JP2008150452A - Liquid crystal composition, liquid crystal optical element and optical pickup device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal composition for a high-density optical disk using a blue laser beam having high Δn, and having high light resistance to the blue laser beam. <P>SOLUTION: The liquid crystal composition has ≤320 nm maximum absorption wave length, and ≥10 GΩcm initial value of the specific resistance of a liquid crystal layer in a liquid crystal element. Especially the liquid crystal composition contains ≥90 wt.% of one or more kinds of compounds represented by general formula (1): R<SP>1</SP>-Cy-Ph-Ph-(Ph)<SB>n</SB>-R<SP>2</SP>(1) (wherein, R<SP>1</SP>is a 1-12C alkyl group; R<SP>2</SP>is a fluorine atom or a 1-12C alkyl group; Cy is a trans-1,4-cyclohexylene group; Ph is a 1,4-phenylene group; n is 0 or 1; at least one hydrogen atom of at least one 1,4-phenylene group at 2-, 3-, 5- or 6-position is substituted with a fluorine atom). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a liquid crystal composition constituting a liquid crystal optical element for an optical pickup device for a blue laser, a liquid crystal optical element encapsulating the liquid crystal composition, and an optical pickup device using the same.

  A DVD that is an optical disk records digital information at a higher density than a CD that is also an optical disk. Therefore, in an optical pickup device for reproducing a DVD, the wavelength of the light source is set to 650 nm or 635 nm shorter than 780 nm of CD, and the numerical aperture (NA) of the objective lens is set to 0.6 which is larger than 0.45 of CD. The spot diameter collected on the optical disk surface is reduced.

  Furthermore, in BD and HDDVD, which are optical discs that require high-density recording, an objective lens having a NA greater than 0.6 is used as a light source using a semiconductor laser that emits blue laser light with a shorter wavelength of 405 nm. By providing this, a higher recording density is obtained. Accordingly, not only conventional optical elements made of inorganic materials but also various liquid crystal optical elements that control and correct phase differences and wavefronts by voltage have been proposed.

  Patent Document 1 discloses a liquid crystal lens for blue laser that performs spherical aberration correction as a liquid crystal optical element for an optical pickup device for blue laser. A liquid crystal material having optical characteristics required for the liquid crystal optical element, that is, Δn capable of generating a phase difference, and sufficiently resistant to blue laser light is appropriately used. However, the liquid crystal material used in the conventional liquid crystal optical element has low resistance to blue laser light and has a problem with long-term durability. In particular, in a liquid crystal lens, it is necessary to generate a large phase difference corresponding to a power component that changes the focal point of incident light, and a liquid crystal material is also required to exhibit a high Δn. There was a tendency to reduce light resistance, and it was difficult to ensure long-term durability.

In general, a liquid crystal material having a large Δn has (A) a strong electron-withdrawing group (for example, a cyano group, a nitro group, a thioisocyanate group, etc.) and an electron donating group such as an amino group. Those having a functional group introduced, and (B) those having increased conjugation by introducing -C≡C- into the molecular skeleton as found in tolan liquid crystals are used. These simultaneously shifted the absorption of the compounds and compositions composed of these compounds to the longer wavelength side and tended to cause absorption in the wavelength band of blue laser light and the accompanying decomposition.
In Patent Document 2, when a laser beam having a wavelength of 400 to 500 nm is used, the liquid crystal material is easily damaged by light irradiation and has a unsaturated bond (excluding an unsaturated bond in an aromatic ring) as the liquid crystal material. It is shown that it is preferable to use a liquid crystal material having a content ratio of 20 mol% or less.

  Thus, when it is going to ensure the light resistance of the liquid crystal material with respect to a blue laser, consideration is given to the absorption coefficient of the liquid crystal material in the wavelength band of the blue laser light. For this reason, generally, a small Δn of 0.1 or less is used. For example, in Patent Document 3, light resistance is improved by using a liquid crystal material for a liquid crystal projector that is irradiated with relatively strong light in the entire visible range including blue light and exposed to a high temperature, with Δn of 0.08. Secured.

JP 2006-85801 A JP 2003-90990 A Japanese Patent Laid-Open No. 07-18258

  However, unlike the case of a display element, it is difficult to generate a sufficient power or phase difference in a liquid crystal optical element such as a liquid crystal lens used in an optical pickup device at Δn of the above level. On the other hand, there is a method of increasing the thickness of the liquid crystal layer by enlarging the uneven step to ensure the phase difference, but this reduces the response time to the voltage and causes a problem in drive characteristics. In the case of a polarization diffraction grating element, it is necessary to increase the height of the concave and convex portions of the diffraction grating, which causes the same problem that it is difficult to form an accurate grating shape and the thickness of the liquid crystal layer is increased. For this reason, liquid crystal materials have been required to ensure as high Δn as possible and light resistance against blue laser light.

  The present invention aims to solve the above-mentioned problem, that is, the problem of securing light resistance in the wavelength band of a blue laser while exhibiting a somewhat high Δn, a novel liquid crystal composition, and a liquid crystal optical element encapsulating the same An optical pickup device using the same is provided.

[1] In a liquid crystal composition for a liquid crystal optical element disposed in an optical pickup device having a blue laser light source, the maximum absorption wavelength is 320 nm or less, and the initial value of the specific resistance of the liquid crystal layer in the liquid crystal optical element Is a liquid crystal composition characterized by being 10 GΩcm or more.

[2] The liquid crystal composition according to [1], wherein an absorption coefficient at 400 to 420 nm is 10 ⁻⁵ μm ⁻¹ or less.

[3] The liquid crystalline composition according to the above [1] or [2], comprising 90% by weight or more of one or more compounds represented by the following general formula (1).
R ¹ -Cy-Ph-Ph- (Ph) _n -R ² (1)
(However, R ¹ is an alkyl group having 1 to 12 carbon atoms, R ² is a fluorine atom or an alkyl group having 1 to 12 carbon atoms, Cy is a 1,4-cyclohexylene group, and Ph is a 1,4-phenylene group. , N means 0 or 1, and at least one hydrogen atom of at least one 1,4-phenylene group is substituted with a fluorine atom.)

[4] The liquid crystalline composition according to the above [3], comprising 90% by weight or more of each of one or more compounds represented by the following general formulas (1A) and (1B).
R ¹ -Cy-Ph-Ph-R ² (1A)
R ¹ -Cy-Ph-Ph-Ph-R ² (1B)
(However, R ¹ , R ² , Cy and Ph have the same meaning as described above, and General Formula (1A) and General Formula (1B) may be the same or different from each other.)

[5] The above-mentioned composition containing 80 to 90% by weight of one or more compounds represented by the following general formula (2), and further containing 10 to 20% by weight of one or more compounds represented by the following general formula (3) The liquid crystalline composition according to [4].

(However, R ³ , R ⁴ , and R ⁵ each independently represents an alkyl group having 1 to 12 carbon atoms).

[6] The liquid crystal composition according to any one of [1] to [5], in which a refractive index anisotropy in a wavelength band of blue laser light of 400 to 420 nm is 0.19 or more.

[7] A liquid crystal optical element for an optical pickup device having a blue laser light source, wherein the liquid crystal composition according to any one of [1] to [6] is used.

[8] A liquid crystal optical element encapsulating the liquid crystal composition according to any one of [1] to [6], a light source emitting at least a blue laser light, and a photodetector. Optical pickup device.

  The liquid crystal composition of the present invention is used by being encapsulated in a liquid crystal optical element disposed in an optical pickup device using a blue laser that emits light having a wavelength of 400 to 420 nm as a light source.

  The liquid crystal composition used for the liquid crystal optical element in the present invention has a maximum absorption wavelength of 320 nm or less and an initial value of specific resistance of the liquid crystal layer in the liquid crystal optical element of 10 GΩcm or more. The wavelength actually used is light of 400 to 420 nm. However, by setting the maximum absorption wavelength to 320 nm or less, absorption in the use wavelength region is low, and optical damage can be reduced. Furthermore, by setting the specific resistance of the liquid crystal layer to 10 GΩcm or more, the light resistance is further improved, and the deterioration of the liquid crystal optical element can be reduced even when exposed to a high amount of light (per unit area) for a long time.

In particular, a liquid crystal composition having an absorption coefficient at 400 to 420 nm of 10 ⁻⁵ μm ⁻¹ or less is preferable with little deterioration with respect to light used.

For this liquid crystal composition, it is preferable to use a liquid crystal composition containing 90% by weight or more of one or more compounds represented by the following general formula (1).
R ¹ -Cy-Ph-Ph- (Ph) _n -R ² (1)
(However, R1 an alkyl group having 1 to 12 carbon atoms, ^{R 2} is a fluorine atom or an alkyl group having 1 to 12 carbon, Cy is trans-1,4-cyclohexylene group, Ph is 1,4-phenylene Group n represents 0 or 1, and at least one hydrogen atom of at least one 1,4-phenylene group is substituted with a fluorine atom.)

That is, the compound of the general formula (1) is a compound in which at least one hydrogen atom in the plurality of 1,4-phenylene groups is substituted with a fluorine atom. This fluorine atom is present at the 2, 3, 5, 6 position of the 1,4-phenylene group, and is different from the fluorine atom of R ² . By using this liquid crystal material, it is possible to obtain a liquid crystal composition with little deterioration due to light of 400 to 420 nm and having a high Δn. Although the compound of this general formula (1) can also be used by 1 type, in order to adjust various characteristics, it is preferable to use in combination of 2 or more types. Other liquid crystal materials combined with this may be added in an amount of 10% by weight or less for the purpose of adjusting the refractive index, the operating temperature range, etc., and various known materials can be used, but deterioration due to light of 400 to 420 nm is possible. It is preferable to use less material.

In the present invention, as the compound of the general formula (1), it is preferable to use a liquid crystalline composition containing 90% by weight or more of each of one or more compounds represented by the following general formulas (1A) and (1B). This is preferable from the viewpoint of durability to laser.
R ¹ -Cy-Ph-Ph-R ² (1A)
R ¹ -Cy-Ph-Ph-Ph-R ² (1B)
(However, R ¹ , R ² , Cy and Ph have the same meaning as the compound of the general formula (1), and the general formula (1A) and the general formula (1B) are the same or different from each other). May be good.)

Examples of the compound of the general formula (1) include the following compounds. As for R ¹ and R ² , as in the case of the general formula (1), R ¹ represents an alkyl group having 1 to 12 carbon atoms, and R ² represents a fluorine atom or an alkyl group having 1 to 12 carbon atoms. To do. There are compounds having a tricyclic structure and a tetracyclic structure where n is 0 or 1. Here, a compound in which only a hydrogen atom of one phenylene group is substituted with a fluorine atom is exemplified, but a compound in which a hydrogen atom is substituted with a fluorine atom in a plurality of phenylene groups can also be used.
In the case of n = 0, that is, in the case of the compound of the general formula (1A) (tricyclic structure), the following compounds are exemplified.

  In the case of n = 1, that is, in the case of the compound of the general formula (1B) (tetracyclic structure), the following compounds are exemplified.

  The above-mentioned compounds can be used even if one kind of compound has desired physical properties, but it is preferable to use a mixture of two or more kinds of compounds in order to adjust the characteristics. In the case of using a plurality of compounds of the above general formula (1), in particular, it contains 80 to 90% by weight of one or more compounds represented by the following general formula (2), and is further represented by the following general formula (3). It is preferable in terms of high Δn and high durability to contain 10 to 20% by weight of one or more compounds.

However, R < ³ >, R < ⁴ >, R < ⁵ > means a C1-C12 alkyl group each independently.
When this liquid crystal composition is used, Δn tends to be high, but it is particularly preferable that Δn is 0.19 or more by selecting a compound having a composition. Thereby, when used as a liquid crystal lens or a polarization diffraction element, the height of the unevenness can be lowered, the manufacturing can be facilitated, and the driving voltage of the liquid crystal can be reduced.

  In an optical pickup device having an oscillation wavelength of 400 to 420 nm and a light source having a maximum light output of 30 mW or more for recording, a liquid crystal optical element in which the liquid crystal composition of the present invention is enclosed is used in the optical path. Become. In this case, the liquid crystal optical element using the liquid crystal composition of the present invention has an effect that it is not easily damaged by light from the light source. In the optical pickup device, the total amount of laser light itself is small, but the laser light passes through a very small area, so the amount of light per unit area is extremely large, and the light resistance of the liquid crystal optical element is low. Deterioration progresses quickly.

  The liquid crystal compound suitable for the liquid crystal composition of the present invention has a structure in which one to a plurality of hydrogen atoms in the ring of the molecule are substituted with fluorine atoms. Due to the steric hindrance of the fluorine atom, the intermolecular interaction is small even in the aggregated system, and as a result, there is an effect of suppressing the spread of absorption toward the long wavelength side.

  The liquid crystal optical element in the present invention has a configuration in which a liquid crystal composition is sealed between substrates. Usually, a transparent substrate may be used for both of the two substrates. However, in the case of a polarization diffraction grating, since it can be a reflection type, one of the substrates can be an opaque substrate. An electrode, a concavo-convex structure, an alignment film, and the like are formed on the substrate as necessary. Such a substrate is peripherally sealed with a sealing material to form an empty cell, and a liquid crystal composition is injected from the injection port to seal the injection port, thereby completing a liquid crystal optical element. The liquid crystal composition can be dropped on the substrate to simultaneously cure the sealing material and encapsulate the liquid crystal. For the structure and manufacturing method of such a liquid crystal optical element, a known method for manufacturing a liquid crystal display element can be used.

  In this liquid crystal optical element, a plurality of liquid crystal layers can be provided for the purpose of eliminating the polarization dependence, increasing the effect, and increasing the variety of changes. When two liquid crystal layers are provided, the number of substrates may be three or four. If necessary, a diaphragm, a non-reflective layer, a diffraction grating, or the like may be formed on the outer surface of the liquid crystal optical element, or a phase plate or the like may be laminated. 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.

  The liquid crystal optical element of the present invention can be used for a liquid crystal lens, a polarization diffraction grating, a phase plate, and the like, and is particularly suitable for applications in which the characteristics of the liquid crystal optical element can be changed depending on the state of voltage application to the electrodes. Specifically, there are aberration control, diffraction on / off control, on / off control of the function as a phase plate, etc. by changing the wavefront by applying a voltage.

Hereinafter, the present invention will be described in detail by way of examples and comparative examples.
As examples and comparative examples, compounds represented by the following formulas (9) to (13) were prepared to give compositions. Table 1 shows the blending ratio (% by weight) of Composition 1 (Example) and Compositions 2 and 3 (Comparative Example), the phase at room temperature, the liquid crystal phase-isotropic phase transition temperature, the composition by the wedge method. Δn at a wavelength of 405 nm is shown.

  In order to perform the light resistance test of the liquid crystal compositions in the present examples and comparative examples, liquid crystal optical elements were prepared. A transparent conductive film made of ITO is formed on a glass substrate, a commercially available polyimide alignment film is applied on the glass substrate, cured, and then rubbed by a rubbing method to form an alignment film. A substrate was manufactured. Thereafter, a sealing material is applied to the periphery of one glass substrate, a spacer having a diameter of 3.5 μm is spread on the surface of the alignment film, and the other glass substrate is stacked so that the alignment processing directions of both glass substrates are parallel. An empty cell was produced. The liquid crystal optical element 1 and the liquid crystal optical element are prepared by injecting the composition 1, the composition 2 and the composition 3 from the injection port, which are notches provided in the sealing material of the empty cell, respectively, and sealing the injection port. 2 and liquid crystal optical element 3 were produced.

  As a light resistance test, linearly polarized light of a Coherent Kr laser (wavelength: 405 nm and 413 nm, intensity: 40 mW) was used as the light source, and the light was incident on the liquid crystal element so that the polarization direction was parallel to the rubbing direction of the liquid crystal optical element. I let you. This light resistance test was conducted at a temperature of 70 ° C. while applying an alternating voltage (frequency: 1 kHz, voltage: 9 V) between the transparent electrodes of the liquid crystal optical element. The laser resistance of the liquid crystalline composition after 380 hours from the start of laser irradiation was evaluated by measuring the specific resistance of the liquid crystal layer before laser irradiation (initial value) and after irradiation (impedance analyzer “SI1260” manufactured by Solartron). ). The element function of the liquid crystal optical element was estimated by a change in the threshold voltage of the voltage-transmittance characteristic of the liquid crystal optical element. The results are shown in Table 2.

  The panel of the liquid crystal optical element 2 (composition 2) was seized after laser irradiation, and voltage-transmittance characteristics could not be measured. Further, the panel of the liquid crystal optical element 3 (composition 3) did not show great deterioration in specific resistance and threshold voltage after laser irradiation, but a large number of alignment defect regions were generated in the panel. On the other hand, the panel of the liquid crystal optical element 1 (composition 1) was stable without the occurrence of image sticking and alignment defects even after laser irradiation.

  As described above, according to the present invention, a liquid crystal composition having high light resistance to blue laser light and having a relatively high Δn can be obtained. By using a liquid crystal optical element encapsulating the liquid crystal composition in an optical pickup device, a high-density optical disk device using blue laser light can be obtained. By selecting the material to be used, it can be easily made high Δn, so that the wavefront correction and the height of the concavo-convex of the diffraction grating can be reduced, the concavo-convex processing becomes easy, and the driving voltage can be lowered.

Claims (8)

  In a liquid crystal composition for a liquid crystal optical element disposed in an optical pickup device having a blue laser light source, the maximum absorption wavelength is 320 nm or less, and the initial value of the specific resistance of the liquid crystal layer in the liquid crystal optical element is 10 GΩcm or more A liquid crystal composition characterized by the above. The liquid crystal composition according to claim 1, wherein an absorption coefficient at 400 to 420 nm is 10 ⁻⁴ μm ⁻¹ or less. The liquid crystalline composition according to claim 1 or 2, comprising 90% by weight or more of one or more compounds represented by the following general formula (1).
R ¹ -Cy-Ph-Ph- (Ph) _n -R ² (1)
(Wherein R ¹ is an alkyl group having 1 to 12 carbon atoms, R ² is a fluorine atom or an alkyl group having 1 to 12 carbon atoms, Cy is a trans-1,4-cyclohexylene group, and Ph is 1,4- A phenylene group, n means 0 or 1, and at least one hydrogen atom at the 2, 3, 5, 6 position of at least one 1,4-phenylene group is substituted with a fluorine atom. The liquid crystalline composition according to claim 3, comprising 90% by weight or more of one or more compounds represented by the following general formulas (1A) and (1B).
R ¹ -Cy-Ph-Ph-R ² (1A)
R ¹ -Cy-Ph-Ph-Ph-R ² (1B)
(However, R ¹ , R ² , Cy and Ph have the same meaning as described above, and General Formula (1A) and General Formula (1B) may be the same or different from each other.) The compound according to claim 4, comprising 80 to 90% by weight of one or more compounds represented by the following general formula (2), and further comprising 10 to 20% by weight of one or more compounds represented by the following general formula (3). The liquid crystalline composition as described.

(However, R ³ , R ⁴ , and R ⁵ each independently represents an alkyl group having 1 to 12 carbon atoms).   The liquid crystal composition according to any one of claims 1 to 5, wherein the refractive index anisotropy at 400 to 420 nm which is a wavelength band of blue laser light is 0.19 or more.   A liquid crystal optical element for an optical pickup device having a light source for blue laser, wherein the liquid crystal composition according to claim 1 is used.   An optical pickup device comprising: a liquid crystal optical element in which the liquid crystal composition according to claim 1 is sealed; a light source that emits at least blue laser light; and a photodetector.