JPH06230412A - Optical modulation element - Google Patents

Abstract

PURPOSE:To provide the optical modulation element having extremely outstanding electro-optical characteristics (steep threshold characteristic) and a high-speed response characteristic. CONSTITUTION:A transparent electrode 3a and a liquid crystal oriented layer 6a are laminated on a substrate 2a. A transparent electrode 3b, a photoconductive layer 4, a light reflection layer 5 and a liquid crystal oriented layer 6b are laminated on a substrate 2b. A liquid crystal layer 7 is sealed between the substrates 2a and 2b with spacers 8a, 8b. The liquid crystal material of the liquid crystal layer 7 is an antiferroelecric liquid crystal and the tilt angles of the liquid crystal molecules in the liquid crystal layer 7 on the liquid crystal oriented layers 6a, 6b are respectively within a 60 to 90 deg. range.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator used in a projection type display device and the like having excellent electro-optical characteristics and response characteristics.

[0002]

2. Description of the Related Art Display devices include a direct-view type device and a projection type device, and as the projection type device, a device using a light modulation element using liquid crystal has already been put into practical use. FIG. 4 is a principle diagram of a projection display device using a light modulation element. In FIG. 4, an image is written on the light modulation element 10 by the writing light 11 emitted from the writing optical system. On the other hand, light source 1
The light emitted from 3 is collimated by the condenser lens 14 and then incident on the polarization beam splitter 15. The S-polarized component of the incident light is reflected at the joint surface in the traveling direction and bent at a right angle. , And enters the light modulation element 10 as the reading light 12.

Here, when an image is drawn on the liquid crystal layer of the light modulation element 10, the reflected light reflected by the light modulation element 10 is modulated in accordance with the light and shade of the image of the liquid crystal layer, and P polarized light is obtained. Ingredients will be included. The P-polarized component in the reflected light passes through the polarization beam splitter 15 as it is, and the projection lens 1
An image is projected on the screen 17 via 6. Therefore, the image written on the light modulation element 10 is displayed on the screen 1
7 will be projected.

FIG. 5 is a view showing the structure of an example of a conventional light modulation element 10 used in this projection type display device. Figure 5
In, the spacers 8a and 8b are arranged around the liquid crystal layer 9, and the liquid crystal alignment layers 6a and 6b are provided on both surfaces of the liquid crystal layer 9. A light reflecting layer 5 made of a dielectric material is laminated on the outer side of the liquid crystal alignment layer 6b, and a photoconductive layer 4 made of, for example, amorphous hydrogenated silicon (a-Si: H) is placed on the outer side of the light reflecting layer 5. It is stacked. Then, transparent electrodes 3a and 3b made of tin oxide (SnO ₂ ) or indium oxide (In ₂ O ₃ ) are arranged outside the liquid crystal alignment layer 6a and the photoconductive layer 4 and sealed by the transparent substrates 2a and 2b. It has a different structure.

In the light modulation element 10 having such a structure, an AC drive voltage is applied between the transparent electrodes 3a and 3b, and the internal impedance of the photoconductive layer 4 in a state where the writing light 11 is not irradiated from the writing side. Is set to a value sufficiently larger than that of the liquid crystal layer 9, the drive voltage is mainly applied to the photoconductive layer 4. And
When the writing light 11 is irradiated and an image is drawn on the photoconductive layer 4 by the writing light 11, the internal impedance of the photoconductive layer 4 locally decreases according to the shading of the image.
The transparent electrodes 3a, 3 are provided on the liquid crystal layer 9 adjacent to the lowered portion.
The drive voltage between points b is spatially modulated according to the density of the image and applied, and the image is written.

A nematic liquid crystal having a negative dielectric anisotropy is used as the liquid crystal material of the liquid crystal layer 9 used in the conventional light modulation element 10. Further, regarding the alignment direction of the liquid crystal molecules of the liquid crystal layer 9 and the polarization direction of the readout light, E
There is known a light modulation element in which the initial liquid crystal molecule alignment is vertical alignment utilizing the CB (electrically controlled birefringence) effect. The liquid crystal molecules of the vertically aligned liquid crystal layer 9 are arranged so that their long axes have a pretilt angle slightly tilted from the perpendicular to the substrates 2a and 2b and are substantially parallel to each other. Further, the liquid crystal molecules of the liquid crystal layer 9 are aligned in parallel on the liquid crystal alignment layers 6a and 6b.

The light modulation element 10 having such a vertically aligned liquid crystal layer 9 is used in the projection display device shown in FIG. 4, and the polarization axis of the polarization beam splitter 15 between the light modulation element 10 and the reading light source 13 is set. 4 and the liquid crystal molecular axis of the liquid crystal layer 9 on the incident side
When arranged so as to form 5 degrees, the linearly polarized incident light passes through the liquid crystal layer 9, the light reflected by the light reflection layer 5 passes through the liquid crystal layer 9 again, and is emitted, the liquid crystal is vertically aligned. From the above, linearly polarized light in the same direction as the incident light is received without being affected by the birefringence, is reflected by the joint surface of the polarization beam splitter 15, the traveling direction is bent at a right angle, and is returned to the light source 13 side.

On the other hand, the transparent electrode 3 of the light modulation element 10
When an electric field is applied between a and 3b, the liquid crystal molecules having a negative dielectric anisotropy form a tilted array that approaches the substrates 2a and 2b horizontally. In this way, since the effective refractive index anisotropy changes as the molecular axis direction changes with respect to the substrates 2a and 2b, the birefringence changes, which affects the polarization of light. The light emitted from the light modulation element 10 after being reflected by the reflective layer 5 is not linearly polarized light at the time of incidence,
The light is transmitted through the polarization beam splitter 15. As a result, the birefringence of the liquid crystal layer 9 changes due to the electric field,
Since the ratio of the P polarization component and the S polarization component of light can be controlled,
By applying an electric field, the intensity of the light transmitted through the polarization beam splitter 15 changes, and an image is projected on the screen 17.

Table 1 shows conditions of an example of the conventional light modulation element 10 described above.

[0010]

[Table 1]

FIG. 6 shows the optical characteristics when an electric field is applied to the conventional optical modulator 10 under the conditions shown in Table 1, and FIG. 7 shows the response characteristics when an electric field is applied. In FIG. 6, the horizontal axis is the applied voltage, and the vertical axis is the ratio of the incident light amount (light incident on the polarization beam splitter 15 in FIG. 4) to the emission light amount (light emitted from the polarization beam splitter 15 in FIG. 4). In FIG. 7, the horizontal axis shows the elapsed time after the electric field is applied, and the vertical axis shows the emitted light quantity / incident light quantity.

[0012]

As described above, when an electric field is applied to the light modulation element 10, the liquid crystal molecules of the liquid crystal layer 9 reach a certain tilt with the tilt angle formed between the liquid crystal layer 9 and the substrates 2a and 2b reduced. The sharpness γ of the threshold value characteristic using applied voltages V1 and V99 corresponding to relative light transmittances of 1% and 99% is defined as γ = V99 / V1 and used. As shown in FIG. 6, since V1 = 3.61 and V99 = 4.86, .gamma. = 1.35, and the incident writing light 11
If the amount of change in the internal impedance of the photoconductive layer 4 due to the change is small, the amount of change in the applied voltage is also small, so that the contrast ratio cannot be increased. Also, FIG.
As shown in, the time from the application of the electric field to the desired inclination at which the contrast ratio can be maximized, that is, the writing time is 36 m.
There is also a problem that it takes about s and cannot be followed when displaying a moving image or the like. Therefore, in view of such problems, an object of the present invention is to provide an optical modulation element having abrupt threshold characteristics and fast response characteristics by using a liquid crystal material different from the conventional optical modulation element. .

[0013]

In order to solve the above-mentioned problems of the prior art, the present invention provides a liquid crystal layer sandwiched between first and second liquid crystal alignment layers and a second liquid crystal alignment layer. And a photoconductive layer laminated via a light reflecting layer between the pair of transparent electrodes facing each other, and writing light is irradiated from the photoconductive layer side to write an image, In the light modulation element for irradiating the reading light from the liquid crystal alignment layer side to read the written image, the liquid crystal material of the liquid crystal layer is an antiferroelectric liquid crystal, and the first and second liquid crystal alignment layers. The above-mentioned liquid crystal layer has a tilt angle of liquid crystal molecules in the range of 60 to 90 degrees, respectively.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The light modulator of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a diagram showing the structure of an embodiment of the light modulation element of the present invention, and FIG. 2 is a diagram showing the relationship between the applied voltage and the emitted light quantity / incident light quantity in the light modulation element of the present invention,
FIG. 3 is a diagram showing the relationship between the elapsed time after application of an electric field and the emitted light amount / incident light amount in the light modulation element of the present invention. In FIG. 1, the same parts as those in FIG. 5 are designated by the same reference numerals.

The light modulation element 1 of the present invention has the same basic structure as the conventional light modulation element 10, but the liquid crystal material in the liquid crystal layer is different. The liquid crystal layer in the light modulation element 1 of the present invention will be described in detail later. As shown in FIG. 1, the light modulation element 1 of the present invention includes transparent substrates 2a and 2b.
Are parallel to each other at substantially equal intervals, and a transparent electrode 3a is provided on the side of the substrate 2a facing the substrate 2b, and a transparent electrode 3b is provided on the side of the substrate 2b facing the substrate 2a. Then, on the transparent electrode 3b, the photoconductive layer 4 whose specific resistance changes according to the light intensity of the writing light 11 is laminated and formed.
Further, a light reflection layer 5 is laminated on the photoconductive layer 4. A liquid crystal alignment layer 6a is provided on the transparent electrode 3a, and a liquid crystal alignment layer 6b is provided on the light reflection layer 5. In this way, the transparent electrode 3a and the liquid crystal alignment layer 6a are laminated and provided on the substrate 2a, and the transparent electrode 3 is provided on the substrate 2b.
b, the photoconductive layer 4, the light reflection layer 5, and the liquid crystal alignment layer 6b are stacked, and a liquid crystal layer 17 of a liquid crystal material different from the conventional liquid crystal layer 9 is sandwiched between the substrates 2a and 2b. Has been done. It should be noted that 8a and 8b are the same as those described above for the substrates 2a and 2b.
It is a spacer for enclosing the liquid crystal with a predetermined space between them.

In this embodiment, the substrates 2a and 2b are made of transparent glass plates, for example, but transparent resin plates can also be used. The transparent electrodes 3a and 3b are generally formed of an indium oxide film, a tin oxide film, or the like. For the photoconductive layer 4 formed on the transparent electrode 3b, various photoconductive materials such as amorphous silicon hydride, cadmium selenide (CdSe), and lead sulfide (PbS) are used alone or in combination. You can The light reflection layer 5 laminated on the photoconductive layer 4 is a dielectric multilayer film such as a zinc sulfide (ZnS): magnesium fluoride (MgF ₂ ) multilayer film or a silicon oxide (SiO): silicon dioxide (SiO ₂ ) multilayer film. To use.

Next, the liquid crystal layer 7 and the liquid crystal alignment layers 6a and 6b in the light modulation element 1 of the present invention will be described in detail. The liquid crystal material of the liquid crystal layer 7 is an antiferroelectric liquid crystal that switches at high speed from an antiferroelectric phase to a ferroelectric phase when a voltage is applied. For example, MHPOBC (4- (1-methyl h
epthyloxy carbonyl) phenyl-4'-octyloxy bipheny
l-4-carboxylate) can be used. The liquid crystal molecules in the liquid crystal layer 7 are vertically aligned,
In the light modulation element 1 of the present invention, the liquid crystal alignment layers 6a, 6
The tilt angle of the liquid crystal molecules on b is in the range of 60 degrees to 90 degrees. Further, the liquid crystal alignment layers 6a and 6b can be realized by a method similar to that of a liquid crystal alignment layer in a nematic liquid crystal of a known liquid crystal display device in which liquid crystal molecules of the liquid crystal layer 7 are vertically aligned, and long-chain alcohol, Silane coupling agent, chromium complex, CTAB (hexadecyl trim)
It is formed using a solution containing ethyl ammonium bromide) or lecithin.

Furthermore, in order to uniquely determine the vertical alignment of the liquid crystal molecules in the liquid crystal layer 7, the liquid crystal layer 7 formed by filling the liquid crystal material between the liquid crystal alignment layers 6a and 6b is once heated to the isotropic phase. After that, it is necessary to gradually cool to the smectic phase. Since the layer structure of MHPOBC is formed in the smectic A phase, if the isotropic phase is gradually cooled to the smectic A phase, the liquid crystal layer structure can be aligned substantially parallel to the liquid crystal alignment layers 6a and 6b. The major axis is tilted substantially perpendicular to the liquid crystal alignment layers 6a and 6b and rightward and leftward from the normal direction to be in a bistable state. By further slowly cooling the liquid crystal layer 7, a layer structure in the smectic C phase can be obtained. At this stage, even if the liquid crystal molecule long axis is mixed with a state in which it is tilted to the right and a state in which it is tilted to the left, if the electric field is applied to the transparent electrodes 3a and 3b, the entire state will be uniformly tilted. be able to.

As described above, a book including the liquid crystal layer 7 in which the liquid crystal material is antiferroelectric liquid crystal and the tilt angles of the liquid crystal molecules on the liquid crystal alignment layers 6a and 6b are in the range of 60 to 90 degrees, respectively. The light modulation element 1 of the invention is used as an image forming element of a projection display device as shown in FIG. In this projection display device, for example, the light emitted from the CRT screen is incident on the light modulation element 1 as the writing light 11 to write an image. On the other hand, the light emitted from the light source 13 is collimated by the condenser lens 14, and then is irradiated onto the light modulation element 1 via the polarization beam splitter 15. The spatially modulated light reflected by the light reflection layer 5 of the light modulation element 1 passes through the polarization beam splitter 15 and forms an image on the screen 17 via the projection lens 16.

Here, the light modulation element 1 is reflected by the light reflection layer 5 and polarized with an electric field being applied so that the entire liquid crystal molecules of the liquid crystal layer 7 are uniformly tilted to the right or left from the normal direction. The angle formed by the polarization axis of the polarization beam splitter 15 and its liquid crystal molecules is set so that the light intensity of the light passing through the beam splitter 15 becomes the lowest (or the highest).

Further, the operation of the light modulation element 1 of the present invention will be described. In FIG. 1, an external power source (not shown) is connected between the transparent electrodes 3a and 3b, and a drive voltage of about 5 to 40 V is applied. Here, since the internal impedance of the photoconductive layer 4 in the state where the writing light 11 is not irradiated from the writing side is set to a value sufficiently higher than that of the liquid crystal layer 7, the driving voltage is mainly the photoconductive layer. 4 will be applied. Then, when the writing light 11 is irradiated and an image is drawn on the photoconductive layer 4 by the writing light 11, the internal impedance of the photoconductive layer 4 is locally reduced according to the shading of the image, and thus the lowered portion is generated. The drive voltage between the transparent electrodes 3a and 3b is spatially modulated and applied to the adjacent liquid crystal layer 7 according to the contrast of the image, and the liquid crystal molecules in the portion of the liquid crystal layer 7 where the applied voltage is increased change the alignment state. .

When the applied voltage increases, the major axis direction of the liquid crystal molecules having dielectric anisotropy becomes an array in which the substrates 2a and 2b are inclined to the right, for example. If the polarity of the applied voltage is reversed, the entire array will be inclined to the left.
In the light modulation element 1 in such a state, since the effective refractive index anisotropy changes as the axial direction of the liquid crystal molecules changes, the birefringence changes and the light is reflected by the light reflection layer 5 and again the liquid crystal. Layer 7
The light emitted after passing through is no longer linearly polarized when it enters the liquid crystal layer 7 with its traveling direction bent at a right angle by the polarization beam splitter 15, so that light that passes through the polarization beam splitter 15 can be obtained. become.

Table 1 shows the conditions of one example of the conventional light modulation element 10 and the conditions of one example of the light modulation element 1 of the present invention. The optical characteristics when an electric field is applied to the optical modulator 1 of the present invention under the conditions shown in Table 1 are shown in FIG. 2, and the response characteristics when an electric field is applied are shown in FIG. 2 is similar to FIG.
The horizontal axis represents the applied voltage, the vertical axis represents the emitted light amount / the incident light amount, and FIG. 3 is the same as in FIG. 7, the horizontal axis represents the elapsed time after the electric field is applied, and the vertical axis represents the emitted light amount / the incident light amount. From FIG. 2, the relative light transmittance is 1
% And 99%, the sharpness γ of the threshold characteristic using the applied voltages V1 and V99 is V1 = 18.8, V99 = 23.5.
Therefore, γ is about 1.25, and the optical modulator 1 of the present invention is
It can be seen that has a very steep threshold characteristic. Further, as shown in FIG. 3, the time from the application of the electric field to the desired inclination at which the contrast ratio can be maximized, that is, the writing time is 0.
It is about 15 ms, and it can be seen that the light modulation element 1 of the present invention has a remarkably high-speed response characteristic.

In this embodiment described above, MHPOBC was used as the liquid crystal material in the liquid crystal layer 7 and the initial alignment of the liquid crystal molecules was described as vertical alignment. However, the alignment is not limited to this, and various antiferroelectric properties can be used. It is needless to say that the same effect can be obtained by appropriately selecting and using the liquid crystal material exhibiting the above and the liquid crystal alignment layer having various vertical alignment properties, and the idea of the present invention is not changed at all. As described above, the light modulation element 1 of the present invention can have various suitable embodiments without departing from the scope of the present invention.

[0025]

As described in detail above, in the light modulation element of the present invention, the liquid crystal material is antiferroelectric liquid crystal, and the tilt angle of the liquid crystal molecules on the liquid crystal alignment layer is in the range of 60 to 90 degrees. Since it has a liquid crystal layer which is inside, it has an extremely excellent threshold characteristic, by which the amount of change in the internal impedance of the photoconductive layer may be small, which facilitates the formation of the photoconductive layer, and , Display with a large contrast ratio becomes possible. Further, it has an extremely excellent high-speed response characteristic, which makes it possible to shorten the writing time of an image, so that it is effective when displaying a moving image or the like.

[Brief description of drawings]

FIG. 1 is a diagram showing a structure of an embodiment of a light modulation element of the present invention.

FIG. 2 is a diagram showing a relationship between an applied voltage and an amount of emitted light / amount of incident light in the light modulation element of the present invention.

FIG. 3 is a diagram showing the relationship between the elapsed time after application of an electric field and the emitted light amount / incident light amount in the light modulation element of the present invention.

FIG. 4 is a principle view of a projection display device using a light modulation element.

FIG. 5 is a diagram showing a structure of an example of a conventional light modulation element.

FIG. 6 is a diagram showing the relationship between the applied voltage and the amount of emitted light / the amount of incident light in a conventional light modulator.

FIG. 7 is a diagram showing the relationship between the elapsed time after application of an electric field and the amount of emitted light / the amount of incident light in a conventional light modulator.

[Explanation of symbols]

 1, 10 Light modulation element 2a, 2b Substrate 3a, 3b Transparent electrode 4 Photoconductive layer 5 Light reflection layer 6a, 6b Liquid crystal alignment layer 7 Liquid crystal layer 8a, 8b Spacer

Claims (1)

[Claims] 1. A pair of a liquid crystal layer sandwiched between first and second liquid crystal alignment layers and a photoconductive layer laminated on the second liquid crystal alignment layer with a light reflection layer interposed therebetween. It is sandwiched between transparent electrodes, and writing light is irradiated from the photoconductive layer side to write an image, and reading light is irradiated from the first liquid crystal alignment layer side to read the written image. In the light modulation element, the liquid crystal material of the liquid crystal layer is antiferroelectric liquid crystal, and the tilt angles of the liquid crystal molecules in the liquid crystal layers on the first and second liquid crystal alignment layers are in the range of 60 degrees to 90 degrees, respectively. An optical modulation element characterized by being inside.