JPH0792487A - Optical modulation element - Google Patents

Abstract

PURPOSE:To provide an optical modulation element having extremely outstanding electro-optic characteristics, such as small intensity of transmitted light, sharp threshold characteristics and high contrast ratio when the element is applied with a voltage. CONSTITUTION:A transparent electrode 3a and a liquid crystal oriented layer 6a are successively laminated on a transparent substrate 2a. A transparent electrode 3b, a photoconductive layer 4, a light reflection layer 5 and a liquid crystal oriented layer 6b are successively laminated on a transparent substrate 2b. A liquid crystal layer 7 is clamped between these liquid crystal oriented layers 6a and 6b and a phase plate 9 is disposed on the outer side of the transparent substrate 2a. The tilt angle of the liquid crystal molecules of the liquid crystal layer 7 is within a range of 0 to 10'. The absolute value of a phase difference of the phase plate 9 is set in a range of >=(lambda/2) to <=(DELTAn.d-lambda/2)mum with respect to the product deltan.d of the thickness d(mum) and refractive index anisotropy DELTAn of the liquid crystal layer 7 when the wavelength of reading out light 12 is defined as lambda(mum).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light modulation element having extremely excellent electro-optical characteristics used in a projection type display device or the like.

[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 7, and the liquid crystal alignment layers 6a and 6b are provided on both surfaces of the liquid crystal layer 7. 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 7, 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 7 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.

Here, regarding the alignment direction of the liquid crystal molecules of the liquid crystal layer 7 used in the conventional light modulation element 10 and the polarization direction of the read light, an ECB (electrically controlled birefrin) is used.
There is known a light modulator in which the initial liquid crystal molecule orientation is vertically or horizontally aligned by utilizing the gence) effect. The liquid crystal molecules of the horizontally aligned liquid crystal layer 7 have the long axis of the substrate 2
A and 2b have slight pretilt angles and are arranged so as to be substantially parallel. Further, the liquid crystal molecules of the liquid crystal layer 7 are aligned in parallel on the liquid crystal alignment layers 6a and 6b.

The light modulation element 10 having such a horizontally aligned liquid crystal layer 7 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 7 on the incident side
When arranged so as to form 5 degrees, the linearly polarized incident light has a retardation (phase lag) R and a phase difference δ between the extraordinary light and the ordinary light generated by passing through the liquid crystal layer 7.
Where R = Δn · d and δ = 2πR /, where Δn is the refractive index anisotropy of the liquid crystal molecules, d is the liquid crystal layer thickness, and λ is the wavelength of the incident light.
It is represented by λ, and when it is reflected by the light reflection layer 5 and again passes through the liquid crystal layer 7, the retardation R and the phase difference δ are generated again. The transmitted light intensity J that passes through the polarization beam splitter 15 is represented by Formula 1.

[0008]

[Equation 1]

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 positive dielectric anisotropy are in a tilted array so as to approach the substrates 2a and 2b in a direction perpendicular to them. In this way, the liquid crystal molecules are
Since the effective refractive index anisotropy changes as the molecular axis direction changes with respect to a and 2b, the retardation R changes. As a result, the birefringence of the liquid crystal layer 7 is changed by the electric field, and the ratio of the P-polarized component and the S-polarized component of light can be controlled.
The intensity of the light passing through 5 changes and an image is projected on the screen 17.

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

[Table 1]

FIG. 6 shows optical characteristics when an electric field is applied to the conventional optical modulator 10 under the conditions shown in Table 1. 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). A certain amount of emitted light / amount of incident light is shown. In the example shown in FIG. 6, operation is performed in the range of applied voltages V1 (= 1.893) and V99 (= 2.028) corresponding to relative light transmittances of 1% and 99%.

[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 7 increase the angle formed with the substrates 2a and 2b and reach a certain inclination, but the electric field is increased. 6, the transmitted light intensity T0 at which the relative light transmittance is minimized is as large as 0.0481, that is, a so-called black floating phenomenon occurs when the writing light is not irradiated. The reason for this is that when the liquid crystal molecules are horizontally aligned and light is incident at 45 degrees with respect to the liquid crystal molecule axis, the application of the electric field to the liquid crystal is small in the practical range, and therefore the range of the retardation value is large. The transmitted light intensity T100 that maximizes the relative light transmittance is 0.4701.
Therefore, the contrast ratio CR is about 9.8 than T100 / T0, and there is a problem that the contrast ratio is small. Further, the sharpness γ of the threshold value characteristic using the applied voltages V1 and V99 corresponding to the relative light transmittances of 1% and 99% is defined as γ = V99 / V1 and used. As shown in FIG. 6, V1 = 1.893, V99 =
From 2.028, γ is about 1.071, and when the change amount of the internal impedance of the photoconductive layer 4 due to the incident writing light 11 is small, the change amount of the applied voltage is also small, and the contrast ratio cannot be increased. There was a problem.

The present invention has been made in view of the above problems.
By adopting an element configuration different from that of the conventional light modulation element, the intensity of transmitted light when a voltage is applied can be made extremely small, and the threshold value characteristic is sharp, and the light modulation element having an extremely excellent electro-optical characteristic with a large contrast ratio. The purpose is to provide.

[0014]

In order to solve the above-mentioned problems of the prior art, the present invention writes an image by transitioning the alignment state of liquid crystal molecules by writing light, and reads the written image. In a light modulation element for projecting with light, a first transparent electrode and a first transparent electrode are provided on a first transparent substrate.
And a liquid crystal alignment layer are sequentially laminated to form a second transparent substrate on the second transparent substrate.
The transparent electrode, the photoconductive layer, the light reflecting layer, and the second liquid crystal alignment layer are sequentially laminated, and the liquid crystal layer is sandwiched between the first and second liquid crystal alignment layers, and further, the first transparent substrate. A phase plate is provided outside the liquid crystal layer, and the liquid crystal layer has a tilt angle of 0 to 10 degrees of the liquid crystal molecules on the first and second liquid crystal alignment layers when the liquid crystal molecules are initially aligned uniformly. The phase plate has a thickness d (μm) of the liquid crystal layer when the wavelength of the incident read light is λ (μm).
And the refractive index anisotropy Δn, the absolute value of the phase difference is (λ / 2) μm or more (Δn · d−λ / 2).
The present invention provides a light modulation element characterized by being set in a range of μm or less.

[0015]

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 applied voltage and the retardation for explaining the operation of the light modulation element of the present invention. In addition,
1, the same parts as those in FIG. 5 are designated by the same reference numerals.

The optical modulator 1 of the present invention has a structure different from that of the conventional optical modulator 10, and is provided with a phase plate 9 outside the substrate 2a and on the incident side of the readout light 12. Light modulation element 1 of the present invention
As shown in FIG. 1, transparent substrates 2a and 2b face each other in parallel at substantially equal intervals.
The transparent electrode 3a on the side facing the substrate 2a of the substrate 2b.
A transparent electrode 3b is provided on the side opposite to. Then, on the transparent electrode 3b, a photoconductive layer 4 whose specific resistance changes according to the light intensity of the writing light 11 is laminated and further formed.
A light reflection layer 5 is laminated on the photoconductive layer 4. Further, the liquid crystal alignment layer 6a is provided on the transparent electrode 3a, and the light reflection layer 5 is formed on the transparent electrode 3a.
A liquid crystal alignment layer 6b is provided on top of it. Thus, the transparent electrode 3a and the liquid crystal molecule alignment layer 6 are formed on the substrate 2a.
a is laminated and a transparent electrode 3b, a photoconductive layer 4, a light reflection layer 5 and a liquid crystal alignment layer 6b are laminated on the substrate 2b. A liquid crystal layer 7 is provided between the substrates 2a and 2b. It is sandwiched. In addition, 8a and 8b are the same as the above
It is a spacer for enclosing a liquid crystal with a predetermined space between a and 2b. Further, the phase plate 9 is laminated on the outside of the substrate 2a or on the incident side of the readout light 12 or provided with a gap.

In this embodiment, the substrates 2a and 2b are made of transparent glass plates, for example, but transparent resin plates may 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.

Further, as the liquid crystal alignment layers 6a and 6b, the liquid crystal alignment layers of the liquid crystal display elements known so far can be used, and the liquid crystal alignment layers can be appropriately selected depending on what kind of liquid crystal molecules are arranged and how. It may be used, and in this embodiment, it is formed by using a polymer film such as polyimide, polyamide or polyvinyl in which liquid crystal molecules are horizontally aligned. As the liquid crystal material of the liquid crystal layer 7, nematic liquid crystal having positive dielectric anisotropy is used. This is fluorine, Schiff, azo, azoxy, ester, biphenyl, terphenyl,
Various liquid crystal substances such as cyclohexane type, pyrimidine type and dioxane type can be used alone or in combination.

Further, the phase plate 9 laminated on the outside of the substrate 2a or provided with a gap has the rotation angle (also referred to as the slow axis direction or the deceleration axis direction) of the read light 12 incident thereon. The angle is about 90 degrees with respect to the axial direction of the liquid crystal molecules on the liquid crystal molecule alignment layer 6a on the substrate 2a side. A liquid crystal layer, for example, can be used for the phase plate 9, and the liquid crystal material used for the liquid crystal layer 7 is used in this embodiment. Then, a liquid crystal alignment layer is formed on a pair of transparent substrates, and a liquid crystal layer is sandwiched between them. In this case, the phase plate 9
The liquid crystal molecule length (short) axis direction of the liquid crystal layer used for is the rotation angle of the phase plate 9.

The operation of the optical modulator 1 of the present invention having such a phase plate 9 will be described with reference to FIG. Liquid crystal layer 7
Let RLC be the retardation (phase lag) of R, and let Rr be the retardation of the phase plate 9, and if the retardation of one is positive, the retardation of the other works negatively when the rotation angle is 90 degrees. As shown. Therefore, conventionally, that is, in the light modulation element 10 not provided with the phase plate 9, the range of the portion where the retardation value is large as shown by the retardation RLC has to be used, but in the light modulation element 1 of the present invention, And retardation R
It is possible to use the range of the portion where the retardation value is small, including the point where = 0.

When the sum R of the retardations is an integral multiple of 1/2 of the wavelength λ (μm) of the reading light 12, that is, R = (RLC + Rr) / λ = m / 2, the wavelength of the reading light 12 is reciprocated. It becomes an integer (m) times λ, and the reflected light of the readout light 12 is reflected by the same linearly polarized light as the incident linearly polarized light, so that it becomes a dark state. In addition, in particular, in that the sum R of the retardations becomes 0 by applying a voltage,
It can be seen that the reflected light is in the dark state regardless of the wavelength λ of the read light 12.

On the other hand, when the sum R of the retardations is an odd multiple of λ / 4, that is, when R = (RLC + Rr) / λ = (2m + 1) / 4, it becomes an odd multiple of λ / 2 of the read light 12 in a round trip and is reflected. The light becomes orthogonally polarized with respect to the incident linearly polarized light, and becomes a bright state.

Further, the relationship between the liquid crystal layer 7 and the phase plate 9 will be considered. The retardation RLC of the liquid crystal layer 7 is the liquid crystal layer 7
It is represented by the product Δn · d of the thickness d (μm) and the refractive index anisotropy Δn. The condition for making the phase plate 9 have a perfect phase difference is λ / 2 ≦ Rr, and in order to make the liquid crystal layer 7 and the phase plate 9 have a perfect phase difference, the sum of retardations R = RLC +
It is necessary that Rr = Δn · d + Rr is λ / 2 ≦ Δn · d + Rr. Therefore, it is necessary to set the absolute value | Rr | of the retardation Rr of the phase plate 9 within the range of λ / 2 ≦ | Rr | ≦ Δn · d−λ / 2.

The light modulation element 1 of the present invention provided with such a phase plate 9 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 used as the writing light 11
Then, the light enters the light modulation element 1 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. Light reflection layer 5 of light modulation element 1
The spatially-modulated light reflected by is transmitted through the polarization beam splitter 15 and passes through the projection lens 16 and the screen 17
Image on top.

Here, when the polarization axis of the polarization beam splitter 15 between the light modulation element 1 and the reading light source 13 is arranged to be 45 degrees with the rotation angle of the phase plate 9, linear polarization is performed by the polarization beam splitter 15. The incident light is polarized by passing through the phase plate 9, the incident light passes through the liquid crystal layer 7, is reflected by the light reflection layer 5 and passes through the liquid crystal layer 7 again, and then the circularly polarized light is reflected by the phase plate 9 Is again converted into linearly polarized light, and the transmitted light intensity J is minimized by the polarization beam splitter 15.

Further, the operation of the light modulation element 1 of the present invention will be described. In FIG. 1, an external power supply (not shown) is connected between the transparent electrodes 3a and 3b, and an optimally set AC drive voltage 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,
Since the internal impedance of the photoconductive layer 4 is locally reduced according to the contrast of the image, the liquid crystal layer 7 adjacent to the lowered portion.
The drive voltage between the transparent electrodes 3a and 3b is spatially modulated and applied 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 long axis direction of the liquid crystal molecules having a positive dielectric anisotropy changes in the direction of the substrates 2a, 2b.
It becomes a slanted array that approaches vertically to. 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 emitted by the sum of the retardation with the phase plate 9. The linearly polarized light of the polarized light has a certain angle, and the light that passes through the polarization beam splitter 15 can be obtained.

The light modulation element 1 utilizes that the arrangement state of the liquid crystal molecules of the liquid crystal layer 7 reversibly transits in response to the change of the internal impedance of the photoconductive layer 4 by the incident writing light 11. Therefore, when the writing light 11 is no longer irradiated, the initial alignment state is automatically restored, and the stored image is erased with a time delay.

Table 1 shows the conditions of an example of the light modulation element 1 of the present invention together with the conditions of the example of the conventional light modulation element 10. 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. Similar to FIG. 6, in FIG. 2, the horizontal axis represents the applied voltage and the vertical axis represents the emitted light amount /
It is the amount of incident light. From FIG. 2, the relative light transmittance is maximum,
The transmittance corresponding to the minimum is T100 = 0.4975, T0
= 0.0041, the contrast ratio CR is about 121, and the light modulation element 1 of the present invention has a remarkably steep threshold characteristic. Furthermore, relative light transmittance is 1% and 99%
The sharpness γ of the threshold characteristic using the applied voltages V99 and V1 corresponding to γ is given by V1 = 1.903 and V99 = 2.027.
= 1.065, it can be seen that the light modulation element 1 of the present invention has a remarkably steep threshold characteristic.

In this embodiment described above, the phase plate 9 is made of the liquid crystal material used for the liquid crystal layer 7, but the present invention is not limited to this, and the natural material such as quartz and mica, or the high material such as polycarbonate. Molecular materials may be used. In the present invention, by providing the phase plate 9, it is possible to realize black display in which light leakage depending on the wavelength of the read light is small, and it is possible to increase the black purity. The light modulation element 1 of the present invention
The embodiments can be variously changed without departing from the scope of the present invention.

[0031]

As described in detail above, the light modulation element of the present invention includes the first transparent electrode and the first transparent electrode on the first transparent substrate.
And a liquid crystal alignment layer are sequentially laminated to form a second transparent substrate on the second transparent substrate.
Of the transparent electrode, the photoconductive layer, the light reflecting layer, and the second liquid crystal alignment layer are sequentially laminated, and the liquid crystal layer is sandwiched between the first and second liquid crystal alignment layers, and further outside the first transparent substrate. When a liquid crystal layer is initially aligned uniformly as a liquid crystal layer, the tilt angle of the liquid crystal molecules on the first and second liquid crystal alignment layers is within the range of 0 to 10 degrees. The wavelength of the incident read-out light is λ (μ
m), the absolute value of the phase difference is (λ / 2) μm or more (Δn · d) with respect to the product Δn · d of the thickness d (μm) of the liquid crystal layer and the refractive index anisotropy Δn. Since the one set in the range of −λ / 2) μm or less is used, when the written image is black, the reflected light of the reading light is extremely small, and an image with a significantly improved contrast ratio can be stably obtained. You can do it. Furthermore, it has an excellent threshold characteristic, and since the change amount of the internal impedance of the photoconductive layer may be small by this, the photoconductive layer can be easily formed.

[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 applied voltage and retardation for explaining the action of 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.

[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 9 Phase plate 11 Writing light 12 Reading light

Claims (1)

[Claims] 1. A light modulation element in which an image is written by causing the alignment state of liquid crystal molecules to transition by writing light and the written image is projected by reading light. An electrode and a first liquid crystal alignment layer are sequentially laminated, and a second transparent electrode, a photoconductive layer, a light reflection layer, and a second liquid crystal alignment layer are sequentially laminated on a second transparent substrate, And a liquid crystal layer sandwiched between the second liquid crystal alignment layers, and a phase plate is provided outside the first transparent substrate, wherein the liquid crystal layer is formed when liquid crystal molecules are initially aligned uniformly. The tilt angle of the liquid crystal molecules on the first and second liquid crystal alignment layers is in the range of 0 to 10 degrees, and the phase plate sets the wavelength of the incident read light to λ (μ).
m), the absolute value of the phase difference is (λ / 2) μm or more (Δn · d) with respect to the product Δn · d of the thickness d (μm) of the liquid crystal layer and the refractive index anisotropy Δn. An optical modulation element characterized by being set in a range of d-λ / 2) μm or less.