JP2000285681A - Hologram memory element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make suppressible photovoltaic effect as well as to enhance sensitivity to radiated light and to make normal optical recording performable by incorporating a specified total amount of at least one selected from among Mg, Sc, Zn and In and a group VIII element into a lithium niobate or lithium tantalate single crystal. SOLUTION: The hologram memory element 1 uses a lithium niobate or lithium tantalate single crystal containing 1-5 mol%, in total, of one or more selected from Mg, Sc, Zn and In and one or more group VIII elements such as Fe, Co, Ni, Ru, Rh and Pd. For example, the single crystal is used as a rectangular solid, a face 1a of the single crystal on which light L is made incident and a face 1b from which light emerges are optically polished and antireflection films 2a, 2b effective to the wavelength of laser light used for hologram recording and reproduction are formed on the faces 1a, 1b.

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 optical memory element for writing (recording) and reading (reproducing) information by using light, and which is suitable as a digital holographic memory.

[0002]

2. Description of the Related Art Conventionally, floppy disks, hard disks and the like have been mainly used as storage media for personal computers. Recently, in addition to these storage media,
CD-ROM, MO (Magneto-optical Disk), DVD
(Digital Video Disk) and various storage media are used. In recent years, a hologram memory device has been considered as a next-generation memory device following the DVD, and has been studied.

[0003] As a material of this hologram memory element,
Various materials such as lithium niobate, barium titanate, strontium barium titanate, and organic photosensitizers have been proposed so far. Among these materials, lithium niobate single crystal is said to be the most practical. I have.

The main reasons for this are that lithium niobate single crystal can be expected to have a high read (reproduction) efficiency, can be repeatedly recorded and erased, and there is no deterioration in the characteristics, and can perform multiplex recording. This is because resolution can be expected. In addition, when irradiated with high-power laser light, the lithium niobate single crystal has a so-called photorefractive effect in which the refractive index of the light-irradiated portion changes locally. Recording and reproduction become possible. Moreover, it is known that sensitivity (efficiency) is improved by adding Fe (iron).

However, in order to realize a storage capacity of 10 gigabytes or more, it is necessary to further improve the sensitivity. However, the improvement in the sensitivity causes a photovoltaic effect and increases the charge in the crystal. Then, the recorded image (data) tends to be distorted. As a result, normal recording cannot be performed, that is, BE
There is a problem that R (Bit error rate: recording error rate) increases.

Accordingly, an object of the present invention is to provide an excellent hologram memory element which can improve the sensitivity to irradiation light, can suppress the photovoltaic effect, and can perform optical recording normally. And

[0007]

A hologram memory device according to the present invention comprises a single crystal of lithium niobate or a single crystal of lithium tantalate containing Mg (magnesium), Sc,
(Scandium), Zn (zinc), In (indium)
And any one or more of the group VIII elements (Fe (iron), Co
(Cobalt), Ni (nickel), Ru (ruthenium), Rh (rhodium), Pd (palladium), Os
(Osmium), Ir (iridium) and Pt (platinum) in total of 1 to 5 mol%.
More preferably, the content is 2 to 4 mol%.

In particular, it is assumed that the content of Fe in the single crystal body is 0.001 to 0.1 mol%.

Further, the light absorption coefficient α at a light wavelength of 400 to 600 nm is 2.5n + 0.325 ≦ α ≦ 2.5.
n + 1.125 (where, n: Fe content (mol%)).

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the hologram memory device according to the present invention will be described in detail with reference to the drawings.

[0011] The hologram memory element of the present invention comprises Mg,
A single crystal of lithium niobate or a single crystal of lithium tantalate containing at least one of Sc, Zn, and In and a group VIII element in a total amount of 1 to 5 mol% is used. Further, as shown in FIG. 1, the hologram memory element 1 is used for optically polishing the light incident surface 1a and the light emitting surface 1b of the light L of, for example, a rectangular parallelepiped single crystal, and uses these surfaces for hologram recording and reproduction. Antireflection film 2 for laser wavelength
a and 2b are formed. This antireflection film 2a,
2b is a dielectric multilayer film such as TiO ₂ having a thickness of 100 to
It is manufactured by a thin film forming method such as a vacuum evaporation method at about 200 °. Note that the antireflection film may be formed on all surfaces of the single crystal body, and the antireflection film may not be provided as long as it can function sufficiently as a hologram memory element.

Further, the atomic composition ratio of the single crystal is 0.1.
937 ≦ Li / Nb ≦ 0.943 or 0.942 ≦
Li / Ta ≦ 0.961. The reason for this is that when the value is outside these ranges, noise and BER due to crystal defects increase significantly, and the recording sensitivity due to the photorefractive effect deteriorates. Further, in particular, Mg, Sc, Zn,
In at least one of In and a group VIII element in total of 2 to 4
The best sensitivity is obtained by containing mol%. In particular, when Mg and Fe are contained in an amount of 2 to 4 mol%, the effect of suppressing the photovoltaic effect is maximized, which is optimal.

The above atomic composition ratio can be analyzed and measured by measuring the phase matching temperature of a second harmonic (second harmonic wave generation) of a single crystal. This is because N
d: When a laser beam (fundamental wave) such as a YAG laser is incident, light having a frequency twice as high as this laser beam (second harmonic)
(Phase matching temperature) can be determined strictly, and the phase matching temperature depends on the composition of the single crystal, for example, L
This is due to a slight change depending on the atomic composition ratio of i / Nb or Li / Ta. Here, the content of impurities such as Fe and Mg can be analyzed by ICP, EPMA, or the like.

Further, among the Group VIII elements, Fe has a large photorefractive effect, a sufficient sensitivity can be obtained at a content of 0.1 mol% or less, and it can be distributed almost uniformly in a single crystal. .

The content of Fe is 0.001-0.
1 mol% is preferred. The reason is that if it is less than 0.001 mol%, the sensitivity due to the photorefractive effect is not improved, and if it exceeds 0.1 mol%, it becomes difficult to grow a single crystal, and it is difficult to dope Fe uniformly. This is because the transmittance is undesirably reduced.

The Fe content of the lithium niobate single crystal or lithium tantalate single crystal is n, and the absorption coefficient is α
Then, 2.5n + 0.325 ≦ α ≦ 2.5n + 1.
It is preferably 125. This is for the following reason.
When the content of Fe ²⁺ in the single crystal increases, the light wavelength of 400 to 6
The light absorption around 00 nm increases, and the sensitivity due to the photorefractive effect depends on the Fe content n and Fe ²⁺ / Fe ^3+.
Is involved. Further, as the Fe content n increases,
Since the absolute amount of e ²⁺ is large, the absorption coefficient α 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 photorefractive effect can be controlled.

If α is less than 2.5n + 0.325, the sensitivity due to the photorefractive effect deteriorates,
If α exceeds 2.5n + 1.125, the Fe content n
And the content of Fe ²⁺ is increased, 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 light wavelengths 400 to 6
Within the wavelength band of 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 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, the manufacturing method of the present invention contains 0.001 to 0.1 mol% of Fe and the atomic composition ratio Li / Nb of Li / Nb is 0.937 ≦ L by a rotation pulling method.
a lithium niobate single crystal wherein i / Nb ≦ 0.943,
Or, the atomic composition ratio Li / Ta is 0.942 ≦ Li / Ta
A single crystal of lithium niobate satisfying ≦ 0.961 is grown.

Next, in the case of a lithium niobate single crystal, the temperature is 200 ° C. to 700 ° C. or 1000 ° C. to 1150 ° C.
In the case of a lithium tantalate single crystal, heat treatment is performed in a temperature range of 200 to 600 ° C. in an oxidizing atmosphere or an inert gas atmosphere. The absorption coefficient α can be controlled by a heat treatment in which the grown single crystal is heated to the above temperature range in an oxidizing atmosphere or an inert gas atmosphere such as nitrogen. Here, when the temperature is lower than 200 ° C., the light transmittance and the absorption coefficient α hardly change. Particularly, in the case of a lithium niobate single crystal,
At 700 ° C. to 1000 ° C., absorption due to oxygen vacancies increases, and a different phase such as LiNb ₃ O ₈ easily precipitates.
If the temperature exceeds 0 ° C., the temperature will be higher than the Curie temperature or higher, and a domain will be generated. In the case of a lithium tantalate single crystal, the Curie temperature is around 600 ° C., and if it exceeds 600 ° C., a domain is generated, resulting in a multi-domain and light scattering, which is not preferable.

The oxidizing atmosphere is, specifically, air,
A gas containing 1% by volume or more of oxygen is used, and an 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, and when heat treatment is performed in an oxidizing atmosphere, α decreases.

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

The heat treatment time is preferably 20 hours to 50 hours. If the heat treatment time is less than 20 hours, oxygen does not diffuse to the inside of the single crystal and color unevenness is likely to occur. Disappears.

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 single crystal, Residual stress is eliminated, and heat treatment in an oxidizing atmosphere has an effect of reducing oxygen defects generated during the growth of the single crystal.

The single crystal of the present invention is preferably grown by a pulling method using a resistance heating method, whereby a wide uniform temperature range can be secured with a simpler apparatus than, for example, a pulling method using a high frequency heating method. Therefore, the defects in the single crystal can be reduced, and the distortion of the crystal lattice when Fe is added can be reduced as much as possible, and the generation of cracks can be suppressed. Furthermore, since a simple device can be used, it can be manufactured at low cost.

FIG. 2 is a block diagram of an optical system H1 configured to irradiate the hologram memory element 1 with a laser beam to record and / or reproduce information.

Here, reference numeral 2 denotes a laser device that emits a laser beam L1 having a wavelength of 500 to 600 nm, a laser device such as a semiconductor laser or a gas laser, or a laser device that converts the wavelength of the laser beam. A beam splitter such as a glass prism for splitting into a signal light L2 and a reference light L3, 4 is a spatial light modulator for inputting information, 5 is a shutter for blocking the signal light L2, and 6 is diffracted by two light wave measurement. A photodiode using silicon or the like for measuring a gain; 7 a CCD camera for measuring a diffraction gain or reproducing information;
Is a personal computer with a display for calculating and measuring diffraction gain from signals from the photodiode 6 and the CCD camera 7, 9 is a receiver for displaying a reproduced image of information from the signal from the CCD camera 7, and 10 is a reference. A mirror for changing the traveling direction of the light L3, a diffraction grating 11 for resolution, and a mirror 12 for beam scanning.

Next, the operation of the optical system H will be described. 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 L 3 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, passes through the diffraction grating 11, and is further changed by the mirror 12 for beam scanning which can be changed, and is incident on the hologram memory element 1. Here, the intersection f between the signal light L2 and the reference light L3
At 0, information is recorded by forming an optically damaged area. 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 are not required.

The laser beam L1 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 passes through the hologram memory device 1. The signal light L2 is incident on the CCD camera 7, and by inputting a signal from the CCD camera 7 to the image receiver 9, information recorded in the hologram memory element 1 can be displayed as an image.

Thus, the present invention comprises a high-quality lithium niobate single crystal or lithium tantalate single crystal in which group VIII elements are uniformly distributed, wherein the lithium niobate single crystal or the lithium tantalate single crystal comprises Mg, S
Since an optimal amount of at least one of c, In, and Zn and a Group VIII element is contained, atoms of Mg, Sc, In, and Zn can suitably fill lattice defects, and are necessary for the photorefractive effect. The number of free electrons can be increased, the sensitivity of the hologram memory element can be improved, and the photovoltaic effect can be suppressed. As a result, a hologram memory element having excellent characteristics such as the sensitivity of the optical damage effect and the BER can be obtained.

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]

Next, a more specific embodiment according to the present invention will be described.

Example 1 Lithium niobate single crystal and hologram memory element 1 for hologram memory element 1 of FIG.
Was prepared as follows.

First, L of purity 4N (99.99% by weight)
Li / Nb = i ₂ CO ₃ , Nb ₂ O ₅ , Fe ₂ O ₃
0.94, Fe = 0.03 mol%, so that Mg = 3 mol%, Nb ₂ O ₅ starting material 2000g against Li ₂ O
₃ , 521.075 g, Fe ₂ O ₃ 0.720 g, M
18.175 g of gO was prepared, put into a 10-liter resin pot, and rotated and mixed by a pot mill for 10 hours.
Similarly, for comparison, Li / Nb = 0.93, 0.
At 95, a raw material of Fe = 0.03 mol% was also prepared and mixed.

Next, each of these materials was calcined at about 750 ° C. for 3 hours, and then calcined at about 1100 ° C. for 3 hours.

Further, 2300 g of each of these fired products was filled into a platinum crucible having a circular cross section of φ (diameter) of 100 mm and a height of 100 mm, and set in a single crystal production apparatus by a rotational pulling method (Czochralski method: CZ method). did. A single crystal was grown at a rotation speed of 10 rpm and a pulling speed of 1.0 mm / hour to produce a columnar lithium niobate single crystal having a diameter of 60 mm and a length of 80 mm. Similarly, Mg,
0, 1, 3, 5, 7 mol% of Sc, In, Zn respectively
The added single crystal was grown.

After the lithium niobate single crystal was subjected to a single domain treatment, a block of 15 × 15 × 10 mm was cut out and placed in an atmosphere of 10% by volume of oxygen and 90% by volume of nitrogen at 1030 to 1050 ° C. Heat treatment was performed for 30 hours to adjust the absorption coefficient α 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 a large number of hologram memory elements 1.

When recording and reproduction were performed on these hologram memory elements 1 by the optical system shown in FIG. 1, respectively,
For a lithium / niobate single crystal of Li / Nb = 0.94, BER ≒ 2.0 × 1 in 256 kbit recording
0 ^-10, BER ≒ 7.1 × 1 recording of 1024kbit
0 ^-5 and showed good values, Li / Nb = 0.93 and Li
/Nb=0.95 for both 256 kbit and 1024 kbit recordings,
R deteriorated on the order of two digits.

Next, the portions of 10 mm, 40 mm and 80 mm from the top of each lithium niobate single crystal were subjected to ICP-MS
(Inductively Coupled Plasma-Mass Spectrometry)
As a result of measuring the Fe content by the method, Li / Nb = 0.9
It was confirmed that the crystal of No. 4 was uniform at 0.03 ± 0.002 mol% within the measurement accuracy.

On the other hand, in the crystal of Li / Nb = 0.93, 0.95, variation was observed at 0.03 ± 0.006 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, 0.9
Crystal No. 5 could not be used as a hologram memory element.

Next, the effect of the change in the refractive index due to the added element measured by the optical system H2 shown in FIG. 3 will be described. As shown in FIG. 3, the sample S, which is a hologram memory device, is disposed on the Peltier device 31 so that the optical axis direction of the sample S matches the polarization direction of the polarizer 26, and the temperature of the sample S is controlled. The light emitted from the He—Ne laser 22 for measuring the change in the refractive index is transmitted through the mirror 23 to the λ / 4 plate 2.
After the light is once circularly polarized in 5a, the light is linearly polarized by the polarizer 26 via the half mirror 24, and is incident on the sample S. By aligning the optical axis direction of the sample S with the polarizer 26, the emitted light becomes linearly polarized light. Therefore, the polarization direction of the analyzer 27 is set to be perpendicular to the polarizer 26, and the power meter 32 Light emitted from the He-Ne laser 22 is not detected. Next, He-Cd
Light emitted from the laser 21 is condensed by a lens 28 via a λ / 4 plate 25b and the like, and is incident on a sample S.
Is cut off by the cut filter 30 for the light emitted from the He-Cd laser 21.

Here, when optical damage occurs in the sample S, the refractive index changes, and He-Ne emitted from the sample S is changed.
The light emitted from the laser 22 becomes elliptically polarized light, and the He-Ne laser 22 is detected by the power meter 32. The detected outgoing light of elliptically polarized light is linearly polarized by the λ / 4 plate 25c. Then, the analyzer 27 is rotated, and the power meter 32 is turned on.
Measure the rotation angle that minimizes the amount of light detected in. From the rotation angle (Δθ) of the analyzer 27 at that time, the refractive index change amount (Δ
n) can be calculated. That is, it can be calculated from the following equation.

Δn = Δθ × (λ / 180) × L (L: sample length 10 mm, λ: wavelength 633 nm) Table 1 shows the refractive index change measured by the above method. As is clear from Table 1, the amount of change in the refractive index of the crystal containing 1 to 5 mol% of Mg, Sc, Zn, and In is larger than that of the crystal not doped at all, and the content of Mg is 1 to 3 mol%. The contained crystal was considerably larger than the other elements. In addition, it has been found that the amount of change becomes maximum when Mg, Sc, Zn, and In are contained in 3 mol%. Thereby, in particular, the content of Mg, Sc, Zn, and In is 2 to 2.
4 mol% seems to be optimal. In particular, Mg is suitable because the crystal can be grown more easily than other elements and the grown crystal has good crystallinity.

[0048]

[Table 1]

Next, the result of confirming the photovoltaic effect using the optical system H3 shown in FIG. 4 will be described. As shown in FIG. 4, the emitted light L5 of the He-Ne laser is split into light L6 and L7 by the half mirror 43, the light L6 is made incident on the sample S, and the light L7 is split by the half mirrors 41, 42 and 44. The beams are multiplexed, condensed by a lens 45, and made incident on a CCD camera 46. Then, the interference image is observed by the CCD camera 46. In the drawings, P1 and P2 indicate polarization directions, and P1 is a direction perpendicular to the paper surface. By changing the polarization direction between the He—Ne laser and the Ar laser in this manner, interference fringes that occur separately are prevented.

Next, the sample S is irradiated with the emitted light L4 from the Ar laser through the half mirror 41. When the sample S has a refractive index distribution due to the photovoltaic effect, distortion occurs in the interference image observed by the CCD camera as shown in FIG.

As a result of the above observation, it was found that Mg, Sc, Zn, In
In the crystal containing 1 to 5 mol%, no photovoltaic effect was observed, and a very good interference image shown in FIG. 6 was observed. An interference image was obtained.

Example 2 An example using a lithium tantalate single crystal for the hologram memory device 1 of FIG. 1 will be described.

The single crystal and the hologram memory element were manufactured as follows.

First, L of purity 4N (99.99% by weight)
i ₂ CO ₃ , Ta ₂ O ₅ , Fe ₂ O ₃ and MgO are converted to Li /
L is added to 3000 g of a Ta ₂ O ₅ raw material so that Ta = 0.95, Fe = 0.03 mol%, and Mg = 3 mol%.
476.528 g of i ₂ CO ₃ and 0.65 Fe ₂ O ₃
4 g and 16.507 g of MgO were mixed, put into a 10-liter resin pot, and rotated and mixed with a pot mill for 10 hours. Similarly, Li / Ta = 0.9 for comparison.
Raw materials of 4,0.96 and Fe = 0.03 mol% were also prepared and mixed.

Then, each of these raw materials was heated at about 750 ° C. for 3 hours.
After calcining for 1 hour, it was calcined at about 1350 ° C. for 15 hours.

Next, 3000 g of each of these fired products is filled into an iridium crucible having a circular cross section of φ (diameter) of 100 mm and a height of 100 mm, and the baked material is applied to a single crystal manufacturing apparatus by a rotational pulling method (Czochralski method: CZ method). I set it. A single crystal was grown at a rotation speed of 10 rpm and a pulling speed of 1.0 mm / hour to produce a columnar lithium tantalate crystal having a diameter of 60 mm and a length of 60 mm.

Next, after these lithium tantalate crystals were subjected to a single domaining treatment, a block of 15 × 15 × 10 mm was cut out and placed in an atmosphere of 15% by volume of oxygen and 85% by volume of nitrogen at 500 to 550 ° C. Heat treatment was performed for 30 hours to adjust the absorption coefficient α to 0.9. 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 a large number of hologram memory elements 1.

When information was recorded and reproduced on each of these hologram memory elements 1 using the optical system shown in FIG. 1, a BER = 1 with a recording of 256 kbits for a lithium / tantalate single crystal of Li / Ta = 0.95. .9 × 1
0 ^-10 , BER = 6.5 × 1 with 1024 kbit recording
0 ^-5 and showed good values, Li / Ta = 0.94, and L
In all the cases of i / Ta = 0.96, the BE value was larger than the above value in the recording of 256 kbits and 1024 kbits.
R deteriorated on the order of two digits.

Next, the portions of 10 mm, 30 mm and 60 mm from the top of each lithium tantalate single crystal were subjected to ICP-M
S (Inductively Coupled Plasma-Mass Spectrometry)
As a result of measuring the Fe content by the method, Li / Ta = 0.9
It was confirmed that the crystal of No. 5 was uniform at 0.03 ± 0.002 mol% within the measurement accuracy.

On the other hand, in the crystal of Li / Ta = 0.94 and 0.96, variation was observed at 0.03 ± 0.006 mol%. Due to this variation, the variation in the light transmittance near the light wavelength of 500 nm was about 10 to 20%, and the variation in the light transmittance of the product of the present invention was considerably larger than 1 to 2% or less. Therefore, Li / Ta = 0.94, 0.9
Crystal 6 could not be used as a hologram memory element.

When the influence of the change in the refractive index due to the added element was measured, the results were almost the same as those of the lithium niobate single crystal.

[0062]

According to the hologram memory element of the present invention, any one of Mg, Sc, In, and Zn is contained in the lithium niobate single crystal or the lithium tantalate single crystal.
Since the optimum amount of at least one species and the group VIII element is contained, the sensitivity of the hologram memory element can be improved and the photovoltaic effect can be suppressed. This makes it possible to provide a highly reliable hologram memory device that can perform optical recording accurately and without error.

[Brief description of the drawings]

FIG. 1 is a perspective view of a hologram memory device according to the present invention.

FIG. 2 is a block diagram of an optical system H1 using the hologram memory device according to the present invention.

FIG. 3 is a configuration diagram illustrating an optical system H2 that measures a change in a refractive index due to a photorefractive effect.

FIG. 4 shows an optical system H3 for measuring a photovoltaic effect.
FIG.

FIG. 5 is an interference image observed by an optical system H3.

FIG. 6 is an interference image observed by an optical system H3.

[Explanation of symbols]

 1: hologram memory element 2a, 2b: anti-reflection film H1 to H3: optical system

Claims (3)

[Claims] 1. A hologram memory element comprising a lithium niobate single crystal or a lithium tantalate single crystal containing at least one of Mg, Sc, Zn, and In and a Group VIII element in an amount of 1 to 5 mol%. 2. The hologram memory device according to claim 1, wherein the content of Fe is 0.001 to 0.1 mol%. 3. The hologram memory device according to claim 2, wherein a light absorption coefficient α at a light wavelength of 400 to 600 nm satisfies the following expression. 2.5n + 0.325 ≦ α ≦ 2.5n + 1.125 (however, n: Fe content (mol%))