fundamental wave of 4 nm near-infrared light was created was described in detail, but according to the present invention, the fundamental wave is not limited to these two wavelengths. However, it can be applied to a wavelength range in which an LN single crystal is transparent and phase matching is possible. Furthermore, the optical function element for periodically reversing the polarization structure of the lithium niobate single crystal of the present invention to shorten or lengthen the wavelength of the incident laser having a wavelength in the visible to near infrared region is the second harmonic. Not only the wave generation element but also optical parametric oscillator element and the like can be applied in various application fields such as remote sensing and gas detection.

(Example 6) Commercially available high-purity Li₂O, Nb₂
O_FiveThe raw material powder of₂O: Nb₂O_FiveRatio of 0.
56-0.60: 0.44-0.40 Li component excess raw material
And Li₂O: Nb ₂O_Five= 0.50: 0.50 stoichiometric composition
The raw materials were mixed. Next, 1 ton / cm²By the hydrostatic pressure of
Bar press molding, each in the atmosphere of about 1050 ℃
A raw material rod was prepared by sintering. In addition, a mixed stoichiometric composition source
Used as a raw material for continuous supply in the atmosphere at about 1150 ° C
Knotted, crushed, size 50 micron or more 500 micro
It was classified in the range of size. Next, by the double crucible method
For growing a single crystal,
Pre-fill the inner and outer crucibles with
The crucible was heated to prepare a melt containing excess Li component. Sc
In the experiment to confirm the effect of addition, when filling this,
High purity Sc₂O₃Pre-fill the inner and outer crucibles
It was Sc to fill₂O₃Is the weight of the Sc concentration in the melt.
0.1, 0.2, 0.5, 1.0 for Nb in the liquid,
Five experiments with 3.0 mol% were performed.

The crucible used for the growth was made of platinum. The shape of the double crucible used is the height of the outer crucible
/ The diameter ratio was 0.45, and the diameter ratio of the inner crucible / outer crucible was 0.8. The outer crucible has a diameter of 1
50mm height 67.5mm, inner crucible 36 diameter 120
mm and height 75 mm. There was a space of about 15 mm on each side between the inner crucible and the outer crucible, and the raw material dropped smoothly there.

Crystals were grown from the melt of the inner crucible with excess Li component. After stabilizing the temperature of the melt to a predetermined temperature with the input power to the high frequency oscillator and the high frequency induction coil,
A 5 mm x 5 mm x 70 mm long LN single crystal in a single polarization state cut out in the Z-axis direction was connected as a seed crystal to the lower part of the rotary support rod and attached to the melt to form a crystal while controlling the melt temperature. The LN single crystal added with Sc was grown by rotating and pulling upward. The growing atmosphere was the atmosphere. The rotation speed of the LN single crystal was kept constant within the range of 5 to 8 rpm, and the pulling speed was changed within the 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 diameter wafer could be produced from the grown crystal. The grown crystal growth weight is measured with a load cell,
An amount of Li ₂ O / (Nb ₂ O ₅ corresponding to the crystallized growth amount)
A raw material having a stoichiometric ratio of + Li ₂ O) of 0.500 was fed to the outer crucible. Since the change in the growth amount of the LN single crystal is obtained by a computer here, the supply of the raw material powder was started at the time when the growth of the single crystal started from the LN seed crystal and the diameter control was stabilized.

The supply of the raw material was carried out through a supply pipe made of ceramics, which was previously stored in a hermetically sealed container equipped with a weight measuring sensor which was previously installed on the upper part of the growth furnace. Gas was flowed into the supply pipe and the sealed container at a rate of 200 to 500 cc / min through a gas pipe equipped with a valve. The gas flow rate was optimized according to the amount of the raw material powder supplied per unit time and the particle size. As a result, the raw material was smoothly supplied without scattering and clogging in the supply pipe. By rotating the noble metal double crucible during the growth, at the same time as homogenizing the supplied powder raw material with the melt, the convection of the melt is forced to make the crystal growth interface flat or convex with respect to the liquid surface. Controlled. Each composition was grown for about 1.5 weeks to obtain an LN crystal body having a diameter of 60 mm, a length of 110 mm, and colorless and transparent Sc added without cracks.

Regarding the obtained crystal, Li ₂ O / (Nb ₂
The mole fraction of O ₅ + Li ₂ O) was determined by a chemical analysis method and a differential thermal analysis method. The measurement position of the sample is 15m from the seed crystal
The center of the axis of the crystal separated by m is defined as the measurement position a, and 10 m in the direction away from the seed crystal along the center of the axis from the measurement position a.
Three positions were taken for each m, and the measurement parts b, c, and d were set in order.
The measurement sample was cut into a 7 mm square cube centered on the measurement position.

It is difficult to accurately obtain the absolute value of the composition ratio by chemical analysis, and in the case of LN crystal, Li ₂ O / (Nb ₂ O
The mole fraction of ( ₅ + Li ₂ O) contains an error of about 0.001 to 0.005. Therefore, the composition of the LN crystal having a composition close to the stoichiometry was analyzed very carefully, and the Curie temperature, which is a physical quantity sensitive to the crystal composition, was determined by the differential thermal analysis method. As a result, in the case of the LN single crystal, even if the composition is close to the stoichiometric ratio, the crystal added with Sc or the like is Li ₂ O / (N
The value of the molar fraction of (b ₂ O ₅ + Li ₂ O) never exceeded 0.005. In addition, the content of Sc element in these samples was also measured, and the content of crystals was S added to the melt.
It was confirmed that the segregation coefficient was almost 1 at the same concentration as c.

Next, regarding each of the obtained single crystals, a cross section of 10 mm × 10 mm was obtained from each of the measurement positions a to d.
Then, a z-plate sample having a thickness of 1.0 mm was cut out. After forming electrodes on both z-axis surfaces, a voltage was applied, and the voltage at which the crystal caused polarization reversal was measured. Sc addition amount is 0.1 mol%
In the case of the above additions, all values were 3.4 kV / mm or less. In these crystals, the internal electric field is hardly seen, the hysteresis curve (PE curve) of the ferroelectric is excellent, and the PE curve rises well in the vicinity of the coercive field. it is conceivable that.

On the other hand, when the content is less than 0.1 mol%, the polarization reversal voltage tends to be slightly higher than that of the added crystal in a larger amount. On the other hand, the added amount of Sc is 3 mol%
When the above additions were made, the polarization reversal was reduced, but there was a tendency for the variation to be increased for each sample. This is the P in the vicinity of the coercive field of the hysteresis curve (PE curve) of the ferroelectric substance.
It is considered that the cause is that it is difficult to measure the absolute value of the polarization reversal voltage because the rise of the -E curve is smooth and bad, and also due to the electrical resistance of the material. Incidentally, when the reversal voltage of the melt coincidence composition crystal was measured under the same sample shape and measurement conditions, the measurement was difficult in some cases. Measurement is possible with a thin sample with a sample thickness of 0.2 to 0.5 mm. 2
It was a very high value of 1.0 kV / mm.

Next, the nonlinear optical constants of these samples were measured.
did. We make absolute measurements using the wedge method and measure
The data should be analyzed considering the effect of multiple reflections.
And to accurately determine the absolute value of the nonlinear optical constant.
It was As a result, a material with a high refractive index (n
Most of the traditional values for> 2) are overvalued
It was clarified that the d₃₃To
When measured, it agrees well with the results required in the literature.
A value of 25.1 pm / V was obtained. Used for measurement
Laser light has a wavelength of single longitudinal mode continuous oscillation of 1.06
It is 4 microns. Sc addition amount is 0.1 mol% or more
When added, crystalline Li₂O / (Nb ₂O_Five+ Li₂
O) mole fraction is large between 0.489 and 0.499
Despite variations, everything is 30.0 pm / V
While the values are above, if less than 0.1 mol%,
It tends to be slightly inferior to this.

Next, with respect to each single crystal obtained in the same manner as above, x, y, z were measured from the respective measurement positions a to d.
A sample of 5 mm × 3 mm × 2 mm was cut out in the azimuth direction. After forming electrodes on both z-axis surfaces, the electro-optical constant of the sample was measured by using the Mach-Zehnder interferometry. It was revealed that some of these constants are very sensitive to crystal composition. That is, crystalline Li ₂ O / (Nb ₂ O ₅ +
In the LN single crystal having a molar fraction of Li ₂ O) of 0.490 or more and less than 0.500, the electro-optic constant r ₁₃ does not increase as compared with the conventional congruent melting composition LN single crystal, but r ₃₃ is about 20%. More than about 36 pm / V or more, and the consistent melt composition LN
It was revealed to be very large compared with the value of single crystal of about 31.5 pm / V. In particular, the electro-optic constant tended to increase as the composition became closer to the stoichiometric composition. Further, in the crystal to which Sc was added, when the addition amount was 0.1 mol% or more, a further increase was observed at 38 pm / V or more, and particularly, in the crystal to which about 1 mol% was added, the maximum was 39.
5 pm / V was obtained. On the other hand, the addition amount of Sc is 1 mol
There was also a tendency that the electro-optical constant gradually decreased when the content was more than%.

Next, regarding each single crystal obtained in the same manner as described above, x, y, and z were measured from the respective measurement positions a to d.
A 2 mm × 10 mm × 10 mm sample was cut out in the azimuth direction. An ultraviolet-green laser having a wavelength of 350, 488, 532 nm was made incident on the crystal in the X direction, and its light damage resistance was evaluated.
When optical damage occurs, a diffraction grating with a refractive index is formed in the crystal and the profile of the passing beam is greatly distorted.
The presence or absence of light damage can be easily determined. LN crystal without addition of Sc, etc., has several kW / c regardless of the crystal composition.
Light damage occurred at an incident intensity of about m ² .

In the crystal to which Sc is added, the light damage resistance is greatly improved, and the LN single crystal having a Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) mole fraction of 0.490 or more and less than 0.500. Then, even if the addition amount of Sc was as small as 0.1 mol%, it was stable against the incidence of light with an intensity of several MW / cm ² . This is characterized by a large improvement effect at a lower concentration than when Mg is added, and a low concentration is a great merit in improving the material performance without deteriorating the crystal quality. . On the other hand, it has been confirmed that in the crystal added with Sc, if 3 mol% or more is added, it becomes relatively difficult to grow the crystal, and the crystal quality is deteriorated, so that large material performance cannot be exhibited.

Example 7 Next, various optical functional elements were manufactured by periodically reversing the polarization of the LN single crystal produced in the same manner as in Example 1. The production of a QPM element that emits blue or green light with respect to the fundamental wave of near infrared light of 840 nm or 1064 nm will be described. S obtained in Example 1
A wafer was cut out from the crystal added with c. The cut wafer has a diameter of 2 inches and a thickness of 0.
3mm, 0.5mm, 1.0mm, 2.0mm, 3.0mm
Prepared. , Cut in the z-axis direction with both sides polished, and using a lithograph on the + z plane, C with a thickness of 500 nm
A comb-shaped pattern was formed using the r film as an electrode. The electrode period was set to 3.0 μm and 6.8 μm so as to form a first-order QPM structure in order to generate blue and green light harmonics with high efficiency. Next, thickness +0 on the + z plane.
A 5 micron insulating film was overcoated and stored at 350 ° C. for 8 hours. Next, a high voltage pulse was applied to both z-planes of the crystal by sandwiching them between electrodes via a lithium chloride aqueous electrolytic solution. The current flowing through the LN single crystal was monitored through a 1 kilohm resistance.

After forming the domain-inverted lattice, the y-plane of the crystal to be the side surface was polished and etched with a mixed solution of hydrofluoric acid / nitric acid, and the state of the domain-inverted lattice was observed. For each sample,
By repeating this observation and the polarization inversion, the pulse width of the applied voltage and the current are optimized, and the polarization inversion lattice width ratio and the shape of the polarization inversion are 1: 1 (1: 0), respectively, over the entire sample. .95-1).

As a result of the experiment, the thickness of the sample is 0.3 mm and 0.1 mm.
In all cases of 5 mm, 1.0 mm, 2.0 mm and 3.0 mm, a polarization inversion lattice width ratio of almost 1: 1 could be obtained for most samples, but the Sc concentration was 3 mol%.
It could not be obtained with higher concentration of crystals. Specifically, the linearity of polarization reversal was poor, and there was a tendency that inversion lattices with adjacent neighbors were formed in many places. This is because the Sc concentration became too high, which made it difficult to apply a fine periodic voltage due to a decrease in electric resistance, resulting in crystal inhomogeneity, and particularly where Sc was contained in a large amount, the polarization reversal progressed straight. Probably because it interfered. In other words, when considering the production of an element for reversing the polarization, the Sc addition concentration is 3 mol.
% Or less is desirable.

(Embodiment 8) Next, L prepared in Embodiment 1 is used.
Optical elements such as a deflection element, a cylindrical lens, a beam scanner, and a switch, which utilize the electro-optical effect, are produced by producing a polarization inverting structure of a lens or prism on N single crystal. A z-cut LN single crystal with a diameter of 2 inches and a thickness of 0.2 to 2.0 mm, which was polished on both sides, was prepared, and Al electrodes with a thickness of about 200 μm were formed on both z surfaces by sputtering. A lens or prismatic pattern was formed. After that, a pulsed voltage of about 3.5 KV / mm was applied to the + z plane to reverse the polarization. 50 more
Heat treatment was performed in air at 0 ° C. for about 5 hours. As a result, the nonuniformity 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 input / output surface. The performance of the optical element utilizing the electro-optic effect of the LN single crystal in which the refractive index inversion is formed by the prototyped polarization inversion structure depends on the accuracy of the design of the lens and prism polarization inversion structure and the manufacturing process of the polarization inversion structure, and It was determined by the magnitude of the electro-optic constant of the material.

In the prototyped lens and the polarization inversion structure of the prism-shaped pattern, what is remarkable is that the control of the polarization inversion property is very easy and good element characteristics are obtained. Further, since the present crystal has a larger electro-optic constant r ₃₃ than that of the crystal having the congruent melting composition, superior device performance was obtained at a smaller operating voltage. For example, in the case of a deflection element, a voltage of about 600 V / mm results in about 6
A large deflection angle of 0 ° was obtained. Also, about 100 V / mm
A lens operating in the vicinity and a switching operation at about 500 V / mm were also obtained.

In this example, the voltage application method was described in detail as an example of polarization reversal at a temperature below the Curie temperature, but according to the present invention, 1) Ti diffusion method. 2) Si
O ₂ loading heat treatment method. 3) Proton exchange heat treatment method. 4) Electron beam scanning irradiation method. Even when using other methods such as
Stoichiometry composition L with excellent crystal perfection and controllability
By using the N single crystal, it is possible to realize an optical element in which the periodically poled grating is formed with high accuracy.

In this case, 840 nm or 106
An example in which a QPM element that generates blue or green light with respect to the fundamental wave of 4 nm near-infrared light was created was described in detail, but according to the present invention, the fundamental wave is not limited to these two wavelengths. However, it can be applied to a wavelength range in which an LN single crystal is transparent and phase matching is possible. Furthermore, the optical function element for periodically reversing the polarization structure of the lithium niobate single crystal of the present invention to shorten or lengthen the wavelength of the incident laser having a wavelength in the visible to near infrared region is the second harmonic. Not only the wave generation element but also optical parametric oscillator element and the like can be applied in various application fields such as remote sensing and gas detection.

[0110]

As described above in detail, according to the present invention, the Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) mole fraction of the LN crystal is completely reduced to 0.500. By adding the third element, the nonlinear optical constant of the LN crystal whose molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.500,
An LN crystal having a polarization reversal voltage and an electro-optical constant can be provided with high efficiency. By utilizing this, it is possible to grow a stoichiometric LN crystal having the best wavelength conversion characteristics and electro-optical characteristics throughout the crystal.

[Brief description of drawings]

FIG. 1 is an example showing a growth furnace for an LN single crystal used in the present invention.

FIG. 2 is a diagram showing a phase diagram of Li and Nb.

FIG. 3 is a schematic view showing an aspect of an LN single crystal when a single crucible is used.

[Explanation of symbols]

1 growth furnace 35 Outer crucible 36 Inner Crucible 37 Raw material supply pipe 40 seed crystals 41 Melt 42 LN single crystal 43 high frequency induction coil 45 raw materials 47 Growth furnace body 51 gas 52 load cell 61 Single crystal part 62 ceramic layer

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yasunori Furukawa             1-1, Namiki, Tsukuba City, Ibaraki Prefecture             Agency Inorganic Materials Research Center (72) Inventor Kenji Kitamura             1-1, Namiki, Tsukuba City, Ibaraki Prefecture             Agency Inorganic Materials Research Center (72) Inventor Shunji Takekawa             1-1, Namiki, Tsukuba City, Ibaraki Prefecture             Agency Inorganic Materials Research Center (72) Inventor Akio Miyamoto             5200 Mikkajiri, Kumagaya-shi, Saitama Hitachi Metals Co., Ltd.             Ceremony Company Magnetic Materials Research Center (72) Inventor Masaki Terao             346 Miyanishi, Harima-cho, Kako-gun, Hyogo Prefecture             Gaku Co., Ltd. (72) Inventor Noboru Suda             3-5 Hikaridai, Seika-cho, Soraku-gun, Kyoto, Kyoto             Central Research Laboratory F-term (reference) 2H079 AA02 AA12 BA01 CA05 CA17                       DA02 HA12                 2K002 AB12 BA01 CA03 FA11 FA27                       HA20 HA21                 4G077 AA02 AB01 BC32 CF10 HA06                       PD08

Claims (9)

[Claims] 1. A lithium niobate single crystal grown from a melt having a composition of Li in excess of the stoichiometric composition, wherein M
0.1 to 3.0 mol% of at least one element of g, Zn, Sc and In based on the lithium niobate single crystal
Li ₂ O / (Nb ₂ O ₅ + Li ₂ characterized by containing
A lithium niobate single crystal having a molar fraction of O) of 0.490 or more and less than 0.500. 2. The lithium niobate single crystal according to claim 1, wherein the nonlinear optical constant d ₃₃ is 26 pm / V or more at a wavelength of 1.064 μm. 3. The lithium niobate single crystal according to claim 1, wherein an applied voltage required for polarization reversal at room temperature is 3.
A lithium niobate single crystal having a voltage of less than 7 kV / mm. 4. The lithium niobate single crystal according to claim 1, which has an electro-optic constant r at a wavelength of 0.633 μm.
₃₃ is 36 pm / V or more, a lithium niobate single crystal. 5. 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 claim 1 is inverted. An optical element characterized by performing quasi-phase matching with. 6. The optical element according to claim 5, wherein the thickness of the element in the z-axis direction is 1.0 mm or more and the period of polarization reversal is 30 μm or less. 7. The optical device according to claim 5, wherein the period of polarization inversion is 5 μm or less. 8. An optical element for controlling a laser beam incident into a single crystal by utilizing an electro-optical effect of the single crystal,
The deflection, focusing, and switching of light are performed by utilizing a large change in the refractive index of the structure in which the ferroelectric polarization of the lithium niobate single crystal according to claim 1, 3 or 4 is inverted. Optical element. 9. A lithium niobate single crystal having a molar ratio of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) of 0.490 or more and less than 0.500 is used, in which Li is in excess of the stoichiometric composition. In a growing method of pulling up from a melt having a different composition, preventing characteristic deterioration due to non-stoichiometric defects in the crystal resulting from composition change of the melt,
In addition, in order to grow a high quality crystal, a crystal containing 0.1 to 3.0 mol% of any element of Mg, Zn, Sc and In, which has substantially no absorption in the visible light region, is deposited. A method for producing a lithium niobate single crystal, which comprises using a composition melt.