JPH11191239A - Hologram mfmory device and manufacture therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve a hologram memory element excellent in such characteristics as sensitivity of optical damage effect, recording error rate, etc., by composing the element of a single crystal of litium niobate made to contain transition metal elements with an atomic composition rate of Li to Nb limited within a specific range. SOLUTION: A hologram memory element is composed of a single crystal of LN containing, for example, Fe, Cu, Mn, Ni, Rh, Co, Ir, Pt, Mo, Cr, etc., and an atomic composition rate of Li to Nb is assumed to be in a range of 0.937<=Li/Nb<=0.943. As transition metal, it is desirable to contain 0.001-0.1 mole % of Fe. Moreover, assuming that a content of iron contained in the lithium niobate monocrystal is n moles % and an absorption coefficient of light is α in the light wavelengths 400-600nm, it is desirable to satisfy 2.5n+0.575<=α<=2.5n+0.875. It is further desirable to bring up the lithium niobate monocrystal by the rotary Czochralski method, and then, process this monocrystal by heat-treatment in an oxide atmosphere or an inert gas atmosphere at 200-700 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hologram memory element which is a kind of an optical memory element for writing (recording) or reading (reproducing) information by using light, and which is suitable as a digital holographic memory and a method of manufacturing the same. .

[0002]

2. Description of the Related Art Conventionally, floppy disks, hard disks (magnetic disks), and the like have been mainly used as recording media for computer devices such as personal computers. Recently, in addition to these recording media, CD-ROM, M
OD (Magneto-optical Disk), DVD (Digital Video
Various recording media such as disks have been used, and research and development of other recording media have been actively conducted.

[0003] Also, due to the lower price of hard disks,
The cost of memory has been spurred to decrease, and at present it is said that the memory cost is about 10-20 yen per megabyte of storage capacity.
With VD, it is predicted that the cost will be several yen per megabyte.

Further, in recent years, hologram memory devices, which are regarded as promising next-generation memory devices following DVDs, have improved characteristics such as a higher CPU speed, a larger data capacity, and a higher access speed in the future. Thereby, a price reduction of about 5 to 10 yen per megabyte can be realized.

Various materials such as a single crystal of lithium niobate, a single crystal of barium titanate, a single crystal of strontium barium titanate and an organic photosensitizer have been proposed as materials for the hologram memory element. It is said that lithium niobate (LiNbO ₃ ) single crystal (hereinafter abbreviated as LN single crystal) is said to have a high possibility of practical use. This is because high reproduction efficiency can be expected, recording and erasure can be repeated, and there is no deterioration of the characteristics due to this. In addition, multiplex recording can be performed and high resolution can be expected.

Further, the LN single crystal has a so-called photorefractive effect in which the refractive index of the light-irradiated portion changes locally when irradiated with high-power laser light. Recording and reproduction are possible. Moreover, it is known that the sensitivity (efficiency) of the optical damage effect is improved by adding Fe (iron).

[0007]

However, the conventional manufacturing method cannot provide a large and high quality LN single crystal, it is difficult to dope Fe uniformly, and
Addition of e has not been put to practical use due to factors such as distorting the crystal lattice to induce cracks, and the inexperience of measurement and evaluation technology. In addition, the conventional optimum amount of Fe is not known. In particular, when Fe is added non-uniformly in the crystal, there is a problem that small-angle grain boundaries are likely to be generated, and a crystal with good crystal quality is still obtained. Not.

Accordingly, the present invention has been completed in view of the above circumstances, and an object thereof is to provide an LN for a hologram element.
By defining the optimum Li / Nb ratio of the single crystal, a high-quality LN single crystal in which transition metal elements such as iron are uniformly distributed can be manufactured, and the sensitivity of the photodamage effect and the BER (Bit error rat)
e: recording error rate) and the like, so as to easily provide a hologram memory element having excellent characteristics such as the following.

[0009]

The hologram memory device of the present invention has an atomic composition ratio Li / Nb of Li / Nb of 0.9.
37 / Li / Nb / 0.943, and made of a single crystal of lithium niobate containing a transition metal element, whereby a transition metal element such as iron for improving the photodamage effect is reduced to niobate. It can be uniformly distributed in the lithium single crystal.

[0010] In the present invention, preferably, 0.001 to 0.1 mol% of iron is contained as the transition metal element. Further, assuming that the iron content of the lithium niobate single crystal is n mol% and the light absorption coefficient at a light wavelength of 400 to 600 nm is α, 2.5n + 0.575 ≦ α ≦ 2.5n +
0.875.

Further, a method of manufacturing a hologram memory element according to the present invention is a method of manufacturing a hologram memory element comprising a single crystal of lithium niobate, wherein the atomic composition ratio Li / Nb of Li / Nb is 0.937 ≦ Li. /Nb≦0.94
3, and a lithium niobate single crystal containing 0.001 to 0.1 mol% of iron is grown by a rotation pulling method. Then, the lithium niobate single crystal is grown at 200 ° C. to 70 ° C.
The heat treatment is performed in an oxidizing atmosphere or an inert gas atmosphere at 0 ° C., and with the above structure, the concentration of coloring and the color unevenness of the lithium niobate single crystal can be controlled, and thus the absorption coefficient α can be adjusted.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A hologram memory device according to the present invention will be described below.

The hologram memory device according to the present invention provides a transition metal element such as Fe, Cu, Mn, Ni, Rh, Co,
It is made of an LN single crystal containing Ir, Pt, Mo, Cr and the like, and satisfies 0.937 ≦ Li / Nb ≦ 0.943.
If the value of Li / Nb is out of the above range, noise and BER due to crystal defects increase, and the recording sensitivity due to the photoreactive effect deteriorates.

The above-mentioned atomic composition ratio of Li / Nb is determined by the second harmonic wave generation of the LN single crystal.
The analysis and measurement can be performed by measuring the phase matching temperature in (). This is because, when laser light (fundamental wave) such as an Nd: YAG laser is incident on the LN single crystal, light (second harmonic) having twice the frequency of this laser light is generated (phase matching temperature). Can be determined strictly, and the phase matching temperature is delicately changed depending on the composition of the LN single crystal, for example, the atomic composition ratio of Li / Nb.

As a transition metal element, Fe is preferable.
Fe has a large photoreactive effect, and the addition amount of Fe is 0.
Sufficient sensitivity can be obtained with a content of 1 mol% or less.
It can be distributed almost uniformly in the crystal.

The content of Fe is 0.001-0.
If it is less than 0.001 mol%, the sensitivity due to the photoreactive effect is not improved, and if it exceeds 0.1 mol%, it becomes difficult to grow an LN single crystal, and F
e becomes difficult to be uniformly doped.

If the Fe content of the LN single crystal is n and the absorption coefficient is α, then 2.5n + 0.575 ≦ α ≦ 2.5
Preferably, n + 0.875 is used for the following reason. When the content of Fe ²⁺ in the LN single crystal increases, the light absorption near the light wavelength of 400 to 600 nm increases, and the sensitivity due to the photoreactive effect shows that the Fe content n and F
e ²⁺ / Fe ³⁺ is involved. Further, when the Fe content n increases, the absorption coefficient α increases because the absolute amount of Fe ²⁺ increases. Therefore, by measuring and adjusting the absorption coefficient α, the content of Fe ²⁺ can be adjusted, and as a result, the sensitivity due to the photoreactive effect can be controlled.

If α is less than 2.5n + 0.575, the sensitivity due to the photorefractive effect deteriorates,
If α exceeds 2.5n + 0.875, the Fe content n and the Fe ²⁺ content increase, so that the sensitivity is improved but the light transmittance is reduced. Further, if the sensitivity is too high, there is a problem that data is easily erased during reproduction.

The light absorption coefficient α is between 400 and 6
Within the wavelength band at 00 nm, the above range is set. If the range is out of this range, it is difficult to control the light transmittance of the LN single crystal. Preferably, the wavelength is 500 to 550 nm, which is one of the bands in which the controllability of the light transmittance is the best and is practical as a hologram memory element.

Further, according to the production method of the present invention, the content of iron is 0.001 to 0.1 mol% and the atomic composition ratio Li / Nb of Li / Nb is 0.937 ≦ Li by a rotation pulling method.
/Nb≦0.943, and growing the lithium niobate single crystal at 200 ° C. to 7 ° C.
The heat treatment is carried out in an oxidizing atmosphere or an inert gas atmosphere at 00 ° C., and the absorption coefficient α is set at 20 ° C. in an oxidizing atmosphere or an inert gas atmosphere such as nitrogen.
It can be controlled by a heat treatment of heating to 0 to 700 ° C. If the temperature is lower than 200 ° C., there is almost no change in the light transmittance and the absorption coefficient α. If the temperature exceeds 700 ° C., the absorption due to oxygen vacancies increases, and a different phase such as LiNb ₃ O ₈ tends to precipitate.

The oxidizing atmosphere is, specifically, air,
This is a gas containing 1% (vol%) or more of oxygen, and the inert gas atmosphere is Ar, N ₂ , H ₂ , CO ₂ , He, or the like.
When heat treatment is performed in an inert gas atmosphere, α increases,
When heat treatment is performed in an oxidizing atmosphere, α decreases.

The above heat treatment can be performed under the atmospheric pressure or a desired degree of vacuum. In the heat treatment in an inert gas atmosphere, the gas is once replaced by 10 ^-5 torr or less in order to control the oxygen partial pressure. Is good.

The heat treatment time is preferably from 20 hours to 50 hours. If the heat treatment time is less than 20 hours, oxygen is not diffused into the LN single crystal and color unevenness tends to occur. If the heat treatment time exceeds 50 hours, the light transmittance changes. Become dull.

By performing such a heat treatment, 4F
eO + O ₂ ← → 2Fe ₂ O ₃ (Fe ²⁺ ← → Fe ³⁺ ), that is, oxidation and reduction reactions of Fe occur, the concentration of coloration and color unevenness can be controlled, and thermal distortion remaining in the LN single crystal ,
Residual stress is eliminated, and heat treatment in an oxidizing atmosphere is also effective in reducing oxygen defects generated during the growth of the LN single crystal.

The LN single crystal of the present invention is preferably grown by a resistance heating type pulling method, whereby it is possible to secure a wide soaking area with a simpler apparatus than, for example, a high frequency heating type pulling method. Therefore, the defects in the LN single crystal can be reduced, and the distortion of the crystal lattice when iron is added can be reduced as much as possible, and the generation of cracks can be suppressed. Furthermore, since it is a simple device, it can be manufactured at low cost.

FIG. 1 shows a block diagram of an optical system H configured to perform recording and / or reproduction by irradiating the hologram memory element 1 with a laser beam.

Here, reference numeral 2 denotes a laser device such as a semiconductor laser or a gas laser which emits a laser beam L1 having a wavelength of 500 to 600 nm, or a laser device which has converted the wavelength of the laser beam. A beam splitter such as a glass prism for splitting into light L3;
Is a spatial light modulator for inputting information, and 5 is a signal light L2.
A shutter for blocking, 6 is a photodiode using silicon or the like for measuring diffraction gain by two-light wave measurement, 7 is a CCD camera for measuring diffraction gain or reproducing information, and 8 is a photo camera. Diode 6
A personal computer with a display for calculating and measuring the diffraction gain from the signal from the CCD camera 7; 9, a receiver for displaying a reproduced image of information from the signal from the CCD camera 7; A mirror for changing the direction, 11 is a diffraction grating for resolution, and 12 is a mirror for beam scanning.

Next, the operation of the optical system H will be described with reference to the drawings. First, a case where a diffraction gain which is an index of the resolution of the hologram memory element 1 is measured will be described.

Laser light L1 emitted from laser device 2
Is split by the beam splitter 3 into light in two directions, that is, signal light L2 and reference light L3. Then, the signal light L2 passes through the spatial light modulator 4 and the shutter 5, and enters the hologram memory element 1. On the other hand, the traveling direction of the reference light L3 is changed by the mirror 10, passes through the diffraction grating 11, is changed again by the mirror 12 for beam scanning, and is incident on the hologram memory element 1. The signal light L2 transmitted through the hologram memory element 1 is incident on the CCD camera 7, the reference light L3 is incident on the photodiode 6, and the signals from the photodiode 6 and the CCD camera 7 are transmitted to the personal computer 8 respectively.
Is input to

Next, a case where a recording laser beam is incident on the hologram memory element 1 and information is recorded on the hologram memory element 1 will be described. In this case, the photodiode 6 and the personal computer 8 become unnecessary.

The recording laser light L1 emitted from the laser device 2 is separated by the beam splitter 3 into signal light L2 and reference light L3. Then, the signal light L
Numeral 2 passes through the spatial light modulator 4 and the shutter 5 and enters the hologram memory element 1. On the other hand, the traveling direction of the reference light L 3 is changed by the mirror 10, passed through the diffraction grating 11, changed in the traveling direction by the variable beam scanning mirror 12, and is incident on the hologram memory element 1.
Here, information is recorded by forming an optically damaged area at an intersection f0 between the signal light L2 and the reference light L3. Note that information is recorded by changing the angle of the mirror 10, moving the hologram memory element 1, and the like.

Next, a case where a reproduction laser beam is incident on the hologram memory element 1 and information is recorded on the hologram memory element 1 will be described. In this case, the reference light L3
Are not incident on the hologram memory element 1, the mirrors 10, 12 and the diffraction grating 11, the photodiode 6, and the personal computer 8 become unnecessary.

The laser beam L 1 for reproduction emitted from the laser device 2 passes through the beam splitter 3, the spatial light modulator 4 and the shutter 5, enters the hologram memory device 1, and transmits the signal transmitted through the hologram memory device 1. Light L2 is C
The hologram memory element 1 is input to a CD camera 7 and a signal from the CCD camera 7 is input to a receiver 9.
Can be displayed as an image.

Thus, according to the present invention, a high-quality LN single crystal in which iron is uniformly distributed can be manufactured, and the sensitivity of the photodamage effect, the B
This has the effect that a hologram memory element having excellent characteristics such as ER can be obtained at low cost.

It should be noted that the present invention is not limited to the above embodiment, and various changes may be made without departing from the scope of the present invention.

[0036]

Embodiments of the present invention will be described below.

Example 1 An LN single crystal and a hologram memory element 1 for the hologram memory element 1 of FIG. 1 were produced as follows.

(1) Li ₂ CO ₃ , Nb ₂ O having a purity of 4N
₅ , Fe ₂ O ₃ with Li / Nb = 0.94, Fe = 0.0
521.0642 g of Li ₂ O ₃ and 0.6985 g of Fe ₂ O ₃ are mixed with 2000 g of Nb ₂ O ₅ raw material so as to be 3 mol%, put in a 10-liter resin pot, and rotated by a pot mill for 10 hours. And mixed. Similarly, when Li / Nb = 0.93 and 0.95 and Fe = 0.0
A 3 mol% raw material was also prepared and mixed.

(2) Each of these materials was calcined at 750 ° C. for 3 hours and then calcined at 1100 ° C. for 3 hours.

(3) Each of these fired products is filled into a platinum crucible having a circular cross section of φ (diameter) of 100 mm and a height of 100 mm in an amount of 2300 g, and a single crystal production apparatus is prepared by a rotational pulling method (Czochralski method: CZ method). Set to Rotation speed 10 rpm, lifting speed 0.4 mm / h (h: 1
Time) to grow an LN single crystal, 55mm in diameter and 80 in length
mm cylindrical LN single crystal was produced.

(4) After subjecting this LN single crystal to a single domain treatment, a block of 10 × 10 × 10 mm is cut out and subjected to a heat treatment at 500 to 700 ° C. for 20 hours in a nitrogen atmosphere to obtain an absorption coefficient α = Adjusted to 0.8. This block was mirror-polished, and an anti-reflection film made of a dielectric multilayer interference film was formed on the polished surface, thereby producing three types of hologram memory devices 1.

When recording and reproduction were performed on these hologram memory elements 1 by the optical system shown in FIG. 1, respectively,
For an LN single crystal with Li / Nb = 0.94, 256 k
BER ≒ 2 × 10 ^-10 , 1024 kbi
t shows a good value of BER ≒ 7 × 10 ⁻⁵ and Li
/Nb=0.93 and Li / Nb = 0.95
In each of the recordings of 256 kbits and 1024 kbits, the BER deteriorated by two digits from the above value.

Next, 10 mm from the top of each LN single crystal,
ICP-MS (Inductivel
As a result of measuring the iron content by y Coupled Plasma-Mass Spectrometry) method, the crystal of Li / Nb = 0.94 was 0.1%.
It was confirmed that it was uniform within the measurement accuracy at 03 ± 0.001 mol%.

On the other hand, in the crystal with Li / Nb = 0.93, 0.95, variation was observed at 0.03 ± 0.02 mol%. Due to this variation, the variation of the light transmittance near the light wavelength of 500 nm was about 10 to 20%, which was considerably larger than the variation of the light transmittance of the product of the present invention of 1 to 2% or less. Therefore, Li / Nb = 0.93, 0.95
Could not be used as a hologram memory element.

Example 2 (1) Li ₂ CO ₃ , Nb ₂ O ₅ , Fe ₂ O having a purity of 4N
₃ Li / Nb = 0.94, Fe = to be 0.03 mol%, Li ₂ against Nb ₂ O ₅ starting material 2000g
521.0642 g of O ₃ and 0.6985 g of Fe ₂ O ₃
g, mixed in a 10-liter resin pot, and rotated by a pot mill for 10 hours to mix.

(2) The raw material was calcined at 750 ° C. for 3 hours and then at 1100 ° C. for 3 hours.

(3) The fired product is φ (diameter) 100 m
2300 g in a 100 mm high platinum crucible with a circular cross section of m
It was filled and set in a single crystal manufacturing apparatus by a rotation pulling method. Rotation speed 10 rpm, lifting speed 0.4 mm / h
(H: 1 hour) to grow an LN single crystal, 55 mm in diameter,
A columnar LN single crystal having a length of 80 mm was produced.

(4) After subjecting this LN single crystal to a single domain treatment, a block of 10 × 10 × 10 mm is cut out and mirror-polished, and an antireflection film made of a dielectric multilayer interference film is formed on the polished surface. The hologram memory device 1 was manufactured. Α of this hologram memory element 1 (comparative example: element A) was calculated from the measurement results of the light transmittance and the reflectance.
= 0.32 ± 0.02, the variation is large, α <2.5
× 0.03 + 0.575.

(5) Another 10 × 10 × 10 mm block cut out of the LN single crystal was placed in a nitrogen atmosphere to form a block.
Heat treatment was performed at 00 ° C. for 24 hours, mirror polishing was performed, and an anti-reflection film made of a dielectric multilayer interference film was formed on the polished surface to produce a hologram memory device 1 (Example: Device B). As a result of measuring α of the element B, α = 0.825 ±
0.001 and almost uniform within the measurement accuracy, 2.5 ×
0.03 + 0.575 <α <2.5 × 0.03 + 0.8
75.

When recording and reproduction were performed by the optical system H shown in FIG. 1 using the two types of elements A and B,
BER ≒ 3 × 10 ^-10 and 1024k with 6 kbit recording
It shows a good value of BER × 8 × 10 ^{-5 in} bit recording,
The BER of the element A deteriorated by two digits from the above value. .

Next, 10 minutes from the top of the LN single crystal of both devices.
mm, 40mm and 80mm parts, respectively ICP
As a result of measuring the Fe content by the -MS method,
3 ± 0.001 mol%, which was uniform within the measurement accuracy.

Example 3 (1) Li ₂ CO ₃ , Nb ₂ O ₅ , Fe ₂ O having a purity of 4N
₃ with Li / Nb = 0.935, Fe = 0.03 mol%
To 2000 g of Nb ₂ O ₅ raw material
519.6322 g of ₂ O ₃ and 0.696 of Fe ₂ O ₃
7 g was mixed, put in a 10-liter resin pot, and rotated and mixed by a pot mill for 10 hours.

(2) This raw material was placed in an alumina ceramic crucible and set in an electric furnace. 750 at 80 ° C / h
℃, hold for 3 hours to remove carbonic acid, then 8
After the temperature was raised to 1100 ° C. at 0 ° C./h and held for 3 hours to sinter the raw material, the temperature was lowered to room temperature at 100 ° C./h to prepare a raw material for growing an LN single crystal.

(3) Using this raw material by the CZ method, LN
Single crystals were produced as follows. First, the raw materials were set in a single crystal manufacturing apparatus, and the temperature was raised and melted. At this time, the hot zone structure was such that the temperature gradient immediately above the liquid surface of the melt was 50 ° C./cm.

(4) The shaft on which the seed crystal is fixed so as to be pulled up in the c-axis direction of the crystal is lowered into the melt, and the tip of the seed crystal is placed on the liquid surface to confirm that the seed crystal does not melt.
It was pulled up at 1 mm / h while rotating at 10 rpm.
Until it is pulled up by 3 mm, the temperature is controlled so that the diameter of the LN single crystal becomes 4 mm or less, so-called necking is performed.

(5) After about 3 hours, the LN single crystal
0mm, then control the diameter to 60mm,
A columnar LN single crystal having a length of 70 mm was grown.

(6) This LN single crystal had a dark brown upper portion (a neck portion on the seed crystal side) and became thinner toward the lower portion. In addition, by ICP-MS method, the F, 10 mm, 30 mm, and 60 mm portions from the top
When the amount of e was analyzed, all were 0.03 mol% and were uniform. An 8 × 8 × 8 mm block was cut out from the LN single crystal, and the light transmittance in the c-axis direction was measured. As a result, absorption was observed at a light wavelength of 600 nm or less, and the amount of absorption was as shown in FIG. The corresponding side was big.

(7) Next, when the block was subjected to a single domaining treatment at 1200 ° C., the color became lighter than that of the untreated one after the growth, but the upper part of the LN single crystal was still The color was dark and uneven color was observed.

(8) Four blocks are cut out, and each is cut at 200 ° C., 400 ° C., 550 ° C., 70 ° C. in a nitrogen atmosphere.
After performing four types of heat treatment at 0 ° C. for 24 hours each,
There was no change at 00 ° C., and at 400 ° C. and 550 ° C., the color became dark and the color unevenness disappeared. At 700 ° C., the color changed to black as a whole. When the light transmittances of these blocks were measured, those subjected to heat treatment at 200 ° C. showed no change (FIG. 3).
Heat treatment at 400 ° C. and 550 ° C. increased the absorption (FIG. 4), and heat treatment at 700 ° C. increased the absorption in the entire light wavelength range (380 to 850 nm) (FIG. 5).

As for α, α = 0.81 when heat-treated at 400 ° C. and α when heat-treated at 550 ° C.
= 0.92.

(9) Next, another three blocks are cut out, and each is cut at 200 ° C. and 400 ° C. in an oxidizing atmosphere (air).
When heat treatment was performed at 700 ° C. and 700 ° C. for 24 hours, heat treatment at 200 ° C. did not change.
The color which was heat-treated at ℃ became light and the color unevenness disappeared. Those heat-treated at 700 ° C. became almost transparent to the naked eye. When the light transmittances of these blocks were measured, those subjected to heat treatment at 200 ° C. showed no change (FIG. 6).
Heat-treated at 400 ° C. and 700 ° C., the absorption was reduced below 600 nm (FIG. 7).

Further, regarding α, 400 ° C., 700 ° C.
Α was 0.4 or less for those heat-treated at 200 ° C., and α exceeded 0.4 for those heat-treated at 200 ° C.

[0063]

According to the present invention, 0.937 ≦ Li / Nb ≦
By setting the ratio to 0.943, crystal defects of the LN single crystal are small, and the transition metal element is uniformly distributed in the LN single crystal.
In addition, the absorption coefficient α is also uniform, which has the effect of improving the sensitivity due to the photoreactive effect. Further, since the absorption coefficient α is uniform, the BER is small and the diffraction efficiency is large.

Further, by adjusting the absorption coefficient α,
Fe ²⁺ / Fe ³⁺ in the LN single crystal can be controlled, and as a result, the sensitivity due to the photoreactive effect can be controlled. In addition, the absorption coefficient α can be adjusted by performing a heat treatment, and the thermal distortion and residual stress remaining in the LN single crystal are eliminated by the heat treatment. Further, by performing the heat treatment in an oxidizing atmosphere, there is an effect of reducing oxygen defects generated during the growth of the LN single crystal.

[Brief description of the drawings]

FIG. 1 is a block diagram of an optical system H using a hologram memory device of the present invention.

FIG. 2 is a graph of light wavelength and light transmittance showing that light absorption is large above an LN single crystal.

FIG. 3 is a graph of light wavelength and light transmittance when an LN single crystal is heat-treated in an N ₂ atmosphere at 200 ° C. for 24 hours.

FIG. 4 is a graph showing the light wavelength and light transmittance of an LN single crystal under each of conditions of a heat treatment in an N ₂ atmosphere at 400 ° C. for 24 hours and a heat treatment in a N ₂ atmosphere at 550 ° C. for 24 hours before heat treatment.

FIG. 5 is a graph of light wavelength and light transmittance of an LN single crystal under heat treatment at 700 ° C. for 24 hours in an N ₂ atmosphere before heat treatment.

FIG. 6 is a graph of light wavelength and light transmittance of an LN single crystal under each condition of heat treatment at 200 ° C. for 24 hours before heat treatment.

FIG. 7 is a graph showing the light wavelength and the light transmittance of the LN single crystal before heat treatment, in an atmosphere at 400 ° C. for 24 hours, and in an atmosphere at 700 ° C. for 24 hours.

[Explanation of symbols]

 1: Hologram memory element 2: Laser device 3: Beam splitter 4: Spatial light modulator 5: Shutter 6: Photodiode 7: CCD camera 8: Personal computer 9: Image receiver 10, 12: Mirror 11: Diffraction grating H: Optical system

Claims (4)

[Claims] 1. An atomic composition ratio Li / Nb of Li: Nb of 0.1.
A hologram memory element, wherein 937 ≦ Li / Nb ≦ 0.943 and made of a lithium niobate single crystal containing a transition metal element. 2. The method according to claim 1, wherein the transition metal element is iron at 0.001.
The hologram memory device according to claim 1, wherein the content of the hologram memory device is from 0.1 to 0.1 mol%. 3. The method according to claim 1, wherein the lithium niobate single crystal has an iron content of n
When the absorption coefficient of light at a light wavelength of 400 to 600 nm is α, the ratio is 2.5n + 0.575 ≦ α ≦ 2.5n
3. The hologram memory device according to claim 2, wherein the value is +0.875. 4. A method for producing a hologram memory element comprising a lithium niobate single crystal, wherein the atomic composition ratio Li / Nb of Li and Nb is 0.937 ≦ Li / Nb ≦ 0.9.
43, and a lithium niobate single crystal containing 0.001 to 0.1 mol% of iron is grown by a rotation pulling method, and then the lithium niobate single crystal is heated to 200 ° C to 7 ° C.
A method for manufacturing a hologram memory element, comprising performing heat treatment in an oxidizing atmosphere or an inert gas atmosphere at 00 ° C.