JP2003167127A - Birefringent holographic optical element and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical element featuring that the element is produced by irradiating a mixture material film of a photosensitive polymer and a low molecular weight compound with two interfering luminous fluxes and that the element has functions to separate natural light into two polarized light components perpendicular to each other, to transmit one component and to reflect the other and to provide a method for manufacturing the optical element. <P>SOLUTION: A mixture material of a photosensitive polymer and a low molecular weight compound is applied on a substrate to form a film. The film is irradiated with two laser beams each divided into two luminous fluxes by beam splitters to interfere with each other, or the film is irradiated with light using a mask pattern and then irradiated with two interfering luminous fluxes. The birefringent holographic optical element manufactured by this method has functions to separate natural light into two polarized light components perpendicular to each other, to transmit one component and to reflect the other component, and to refract the transmitted polarized light component. The obtained element is used as an optical anisotropic element effective to improve the use efficiency of light of a surface light source device. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a birefringent holographic optical element suitable for use in, for example, improving the utilization efficiency of light emitted from a surface light source device, and a manufacturing method thereof.

[0002]

2. Description of the Related Art In a sidelight type surface light source device used in a conventional liquid crystal display device or the like, in order to direct light emitted from a light guide in a substantially normal direction, Japanese Patent Laid-Open No. 62-14441.
A sheet (hereinafter, referred to as a prism sheet) in which prisms each having an isosceles right triangle are linearly arranged on the surface of a thin elastic film as described in No. 02 is attached to the surface light source device. Further, in the liquid crystal display device, natural light is polarized by a polarizing plate and used for image display. Therefore, one of the two orthogonal polarization components of natural light is absorbed by the polarizing sheet, and the light utilization efficiency is 50% at maximum, and the loss of light emitted from the surface light source device is large. In order to eliminate this light loss, US Pat. No. 3,610,792 and US Pat.
No. 12820 and US Pat. No. 5,486,946, an optical element which is a multilayer structure made of a transparent material, in which a difference in refractive index between layers is close to 0 in a certain direction in a plane and a difference in refractive index in a direction perpendicular to the direction, JP-A-8-2718
No. 37 proposes a linear polarization element that separates natural light into two orthogonal polarization components with an optical element that uses a cholesteric liquid crystal and a quarter-wave plate, transmits one, and reflects the other. Has been done. In such an element, of the light emitted from the surface light source device, the light of one polarization component is reflected and returned to the light guide of the surface light source device, and this light is reflected and scattered and the polarization state is disturbed and emitted again. To be done. By repeating this, the utilization efficiency of light is improved. A polarization separation element using holography has been proposed in US Pat. No. 5,161,039, in which the refractive index in the direction perpendicular to the paper surface shown in FIG. 4 is periodically changed (a high refractive index region (42) and a refractive index). A method of separating natural light (L ″) into two polarization components (L1 ″, L2 ″) and separating the emission direction by a polarization separation element (41) having a structure in which low regions (43) are alternately layered) is proposed. This periodic structure is manufactured by utilizing the property that the part that is irradiated with two lights and strengthened by interference undergoes a two-photon absorption process to change its refractive index.

[0003]

However, in the prism sheet in which minute prisms are arranged on the surface of the sheet, the light from the surface light source device is refracted in a desired direction by refracting the light from the surface light source device to improve the front brightness. Although it is possible to have a directional direction, one polarization component of the light from the surface light source device is absorbed by the polarizing plate, resulting in a large light loss. On the other hand, an optical element having a multilayer structure made of a transparent material or an optical element having a structure using a cholesteric liquid crystal and a quarter-wave plate is used to reflect light of one polarization component to improve light utilization efficiency. However, in some cases, the light emitted from the surface light source device cannot be refracted and, as a result, the light cannot be fully utilized, and in the end, the prism sheet is often used together with the surface light source device. As a result, there is a problem that the structure of the liquid crystal display device becomes complicated.
Further, in an optical element using a multilayer structure made of a transparent material, it is necessary to form a multilayer structure in which the film thickness per layer is precisely controlled because light interference is used, and it is difficult to manufacture at low cost. Is. Further, it is difficult to reflect the light of one polarization component in a hologram element that simply has a periodic structure in the plane, and it cannot be expected to improve the utilization efficiency.

[0004]

In the birefringent holographic optical element and the method for manufacturing the same according to the present invention, a film of a material that causes optical modulation by light irradiation is formed on a substrate, and the film is irradiated with at least two light beams to interfere with each other. Then, by periodically expressing the optical modulation in the film, it is possible to manufacture a holographic optical element having a function of reflecting the light of one polarization component and refracting the transmitted polarization component. Issues are solved.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The details of the present invention will be described below. For the production of the optical element of the present invention, materials such as those described by the present inventor in JP-A No. 11-189665 and Japanese Patent Application No. 2000-400356 that cause optical modulation by light irradiation are preferably used. This material is a photosensitive material that undergoes molecular orientation and birefringence when irradiated with linearly polarized light. However, the material is not limited to such a material as long as the material causes optical modulation by irradiation of light. As shown in FIG. 2, when a photosensitive material that causes optical modulation by light irradiation as described above is formed into a film, and a linearly polarized light flux with good coherence such as laser light is applied to the film from two different directions. In the interference of two light fluxes (L, L '), optical modulation is promoted in the portion (21) where the wavefronts (a, a') of the two light fluxes interfere and strengthen each other, and the two light fluxes interfere and weaken each other (22).
Since no optical modulation occurs, the resulting photosensitive material thus formed undergoes birefringent periodic changes. As a manufacturing method, for example, two light sources (31, 31 ') and beam splitters (32, 32') as shown in FIG.
When an optical system consisting of mirrors (33, 33 ') is used and two light beams are interfered and a photosensitive material (35) formed on a transparent substrate (34) is irradiated, a periodic structure of optical modulation appears. By fixing this structure in the film, a birefringent holographic optical element can be manufactured.

FIG. 1 shows the birefringent holographic optical element (11) of the present invention obtained in this way. In the birefringent holographic optical element (11) shown in FIG. 1, a layer having an optical micro-modulation structure composed of a portion (12) with a large birefringence and a portion (13) with a small or no birefringence. This layer is inclined with respect to the film thickness direction.
The birefringent holographic optical element (11) of the invention splits natural light (L) into two orthogonal polarization components (L1, L2), transmits one polarization component (L1), and transmits the other polarization component (L2). ) Is reflected, and the function of refracting the light of the polarized component that transmits can be added. (On the other hand, in the conventional holographic optical element, the function of separating natural light into two orthogonal polarization components and dividing the refraction directions of the two polarization components is limited.)

The optical element of the present invention splits natural light into two orthogonal polarization components, transmits one of them and transmits the other, even if it does not have a birefringent ellipsoidal modulation structure uniformly throughout the film formation. It has the function of reflecting light and the function of changing the direction of incident light and emitted light in light transmitted through this element.

An example of the method of manufacturing the optical element of the present invention is the method of FIG. 6 which uses the interference of two light beams having different wavelengths. This method utilizes the difference in absorbance depending on the wavelength of the photosensitive material irradiated with the interference light. To further explain, when a wavelength of light having a low absorbance of the photosensitive material is irradiated, a modulation structure is formed in a deep portion of the film or the whole film, and when a wavelength of light having a high absorbance of the photosensitive material is irradiated, a relatively shallow portion of the film is irradiated. Only the modulation structure is formed. In FIG. 6, when a light (L71) having a wavelength lower than that of the light source (71) is made to interfere and a photosensitive material (75) formed on a substrate (not shown) is irradiated, a minute structure of optical modulation is generated. A layered structure, a so-called Lippmann hologram, can be applied to the entire film formation (75). On the other hand, the light source (7
By interfering and irradiating with a wavelength having a higher absorbance than the arrangement of 1 ′), a periodic structure can be provided in the plane in the vicinity (75 ′) of the film surface.

As another manufacturing method, as shown in FIG. 7, a photosensitive material (85) formed on a substrate is irradiated with light (L82) having a wavelength of high absorbance using a mask pattern (86). In addition to the one in which the modulation structure of the birefringent ellipsoid is formed in the vicinity of the surface of the film formation in accordance with the mask pattern, in addition to the film formation, the light (L81) having the wavelength of the low absorption of the photosensitive material is interfered and irradiated. There is an example in which a birefringent ellipsoidal modulation structure is additionally formed on the entire film formation. This method also produces a film (85)
The entire Lippmann hologram and the vicinity of the film surface (8
5 ') can be provided with a periodic structure in the plane. The optical element having such a structure has a function of separating natural light into two orthogonal polarization components by the Lippmann hologram provided on the entire film formation, transmitting one and reflecting the other,
Since the periodic structure in the vicinity of the film-forming surface can play the function of refracting the polarized component to be transmitted, the function of separating and reflecting the polarized light and the function of refracting the transmitted light can be enhanced.

The above-mentioned photosensitive materials include, for example, substituents such as biphenyl, terphenyl, phenylbenzoate, and azobenzene, which are often used as mesogenic components of liquid crystalline polymers, and cinnamic acid groups (or their derivative groups). Hydrocarbon having a side chain containing a structure in which a photosensitive group of is bonded,
It is a polymer having a structure such as acrylate, methacrylate, maleimide, N-phenylmaleimide, and siloxane in the main chain. The polymer may be a homopolymer of the same repeating unit or a copolymer of units having side chains with different structures, or it is also possible to copolymerize units having side chains containing no photosensitive group. . The low-molecular compound to be mixed also has a substituent such as biphenyl, terphenyl, phenylbenzoate, and azobenzene which are often used as a mesogenic component, and the mesogenic component and allyl, acrylate, methacrylate, cinnamic acid group (or its A compound having crystallinity or liquid crystallinity in which a functional group such as a derivative group) is bonded with or without a flexible component. When mixing these low molecular weight compounds,
Not only a single compound but also a plurality of types of compounds can be mixed. In order to cause molecular orientation and birefringence in the material by irradiation with linearly polarized light, irradiation with light having a wavelength with which the photosensitive group moiety can react is necessary. This wavelength is generally 200-500 nm, especially 250-400 nm, although it varies depending on the type of the photosensitive group.
Is often highly effective.

Examples of materials suitably used for producing the optical element of the present invention are shown below together with the method of synthesizing the raw material compounds. (Monomer 1) 4,4′-biphenyldiol and 2-chloroethanol were heated under alkaline conditions to synthesize 4-hydroxy-4′-hydroxyethoxybiphenyl. This product is
6-Dibromohexane was reacted to synthesize 4- (6-bromohexyloxy) -4′-hydroxyethoxybiphenyl. Then, lithium methacrylate was reacted to synthesize 4- (2-hydroxyethoxy-4 ′-(6-methacryloyloxyhexyloxy) biphenyl.Finally, cinnamoyl chloride was added under basic conditions to give Formula 1. The indicated methacrylic acid ester was synthesized.

[Chemical 1]

(Polymer 1) Polymer 1 is prepared by dissolving Monomer 1 in tetrahydrofuran, adding AIBN (azobisisobutyronitrile) as a reaction initiator and polymerizing.
Got This polymer 1 exhibited liquid crystallinity in the temperature range of 47 to 75 ° C.

(Low molecular weight compound 1) 4,4'-biphenyldiol and 1,6-dibromohexane were reacted under alkaline conditions to synthesize 4,4'-bis (6-bromohexyloxy) biphenyl. Next, the low molecular compound 1 represented by the chemical formula 2 was synthesized by reacting with lithium methacrylate and purifying the product with a column.

[Chemical 2]

(Example) An example of a birefringent holographic optical element manufactured by the manufacturing method of the present invention will be described below. The birefringent holographic optical element of the embodiment is used in a surface light source device having a light guide plate having a property that the emission peak intensity of emitted light is inclined by 60 ° from the normal direction, and the emission peak intensity of light is emitted in a specific direction. Designed to convert. In general, due to the interference of two light beams incident from the incident angles P and Q, the photosensitive material has λ / {2 in the direction of (P + Q) / 2.
Interference manuscripts are formed at intervals of sin ((P + Q) / 2)}. The light is extracted in the direction of the emission angle Q by making the light incident on the hologram in which the interference pattern is recorded at the incident angle P.

Example 1 (Production Method) 3.75% by Weight of Polymer 1 and 1.25
A low molecular weight compound 1 (wt%) was dissolved in dichloroethane, and the solution was applied onto a substrate to a thickness of about 4 μm to form a film. The substrate is irradiated with light from a YAG laser (355 nm) as a light source in two directions at the same time from the upper surface to the surface normal direction and from the lower surface at 60 ° from the normal direction by the optical system shown in FIG. , So that the two lights overlap and interfere with each other in the film. Two YAG lasers (35
(5 nm) is irradiated with 200 mJ / cm ² as interference light of two light fluxes by the optical system shown in FIG.
After heating to ℃, it was cooled to room temperature. Then, for the purpose of fixing the birefringence modulation structure, ultraviolet rays were irradiated at 1 J / cm ² using a low pressure mercury lamp. (Evaluation) As shown in FIG. 5, the substrate (50) of the example was
Transparent acrylic resin wedge-shaped light guide plate (5
1), a cold cathode tube (52), and a reflection sheet (53). The peak intensity of light emitted from the light guide plate of this surface light source device is inclined by 60 ° from the normal direction. When the substrate of Example 1 is mounted here, the peak intensity of emitted light is in a direction inclined by 30 ° from the normal direction, and the degree of polarization V (V = (I0-I90) / (I0 + I90): where I0 Was 0.70, and I90 was the intensity of the polarized component having the smaller intensity, and I90 was the intensity of the polarized component having the smaller intensity. Further, the luminance in the normal direction of the liquid crystal display device equipped with the surface light source device using this substrate was enhanced by 10% as compared with the case where the prism sheet was used.

Example 2 (Production method) 3.75% by weight of Polymer 1 and 1.25
A low molecular weight compound 1 (wt%) was dissolved in dichloroethane, and the solution was applied onto a substrate to a thickness of about 4 μm to form a film. Light from a YAG laser (355 nm) was used as a light source on the substrate as shown in FIG.
From the arrangement of the light source (31) shown in Fig. 2, 200 mJ / cm 2 is irradiated as interference light of two light fluxes, and A is used as the other light source.
The light from the r laser (300 nm) is 100 m from the arrangement of the light source (31 ′) shown in FIG.
After irradiation with J / cm ² , the temperature was raised to 100 ° C. and then cooled to room temperature. Subsequently, for the purpose of fixing the hologram structure, ultraviolet rays of 1 J / cm ² were applied using a low pressure mercury lamp.
Irradiated. (Evaluation) When the substrate of Example 2 was placed on the surface light source device in the same manner as in Example 1, the peak intensity of the emitted light was in the direction inclined by 5 ° from the normal direction, and the polarization degree V (V = ( I0-I
90) / (I0 + I90): Here, I0 is the intensity of the polarization component with the higher intensity of the two complete polarization components, and I90 is the intensity of the polarization component with the lower intensity. ) Was 0.85. Further, the luminance in the normal direction of the liquid crystal display device equipped with the surface light source device using this substrate was enhanced by 23% as compared with the case where the prism sheet was used.

Example 3 (Production Method) 3.75% by Weight of Polymer 1 and 1.25
A low molecular weight compound 1 (wt%) was dissolved in dichloroethane, and the solution was applied onto a substrate to a thickness of about 4 μm to form a film. The substrate was irradiated with parallel light from a low-pressure mercury lamp using a Glan-Taylor prism as linearly polarized light and using a mask having a grating pattern, followed by YAG laser (355 nm).
From the arrangement of the light source (31) shown in FIG. 3 as interference light of two light fluxes at 200 mJ / cm ² and then 10
After heating to 0 ° C., it was cooled to room temperature. Then, for the purpose of fixing the hologram structure, ultraviolet rays were irradiated at 1 J / cm ² using a low pressure mercury lamp. (Evaluation) When the substrate of Example 3 was placed on the surface light source device in the same manner as in Example 1, the peak intensity of the emitted light was in the direction inclined by 2 ° from the normal direction, and the polarization degree V (V = ( I0-I
90) / (I0 + I90): Here, I0 is the intensity of the polarization component with the higher intensity of the two complete polarization components, and I90 is the intensity of the polarization component with the lower intensity. ) Was 0.85. Further, the luminance in the normal direction of the liquid crystal display device equipped with the surface light source device using this substrate was enhanced by 26% as compared with the case where the prism sheet was used.

[0018]

In the conventional surface light source device, in order to improve the light utilization efficiency, natural light is separated into two orthogonal polarization components, and one is transmitted through the optical element having the function of transmitting one and reflecting the other. The polarized component cannot be refracted, and it is necessary to use a prism sheet together to direct the light emitted from the surface light source device to the front direction, and a complicated process was required to manufacture the optical element. According to the present invention, natural light is emitted in a simple process of irradiating a film of a photosensitive material with two beams of interference light, or irradiating two beams of interference light and then irradiating with light using a mask pattern. It is possible to manufacture an optical element that has the function of separating two orthogonal polarization components, transmitting one, and reflecting the other, and the function of refracting the transmitted polarization component to improve the light utilization efficiency of the surface light source device. It became.

[0019]

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing separation of polarization components by a hologram element of the present invention.

FIG. 2 is a schematic diagram showing interference of two light fluxes.

FIG. 3 is a conceptual diagram showing a method for manufacturing a hologram element of the present invention.

FIG. 4 is a conceptual diagram showing separation of polarization components by a holographic optical element having a periodic structure in a plane.

FIG. 5 shows a surface light source device used for performance evaluation of an example birefringent holographic optical element.

FIG. 6 shows an example of a method for manufacturing the optical element of the present invention.

FIG. 7 shows another example of a method for manufacturing an optical element of the present invention.

[Explanation of symbols]

11 ... Optical element of the present invention L, L '... natural light L1 ... Polarization component that transmits L2: Reflected polarization component

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) // C08L 101: 00 C08L 101: 00 F term (reference) 2H049 AA25 AA34 AA43 AA48 AA60 BA05 BA42 BA45 BC05 BC22 CA05 CA22 CA28 CA30 2H099 BA17 CA05 DA00 4F071 AA33X AA77X AF31 AF35 AG02 AG15 BA02 BA09 BB02 BB12 BC01 BC02

Claims (4)

[Claims] 1. A film of a photosensitive material is irradiated with linearly polarized light beams from two different directions so as to interfere with each other, so that the degree of interference of the irradiated light beams can be adjusted in the plane of the film. A birefringent holographic optical element formed by forming a modulated structure of a birefringent ellipsoid, wherein the modulated structure of the birefringent ellipsoid is formed inhomogeneously in the thickness direction of film formation. Birefringent holographic optical element. 2. The birefringent holographic optical element according to claim 1, wherein the birefringent ellipsoidal modulation structure is formed as a layer inhomogeneous in the thickness direction of film formation. . 3. The method of manufacturing a birefringent holographic optical element according to claim 1, wherein the wavelength of the light beam with which the film is irradiated is set to a wavelength at which the light-absorbing property of the photosensitive material is low. A birefringent ellipsoidal modulation structure is formed in the deep part or the entire film formation, and in addition, the wavelength of the light flux irradiating the film formation is set to a wavelength where the light absorption of the photosensitive material is high, and the birefringence is caused in a relatively shallow part of the film formation. A method of manufacturing a birefringent holographic optical element, which comprises forming an ellipsoidal modulation structure. 4. A method of manufacturing a birefringent holographic optical element according to claim 1 or 2, which comprises:
In addition to the one in which the modulation structure of the birefringent ellipsoid is formed to the mask pattern by irradiating the film formation with a light flux having a high absorbance of the photosensitive material through the mask pattern, in a relatively shallow portion of the film formation, , A birefringent ellipsoidal modulation structure is additionally formed on the entire film formation by irradiating the film formation with the wavelength of the light flux in a state of interference as light having a wavelength at which the photosensitive material has low absorbance. Method for manufacturing birefringent holographic optical element.