JP5717349B2 - Light modulation material and method for producing the same - Google Patents

Description

The present invention relates to an optical modulation material used for optical modulation, optical switching, an isolator and the like in the field of information communication and a method for manufacturing the same. Specifically, the present invention relates to a transparent light modulation material made of a composite material of Sr _x Ba _1-x Nb ₂ O ₆ crystal and glass and a method for producing the same.

In recent years, an external optical modulator using a transparent oxide ferroelectric as a base material has been put into practical use in the field of optical communication. These base materials are typically lithium niobate. These operating principles stem from the electro-optic effect of crystals that do not have inversion symmetry. By applying a modulated electric signal to the base material, the refractive index in the base material changes, and the phase and intensity of the transmitted light are changed.

A single crystal material is generally used because an oxide ferroelectric that is a base material of a conventional external light modulator requires transparency. However, it is not easy to impart a good-quality waveguide structure to a single crystal material, and as a result, a sufficient refractive index change cannot be provided unless the action length is lengthened, which is disadvantageous for cost reduction and miniaturization of the element. It is.

In recent years, tungsten bronze-type SBN (Sr _x Ba _1-x Nb ₂ O ₆ ) crystals have attracted attention as a material that far exceeds the electro-optic effect of lithium niobate (see, for example, Non-Patent Documents 1 and 2). In SBN crystals, the Curie point changes according to the Sr / Ba ratio, and when x = 0.61, the dielectric constant near room temperature is maximized. However, since an SBN crystal requires a high technology for a single crystal growth process and it is difficult to grow a large crystal, an inexpensive manufacturing method has not been found.

  As methods for obtaining transparent crystals other than single crystals, synthesis of ceramics by solid phase reaction and synthesis method by crystallization of glass have been proposed. These synthesis methods are simpler and less expensive than single crystal fabrication. The following literature is reported as an example of forming SBN crystals. (Non-patent documents 3 to 5, Patent document 1)

Japanese Patent No. 4166362

S. Takekawa, Y. Furukawa, M. Lee and K. Kitamura; “Double crucible Stepanov technique for thegrowth of striation-free SBN single crystal”, J. Crystal Growth, 229 (2001) 238. S. Takekawa, Y. Furukawa, N. Kaneko and K. Kitamura; “Single Crystal Growth of SBN by the Floating Zone Method”, J. Crystal Growth, 229 (2001) 212. AR Kortan et al., Novel ferroelectric ceram-glass of [(SrO) (y) (BaO) (1-y) (Nb2O5)] (1-x) tellurite, J. Mater. Res., Vol. 17, No. 5 , 1208, 2002. Jiin-Jyh Shyu et al., Crystallization and Dielectric Propertiesof SrO-BaO-Nb2O5-SiO2 Tungsten-BronzeGlass-Ceramics, J. Am. Ceram. Soc., 83 3135, (2000). Nobuko S. VanDamme et al., Fabrication of optically Transparent and Electro-optic Strontium Barium Niobate Ceramics, J. Am. Ceram. Soc., 74 1785 (1991).

However, in order for the obtained SBN crystal to achieve optical transparency comparable to that of a single crystal and to exhibit an electro-optic effect, (1) the volume fraction of the crystal constituent oxide is large, and (2) the size of the crystal Is smaller than the wavelength of light, and (3) it is important to reduce the difference in refractive index between the crystal and the glass layer. That is, it is necessary to control the process management of the glass composition and crystallization.

In Patent Document 1, there is a report that a translucent crystallized glass was obtained in terms of mol% in 12.5SrO-12.5BaO-8Nb ₂ O ₅ -67B ₂ O ₃ . In addition, there is a report that a transparent crystallized glass was obtained even with the TeO ₂ -based crystallized glass of Non-Patent Document 3. However, the reason why these crystals are transparent is that the volume fraction of the crystals is small, and with this, it is difficult to exhibit a sufficient electro-optic effect. In Non-Patent Document 5, birefringence is measured with an SBN sintered body whose volume fraction is almost 100. However, in such an SBN sintered body, it is extremely difficult to completely remove voids existing in grain boundaries that cause scattering and improve transparency.

As described above, in the prior art, a material having high transparency and a large volume fraction of SBN crystal that can withstand the practical use of an optical modulator has not been found. Therefore, an object of the present invention is to provide a transparent light modulation material comprising a composite material of Sr _x Ba _1-x Nb ₂ O ₆ (0.25 ≦ x ≦ 0.75) crystal and glass, and a method for producing the same.

The present inventors have found that a transparent light modulation material can be obtained by using a mother glass that precipitates Sr _x Ba _1-x Nb ₂ O ₆ (0.25 ≦ x ≦ 0.75) crystal as a raw material. It is proposed as an invention.
That is, in the present invention, the following 1. ~ 6. The configuration is adopted.
1. In a glass composed of an oxide having the chemical composition formula aSrO-bBaO-cNb ₂ O ₅ -dB ₂ O ₃ and a + b + c + d = 100 and (a + b + c)> 60, Tungsten bronze type Sr _x Ba _1-x Nb ₂ O ₆ (0.25 ≦ x ≦ 0.75) light modulation material composed of glass ceramics that exists uniformly over the entire glass region, the tungsten bronze in the glass ceramic Type Sr _x Ba _1-x Nb ₂ O ₆ (0.25 ≦ x ≦ 0.75) crystal has a volume fraction of 20 to 75%, the crystal has an average particle size of 0.1 to 60 nm, and the wavelength at 1550 nm A light modulation material characterized in that the light transmittance of glass ceramics is 50% or more.
2. 1. Electrodes for applying an electric field are provided on both side surfaces of a region where light of the glass ceramics travels . The light modulation material described in 1.
3. 1. The electrode is formed of a metal thin film. The light modulation material described in 1.
4). A glass composed of an oxide having a chemical composition formula aSrO-bBaO-cNb ₂ O ₅ -dB ₂ O ₃ and a + b + c + d = 100 and (a + b + c)> 60, Heat treatment is performed at a temperature not lower than a transition temperature of −80 ° C. and lower than a crystallization start temperature until the light transmittance of the glass ceramic at a wavelength of 1550 nm becomes 50% or higher. The manufacturing method of the light modulation material as described in any one of Claims 1-3.
5. Furthermore, electrodes for applying an electric field are provided on both side surfaces of a region where light travels in glass ceramics. The manufacturing method of the light modulation material as described in any one of Claims 1-3.
6). 4. The electrode is provided by forming a metal thin film on both side surfaces of a region where light travels in glass ceramics . The manufacturing method of the light modulation material as described in any one of Claims 1-3.

  According to the present invention, it is possible to obtain a light modulation material having high transparency, a large volume fraction of a crystal constituent oxide, and a small difference in refractive index between a glass layer and an SBN crystal. Conventional SBN crystallized glass is often opaque and unsuitable for use as an electro-optic material. The light modulation material of the present invention has light transmittance comparable to that of a single crystal and can be manufactured at low cost, and therefore can be widely used for applications such as light modulation, light switching, and isolator.

It is a figure which shows the differential thermal analysis (DTA) pattern of the bulk body A of the glass produced by manufacture example 1. FIG. It is a figure which shows the X-ray-diffraction pattern about the sample D obtained by heat-processing the sample A of Example 1, and the sample A. FIG. It is a figure which shows the X-ray-diffraction pattern about the sample B of Example 2, and the samples JO obtained by heat-processing the sample B. FIG. It is a figure which shows the X-ray-diffraction pattern about Sample C of Example 3, and Sample PU obtained by heat-processing Sample C. 2 is a transmission electron micrograph of sample N powder obtained in Example 2. FIG. In Example 4, it is a schematic diagram which shows the system which measures the light modulation characteristic of the produced light modulation material. In Example 4, it is a figure which shows the result of the light modulation measurement of the sample B and the sample M.

The light modulation material of the present invention has a chemical composition formula aSrO-bBaO-cNb ₂ O ₅ -dB ₂ O ₃ , a + b + c + d = 100 and (a + b + c)> 60 Further, it can be manufactured by performing a heat treatment. In the present invention, the “mother glass” means a glass that crystallizes by heat treatment and precipitates a target crystal.
When this mother glass is heat-treated, glass ceramics in which tungsten bronze type Sr _x Ba _1-x Nb ₂ O ₆ (0.25 ≦ x ≦ 0.75) crystals are present uniformly over the entire glass can be obtained.

The method for producing the mother glass is not particularly limited as long as it is a practical method for producing a glass material. However, in view of mass production and manufacturing costs, a melt quench method in which a glass body is obtained by rapidly cooling a glass melt is particularly preferable.
Examples of the raw material include inorganic salts containing strontium, barium, niobium, and boric acid (particularly, oxides and carbonates are preferred), organometallic compounds, and composite oxides such as SBN crystal powder. However, the type of the reagent is not particularly limited as long as the reagent can be adjusted to have a desired composition.

Although the above Patent Document 1 discloses the production of crystallized glass by adding a nucleating agent material, it can be easily assumed that crystallization can be promoted by adding a generally known nucleating agent. In addition, ions known as nucleating agents dissolve in the crystal, and as a result, the electrical characteristics inherent in the crystal may not be exhibited. Moreover, it is not preferable to add oxides other than the oxides (SrO, BaO, Nb ₂ O ₅ ) constituting the target crystal to the crystallized glass except for the glass constituting oxides (B ₂ O ₃ ). .

  The melting of the glass raw material is not particularly limited as long as the temperature is equal to or higher than the liquid phase of the SBN crystal. Considering homogenization of the melt and volatilization of boric acid, a suitable melting temperature is generally 1000 to 1450 ° C. When the melting temperature is higher than 1450 ° C., boric acid is volatilized and it is difficult to obtain a homogeneous glass.

As the glass composition, a composition in which an SBN crystal exhibiting high transparency and sufficiently exhibiting an electrooptic effect is precipitated is preferable. aSrO-bBaO-cNb ₂ O ₅ -dB ₂ O ₃ and when a + b + c + d = 100, 60 ≤ (a + b + c) <
A composition of 80, 20 ≦ d ≦ 40 is preferable (the numerical values of a, b, c, d correspond to the mol% representation of the glass composition). In particular, a composition of 25 ≦ d ≦ 38 is preferable. If d is less than 25, vitrification becomes difficult. On the other hand, if d is larger than 38, vitrification is easy, but the volume fraction of the SBN crystal is small, and the action length of the optical modulator may be long.

The composition range of Sr _x Ba _1-x Nb ₂ O ₆ (0.25 ≤ x ≤ 0.75) has a tungsten bronze structure and high electro-optic coefficient because the Sr / Ba ratio of the mother glass and the Sr / Ba ratio of the crystal are almost equal. Is preferred. A composition in the vicinity of x = 0.61 in which the electro-optic coefficient is highest is particularly preferable.

In order to obtain a transparent SBN crystal from glass, it is necessary to perform heat treatment in a temperature region in which crystal grains are difficult to grow. This is because the light transmission is reduced unless the crystal grain size is smaller than the wavelength of light. The crystal grain size (average particle size) is preferably about 0.05 to 100 nm, and particularly preferably about 0.1 to 60 nm.
Further, the volume fraction of crystal grains in the sample after the heat treatment is preferably 20 to 75%, particularly preferably 25 to 70%. When the volume fraction is less than 20%, the amount of change in the refractive index due to the electro-optic effect becomes remarkably small, so that the action length of the optical modulator may be increased. On the other hand, if the volume fraction exceeds 75%, the difference in refractive index from the remaining glass becomes large, which may increase light scattering.

In order to achieve such crystallization, the heat treatment temperature is preferably in the region from the glass transition temperature −80 ° C. of the mother glass to the crystallization start temperature. Particularly preferred is a temperature range from the crystallization start temperature −60 ° C. to the crystallization start temperature. Heat treatment is preferably performed at a constant temperature in this temperature range. The heat treatment time is not particularly limited, but generally requires 10 to 72 hours to obtain a sufficient volume fraction. Through the above steps, crystallized glass with high transparency can be obtained.

  In order to fabricate the optical modulator, as shown in FIG. 6, the electrodes 2 and 2 are formed on both side surfaces of the region (optical waveguide) where the light of the light modulation material 1 made of crystallized glass travels. Then, by applying a voltage having a desired modulation signal to this optical modulator, the optical modulator can be optically modulated. In order to provide a homogeneous electrode on the light modulation material 1, it is preferable to form a conductive metal thin film such as gold, platinum, silver, copper, nickel, chromium, etc. by sputtering, coating, vacuum deposition or the like. By entering linearly polarized light into the optical modulator through the polarizer 4 and providing an analyzer 7 orthogonal to the polarization direction at the output end, a phase modulator can be manufactured. Modulation can be detected by changing the phase difference of incident light due to the application of an electric field.

EXAMPLES Next, although this invention is demonstrated in detail based on an Example, this invention is not limited to this Example. In the following examples, DTA (differential thermal analysis) was measured by a conventional method using “TG8120” manufactured by Rigaku Corporation. X-ray diffraction (XRD) was measured using “M03XHF22” manufactured by Mac Science.
(Production Example 1: Production of mother glass)
SrCO ₃ , BaCO _3,
Nb ₂ O ₅ and B ₂ O ₃ are used as raw materials and weighed so as to be 16SrO-16BaO-32Nb ₂ O ₅ -36B ₂ O ₃ (Sample A) in mol%, and melted at 1350 ° C for 30 minutes did. After press-cooling the melt, it was confirmed by an X-ray diffraction (XRD) method that all compositions were in a glass state. The thermal properties of the obtained mother glass were measured by DTA (differential thermal analysis). FIG. 1 shows the DTA pattern of the bulk material of glass A produced. In this glass A, the glass transition temperature _Tg was 599 ° C., and the crystallization start temperature Tx was 669 ° C.

(Production Examples 2 and 3)
Transparent mother glass B (Production Example 2) and C (Production Example 3) were obtained in the same manner as in Production Example 1 except that the composition of the raw materials was changed as shown in Table 1. The thermophysical properties of the obtained mother glass are shown in Table 1, and the DTA pattern is shown in FIG.

Glasses A to C obtained in the above Production Examples 1 to 3 were all confirmed in the temperature range of 650 to 680 ° C. by differential thermal analysis derived from crystallization of Sr _x Ba _1-x Nb ₂ O _5. It has an exothermic peak. (See Figure 1)

(Example 1: Production of crystallized glass)
The glass A produced in Production Example 1 was polished. Next, the glass A was subjected to a crystallization treatment by performing a heat treatment for 10 hours at a temperature shown in Table 2 from the vicinity of the glass transition temperature to the crystallization temperature. The thermophysical properties of the obtained samples D to I are shown in Table 2, and the X-ray diffraction pattern is shown in FIG. The numbers in parentheses in the figure indicate the Miller index of Sr _0.5 Ba _0.5 Nb ₂ O ₆ crystal.
The diffraction peak positions confirmed in the samples F to I were the same as those of the known Sr _0.5 Ba _0.5 Nb ₂ O ₆ crystal (ICSD # 240388). When the lattice constant of the crystal was estimated using the Scherrer equation, it was found to have an average particle diameter of 20 to 40 nm. In the samples of D and E having a low heat treatment temperature, crystallization was insufficient with the heat treatment for 10 hours, but the crystallization proceeded in the same manner as the samples of F to I by further extending the heat treatment time.

(Example 2)
In Example 1, the glass B obtained in Production Example 2 was used in place of the glass A. The glass B was subjected to crystallization treatment by performing a heat treatment for 10 hours at a temperature shown in Table 3 from the vicinity of the glass transition temperature to the crystallization temperature. The thermophysical properties of the obtained samples J to O are shown in Table 3, and the X-ray diffraction pattern is shown in FIG. The numbers in parentheses in the figure indicate the Miller index of Sr _0.61 Ba _0.39 Nb ₂ O ₆ crystal.
The diffraction peak positions confirmed in the samples of L to O were the same as the known Sr _0.61 Ba _0.39 Nb ₂ O ₆ crystal (ICSD # 240389). When the lattice constant of the crystal was estimated using the Scherrer equation, it was found to have an average particle diameter of 20 to 40 nm. In the samples of J and K having a low heat treatment temperature, crystallization was insufficient with the heat treatment for 10 hours, but the crystallization proceeded in the same manner as the samples L to O by further extending the heat treatment time.

(Example 3)
In Example 1, the glass C obtained in Production Example 3 was used in place of the glass A. Glass C was subjected to a crystallization treatment by performing a heat treatment for 10 hours at a temperature shown in Table 4 from the vicinity of the glass transition temperature to the crystallization temperature. The thermophysical properties of the obtained samples P to U are shown in Table 4, and the X-ray diffraction pattern is shown in FIG. The numbers in parentheses in the figure indicate the Miller index of Sr _0.75 Ba _0.25 Nb ₂ O ₆ crystal.
The diffraction peak positions of the samples R to U were partially confirmed by diffraction derived from SrNb ₂ O ₆ (marked with ● in the figure), but otherwise known Sr _0.75 Ba _0.25 Nb ₂ O ₆ crystals (ICSD # 15614) Was the same. When the lattice constant of the crystal was estimated using the Scherrer equation, it was found to have an average particle diameter of 20 to 40 nm. In the samples of P and Q having a low heat treatment temperature, the crystallization was insufficient with the heat treatment for 10 hours, but the crystallization proceeded in the same manner as the RU samples by further extending the heat treatment time.

According to each of the above examples, the average particle diameter of the crystals in samples I, O, and U obtained by heat-treating each of the glasses A, B, and C at the respective crystallization start temperatures (Tx) exceeds 60 nm. It was found that the particle size increases with increasing heat treatment temperature. FIG. 5 is a transmission electron micrograph taken using “JEM-2010” manufactured by JEOL Ltd. for the sample N powder subjected to the crystallization treatment in Example 2. From FIG. 5, it can be seen that one particle is composed of an elliptical Sr _0.61 Ba _0.39 Nb ₂ O ₆ crystal single phase of about 34 nm in the longitudinal direction and about 16 nm in the minor axis direction. The other samples D to U were evaluated in the same manner, and it was confirmed that the average grain size of the crystals was approximately the same as that obtained using the Scherrer equation.

(Measurement of optical properties of crystallized samples)
The light transmittance of each sample crystallized by heat treatment in Examples 1 to 3 was measured with a visible / near infrared spectrophotometer ("Impact 400" manufactured by Nicorey Japan). The sample size was 10 mm × 10 mm × 1 mm thickness, and a sample subjected to mirror polishing was measured. Tables 2 to 4 show light transmittances at wavelengths of 650 nm and 1550 nm per 1 mm of the sample. Since this value includes the loss on the surface due to Fresnel reflection, the actual transmittance can be predicted to be larger than this. As the size of the crystal grains increased as the heat treatment temperature increased, the transparency was lowered in the visible region due to the influence of light scattering. However, in the vicinity of 1550 nm which is the communication wavelength band, it showed transparency exceeding 60%, and it was found that it has the necessary characteristics as a light modulation material.

(Example 4: Formation of electrodes and measurement of light modulation characteristics)
Sample B used as a raw material in Example 2 and Sample M crystallized by heat treatment were cut into 1 mm × 1.5 mm × 10 mm. After all surfaces were mirror-polished, a silver paste was applied to both sides of 1.5 mm × 10 mm and dried to form thin film electrodes.
FIG. 6 is a schematic diagram showing a system for measuring the light modulation characteristics of the produced light modulation material. In this system, the electrodes 2 and 2 of the sample (light modulation material) 1 obtained above were connected to a high-voltage AC power source 3. In addition, a linearly polarized semiconductor laser 5 (wavelength 650 nm) that passes through the polarizer 4 is introduced from one of the 1 mm × 10 mm surfaces of the sample 1 and enters a crossed Nicols state through the quarter-wave plate 6 and the analyzer 7. Arranged. The applied direction of the electric field and the vibration direction of the electric field vector of the linearly polarized light are arranged to be inclined by 45 °. The amount of light transmitted through the analyzer 6 was detected by a Si photodiode 8, amplified by a signal amplifier 9, and a change in output light intensity due to application of an electric field was measured with an oscilloscope 10.

FIG. 7 shows the results of light modulation measurement of samples B and M. This figure shows a change in output light intensity when a ± 3 kV sine wave is applied at 1 kHz with respect to a distance between electrodes of 1 mm. In sample B that had not been crystallized, no change in strength appeared. In contrast, the crystallized sample M showed a 2 kHz waveform that was twice 1 kHz, indicating that phase modulation due to the electro-optic effect occurred.
Similarly, when light modulation measurement was performed on each sample obtained in Examples 1 to 3, a clear change in retardation was similarly shown. When the electric field induced birefringence was quantified by the Senarmon method, the effective value of the electro-optic coefficient was about 1.5 pm / V.

According to the present invention, glass ceramics in which tungsten bronze SBN crystals having a crystal diameter of several tens of nanometers are uniformly dispersed in glass can be obtained. In the present invention, the scattering loss is reduced by making the crystal diameter of the SBN polycrystal smaller than the wavelength of light, and even when it becomes a nanocrystal, the electro-optic coefficient can be expressed as an effective value of several pm / V. It was.
A conventional light modulation material using single crystal glass does not respond at all to light orthogonal to the polarization axis. In contrast, the light modulation material of the present invention can eliminate the polarization dependence of the conventional light modulation material.

  The light modulation material obtained by the present invention is suitably used for optical components such as an optical modulator, an optical switching element, a waveguide, and an attenuator.

1 Sample (light modulation material)
2, 2 Thin-film electrode 3 High-voltage AC power supply 4 Polarizer 5 Semiconductor laser 6 1/4 wavelength plate 7 Analyzer 8 Photodiode 9 Signal amplifier 10 Oscilloscope

Claims (6)

In a glass composed of an oxide having the chemical composition formula aSrO-bBaO-cNb ₂ O ₅ -dB ₂ O ₃ and a + b + c + d = 100 and (a + b + c)> 60, Tungsten bronze type Sr _x Ba _1-x Nb ₂ O ₆ (0.25 ≦ x ≦ 0.75) light modulation material composed of glass ceramics that exists uniformly over the entire glass region, the tungsten bronze in the glass ceramic Type Sr _x Ba _1-x Nb ₂ O ₆ (0.25 ≦ x ≦ 0.75) crystal has a volume fraction of 20 to 75%, the crystal has an average particle size of 0.1 to 60 nm, and the wavelength at 1550 nm A light modulation material characterized in that the light transmittance of glass ceramics is 50% or more. 2. The light modulation material according to claim 1, wherein electrodes for applying an electric field are provided on both side surfaces of the region of the glass ceramic where light travels . The light modulation material according to claim 2 , wherein the electrode is made of a metal thin film . The chemical composition formula is aSrO-bBaO-cNb ₂ O _Five -dB ₂ O _Three Thus, a glass composed of an oxide with a + b + c + d = 100 and (a + b + c)> 60 is lower than the crystallization start temperature at a temperature not lower than a glass transition temperature of −80 ° C. or lower. The method for producing a light modulation material according to claim 1, wherein heat treatment is performed until the light transmittance of the glass ceramic at a wavelength of 1550 nm becomes 50% or more at a temperature. 5. The method for producing a light modulation material according to claim 4 , further comprising providing an electrode for applying an electric field on both side surfaces of a region in the glass ceramic where light travels . 6. The method of manufacturing a light modulation material according to claim 5, wherein the electrode is provided by forming a metal thin film on both side surfaces of a region where light travels in the glass ceramic .

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