JPH11199391A - Cerium-containing lithium niobate single crystal, its production and optical element containing the single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a single crystal having increased photoabsorption from a visible light to a near infrared and increased photorefractive sensitivity, by making a single crystal containing a specific amount of Ce, having a specified mol fraction of lithium oxide and a (semi)fixed ratio composition. SOLUTION: This single crystal contains >=18 ppm Ce and has a mol fraction of Li2 O/(Nb2 O5 +Li2 O) of 0.495-0.50 and a (semi)fixed ratio composition. The single crystal can be produced by making the composition of a lithium niobate melt to which Ce is added into a melt composition obtained adding >=5 wt.% of K to Li2 O/(Nb2 O5 +Li2 O)=0.40-0.60 or a melt composition obtained by no addition of K to Li2 O/(Nb2 O5 +Li2 O)=0.56-0.60 and growing the melt composition to give a crystal having the mol fraction of Li2 O/(Nb2 O5 +Li2 O5 ) of 0.495-0.50 and a (semi)fixed ratio composition from the melt composition. The single crystal has excellent optical uniformity and no crystal defects such as voids to cause light scattering, striation, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical information processing technology using a laser beam, optical processing techniques, photochemical reaction technology, lithium niobate utilized in the field of so optical measurement control technology (LiNbO ₃₎ (hereinafter "LN Abbreviated as "). More specifically, the present invention relates to an LN single crystal for an optical element having improved hologram diffraction efficiency and response speed, a method for manufacturing the same, 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, a local change in the refractive index (light-induced refractive index change) appears when an intense laser beam is applied, and this light-induced refractive index change is actively utilized. Therefore, interest in application to a phase-type hologram recording element as a high-sensitivity optical memory is increasing. In particular, the LN single crystal is superior to other photorefractive crystals in that a relatively inexpensive and large crystal can be easily obtained among many electro-optic crystals and a hologram can be recorded for several months or more. Because of its features, it is used in applied research on phase-type hologram recording memory devices. On the other hand, an LN single crystal having a consistent melting composition without the addition of a transition metal is not suitable for these applications because the hologram diffraction efficiency is extremely low. 100 ppm of F
Crystals to which e has been added have been mainly used.

The LN single crystal has 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 phase diagram. Because
It was difficult to grow a stoichiometric LN crystal by the ordinary Cz method. Here, “stoichiometric LN crystal” refers to Li ₂ O (N
(b ₂ O ₅ + Li ₂ O) means a crystal having a molar fraction of 0.50. “Congruent composition LN crystal” is Li ₂ O
It is a crystal in which the molar fraction of (Nb ₂ O ₅ + Li ₂ O) is 0.485. Until now, 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 a Li component excess melt (see “Applied Physics” 65).
(1996) 931), it is known that even when K ₂ O is added to a melt, an LN single crystal having a nearly stoichiometric composition can be similarly grown (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 using a visible laser beam, a hologram is obtained by adding a transition metal such as Fe to an LN single crystal. L with improved diffraction efficiency
N single crystals were used in research, but crystals grown by adding a large amount of iron to the congruent composition incorporated crystal defects such as striations and light scattering into the crystals, which caused the optical inhomogeneity of the crystals. As a result, the diffraction efficiency of the hologram fluctuates greatly depending on the location of the crystal, so that a stable hologram diffraction characteristic cannot be obtained.

[0005] Further, in a crystal in which iron is added to the congruent composition, there is a problem that the time until the hologram diffraction efficiency reaches a certain value (response time of hologram writing) is slow. It was said that there was a drawback that the writing speed was slow. Furthermore, the present inventors have recently found that the hologram diffraction efficiency of an iron-added LN single crystal can be improved by bringing the composition of the LN single crystal closer to the stoichiometric composition. Is mainly obtained in the visible light region near the wavelength of 532 nm. Therefore, it has been considered that there is a problem in application to a hologram memory device using a near-infrared laser having a longer wavelength region.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems. By growing the composition and stoichiometric LN crystals, the photo-induced refractive index change characteristics are controlled, the diffraction efficiency in the visible to near-infrared light region is high, and the writing speed is also high.
In addition, they have found that an LN single crystal excellent in crystal quality can be obtained without crystal defects such as voids and striations that cause light scattering, and the present invention has been made here.

As is well known, LN single crystals have a wide non-stoichiometric composition, and therefore, it is usually not easy to grow a stoichiometric crystal. The composition may deviate from the stoichiometric ratio. Here, this is referred to as “quasi-stoichiometric crystal”. The stoichiometric composition LN crystal and the quasi-stoichiometric composition L according to the present invention
In the case of the N crystal, the addition of Ce increased the diffraction of the He-Ne laser, and a large change in the refractive index and a photorefractive sensitivity were obtained. In particular, it was found that when the Ce concentration was 18 ppm or more, an excellent photorefractive sensitivity of 2 ^* 10 ^-7 or more was obtained.

That is, according to the present invention, cerium is supplied at 18 pp.
wherein more than m, it is one having an 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.
The present invention relates to a quasi-stoichiometric composition and a stoichiometric composition of lithium niobate single crystal to which cerium is added, which is characterized by being 50.

Further, according to the present invention, in producing a lithium niobate single crystal to which cerium is added, Li ₂ O / (N
b ₂ O ₅ + Li ₂ O) = 0.40 to 0.60 and K is 5 w
The composition of the melt added at least t% or Li ₂ O / (Nb ₂
O ₅ + Li ₂ O) = 0.56 to 0.60, from the melt composition to which K is not added, Li ₂ O / (Nb ₂₎ having excellent optical homogeneity is obtained.
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, the present invention relates to a memory element for recording information by writing a hologram diffraction grating in a single crystal using laser light in a visible to near infrared region, wherein the single crystal has a cerium content as set forth in claim 1. The present invention relates to an optical element using a lithium niobate single crystal to which is added. This optical element is extremely promising for application to an optical amplifier such as a phase-type hologram memory.

[0011]

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

The method for producing an LN single crystal according to the present invention
The composition of the LN melt to which e is added is the same as the conventional melt composition (Li ₂ O /
_{(Nb 2 O 5 + Li 2} O) molar fraction of 0.485) is _{not, (Li 2 O / (Nb} 2 O 5 + Li 2 O) = 0.40~
0.60, melt composition in which K is added at 5 wt% or more, or Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) = 0.56-0.
A melt composition without adding K of 60 is used. From this melt composition, Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) having excellent optical homogeneity is obtained.
The crystals having a quasi-stoichiometric composition and a stoichiometric composition having a molar fraction of 0.495 to 0.50 are grown.

Although a crystal having such a composition can be produced by a floating zone method or a top seeding method, it is preferable to grow a crystal by a double crucible method to obtain a crystal of higher quality and a larger diameter. can get. Also,
When grown from a melt of the present composition, the Curie temperature of the crystal composition to be crystallized is approximately 1200 ° C., which is higher than the normal congruent composition crystal of 1150 ° C., and is close to the crystal growth temperature. The part is in a single domain state. For this reason, it is significantly different from the multi-domain crystal grown from the conventional congruent composition melt,
There is an advantage that there is no need to perform a post-growth poling treatment required for a multi-domain crystal. Furthermore, by using the 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 very small. Conventionally, it is possible to produce an LN single crystal having a very uniform composition and excellent optical homogeneity without any voids or striations which cause light scattering as observed in a congruent composition crystal.

Here, the double crucible method will be briefly described. The coincident melting composition of the LN single crystal is Li ₂ O / (Nb ₂ O
_Since the molar fraction of ( ₅ + Li ₂ O) is 0.485, the LN single crystal obtained by the ordinary pulling method from the melt having the same molten composition has an excessive Nb component, but the composition of the melt has a remarkably excessive 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), it is close to the stoichiometric composition, that is, non-stoichiometric defects A single crystal having a concentration as low as possible can be obtained. However, if the composition of the crystal to be grown is different from the composition of the melt, the crystal becomes difficult because the composition of the melt and the crystal is further separated as the growth proceeds by the ordinary pulling method. Therefore, in order to precisely control the density and structure of nonstoichiometric defects, single crystals were grown by the double crucible method.

The optical element according to the present invention is used for an optical amplifier for writing a three-dimensional hologram in a single crystal using a laser beam from visible light to near-infrared light. Due to its high speed and potentially large storage capacity, it has recently been expected to have a future development as a new multimedia-related recording method. Various materials such as silver halide photography are used for holograms, but photo-induced refractive 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 photo-induced refractive crystals, and LN single crystals whose diffraction efficiency has been increased by adding most of Fe, which is a transition metal, have been used in most memory experiments. . However, the congruent LN single crystal to which Fe was added could only be used as a hologram recording material in a visible light region of about 400 to 600 nm.

The present inventors have proposed Li ₂ O / (N
_{b 2 O 5 + Li 2 O} ) molar fraction of 0.495 to 0.50
Diffraction efficiency is a crystal of quasi-stoichiometric and stoichiometric composition that is optically homogeneous and of high quality, and large enough to be required for laser equipment that uses near-infrared laser light with a wavelength longer than visible light. And a phenomenon that a sufficiently fast writing speed can be obtained. The optical element for writing a three-dimensional hologram using the LN single crystal according to the present invention has been made possible for the first time by the present inventors.

[0017]

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

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 between the as-grown state, the oxidation treatment, and 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., absorption in the visible to near-infrared region is hardly observed. Ce in the crystal is 3
It is well known that the valence changes easily between valence and valence. However, it is not yet well understood what the state of Ce is in the LN single crystal, but the light absorption characteristics by heat treatment are not clear. From the appearance of the behavior change, the absorption in the visible to near-infrared region is due to the valence of Ce, and by sufficient oxidation of the crystal, the valence changes to Ce4 without absorption in this wavelength region. It is. In addition, as-grown crystals show some absorption even in the visible to near infrared region,
It appears to include both valency and tetravalence. From the above light absorption characteristics, Ce ions contained in the LN single crystal contribute to an increase in light absorption in the visible to near-infrared region, and the optically active C
The e-ion is expected to play an important role in improving photorefractive sensitivity in this wavelength range.

Then, the photorefractive characteristics of the LN crystal grown in the present invention were evaluated by a light wave mixing experiment. The optical system of the experiment is schematically shown in FIG. As shown in FIG. 3, two coherent light waves called pump light and probe light, respectively, are crossed in the LN single crystal 2 which is a photo-induced refractive crystal by the laser light source 1 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 second harmonic of an Nd: YAG laser, was used as the pump light and the probe light. In the experiment, the beam diameter was 10 mm, respectively, and the light intensity ratio between the pump light and the probe light was fixed at approximately 1: 1. In this case, the crossing angle of the two waves was set to about 15 ° so as to maximize the diffraction efficiency, so that the interval between the interference fringes was about 0.9 μm. Further, the diffraction efficiency measuring laser 3 emits a wavelength of 633n from another angle with respect to the refractive index grating formed in the crystal.
m-He-Ne laser light is incident, and the diffraction efficiency when the He-Ne light passes through the photo-induced refractive crystal is detected and obtained by the detector 4, so that the photorefractive characteristic formed in the crystal can be obtained. evaluated.

As a result of the formation of the photorefractive refractive index grating in the crystal, as shown in FIG. 4, an increase in the diffracted He-Ne laser light is observed on the oscilloscope 5, and before and after the two-wave mixing. Of the diffraction efficiency and its speed were determined. By this measurement, the magnitude Δn of the change in the refractive index induced in the crystal by the photorefractive effect was obtained from equation (1), and the photorefractive sensitivity S was obtained from 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 beam intensity I, and the writing time t.

As shown in FIG. 4, in a LN single crystal having a stoichiometric composition containing about 40 ppm of Ce, He-
Ne laser light is diffracted, about 60% by irradiation for about 20 seconds
Thus, a very high 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 , the photorefractive sensitivity S is 2.5 ^* 10 ⁻⁷ ,
It was found that a stoichiometric LN single crystal containing Ce exhibited excellent photorefractive sensitivity. Therefore, the same measurement was performed on single crystals having various compositions different from each other to obtain a crystal composition preferable as a photorefractive material. Table 1 shows the results.

As shown in Table 1, no change in refractive index was observed in the non-added congruent composition LN crystal.
In contrast, in the stoichiometric and semi-stoichiometric crystals,
He-Ne laser diffraction can be seen even with no additive,
Refractive index change and photorefractive sensitivity were obtained. L
The reason why the photorefractive characteristics differ greatly depending on the composition of the N single crystal has not been completely elucidated yet, but in the stoichiometric and quasi-stoichiometric crystals, the non-stoichiometric defects have a larger number of defects than the congruent crystal. It is a crystal which is smaller by one digit or more, and it is clear that this non-stoichiometric defect greatly affects the photorefractive property.

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 stoichiometric crystals, Li ₂
When the ratio of CO ₃ : Nb ₂ O ₅ is 0.40: 0.60 to 0.6
K ₂ CO ₃ is added to the raw material mixed at a ratio of 0: 0.40 to 2-1.
What added and mixed 0 mol% was created. The raw materials of these various compositions were each subjected to rubber press molding under a hydrostatic pressure of 1 ton / cm ² , and each was sintered in oxygen at about 1050 ° C. Next, at the time of growing a single crystal, the obtained sintering raw material was previously filled in a crucible, 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:
Growth was attempted by using a melt having a composition obtained by adding 5 mol% of K ₂ CO ₃ to 0.50. Before growing, the melt was held for about 20 hours, and in order to homogenize the melt composition, the crucible was slowly rotated in the direction opposite to the seed crystal at a speed of 0.2 rpm during growth. The growth conditions were such that the crystal rotation speed was 10 rpm, the pulling speed was constant at 0.1 mm / h, and the growth atmosphere was air. A colorless, transparent, crack-free, stoichiometric LN crystal having a diameter of about 25 mm and a length of about 30 mm was obtained by growing for about 2 weeks by the TSSG method. Further, the range of the raw material composition is set such that the ratio of Li ₂ CO ₃ : Nb ₂ O ₅ is 0.40:
0.60 to 0.60: 0.40, and K ₂ CO ₃ is 2 to 10
When grown by adding mol%, the same optically transparent single crystal was obtained by growing under the same conditions.

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

Next, when growing a single crystal by the double crucible method, the obtained excess Li component material is charged into the inner crucible and the stoichiometric composition material is charged into the outer crucible in advance, and then the crucible is heated and melted. A liquid was made. In the double crucible method, the crucible has a double structure, and a hole is provided at the bottom of the inner crucible to communicate from the outer crucible to the inner crucible. Further, Li ₂ O / (Nb ₂ O ₅
+ Li ₂ O) The molar weight of the crystal grown from the melt in the inner crucible having a molar excess of 0.56 to 0.60 and an excess of the Li component was measured with a load cell, and the amount of Li corresponding to the crystallized growth amount was measured. The molar fraction of ₂ O / (Nb ₂ O ₅ + Li ₂ O) is 0.
Raw materials with a stoichiometric composition of 50 were automatically fed into the outer crucible.

According to this method, it is possible to grow a large single crystal having a uniform composition for the first time since a crystal can always be grown from a melt having a constant composition at a constant depth by mixing the flow of the raw material from the outside to the inside. It became. Here, the crucible used for growing was made of platinum, and the seed crystal used was a single domain LN single crystal of 5 mm × 5 mm × 60 mm in length 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 in the atmosphere. Further, in order to make the melt composition uniform, the crucible was slowly rotated in the direction opposite to the seed crystal at a speed of 0.2 rpm. Approximately 3 weeks in diameter after 1.5 weeks of growth
A Ce-added LN crystal having a thickness of about 6 mm and a length of about 50 mm and having no cracks was obtained, which was colored in pale reddish purple.

The as-grown crystal obtained by both the TSSG method and the double crucible method was cut into various orientations, and the internal domain state was observed. It was observed that the region was uniformly in a single domain. The obtained LN single crystal was close to a stoichiometric composition by chemical analysis, had a molar fraction of Li ₂ O / (Nb ₂ O ₅ + Li ₂ O) of 0.495 to 0.500, and had a non-stoichiometric defect concentration. It was confirmed that the crystal was a stoichiometric crystal and a quasi-stoichiometric single crystal which were suppressed as much as possible. Next, when the Curie temperature of the grown crystal was measured by differential thermal analysis, the Curie temperature of each sample cut from each part of the crystal was 1
The temperature was in the range of 196 to 1200.0 ° C., and the Curie temperature of a sample cut from one crystal was constant irrespective of the cut position of the sample, and it was confirmed that 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. However, the composition was controlled to be uniform, and the distribution of Ce in the crystal was good. It was confirmed that. Conventional crystals
In order to form a single domain, it is necessary to perform poling, which is a process time of several tens of hours, called poling. Since Ce added to the LN single crystal moves in the crystal due to the poling,
Although there was a problem that a remarkable concentration gradient was formed in the crystal and the optical characteristics of the crystal became non-uniform, the stoichiometric composition and the quasi-stoichiometric LN crystal of the present invention require a polarization treatment. It was clarified that there was an advantage that such a problem could be solved because there was no.

Further, when the optical homogeneity of the sample was observed with a laser interferometer, no defects such as striations and voids caused by crystal defects were observed, and a high refraction of 1 × 10 ⁻⁵ or more over the entire sample. It was confirmed that there was rate homogeneity. Further, scattering contained in the crystal was evaluated by a 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 using laser light of visible light and near-infrared light was prototyped. FIG. 5 shows a schematic configuration of the apparatus. This device is a volume hologram memory device of the present invention based on the angle multiplexing method using the LN single crystal 6 having high hologram diffraction efficiency. Digital image input data is developed as a graphic on the spatial light modulator 9. Next, this was read out with a laser beam to obtain a hologram object wave. A reference wave was incident substantially perpendicular thereto, 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 and wavelength 780
nm semiconductor laser was used. At any wavelength, the stoichiometric and quasi-stoichiometric LN single crystals containing Ce exhibit excellent photorefractive sensitivity, so that photorefractive interference fringes are formed with high efficiency.

Here, the LN crystal is placed on a stage 8 which is arranged so that the c-axis of the crystal is perpendicular to the direction of the interference fringes and can be rotated with high precision. The crystal size is 1 × 1 × 1 cm ³ . While changing the crystal little by little, about 100 data were multiplex-recorded using the selectivity of black diffraction. These data were reproduced by a reference wave and converted into electric signals by the two-dimensional photodetector 10.
The feature of hologram recording here is that it is a phase hologram in which the refractive index changes, so that high diffraction efficiency is expected, and it is possible to write a diffraction grating simply by irradiating interference fringes without requiring development processing. And the once written hologram can be retained. The stoichiometric and quasi-stoichiometric LN single crystals to which Ce was added according to the present invention could be written in even shorter time than conventional crystals and using near-infrared laser light. 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 non-uniformity of the LN single crystal has been a problem. However, the LN single crystal according to the present invention has high diffraction efficiency from visible to near-infrared light, and has a high crystal efficiency. It was confirmed that the noise was significantly reduced and the recording density could be improved because of the uniform and no scattering.

[0034]

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

[Brief description of the drawings]

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

FIG. 2 is a diagram showing a change 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 apparatus for writing a hologram in an LN single crystal by light wave mixing and obtaining diffraction efficiency.

FIG. 4 is a diagram showing the time dependence of the diffraction efficiency of a He—Ne laser beam due to light wave mixing.

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

[Explanation of symbols]

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

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI G11B 7/24 511 G11B 7/24 511 G11C 13/04 G11C 13/04 B H01B 3/12 313 H01B 3/12 313Z (72) Inventor Kazuo Niwa 1-1-1, Namiki, Tsukuba, Ibaraki Pref.

Claims (4)

[Claims] Claims: 1. A light-emitting device comprising at least 18 ppm of cerium and having enhanced light absorption and photorefractive sensitivity in the visible to near-infrared region, wherein Li ₂ O / (Nb ₂ O ₅ + Li
₂ O) quasi stoichiometric and stoichiometric lithium niobate single crystal of the composition with the addition of cerium, wherein the molar fraction of 0.495 to 0.50 of. 2. The method of producing a cerium-doped lithium niobate single crystal according to claim 1, wherein Li ₂ O / (Nb ₂ O
₅ + Li ₂ O) = 0.40 to 0.60, and from the melt composition obtained by adding 5 wt% or more of K, Li ₂ having excellent optical homogeneity was obtained.
The molar fraction of O / (Nb ₂ O ₅ + Li ₂ O) is 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. The method of producing a cerium-doped lithium niobate single crystal according to claim 1, wherein Li ₂ O / (Nb ₂ O
₅ + Li ₂ O) = 0.56 to 0.60 from the melt composition to which K is not added, Li ₂ O / (Nb ₂ O) having excellent optical homogeneity
₅ + Li ₂ O) method for producing a lithium niobate single crystal, wherein a molar fraction to grow a crystal of quasi stoichiometric and stoichiometric composition of 0.495 to 0.50 in. 4. A memory element 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 niobium to which the cerium according to claim 1 is added as the single crystal. An optical device using a lithium oxide single crystal.