JP3390905B2 - Cerium-doped lithium niobate single crystal, method for producing the same, and optical element including the single crystal - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to lithium niobate (LiNbO ₃ ) (hereinafter referred to as “LNB0 ₃ ”) used in the fields of optical information processing technology using laser light, optical processing technology, photochemical reaction technology, optical measurement control technology, and the like. Abbreviated as "). More specifically, the present invention relates to an LN single crystal for an optical element having improved hologram diffraction efficiency and its response speed, a manufacturing method thereof, and a hologram memory element including the LN single crystal.

[0002]

2. Description of the Related Art When an LN single crystal is used for optical applications, when a strong laser beam is applied, a local change in the refractive index (light-induced refractive index change) appears, and this light-induced refractive index change is positively utilized. There is a growing interest in applying it to a phase hologram recording element as a high-sensitivity optical memory. In particular, LN single crystals are superior to other photorefractive crystals in that, among the many electro-optic crystals, relatively inexpensive and large crystals are easily available, and hologram recording time is several months or more. Because of its characteristics, it is used for applied research of phase hologram recording memory devices. On the other hand, the LN single crystal of the congruent melting composition to which the transition metal is not added is not suitable for these applications because the hologram diffraction efficiency is extremely low. 100 ppm F
Crystals to which e has been added have been mainly used.

LN single crystals have a relatively wide non-stoichiometric composition near the melting point, and the non-stoichiometric region spreads only to the Nb-excess side on the Li ₂ O--Nb ₂ O ₅ pseudo binary system phase diagram. Because
It was difficult to grow a stoichiometric LN crystal by the usual Cz method. Here, “stoichiometric LN crystal” means Li ₂ O (N
b ₂ O ₅ + Li ₂ O) means a crystal having a mole fraction of 0.50. "Congruent composition LN crystal" is Li ₂ O
It is a crystal having a molar fraction of (Nb ₂ O ₅ + Li ₂ O) of 0.485. So far, the present inventors have developed a fully automatic raw material continuous supply double crucible pulling method, and have grown a stoichiometric LN single crystal from an Li component excess melt (“Applied Physics” 65
(1996) 931), it is known that an LN single crystal having a similar stoichiometric composition can be grown even when K ₂ O is added to the melt (GI Malovichko et al. Appl. Phys. A 56 (1993). ) 103
).

[0004]

As a single crystal used in an optical laser device for writing a three-dimensional hologram in a single crystal by using visible laser light, a hologram is obtained by adding a transition metal such as Fe to LN single crystal. L with improved diffraction efficiency
N single crystal was used for research, but in the crystal grown by adding a large amount of iron to the congruent composition, crystal defects such as striation and light scattering are incorporated into the crystal, which causes optical inhomogeneity of the crystal. As a result of the unevenness and the non-uniform scattering of laser light, there is a problem in that the hologram diffraction efficiency varies greatly depending on the crystal position, and stable hologram diffraction characteristics cannot be obtained.

Further, in the crystal in which iron is added to the congruent composition, there is a problem that the time until the hologram diffraction efficiency reaches a constant value (hologram writing response time) is slow, so that it is not suitable for application to a hologram memory device. It was said that the writing speed was slow. Furthermore, the present inventors have recently found that it is possible to improve the hologram diffraction efficiency of the iron-added LN single crystal by bringing the composition of the LN single crystal closer to the stoichiometry composition, but in this case, a large diffraction efficiency is obtained. It is said that there is a problem that the above is mainly obtained in the visible light region near a wavelength of 532 nm, and therefore there is a difficulty in application to a hologram memory device using a near infrared laser in a longer wavelength region.

[0006]

Means for Solving the Problems As a result of earnest research for solving the above problems, the present inventors have studied the crystal growth in which the crystal composition of LN single crystal was controlled to find a quasi-stoichiometric ratio containing Ce. By growing LN crystal of composition and stoichiometric composition, the photo-induced refractive index change property is controlled, the diffraction efficiency in the visible to near infrared light region is high, and the writing speed is fast,
Moreover, it was found that an LN single crystal having extremely excellent crystal quality without crystal defects such as voids and striations that cause light scattering can be obtained, and the present invention has been made here.

As is well known, since LN single crystals have a wide range of non-stoichiometric compositions, it is usually not easy to grow stoichiometric crystals, and even if it is attempted to grow stoichiometric crystals, it is only slightly possible. The composition may deviate from the stoichiometric ratio. Here, this is referred to as “quasi-stoichiometric crystal”. Stoichiometric LN crystal and quasi-stoichiometric L according to the invention
In the N crystal, the addition of Ce increased the diffraction of the He—Ne laser, and a large refractive index change and photorefractive sensitivity were obtained. In particular, it has been found that when the Ce concentration is 18 ppm or more, excellent photorefractive sensitivity of 2 ^* 10 ⁻⁷ or more can be obtained.

That is, according to the present invention, cerium is added at 18 pp
m to 1000 ppm or less to increase light absorption and photorefractive sensitivity in the visible to near-infrared region, and the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.495 to The present invention relates to a cerium-added quasi-stoichiometric composition and stoichiometric composition lithium niobate single crystal characterized by being 0.50.

Further, the present invention provides a method for producing Li ₂ O / (N
a _{b 2 O 5 + Li 2 O} ) = 0.40~0.60, 5w and K
Melt composition added at t% or more, or Li ₂ O / (Nb ₂
O ₅ + Li ₂ O) = 0.56-0.60 from the melt composition without adding K, Li ₂ O / (Nb ₂₎ having excellent optical homogeneity.
O ₅ + Li ₂ O) molar fraction of a method of manufacturing a lithium niobate single crystal, which comprises growing a crystal of a quasi-stoichiometric and stoichiometric composition of 0.495 to 0.50.

Further, according to the present invention, in a memory element for recording information by writing a hologram diffraction grating in a single crystal by using a laser beam in a visible to near infrared region, the cerium according to claim 1 as the single crystal. The present invention relates to an optical device characterized by using a lithium niobate single crystal to which is added. This optical element is extremely promising for application to optical amplification devices such as phase hologram memories.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The LN single crystal according to the present invention contains 18 ppm or more of cerium and has increased light absorption in the visible to near infrared region and photorefractive sensitivity, and is composed of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) has a molar fraction of 0.
495 to 0.50, and a quasi-stoichiometric composition and a stoichiometric composition. The LN single crystal according to the present invention is Li ₂ O / (Nb ₂ O ₅
+ Li ₂ O) has a mole fraction of 0.495 to 0.5 as compared with a normal congruent composition having a mole fraction of 0.485.
Since it has a characteristic of 0 and a quasi-stoichiometric composition and a stoichiometric composition, the crystal perfection is high and the defect density is low. Further, with this crystal composition, since single domain treatment is not required, crystal defects such as light scattering voids and striations are very small, and the crystal composition and optical homogeneity are excellent.

The method for producing an LN single crystal according to the present invention comprises C
The composition of the LN melt containing e is the same as the conventional melt composition (Li ₂ O / Li ₂ O /
The molar fraction of (Nb ₂ O ₅ + Li ₂ O) is not 0.485, but (Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) = 0.40 to
Melt composition in which K is added to 0.60 by 5 wt% or more, or Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) = 0.56-0.
A melt composition of 60 K was not added, and from this melt composition, Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) excellent in optical homogeneity was obtained.
A crystal having a quasi-stoichiometric composition and a stoichiometric composition having a molar fraction of 0.495 to 0.50 is grown.

Crystals having such a composition can be produced by a floating zone method or a top seeding method, but preferably, a crystal having a higher quality and a larger diameter is grown by growing the crystal by a double crucible method. can get. Also,
If grown from a melt of this composition, the Curie temperature of the crystal composition to be crystallized is about 1200 ° C., which is higher than the normal congruent composition crystal of 1150 ° C., and close to the crystal growth temperature. The part is in a single domain state. Therefore, it is significantly different from the conventional multi-domain crystal grown from the congruent composition melt,
There is an advantage that it is not necessary to perform the poling treatment after the growth which is necessary for the multi-domain crystal. Further, by adopting a double crucible method as the crystal growth method, the composition of the melt in which the crystal is grown is always kept constant, and the temperature fluctuation of the melt in the inner crucible is extremely small. It is possible to produce an LN single crystal having an extremely uniform composition and good optical homogeneity, without the voids and strays that cause light scattering that have been conventionally observed in congruent composition crystals.

Here, the double crucible method will be briefly described. The congruent melting composition of LN single crystal is Li ₂ O / (Nb ₂ O
₅ + Li ₂ O) has a molar fraction of 0.485, the LN single crystal obtained from the congruent melting composition melt by a normal pulling method has an excess of Nb component, but the composition of the melt has a significant excess of Li component. (For example, when the molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.56 to 0.60, preferably 0.58), the composition is close to the stoichiometric composition, that is, nonstoichiometric defects. It is possible to obtain a single crystal whose concentration is suppressed as much as possible. However, if the composition of the growing crystal and the composition of the melt are different, the composition becomes more distant from the melt and the crystal becomes more difficult as the growth progresses by the normal pulling method, which makes the crystal difficult. Therefore, in order to precisely control the density and structure of non-stoichiometric defects, single crystal growth was performed by the double crucible method.

Further, the optical element according to the present invention is used in an optical amplifying device for writing a three-dimensional hologram in a single crystal by using a laser beam of visible light to near infrared light. Since it is high-speed and has a large storage capacity, it is expected to be developed in the future as a new recording method related to multimedia. Although various materials such as silver salt photographs are used for holograms, photoinduced refraction crystals are considered to be the most effective for memory applications. In particular, LN single crystals are numerous. It is the material with the longest retention time among the photo-induced refraction crystals, and so far, most of the memory experiments have used LN single crystals whose diffraction efficiency is increased by adding Fe, which is a transition metal. . However, the congruent LN single crystal containing Fe was usable as a hologram recording material only in the visible light region of about 400 to 600 nm.

The present inventors have found that Li ₂ O / (N containing Ce is used.
b ₂ O ₅ + Li ₂ O) has a mole fraction of 0.495 to 0.50.
Crystals of quasi-stoichiometric composition and stoichiometric composition, which are optically homogeneous and of high quality, and have a diffraction efficiency sufficiently high for a laser device that uses laser light in the near-infrared region, which has a longer wavelength than the visible light region. For the first time, I found a phenomenon that a sufficiently high writing speed can be obtained. The optical element for writing a three-dimensional hologram using the LN single crystal according to the present invention was made possible for the first time by the present inventor.

[0017]

EXAMPLE First, the optical characteristics of the LN crystal grown in the present invention were evaluated. FIG. 1 is a diagram showing the light absorption characteristics of a single crystal in the ultraviolet to near-infrared light region (horizontal axis represents wavelength, vertical axis represents light absorption coefficient). As shown in the figure, by adding Ce, good light absorption, which was not seen in the stoichiometric LN crystal without addition, was observed in the visible to near-infrared light region. For comparison, the light absorption characteristics of stoichiometric single crystals to which iron is added are shown in the figure. The crystal added with Ce is about 5 times more than the crystal added with iron.
It can be seen that light absorption in the near-infrared region is increased from visible light of 20 to 820 nm.

FIG. 2 shows the light transmission characteristics of the LN single crystal containing 81 ppm of Ce in the ultraviolet to near infrared light region. As can be seen from the difference in the as-grown state, after the oxidation treatment, and after the reduction treatment shown in the figure, the light transmission characteristics greatly change depending on the heat treatment state of the crystal. When the growth is heat-treated (oxidized) in oxygen at 1000 ° C., almost no absorption is observed in the visible to near-infrared region. Ce in the crystal is 3
It is well known that the valence of tetravalent or tetravalent is easily changed, but it is not yet clear what kind of state Ce is in the LN single crystal, but the light absorption property by heat treatment is unknown. From the state of change in behavior, it is considered that absorption in the visible to near-infrared region is due to the trivalence of Ce, and due to sufficient oxidation of the crystal, the valence changes to Ce4 valence without absorption in this wavelength region. Be done. In addition, as-grown crystals show some absorption in the visible to near-infrared region.
It seems to include both valence and valence. From the above light absorption characteristics, Ce ions contained in the LN single crystal contribute to the increase of light absorption in the visible to near-infrared region, and are optically active C
The e-ions are expected to play an important role in improving the photorefractive sensitivity in this wavelength range.

Then, next, the photorefractive characteristic of the LN crystal grown in the present invention was evaluated by a light wave mixing experiment. The experimental optical system is schematically shown in FIG. As shown in FIG. 3, the laser light source 1 intersects two coherent light waves, which are respectively called pump light and probe light, within the LN single crystal 2 which is a photoinduced refractive crystal to form a plurality of interference fringes. . A spatial electric field corresponding to the spatial intensity change of the interference fringes was formed, and as a result, a refractive index grating was formed in the crystal. Here, green light having a wavelength of 532 nm, which is a double wave of an Nd: YAG laser, was used as the pump light and the probe light. In the experiment, the beam diameter was 10 mm, and the light intensity ratio between the pump light and the probe light was kept constant at about 1: l. Further, the crossing angle of the two waves at this time was set to about 15 ° so that the diffraction efficiency was maximized, so that the interval of the interference fringes was about 0.9 micron. Further, the wavelength 633n is measured by the laser 3 for measuring the diffraction efficiency from another angle with respect to the refractive index grating formed in the crystal.
The He-Ne laser light of m is incident, and the diffraction efficiency when the He-Ne light passes through the photoinduced refractive crystal is detected by the detector 4 to obtain the photorefractive characteristic formed in the crystal. evaluated.

As a result of the formation of the photorefractive refractive index grating in the crystal, an increase in the diffracted He-Ne laser light was observed on the oscilloscope 5, as shown in FIG. The magnitude of the diffraction efficiency and its speed were obtained. By this measurement, the magnitude Δn of the refractive index change induced in the crystal by the photorefractive effect was obtained from the equation (1), and the photorefractive sensitivity S was obtained from the equation (2). η = (π ^* Δn ^* d / λ) ² (1), S = (α ^* I ^* t / Δn) ^-1 (2), where diffraction efficiency η, crystal thickness d, He-Ne laser wavelength λ, The absorption coefficient α of the crystal, the laser light intensity I, and the writing time t.

As shown in FIG. 4, in the LN single crystal having a stoichiometric composition containing about 40 ppm of Ce, He--
Ne laser light is diffracted, and about 60% by irradiation for about 20 seconds
And a very large diffraction efficiency was obtained. The magnitude Δn of the change in the refractive index of the crystal obtained from this is 1.5 ^* 10 ⁻⁵ , and the photorefractive sensitivity S is 2.5 ^* 10 ⁻⁷ .
It was found that LN single crystals with a stoichiometric composition containing Ce exhibit excellent photorefractive sensitivity. Therefore, the same measurement was performed on single crystals having different compositions to determine a preferable crystal composition as a photorefractive material. The results are shown in Table 1.

As can be seen from Table 1, no change in refractive index was observed in the LN crystal of the additive-free congruent composition.
On the other hand, in the stoichiometric crystal and the quasi-stoichiometric crystal,
He-Ne laser diffraction can be seen even without addition,
The refractive index change and photorefractive sensitivity were obtained. L
The reason why the photorefractive properties greatly differ depending on the composition of N single crystal has not been completely clarified yet, but in the crystal of stoichiometric composition and quasi-stoichiometric composition, the non-stoichiometric defect is compared with the congruent composition crystal. The number of crystals is one digit or less, and it is clear that this non-stoichiometric defect greatly affects the photorefractive characteristics.

Example 1 Commercially available raw material powders of high purity Li ₂ CO ₃ , Nb ₂ O ₅ and K ₂ CO ₃ (each having a purity of 99.99%) were prepared. Next, as a raw material for growing the stoichiometry crystal, Li ₂
CO _3: The ratio of Nb ₂ O ₅ is 0.40: 0.60 to 0.6
0: the raw materials are mixed at a ratio of 0.40 to K ₂ CO ₃ 2 to 1
A mixture was prepared by adding 0 mol%. Each of these raw materials having various compositions was rubber-press molded under a hydrostatic pressure of 1 ton / cm ² , and each was sintered in oxygen at about 1050 ° C. Next, when growing a single crystal, the obtained sintering raw material was filled in the crucible in advance, and then the crucible was heated to form a melt. Here, a platinum crucible was used as the crucible.
As the seed crystal, an LN single crystal cut in the z-axis direction was used.

First, Li ₂ CO ₃ : Nb ₂ O ₅ = 0.50:
Attempts were made to grow using a melt having a composition in which 5 mol% of K ₂ CO ₃ was added to 0.50. The melt was held for about 20 hours before the growth, and in order to make the melt composition uniform, the crucible was slowly rotated at a speed of 0.2 rpm in the direction opposite to the seed crystal during the growth. As for the growth conditions, the crystal rotation speed was 10 rpm, the pulling speed was constant at 0.1 mm / h, and the growth atmosphere was the atmosphere. By the TSSG method, the colorless and transparent stoichiometric LN crystal having a diameter of about 25 mm and a length of about 30 mm was obtained by growth for about 2 weeks. In addition, in the range of raw material composition, the ratio of Li ₂ CO ₃ : Nb ₂ O ₅ is 0.40:
0.60-0.60: 0.40, K ₂ CO ₃ 2-10
When grown with the mol% added, the same optically transparent single crystal was obtained by growing under the same conditions.

In the same manner, single crystals with different Ce addition concentrations were grown. The amount of Ce added is 20 to 1000.
Growing was performed in the range of ppm. The crystal to which Ce is added is
It was observed that the color gradually changed to pale reddish purple with an increase in the amount of Ce added. Next, an attempt was made to grow an LN single crystal having a stoichiometric composition to which Ce was added by the continuous raw material supply double crucible method. Commercially available raw material powders of high-purity Li ₂ CO ₃ and Nb ₂ O ₅ (each having a purity of 99.999%) were prepared, and the ratio of Li ₂ CO ₃ : Nb ₂ O ₅ was 0.56: 0 as the Li component excess raw material. .4
4 to 0.60: in a ratio of 0.40, Li ₂ CO ₃ as a stoichiometric composition _{material: Nb 2 O 5 = 0.50:} 0.5
Mixed at a ratio of 0. Add to this cerium oxide 20 to 5
The raw material added in the range of 00 ppm was prepared. Then 1
Rubber press molding was performed at a hydrostatic pressure of ton / cm ² , and each was sintered in oxygen at about 1050 ° C. to prepare a raw material rod.
A stoichiometric composition raw material was prepared by sintering the mixed stoichiometric composition raw material as a powder material for continuous supply in oxygen at about 1050 ° C.

Next, when growing a single crystal by the double crucible method, the obtained Li component excess raw material is pre-filled in the inner crucible and the stoichiometric composition raw material is filled in the outer crucible in advance, and then the crucible is heated to melt. A liquid was created. In the double crucible method, the crucible has a double structure, and the bottom of the inner crucible is provided with a hole leading from the outer crucible to the inner crucible. Furthermore, Li ₂ O / (Nb ₂ O ₅
+ Li ₂ O) has a molar fraction of 0.56 to 0.60, the weight of crystal growth grown from the melt of the inner crucible having an excess of Li component is measured by a load cell, and an amount of Li corresponding to the amount of crystallized growth The molar fraction of ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.
A raw material having a stoichiometric composition ratio of 50 was automatically fed to the outer crucible.

According to this method, a crystal can be grown from a melt having a constant composition at a constant depth due to the mixture of raw materials from the outside to the inside, so that a large single crystal having a homogeneous composition can be grown for the first time. Became. Here, the crucible used for growth was made of platinum, and the seed crystal used was an LN single crystal in a single domain state of 5 mm × 5 mm × length 60 mm cut out in the z-axis direction. The growth conditions were a crystal rotation speed of 5 rpm, a pulling speed of 0.3 to 0.5 mm / h, and an atmosphere of air. Further, in order to make the melt composition uniform, the crucible was slowly rotated at a speed of 0.2 rpm in the direction opposite to the seed crystal. Approximately 3 diameters after growing for about 1.5 weeks
A Ce-added LN crystal body having a length of 6 mm and a length of about 50 mm and colored in a light reddish purple color without cracks was obtained.

The as-grown crystals obtained by both the TSSG method and the double crucible method were cut in various orientations, and the internal domain state was observed. The inside of the crystal was observed except for a small part near the surface of the crystal. It was confirmed that the cells were uniformly in a single domain. The obtained LN single crystal was close to a stoichiometric composition by chemical analysis, had a Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) mole fraction of 0.495 to 0.500, and had a nonstoichiometric defect concentration. It was confirmed that the crystals were stoichiometric composition crystals and quasi-stoichiometric composition single crystals that were suppressed as much as possible. Next, when the Curie temperature of the grown crystal was measured by a differential thermal analysis method, the Curie temperature of the sample cut out from each part of the crystal was 1
It was confirmed that the Curie temperature of the sample cut out from one crystal was in the range of 196 to 1200.0 ° C. and was constant regardless of the cutting position of the sample, and the homogeneity of the crystal composition was extremely good.

Further, the amount of Ce contained in the crystal was determined by chemical analysis. When the distribution of Ce in the grown crystal was examined, the segregation coefficient of Ce was about 0.2, which was smaller than 1, but the composition was controlled homogeneously, and the distribution of Ce in the crystal was also uniform. It was confirmed. Conventional crystals are
It is necessary to perform poling, which requires a process time of several tens of hours, such as poling in order to form a single domain, and since Ce added in the LN single crystal moves in the crystal by the poling treatment,
There is a problem that a remarkable concentration gradient occurs in the crystal and the optical characteristics of the crystal become non-uniform. However, in the present invention, the Ce-added stoichiometric composition and quasi-stoichiometric LN crystal require polarization treatment. It was revealed that there is also the merit that such a problem will be solved because there is no.

Further, when the optical homogeneity of the sample was observed by a laser interference device, defects such as striations and voids caused by crystal defects were not observed, and high refraction of 1 × 10 ^-5 or more was observed in the entire sample. It was confirmed that there was rate homogeneity. Furthermore, the scattering contained in the crystal was evaluated by the laser microprobe method. In the LN crystal grown in the present invention, no laser scattering was observed and it was confirmed that the crystal quality was excellent.

Example 2 Next, an optical laser device for writing a three-dimensional hologram in a single crystal by using visible and near-infrared laser beams was manufactured. A schematic configuration of the device is shown in FIG. This device is a volume hologram memory device of an angle multiplexing system using the LN single crystal 6 having a high hologram diffraction efficiency of the present invention. The digital image input data is developed as a figure on the spatial light modulator 9. Next, this was read out with a laser beam and used as a hologram object wave. A reference wave was incident on this at a substantially right angle, and interference fringes were written in the LN single crystal 6 as a recording medium. Wavelength 532 as laser for writing
nm Nd: YAG-SHG laser 7, wavelength 780
A semiconductor laser of nm was used. At any wavelength, the stoichiometric composition and quasi-stoichiometric composition LN single crystal containing Ce exhibit excellent photorefractive sensitivity, so that photorefractive interference fringes are formed with high efficiency.

Here, the LN crystal is placed so that the c-axis of the crystal is orthogonal to the direction of the interference fringes, and is placed on the stage 8 which can be rotated with high precision. The crystal size is 1 × 1 × 1 cm ³ . While changing the crystal little by little, the data of about 100 sheets were multi-recorded by utilizing the selectivity of black diffraction. These data were reproduced by the reference wave and converted into an electric signal by the two-dimensional photodetector 10.
The feature of hologram recording here is that it is a phase-type hologram in which the refractive index changes, so high diffraction efficiency is expected, and it is possible to write a diffraction grating simply by irradiating interference fringes without requiring development processing. That is, the hologram once written can be held. The LN single crystal having a stoichiometric composition and a quasi-stoichiometric composition to which Ce is added according to the present invention can be written in a shorter time than a conventional crystal and even by using a near infrared laser beam. The data recording density in this configuration is determined by the diffraction efficiency and the magnitude of noise. Conventionally, noise caused by random light scattering due to inhomogeneity of the LN single crystal has been a problem, but the LN single crystal according to the present invention has a high diffraction efficiency from visible light to near infrared light, and the crystal It was confirmed that the noise was drastically reduced and the recording density could be improved because it was homogeneous and had no scattering.

[0034]

As described above in detail, according to the present invention, the absorption of light in the visible to near infrared region can be achieved by using the composition of LN single crystal having a stoichiometric composition and a quasi-stoichiometric composition to which Ce is added. And an excellent LN single crystal having high photorefractive sensitivity can be obtained. Furthermore, it is possible to provide an LN single crystal that controls the amount of non-stoichiometric defects, is crystallographically homogeneous and has high quality, and has an improved photorefractive response speed. From these facts, by utilizing this characteristic, it is possible to provide an optical element using a LN single crystal, which uses a visible to near-infrared laser at a high speed with a large storage capacity and at a high speed. is there.

[Brief description of drawings]

FIG. 1 is a diagram showing the light absorption coefficient in the ultraviolet to near infrared light region of a stoichiometric LN single crystal to which Ce is added.

FIG. 2 is a diagram showing changes in light transmission characteristics of a stoichiometric LN single crystal to which Ce is added depending on a heat treatment state.

FIG. 3 is a diagram schematically showing an experimental device for writing a hologram in an LN single crystal by light wave mixing to obtain a diffraction efficiency.

FIG. 4 is a diagram showing time dependence of diffraction efficiency of He—Ne laser light by light wave mixing.

FIG. 5 is a diagram schematically showing the configuration of an optical element for writing a three-dimensional hologram in a single crystal by using visible light and near infrared laser light.

[Explanation of symbols]

6 LN single crystal 7 Nd: YAG-SHG laser 10 Two-dimensional photodetector

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JING. et al. , Grow of of Ce: LiNbO3 crystal and rehearse of phase conjucation, Journal of synthetical crystals, 1992, Vol. 21, No. 4, Nov. 1992, pp. 362-366 MALOVICHKO G. I. et al. , Characterization of stoichiometry LInbO3 grown from melts contentin K K, Applied Physics A, 1993, Vol. 56, pp. 103-108 Kenji Kitamura et al., Nonstoichiometric defects and optical properties of LiNbO3, Applied Physics, 1996, Vol. 65, No. 9, pp. 931-935 (58) Fields surveyed (Int.Cl. ⁷ , DB name) C30B 1/00-35/00 CA (STN) JISST file (JOIS)

Claims (4)

(57) [Claims] 1. Cerium containing 18 ppm or more and 1000 pp
m comprises the following, there is with increased light absorption and photorefractive sensitivity in the near infrared region from visible, Li ₂ O /
_{(Nb 2 O 5 + Li 2} O) molar fraction of 0.495 to 0.
A single crystal of lithium niobate having a quasi-stoichiometric composition and a stoichiometric composition to which cerium is added, which is 50. 2. When producing the cerium-doped lithium niobate single crystal according to claim 1, Li ₂ O / (Nb ₂ O)
₅ + Li ₂ O) = 0.40 to 0.60, and from the melt composition in which K is added in an amount of 5 wt% or more, Li ₂ having excellent optical homogeneity is obtained.
_{O / (Nb 2 O 5 +} Li 2 O) molar fraction of 0.495～
A method for producing a lithium niobate single crystal, which comprises growing a crystal having a quasi-stoichiometric composition and a stoichiometric composition of 0.50. 3. When producing the cerium-doped lithium niobate single crystal according to claim 1, Li ₂ O / (Nb ₂ O)
₅ + Li ₂ O) = 0.56-0.60 from the melt composition without addition of K, from which Li ₂ O / (Nb ₂ O) having excellent optical homogeneity was obtained.
_A method for producing a lithium niobate single crystal, which comprises growing a crystal having a quasi-stoichiometric composition and a stoichiometric composition having a mole fraction of ₅ + Li ₂ O) of 0.495 to 0.50. 4. A memory device for recording information by writing a hologram diffraction grating in a single crystal using a laser beam in the visible to near infrared region, wherein the cerium-added niobium as claimed in claim 1 is used as the single crystal. An optical device characterized by using a lithium oxide single crystal.