JPH0776814B2 - Light modulator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a light modulation element, in particular, an element that combines a liquid crystal and a diffraction grating to change the refractive index of the liquid crystal with respect to the incident light to exert a desired diffracting action on the incident light. In addition, the present invention relates to an optical modulation element suitable for a display element and a device such as optical recording and optical communication.

<Prior Art> Conventionally, as a light modulation element in which a liquid crystal and a diffraction grating are combined, there are a display element and a variable color filter element in Japanese Patent Publication No. 53-3928 and USP 4,251,137. The light modulation elements disclosed in these documents obtain a decorative effect and a color filter effect by utilizing a diffraction phenomenon, and the relief pattern formed on the substrate functions as a diffraction grating.

On the other hand, USP3,787,110 and USP4,256,787 show that the liquid crystal is aligned by a fine regular relief pattern having the same shape as the diffraction grating, and the diffraction function is given by giving the diffraction grating a predetermined size and shape. It is also possible to satisfy both the liquid crystal alignment function and the liquid crystal alignment function at the same time in view of the conventional technique.

However, since the orientation control force by the relief pattern as in USP3,787,110 and USP4,256,787 exerts the force only within the region surrounded by the side surface and the bottom surface of the recess of the relief pattern, the Japanese Patent Publication No. 53-3928. Even if it is applied to devices such as Japanese Patent Publication No. US Pat. No. 4,251,137, etc., the tilt direction of liquid crystal molecules is not uniform with respect to the electric field, and the reliability is poor. Further, when this type of element is used as a display element, there is a drawback that display unevenness occurs in a static state and response unevenness occurs in a dynamic state. That is, there is a problem that the orientation of the liquid crystal molecules is locally changed depending on the interface state due to the thermal motion or the energy of the electric field because there is no regulation force for the pre-tilt of the liquid crystal molecules.

<Summary of the Invention> In view of the above-mentioned conventional problems, an object of the present invention is to provide a light modulation element having an improved function relating to liquid crystal alignment by a diffraction grating and improved electro-optical characteristics and temperature dependence thereof. is there.

In order to achieve the above object, the light modulation element according to the present invention,
A pair of substrates, a diffraction grating structure provided between the pair of substrates, in which transparent members and liquid crystals having different refractive indexes in a certain state are alternately arranged, and an alignment state of molecules of the liquid crystal are changed. Means for tilting the molecules of the liquid crystal with respect to the surface of the substrate and projecting the surface of the substrate substantially parallel to the groove direction of the diffraction grating structure. The surface of the diffraction grating structure side is subjected to an orientation treatment. Further characteristics of the present invention will be apparent from the examples shown below.

Further, in the present invention, the shape of the diffraction grating is not limited, and in addition to the rectangular diffraction grating shown in the examples described later, various types of diffraction gratings such as triangular wave shape, sine wave shape and asymmetric shape are applied. .

<Example> FIG. 1 shows a basic configuration of an optical modulator according to the present invention.
It also serves as a functional explanatory view of the present light modulation element. In the figure, 1 is a liquid crystal, and 2 is a diffraction grating made of a member transparent to a used wavelength, and the liquid crystals 1 are alternately arranged along the substrate surface.
And each part of the diffraction grating 2 form a diffraction grating structure. Reference numeral 3 is a transparent electrode, 4 is a transparent substrate made of a transparent optical member, 5 is incident light having a predetermined polarization characteristic, 6 and 6'are polarization components of the incident light 5 which are orthogonal to each other, 6 is a direction perpendicular to the paper surface, and 6'is Indicates the direction parallel to the paper surface.

In the present light modulation element, a transparent electrode 3 is formed on opposite surfaces of a pair of transparent substrates 4, and a rectangular diffraction grating 2 made of a transparent material is provided on one transparent electrode 3 of the pair of transparent substrates 4. The refractive index variable substance 1 is arranged in the groove (concave portion) of the diffraction grating 2 and its refractive index is variable by applying an electric field through the transparent electrode 3.

The principle of modulation of the present light modulator will be described below with reference to FIG.

Here, it is assumed that a predetermined diffracting action is generated by controlling the alignment state of the liquid crystal 1 by applying an electric field.

In a static state where no electric field is applied as shown in FIG. 1, the liquid crystal 1 is aligned in the groove direction of the diffraction grating 2, that is, in the direction perpendicular to the paper surface, and maintains the homogeneous alignment state. Therefore, the incident light 5 entering the present light modulation element in this static state
Of the polarization components 6 and 6 ′ of the liquid crystal 1, the polarization component 6 ′ which is a component orthogonal to the alignment direction of the liquid crystal 1 senses the ordinary refractive index n _o of the liquid crystal 1,
The polarization component 6 that is a component parallel to the alignment direction of the liquid crystal 1 feels the extraordinary refractive index n _e of the liquid crystal 1. Here, if the refractive index of the substance forming the diffraction grating 2 is n _g , the wavelength of the incident light 5 is λ, and the thickness of the diffraction grating 2 is T, the polarization component of the incident light 5 is obtained in the case of a rectangular diffraction grating. The diffraction efficiency η _o of the zero-order transmitted diffracted light with respect to each of 6,6 ′ can be approximately expressed by the following equation (1).

Where Δn is the refractive index n _g of the diffraction grating 2 and the refractive index n _{e of the} liquid crystal 1.
Or the difference in refractive index from n _o , Δn = | n _e −n _g | for the polarization component 6 of the incident light 5 and Δn = | n _g −n _o for the polarization component 6 ′. |

Therefore, from the equation (1), when Δn = 0, that is, n _e = n _g or n _o = n _g , the diffraction efficiency η _o of the zero-order transmitted diffracted light becomes η _o = 1
or, At that time, the diffraction efficiency η _o is η _o = 0.

Next, when an electric field is applied to the liquid crystal 1 through the transparent electrode 3, the alignment direction (optical axis direction) of the liquid crystal 1 gradually changes,
於Ru polarized component 6 in the incident light 5 'felt ordinary refractive index n _o of constantly LCD 1 regardless of the electric field is applied, the polarization component 6 of the liquid crystal 1 according to electric field application amount extraordinary refractive index n _e and ordinary index n _o And feel the combined refractive index n _θ combined at a predetermined ratio. Needless to say, the combined refractive index n _θ changes as the orientation of the liquid crystal 1 changes.
Changes. When the amount of applied electric field is further increased, the liquid crystal 1 is aligned perpendicularly to the substrate 4 (transparent electrode 3) and enters a homeotropic alignment state, so that the polarization components 6 and 6 ′ of the incident light 5 are both normal refractive index n _{o of the} liquid crystal. Feels saturated.

Even in this state, the incident light 5 is modulated according to the equation (1).

FIG. 2 is a schematic perspective view showing an embodiment of a light modulation element according to the present invention. Light obtained by superposing elements having the basic structure shown in FIG. 1 so that the arrangement directions of diffraction gratings are orthogonal to each other. It is a modulator. In the figure, the same members as those in FIG. 1 are designated by the same reference numerals, and 7 is a transparent substrate forming a spacer, on which transparent electrodes 3 are formed.

As described with reference to FIG. 1, when light modulation is performed using a single diffraction grating 2, for example, when the alignment state of the liquid crystal 1 is changed from homogeneous alignment to homeotropic alignment by an electric field or the like, FIG. The polarized light component 6 of the incident light 5 is modulated, but the polarized light component 6'is not modulated because the perceived refractive index does not change, and when the incident light 5 has a random polarization characteristic, it is substantially 50%. Only% light can be modulated. However, by adopting the superposed structure as shown in FIG. 2, each diffraction grating 2 independently acts on the respective polarization components 6, 6 ′ of the incident light 5 to diffract, and all the polarization components The light can be modulated.

As shown in FIGS. 1 and 2, the liquid crystal 1 is oriented in the groove direction of the diffraction grating 2, but in the conventional method, the regulating force for orienting the liquid crystal 1 is the groove portion (recessed portion) of the diffraction grating 2. ) Is only the physical orientation control force by the side wall, and when actually driving the device, the orientation of liquid crystal molecules changes depending on the interface state etc. due to the energy of thermal motion or electric field, and when applied as a display device And uneven response occur, resulting in poor reliability.

However, in the present invention, the bottom of the diffraction grating 2 of the element as shown in FIGS. 1 and 2 or the opposing substrate 4 (3),
Alignment treatment that regulates the tilt direction of the liquid crystal molecules is applied to one of the liquid crystal molecules 7 to keep the tilt direction of each liquid crystal molecule constant. As the orientation treatment, generally known rubbing treatment, oblique vapor deposition of SiO ₂ , and the like are suitable.

3 (A), (B), and (C) are schematic diagrams for explaining the effect of the alignment treatment for defining the tilt of the liquid crystal molecules in the present invention, and FIG. 3 (A) shows the alignment treatment. The state of liquid crystal molecules at the interface when not applied, FIG. 3 (B) is the state of liquid crystal molecules at the interface when alignment processing is applied, and FIG. 3 (C) is the orientation processing to define the tilt. The behavior of liquid crystal molecules when an electric field is applied is shown.

As can be seen by comparing FIGS. 3 (A) and 3 (B), the tilt direction of the liquid crystal molecules is random at the interface not subjected to the alignment treatment, and the element is applied by applying an electric field or the like. In addition to the above-mentioned problems during driving, incident light is extraordinarily polarized due to disturbance of liquid crystal molecules at the interface even in a static state, and display performance is deteriorated. On the other hand, at the interface subjected to the alignment treatment, the liquid crystal molecules are aligned at the interface with a uniform inclination, that is, a pre-tilt angle θ mainly due to the chemical alignment regulating force. Therefore, since the pre-tilt directions are aligned, abnormal polarization or the like does not occur even in the static state. Naturally, as shown in FIG. 3C, the behavior of liquid crystal molecules when an electric field is applied is different from each other. The molecules match and change the orientation direction. Therefore, as compared with the conventional method in which only the diffraction grating is used for orientation, the response is good and display unevenness does not occur.

Hereinafter, specific examples of the present light modulation element will be described together with a simple manufacturing method.

First, a diffraction grating having a thickness of 1.5 μm and a pitch of 1.5 μm was formed in a display pattern shape on a transparent electrode patterned on the substrate in a display pattern shape.

On the other hand, after forming a transparent electrode on the substrate surface of the counter substrate to be bonded to the substrate having this diffraction grating, a polyimide alignment film is provided on the transparent electrode, and the groove direction of the diffraction grating and the tilt direction of liquid crystal molecules are in the same direction. Alignment treatment was performed by rubbing treatment so that Then, the two transparent substrates produced by the above method were brought into close contact with each other and the liquid crystal was sealed therein to produce an optical modulation element as shown in FIG.

In the light modulation element according to the present embodiment, the liquid crystal has a so-called homogeneous alignment in which the liquid crystal has a pre-tilt in a predetermined direction and a director (alignment direction) is provided in the groove direction of the diffraction grating.

Therefore, the liquid crystal molecules can change their directions uniformly and rapidly by turning on / off the electric field application, and can respond at high speed. Further, the rearrangement time of liquid crystal molecules when the electric field is turned off at a low temperature is shortened, and the temperature dependence of the refractive index defining the display color of the present light modulator is also improved. Also, the color variation due to temperature was reduced.

Furthermore, there is a gap between the counter substrate and the upper part of the diffraction grating,
Even if liquid crystal molecules are present in the voids, the liquid crystal molecules are not disturbed, reliability with respect to an electric field and temperature can be obtained, display unevenness and display delay can be reduced, and display quality and reliability can be improved. It is possible.

In the present embodiment, the rubbing method is used as the orientation treatment method for the substrate, but anisotropic etching such as ion beam or plasma or oblique vapor deposition of SiO ₂ can also be used. Needless to say, on the contrary, the alignment treatment may be performed on the surface of the substrate having the diffraction grating by using the method as described above.

Hereinafter, an example in which an alignment regulating force different from the above example is generated will be described.

0.5 w of octadecyltriethoxysilane was used as an aligning agent on the counter substrate of the substrate on which the diffraction grating was formed in the above example.
Apply t% ethanol solution by spin coating and 100 ℃
After heat treatment for 1 hour, a rubbing treatment was performed so that the groove direction of the diffraction grating and the tilt direction of the liquid crystal molecules were aligned, and a light modulation element was prepared in the same manner as in the above-mentioned embodiment. By this alignment treatment, the liquid crystal molecules on the surface of the alignment substrate are aligned in the thickness direction of the diffraction grating, that is, in the direction perpendicular to the substrate surface, and exhibit so-called hybrid alignment in a static state. That is, the change has a molecular arrangement in which the homeotropic state shifts to the homogeneous state, and the tilt of the liquid crystal molecules at the interface can be defined.

In the above-mentioned embodiments, a transmissive optical modulator is shown, but it is also possible to provide a light reflective film on one of the substrates to form a reflective element. However, in the case of the reflection type, the behavior of the diffracted light inside the element becomes complicated. Therefore, in consideration of the design and the practical application of the display element and the like, in the present invention, the transmission type light modulation element is configured. preferable. in this case,
As a matter of course, the diffraction grating, the liquid crystal, the substrate, and the like use members having transparency to the used wavelength.

<Effects of the Invention> As described above, in the light modulation element according to the present invention, in addition to the alignment regulating force by the diffraction grating, the alignment treatment is performed on the surface of one of the pair of substrates sandwiching the diffraction grating structure on the diffraction grating structure side. Particularly, it is an optical modulation element capable of improving display contrast when used as a display element, preventing uneven response and uneven display, and obtaining high display quality and responsiveness.

[Brief description of drawings]

FIG. 1 is a modulation principle diagram of an optical modulator according to the present invention. FIG. 2 is a schematic perspective view showing an embodiment of the light modulation element according to the present invention. 3 (A), (B) and (C) are explanatory views showing the effect of the alignment treatment in the present invention. 1 ... Liquid crystal or liquid crystal molecule 2 ... Diffraction grating 3 ... Transparent electrode 4 ... Transparent substrate 5 ... Incident light 6,6 '... Polarization components orthogonal to each other

Claims (1)

[Claims] 1. A pair of substrates, a diffraction grating structure provided between the pair of substrates, wherein transparent members and liquid crystals having different refractive indices in a certain state are alternately arranged, and molecules of the liquid crystal. And a means for changing the orientation state of the liquid crystal, wherein the molecules of the liquid crystal are inclined with respect to the surface of the substrate and the projection on the surface of the substrate is substantially parallel to the groove direction of the diffraction grating structure.
An optical modulation element, wherein an alignment treatment is applied to a surface of one of the pair of substrates on the side of the diffraction grating structure.