JP2000191398A - Barium titanate single crystal, and optical part and optical recording and reproducing device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a barium titanate single crystal having excellent characteristics such as high refractive index and high transmittance. SOLUTION: A condenser lens 5 consisting of a barium titanate single crystal expressed by the compsn. formula of (Nax,Bay)(Nbx,Tiy)O3 (wherein (x) and (y) satisfy 0.35<=x<=0.40 and y=1-x) is produced and assembled into an optical head 3. In this method, the spot diameter can be decreased to about 1/n×λ/NA (wherein (n) is the refractive index of the condenser lens 5), and the recording density of an optical disk D can be increased by about n2 times.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a barium titanate single crystal used for an electronic device such as an optical component and a piezoelectric element, an optical component using the same, an optical disk, a magneto-optical disk and a DVD (digital video disk). The present invention relates to an optical recording / reproducing apparatus for recording / reproducing information on / from an optical recording medium such as an optical recording medium.

[0002]

2. Description of the Related Art Conventionally, barium titanate (BaTi)
O ₃ ) single crystals have been studied for application to electronic devices having high piezoelectricity and electro-optic effect, and optical components having high refractive index. However, barium titanate single crystals have a high melting point exceeding 1600 ° C. 120 ° C just below the melting point
Because of the phase transition around 0 ° C. and −120 ° C., single crystallization cannot be performed by a simple melting method, and application to polycrystals has been made. Further, as a method for producing a barium titanate single crystal, a flux method or a vapor phase method utilizing a melting point drop phenomenon caused by a composition shift of a melting point has been studied in order to avoid a phase transition.

In order to improve the recording density of an optical recording medium such as an optical disk, a magneto-optical disk, and a DVD, it is necessary to reduce the spot diameter of a recording / reproducing laser beam as much as possible. And the numerical aperture NA of the objective lens, and is approximately 0.8 × λ / NA. Therefore, it is conceivable to shorten the wavelength of the light source. However, compared to the conventional red semiconductor laser, short-wavelength lasers such as a blue semiconductor laser that can continuously oscillate for a long time have not yet become widespread. It is difficult to use a suitable light source. The upper limit of the numerical aperture NA of the objective lens is about 0.7, and if it is larger than that, the optical system becomes unstable. That is, the spherical aberration generated when a laser beam is incident on a flat substrate is NA
When the spherical aberration increases, it becomes difficult to control the focus of the objective lens. Similarly, coma aberration generated when laser light is incident on a substrate inclined with respect to the optical axis increases in proportion to the cube of NA, and when the coma aberration increases, the modulation degree of a signal rapidly decreases.

In order to improve the recording density in addition to the above method, a solid immersion lens (Solid Immersion Lens) has been proposed.
It is proposed to reduce the spot diameter to 1 / n (n is the refractive index of the SIL) by installing a hemispherical lens called an Immersion Lens (hereinafter referred to as SIL) between the optical recording medium and the objective lens. Have been. In this case, the refractive index n of the SIL needs to be about 2.4 or more.

[0005]

However, the method for producing a barium titanate single crystal has a melting point of 160.
Due to the high melting point exceeding 0 ° C., melting using a chemically stable platinum crucible cannot be performed, and a high melting point iridium crucible must be used. However, the crucible made of iridium has problems that components are volatilized during use, so that atmospheric components are limited, and the crucible is expensive.

In addition, when barium titanate single crystal is melted at a low oxygen partial pressure, the titanium component is reduced near the melting point, and metal is precipitated from the oxide to hinder crystallization.
Although it is necessary to increase the oxygen partial pressure, the use of a crucible made of iridium limits the atmospheric components. As a result, the oxygen partial pressure cannot be increased, and single crystallization is difficult. Further, the barium titanate single crystal has a melting point of just below the melting point of 12%.
Due to the phase transition around 0 ° C, 0 ° C, and -120 ° C, single crystallization by a simple melting method cannot be performed, and production by a flux method or a gas phase method is being studied. Therefore, it was difficult to obtain a large single crystal stably. Also, several meters
A barium titanate single crystal of about m also has a problem that it easily becomes cloudy or cracked due to phase transition at room temperature.

[0007] Glass is generally considered as a material for the SIL, but the upper limit of the refractive index of glass is about 1.8, and rare earth glass to which a rare earth element is added in order to obtain a refractive index higher than 1.8. However, even with this rare earth glass, the upper limit of the refractive index was about 2.0.
For this reason, the refractive index of about 2.0 gives an optical limit to the recording density of the optical recording medium.

Accordingly, the present invention has been completed in view of the above circumstances, and an object of the present invention is to provide a barium titanate single crystal having an extremely high refractive index and excellent light transmittance, an optical component using the same, and An object of the present invention is to provide an optical recording / reproducing apparatus.

[0009]

Barium titanate single crystal of the present invention According to an aspect of the composition formula _{(Na x, Ba y) (} Nb x, T
_{i y) O 3 (0.35 ≦} x ≦ 0.45, characterized by being represented by y = 1-x).

The present invention provides a platinum crucible because the melting point is as low as about 1350 ° C. by adding sodium niobate to barium titanate and making the above composition range utilizing the solid solution relationship between barium titanate and sodium niobate. Can be used, and the phase transition hardly occurs, and as a result, the occurrence of cracks due to the phase transition is prevented.

An optical component according to the present invention is characterized by comprising the above-mentioned barium titanate single crystal. This results in an optical component having an extremely high refractive index and excellent light transmittance.

An optical recording / reproducing apparatus according to the present invention comprises: a light source device for irradiating an optical recording medium with recording / reproducing light; an optical head having a converging lens made of the above barium titanate single crystal; And characterized in that: According to the above configuration, the spot diameter is set to 1 with the condenser lens as the SIL.
By narrowing to / n (n is the refractive index of the SIL), the recording density of the optical recording medium can be increased to about n ² times.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below.
Barium titanate single crystal of the present invention (hereinafter, abbreviated as single crystals), the compositional formula _{(Na x, Ba y) (} Nb x, T
i _y ) O ₃ (0.35 ≦ x ≦ 0.45, y = 1−x). When x <0.35, it becomes cloudy due to phase transition, and x>
At 0.45, the difference in refractive index between the upper and lower portions of the grown single crystal exceeds 0.01, and homogeneity is lost. Also, preferably, 0.37 ≦ x ≦ 0.43.

The above composition can be measured by the following analytical method. Na / Ba by wet chemical quantitative analysis
The composition ratio can be determined by specifying the ratio and the Nb / Ti ratio. Further, the single crystal of the present invention is dissolved in a high-purity K ₂ B ₄ O ₇ melt, which is dissolved in an alkali, and the main component ratio can be specified by atomic absorption and ICP emission analysis.

Further, the single crystal of the present invention is manufactured as follows. Raw material powder A of predetermined amount of barium titanate
And raw material powder B of sodium niobate,
_{x, Ba y) (Nb x} , in _{Ti y) O 3 x = 0} .
40 and these raw material powders were 900
Firing at a temperature of not less than 3 ° C. for about 3 hours to produce a polycrystalline raw material powder. Next, the obtained raw material powder is filled in a platinum crucible,
It is melted at 1350 ° C. in an air atmosphere, and a single crystal is grown by the Czochralski method (rotary pulling method). At this time, the single crystal is pulled at a rotation speed of 5 to 10 rpm and a pulling speed of 1 mm or less per hour. At the time of pulling, the temperature gradient is set so that the temperature gap between the molten liquid surface and the height of 2 cm from the liquid surface is 60 ° C. or more, and the seed crystal is set so that the <111> direction is the pulling direction. .

When the pulling speed is more than 1 mm / hour and / or the temperature gap is less than 60 ° C., a large amount of bubbles are generated in the single crystal and the single crystal becomes cloudy.
m / hour or less and / or a temperature gap of 60 ° C. or more are preferred. Since the barium titanate and sodium niobate are continuous solid solutions, single crystallization can be performed over a wide temperature range.However, by setting the composition range of the present invention, a stable phase transition does not occur at room temperature or higher. .

FIG. 1 shows a basic structure of an optical head 3 and an optical recording / reproducing apparatus using a condensing lens made of a single crystal according to the present invention.
FIG. 2 is a sectional view taken along line AA of the optical head 3 and an optical disk D as an optical recording medium. An optical disc D having a predetermined reflective layer, a phase-change type recording layer, a magneto-optical recording layer, or the like as the recording layer 2 formed on the surface of the substrate 1 has an optical wavelength of about 400 to about 800.
The laser beam L of nm is condensed by the optical head 3 and is radiated to a desired spot diameter. Thus, information is written to and read from the recording layer 2 of the optical disc D.

For example, in a magneto-optical disk, information is written as follows. The laser beam L is condensed by an objective lens 4 which is a convex lens, and is further narrowed down to a predetermined spot diameter (about 1 / n × λ / NA) by a hemispherical condenser lens (SIL) 5 which focuses on a lower surface 5a. Can be Then, the recording layer 2 in the spot of the laser beam L applied to the optical disk D is heated, and a modulation magnetic field is applied by the magnetic field modulation coil 6 until the heated area is cooled. In the above, digital information (0, 1) is obtained by changing the magnetic polarity of the recording layer 2 in either the up or down direction.
Record. In FIG. 1, reference numeral 7 denotes an arm for holding the optical head 3.

The condenser lens 5 of the present invention is made of a single crystal having the above composition range. Thereby, a high refractive index of about 2.10 to 2.15 is obtained, and the spot diameter is reduced to 1 / n × λ / NA.
And the recording density of the optical disc D can be reduced to n ²
It is possible to increase it about twice. Also, the condenser lens 5
Is made to be a super hemisphere closer to a true sphere than a hemisphere, the spot diameter can be reduced to about λ / n ² , and the recording density of the optical disc D can be increased to about n ⁴ times.

The single crystal of the present invention includes various optical components such as a prism, a triangular prism, a beam splitter, a polarizing beam splitter, a mirror, an optical waveguide substrate, a piezoelectric element, an optical direction control element, an optical phase control element, and a light refraction. It can be applied to various light modulation elements such as a rate control element.

The optical recording medium of the present invention is for recording or reproducing digital information optically or magneto-optically, such as an optical disk such as a CD (compact disk), a phase change optical disk, a DVD, or a magneto-optical disk. . As a light source of the optical recording / reproducing apparatus, a semiconductor laser, various solid-state laser devices such as a YAG laser, a liquid laser device, a gas laser device such as an Ar laser, a carbon dioxide gas laser, and a He—Ne laser can be used.

Thus, the present invention provides a single crystal and an optical component having an extremely high refractive index and excellent light transmittance, and as a result, an optical recording / reproducing apparatus having an optical recording medium with a high recording density can be obtained. Has an effect.

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.

[0024]

Embodiments of the present invention will be described below.

Example 1 A single crystal of the present invention was produced by the following steps [1] and [2].

[1] A raw material powder A of barium titanate is mixed with 9
34.4 g and 43 parts of raw material powder B of sodium niobate
Were mixed with _{7.6g, (Na x, Ba y} ) (Nb x, T
i _y ) Prepared so that x = 0.40 in O ₃ , and fired these raw material powders at 950 ° C. to produce polycrystalline raw material powders.

[2] The obtained raw material powder was filled in a platinum crucible, melted at 1350 ° C. in the air atmosphere, and a single crystal was grown by the Czochralski method (rotary pulling method). At this time, the single crystal was pulled at a rotation speed of 5 rpm and a pulling speed of 1 mm per hour. At the time of lifting, a temperature gradient is set so that the temperature gap between the molten liquid surface and a height of 2 cm from the liquid surface is 60 ° C. or more,
The seed crystal was set so that the <111> direction was the pulling direction. The obtained single crystal is φ (diameter) 50 mm, length 1
00 mm. In this case, the lifting speed is 1 mm /
If over time and / or the temperature gap is less than 60 ° C.,
In each case, a large amount of air bubbles were generated in the single crystal and became cloudy.

This single crystal ingot is sliced and cut into a cube of 1.2 mm × 1.2 mm × 1.2 mm square, and then subjected to spherical polishing using SiC abrasive grains to form a hemispherical 0.1 mm diameter. SIL was prepared. Next, when a laser beam of a semiconductor laser having an optical wavelength of 633 nm is incident on the optical head 3 composed of the objective lens 4 and the SIL of the present embodiment, the spot diameter of the focal point is 30 which is about half of the optical wavelength.
It became 0 nm or less.

(Example 2) In step [1] of Example 1, when mixing the raw material powder A and the raw material powder B, the molar ratio of sodium niobate to barium titanate was 3
Twenty types of raw material powders were prepared at 0 mol% to 50 mol% and set at every 1 mol%. Next, the process [2] of Example 1
In the same manner as described above, 20 types of single crystals were grown. FIG. 3 shows the results of measuring the difference in the refractive index between the upper part and the lower part of these single crystals.

As shown in the figure, the molar ratio is 40 ± 5.
If it exceeds mol%, the refractive index difference between the upper and lower portions of the single crystal becomes 0.0
Above 1, homogeneity was lost. Further, when the molar ratio is less than 35 mol%, the single crystal is easily turbid due to phase transition, and when the molar ratio is 30 mol% or less, the whole becomes completely turbid.

[0031]

According to the present invention, by using a barium titanate single crystal having a predetermined composition range, a barium titanate single crystal which has almost the same characteristics as a pure barium titanate single crystal, which has been difficult to produce conventionally, can be easily produced. Thus, a barium titanate single crystal having a high refractive index and a high light transmittance can be obtained. In addition, the barium titanate single crystal of the present invention has high piezoelectricity and electro-optical effect, has no phase transition from the melting point to room temperature, and can be easily manufactured by a general Czochralski method, and thus has excellent mass productivity. Further, since it is cubic, there is no optical anisotropy, so that it is a material suitable for optical components.

Further, the barium titanate single crystal of the present invention has a high refractive index n of 2.10 or more, and as a result, it has an optical head using a converging lens made of the barium titanate single crystal of the present invention. In the case of the optical recording / reproducing apparatus, there is an operational effect that the recording density of the optical recording medium can be increased to about n ² times.

[Brief description of the drawings]

FIG. 1 is a perspective view of a basic configuration of an optical recording / reproducing apparatus according to the present invention.

FIG. 2 is a sectional view of an optical head and an optical recording medium of the present invention.

FIG. 3 shows the barium titanate single crystal of the present invention using N
5 is a graph showing the relationship between the amounts of a and Nb added and the refractive index.

[Explanation of symbols]

 1: substrate 2: recording layer 3: optical head 4: objective lens 5: condenser lens (SIL) 5a: lower surface of the condenser lens 6: magnetic field modulation coil 7: arm for holding the optical head D: optical disk L: laser beam

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4G077 AA02 BC42 5D075 CD17 5D119 AA22 AA43 JA02 JA43 JB03 NA05 NA10

Claims (3)

[Claims] 1. A composition formula _{(Na x, Ba y) (} Nb x, T
i _y ) A barium titanate single crystal represented by O ₃ (0.35 ≦ x ≦ 0.45, y = 1−x). 2. An optical component comprising the barium titanate single crystal according to claim 1. 3. A light source device for irradiating recording / reproducing light to an optical recording medium, an optical head having a converging lens made of barium titanate single crystal according to claim 1, and an optical recording medium. An optical recording / reproducing apparatus characterized by the following.