JPH06230408A - Optical modulation element - Google Patents

Abstract

PURPOSE:To provide the optical modulation element which has electro-optical characteristics of the sharp threshold characteristic and small dependency on wavelength and enables a normally white mode and normally black mode to be inversed by providing a phase plate on the outer side of a first transparent substrate. CONSTITUTION:A first transparent electrode 3a and a first liquid crystal oriented layer 6a are successively laminated on the first transparent substrate 2a. A second transparent electrode 3b, a photoconductive layer 4, a light reflection layer 5 and a second liquid crystal oriented layer 6b are successively laminated on a second transparent substrate 2b. A liquid crystal layer 7 is interposed between the first and second liquid crystal oriented layers 6a and 6b. The liquid crystal molecules of the liquid crystal layer 7 are initially arranged uniformly and the phase plate 9 is provided on the outer side of the first transparent substrate 2a. The phase plate 9 is so provided that its rotating angle attains about 90 deg. to the axial direction of the liquid crystal molecules on the liquid crystal oriented layer 6a on the substrate 2a side on which reading-out light 12 is made incident. The inversion of the normally white mode and the normally black mode is realized by providing such phase plate 9.

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, with respect to 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-out light, the light modulation element using the hybrid field effect mode by the liquid crystal layer having the twist alignment of 45 degrees. Or ECB (el
An optical modulator is known in which the initial alignment of liquid crystal molecules is made vertical or horizontal by utilizing the effect of electrical controlled birefringence. The liquid crystal molecules of the horizontally aligned liquid crystal layer 7 are arranged so that their long axes have a slight pretilt angle on the substrates 2a and 2b and are substantially parallel to each other. 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 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 wavelength dependence when no 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 indicates the wavelength, and the vertical axis indicates the output light amount / the input light amount. In the example shown in FIG. 6, applied voltages V99 (= 1.18) and V1 (= 1) corresponding to relative light transmittances of 99% and 1% in the first half (front slope) of the downwardly convex portion. .79). In this case, when no electric field is applied, it is white, which is a so-called normally white mode.

Further, although not shown in FIG. 6, by using a singular point at which the transmitted light intensity J becomes maximum, the transmitted light intensity J can be reduced according to the magnitude of the applied voltage above the voltage. As described above, a method of operating in the normally white mode by using the latter half portion (back slope) of the convex portion is described in US Pat.
No. 378,955. According to this, not only the condition that the transmitted light intensity J becomes maximum when the electric field is not applied but also the singular point where the transmitted light intensity J becomes maximum at a certain voltage can be used, so that many conditions can be used. Is.

[0013]

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 Using the applied voltages V99 and V1 corresponding to the light transmittances of 99% and 1%, the sharpness γ of the threshold value characteristic is
It is used by defining it as γ = V1 / V99. As shown in FIG. 6, from V1 = 1.79 and V99 = 1.18, γ =
If the amount of change in the internal impedance of the photoconductive layer 4 due to the incident writing light 11 is small, the amount of change in applied voltage is also small, so that the contrast ratio cannot be increased. Further, as shown in FIG. 7, when light having a large wavelength dependency and a certain wavelength range is modulated, there is a problem that a difference in light transmittance occurs depending on the wavelength, which causes a change in hue.

Further, in the normally white mode, when writing is performed by pulsed light such as a cathode ray tube,
There is also a problem that the transmitted light intensity J increases during the time when the writing light is not irradiated, so-called black floating phenomenon occurs, and the contrast ratio is lowered. Therefore, in view of such a problem, the present invention has a sharp threshold value characteristic by taking an element configuration different from that of the conventional light modulation element, and has extremely excellent electro-optical characteristics with small wavelength dependence, and is normally white. It is an object of the present invention to provide an optical modulator capable of reversing a mode and a normally black mode.

[0015]

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.
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 the liquid crystal molecules of the liquid crystal layer are As described above, the present invention provides an optical modulation element characterized in that a phase plate is provided outside the first transparent substrate in the initial arrangement.

[0016]

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 wavelength and the emitted light amount / incident light amount in the light modulation element of the present invention. In addition, in FIG.
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.

Here, although the substrates 2a and 2b are made of, for example, transparent glass plates in this embodiment, 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.

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 are 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 a rotation angle of the liquid crystal molecule alignment layer 6a on the substrate 2a side on which the reading light 12 is incident.
The angle is about 90 degrees with respect to the axial direction of the upper liquid crystal molecules. As the phase plate 9, for example, a wave plate that gives a phase difference of π / 2 or π can be used. In this embodiment, a λ / 4 plate (1/4 wave plate) that gives a phase difference of π / 2 is used. I am using. For the λ / 4 plate, for example, the mica may be cleaved (cleaved) to a thickness corresponding to a desired phase difference. Since the α axis of the mica is almost perpendicular to the cleavage plane, the polarization directions are β axis and γ axis,
Since the refractive indexes are 1.590 and 1.594, respectively,
The thickness is 36 μm.

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 type 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 converted into circularly polarized light by passing through the phase plate 9, and the incident light is converted into the liquid crystal layer 7.
After being transmitted through the liquid crystal layer 7, the circularly polarized light is converted into linearly polarized light by passing through the phase plate 9 again, and the transmitted light intensity J is minimum by the polarization beam splitter 15. It is designed to be.

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 is increased, the long axis direction of the liquid crystal molecules having positive dielectric anisotropy is changed to 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 reflected by the light reflection layer 5 and again the liquid crystal. The light emitted after passing through the layer 7 is converted into circularly polarized light or elliptically polarized light, and when passing through the phase plate 9 again, only the component in which the vibration direction of the principal axis of the elliptically polarized light and the rotation angle of the phase plate 9 are linearly polarized light. become,
The light transmitted 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 due to 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. 2 shows 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, and FIG. 3 shows the wavelength dependence when no electric field is applied. In FIG. 2, as in FIG. 6, the horizontal axis represents applied voltage, the vertical axis represents emission light amount / incident light amount, and in FIG. 3, as in FIG. 7, the horizontal axis represents wavelength and the vertical axis represents emission light amount / incident light amount. is there. From FIG. 2, the relative light transmittance is 1
% And 99%, the sharpness γ of the threshold characteristic using the applied voltages V99 and V1 is V1 = 1.18, V99 = 1.71.
Therefore, γ = 1.45 (in this case, γ = V99 / V1)
Therefore, it can be seen that the light modulation element 1 of the present invention has a remarkably steep threshold characteristic. Furthermore, as shown in FIG. 3, it can be seen that the difference in the light transmittance depending on the wavelength is small, and the light modulation element 1 of the present invention has a characteristic that the wavelength dependence is small.

The optical modulator 1 of the present invention is provided with a phase plate 9
With the provision of, the inversion of the normally white mode and the normally black mode is realized. In this way, by inverting the normally white mode to the normally black mode, the transmitted light intensity J does not increase during black display, and for example, light modulated by a thin film transistor (TFT) driven liquid crystal panel or the like is used as a writing light source. Of course, the writing light source is a CRT,
This is effective for writing light by pulsed light such as EL (electroluminescence element), LED (light emitting diode), and LD (laser diode).

In this embodiment described above, the case where the normally white mode is inverted to the normally black mode by providing the phase plate 9 has been described.
On the contrary, reversing the normally black mode to the normally white mode can be realized by the same means. Inverting to the normally white mode, for example, when the light modulated by a liquid crystal panel driven by a thin film transistor is used as the writing light, the liquid crystal panel is easily inverted in black and white by using a reverse signal as an input signal. It can be effectively used for such a writing light source that can hold writing light for a certain period of time, and this makes it possible to realize black display with less light leakage depending on the wavelength of the reading light and to improve black purity. To enable. Furthermore, although the initial liquid crystal molecule alignment in the liquid crystal layer 7 is described as horizontal alignment in the thickness direction of the liquid crystal layer 7, the present invention is not limited to this.
Of course, the same effect can be obtained even in the vertical alignment, the tilted alignment, and the alignment having the twist angle, and the idea of the present invention is not changed at all. in this way,
The embodiment of the light modulation element 1 of the present invention can be changed in various ways without departing from the scope of the present invention.

[0029]

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 the liquid crystal molecules of the liquid crystal layer are As described above, since the initial arrangement is performed and the phase plate is provided outside the first transparent substrate, it has an extremely excellent threshold characteristic, and thus the change amount of the internal impedance of the photoconductive layer may be small. The formation of the photoconductive layer is facilitated, and display with a large contrast ratio is possible. In addition, since the wavelength dependence can be reduced, stable display can be achieved in which the hue change of the display color due to the difference in light transmittance depending on the wavelength is small.
Further, since the normally white mode and the normally black mode can be reversed, it is possible to select an effective mode according to the characteristics of the writing light.

[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 shows the wavelength and the amount of emitted light in the optical modulator of the present invention.
It is a figure which shows the relationship with the amount of incident light.

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 a relationship between a wavelength and an amount of emitted light / an 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

Claims (2)

[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, the liquid crystal molecules of the liquid crystal layer are initially aligned uniformly, and a phase plate is provided outside the first transparent substrate. element. 2. The phase plate provides a phase difference of π / 2.
The light modulation element according to claim 1, which is a quarter-wave plate.