JPH06202143A - Spatial optical modulator - Google Patents

Abstract

PURPOSE:To make image display with a high sensitivity and high contrast ratio by preventing the generation of dynamic scattering(DS) without using the heterojunction of photocapacitors and metallic lattice. CONSTITUTION:A liquid crystal constituting the optical modulator is set at sigma>=10<9>OMEGAcm in its resistivitysigma, 4<=epsilon¦¦<=10 dielectric constant of a major axis direction and 2<=T<=4mum cell thickness. As a result, the impedance of a liquid crystal cell is lowered and the impedance matching with a photoconductor is improved. The generation of the DS is, therefor, prevented and the image display is executable with a high sensitivity and high contrast. Since the impedance of the liquid crystal cell is low, the use of the photoconductive films consisting of a-Si:H and a-SiC:H having excellent mass productivity is possible.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spatial light modulator in which a light modulator using a liquid crystal is combined with a photoconductor,
More specifically, the present invention relates to improvement of a spatial light modulator suitable for an image projection device such as a projection television or a video projector that requires high sensitivity and high contrast.

[0002]

2. Description of the Related Art A spatial light modulator used in a projector or the like has a structure as shown in FIG. In the figure, the writing light incident side of the optical modulator 10 (arrow FA
Side), the dielectric mirror 12, the light shielding layer 13, the photoconductor 1
4, the transparent electrode 16, and the transparent glass substrate 20 are sequentially stacked. In addition, the read light incident side of the optical modulator 10 (arrow F
(See B), the transparent electrode 18 and the transparent glass substrate 24 are sequentially laminated. An antireflection film is formed on each of the transparent glass substrates 20 and 24 on the light incident side from the outside as needed. The light shielding layer 13 is also provided as needed. A power supply 26 is connected to the transparent electrodes 16 and 18, and a drive voltage output from the power supply 26 is applied to the light modulator 10 and the photoconductor 14.

Of the above-mentioned respective parts, the light modulator 10 has, for example, a structure in which a cell sandwiched between the liquid crystal alignment layers 30 and 34 is filled with liquid crystal. The dielectric mirror 12 has a structure in which, for example, Si and SiO ₂ (or TiO ₂ and SiO ₂ ) are alternately laminated by a method such as vapor deposition a plurality of times.
Further, as the photoconductor 14, for example, a-Si: H (hydrogenated amorphous silicon) or a-SiC: H (hydrogenated amorphous silicon carbide) is used. As the transparent electrodes 16 and 18, for example, ITO (Indium Tin O
xide) or SnO ₂ is used.

Next, the operation of the above spatial light modulator will be described. An AC voltage is applied in advance between the transparent electrodes 16 and 18 by a power source 26. This applied voltage is applied to the light modulator 10, the dielectric mirror 12, the light shielding layer 13, and the photoconductor 14.
It is distributed to each layer according to the impedance of. In this state, when the writing light including the image information is incident on the spatial light modulator as indicated by the arrow FA, the writing light passes through the transparent glass substrate 20 and the transparent electrode 16 in order and reaches the photoconductor 14. . In the photoconductor 14, the writing light is absorbed and its impedance is reduced. Then, the drive voltage distributed to the light modulator 10 increases.
That is, an electric field corresponding to the intensity distribution of the writing light is formed in the optical modulator 10.

In this state, when the reading light is incident on the spatial light modulator as shown by the arrow FB, the reading light passes through the transparent glass substrate 24 and the transparent electrode 18 in order and reaches the light modulator 10. The read light is modulated by the birefringence of the liquid crystal or the like according to the write light intensity distribution, is further reflected by the dielectric mirror layer 12, and is output from the spatial light modulator as indicated by the arrow FC. In this way, the image information written in the spatial light modulator is read. The reading light is
For example, it is projected on the screen. By using the spatial light modulator, it is possible to display an image with high brightness and high resolution. In particular, when a vertically aligned liquid crystal is used as the light modulator 10, it is possible to display an image with good contrast.

By the way, in such a spatial light modulator, impedance matching between layers is important, as pointed out in Japanese Patent Publication No. 30508/53. That is, as pointed out in the publication, the impedance of the photoconductor 14 is sufficiently larger than the impedance of the light modulator 10 when the light is not irradiated, and the impedance of the light modulator 10 when the light is irradiated. It is necessary to perform impedance matching so as to be smaller. According to the liquid crystal light valve disclosed in the publication, impedance matching is realized by optical capacitance type heterojunction, use of low-impedance liquid crystal, and attachment of a metal grid when matching is not successful with a simple laminated structure. There is.

First, according to the example of FIG. 2, the optical capacitance type heterojunction is formed between the light-shielding layer 13 and the photoconductor 14, whereby the capacitance of the junction portion is changed and the capacitance of the photoconductor 14 is changed. Optically modulate the impedance. Next, when a low impedance liquid crystal of about 10 ⁵ Ω / cm ² is used, the impedances of the light modulator 10 and the photoconductor 14 become appropriate values. The metal grid is also attached to the writing light incident side of the photoconductor 14. As a result, the optical modulator 10 of the AC electric field
The combination of impedances can be improved by controlling the transmission of light to the.

[0008]

However, the above conventional techniques have the following disadvantages. (1) When using the optical capacitance type heterojunction, the materials that can be used for the light shielding layer 13 and the photoconductor 14 are limited to those capable of forming the heterojunction. In particular, a-Si: H or a-SiC: H is suitable for mass production as the photoconductor 14, but a good combination capable of forming an appropriate optical capacity type heterojunction with these a-Si materials. There is no light shielding film material.

(2) Next, when a low impedance liquid crystal is used, in the above publication, the impedance of the liquid crystal is lowered by mixing impurities into the liquid crystal to lower the resistivity. This is a method that is possible because the modulation mode of the liquid crystal is a scattering method by the DS (dynamic scattering) mode and it is premised that a current flows through the liquid crystal. However, this method has the disadvantage that the life of the liquid crystal is short and the contrast ratio is low.

(3) Further, when the metal grid is provided, fine processing is required, which complicates the manufacturing process. Further, when the optical signal has a high density, there is a disadvantage that moire occurs due to interference with the metal grating. The present invention focuses on these points, and it is a spatial light that can prevent the occurrence of DS and can perform image display with high sensitivity and high contrast ratio without using a light-capacitance dissimilar junction or a metal grating. It is an object to provide a modulator.

[0011]

In order to achieve the above object, the present invention provides a light modulating layer provided between transparent electrodes together with a photoconductive layer and a mirror layer, in which the major axis of the molecule is oriented substantially perpendicularly to the substrate. The liquid crystal is composed of a liquid crystal having a negative dielectric anisotropy, and the read-out light modulated by the electric field control type birefringence effect by the liquid crystal is reflected by the mirror layer and is output, which is a reflective read-out spatial light modulation. In the container, the dielectric constant ε∥ of the liquid crystal in the major axis direction is 4 ≦ ε‖ ≦ 10, the cell thickness T is 2 ≦ T ≦ 4 μm, and the resistivity σ is σ ≧ 10 ⁹
It is characterized in that it is Ωcm.

[0012]

According to the present invention, the liquid crystal cell constituting the light modulator of the spatial light modulator has a structure in which nematic liquid crystals having a negative dielectric anisotropy having an electric field control type birefringence effect are substantially vertically aligned. Has become. The device is devised from the viewpoint of its resistivity, cell thickness and permittivity, and the resistivity is 10 ⁹ Ωc.
The dielectric constant in the major axis direction of the liquid crystal is in the range of 4 to 10, and the liquid crystal cell thickness is in the range of 2 to 4 μm. Hereinafter, they will be described in order.

When the DS mode modulation operation occurs in the liquid crystal,
The projection image becomes defective and the life of the liquid crystal is significantly reduced. Therefore, the spatial light modulator of the present invention is used in a drive condition range where such a DS mode modulation operation does not occur. As a region where the DS mode does not occur, the spatial light modulator may be driven by setting the drive frequency of the power supply 26 to 1 KHz or higher.

Further, the resistivity σ of the liquid crystal is σ ≧ 10 ⁹ Ωcm
By so doing, it is possible to suppress the presence of ionic impurities that cause the DS mode modulation operation.
However, in this case, the impedance of the liquid crystal increases, and the impedance matching balance with the photoconductor is lost. Therefore, it is necessary to reduce the impedance of the liquid crystal by another method.

Therefore, in the present invention, the dielectric constant ε ∥ in the major axis direction of the liquid crystal is set to 4 or more to increase the resistivity so that the conditions of low impedance and no DS mode can be satisfied at the same time. is doing. However, increasing the permittivity ε‖ of the liquid crystal in the major axis direction means that unless the permittivity ε⊥ in the minor axis direction is increased further, if the dielectric anisotropy Δε = ε‖−ε⊥ is negative. It's gone.

In the vertical alignment type liquid crystal, other characteristics are prioritized over impedance, so that the dielectric constant ε∥ in the major axis direction is usually about 3 to 4. Therefore, in order to increase ε | even more, it is sufficient to mix a material having a large dielectric constant ε⊥ in the short axis direction. Specifically, the materials shown in Chemical formula 1 may be mixed.

[0017]

[Chemical 1]

However, in general, such a material having a large dielectric constant ε⊥ in the short axis direction has a large viscosity, so that the actual response of the liquid crystal cell becomes slow. However, this can be solved by reducing the liquid crystal cell thickness T to 2 to 4 μm. Further, reducing the liquid crystal cell thickness T is also an effective method for reducing the impedance of the liquid crystal cell.

When the liquid crystal cell thickness T is reduced, the degree of modulation of light passing through the cell, that is, the retardation birefringence phase difference obtained by multiplying the cell thickness T by the birefringence anisotropy of the liquid crystal becomes small. Normally, the cell thickness T cannot be reduced so much. However, the spatial light modulator of the present invention is of the reflective reading type in which the dielectric mirror 12 is provided (see FIG. 2). Therefore, the reading light shown by the arrow FB in the figure passes through the liquid crystal cell (light modulator 10) twice and is output as shown by the arrow FC. Therefore, as compared with the case of the transmissive readout method, there is an advantage that a birefringence phase difference that is twice the liquid crystal cell thickness T can be obtained.

Next, if the dielectric constant ε∥ of the liquid crystal in the long axis direction is 4 or more, the effect of increasing the sensitivity of the spatial light modulator is obtained.
If it becomes too large, the voltage applied to the liquid crystal (light modulator 10) of the entire drive voltage of the power supply 26 is decreased. Therefore, there is a disadvantage that the power supply voltage for driving the spatial light modulator must be increased. Therefore, the dielectric constant ε || in the major axis direction of the liquid crystal is at most about 10 as shown in Examples described later,
4 to 10 is an appropriate range.

Further, the same applies to the liquid crystal cell thickness T, and if the liquid crystal cell thickness T becomes too thin and is 2 μm or less, the entire drive voltage (power supply voltage) becomes high.
Then, a slight change in the cell thickness T has a great effect on the change in the characteristics, and the uniformity of the spatial light modulator deteriorates. Therefore, the cell thickness T is 2 to 4 μm.
The range is appropriate.

As described above, in the spatial light modulator according to the present invention, the resistivity σ of the liquid crystal forming the light modulator is σ
≧ 10 ⁹ Ωcm, permittivity ε‖ in the major axis direction is 4 ≦ ε‖ ≦
10. By setting the cell thickness T to 2 ≦ T ≦ 4 μm, the impedance of the liquid crystal cell is reduced and impedance matching with the photoconductor is achieved. Therefore, it is possible to prevent the occurrence of DS and display an image with high sensitivity and a high contrast ratio without using the different-capacitance heterojunction or the metal grating as in the above-mentioned conventional technique. In addition, since the impedance of the liquid crystal cell is low, aS
It is possible to use a photoconductive film made of i: H or a-SiC: H. Further, although the dielectric constant and the resistivity are large, the impedance is small, so that the liquid crystal is easy to move and the response is excellent.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the spatial light modulator according to the present invention will be described below with reference to FIG. In addition, when corresponding, the code of the spatial light modulator shown in FIG. 2 will be used. A transparent electrode 16 made of ITO is formed on the glass substrate 20 by vapor deposition to a thickness of about 1000 Å, and a photo conductor 1 made of a-Si: H containing a trace amount of B (boron) is formed thereon.
4 was formed by plasma CVD to a thickness of 15 μm, and further 11 layers of Si and SiO ₂ were alternately formed as the dielectric mirror 12 thereon. Also, as the other substrate, ITO
A glass substrate 24 with 18 was prepared.

Next, an obliquely vapor-deposited film of SiO ₂ was formed on each of the substrates and a treatment with a silane coupling agent was performed to form alignment films 30 and 34. Then, these two substrates were assembled by an ordinary method for assembling a liquid crystal cell to produce a liquid crystal cell having a substantially vertical alignment.
The liquid crystal cell thickness T was 3 μm. When liquid crystal was injected into the sample cell thus obtained by changing the dielectric constant ε || in the direction of the major axis and the image writing sensitivity and driving voltage were measured, the results shown in Table 1 below were obtained. .

[0025]

[Table 1]

The results of Table 1 are shown in a graph in FIG. First, considering the writing sensitivity, the value increases even if the permittivity ε∥ in the major axis direction is small or large. Is good. As a whole, good sensitivity is obtained in a region where the dielectric constant ε || in the major axis direction is 4 or more.

However, the driving voltage increases with an increase in the dielectric constant ε∥ in the major axis direction, and from this point, it is not preferable to increase the dielectric constant ε∥ in the major axis direction too much. Considering this, the dielectric constant in the major axis direction is limited to about 10 at the maximum. That is, it is appropriate that the dielectric constant in the major axis direction is in the range of 4 to 10.

<Other Embodiments> The present invention is not limited to the above embodiments. For example, in the structure of the spatial light modulator shown in FIG. 2, a light shielding layer and an antireflection film are provided. It is optional and may be changed as needed. Also,
Photoconductors and dielectric mirrors that make up the spatial light modulator,
The material of each part is also arbitrary.

[0029]

As described above, according to the spatial light modulator of the present invention, the resistivity σ of the liquid crystal forming the optical modulator of the spatial light modulator is σ ≧ 10 ⁹ Ωcm in the long axis direction. Dielectric constant ε ‖ is 4 ≤ ε ‖ ≤ 10, cell thickness T is 2 ≤
Since T ≦ 4 μm, there is an effect that it is possible to prevent the occurrence of dynamic scattering and to display an image with high sensitivity and high contrast ratio without using a light capacity different junction or a metal grating.

[Brief description of drawings]

FIG. 1 is a graph showing a measurement example of a relationship between a dielectric constant in a long axis direction of a liquid crystal, a writing sensitivity, and a driving voltage in a spatial light modulator according to the present invention.

FIG. 2 is an explanatory diagram showing a general configuration of a spatial light modulator.

[Explanation of Codes] 10 ... Optical Modulator, 12 ... Dielectric Mirror, 13 ... Light Shielding Layer,
14 ... Photoconductor, 16, 18 ... Transparent electrodes, 20, 24 ...
Transparent glass substrate, 26 ... Power source, 30, 34 ... Alignment film, F
A ... Incident direction of writing light, FB ... Incident direction of reading light,
FC: Output direction of read light.

Claims (1)

[Claims] 1. A light modulation layer provided together with a photoconductive layer and a mirror layer between transparent electrodes is composed of a liquid crystal having a negative dielectric anisotropy in which a major axis of a molecule is aligned substantially perpendicularly to a substrate. In the reflection read type spatial light modulator in which the read light modulated by the electric field control type birefringence effect by the liquid crystal is reflected by the mirror layer and output, the dielectric constant ε∥ of the liquid crystal in the long axis direction is 4 ≦ ε‖ ≦ 10, the cell thickness T is 2 ≦ T ≦ 4 μm, and the resistivity σ is σ ≧ 10 ⁹
A spatial light modulator having an Ωcm.