JPH04149519A - Optical modulation element and display device using the element - Google Patents

Abstract

PURPOSE:To obtain the optical modulating element which is prevented from decreasing in contrast while the reflection of light passing through films differing in refractive index is suppressed by minimizing the difference in refractive index between adjacent films to plural boundary surfaces. CONSTITUTION:On a border surface between layers which differ in refractive index, the reflective indexes become smaller as the difference in refractive index between the two adjacent layers grow smaller and when incident light is reflected by plural border surfaces, the refractive indexes of the respective layers are most preferably intermediate between the refractive indexes of the two adjacent layers so that the final refractive index is made small. Here, n1>n2>n3, and n2 is intermediate between n1 and n3, where n1, n2, and n3 are the refractive indexes of a transparent electrode layer 302, an insulating layer 303, and a ferroelectric liquid crystal layer 305 in the thin film laminate structure of the optical modulating element. Consequently, the reflection of light passing through the films differing in refractive index is evaded as much as possible and the decrease in contrast is greatly reduced.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical modulation element using a material having refractive index anisotropy, particularly an optical modulation element using a ferroelectric liquid crystal, and an optical modulation element using the optical modulation element. Regarding the display device used.

(Prior art) In an optical modulation element using ferroelectric liquid crystal, a liquid crystal layer is formed between two parallel plates with a very thin gap (for example, 1 to 2 μm) between the two plates. A method of creating a bistable state using surface action (5SFLC, ^pp1.
Phys, Lett, 3B (1980) 899) is expected to find a variety of applications due to its high-speed response, memory performance, etc.

The above-mentioned bistable ferroelectric liquid crystal element is an agricultural tool having a certain angle with respect to the axial direction (rubbing direction, etc.) of the alignment working surface formed by rubbing or the like on the liquid crystal layer side of the plates on both sides sandwiching the liquid crystal layer. It shows two stable states in the direction. This angle is called the cone angle (hereinafter expressed as θ).

When a voltage is applied in a direction perpendicular to the liquid crystal layer surface of the element,
The ferroelectric element moves from one stable state to another. This change corresponds to rotating the optical axis of the material having refractive index anisotropy by an angle of 2θ. Therefore, when polarized light is incident on the ferroelectric liquid crystal element having a thickness corresponding to the effect of a narrow wavelength plate, the polarization rotation effect on the incident polarized light due to the two bistable states is 4θ with respect to each other.
Only C is different. When the strong electroconductive liquid crystal element is sandwiched between polarizing elements (polarizing plates, etc.) with crossed Nicols or parallel Nicols arrangement, when 4θo=90° (θ.=225°), the amount of transmitted light in both bistable states is turned on.・Off ratio (
(transmittance ratio, contrast) is the highest.

However, the cone angle strongly depends on the characteristics of the liquid crystal material and the alignment surface, and a strong electrostatic liquid crystal element with a sufficient cone angle has not yet been realized. The degree of modulation is insufficient.

As a method for solving the above-mentioned problems, a method is known in which two optically modulated strong liquid crystal elements and a local wavelength plate are combined. The outline is shown below. FIG. 4 shows its structure and operation. Figure 4(a) is composed of an identical bistable ferroelectric element 1.3 and a local wavelength plate 2 whose alignment axes are aligned, and the liquid crystal molecules in one of the two ferroelectric elements 1.3 in a stable state. Long axis direction n I +
13 (strictly speaking, the life axis direction of the refractive index ellipsoid of the liquid crystal) and %
The three life axis directions n2 of the refractive index ellipsoids of the wave plate 2 face the same direction. The three plates 1, 2.3 are parallel to each other, and each acts as a % wave plate with respect to the dominant wavelength. For the liquid crystal element of this configuration, n1□ n2. When an electromagnetic wave E with an oscillating electric field parallel to n3 is incident, the oscillating electric field E, , E2 . after passing through each plate 1, 2.3. The direction of the oscillating electric field of E3 and the output light E does not change (E person E1 E)
2mu E1 out (=E3)).

On the other hand, FIG. 4(b) shows the configuration when bistable ferroelectric element 1.3 is kept in the other stable state, and elements 1, 3
The long axis direction of the liquid crystal is rotated in the same direction by 2θC with respect to the direction of the oscillating electric field of the incident light E. First, the electric field E1 of the light that has passed through the liquid crystal element 1 of 581 is 4θ with respect to the incident light.
Rotates 0 times. Next, the electric field E of the light that has passed through the wavelength plate 2
2 is a direction rotated by -400 with respect to the 1 principal axis n2 of the refractive index. Finally, 6θ for E2, (=40
The electric field E of the light that has passed through the second liquid crystal element 3, which has the long axis n3 of the liquid crystal molecules in the direction rotated by c+2θC), (=E output)
is 6θ with respect to the liquid crystal molecule long axis n3. The direction is the direction of rotation. Therefore, the electric field direction of the emitted light will be rotated by 8θc (=20, +6θC) with respect to the electric field direction of the incident light, making it possible to rotate polarization twice as much as with the current liquid crystal element technology. It is shown that. That is, θ,=1
The on-off ratio (transmittance ratio, contrast) of the amount of transmitted light can be maximized at a no cone angle of 1.25'.

For example, the optical modulation element proposed by the present applicant includes: (1) a bistable ferroelectric liquid crystal element capable of optical modulation (having an action equivalent to a % wave plate), (2) a Figure 0 wavelength plate, (2) a reflector, and a polarized light It is characterized by being arranged in the following order from the light incident side: the modulated part (i.e., the liquid crystal layer), 2, and 2 from the light incident side.

As a result, the light rays pass through the liquid crystal element twice through the reflective surface, and the light rays pass through the radio wave plate twice through the reflective surface during that time, so it functions as a % wave plate, and therefore the liquid crystal Twice as much polarization rotation is possible compared to the element alone. Therefore,
Only one optically modulated liquid crystal element is required, and the configuration is simple.
Another advantage is that highly accurate registration is not required. or,
By using polymeric liquid crystal as a radio wave plate, the refractive index anisotropy is greater than that of ordinary anisotropic materials (crystal, mica, stretched film, etc.), so the thickness can be made thinner, and the aperture ratio can be reduced.
It can be used in a temperature range where inter-pixel crosstalk is alleviated or alignment is stabilized after alignment in high-temperature conditions with high fluidity, so low It has the advantage of being easier to handle than molecular liquid crystals.

Fig. 5 shows the configuration when a conventional optical modulation element is applied to a projection display device, Fig. 6 is a schematic explanatory diagram of the operation of the optical modulation element section, and Fig. 3 shows its details. This shows the configuration.

In FIG. 5, light emitted from a light source 6 that emits undefined polarized light is reflected by a beam splitter 7, collimated by a condenser lens 8, and then enters a polarizing beam splitter 9, and the P polarized light component is transmitted to the polarizing beam splitter 9. S f
The Ji light component is reflected in the vertical direction. The above S-polarized light component is a bistable ferroelectric state crystal equivalent to a % wave plate. The light passes through the wavelength plate 4, is reflected by the reflection plate 5, and passes through the wavelength plate 4 and the bistable ferroelectric state crystal 1 again.

The S(j! light component generates a P-polarized light component depending on the state of the liquid crystal element 1, and when it enters the polarization beam splitter 9 again, the S(I4 light component is reflected, and the transmitted P-polarized light component is reflected by the projection lens 10. As a result, an image is formed and projected onto an image projection screen (not shown).In this configuration, the polarizing beam splitter 9 serves as both a polarizer and an analyzer.

In FIG. 6, the incident light is linearly polarized light with an oscillating electric field. In a stable state in which the long axis n of the ferroelectric liquid crystal 1 is parallel to the long axis n4 of the wave plate 4, no polarization rotation occurs. On the other hand, as shown in FIG. 6, when the long axis n of the liquid crystal 1 is rotated by 2ec with respect to the person E, the electric field E1 of the light passing through the liquid crystal element 1 is initially Rotates by 4θC. Next, the electric field E2 of the light that has passed through the wave plate 4 becomes circularly polarized light, is opposed by the reflection plate 5, and enters the wave plate 4 again. The electric field E3 that has passed through the wavelength plate 4 is −
40, the direction is rotated by 40 degrees. Finally 6 for I3
The direction rotated by 0c (=400+2θC) is the direction rotated by 6θC with respect to the long axis of the liquid crystal molecules. Therefore, the electric field direction of the emitted light is 800 (
= 20° + 6θC).

With this reflective configuration, a polarization rotation angle twice as large can be obtained with one modulation element, which eliminates the need for registration and is advantageous in terms of cost.

FIG. 3 shows the part shown in FIG. 6 in more detail, and shows, from the incident light side, a transparent glass substrate 301 (approx. 1 mm), a transparent ITO film 302 having an electrode function (
1,500 mm thick), an insulating film 303 (approximately 1200 mm thick) to avoid short circuit with the counter electrode, and a polyimide film 304 (approximately 2 mm thick) subjected to rubbing treatment to orient the liquid crystal.
00 persons), a liquid crystal layer 305 injected into a gap held by beads with a diameter of 1 to 2 μm (not shown), and a polyimide film 30.
A polyimide film 306 (thickness of approximately 200 mm) similar to 4, a thin transparent layer 307 (for example, a glass plate) that serves as a substrate for the polyimide film 306, and a polymer liquid crystal layer that has refractive index anisotropy and functions as a wave plate. 308 (thickness 1 μm or less), polyimide film 309 (thickness approx. 200) subjected to rubbing treatment to facilitate alignment of polymer liquid crystal, rTO film 302C
Aluminum vapor deposition @ 310 (thickness of several μm) that serves as a counter electrode and a light reflection function, and a glass substrate 31
1 (thickness approximately 1.mm).

Here, in the optical modulation element, after necessary layers are sequentially formed on the glass substrates 301 and 311, a liquid crystal material is injected into the gap 305 between the two, and the layer is further brought into a bistable ferroelectric state by heat treatment or the like. Modulation of the polarization state of the emitted light is performed by applying signals to the electrodes 302 and 310. The optical modulation element can be easily applied to image display by independently modulating a plurality of pixels. For example, ITO electrode 302 and aluminum electrode 310
This is a case where a plurality of independent electrodes each having a strip shape are used, and both are orthogonal to each other to form a matrix type structure (so-called simple matrix drive). When displaying images in this way, it is necessary to reduce the size of each pixel in order to improve resolution at the same screen size.
When using a small liquid crystal display element such as a projection display device, for example, one pixel is approximately 60 μm wide on a 3-inch diagonal.
(for EDTV).

In the above element configuration, as a wave plate, the refractive index anisotropy is much larger than that of quartz, mica, stretched film, etc.
By using a polymer liquid crystal (Δn~0.2) (2 digits), the thickness of the wave plate can be suppressed to 1 μm or less.

In addition, a glass plate 307 serves as a substrate for the polyimide film 306.
is a glass substrate 3 with sufficient thickness to hold the element.
11, a very thin glass material can be used. Pixel chamber 8302. The other layers between 310 and 310 are sufficiently thin compared to the size of one pixel even at the conventional technology level, so the wavelength plate 308 made of polymer liquid crystal and the ultra-thin glass plate 307 improve the effective aperture ratio. It exerts great power in preventing crosstalk between pixels.

(Problem to be Solved by the Invention) However, since the above-mentioned optical modulation element configuration has a thin film laminated structure, the incident light is reflected at the interface between two layers having different refractive indexes due to the difference in refractive index between two adjacent layers. As a result, the amount of incident light decreases, resulting in a decrease in contrast. In addition, interference effects due to the extremely thin film structure and effects of scattered light due to the surface roughness of the interface are also considered, but these are extremely small compared to the amount of reflected light due to the difference in refractive index as described above.

In conventional optical modulation elements, in the thin film laminated structure, the refractive index difference is small at certain interfaces and the reflectance is kept low, but at other interfaces the refractive index difference becomes large and the amount of reflected light increases. No consideration was given to reducing the amount of reflected light and preventing a decrease in contrast by reducing the refractive index difference at multiple interfaces.

The present invention has been made in view of the above-mentioned drawbacks of the prior art, and an object of the present invention is to provide an optical modulation element that suppresses the reflection of light passing through films having different refractive indexes and prevents a decrease in contrast.

[Means and effects for solving the problem] In order to achieve the above object, the present invention provides at least three optical modulation elements.
In a thin film laminated structure consisting of more than one layer, the reflectance of each interface and the total reflectance of the laminated film are reduced by minimizing the difference in refractive index between adjacent films for multiple interfaces,
Achieved improved contrast.

(Example) FIG. 1 is a sectional view of an example of the present invention. In the figure, 301 is a transparent glass substrate (thickness approximately 1 mm), 302 is a transparent electrode ITO (thickness approximately 1500 mm), refractive index r), =
1.92.303 is an insulating film (thickness of about 1200 nm) (e.g. 5io2/Ti02), and refractive index 1.74.305 is a strongly galvanic liquid crystal layer (thickness of about 2 μm) (e.g. (S)2-methylbutyl- p[(p-n-decyloxybenzylidene)aminococinnamate, commonly known as DOBAMBC) with a refractive index of 1.
58. 306 is a polyimide film (about 200 mm thick) for orienting the strong electrolytic liquid crystal layer 305, and 307 to 311 have the same structure as that shown in FIG.

At the interface between layers with different refractive indexes, the smaller the difference in refractive index between two adjacent layers, the lower the reflectance, and the reflectance R when incident light passes from the refractive index n1 layer to the refractive index nb layer is mainly can be expressed as . In addition, when incident light is reflected at multiple interfaces, in order to reduce the final reflectance, the refractive index of each layer should be approximately midway between the refractive index of two adjacent layers. It is most desirable to have.

In the configuration of this embodiment, n1=1.92. n2”1.7
4. Since ns = 1.58, the reflectance at the transparent electrode layer/insulating film layer is approximately 2.42x10-' The reflectance at the insulating film/ferroelectric liquid crystal layer is 2.32x10. This reduces the amount of incident light to 99.5%. however,
If n is not set somewhere between n and 03, but is very close to nl, for example, n=1.9, the amount of incident light will further decrease to 99.2%.

Furthermore, when there is no intermediate layer, the reduction is significantly reduced to 9905%.

FIG. 2 is a diagram of another embodiment of the invention. In the figure,
301 is a transparent glass substrate (thickness about 1 mm>), 302 is a transparent electrode ITO (thickness about 1500 mm) with a refractive index n, = 1.
92.303 is an insulating film (approximately 1200 mm thick) (for example, E
T120s formed by B evaporation) n2 = 1.78
．． 304 is an alignment film of polyimide (about 200 layers thick) for aligning the ferroelectric liquid crystal layer 305, with a refractive index n5=1.6.
8.305 is a ferroelectric liquid crystal layer (approximately 2 μm thick) with n, = 1
．． 58. 306 to 311 have the same structure as in FIG.

In this example, as a process for aligning the ferroelectric liquid crystal layer, a polyimide alignment film is formed as shown in the configuration in the figure, but this does not contradict the intention of the present invention (nl>n2>ns) .

In the configuration of this embodiment, transparent electrode layer/insulating film layer, insulating film layer/
When passing through the three interfaces of the polyimide alignment film and polyimide alignment @/ferroelectric liquid crystal layer, the incident light on the transparent π-pole layer becomes 9
It becomes 97%.

〔Effect of the invention〕

As explained above, in the thin @laminated structure of the optical modulation element, when the refractive index of each of the transparent electrode layer, insulating layer, and ferroelectric liquid crystal layer is nl + n2 and n3, n H>
By setting n 2 > ns and more preferably setting n 2 to a value approximately between nl and n, reflection of light passing through films with different refractive indexes is avoided as much as possible, and contrast degradation is significantly reduced. This effect can be obtained.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the configuration of an embodiment of the present invention, FIG. 2 is an explanatory diagram of the configuration of another embodiment of the invention, FIG. 3 is an explanatory diagram of the configuration of a conventional optical modulation element section, and FIG. ) and (b) are schematic explanatory diagrams of the conventional examples, FIG. 5 is a basic configuration diagram of the conventional technology, and FIGS. 6(a) and (b) are diagrams of the operation of the prior art, respectively. 301: Transparent substrate, 302: Transparent! Pole, 303 = Insulating film, 304: FLC alignment film, 3o5: Ferroelectric liquid crystal (FLC
), 306: FLC alignment film, 307; transparent substrate, 3
08: Figure wavelength plate, 309: PLC alignment film, 310: Opposing pole and reflection plate, 311 Ni glass substrate.

Claims (4)

[Claims] (1) Consisting of a laminate of three or more layers including a transparent electrode layer, an insulating layer, and a ferroelectric liquid crystal layer, when the refractive index of the three adjacent layers is n_1, n_2, and n_3 in order, n_1>n_
An optical modulation element characterized in that 2>n_3 or n_1<n_2<n_3. (2) When the refractive indices of the transparent electrode layer, insulating layer, and ferroelectric liquid crystal layer are n_1, n_2, and n_3, n_1>n_2
The optical modulation element according to claim 1, wherein n_3. (3) The optical modulation element according to claim 1, wherein the laminate further includes a polymer liquid crystal layer having a quarter-wave plate function and a reflective layer. (4) A display device using the optical modulation element according to claim 1, wherein a plurality of optical modulation elements are arranged in a matrix and a voltage is selectively applied to perform modulation control and image display. .