Hologram recording material

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Industrial application field]

The present invention relates to hologram recording materials. Specifically, it relates to holography technology in the field of optical information processing.

【0002】

[Conventional technology]

The phenomenon in which the refractive index changes when light is applied to a certain material is called the photorefractive effect. When the interference fringes of light are irradiated, the refractive index is modulated as it is in the shape of the interference fringes, and even if the light irradiation is stopped, the refractive index modulation is preserved and functions as a kind of diffraction grating. Materials exhibiting such effects are used as hologram recording media in holography technology.

【0003】

That is, when a material exhibiting a photorefractive effect is irradiated with "object light" and "reference light" referred to in holographic technology to form interference fringes in the material, the interference fringes are recorded as refractive index modulation of the material. Will be done. The recorded index of refraction modulation is a kind of diffraction grating as described above, and is a "hologram" as referred to in holographic technology. When the material is irradiated with light corresponding to the reference light (referred to as regenerated light) again after the irradiation of the reference light and the object light is stopped, the object light is regenerated by the diffraction phenomenon of the hologram.

【0004】

Examples of the material exhibiting the photorefractive effect include single crystals such as lithium niobate, strontium barium niobate, barium titanate, potassium niobate, and potassium niobate tantalate, which are known as electro-optical crystals. In particular, crystals of potassium niobate tantalate or a material to which other elements are added are currently the only materials that exhibit a photorefractive effect because the diffraction efficiency of holograms can be controlled by electrical signals. It is a known material. This material is a promising material in that it is possible to fabricate a new device that has controllability by an electric signal in addition to the wave surface conversion function and wavelength selection function peculiar to a conventional device (called a holographic device) using the principle of holography. ..

【0005】

Specific fields of use of holograms include image recording, phase-conjugated light generation, which has been attracting attention in recent years, and filters that utilize the wavelength selectivity of recorded diffraction gratings. These filters are already on the market. There is. The mechanism of the manifestation of the photorefractive effect is explained step by step as shown in (1), (2) and (3) below.

【0006】

(1) Interference fringes are formed in the crystal material, (2) Carriers (electrons or holes) are rearranged in the crystal through the process of photoexcitation, transfer, and recombination according to the light intensity distribution of the interference fringes, resulting in space. Charges are generated. (3) Since space charges are accompanied by a space electric field, the refractive index is modulated by the electro-optical effect. However, even in the same spatial electric field, the method of modulating the refractive index differs slightly depending on the material. The amplitude of the spatial electric field is E_SCAt the time, the amplitude of the refractive index modulation of the hologram n₁Is the result shown by the following equation (1) in the case of an electro-optical crystal having a primary electro-optical effect such as lithium niobate, strontium barium niobate, barium titanate, and potassium niobate.

【0007】

n₁= (1/2) n₀³rE_SC ... (1) In the case of cubic potassium niobate tantalate, the result is shown by the following equation (2). n₁= (1/2) n₀³gE_SCE ... (2) In addition, n₀Is the refractive index of each electro-optical crystal, r and g are the first-order and second-order electro-optic coefficients, respectively, and E is the voltage applied from the outside. The magnitude of the hologram effect is this n₁It is represented by the size of. As is clear from these equations, the cubic potassium niobate tantalate differs from other materials in that the magnitude of the hologram effect depends on the externally applied electric field.

【0008】

Holograms change exponentially with time with a certain time constant. The time constant τ is generally represented by the dielectric relaxation time. τ = ε / σ In addition, ε is a dielectric constant and σ conductivity. Figure 2 shows the spatial electric field E of the hologram._SCAn example of the time change of is shown. When light irradiation is started at time t = 0 and interference fringes are formed, the time constant τ₁It rises with, and eventually reaches saturation. Time constant τ at this time₁Is σ as the conductivity due to photoexcitation of carriers. Since the light conductivity is proportional to the light intensity, the rising speed differs depending on the light intensity, but when compared at the same light speed, the time constant τ₁When is short, the sensitivity is considered to be good.

【0009】

Time t₁When the light irradiation is stopped with, this time the slow time constant τ₂It gradually disappears. Time constant τ at this time₂Is the dark conductivity of the carrier due to thermal excitation. Time t₁Instead of stopping the light irradiation with, if you irradiate a uniform light without an intensity distribution like interference fringes, the time constant τ is very short.₃Disappears with. This is because the conductivity due to photoexcitation is much higher than the dark conductivity.

【0010】

From these phenomena, one problem is pointed out when using an electro-optical crystal showing a photorefractive effect as a hologram recording material. That is, the stability of the hologram. As the reproduced light emitted when reproducing the recorded hologram, a uniform light having no information is usually used. Therefore, when the hologram is reproduced, the hologram disappears (referred to as reproduction destruction). In addition, the hologram is erased by the thermal excitation of the carrier even if it is left at room temperature for a while even if it is not irradiated with light. The life of a hologram is usually as long as several days. Therefore, in order to use it for an optical element such as a filter, it is necessary to prevent it from being erased by some method. This operation is called fixing.

【0011】

Fixing methods include methods such as heat treatment and electrical treatment. In the former, as compared with the above-mentioned carriers in the state of not irradiating light, ions that are easier to move (large σ) at high temperature and harder to move (small σ) at room temperature, for example, hydrogen ions, are contained in the crystal. It is possible when you are. When a hologram is recorded on such a crystal (there is a spatial distribution of carriers), raising the temperature of this crystal sufficiently makes it easier to move before the carriers move due to thermal excitation and the spatial distribution disappears. The ions move to cancel the spatial electric field due to the spatial distribution of the carriers. When the temperature is returned to room temperature in this state, the charge distribution due to the carriers gradually disappears, but the charge distribution due to the ions formed so as to compensate for this charge distribution is very difficult to disappear because the ions are difficult to move. In addition, since the ions do not move easily (sigma increases) due to photoexcitation, they do not disappear even when irradiated with light.

【0012】

The latter is possible in the case of ferroelectric electro-optical crystals. Since ferroelectrics have spontaneous polarization, this is a method of replacing the spatial electric field distribution with the distribution of spontaneous polarization. In this case as well, the spontaneous polarization is stable even at room temperature and has nothing to do with light irradiation, so that the life of the hologram becomes very long. An example of utilizing fixing by heat treatment is a filter using lithium niobate crystals. This is because the hologram was fixed by heat treatment focusing on the fact that hydrogen ions are taken in during crystal growth.

【0013】

[Problems to be Solved by the Invention]

Cubic potassium niobate tantalate single crystal (KTa)_1-xNb_xO₃, 0 x 0.35 is cubic at 0 ° C), as can be seen from the above equation (2), diffraction of the recorded hologram by changing the magnitude of the electric field applied from the outside. It is a material whose efficiency can be changed from 0% to 100%. It is a promising material in that it is possible to fabricate an element that has both the wave surface conversion function and wavelength selection function peculiar to holography and the controllability by an electric signal. For example, as inferred from the application example to the filter, it is possible to manufacture a filter in which the wavelength to be passed can be freely set by an electric signal.

【0014】

When used as a material for a holographic element such as a filter, the function as an element is lost when the recorded hologram is erased, so it is indispensable to perform a fixing operation so that the hologram does not disappear. However, since the conventional potassium niobate tantalate does not contain ions that can move in the crystal, thermal fixing could not be performed. Moreover, since it is not a ferroelectric substance at room temperature, it could not be electrically fixed. The present invention has been made in view of the above prior art, and an object of the present invention is to provide a hologram recording material made of potassium niobate tantalate capable of fixing a hologram.

【0015】

[Means for solving problems]

The constitution of the hologram recording material of the present invention that achieves such an object is in the crystal of potassium niobate tantalum or a compound in which a part of tantalum, niobium or potassium which is a constituent element of the substance is replaced with another element. , The crystal is characterized by containing hydrogen ions.

【0016】

[Action]

Potassium niobate tantalate is ABO₃It is one of the oxides having a perovskite type crystal structure represented by the chemical formula of. Potassium ions are contained in the A site, and tantalum or niobium ions occupy the B site. Since potassium is monovalent in the crystal, it can be replaced with an alkali metal such as lithium which is also monovalent in the crystal. The hologram recording material of the present invention is characterized in that monovalent hydrogen ions (protons, protons) are also contained in a part of the A site to be occupied by potassium ions or at interstitial positions.

【0017】

The process of the fixing operation will be described below. Hologram recording: Reference light and object light are incident on the crystal of the present invention to form space charges due to electrons and holes. Rearrangement of hydrogen ions: Heat treatment of crystals is performed to move hydrogen ions so as to compensate for the space charge. Rearrangement of electrons and holes: Cool to room temperature again and irradiate with uniform light to evenly distribute electrons and holes throughout the crystal. Hydrogen ions hardly move at room temperature, and the space charge due to these hydrogen ions remains as a replica. By the above operation, the life of the hologram is dramatically improved. The operations of and may be performed at the same time. That is, it is an operation in which the temperature is raised from the beginning and hydrogen ions are moved while forming a spatial electric field by electrons and holes.

【0018】

The methods of inserting hydrogen ions into a single crystal of the above-mentioned material can be roughly divided into two methods as described in (a) and (b) below. (a) This is a method in which a single crystal is grown from a melt composed of oxides such as tantalum, niobium, and potassium, and the melt is reacted with hydrogen gas or water vapor. This method is mainly used as a raw material when growing KTN single crystals.₂CO₃, Ta₂O₅And Nb₂O₅Powder is used. Of these, K₂CO₃Water is incorporated into the mixed powder before melting because the powder absorbs moisture well. Hydrogen ions emitted from this water or water taken in from the atmosphere during growth after melting may enter the single crystal. However, depending on the impurities contained in the raw material, hydrogen ions may not enter at all. However, in order to increase the light sensitivity and enhance the effect of the hologram, it is better to dope impurities. Iron, cobalt, and the like are suitable as impurities that do not prevent hydrogen ions from entering the crystal even when mixed with the raw material.

【0019】

(b) This is a method in which hydrogen ions are added to a crystal after growing a single crystal that does not contain hydrogen. The method of injecting hydrogen ions into a single crystal is a method of immersing a crystal in an acidic liquid and exchanging hydrogen ions with A-site ions in the crystal (called a proton exchange method), water vapor or hydrogen. There are a method of heat treatment in a gas containing gas, a method of accelerating the reaction electrochemically, which is similar to these two methods, and a method of injecting accelerated hydrogen ions. On the contrary, it is possible to remove hydrogen ions in the crystal by heat treatment in vacuum. Thereby, the amount of hydrogen ions contained in the single crystal can be adjusted to an appropriate value, and the lifetime of the hologram can be controlled. If there are too many hydrogen ions, the life of the fixed hologram will be shortened.

【0020】

The hologram recording material of the present invention is based on the fact that potassium niobate tantalate contains hydrogen ions, but the potassium niobate tantalate is obtained by substituting a part of its A site and B site with another material. But it's okay. The A site can be replaced with an alkali metal such as lithium, sodium, rubidium, or francium, and the B site can be replaced with vanadium or the like.

【0021】

Crystals of potassium niobate tantalate have a great effect at room temperature if the composition is appropriately selected, but these crystals have a drawback of being easily cracked. Material K in which a part of potassium is replaced with lithium_1-yLi_yTa_1-xNb_xO₃Then, a crystal having a large effect at room temperature and being hard to break can be produced. The same can be expected when it is replaced with another element.

【0022】

In particular, lithium is promising because it can be replaced with potassium in a limited amount, whereas sodium can be replaced with more. Further, in order to increase the sensitivity to light, a small amount of transition elements such as titanium, chromium, manganese, cobalt, nickel, iron, copper and rhodium are usually added.

【0023】

[Example]

Hereinafter, the present invention will be described in detail with reference to the examples shown in the drawings.

【0024】

[Example 1] This example is an example of producing a single crystal hologram recording material. Lithium potassium niobate tantalate (K) with copper added as an impurity_0.95Li₀_.05Ta_0.65Nb_0.35O₃) A single crystal was cut into cubes in which all planes were equivalent to the (100) plane of the crystal. The size is 2 mm x 2 mm x 2 mm. The single crystal was immersed in nitric acid at a concentration of 69% and reacted at 98 ° C for 100 days. As a result, lithium in the crystal was replaced with almost 100% hydrogen near the crystal surface and 50 to 70% hydrogen in the center of the crystal. A platinum film was deposited on the entire crystal surface by a vapor deposition method or a sputtering method to prevent evaporation of hydrogen ions from the surface, and heat treatment was performed at 1000 ° C. for 10 minutes to uniformly distribute hydrogen ions throughout the crystal.

【0025】

[Example 2] This example is an example of producing a single crystal hologram recording material. Using a single crystal having the same composition and size as in Example 1, 200 KeV of accelerated hydrogen ions was implanted into the entire surface of this single crystal. The surface of this was wrapped with a platinum film having a thickness of 1 μm in the same manner as in Example 1, and heat treatment was performed at 1000 ° C. for 10 minutes to uniformly distribute hydrogen ions throughout the crystal.

【0026】

[Example 3] This example is an example of producing a single crystal hologram recording material. Using a single crystal having the same composition and size as in Example 1, this single crystal was heat-treated at 900 ° C. in steam for 24 hours. As a result, the hydrogen ion concentration in the crystal is 10.²⁰cm^-3It is expected to be about.

【0027】

[Example 4] This embodiment relates to fixing a hologram. Figure 1 shows a schematic diagram of hologram fixation. As shown in the figure, the oscillating light of the Ar ion laser with a wavelength of 514.5 nm is split into two laser beams 3 and 4 by a beam splitter (not shown), and these laser beams 3 and 4 form an angle of 20 ° with each other. In this way, the light was incident on the hologram recording material 1 and interfered with the hologram recording material 1. As the hologram recording material 1, the single crystal of Example 3 was used.

【0028】

Hologram recording was completed by light irradiation for about 1 second, and after heat-treating the hologram recording material 1 at 150 ° C. for 5 minutes, this single crystal was irradiated with uniform Ar ion laser light for about 1 second. After completing the fixing operation in this way, as shown in FIG. 1, the hologram recording material 1 was irradiated with only the laser beam 3, and the hologram recording material 1 was diffracted when a voltage of 20 V was applied through the electrode 2. , And we were able to observe the light traveling in the same direction as the laser beam 4.

【0029】

One week later, when the experiment was carried out under exactly the same conditions, no decrease in diffraction efficiency was observed. Further, even if the fixed hologram recording material 1 was irradiated with a laser from an arbitrary direction for 1 hour or more, no deterioration of the hologram was observed.

【0030】

[Example 5] This example is an example of producing a single crystal hologram recording material. The following is an example of growing a single crystal using a high-purity raw material. KTN single crystals were grown by the static cooling method. The raw material is K with a purity of 99.99%₂CO₃, Ta with a purity of 99.99%₂O₅And 99.99% pure Nb₂O₅Powder. These were weighed and mixed so as to have a ratio of 58 mol%, 29.4 mol%, and 12.6 mol%, respectively, placed in a platinum crucible, heated and melted, and the raw materials were reacted well and then cooled. As a result, a single crystal of about 1 cm square can be obtained.

【0031】

FIG. 3 is an infrared transmission spectrum of this sample obtained by measurement with an FTIR (Fourier Transform Infrared Spectorscopy) device. The horizontal axis is the wave number and the vertical axis is the transmittance. 3480cm^-1A nearby peak indicates that there are hydrogen ions (protons) around the oxygen atoms in the crystal. Absorption coefficient at peak is 0.67 cm^-1And the hydrogen ion concentration is 10 from this^-3mol% (10²³m^-3) It turned out to be about.

【0032】

[Example 6] This example is an example of producing a single crystal hologram recording material. In Example 5, impurities were not doped. However, when impurities are not contained, not only the sensitivity to light is poor (the response is slow), but also the amplitude of the spatial electric field of the hologram is small and the diffraction efficiency is small even if saturation is reached after a sufficient time (see Fig. 2). Therefore, the KTN single crystal containing iron (Fe), chromium (Cr), cobalt (Co), and copper (Cu) as impurities was grown by the same static cooling method. The raw material is exactly the same amount of K as in Example 5.₂CO₃, Ta₂O₅And Nb₂O₅In addition to a small amount of Fe₂O₃, Cr₂O₃, Co₃O₄, CuO powders were mixed one by one.

【0033】

It was found that the grown single crystal was colored by impurities and absorbed light of a specific wavelength. The Fe-doped single crystal was yellow when the doping amount was small and reddish-brown when the doping amount was large. The Cr-doped single crystal was almost colorless. The Co-doped single crystal turned yellow in the early stage of growth and blue as the growth progressed. The Cu-doped single crystal was also discolored from yellow to deep green. In general, the greater the coloration and absorption, the better the sensitivity of the photorefractive effect, and in this respect, iron, cobalt, and copper were better. Infrared light transmission spectra were measured with an FTIR device for each of the grown single crystals. As a result, 0.1 mol%, 0.5 mol%, and 1.0 mol% Fe were added to the raw materials, respectively.₂O₃The presence of hydrogen ions could be confirmed in the single crystal grown by adding. Cr₂O₃The same applies when is added.

【0034】

Co₃O₄In the case of, single crystals were grown from the addition of 0.1 mol% and 2.5 mol%, respectively, but both were 3480 cm.^-1No peaks in the vicinity were observed, and no clear evidence of the presence of hydrogen ions was obtained. For those that detected hydrogen ions, 3480 cm^-1From the absorption peak of, the concentration is 10^-3mol% (10²³m^-3) It turned out to be about. From both the color observation and FTIR measurement results, in order to grow a KTN single crystal having good photosensitivity and containing hydrogen ions, it is more preferable to use iron and then cobalt as impurities than to use chromium and copper as impurities. It turned out.

【0035】

[Example 7] This example is an example of producing a single crystal hologram recording material. Unlike Examples 5 and 6 using the static cooling method, KTN single crystals were grown by the TSSG method. The fixing operation of the single crystal hologram recording material of this example was confirmed in Example 8. The raw material is K with a purity of 99.99%₂CO₃, Ta with a purity of 99.99%₂O₅And 99.99% pure Nb₂O₅Powder. Weigh and mix these so that the ratios are 54.4 mol%, 16.1 mol%, and 29.9 mol%, respectively, put them in a platinum crucible, heat and melt them, react the raw materials well, and then soak the seed crystals to remove them. It was chilled.

【0036】

The obtained crystal was cut into a rectangular parallelepiped having a size of 4.96 × 4.20 × 5.64 mm surrounded by a plane parallel to the {100} plane. The composition of the entire crystal differs slightly depending on the part, but as a result of composition analysis by EPMA (Electron Probe Micro Analysis), the composition is almost constant in the cut out rectangular parallelepiped sample, and the molar ratio of K, Ta, and Nb is about 52. It was: 35:13 and was cubic at room temperature. Absorption coefficient at peak is 1.4 cm^-1It was slightly larger than the single crystals of Examples 5 and 6.

【0037】

[Example 8] This embodiment relates to fixing a hologram. FIG. 4 shows a schematic diagram of hologram fixing. This example is different from Example 4 in which the temperature is raised after generating the spatial distribution of carriers such as electrons, in that the temperature is raised from the beginning. Similar to Example 4, the oscillating light of the Ar ion laser having a wavelength of 514.5 nm is split into two laser beams 3 and 4 by a beam splitter (not shown), and these laser beams 3 and 4 form an angle of 16 ° with each other. In this way, the light was incident on the hologram recording material 1 and interfered with the hologram recording material 1. An oscillating electric field was applied to the hologram recording material 1 using a high-voltage AC power supply 7 and an electrode 2.

【0038】

If the vibration of the electric field is made much faster than the growth rate of the hologram, the spatial distribution of carriers can be equated with the state where no electric field is applied. Further, when the modulation amplitude of the refractive index of the hologram is very small, the light intensity I emitted from the hologram recording material 1 and detected by the optical detector 5 is expressed by the following equation. I = I₁(1 + ξn₁) ... (3) Where I₁Is the light intensity before being incident on the hologram recording material 1, E is the externally applied electric field, ξ is the constant determined by the material constant of the hologram recording material 1 and the experimental conditions, n₁Is the spatial electric field E as described above_SCIs proportional to the externally applied electric field E. The positive and negative signs are determined by the direction of the electric field and the like.

【0039】

The applied voltage is E₀The light intensity detected by the optical detector 5 when vibrating with sinωt is expressed by the following equation. I = I₁{1+ (1/2) ξn₀³gE_SCE₀sinωt} ... (4) When this vibration component is extracted by the lock-in amplifier 6, the output is the spatial electric field E._SCIs proportional to. As the hologram recording material 1, the single crystal of Example 7 was used. Since this single crystal is not impurity-doped, the modulation amplitude of the refractive index of the hologram is quite small. The oscillating electric field applied to the hologram recording material 1 had an amplitude of 610 V / cm and a frequency of 1.61 kHz.

【0040】

Figure 5 shows the time change of the measured output of the lock-in amplifier. The temperature of the single crystal was kept at 90 ° C from beginning to end. Normally, in the photorefractive effect, the spatial electric field increases and saturates, as shown in FIG. However, in Fig. 5, after increasing, it does not saturate and decreases again. This is because the hydrogen ions moved slowly so as to compensate for the spatial distribution of the carriers rearranged by the interference fringes of light. Eventually, the hydrogen ions completely compensate for the spatial distribution of the carriers, so that the spatial distribution is apparently almost zero. Figure 6 shows the results of the exact same experiment at 60 ° C. It can be seen that the rate of decrease of the spatial electric field has become very slow. This is a result of the movement of hydrogen ions becoming very slow. Furthermore, when the temperature was lowered, the hydrogen ions hardly moved.

【0041】

FIG. 7 shows the time constants χ of the rising and falling ends of the spatial electric field seen in FIG.₁, χ₂It depends on the irradiation light intensity. The vertical axis is the reciprocal of the time constant (s)^-1), The horizontal axis is the light intensity (mW / cm)²). The circle indicates the rising time constant χ₁In addition, the square mark is the falling time constant χ₂Corresponds to each. Falling time constant χ₁Is light intensity I₀Depends on the falling time constant χ₂Does not depend at all. The rising edge symbolizes the movement of carriers such as photoactive electrons, while the falling edge symbolizes the movement of hydrogen ions, and the hydrogen ions are not sensitive to light. That is, no matter how strong the light is irradiated, the hydrogen ions cannot be moved to eliminate the distribution.

【0042】

[Effect of the invention]

As described above in detail based on Examples, in the present invention, potassium niobate tantalate or a compound in which a part of tantalum, niobium or potassium which is a constituent element of the substance is replaced with another element. Since hydrogen ions are contained in the crystal of No. 1, the fixing operation of the hologram becomes possible, and a holographic element having a practical electric signal control function can be provided.

[Simple explanation of drawings]

[Figure 1]

FIG. 3 is a schematic diagram of hologram recording according to an embodiment of the present invention.

[Figure 2]

It is a graph which shows the example of the time change of the spatial electric field of a normal photorefractive effect, which is related to the prior art.

[Fig. 3]

FIG. 5 is a graph showing an example of an infrared transmission spectrum of a KTN single crystal containing hydrogen ions according to an embodiment of the present invention.

[Fig. 4]

FIG. 5 is a schematic diagram of hologram recording and fixing according to an embodiment of the present invention.

[Fig. 5]

It is a graph which shows the time change of the space electric field at 90 ° C measured by the system of FIG. 4 according to one Example of this invention.

[Fig. 6]

It is a graph which shows the time change of the space electric field at 60 ° C measured by the system of FIG. 4 according to one Example of this invention.

[Fig. 7]

It is a graph which shows the irradiation light intensity dependence of the rising time constant and the falling time constant in the time change of the spatial electric field of FIG. 5 measured by the system of FIG. 4 according to one embodiment of the present invention.

[Explanation of symbols]

1 Hologram recording material 2 electrodes 3,4 laser beam 5 Optical detector 6 Lock-in amplifier 7 High voltage AC power supply