JP4590531B2 - Optical device comprising lithium niobate single crystal wafer and method for producing lithium niobate single crystal for the wafer - Google Patents

Description

The present invention relates to an optical element such as a wavelength conversion element, an optical modulator, a switch, and a deflector optical element using a lithium niobate single crystal wafer for optical applications excellent in polarization control characteristics, nonlinear optical characteristics, and electro-optical characteristics, and the method of manufacturing of stably growing a lithium niobate single crystal for the wafer.

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 a ferroelectric crystal, and the laser light passing through this 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. The reason for this is that the phase diagram of LN single crystals has been known for a long time. Conventionally, in order to produce LN single crystals with high compositional homogeneity, the crystals and the melt are in equilibrium and coexist in the same 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), which is a composition, by a rotary pulling method. In addition, as shown in a known example (Non-patent Document 2), for the purpose of improving the light damage resistance, LN having a coincident melt composition is used.
Adding 4.5 mol% or more of Mg to a single crystal is also performed.

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 the LN single crystal is one of the most popular nonlinear optical materials, many measurements of nonlinear optical constants have been performed for a long time. In previous reported wavelength 1.064 nonlinear optical constant d ₃₃ is LN single binding <br/> crystals congruent has microns, typically from about 27~34p
Although it is m / V, the variation for each reported value is surprisingly large, reaching 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 LN single crystals with coincidence melting compositions, including those with added Mg, have been corrected to have a nonlinear optical constant ₃₃ of 24.9 to 25.2 pm / V. It has been reported.

In addition, when an LN single crystal is used for an electro-optic element, a large electro-optic constant is desired. L
Although N electrooptical constant itself of the single crystal is not necessarily large in ferroelectric single crystal, since the single crystal of large diameter with high quality can be produced stably at low cost an optical element utilizing various electro-optical effect It has been used as a substrate material. The electro-optic constant of LN single crystals has generally been measured using Mach-Zehnder interferometry. Conventionally used LN with a consistent melt composition
In the single crystal, the electro-optic constants r ₁₃ and r ₃₃ are about 8.0 pm / V and about 32.2 pm / V, respectively.
It is reported that. 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, the study of reducing the existence of non-stoichiometric defects in LN single crystals having a congruent composition, that is, the study of bringing the crystal composition ratio close to the constant ratio, indicates that the existence of these non-stoichiometric defects is the nonlinear optical constant inherent in the LN single crystal. 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 a LN crystal having a stoichiometric composition practical, L
Research on the growth method of N single crystal has been actively conducted. For example, according to known literature (Non-Patent Document 5), by growing crystals from a melt obtained by adding 6 mol% or more of K ₂ O to a coincident melt composition or a stoichiometric composition, this defect density is reduced, It is said that the thing of a composition close | similar to is obtained. In addition, in the known literature (Non-patent literature 6), Mg, Zn, In
A comparison result of light damage when either element of Sc and Sc is added has 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 method for producing no optical damage LN single crystal 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 the Czochralski method (hereinafter abbreviated as CZ method), which has been used as a means for industrially mass-producing large-diameter LN single crystals, is used, A solidification rate of only about 10% can be obtained.

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).
The excellent photorefractive sensitivity and cerium is added over 18ppm in N single crystal and semidefinite ratio composition LN single crystal is obtained, discloses that the continuous feed process is suitable for the manufacture of the cerium-doped LN single 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 etal., 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 single 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) must be fixed very close to 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, if 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. For variation of the crystal composition, to foster the crystals of the same characteristics with a high yield has become very difficult, the defect-free perfect stoichiometric LN single crystal of a high Li concentration melt The breeding technique has not been put into practical use industrially.

In addition, with the LN single crystal having the coincidence melting composition, it is very difficult to form a complete reversal width ratio of 1: 1 with high reproducibility. In other words, in the voltage application method, the LN of the z-cut coincident melt composition
A periodic electrode is provided on one side of a wafer cut out from a single crystal , and a uniform electrode is provided on the opposite side. By applying a pulse voltage through this electrode, the portion directly under the periodic electrode is inverted in polarization toward the z-axis direction. The polarization width and the electrode width do not always coincide with each other, and the manufacturing error is large. In addition, in the middle of forming the polarization inversion in the z-axis direction of the opposite surface, the inversion is interrupted or the polarization inversion width is z
Because there are problems such as differences between both sides of the cut crystal, the ideal polarization inversion width ratio was not realized.

In the case of short wavelength applications such as the visible range to the ultraviolet range, the polarization inversion width necessary for phase matching is about 3 microns, which makes it more difficult to create compared to the long wavelength range. 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 single crystal , the symmetry of the ferroelectric hysteresis curve (PE curve) is poor due to the large internal electric field, and the rise of the PE curve in the vicinity of the coercive electric field is gentle and steep, so that There is a problem that control of the reversal of spontaneous polarization when an electric field in the opposite direction is applied is poor.

Furthermore, in the case of the coincidence melt composition LN single crystal 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 the applied voltage can be delicately controlled. It is difficult, and 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 single 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 in which 5 mol% or more of MgO is added to the coincidence melt composition LN single crystal is light resistant. Expected because of its excellent damage, but it is possible to fabricate high-precision lenses and prismatic domain-inverted structures due to the material property problem of poor control of reversal of spontaneous polarization, similar to fabrication of QPM elements. Was not.

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. Therefore, development techniques LN single crystal of stoichiometric composition with reduced nonstoichiometric defects have been developed. Crystal growth method from a melt of higher Li concentration than congruent with continuous feed process raise the stoichiometric LN single crystal body, when viewed from the industrial point of view, a problem in terms of yield 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 surface of the melt maintained at a high temperature 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. For variation of the crystal composition, to foster the crystals of the same characteristics with a high yield has become very difficult, the defect-free perfect stoichiometric LN single crystal of a high Li concentration melt The breeding technique 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 single 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) must be fixed 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 single 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, the coincidence melt composition LN single crystal is 5 m.
Although crystals with ol% or more of MgO added were expected to be excellent in light damage resistance, the production of highly accurate lenses and prismatic domain-inverted structures due to the problem of material properties that the control of spontaneous polarization inversion was poor It was not realized.

The inventors of the present invention have earnestly studied to achieve the provision of a lithium niobate single crystal for optical devices that has non-stoichiometric defects but maintains the same characteristics as a lithium niobate single crystal having a perfect stoichiometric composition. As a result, by adding Mg element substantially not absorbing in the visible light range in the range of 0.1 to 3 mol%, more preferably in the range of 0.2 mol% or more and 1.0 mol% or less, A small polarization inversion voltage can be obtained without deteriorating the nonlinear optical constant d ₃₃ and the electro-optical characteristic r _33, and the defect portion of Li can be filled with the third element. Even if it is a lithium niobate single crystal having defects, Li ₂ O / (
To realize the same nonlinear optical constant as that of a perfect LN single crystal having a molar fraction of (Nb ₂ O ₅ + Li ₂ O) of 0.500 and the applied voltage and electro-optical constant necessary for creating a periodically polarized structure Furthermore, the present means is applicable 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 the LN single crystal of 0.490 or more and less than 0.500. Thus, 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 single 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) The LN single crystal having a N of 0.500 shows the maximum nonlinearity because there is no Li defect contained. In the case of a crystal having a non-stoichiometric composition, excess Nb element enters the Li defect portion, but since non-linearity hardly occurs in the combination of Nb element and O element, the overall non-linearity is reduced.

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 optics is obtained even if the molar ratio of the Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) of the crystal varies slightly throughout the grown crystal. It is thought that sex is maintained.

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 between 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. When the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is in the range of 0.490 or more and less than 0.500, the required amount of Mg added is 0.1 to 3.0 mol%, more preferably 0.8. Since it is as small as 2 mol% or more and 1.0 mol% or less, there is no sudden drop in electrical resistance.

Further, the effect of Mg addition on the electro-optic constant r ₃₃ is not 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 single 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 mole fraction 0.500 LN single <br/> crystal O) can be expected to show the highest electrooptical constant because there is no Li defects contained.

In the case of an LN single crystal having a non-stoichiometric composition, excess Nb element enters the Li defect portion, but Nb
Since the electro-optical characteristics are hardly generated in the combination of the element and the O element, the electro-optic constant as a whole becomes small. On the other hand, in the case of Mg addition, Mg enters the Li defect portion,
Electro-optical characteristics are generated by the combination of Mg element and 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) is changed, the Mg element present in the melt fills the Li defect,
It can be explained that the maximum electro-optic constant is maintained even if the molar ratio of i ₂ O / (Nb ₂ O ₅ + Li ₂ O) varies somewhat.

In the production of a single crystal used in the optical element of the present invention, for example, in the continuous supply method, Mg is set to a value of 0.00.
Even if the setting of the melt composition ratio at the start of growth is deviated from the desired melt composition by adding 1 mol, more preferably 0.2 mol% or more, Li ₂ O / (Nb ₂ O ₅ + Li ₂ O ) Can grow single crystals with the same nonlinear optical constant, polarization structure creation voltage, and electro-optic constant as the LN single crystal having a mole fraction of 0.500, and as a result, the yield can be greatly improved. is there.

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 Due to the fluctuation, inhomogeneity of the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) occurs in the crystal, but the LN single crystal obtained by the production method of the present invention has a nonlinear optical constant and polarization. Since the structure creation voltage and the electro-optic constant do not depend on the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O),
Heterogeneous of these characteristics does not occur, in which growth conditions for stably producing the LN single <br/> crystal having both superior performance and high homogeneity as a result is 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.

In the case of a crystal having a composition 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, the required Mg addition concentration is less than 3 mol%, more preferably 1.0m
Since it is ol% or less, it is possible to prevent a sudden decrease in electrical resistance as seen in a crystal in which 5.0 mol% of Mg is added to a crystal having a congruent melting composition, and the polarization inversion width ratio is approximately 1. : 1
It is possible to manufacture a very highly efficient QPM device.

With the above configuration, in the optical element that converts the wavelength of the laser light incident on the single crystal, the wavelength 1
. The non-linear optical constant d _{33 at} 064 microns is 26 pm / V or more, and the applied voltage required for polarization reversal at room temperature is less than 3.7 kV / mm, more preferably 3.1 kV / mm.
It is possible to produce a LN single crystal, characterized in that at most. A QPM element having a thickness in the z-axis direction of 1.0 mm or more and a polarization inversion period of 30 microns or less is the light of the present invention.
Is obtained by the first time realized in LN single crystal used in the device, further, with regard QPM element period of the domain inversion is 5 microns or less, is obtained by realizing started by the optical device of 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 manufacture a lithium niobate single crystal, characterized in that it pm / V or more. An optical element characterized in that light deflection, focus, or switching is performed with high efficiency and stability by utilizing a large refractive index change in a structure in which ferroelectric polarization of a lithium niobate single crystal is inverted. it is obtained by realizing started by LN single crystal obtained by the production method.
(Example)

Examples of the present invention are shown below. For LN single crystal, usually of LN single crystal obtained by the pulling method from congruent melt it becomes excessive Nb component, excess significantly Li component composition of the melt (
For example, when a crystal is grown from a melt in which the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.56 to 0.60), Li ₂ O / (Nb ₂₎ close to the stoichiometric composition. O ₅ + Li ₂ O) 0.500 mole fractions, i.e., it is possible to obtain a single crystal body while minimizing the nonstoichiometric defect concentration. Using a double crucible method continuously supplying raw material in this example, it was grown LN single crystal close to the stoichiometric composition from a melt of Li component excessive composition.

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.

Then, when the single crystal grown by the double crucible method, pre-filled with a feed rod consisting of Li component excessive material created inner and outer crucibles were generated Li component excessive melt and then heated crucible . In an experiment for confirming the effect of adding Mg, commercially available high-purity MgCO ₃ was prefilled in the inner and outer crucibles as the Mg element source during the filling. The weight of MgCO _{3 to be} filled is such that the Mg concentration in the melt is 0.1, 0.2, 0.5, 1.0, and 3.0 m, respectively, with respect to Nb in the melt.
Five types of experiments of ol% were conducted. For comparison, the Mg concentration is set to 0, 0.05, 5.0 mo.
The experiment was conducted at 1%.

Here it will be briefly described with reference to FIGS. 1 and 2 the principle of the double crucible method of growing a stoichiometric LN single crystal. FIG. 2 shows the phase diagram of LN. Phase as seen in FIG, LN single crystal body from the congruent melt LN single crystal obtained by the conventional pulling method becomes excessive Nb component,
When the composition of the melt is grown from a melt in which the Li component is excessively large (for example, the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.56 to 0.60), it is close to the stoichiometric composition. Li ₂ O / (Nb ₂ O ₅
+ Li ₂ O), which is 0.500, that is, a single crystal that suppresses the non-stoichiometric defect concentration as much as possible.
You can get a body .

FIG. 1 shows a growth furnace 1 used for producing a single crystal used in the optical element of the present invention. The structure of the double crucible used in this example is 7.5 mm in height inside the outer crucible 35 than the outer crucible.
It has a structure in which a high cylinder 36 (referred to as an inner crucible) is installed, and a hole is provided at the bottom of the inner crucible from the outer crucible to the inner crucible. 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. 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 single crystal having a diameter of 60 mm and a length of 110 mm and having no cracks was obtained.

For all resulting in LN single crystal was determined from the chemical analysis of the mole fraction of _{Li 2 O / (Nb 2 O} 5 + Li 2 O). The measurement position of the sample is 10 mm in the direction away from the seed crystal from the measurement position a along the axial center from the axial center of the LN single crystal body 15 mm away from the seed crystal.
Three points were taken for each position, and the measurement parts b, c, and d were taken in order. The measurement sample is 7mm around the measurement position.
Cut out as a cube of corners.

Table 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 single crystal, Li ₂ O / (Nb ₂ O ₅ +
The molar fraction of Li ₂ O) includes an error of about 0.001 to 0.005. Therefore, the composition of the sample having a composition close to the 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, LN
In the case of a single crystal, an LN single crystal added with Mg or the like has a composition close to a constant ratio , but Li ₂ O /
The value of the molar fraction of (Nb ₂ O ₅ + Li ₂ O) did not exceed 0.005. In addition, we also measured about the Mg content of the sample, the Mg content of the LN single crystal was added to the melt M
It was confirmed that the concentration was almost the same as the g concentration.

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 single crystal melting congruent composition, good agreement with the results that are sought in the literature 25.1
A value of pm / V was obtained. The laser beam used for the measurement has a single longitudinal mode continuous oscillation wavelength of 1.064 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.

Next, with respect to a single crystal of each obtained in the same manner as described above, from the location of the measurement positions to d, in cross-section and a thickness 10 mm × 10 mm was cut out z Plaques of 1.0 mm. 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 applied voltage of 3.7 kV / mm or less, and when it is 0.2 mol% or more, a constant value smaller than 3.1 kV / mm is smaller than that. Is obtained. In these crystals, almost no internal electric field is seen, and the hysteresis curve of ferroelectrics (
It is considered that there is little variation in measured values because of the excellent symmetry of the (PE curve) and the good rise of the PE curve near the coercive electric field.

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.

Next, with respect to a single crystal of each obtained in the same manner as described above, were cut from each location of measurement positions to d, x, y, a sample of 5 mm × 3 mm × 2 mm in the z direction. 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.

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,
Single crucible method, i.e., when the single crystal growing according to the conventional CZ method, pre-filled with a feed rod consisting of Li component excessive material created, then to prepare a Li component excessive melt by heating the crucible. In an experiment for confirming the effect of Mg addition, a commercially available high-purity MgCO ₃ was prefilled in a crucible as an Mg element source during the filling. As for the weight of MgCO _{3 to be} filled, the Mg concentration in the melt is determined with respect to Nb in the melt, and five types of 0.1, 0.2, 0.5, 1.0, and 3.0 mol% are available. The experiment was conducted.

For comparison, the same experiment was conducted except that no addition, 0.05, and 5.0 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 controlled crystal is rotated while the grown single crystal by pulling 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 speed is 0.5 to
It was changed in the range of 3.0 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. It shows a schematic view of the obtained LN single crystal body in FIG.

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 pulled up to 60 mm. The composition of the liquid reached the eutectic point, and the portion pulled after reaching the eutectic point was not the LN single crystal but the 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).
Moreover, the measurement regarding Mg content of these samples was also performed, and it was confirmed that the Mg content in the crystal was almost the same as the Mg concentration added to the melt.

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 addition amount of Mg is 0.1 mol% or more,
There is no increase as seen with less than 0.1 mol%. 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.

Next, to produce a single crystal of each obtained in the same manner as described above, from the location of the measurement positions G to I, cross-section and a thickness 10 mm × 10 mm was cut out z Plaques of 1.0 mm. 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, 0.1 mol%
The difference in reversal voltage is within 0.5 kV / mm even at a distance of 10 mm from the measurement position g to i, and the entire crystal is not less than 0.2 mol%. Shows the minimum value of 3.1 kV / mm almost uniformly.

Next, various optical functional elements were manufactured by periodically reversing the polarization of a wafer cut out from 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 are 2 inches in diameter and 0.3 mm and 0.5 mm in thickness, respectively.
1.0 mm, 2.0 mm, and 3.0 mm were prepared and used as samples .

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 that reverses polarization, the Mg addition concentration is desirably 3 mol% or less, more preferably 1.0 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 of the wafer was 2 inches and the thickness was 1.0 mm. The target of the domain-inverted lattice width was set to every 5 microns so as to approach the ideal 1: 1 (1: 0.95 to 1). As a result of the experiment, a polarization inversion lattice width ratio of about 1: 1 (1: 0.95 to 1) was obtained for most of the samples, but it was not obtained with crystals having an Mg concentration higher than 3 mol%. .

Next, optical elements such as a deflection element, a cylindrical lens, a beam scanner, and a switch using a electro-optic effect by producing a lens or prism-like domain-inverted structure on a wafer cut out from the LN single crystal produced in Example 1 are used. Produced. Diameter 2 inches, thickness 0.2-2.0mm
A z-cut LN single crystal wafer polished on both sides was prepared, Al electrodes having a thickness of about 200 microns were formed on both z planes by sputtering, and a lens or a prism-like pattern was formed using a lithograph. Thereafter, 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 surface of the LN single crystal wafer 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 wafer formed with the reversal of the refractive index by the prototype domain-inverted structure depends on the design of the lens- and prism-shaped domain-inverted structure and the accuracy of the fabrication process of the domain-inverted structure, And 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 single 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, 5 mol of MgO is added to a conventional LN single crystal with a congruent melting composition.
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.

When a lens or prism-like polarization inversion structure is produced on a wafer cut from 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 such problem. Furthermore, since the LN single crystal used in the present optical element has a larger electro-optic constant r ₃₃ than that of the crystal having the coincidence melting composition, a superior device performance can be obtained with a smaller operating voltage. For example, in the case of a deflection element, a large deflection angle of about 6 ° was obtained at a voltage of about 600 V / mm. About 1
A lens operating near 00 V / mm 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 equal to or lower than the Curie temperature. However, according to the LN single crystal obtained by the production method of the present invention , 1) Ti in-diffusion method. 2) SiO ₂ loading heat treatment method. 3) Proton exchange heat treatment method. 4) Electron beam scanning irradiation method.
Even when using other methods such as, stoichiometric composition L with excellent crystal perfection and controllability
By using N single crystal, it is possible to realize an optical element in which a periodically poled lattice is formed with high accuracy.

In addition, although the embodiment in which the QPM element that generates blue or green light with respect to the fundamental wave of near-infrared light of 840 nm or 1064 nm is produced is described in detail, the light element of the present invention is described.
According to the child , the fundamental wave is not limited to these two wavelengths, and the present invention 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, optical elements such as a deflection element, a cylindrical lens, a beam scanner, and a switch using a electro-optic effect by producing a lens or prism-like domain-inverted structure on a wafer cut out from the LN single crystal produced in Example 1 are used. Produced. Diameter 2 inches, thickness 0.2-2.0mm
A z-cut LN single crystal wafer polished on both sides was prepared, Al electrodes having a thickness of about 200 microns were formed on both z planes by sputtering, and a lens or a prism-like pattern was formed using a lithograph. Thereafter, 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 surface of the LN single crystal wafer was mirror-polished to form a laser beam 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 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. More books
Since the single crystal used for the optical element has a larger electro-optic constant r ₃₃ than the crystal of the coincidence melt composition, a better device performance was obtained with a smaller operating voltage. For example, in the case of a deflection element, a large deflection angle of about 6 ° was obtained at a voltage of about 600 V / mm. Also, about 100V /
A lens operating in the vicinity of mm 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 equal to or lower than the Curie temperature. However, according to the LN single crystal obtained by the production method of the present invention , 1) Ti in-diffusion method. 2) SiO ₂ loading heat treatment method. 3) Proton exchange heat treatment method. 4) Electron beam scanning irradiation method.
Even when using other methods such as, stoichiometric composition L with excellent crystal perfection and controllability
By using N single crystal, it is possible to realize an optical element in which a periodically poled lattice is formed with high accuracy.

In addition, although an example 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 produced is described in detail here, the optical element of the present invention is described. Therefore, the fundamental wave is not limited to these two wavelengths, and the present invention can be applied to a wavelength region in which 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 method for producing an LN single crystal used in the optical element of the present invention, L
The N mole fraction of single crystals of _{Li 2 O / (Nb 2 O} 5 + Li 2 O) without a 0.500 fully stoichiometric composition, Li ₂ O by addition of Mg of the third element Device using LN single crystal having non-linear optical constant, polarization reversal voltage and electro-optical constant the same as the size of complete LN single crystal having a molar fraction of / (Nb ₂ O ₅ + Li ₂ O) of 0.500 Can be provided . Ma
In addition, by using the method for producing an LN single crystal of the present invention, the mole of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) having the best wavelength conversion characteristics and electro-optical characteristics in the entire LN single crystal is obtained. min rate, it is possible to grow a LN single crystal close to the stoichiometric composition is between less than 0.490 or 0.500.

It is an example showing the growth furnace of LN single crystal used in the cut of the optical device wafers of the present invention. It is a figure which shows the phase diagram of Li and Nb. It is a schematic view showing the manner of LN single crystal which is grown when using a single crucible.

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

Claims (9)

An optical element for converting the wavelength of laser light incident on a single crystal, wherein the single crystal is L
The molar fraction of i ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.56 to 0.60, Li is a stoichiometric composition
Lithium niobate single crystal grown by a pulling method from a melt with an excessive composition.
What
The melt contains Mg element,
In the lithium niobate single crystal, the Mg element is paired with the lithium niobate single crystal.
0.2 mol% or more and 1.0 mol% or less
The mole fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) in the entire lithium niobate single crystal.
The ratio varies in a range of 0.490 or more and less than 0.500 of the complete stoichiometric composition.
Consisting of a wafer cut from a defected lithium niobate single crystal ,
The applied voltage required for polarization reversal at room temperature is 3.1 kV / mm or less, and the wavelength is 1.0.
Light nonlinear optical constant d ₃₃ in 64 micron using a lithium niobate single crystal is 29.9pm / V or more, and performs quasi-phase matching in the inverted structure of the ferroelectric polarization of the single crystal element. 2. The optical element according to claim 1, wherein the lithium niobate single crystal has a wavelength of 1.064 μm.
A non-linear optical constant d33 in Ron is 26 pm / V or more. 2. The optical element according to claim 1, wherein the lithium niobate single crystal has a wavelength of 0.633 mix.
An optical element having an electro-optic constant r33 of 36 pm / V or more in Ron. The optical element according to claim 1 , wherein the element has a thickness in the z-axis direction of 1.0 mm or more and a period of polarization inversion of 30 μm or less. 2. The optical element according to claim 1 , wherein the period of polarization inversion is 5 microns or less. An optical element that controls a laser beam incident on a single crystal by using an electro-optic effect of the single crystal, wherein the single crystal has a mole fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O). 0.56-0.6
Nio grown by a pulling method from a melt having a composition in which Li is more than the stoichiometric composition.
Lithium butyrate single crystal,
The melt contains Mg element,
In the lithium niobate single crystal, the Mg element is paired with the lithium niobate single crystal.
0.2 mol% or more and 1.0 mol% or less
The mole fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) in the entire lithium niobate single crystal.
The ratio varies in a range of 0.490 or more and less than 0.500 of the complete stoichiometric composition.
Consisting of a wafer cut from a defected lithium niobate single crystal ,
The applied voltage required to reverse the polarization at room temperature is 3.1 kV / mm or less, and the wavelength is 0.6.
Single lithium niobate with an electro-optic constant r ₃₃ of 36 pm / V at 33 microns
An optical element characterized in that light is deflected, focused, or switched using a large refractive index change of a structure in which a single crystal is inverted in ferroelectric polarization. A claim 1 or 6 process for producing lithium niobate single crystal <br/> for cutting out the wafer for optical element described in
In the crucible, the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.56 to 0.60.
a raw material in which Li is excessive in excess of the stoichiometric composition, and an Mg element source satisfying a Mg concentration in the melt satisfying a range of 0.2 mol% to 1.0 mol% with respect to Nb in the melt. Fill and heat to make 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 crucible is a double crucible composed of an inner crucible and an outer crucible, and the Li component excess raw material and the Mg element source are filled in the inner crucible and the outer crucible,
The method according to claim 7 , wherein the stoichiometric composition raw material is supplied to the outer crucible by continuous supply. A claim 1 or 6 process for producing lithium niobate single crystal <br/> for cutting out the wafer for optical element described in
In the crucible, the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.56 to 0.60.
a raw material in which Li is excessive in excess of the stoichiometric composition, and an Mg element source satisfying a Mg concentration in the melt satisfying a range of 0.2 mol% to 1.0 mol% with respect to Nb in the melt. Fill and heat to make melt,
Dipping a seed crystal of lithium niobate single crystal in the melt, pulling it up while rotating,
A method in which the single crystal is grown until the composition of the melt reaches the eutectic point without supplying the raw material during the growth.

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