JP2009098163A - Liquid crystal optical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal optical element which incorporates one-dimensional fine optical structure more excellent in performance and whose characteristics can be electrically controlled as necessary. <P>SOLUTION: The liquid crystal optical element which holds liquid crystal between two sheets of transparent substrates has a configuration provided with a fine optical structure which is set to be such a shape that alignment axis direction of liquid crystal molecules is vertical to the direction of one-dimensional structural change on at least one side transparent substrate of the two sheets of transparent substrates, whereby the damage of a diffraction element structure due to alignment treatment can be reduced. By constituting the liquid crystal optical element thus, the optical element which has the enhanced maximum diffraction efficiency and hardly causes resonance phenomena can be obtained. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a technique of a micro optical element, and more particularly to a liquid crystal optical element that electrically controls the optical characteristics of the micro optical element as necessary. In particular, the present invention relates to a liquid crystal optical element that electrically controls the diffraction characteristics of a one-dimensional multilevel binary diffraction element. Moreover, it belongs to the liquid crystal optical element provided with the one-dimensional fine optical structure below the wavelength in visible light range.

  From the practical use of spherical optical elements with gradual structural changes defined only by the radius of curvature, aspherical optical elements with more complex structures, and elements with fine optical structures such as diffractive optical elements. Systems have been simplified and enhanced. In recent years, elements capable of electrically controlling optical characteristics typified by liquid crystal optical elements have been put into practical use, contributing to higher functionality of optical pickups such as DVD devices.

Recently, multilevel binary diffraction gratings and moth-eye structures having a complicated structure obtained by staircase approximation of a blazed grating as a fine optical structure have been put into practical use. In addition, a liquid crystal optical element having such a fine optical structure has been proposed (see, for example, Patent Document 1). Since this liquid crystal optical element has a one-dimensional moth-eye structure on the liquid crystal layer side of the glass substrate and has a non-reflective function, the incident light incident on the element is highly coherent light such as laser light. Even so, the resonance phenomenon of light due to internal reflection of the element can be prevented.
International Publication No. 2004/086389 (page 7, Fig. 2-4)

  However, in the case of a multi-level binary type diffraction element, its characteristics are sensitive to the shape, and if the stepped edge is broken even a little, its function is significantly degraded. The moth-eye structure is the same, and if the edge shape collapses even a little, its function deteriorates. Here, in the liquid crystal optical element incorporating the moth-eye structure as the fine optical structure shown in Patent Document 1, the alignment process must be performed in the process of aligning the liquid crystal. The most practical method for this alignment treatment is a method of rubbing the organic alignment film, which is performed by rubbing the alignment film with fibers in a direction in which an alignment axis is desired. When the alignment treatment is performed by such a method, it is easy to cause fine damage particularly in the alignment axis direction in the fine optical structure. In the photo-alignment process and the vapor deposition alignment process, which are known as an alternative to the rubbing alignment process, the same phenomenon occurs to some extent. Therefore, in the course of the alignment treatment, the fine optical structure may be damaged particularly in the alignment direction.

  The object of the present invention is to solve the above problems and to provide a liquid crystal element or a micro optical structure with a multi-level binary diffraction element capable of electrically controlling diffraction characteristics, and a liquid crystal with little structural deterioration and hence little performance deterioration. An optical element is provided.

  In order to solve the above problems, the liquid crystal optical element of the present invention basically employs the following contents.

The liquid crystal optical element according to the present invention is a liquid crystal optical element in which liquid crystal is sandwiched between two transparent substrates. At least one transparent substrate of the two transparent substrates has an alignment axis direction of liquid crystal molecules and one-dimensional optics. It is characterized by having a fine optical structure in which the direction in which the structure changes is orthogonal.

  In the liquid crystal optical element of the present invention, the fine optical structure is a one-dimensional multilevel binary diffraction grating, and the one-dimensional one is obtained by making the alignment axis direction of the liquid crystal molecules and the direction of the lattice vector orthogonal to each other. The direction in which the optical structure changes is set.

  In the liquid crystal optical element of the present invention, the fine optical structure is a one-dimensional moth-eye structure.

  The liquid crystal optical element of the present invention is characterized in that the liquid crystal is a parallel alignment type liquid crystal.

  The liquid crystal optical element having the structure of the multi-level binary diffraction element in the present invention can electrically control the diffraction characteristics of the multi-level binary diffraction element, and further reduces the damage of the diffraction element structure due to the alignment treatment. The maximum diffraction efficiency can be increased.

  In addition, in the liquid crystal optical element having a one-dimensional fine optical structure (moth eye structure) of about the wavelength or less in the present invention, the damage of the fine optical structure due to the alignment treatment can be reduced, and an optical element with less resonance phenomenon can be obtained.

  The liquid crystal optical element according to the present invention includes a one-dimensional multilevel binary diffraction element or a one-dimensional moth-eye micro-optical structure on at least one transparent substrate constituting the element, and the one-dimensional structural change in the micro-optical structure. And the orientation axis direction of the liquid crystal molecules are orthogonal to each other. In addition, this one-dimensional multilevel binary diffraction element has a form in which a one-dimensional fine optical structure equal to or less than the wavelength of incident light incident on this element is contained, and the direction of structural change of the fine optical structure and the orientation of liquid crystal molecules The axial directions are orthogonal. The same applies to the moth-eye structure. The specific configuration of the liquid crystal optical element in the present invention will be described below.

  FIG. 1 shows a cross-sectional structure of a liquid crystal optical element having a multilevel binary diffraction element structure corresponding to a one-dimensional fine optical structure in the present invention.

  As shown in FIG. 1, in the liquid crystal optical element 100a of the present invention, the structure of a multilevel binary diffraction element 102 having a fine optical structure is added to a transparent substrate 101a, and the transparent electrode 103a is further formed on the surface of the diffraction element. And an alignment film 104a made of polyimide. Here, the multi-level binary diffraction element 102 has a one-dimensional structure, which will be described later, and has a four-step structure in which the direction of the grating vector is the Y-axis direction and the height per step is d. . The alignment axis 105 of the alignment film 104a provided on the diffraction element surface is the X-axis direction, and is set to be orthogonal to the direction of the grating vector in the multilevel binary diffraction grating 102. The direction of this lattice vector corresponds to the direction in which the one-dimensional optical structure in the present invention changes (the Y-axis direction in the figure).

A transparent electrode 103b and an alignment film 104b are formed on the surface of the transparent substrate 101b opposite to the transparent substrate 101a on which the multilevel binary diffraction element 102 is formed. The transparent electrode 103b is a solid electrode on the entire surface, and the alignment film 104b is an organic alignment film formed of polyimide that has been rubbed and aligned so as to be parallel to the alignment film 104a. In this way, the alignment films 104a and 104b are in a parallel alignment mode, so that the direction of the incident linearly polarized light of the incident light 106 is emitted from the element before and after the voltage is applied, so that the liquid crystal optical element 100a of the present invention is used. Thus, pure phase modulation can be realized.

  In addition, this drawing shows an example in which a transparent electrode 103a is divided into a plurality of parts for each multilevel binary diffraction grating 102 having a four-step staircase structure, and each of these transparent electrodes 103a is individually provided. Power can be supplied.

  With such a configuration, it is possible to arbitrarily form a region that supplies power to the transparent electrode 103a and a region that does not supply power, and the liquid crystal molecules contained in the liquid crystal layer 112 sandwiched between the transparent substrates 101a and 101b The orientation of the liquid crystal molecules 107a oriented in the direction perpendicular to the drawing and the liquid crystal molecules 107b oriented in the horizontal direction can be adopted.

  Next, the structure and characteristics of a one-dimensional multilevel binary diffraction element 102, which is one of the components of the liquid crystal optical element 100a, will be described. FIG. 2 is a diagram showing a Y-axis cross-sectional structure of the multilevel binary diffraction element 102, and shows a one-dimensional diffraction grating having no structural change in the X-axis direction.

  As shown in FIG. 2, the multilevel binary diffraction element 102 has a stepped structure in the Z-axis direction, which is the height direction, as described above. Here, the case of four stages is shown. The four-step staircase structure is a diffraction grating that repeats at a pitch P in the Y-axis direction. This is the direction in which the optical structure changes. The Y-axis direction at this time is defined as the direction of the grating vector.

  In FIG. 2, the staircase height 108 is constant at d. At this time, if the refractive index of the medium constituting the multilevel binary diffraction element 102 is n, the optical path length of the medium portion per step is n × d. On the other hand, if the refractive index of air is 1, the optical path length in the air portion per step of the staircase is d, so the optical path length difference between them is (n−1) × d, and the staircase-shaped optical path length distribution. You can see that

  Moreover, since the staircase structure is composed of four steps, the maximum optical path length difference is represented by 4 × (n−1) × d. If the number of steps is sufficiently large at this time, it can be seen that the staircase structure has a triangular prism shape that smoothly and linearly changes. This is a blazed grating that can theoretically realize 100% diffraction efficiency for a specific wavelength.

Here, it is assumed that the incident light 106 having the wavelength λ ₁ is vertically incident on the multilevel binary diffraction element 102 shown in FIG. At this time, if m is an arbitrary natural number and the following equation (1) is satisfied, 100% primary diffracted light 109 can be obtained theoretically. In the equation (1), the symbol || represents an absolute value. The diffraction angle θ of the first-order diffracted light 109 at this time is expressed by equation (2) using the previous period P.

m × λ ₁ = 4 × | (n−1) | × d (1)
P × sin (θ) = λ ₁ (2)

Here, it is assumed that the incident light 106 having the wavelength λ ₂ is vertically incident on the multilevel binary diffraction element 102 shown in FIG. At this time, if L is an arbitrary natural number and the optical path length difference with the air per step is the same as the wavelength λ ₂ or an integral multiple thereof, as shown in the equation (3), the staircase structure for the light It can be considered the same as not having. This is because light is a wave that repeats with a wavelength as a period. Accordingly, the incident light 106 at this time passes through the liquid crystal optical element 100 a as it is, and becomes zero-order light 110.

L × λ ₂ = | (n−1) | × d (3)

As described above, the binary diffraction element 102 can realize an optical characteristic that functions 100% with respect to a specific wavelength λ ₁ and does not function at all with respect to another specific wavelength λ ₂ .

  Next, the structure and operation of the liquid crystal optical element 100a of the present invention will be briefly described with reference to FIG. FIG. 3A shows the behavior of the liquid crystal molecules in the liquid crystal optical element 100a viewed from the direction rotated by 90 ° from the direction shown in FIG. 1, and FIG. 3B shows the behavior from the same direction as FIG. The state of liquid crystal molecules was shown.

  As shown in FIGS. 3A and 3B, transparent electrodes 103a and 103b are formed on a pair of transparent substrates 101a and 101b, and the transparent electrodes 103a formed on one transparent substrate 101a are vertically arranged in this figure. It is electrically divided into two. Further, it is assumed that an alignment film 104a is further applied on the transparent electrode 103a and the alignment axis 105 is set to be in the X-axis direction.

3A and 3B, it is assumed that no potential difference is given to the liquid crystal layer 112 sandwiched between the transparent electrodes 103a and 103b in the upper region in the drawing. At this time, the rugby ball-shaped liquid crystal molecules 107 a align their molecular major axes 111 in the direction of the alignment axis 105 in the vicinity of the alignment film 104 a. Therefore, the group of liquid crystal molecules 107a that behave as a continuum forms a liquid crystal layer 112 in which the molecular long axes 111 are aligned substantially parallel to the X-axis direction. Here, the liquid crystal molecules 107a shown in FIG. 3A are slightly tilted with respect to the X axis (alignment axis 105) because the pretilt angle is set, and the angle is about several degrees. is there. In this region, the incident light 106 having a linear polarization component in the X-axis direction propagates through the liquid crystal layer 112 while looking at the refractive index n _e of approximately molecular long axis 111.

On the other hand, when a sufficiently high voltage is applied to the liquid crystal layer 112 sandwiched between the transparent electrodes 103a and 103b in the lower region of both drawings, the liquid crystal molecules 107b have a molecular major axis 111 extending in the direction of the electric field Z. Stand still in the axial direction. In this case the incident light 106 propagates while watching the approximate molecular refraction index _{n o} of the minor axis 113. By adjusting the voltage applied transparent electrodes 103a, in 103b, it can be adjusted the tilt of the liquid crystal molecules 107 b, _{n o} the refractive index for incident light 106 of the liquid crystal layer 112 by this voltage, approximately _{n e} Can be changed continuously.

Therefore, it is possible to give a refractive index distribution according to the shape of the transparent electrode 103a formed on the surface of the transparent substrate 101a, and the depth of the refractive index distribution can be controlled by voltage. Here, the polarization direction of the incident light 106 must be the same as the direction of the orientation axis 105, more precisely, the direction of the molecular major axis 111 before applying a voltage. This is because with respect to the X-axis direction of the linearly polarized light regardless of the inclination of the liquid crystal molecules 107 b, propagates while watching the refractive index n _o of the molecular minor axis 113.

This phenomenon is the same as replacing the air layer with the liquid crystal layer 112. Therefore, if the refractive index of the medium constituting the multi-level binary type diffraction element 102 is n, any in the aforementioned (1) (3), instead of the refractive index 1 of air, from n _e to n _o It can be seen that the refractive index can be electrically selected and the design wavelength can be changed electrically.

N is 1.5, which is a typical value as an example, _{n e} is 1.7, _{n o} is 1.5, and d is 2 microns, if not give voltage to the liquid crystal optical element 100a, (3 From the equation (1), the wavelength of 400 nm passes when L is 1, and from the equation (1), when m is 2, almost 100% of the first-order diffracted light can be obtained with respect to the wavelength of 800 nm.

Further, a voltage is applied to the liquid crystal optical element 100a, and liquid crystal molecules 107a with respect to the incident light 106 are
If the effective refractive index of 107b is set to 1.6, similarly, light having a wavelength of 200 nm passes through, and almost 100% primary diffracted light can be obtained for light having a wavelength of 400 nm.

  To be exact, the refractive indexes and thicknesses of the transparent electrodes 103a and 103b and the alignment films 104a and 104b must be taken into account, but these thicknesses are neglected here because they are about 200 nm or less.

  Further, as described above, in the multi-level binary diffraction element 102, a blazed shape, that is, a triangular prism-shaped slope (indicated by a broken line in FIG. 2) is approximated stepwise. It is known that the theoretical maximum diffraction efficiency obtained at this time is about 82% in the case of four stages shown in FIG. 2 and about 95% in the case of eight stages.

  Here, as an example of the effect of the present invention, the maximum diffraction efficiency of the liquid crystal optical element 100a including the multilevel binary diffraction element 102 having a pitch of 10 microns, a step d of 0.285 microns, and the number of steps of 8 was measured. In this measurement, a laser beam having a wavelength of 650 nm was perpendicularly incident on the liquid crystal optical element 100a, and the first-order diffracted light intensity with respect to the incident light intensity was measured to obtain the ratio. In measuring the light intensity, a commercially available laser power meter was used. As a result, in the liquid crystal optical element 100a of the present invention, the value of the first-order diffracted light with respect to the design wavelength was 82%, which was a value relatively close to 95% of the theoretical maximum efficiency, but the direction orthogonal to this orientation direction. In the conventional liquid crystal optical element subjected to the alignment treatment, only about 1% of first-order diffracted light was obtained.

  In this way, if the liquid crystal optical element 100a of the present invention is set so that the alignment axis 105 in the alignment film 104a is orthogonal to the direction of the lattice vector of the multilevel binary diffraction element 102, not only the rubbing alignment process. In the alignment process such as the optical alignment process or the vapor deposition alignment process, it was confirmed that the step structure of the multi-level binary diffraction grating 102 was hardly damaged and the maximum diffraction efficiency was dramatically improved as compared with the conventional one.

  In the above description, the configuration of the liquid crystal optical element 100a in which the alignment films 104a and 104b are aligned in parallel is shown. However, the direction of the alignment axis 105 in the alignment film 104a is orthogonal to the direction of the one-dimensional structural change. If the above condition is satisfied, the same effect as that of the present invention can be obtained even if the orientation direction of the alignment film 104b is changed to form a twist alignment type or hybrid alignment type optical element.

  In the above description, an example in which the multi-level binary diffraction element 102 is disposed only on the transparent substrate 101a is shown, but this may be provided on both surfaces of the transparent substrates 101a and 101b as necessary. Absent. In this way, by providing the multilevel binary diffraction element 102 with the same grating and the same pitch on the surfaces of both transparent substrates 101a and 101b, the depth carved into each grating can be halved, and the manufacture of the element is facilitated. . Further, when different diffraction gratings are provided on both substrates, one element can have two types of diffraction functions.

  Next, another configuration example of the liquid crystal optical element of the present invention will be described with reference to FIG. FIG. 4 shows a cross-sectional structure of a liquid crystal optical element having a fine optical structure (moth eye structure) having a wavelength equal to or smaller than the wavelength of visible light.

  As shown in FIG. 4, in the liquid crystal optical element 100b shown in this embodiment, a moth-eye structure 114 is added to a transparent substrate 101a, and a transparent electrode 103a and an alignment film 104a are formed on the surface of the moth-eye structure 114. . Here, the moth-eye structure 114 is a one-dimensional moth-eye structure 114 shown in FIG. 5 described later. The direction in which the optical structure of the moth-eye structure 114 changes is the Y-axis direction in the figure.

  Further, since the pitch P of the moth-eye structure 114 is set to be smaller than about 400 nm, it can exhibit non-reflection characteristics with respect to incident light having a wavelength including visible light of 400 nm or more. The resonance phenomenon of light due to the internal reflection of 100b does not occur. Therefore, even if the tilt of the liquid crystal molecules 107a is changed by driving, it is possible to realize the liquid crystal optical element 100b with little variation in transmittance.

  Here, in the present invention, the alignment axis 105 of the alignment film 104a applied to the moth-eye structure 114 side is the X-axis direction as in the first embodiment, and is set to a direction in which the structure of the moth-eye structure 114 does not change. In addition, the alignment film 104b provided opposite to this is an organic alignment film that has been subjected to a parallel alignment process in the direction of the alignment axis 105 of the alignment film 104a. Therefore, in the configuration of the present embodiment, similarly to the first embodiment, it is difficult for the moth-eye structure 114 to be damaged due to the alignment treatment, and as a result, the liquid crystal optical element 100b with little resonance phenomenon can be realized.

  Next, the operation of the moth-eye structure 114 applied to this embodiment will be described. FIG. 5 is a diagram showing a Y-axis sectional view of the moth-eye structure 114 which is a component of the present invention, and shows a one-dimensional diffraction grating having no structural change in the X-axis direction.

  As can be seen from FIG. 5, the cross-section of the moth-eye structure 114 has an isosceles triangular structure that repeats at a pitch P, and has a grating vector in the Y-axis direction in the figure. Assume that the pitch P is smaller than the wavelength λ of the incident light 106. The diffraction angle of the primary light at this time can also be calculated by the equation (2) shown in the first embodiment, but sin (θ) becomes 1 or more and there is no real solution. This is because there is no diffracted light higher than the first-order diffracted light, and only the 0th-order light 110 that is transmitted as it is. This 0th-order light 110 is particularly called 0th-order diffracted light.

  The zero-order light 110 is known to behave peculiarly with an evanescent wave. For example, in the case of FIG. 5, the incident light 106 is not reflected and passes through the boundary on the incident side. This is because qualitatively, light is insensitive to an optical structure of a wavelength or less, and the structure is known on average.

  Here, it is assumed that the refractive index of the medium constituting the moth-eye structure 114 is n greater than 1, and the moth-eye structure 114 is placed in the air. At this time, the incident light 106 travels through the air and enters the transparent substrate 101a. When the base of the isosceles triangle that is the boundary of the one-dimensional moth-eye structure 114 (shown by a broken line in FIG. 5) is reached, the refractive index of the incident light 106 is n. As incident light 106 travels beyond this bottom portion, the refractive index felt on average decreases. This is because the ratio of the region having the refractive index n gradually decreases. When the incident light 106 exceeds the apex 116, the refractive index seen by the incident light 106 is 1. Therefore, since the refractive index seen by the incident light 106 continuously changes gradually from n to 1, there is no discontinuity of the refractive index, and the incident light 106 propagates without reflection and emits the 0th-order light 110.

  In addition, when the incident light 106 is incident from the apex 116 side of the moth-eye structure 114, the refractive index gradually and gradually changes from 1 to n after passing through the transparent substrate 101a.

  Further, when it is necessary to form a double-sided non-reflective coating, the moth-eye structure 114 may be formed on both sides of the transparent substrates 101a and 101b.

  Also in the embodiment shown in this embodiment, the same effects as those of the present invention can be obtained not only in the parallel alignment type but also in the twist alignment type or hybrid alignment type optical element as in the first embodiment. it can.

  In addition, the polarization direction of the incident light 106 that is linearly polarized light and is incident on the liquid crystal optical elements 100a and 100b described in the first and second embodiments is limited to a direction in which the optical structure does not change. That is, this direction is the major axis direction of the liquid crystal molecules, and the effective refractive index can be electrically varied. However, particularly in the case of a fine optical structure, it is known that the optical characteristics change significantly depending on the direction of polarization. Therefore, if the direction of the incident linearly polarized light is to be changed to the direction in which the fine optical structure changes, a liquid crystal substance having a property that the minor axis of the liquid crystal molecules is aligned with the alignment direction may be used.

  More precisely, the liquid crystal molecules in the vicinity of the alignment film interface are bound to the interface by a strong anchoring force and do not move at all even when a voltage is applied. However, in general, a region that does not move at all has a thickness of about 10 nm or less, and an experimentally negligible result has been obtained.

It is sectional drawing which shows the structural example of the liquid crystal optical element of this invention. Example 1 It is the figure which showed the structure and effect | action of the one-dimensional multilevel binary type | mold diffraction element which is a component of this invention. Example 1 It is the figure which showed operation | movement of the liquid crystal optical element of this invention. Example 1 It is sectional drawing which shows the other structural example of the liquid crystal optical element of this invention. (Example 2) It is the figure which showed the one-dimensional moth-eye structure which is one of the one-dimensional fine optical structures which are the components of this invention, and an effect | action. (Example 2)

Explanation of symbols

100a, 100b Liquid crystal optical element 101a, 101b Transparent substrate 102 Multi-level binary diffraction element 103a, 103b Transparent electrode 104a, 104b Alignment film 105 Alignment axis 106 Incident light 107a, 107b Liquid crystal molecule 108 Stair height 109 First order diffracted light 110 0th order Light 111 Molecular long axis 112 Liquid crystal layer 113 Molecular short axis 114 Moss eye structure 116 Vertex

Claims (4)

In a liquid crystal optical element that sandwiches liquid crystal between two transparent substrates,
A liquid crystal optical element comprising: at least one of the two transparent substrates, a fine optical structure in which an alignment axis direction of liquid crystal molecules and a direction in which a one-dimensional optical structure changes are orthogonal to each other. The fine optical structure is a one-dimensional multilevel binary diffraction grating,
2. The liquid crystal optical element according to claim 1, wherein the direction in which the one-dimensional optical structure changes is set by making the alignment axis direction of the liquid crystal molecules and the direction of the lattice vector orthogonal to each other. The liquid crystal optical element according to claim 1, wherein the fine optical structure is a one-dimensional moth-eye structure. The liquid crystal optical element according to claim 1, wherein the liquid crystal is a parallel alignment type liquid crystal.

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