JP3476059B2 - Method for producing hologram recording material - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hologram recording material, and more particularly to a method for producing a hologram recording material which is a main material of light control parts based on holography technology in the field of optical information processing. Regarding

[0002]

2. Description of the Related Art The phenomenon in which the refractive index changes when a certain material is exposed to light is called the photorefractive effect. When the interference fringes of light are irradiated, the refractive index is modulated while maintaining the shape of the interference fringes, and even if the irradiation of light is stopped, the refractive index modulation is preserved and it functions as a kind of diffraction grating. A material exhibiting such an effect is used as a hologram recording medium in the holographic technique. That is, when a material exhibiting a photorefractive effect is irradiated with “object light” and “reference light”, which are called by holography technology, and interference fringes are formed in the material, the interference fringes are recorded as a refractive index modulation of the material. To be done. The recorded refractive index modulation is a kind of diffraction grating as described above, and is a “hologram” referred to in holographic technology. After the irradiation of the reference light and the object light is stopped, the light corresponding to the reference light is irradiated again on the material (irradiation of the reproduction light), whereby the object light is reproduced by the diffraction phenomenon by the hologram.

As a material exhibiting a photorefractive effect, lithium niobate known as an electro-optic crystal,
Strontium barium niobate, barium titanate,
Single crystals such as potassium niobate and potassium tantalate niobate are available. Specific fields of application of this material include recording of images, generation of phase conjugate light, which has been attracting attention in recent years, and filters utilizing the wavelength selective control of recorded diffraction gratings.

The photorefractive effect is observed in substances having the following properties. 1) To develop a phenomenon in which the refractive index is modulated by an electric field, that is, an electro-optical effect. 2) There is a trap that accepts carriers (electrons or holes). 3) The carriers trapped in the trap cannot move in the substance, but when irradiated with light, they are excited in the conduction band or valence band and can move around.

Further, such a substance exhibits a photorefractive effect according to the following process. Carriers have electrons or holes, but here, for the sake of simplicity, only electrons move. 1) Interference fringes are formed in the crystalline material, so that high light intensity (bright) portions and low light intensity (dark) portions are alternately formed in stripes in the material. 2) In the bright part, electrons are photoexcited to the conduction band. 3) The electrons excited in the conduction band move around in the substance and are trapped by the trap somewhere. 4) The number of electrons is gradually reduced because the electrons are excited in the bright part, but in the dark part, electrons are newly excited but are not excited, and the electrons are gradually accumulated. 5) Therefore, the bright part is positively charged and the dark part is negatively charged (this modulated charge is called space charge). As a result, an electric field is generated between the bright part and the dark part. This electric field is modulated in the same shape as the interference fringes and is called the spatial electric field. 6) The refractive index is modulated by the electric field. This is a hologram, which has the function of diffracting light.

However, when the space charge formed by the photorefractive effect is again irradiated with light, the carriers are excited again and can move in the substance.
The space charge formed, that is, the hologram, disappears. This has been a fatal problem in devices such as optical switches and filters that constantly emit light.

For this reason, "fixing" is performed so that space charges due to the photorefractive effect are not erased by light irradiation.
There is technology. At present, a technique called thermal fixing is used for commercially available optical filters and optical monitors.
A lithium niobate single crystal, which is a photorefractive material, is used for these elements, and thermal fixing is performed using hydrogen ions (hydrogen nuclei) contained in the crystal. Usually, hydrogen ions in the crystal hardly move at room temperature, but when heated to about 100 ° C., they easily move. When the lithium niobate single crystal is irradiated with light to form a hologram in advance and then heated, the hydrogen ions move so as to cancel the spatial electric field of this hologram, so that the concentration distribution of the hydrogen ions has the same shape as the original hologram. it can. When the temperature is returned to room temperature in this state, hydrogen ions hardly move again and the concentration distribution of hydrogen ions is fixed. Then, when the light is uniformly irradiated, the original space charge due to the electrons or holes disappears, and the space charge of the replica due to hydrogen ions emerges. Since electrons and holes sense light, hydrogen ions do not sense light. Therefore, the space charge due to the hydrogen ions does not disappear even when light is irradiated. This is fixation.

[0008]

However, the technique of fixing the space charge by the photorefractive effect in order to obtain the hologram as described above is actually not easy. That is, it is not easy to obtain a desired concentration distribution by including hydrogen ions in the optical single crystal. In particular, potassium tantalate niobate single crystal having a photorefractive effect,
In particular, the addition of a trace amount of copper can increase the number of the traps described above, and the space electric field becomes large, so that the effect becomes large. If the ratio of the amounts of tantalum and niobium is appropriately selected, the secondary electric field at room temperature The optical effect (a phenomenon in which the change in the refractive index proportional to the square of the electric field appears) becomes large, and in this case,
Since the degree of modulation of the refractive index can be freely controlled by an electric field applied from the outside of the crystal, it can be used as a material for an optical switch. If lithium is added to an appropriate degree, the secondary electro-optical effect is increased and the crystal is less likely to be broken. It has extremely excellent features such as becoming,
Also, in order to obtain hydrogen ions necessary for fixing, a single crystal containing hydrogen ions can be grown at 900 ° C. just by growing it normally.
By performing heat treatment to a degree, hydrogen ions that are compatible with oxygen can be made to enter and leave the crystal during heat treatment to increase hydrogen ions, but if certain raw materials for single crystal growth contain impurities, hydrogen ions Would be extremely difficult to include.

For example, as mentioned above, substituting a part of single crystal potassium with lithium or adding a trace amount of copper is a very important elemental technique for improving the photorefractive effect. Both of these two additives significantly hinder the uptake of hydrogen ions by the single crystal. 3 and 4 explain these circumstances. In FIG. 3, the change in the concentration of hydrogen ions contained in the obtained single crystal was examined by changing the amount of lithium in the raw material for crystal growth. Here, the raw materials Ta ₂ O ₅ , Nb ₂ O ₅ , and K ₂ C are used.
O ₃ and Li ₂ CO ₃ powders were mixed, heated, melted and gradually cooled to grow a single crystal. The raw material mixing ratio was Ta ₂ O ₅
And Nb ₂ O ₅ are fixed at 30.6 mol.% And 13.1 mol.% Respectively, and the remaining 56.3 mol.% With K ₂ CO ₃ and Li ₂ CO ₃ .
And The horizontal axis of FIG. 3 is mol% of Li ₂ CO ₃ .

When the single crystal of potassium tantalate niobate contains hydrogen ions, the wave number by the O--H bond is about 350.
Infrared absorption at 0 cm ^-1 is observed. The vertical axis of FIG. 3 is a value obtained by measuring an optical absorption spectrum with a spectroscope and integrating the absorption coefficient on the wave number axis, and is proportional to the concentration of hydrogen ions in the single crystal. Since a high-purity raw material was used in this experiment, a high hydrogen ion concentration can be obtained when lithium is not added. However, by increasing the mixing ratio of Li ₂ CO ₃ from this figure, the hydrogen ion concentration in the growing single crystal can be increased. It is clear that decreases and approaches zero.

The same was done for copper. K ₂ C
O ₃ is 58.0 mol.%, Ta ₂ O ₅ is 29.4 mol.%, Nb ₂
A single crystal was grown from a mixed powder containing O ₅ at 12.6 mol.% And a small amount of CuO powder added thereto. Figure 4
Is the result obtained by plotting mol% of CuO on the horizontal axis and the integrated absorption coefficient on the vertical axis. It can be seen that the hydrogen ion concentration is significantly reduced even with a slight addition of CuO. Accordingly, lithium-substituted / copper-doped potassium tantalate niobate, which is promising as a photorefractive material, contains almost no hydrogen ions, and thermal fixing cannot be performed.
Then, even if this single crystal was heat-treated at about 900 ° C., hydrogen ions could not be included.

In view of the above-mentioned problems, the present invention provides a method for producing a hologram recording material in which hydrogen ions are surely included in a crystal.

[0013]

The present invention which achieves the above object has the following matters for specifying the invention. A first invention is a method for producing a hologram recording material using an optical crystal having tantalum, niobium, potassium and oxygen as main components and having a perovskite type crystal structure, wherein oxygen vacancies are formed in the optical crystal, and It is characterized by absorbing water vapor.

A second aspect of the present invention is the first aspect of the present invention, which is characterized in that, as a method of generating the oxygen vacancies, a raw material lacking oxygen is used during the growth of a single crystal.

A third aspect of the present invention is the second aspect of the present invention, wherein as the oxygen-deficient raw material, the entire raw material of the optical crystal is used.
It is characterized in that it contains metallic copper in the range of 0.1 mol.% To 1.0 mol.%.

A fourth aspect of the present invention is the method according to the first aspect, wherein at least one single crystal is used as a method for generating the oxygen vacancies.
The heat treatment is performed at a temperature of 000 ° C. or higher and a melting point of the single crystal or lower.

For example, hydrogen ions entering and exiting a potassium tantalate niobate single crystal may enter and exit with oxygen in the form of water vapor. For example, even if heat treatment is performed in hydrogen gas containing no oxygen, the single crystal It has been found difficult to include hydrogen ions in it. In other words, oxygen is considered to play a very important role in the entry and exit of hydrogen ions. From this, if a place (oxygen vacancy) for accepting oxygen is created in the single crystal, it is likely that the hydrogen ion concentration in the single crystal can be increased by introducing water vapor into the place. From this point of view, it has been found that it is difficult to introduce hydrogen ions into potassium tantalate niobate containing lithium and copper because the oxygen vacancy concentration is very low. Therefore, if oxygen vacancies are generated in the single crystal, hydrogen ions can be easily introduced by the subsequent heat treatment in steam.

There are various methods for creating oxygen vacancies in a single crystal. One is a method of growing a single crystal using a raw material lacking oxygen. For example, when growing a single crystal to which copper is added, metallic copper is used as a raw material instead of CuO described above. At this time, the melt during crystal growth is deficient in oxygen, and as a result, the growing single crystal also lacks oxygen. Another is heat treatment.
When the oxide is exposed to a high temperature, the concentration of oxygen vacancies increases, but in the case of potassium tantalate niobate, the heat treatment at about 900 ° C. as described above hardly has any effect. The heat treatment temperature must exceed at least 1000 ° C., and is preferably higher than the temperature at which the crystal was grown. In this case, it is not possible to perform heat treatment at a temperature above the melting point of the single crystal, but potassium tantalate niobate is an incongruent (a solid with a different composition from the original solid can be obtained by melting and solidifying again). ), The temperature at which the solid phase changes to the liquid phase and the temperature at which the liquid phase changes to the solid phase are significantly different, and since the melting point is considerably higher than the crystal precipitation temperature, heat treatment at a temperature higher than the crystal precipitation temperature is necessary. It is possible. Furthermore, as another method, there is a method in which the atmosphere during the growth of the single crystal is a so-called reducing atmosphere such as hydrogen.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Here, embodiments of the present invention will be described. <First Example> A single crystal was grown from a raw material for growing pure potassium niobate tannate to which particles of metallic copper were added. It was confirmed that the obtained single crystal was a perovskite type. FIG. 1 shows the copper addition amount dependency of the integrated absorption coefficient of the obtained O—H bond of the single crystal. For reference,
The data in FIG. 4 was plotted in an overlapping manner. The higher the copper concentration in the single crystal, the larger the space charge of the original hologram, and the higher the concentration of hydrogen ions, the larger the space charge of the replica hologram. Show. Similar to the case of CuO, the hydrogen ion concentration tends to decrease with the increase of the addition amount, but the high hydrogen ion concentration is maintained even with the addition amount of CuO. This means that when metallic copper was used as a raw material instead of CuO, oxygen vacancies were excessively generated in the potassium tantalate niobate single crystal growing due to oxygen deficiency. When hydrogen ions are mainly cooled to room temperature after crystal growth, water vapor in the surrounding air enters the oxygen vacancies and is taken into the crystals.

<Second Example> Ta ₂ O ₂ , Nb ₂ O ₅ , K ₂ CO
_In addition to ₃ , a powder of Li ₂ CO ₃ and CuO was mixed to grow a potassium niobate tantalate single crystal. The obtained single crystal had a perovskite type crystal structure. The filtrate that this single crystal in the same manner as the first example to prepare a sample to measure the hydrogen ion concentration, Li ₂ CO ₃ and Cu
Due to the influence of the addition of both O, only the concentration at the margin of the measurement accuracy was detected. The heat treatment was performed at 900 ° C. for 3 days in an argon gas saturated with water vapor, but no remarkable increase in hydrogen ion concentration was observed. Furthermore, still 900 ℃
The sample was heat-treated in a vacuum at 1, and then heat-treated in an argon gas saturated with water vapor, but no increase in hydrogen ion concentration was observed. However, after the temperature was maintained at 1100 ° C. for 1 day or more in an argon gas saturated with water vapor, the temperature was raised to 8
As a result of lowering the temperature to 00 ° C. and maintaining it as it is for 10 hours or more, the hydrogen ion concentration could be remarkably increased.
The resulting OH bond absorption spectrum is shown in FIG. 2 together with that immediately after single crystal growth. Here, in the final heat treatment, oxygen vacancies are mainly generated at 1100 ° C., and water vapor is absorbed in the single crystal at 800 ° C. Note that the temperature of 1100 ° C. has already exceeded the temperature when this single crystal was grown, and is close to the melting point. Therefore, heat treatment at a temperature higher than this is extremely difficult. 1150 ° C
When heated to, the single crystal melted.

In the above description, potassium tantalate niobate is the basis of the material. This potassium tantalate niobate is a compound mainly composed of tantalum, niobium, potassium and oxygen, and has a perovskite type crystal structure. It is a general term for things. Usually, KTa _1-x Nb _x
It is represented by a chemical formula consisting of O ₃ , and x takes a numerical value within the range of 0 to 1. Therefore, x = 0 and x = 1 are also included in potassium tantalate niobate, and the former is usually called potassium tantalate and the latter is called potassium niobate. In reality, the ratio of the elements represented by the chemical formula KTa _1-x Nb _x O ₃ is not strictly satisfied. For example, potassium is about 10% in excess, or conversely, even if it is deficient, it is a perovskite type. May retain the crystal structure of. Even in such a case, oxygen vacancies always exist, and it is natural that impurities are contained. Therefore, the material handled in the present invention, potassium, tantalum, niobium and oxygen are the main constituent elements,
In addition, all materials having a perovskite type crystal structure are included.

It is common practice to add some impurities to enhance the photorefractive effect. For example, as described above, the material in which a part of potassium is replaced with lithium has a better photorefractive property than pure potassium tantalate niobate, and the effect of the present invention is more remarkable than potassium tantalate niobate. Not only lithium, but also alkali metals such as sodium, rubidium, cesium, and francium, and alkaline earth metals such as beryllium, magnesium, calcium, strontium, barium, and radium are candidates for replacing potassium. Similarly, part of the tantalum or niobium may be replaced with a transition metal element. Copper described in the present invention is one example, but other metal elements such as titanium, vanadium, chromium, manganese, and iron are often used. In addition, as the element substituting tantalum or niobium, in the Periodic Table of the Elements, a row of elements from scandium having 21 atoms to zinc having 30 atoms, an element group having 39 atoms from yttrium to 48 cadmium, and 72 atoms
A row of elements from hafnium to 80 mercury, 5 atoms
The group of elements called rare earths from 7 lanthanum to 71 lattenium and the group of elements from 89 actinium to 103 lorencium atoms are listed as candidates for substitution. other,
Aluminum, silicon, gallium, germanium, indium, tin, lead, bismuth, and all elements listed in the synchronization table, even if a part of potassium or tantalum or niobium is replaced, perovskite type crystal structure What is kept is contained in the potassium tantalate niobate treated in the present invention. For single crystals of all these materials, the hydrogen ion concentration can be increased by the method of the present invention.

In the above case, when intentionally adding a metal element using a metal oxide, it is preferable to add a raw material having a small degree of oxidation. For example, when adding iron, FeO is more effective than Fe ₂ O _{3 and} metallic iron is more effective than FeO. Regarding all the metal elements in the above-mentioned substituting elements, oxygen-deficient raw materials can be obtained by adding them to the raw materials as metals instead of oxides which are general raw materials. Further, tantalum, which is the main constituent element of potassium niobate tantalate handled by the present invention, niobium,
As for potassium, the raw materials Ta ₂ O ₅ and Nb are also used.
₂ O ₅ , K ₂ CO ₃ are partially metal tantalum, metal niobium,
A similar effect can be expected by replacing with metal potassium.

Materials exhibiting a photorefractive effect include lithium niobate, lithium tantalate, barium titanate, etc. in addition to potassium tantalate niobate.
There are oxides represented by the chemical formula ABO ₃ and oxides such as strontium barium niobate having a tungsten bronze type crystal structure which is similar to the oxides, and the effects of the present invention can be expected for these materials. In addition to crystals having the perovskite type crystal structure described above, there are crystals having a tungsten bronze type crystal structure as the crystals containing tantalum, niobium, potassium, lithium and oxygen as main constituent elements. A photorefractive effect is also observed in this material, and the effect of the present invention can be expected.

Regarding the above heat treatment conditions, the higher the melting point is, the shorter the time is.
Although a temperature of 100 ° C. was used, if the temperature was 1000 ° C. or higher even at a slightly lower temperature, the oxygen vacancy concentration could be similarly increased by the long-term heat treatment, and thus the hydrogen ion concentration could be increased. The holding time is not particularly limited, and the hydrogen ion concentration can be improved even for a short time, but the longer the hydrogen ion concentration, the higher the hydrogen ion concentration obtained. Furthermore, the atmosphere should have a low oxygen concentration, but this is not specified. For example, in air 1100
After heat treatment at ℃ to generate oxygen vacancies, it was rapidly cooled and then heat-treated again at 800 ℃ in moist argon to absorb water vapor. The obtained hydrogen ion concentration was as shown in FIG. Although it is lower by about 10%, a sufficient effect was observed.

[0026]

As described above, according to the present invention, hydrogen vacancies, which are hydrogen nuclei, can be surely included in the crystal by forming oxygen vacancies in the optical crystal and then absorbing water vapor. did it.

[Brief description of drawings]

FIG. 1 is one of the methods for producing a hologram recording material of the present invention, in which metallic copper is used as a raw material for addition of copper ions instead of CuO, and a single crystal of potassium tantalate niobate in a reduced state is obtained by this. The figure explaining the effect of the method of raising.

FIG. 2 illustrates the effect of a method of producing oxygen vacancies in a potassium tantalate niobate single crystal by heat treatment in a wet argon atmosphere at 1133 ° C., which is one of the methods for producing a hologram recording material of the present invention. Figure to do.

FIG. 3 is a graph showing changes in the hydrogen ion concentration of a grown single crystal with respect to the ratio of Li ₂ CO _{3 in} the raw material when growing a potassium tantalate niobate single crystal.

FIG. 4 is a diagram showing changes in the hydrogen ion concentration of a grown single crystal with respect to the ratio of CuO in the raw material during the growth of a potassium tantalate niobate single crystal.

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Claims (4)

(57) [Claims] 1. A method for producing a hologram recording material using an optical crystal having a perovskite type crystal structure containing tantalum, niobium, potassium and oxygen as main components, wherein oxygen vacancies are formed in the optical crystal, and then water vapor is formed. A method for producing a hologram recording material, which comprises absorbing 2. The method for producing a hologram recording material according to claim 1, wherein the method of forming the oxygen vacancies uses a raw material devoid of oxygen when growing a single crystal. 3. The oxygen-deficient raw material contains metallic copper within a range of 0.1 mol.% To 1.0 mol.% Of the total raw material of the optical crystal.
A method for producing the hologram recording material described. 4. The method for producing a hologram recording material according to claim 1, wherein the method of forming oxygen vacancies comprises heat-treating the single crystal at a temperature of at least 1000 ° C. and at a temperature below the melting point of the single crystal. .