JP4882737B2 - Liquid crystal element and optical head device - Google Patents

Description

  The present invention relates to a liquid crystal element, and more particularly to a liquid crystal element mounted on an optical head device that records and reproduces information on an optical recording medium such as an optical disk.

  A DVD, which is an optical disk, records digital information at a higher density than a CD. For this reason, an optical head device for reading / writing a DVD includes a 650 nm band semiconductor laser having a wavelength shorter than the CD light source wavelength (780 nm band) and the numerical aperture (NA = 0.45) of the CD objective lens. The diameter of the light spot collected on the optical disk surface is reduced by using an objective lens having a larger numerical aperture (NA = 0.6).

  More recently, an optical disk has been introduced that can obtain a higher recording density by setting the wavelength of light emitted from a light source to about 400 nm and NA larger than 0.6 (Blu-ray, HD-DVD). Here, laser light having a wavelength of 390 to 500 nm is defined as blue laser light. However, as the light source has a shorter wavelength and the objective lens has a higher NA, the tilt angle at which the optical disk surface is tilted from the right angle with respect to the optical axis and the allowable amount of thickness unevenness of the optical disk are required to be small.

  The reason why these tolerances become smaller is that coma aberration occurs when the optical disk is tilted, and spherical aberration occurs when the optical disk is uneven in thickness. This is because deterioration is likely to occur and the accuracy of signal reading is reduced. For this reason, it has been proposed to cancel these aberrations by installing a liquid crystal element in the optical path and changing the wavefront shape of the transmitted light (aberration correction element).

  Further, the liquid crystal element can change the amount of light reaching the optical disk surface without changing the output of light from the light source. For example, there has been proposed a method of changing the amount of light after passing through a polarizing beam splitter by combining a liquid crystal element that changes a polarization state that is transmitted by applying a voltage and a polarization separating element such as a polarizing beam splitter. .

  Here, a configuration example in which a liquid crystal element is disposed between the light source and the polarization beam splitter will be described. A beam splitter that transmits almost 100% of light in a certain linear polarization direction (a direction) and reflects almost 100% of light in a linear polarization direction (b direction) orthogonal to the a direction is used. When the voltage applied to the liquid crystal element is adjusted so that the light transmitted through the liquid crystal element is linearly polarized in the a direction, the light incident on the polarization beam splitter is transmitted almost 100% and the light intensity is not substantially changed. In addition, when the voltage applied to the liquid crystal element is adjusted so that the polarization state of the light transmitted through the liquid crystal element is circularly polarized light, the light incident on the polarization beam splitter is transmitted about 50% and the light intensity is also about 50%.

Thus, by changing the voltage applied to the liquid crystal element, the polarization state of the transmitted light can be changed, and the light intensity after passing through the polarizing beam splitter can be changed (light quantity variable element).

  The liquid crystal elements used in these optical head devices have a horizontal alignment type in which liquid crystal molecules are approximately horizontal alignment (homogeneous alignment) with respect to the substrate in an initial (no electric field applied) state, and an initial (no electric field applied) state. There is a vertical alignment type in which the liquid crystal molecules are approximately vertical alignment (homeotropic alignment) with respect to the substrate. A horizontal alignment type liquid crystal element uses nematic liquid crystal having positive dielectric anisotropy, and is a mode in which liquid crystal molecules aligned horizontally on a substrate are aligned in the direction perpendicular to the substrate by an applied voltage, specifically TN (twisted nematic) mode, homogeneous alignment mode, etc. are used.

  A liquid crystal element used for such an application usually has a configuration in which a liquid crystal layer is sealed between a pair of substrates provided with an alignment film. As the alignment film, a rubbed polyimide film is generally used. However, when laser light having a wavelength of 500 nm or less is incident on a liquid crystal element composed of the polyimide alignment film, the liquid crystal element is damaged by light. . As a cause of this, it is known that the polyimide film is deteriorated by irradiation with light of a short wavelength and affects the orientation of the liquid crystal.

  Therefore, a method for forming an alignment film of a liquid crystal element used for laser light having a wavelength of 500 nm or less with an inorganic material such as silicon oxide has been proposed and implemented. This method is an alignment film that aligns vapor deposition molecules in one direction by supplying an inorganic material such as silicon oxide from a predetermined oblique direction with respect to the normal line of the substrate, and is called an oblique vapor deposition method. Unlike the polyimide film, the obliquely deposited alignment film is rubbing-free, so that no dust is attached or damaged, and since it is an inorganic alignment film, deterioration of the alignment film due to light irradiation can be ignored.

  Normally, when driving a liquid crystal element, it is necessary to incline the liquid crystal molecules in advance at a certain angle (pretilt angle) with respect to the substrate in order to align the rising directions of the liquid crystal molecules when a voltage is applied. When the aberration correction element and the light amount variable element used in the optical head device are used in a horizontal orientation type, the pretilt angle is suitable in the range of about 0.3 ° to 3 °. In the horizontal alignment film formed by the side evaporation method, the pretilt angle is too high, such as 20 ° to 30 °, or not at all, depending on the deposition angle. When the pretilt angle is too high, the refractive index anisotropy (Δn) of the liquid crystal generated by voltage application is reduced, and the performance of the liquid crystal element is reduced. On the other hand, when the angle is not formed at all, alignment unevenness is likely to occur when a voltage is applied.

  Therefore, as an example of obtaining an appropriate pretilt angle by the oblique vapor deposition method, the first layer is formed by the oblique vapor deposition method, and then the second layer is formed by the oblique vapor deposition method by changing the vapor deposition angle and the azimuth angle. Membranes have been proposed.

JP 2004-45784 A

  However, in this method, in the process of forming the first and second oblique deposition films, it is necessary to change the deposition angle and the deposition azimuth angle, and the mechanism of the oblique deposition apparatus is complicated. As a result, there arises a problem that the cost increases.

  The present invention has been made in order to solve the above-mentioned problems, and a horizontal alignment film is formed by forming a single-layer oblique vapor deposition film with an inorganic material such as silicon oxide with a thickness of 5 to 8 nm. Provided is a liquid crystal element excellent in performance for realizing an appropriate pretilt angle without being complicated.

The present invention relates to a liquid crystal element that uses blue laser light having an incident wavelength of 390 to 500 nm, and the liquid crystal element includes a plurality of transparent substrates sandwiching a liquid crystal layer, and the transparent substrate surface in contact with the liquid crystal layer is disposed on the surface of the transparent substrate. There is provided a liquid crystal element comprising a horizontal alignment film obliquely vapor-deposited with an inorganic material, wherein the horizontal alignment film has a thickness of 5 to 8 nm. In addition, a liquid crystal element in which the inorganic material is formed of SiO _x (0 <x <2) is provided.

  A transparent electrode is formed on the transparent substrate, and the transparent electrode is divided into concentric segments centered on the optical axis of the blue laser light. To do. Furthermore, the liquid crystal element according to the above, wherein an insulating layer is laminated on the transparent electrode.

  The liquid crystal element according to the above, wherein the transparent substrate is provided with a planar electrode that changes a wavefront shape of transmitted light of the liquid crystal element by applying a voltage to the liquid crystal layer. Further, the liquid crystal element according to the above, wherein the transparent substrate is provided with a planar electrode that applies a voltage to the liquid crystal layer to change a polarization state of transmitted light of the liquid crystal element.

  A light source; an objective lens for condensing the emitted light from the light source on the optical recording medium; a photodetector for receiving the emitted light collected and reflected by the optical recording medium; An optical head device comprising: the liquid crystal element described above disposed in an optical path between a light source and the optical recording medium or in an optical path between the optical recording medium and the photodetector. provide.

  In the present invention, an appropriate pretilt angle can be easily obtained by forming a single oblique deposition film of an inorganic material such as silicon oxide with a thickness of 5 to 8 nm as a horizontal alignment film of a liquid crystal element. The vapor deposition apparatus is not complicated, the apparatus cost is reduced, and the productivity is improved. Further, since it is formed of an alignment film of an inorganic material, there is no deterioration of the alignment film of the liquid crystal element even when used for laser light having a wavelength of 500 nm or less, and an optical head device equipped with the liquid crystal element can function stably. .

  The present invention includes a plurality of transparent substrates sandwiching a liquid crystal layer, a liquid crystal layer side of the transparent substrate is provided with a horizontal alignment film obliquely deposited by an inorganic material, and the thickness of the horizontal alignment film is 5 to 8 nm. It relates to a certain liquid crystal element. A light source; an objective lens for condensing the emitted light from the light source on the optical recording medium; a photodetector for receiving the emitted light collected and reflected by the optical recording medium; The present invention relates to an optical head device in which the liquid crystal element is arranged in an optical path between a light source and the optical recording medium.

A light source that emits light having a wavelength of 500 nm or less and has an irradiation energy of 1000 kJ / cm ² or more can be used. The liquid crystal element may be disposed in the optical path between the photodetector and the optical recording medium. If there is a collimating lens or beam splitter between the light source and the optical recording medium, the position of the liquid crystal element should be set between the collimating lens and the beam splitter, or between the beam splitter and the objective lens. However, it is preferable because the liquid crystal element can control the plane wave.

  The liquid crystal element according to the present invention includes a plurality of transparent substrates sandwiching a liquid crystal layer, and a horizontal alignment film on which an inorganic material is obliquely deposited is provided on the side of each transparent substrate surface in contact with the liquid crystal layer. By configuring the liquid crystal element in this manner, the horizontal alignment film formed from the inorganic material is not deteriorated even when the light having a wavelength of 500 nm or less, particularly, the wavelength of about 405 nm is continuously irradiated, and the liquid crystal element is configured. Does not affect the alignment of the liquid crystal layer. Therefore, when this liquid crystal element is mounted on an optical head device, there is an effect that a wavefront change of incident light does not occur even if light continues to enter the liquid crystal element.

The number of transparent substrates constituting the liquid crystal element is not limited to two, and may be three or more. In any configuration, a liquid crystal layer is sandwiched between one transparent substrate and another transparent substrate, and a horizontal alignment film on which an inorganic material is obliquely deposited is provided on a surface where the transparent substrate is in contact with the liquid crystal layer. . The number of transparent substrates may be selected according to the purpose of use, and if necessary, a curved surface such as a concave shape or a convex shape may be formed on the surface of the transparent substrate in contact with the liquid crystal layer. Further, the transparent substrate may be provided with a transparent electrode so that a voltage can be applied to the liquid crystal layer, and an insulating layer may be laminated on the transparent electrode to enhance the adhesion of the obliquely deposited film. . It is preferable to use SiO _x (0 <x ≦ 2) for the insulating layer.

  In the following description, it is assumed that two transparent substrates constituting the liquid crystal element are used. On the transparent substrate constituting the liquid crystal element, planar electrodes are formed so that the wavefront shape of light transmitted through the liquid crystal element can be changed by applying a voltage to the liquid crystal layer. An optical head device in which this liquid crystal element is installed is preferable because it can correct spherical aberration due to deviation of the optical disc thickness and coma aberration due to deviation of the tilt angle of the optical disc. Further, it is also preferable in that a desired transmitted wavefront shape can be generated without forming an uneven curved surface on the transparent substrate.

  An example of the optical head apparatus of the present invention shown in FIG. 1 uses a semiconductor laser having a wavelength of 405 nm as a light source. Information is recorded on the optical disk 8 as an optical recording medium, or information recorded on the optical disk 8 is recorded. It is for playback. This optical head device includes a semiconductor laser 1 as a light source, collimating lenses 30 and 31, an objective lens 6, an actuator 7 equipped with the objective lens, an optical disc 8, a prism type beam splitter 2, and a photodetector 9. Further, a liquid crystal aberration correction element, which is a liquid crystal element 100 that changes the wavefront of the light emitted from the semiconductor laser 1, is integrated with the quarter-wave plate 5.

  In addition, as a method of changing the amount of light reaching the optical disk surface, there is a method of changing the amount of light output from the light source. However, depending on the characteristics of the semiconductor laser, there are cases where noise is increased when the output is reduced, and cannot be used. On the transparent substrate constituting the liquid crystal element, planar electrodes are formed so that a voltage can be applied to the liquid crystal layer to change the polarization state of the transmitted light of the liquid crystal element. By constructing an optical head device that combines this liquid crystal element and a polarization separation element such as a polarization beam splitter, the amount of light that reaches the optical disk surface is applied to the liquid crystal element while keeping the output noise of the semiconductor laser at a minimum. This is preferable because it can be changed by adjusting the voltage to be applied.

  In another configuration example of the optical head device of the present invention shown in FIG. 3, the liquid crystal element 101 that changes the polarization state of the light emitted from the semiconductor laser 1 is used, and the liquid crystal element 101 and the polarization beam splitter 2 are combined. The amount of light transmitted to the optical disk surface is changed. 3 that are the same as those in FIG. 1 indicate the same elements as those in FIG.

  An example of the liquid crystal element used in these optical head devices of the present invention will be described with reference to a schematic cross-sectional view shown in FIG. In the liquid crystal element, transparent conductive films 24a and 24b, which are flat electrodes, are formed on the surfaces of two glass substrates 21a and 21b which are opposite transparent substrates, and an inorganic substance is formed on the transparent conductive films 24a and 24b. Horizontal alignment films 25a and 25b formed by oblique deposition of a single layer film of material are laminated, and the peripheral portions of the glass substrates 21a and 21b are sealed with a sealing material 22 to be aligned with the horizontal alignment film 25a. The gap formed by the film 25 b has a structure in which a liquid crystal is filled with a liquid crystal layer 23.

Here, Ta ₂ O ₅ , WO ₃ , Bi ₂ O ₃ are listed as inorganic materials used for the oblique vapor deposition film. Furthermore, oxides such as SiO _x (0 <x <2) and (1-y) SiO _x + yZrO ₂ , (1-y) SiO _x + yTiO ₂ (0 <x <2, 0 <y <1) may be used. . Among these, it is preferable to use SiO _x (0 <x <2) because excellent stability of the alignment state of the liquid crystal molecules can be obtained.

For example, when SiO _x is used as an inorganic material as a method for forming an obliquely deposited film, an angle formed between a vertical line and a normal line of the substrate surface is arranged in a vacuum vapor deposition apparatus on a substrate vertically above the SiO vapor deposition source. Is set to any one point between 88 and 60 °, and the substrate temperature is set between room temperature and 300 ° C., and SiO _x is vacuum deposited. The oxygen amount x can be adjusted by adjusting the oxygen gas concentration. When the angle formed between the vertical line and the normal of the substrate surface is set to any one of 88 to 80 ° and the thickness of the oblique deposition film is adjusted to 5 to 8 nm, the liquid crystal is horizontally aligned. It is preferable because an appropriate pretilt angle of the molecule can be obtained. If the thickness of the oblique deposition film is 10 nm or more, the pretilt angle of horizontally aligned liquid crystal molecules may be 20 to 30 °. Moreover, when the thickness of the oblique deposition film is 4.5 nm or less, it is difficult to obtain uniform orientation. In addition, baking at about 200 ° C. or irradiation with UV light after vapor deposition is preferable because the surface state of the film can be stabilized.

  In this way, the surfaces having the horizontal alignment films of the two transparent substrates on which the horizontal alignment films are formed by the oblique vapor deposition method are opposed to each other so that the horizontal alignment directions of the liquid crystal molecules of the two transparent substrates form a predetermined angle. The liquid crystal element is configured by setting and providing a predetermined gap, sealing the peripheral portion of the transparent substrate with a sealing material, and injecting liquid crystal into the gap.

  When using a liquid crystal element as an aberration correction element, it is preferable to oppose so that the alignment direction of the liquid crystal molecules of two transparent substrates may become parallel. At this time, if the horizontal alignment direction of the liquid crystal molecules and the linear polarization direction of the incident light are set to coincide with each other and a voltage is applied to the liquid crystal layer, the effective refractive index changes, and the polarization state of the light changes almost. Without changing the wavefront of the incident light.

  Also, it is used as a light quantity variable element that changes the amount of light transmitted through the optical disk surface by combining a liquid crystal element that can change the polarization state of light to be transmitted and a polarizing beam splitter that has different transmittance depending on the polarization direction of incident light. You can also.

  As the liquid crystal material to be used, nematic liquid crystal used for a display application or the like is preferably used, and liquid crystal molecules may be twisted by adding a chiral agent. Moreover, as a material of the transparent substrate to be used, glass, polycarbonate resin, acrylic resin, epoxy resin, vinyl chloride resin and the like can be used, but a glass substrate is preferable from the viewpoint of durability.

Further, a phase plate such as a quarter-wave plate may be stacked on the liquid crystal element. As the material of the phase plate, a birefringent single crystal material such as quartz or LiNbO ₃ may be used, or an organic film such as a polymer liquid crystal or polycarbonate may be used.

  An example in which the liquid crystal element according to the present invention is applied to an optical head device will be described. The optical head device of the present embodiment includes a phase correction element that corrects spherical aberration caused by the deviation of the thickness of the optical disk. The wavelength of the emitted light from the semiconductor laser that is the light source is 405 nm. The objective lens generates spherical aberration when the thickness of the optical disc deviates from the design value, and the signal reading accuracy decreases. A phase correction element for correcting this spherical aberration was incorporated at the position of the liquid crystal element 100 of the optical head device shown in FIG.

  This liquid crystal element has the same structure as the schematic cross-sectional view shown in FIG. The liquid crystal used is a nematic liquid crystal, and the horizontal alignment direction of the liquid crystal molecules is set to be parallel to the polarization direction of the light before entering the liquid crystal layer in order to change the wavefront of the light emitted from the semiconductor laser. did.

  As shown in FIG. 2, the liquid crystal element has a configuration in which a liquid crystal layer 23 is surrounded by a sealing material 22 and sandwiched between horizontal alignment films 25a and 25b, transparent conductive films 24a and 24b, and glass substrates 21a and 21b. In this embodiment, a transparent electrode made of ITO divided into concentric segments shown in FIG. 4 so that a voltage can be applied to the liquid crystal layer 23 on two transparent glass substrates sandwiching the liquid crystal layer 23. Formed.

On this transparent electrode, as a horizontal alignment film for liquid crystal, vacuum deposition was performed by an electron beam heating (EB) method at a rate of 0.1 nm / s so that the thickness of the SiO _x oblique deposition film became 5 nm. At this time, the substrate temperature was set to 80 ° C., and the angle between the substrate normal and the vertical (vertical) line of the evaporation source was set to 85 °, and the deposition was performed. After the vapor deposition, in order to obtain good alignment of liquid crystal molecules, this horizontal alignment film was baked at 200 ° C. for 1 hour in the air.

  The two transparent substrates on which the horizontal alignment film is formed by the oblique vapor deposition method as described above are opposed to each other so as to be in anti-para alignment, and a peripheral portion of the substrate is sealed with a sealing material with a gap of 5 μm. A nematic liquid crystal having a positive dielectric anisotropy was injected into a liquid crystal element. At this time, when the liquid crystal element was heat-treated and evaluated, in-plane uniform alignment was obtained, and a pretilt angle of about 1 ° was obtained.

  This liquid crystal element was incorporated at the position of the liquid crystal element 100 of the optical head device shown in FIG. 1, and a desired voltage was applied to each concentric segment so as to correct the spherical aberration due to the deviation of the thickness of the optical disk.

  The objective lens is one with NA of 0.85, and the optical disc is a laser with a wavelength of 405 nm for three types of cover layer thicknesses of 0.09 mm, 0.10 mm and 0.11 mm to the reflecting surface. The reproduction characteristics of recorded information on the optical disk surface when light was emitted were examined. The optical head device used was adjusted so that the spherical aberration was minimized when the thickness of the cover layer of the optical disk was 0.10 mm. With an optical disk having a thickness of 0.10 mm, good reproduction characteristics were obtained without applying a voltage to the liquid crystal layer. On the other hand, in the optical disks having thicknesses of 0.11 mm and 0.09 mm, the reproduction characteristics were not good due to the influence of spherical aberration.

  For optical discs with a thickness of 0.11 mm and 0.09 mm, the voltage applied to the liquid crystal layer is optimally adjusted according to the difference in the thickness of each optical disc, and is generated due to the deviation in the thickness of the cover layer of the optical disc. When spherical aberration having a sign opposite to that of spherical aberration was generated, the light on the optical disk surface showed good condensing characteristics and improved reproduction characteristics. In addition, even after the output of laser light having a wavelength of 405 nm was 30 mW and the light irradiation test for 5000 hours was performed, the reproduction characteristics were not deteriorated, and the light collection characteristics were good.

  In addition, as a comparative example, a liquid crystal element was formed under the same conditions as described above except that the obliquely deposited film was formed to a thickness of 10 nm and a horizontal alignment film was formed. The pretilt angle was about 30 °, and a desired low pretilt angle was obtained. Could not get.

  According to the present invention, an oblique deposition film of an inorganic material formed as a horizontal alignment film of a liquid crystal element can be realized by an easy method, and the liquid crystal element is mounted on an optical head device using a laser beam having a wavelength of 500 nm or less. However, the horizontal alignment film does not deteriorate and can be optically stabilized.

1 is a conceptual cross-sectional view showing an example of the principle configuration of an optical head device using a liquid crystal element of the present invention. Typical sectional drawing which shows an example of the liquid crystal element of this invention Conceptual sectional view showing another example of the principle configuration of the optical head device using the liquid crystal element of the present invention. The typical top view which shows the division | segmentation electrode pattern of the liquid crystal element which correct | amends the spherical aberration of this invention

Explanation of symbols

1: Semiconductor laser 2: Beam splitter 30, 31: Collimating lens 5: Quarter wavelength plate 6: Objective lens 7: Actuator 8: Optical disk 9: Photo detector 100, 101: Liquid crystal elements 21a, 21b: Glass substrate 22 : Sealing material 23: Liquid crystal layer 24a, 24b: Transparent conductive film 25a, 25b: Horizontal alignment film

Claims (7)

  In a liquid crystal element used by incident blue laser light having an incident wavelength of 390 to 500 nm, the liquid crystal element includes a plurality of transparent substrates sandwiching a liquid crystal layer, and an inorganic material is provided on the transparent substrate surface in contact with the liquid crystal layer. A liquid crystal device comprising a horizontal alignment film obliquely vapor-deposited by the method, wherein the horizontal alignment film has a thickness of 5 to 8 nm. The liquid crystal element according to claim 1, wherein the inorganic material is formed of SiO _x (0 <x <2).   3. A transparent electrode is formed on the transparent substrate, and the transparent electrode is divided into concentric segments centered on the optical axis of the blue laser light. The liquid crystal element as described.   The liquid crystal element according to any one of claims 1 to 3, wherein an insulating layer is laminated on the transparent electrode.   The planar electrode which changes the wave front shape of the transmitted light of the said liquid crystal element by applying a voltage to the said liquid crystal layer is formed in the said transparent substrate. The liquid crystal element described in 1.   The planar electrode which applies a voltage to the said liquid-crystal layer, and changes the polarization state of the transmitted light of the said liquid-crystal element is formed in the said transparent substrate. The liquid crystal element described in 1.   A light source, an objective lens for condensing the light emitted from the light source on the optical recording medium, a photodetector for receiving the emitted light collected and reflected by the optical recording medium, and the light source The liquid crystal element according to claim 1, wherein the liquid crystal element is disposed in an optical path between the optical recording medium or an optical path between the optical recording medium and the photodetector. An optical head device.

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