JPH06222383A - Liquid crystal spatial optical modulation element - Google Patents

Abstract

PURPOSE:To provide the liquid crystal spatial optical modulation element which exhibits sufficiently high diffraction efficiency at high spatial frequencies of several hundreds 1p/mm or more necessary for using in a holographic display and is applicable to the holographic display. CONSTITUTION:This liquid crystal spatial optical modulation element has a first glass substrate 1a and second glass substrate 1b formed with transparent electrodes 2a, 2b on the respective surfaces, a photoconductor film 3 formed on the transparent electrodes 2a, a multilayerd film mirror 4 formed on this photoconductor film 3, a first liquid crystal oriented film 5a formed on this multilayerd film mirror 4, a second liquid crystal oriented film 5b formed on the transparent electrode 2b and a liquid crystal layer 6 clamped between the first liquid crystal oriented film 5a and the second liquid crystal oriented film 5b. The multilayerd film mirror 4 is constituted of the multilayerd film mirror alternately laminated with high-refractive index photoconductor films and low- refractive index photoconductor films in multiple layers.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal spatial light modulator used in an image display device, an image processing device, an optical information processing system and the like and applicable to a holographic display.

[0002]

2. Description of the Related Art The function of a liquid crystal spatial light modulation element is to write a two-dimensional pattern such as an image into a liquid crystal spatial light modulation element by writing light and read out the two-dimensional pattern written by reading light. . Thereby, processing such as amplification of image light, threshold value processing, inversion, incoherent / coherent conversion between reading light and writing light, wavelength conversion, and the like can be performed. Further, if the light interference pattern is written in the liquid crystal spatial light modulation element by the writing light and the written light interference pattern is read by the reading light, the liquid crystal spatial light modulation element can be used as a holographic display. At this time, the resolution required for the holographic display is at least several hundreds lp / mm.
That is all.

The structure of a conventional liquid crystal spatial light modulator is shown in FIG. 4, 101a and 101b are glass substrates, 102a and 102b are transparent electrodes, 103 is a photoconductive film, 104 is a multilayer film mirror, 105a and 105b are liquid crystal alignment films, 106 is a liquid crystal layer, 107 is a spacer, and 10 is a spacer.
Reference numeral 8 is a writing light, and 109 is a reading light. here,
The multi-layer film mirror is usually a multi-layered structure in which high-refractive-index dielectric films and low-refractive-index dielectric films are alternately laminated. As the high-refractive-index dielectric film, TiO ₂ , ZnS, or the like is used. SiO ₂ , MgF ₂ , Si ₃ N ₄ or the like is used as the dielectric film.

Next, the operation of the conventional liquid crystal spatial light modulator will be described with reference to FIG. When the liquid crystal layer 106 is made of twisted nematic liquid crystal, photocarriers are generated in the photoconductor film 103 in the portion irradiated with the writing light 108, and the specific resistance of the photoconductor film 103 is reduced. Then, in the portion where the specific resistance of the photoconductor film 103 is lowered, the voltage applied to the liquid crystal layer 106 increases and exceeds the threshold voltage of the nematic liquid crystal, so that the glass substrate 10
The liquid crystal axis oriented along the plane direction of 1a and the glass substrate 101b is oriented in the voltage application direction. Therefore, in this portion, the plane of polarization of the linearly polarized readout light 109 is not rotated.

On the other hand, in the portion where the writing light 108 is not irradiated, the specific resistance of the photoconductor layer 103 does not change, and the liquid crystal axis of the liquid crystal layer 106 remains along the surface direction of the glass substrates 101a and 101b. For reading light 1
The polarization plane of 09 is rotated. Therefore, a pattern corresponding to the writing pattern can be read by reading through an analyzer (not shown), and thus the reading light is modulated by the writing light (Applied Optimal
cs, Vol.26, p.241, 1987 etc.).

Further, as the liquid crystal layer, in addition to the above, a nematic liquid crystal of a tilt arrangement and a nematic liquid crystal of a super twist arrangement (Technical Report of IEICE, Vol.
26, No.431, p.23, 1990), and ferroelectric liquid crystals (Procedures of the 51st Japan Society of Applied Physics, Applide Physics Letters,
A liquid crystal spatial light modulator using Vol.158, No.8, p.787, 1991) has also been proposed.

In such a liquid crystal spatial light modulator, several hundreds of lp
If a spatial frequency pattern of about / mm is written by using coherent light interference or the like, this liquid crystal spatial light modulation element behaves as a diffraction grating and can be used as a holographic display.

[0008]

However, in the above-mentioned conventional liquid crystal spatial light modulator, sufficient diffraction efficiency is not obtained at a high spatial frequency of several hundred lp / mm or more necessary for use in a holographic display. . For example,
Bulletin of the Holographic Display Research Society, 1991, N
In o.2 and p.15, a liquid crystal spatial light modulator using a ferroelectric liquid crystal is shown as a phase modulation type diffraction grating, and the measurement of diffraction efficiency up to 200 lp / mm is reported. However, in the present situation, even the measurement of the diffraction efficiency has not been reported at a spatial frequency higher than this.

As described above, in the conventional liquid crystal spatial light modulator, one of the reasons why sufficient diffraction efficiency is not obtained at a high spatial frequency of several hundred lp / mm or more is a problem in the photoconductor film. . That is, when the write pattern is written in the liquid crystal spatial light modulator, if the photoconductor film is thick, the write pattern is blurred in this film.
It is considered that this is because photocarriers generated by irradiation of writing light in the photoconductor film diffuse in the plane direction of the film. On the other hand, when the thickness of the photoconductor film is thin, the ratio of the voltage applied to the portion of the liquid crystal layer that is irradiated with the writing light and the portion that is not irradiated with the writing light is small, and the modulation amount of the reading light modulated in the liquid crystal layer is reduced. I can't get enough. For this reason, in the conventional liquid crystal spatial light modulator,
The film thickness of the photoconductor film needs to be 2 to 7 μm. Therefore, with the film thickness of the photoconductor film of the conventional liquid crystal spatial light modulator, writing and reading of a high spatial frequency pattern of several hundreds lp / mm or more are performed so that sufficient diffraction efficiency can be obtained. I couldn't.

The present invention has been made to solve the above-mentioned problems, and in a liquid crystal spatial light modulator, a multi-layer film mirror having a high refractive index film and a low refractive index film alternately laminated is provided. By adopting a photoconductive film having a photoconductive effect for the refractive index film, it has sufficient diffraction efficiency and high reflectance at a high spatial frequency of several hundred lp / mm or more necessary for use in a holographic display. However, it is an object of the present invention to provide a liquid crystal spatial light modulator which can be applied to a holographic display.

[0011]

In order to solve the above problems, according to the present invention, a first transparent electrode is formed on each surface.
Glass substrate and second glass substrate, a photoconductor film formed on the transparent electrode of the first glass substrate, a multilayer film mirror formed on the photoconductor film, and the multilayer film. A first liquid crystal alignment film formed on a mirror, a second liquid crystal alignment film formed on a transparent electrode of a second glass substrate, the first liquid crystal alignment film and the second liquid crystal alignment film. In a liquid crystal spatial light modulation element having a liquid crystal layer sandwiched between the multi-layered film mirror and the multi-layered film mirror, the high-refractive-index photoconductor film and the low-refractive-index dielectric film are alternately laminated. There is.

In the present invention, in the above liquid crystal spatial light modulator, preferably, one of the photoconductor film and the high refractive index photoconductor film formed on the transparent electrode of the first glass substrate is used. One or both are hydrogenated amorphous silicon films.

Further, in the present invention, in the above-mentioned liquid crystal spatial light modulator, the liquid crystal layer is preferably a layer made of nematic liquid crystal or ferroelectric liquid crystal.

[0014]

According to the present invention, in the liquid crystal spatial light modulating element, the high-refractive index film of the multi-layer film mirror is formed of the photoconductive film having the photoconductive effect as described above.
Even if the thickness of the photoconductor film is reduced, it is possible to secure a sufficient ratio of the voltage applied to the portion of the liquid crystal layer that is irradiated with the writing light and the portion that is not irradiated with the writing light. Become. Furthermore, according to the present invention, it is possible to prevent the blurring of the writing pattern due to the thick film of the photoconductor film. Therefore, by using the liquid crystal spatial light modulator of the present invention, even if the photoconductor film is thinned, a sufficient amount of read light modulated by the liquid crystal layer can be obtained, and the liquid crystal spatial light modulator is used for a holographic display. It is possible to obtain a sufficiently high diffraction efficiency and a high reflectance at a high spatial frequency of several hundred lp / mm or more required for the above. Therefore, the liquid crystal spatial modulation element of the present invention can be applied to a holographic display.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing the structure of an embodiment of a liquid crystal spatial light modulator according to the present invention. In FIG. 1, 1a and 1b are glass substrates, 2a and 2b are ITO.
Such as transparent electrodes, 3 is a photoconductor film such as hydrogenated amorphous silicon (hereinafter referred to as a-Si: H) film, 4 is a multilayer film mirror, 5a and 5b are liquid crystal alignment films, 6 is a liquid crystal layer, and 7 is Spacer, 8 for writing light, 9 for reading light, 10
Shows the whole of the liquid crystal spatial light modulator comprising the above-mentioned components.

First, the production of the liquid crystal spatial light modulator according to the present invention will be described. In this embodiment, the photoconductor film 3
As the film, an a-Si: H film was used. This a-Si: H film is formed by using a Si target in a mixed gas of Ar (argon) and H ₂ (hydrogen) on the writing side glass substrate 1a on which the transparent electrode (ITO film) 2a is formed. Then, a film was formed by a sputtering method to obtain an intrinsic a-Si: H film. The characteristics of the a-Si: H film thus formed are as follows: dark conductivity is 10 ⁻¹⁰ S / cm, photoconductivity is 10 ⁻⁷ S / cm.
And the film thickness was 1.2 μm. Here, the photoconductor film used in the liquid crystal spatial light modulator of the present invention has a dark conductivity to photoconductivity ratio of two digits or more, and particularly 100H.
When driven at frequencies above z, the photoconductivity is 10 ^-8.
It is preferably S / cm or more. Although the a-Si: H film was formed by the sputtering method in this embodiment,
It may be formed by an appropriate forming method such as plasma CVD.

Next, a multilayer mirror was formed on the glass substrate 1a on the writing side on which the transparent electrode (ITO film) 2a and the photoconductor film (a-Si: H film) 3 were formed. . In this embodiment, an a-Si: H film is used as the high refractive index photoconductor film,
Silicon dioxide (hereinafter referred to as SiO _{2) is used} as a low refractive index dielectric film.
Abbreviated). In order to form these, the photoconductor film (a-Si: H film) 3 formed on the glass substrate 1a on the writing side on which the transparent electrode (ITO film) 2a is formed is formed by sputtering. After a SiO ₂ film is formed on the upper surface by using a SiO ₂ target in argon (Ar) gas, a-Si is formed on the SiO ₂ film by using a Si target in a mixed gas of argon (Ar) and hydrogen (H ₂ ). : H film is formed,
These were alternately formed in the same manner to form a multilayer mirror 4 of 5 layers. Here, the film formation of a-Si: H is performed under the same conditions as the film formation of the a-Si: H film which is the photoconductor film 3, and is a film having a photoconductive effect similar to that of the photoconductor film. is there. In this example, a-Si: H was used as the high-refractive-index photoconductor film, but the present invention is not limited to this, and it has a high refractive index in optical characteristics and has a high photoconductivity. Anything you have will do. Further, as a low-refractive-index dielectric material, MgF ₂ , Si ₃ N ₄
Etc. may be used. Although the sputtering method is used as the method for forming the multilayer mirror in this embodiment, it may be formed by a vacuum vapor deposition method or the like. In this embodiment, the number of laminated multilayer mirrors is five, but the number is not limited to this.

The film thicknesses of the high-refractive-index photoconductor film and the low-refractive-index dielectric film constituting the multilayer film mirror are such that the refractive index of these films and the wavelength of the read light reflected by this multilayer film mirror, It depends on the angle of incidence. A formed in this example
-Si: H film and SiO ₂ film have a refractive index of 1.47, respectively.
3 and 3.19, and as will be described later, since He-Ne laser light is used as the reading light in this embodiment at a vertical incidence, the wavelength of this laser light is 632.8n.
The optical distance of ¼ wavelength of m was calculated from the refractive index of each film, and the film thicknesses of the a-Si: H film and the SiO ₂ film were about 107 nm and about 50 nm, respectively. The film thicknesses of the high-refractive-index photoconductor film and the low-refractive-index dielectric film that form the multilayer mirror may be appropriately set depending on the refractive index of each film, the wavelength of the reading light, the incident angle, and the like. The present invention is not limited to this example.

Next, the transparent electrode (ITO film) 2a, the photoconductor film (a-Si: H film) 3, and the glass substrate 1a on the writing side on which the multilayer mirror 4 is formed, and the transparent electrode (ITO film).
A liquid crystal alignment film 5a and a liquid crystal alignment film 5b for aligning the nematic liquid crystal were formed on each of the read side glass substrate 1b on which 2b was formed. In this embodiment, as the liquid crystal alignment film 5a and the liquid crystal alignment film 5b,
Silicon oxide (hereinafter referred to as SiO) was formed by oblique vapor deposition at an angle of 85 ° with respect to the normal line direction of the glass substrate surface. Although the SiO films formed by oblique vapor deposition were used as the liquid crystal alignment film 5a and the liquid crystal alignment film 5b in this embodiment, a liquid crystal alignment film obtained by rubbing polyimide or polyvinyl alcohol, or another alignment agent may be used. A liquid crystal alignment film or the like formed by coating may be used, and an appropriate one may be used depending on the type and alignment state of the liquid crystal used.

The liquid crystal alignment film 5a and the liquid crystal alignment film 5b thus formed were opposed to each other, a gap was formed by the spacer 7, and the gap was filled with liquid crystal to form the liquid crystal layer 6.
In this embodiment, a nematic liquid crystal is used as the liquid crystal forming the liquid crystal layer 6, and the thickness of the liquid crystal layer 6 is 3 μm. Also,
As the nematic liquid crystal, a liquid crystal exhibiting large birefringence is desirable in order to sufficiently increase the phase shift amount. In this example, a positive dielectric anisotropy in which the orientation direction changes from a state substantially parallel to the glass substrate surface to a state perpendicular to the glass substrate surface depending on the strength of the voltage applied to the nematic liquid crystal Although the nematic liquid crystal exhibiting the above is used in the homogeneous alignment, the nematic liquid crystal exhibiting negative dielectric anisotropy may be used in the homeotropic alignment. Besides, nematic liquid crystals of twist alignment, nematic liquid crystals of super twist alignment, ferroelectric liquid crystals and the like can be used, and the present invention is not limited to this embodiment.

In the structure of the liquid crystal spatial light modulator of the present embodiment, the amount of phase shift is the product of the birefringence of the nematic liquid crystal and the thickness of the liquid crystal layer at a low spatial frequency, and the thickness of the liquid crystal layer is larger. However, the amount of phase shift becomes large, but at high spatial frequencies, the amount of phase shift becomes larger as the liquid crystal layer becomes thinner. Therefore, in order to obtain a sufficient diffraction efficiency at a high spatial frequency of several hundred lp / mm or more required for use in a holographic display, it is necessary to obtain a sufficient amount of phase shift in this high spatial frequency region. ,
It is desirable that the thickness of the liquid crystal layer is thin, and in this embodiment, it is 3 μm.
And

Next, the operation characteristics of the liquid crystal spatial light modulator manufactured according to the present invention as described above will be described. FIG. 2 is a configuration diagram showing a diffraction efficiency measurement system of the liquid crystal spatial light modulator manufactured according to the present invention as described above. In FIG. 2, 10 is a liquid crystal spatial light modulator manufactured according to the present invention as described above, and 20 is an H for writing.
e-Ne laser light source, 21 is an ND filter for adjusting the intensity of writing light, 22 is a beam expander, 23 is a half mirror, 24 is a mirror, and 25 is He-for reading.
Ne laser light source, 26 is N for adjusting the intensity of the read light
D filter, 27 is an optical power meter, 28 is 0th order diffracted light, 29a is + 1st order diffracted light, and 29b is −1st order diffracted light.

The linearly polarized laser light from the writing He-Ne laser light source 20 is attenuated in intensity by the ND filter 21, expanded in beam diameter by the beam expander 22, and separated by the half mirror 23. . Then, the laser light reflected by the half mirror 23 is reflected by the mirror 24 and interferes with the laser light transmitted through the half mirror 23, so that two-beam interference fringes are generated in the liquid crystal spatial light modulation element 1.
It is irradiated to the writing side of 0, and the two-beam interference fringes are written in the liquid crystal spatial light modulation element 10.

On the other hand, the linearly polarized laser light from the He-Ne laser light source 25 for reading is attenuated by the ND filter 26, and then is irradiated to the reading side of the liquid crystal spatial light modulator 10. At this time, the transparent electrode 2a in FIG.
If an AC voltage is applied between the transparent electrode 2b and the transparent electrode 2b,
By writing the light flux interference fringes, a phase modulation type reflection diffraction grating due to the birefringence of the nematic liquid crystal is formed in the liquid crystal layer of the liquid crystal spatial light modulation element 10.

Here, when λ is the wavelength of the reading light,
With respect to the spatial frequency n of the interference fringes of the writing light, the first-order diffracted light of the reading light is generated at the diffraction angle θ of sin θ = nλ.
The diffraction efficiency γ of this first-order diffracted light is defined as follows.

Γ (%) = A1 / A0 × 100 where A1 is the intensity of the 1st-order diffracted light and A0 is the intensity of the 0th-order diffracted light. According to this, by measuring the respective intensities of the + 1st-order diffracted light 29a or the -1st-order diffracted light 29b and the 0th-order diffracted light 28 with the optical power meter 27, the diffraction efficiency at the spatial frequency can be obtained.

FIG. 4 shows the result of measuring the diffraction efficiency of the liquid crystal spatial light modulator manufactured in this example by the above-mentioned measuring system. The measurement conditions at this time were: writing light intensity of 4 mW / cm ² and reading light intensity of 10 mW / c.
m ² , applied voltage was 8 V, and applied frequency was 1 kHz.
From the measurement results, the diffraction efficiency was 0.4% at the spatial frequency of 200 lp / mm, and the diffraction efficiency was 0.01% at the spatial frequency of 400 lp / mm. Further, the reflectance was a sufficiently high value of about 78% at incident light of 632.8 nm.

These results show that the liquid crystal spatial modulation element manufactured in this example has several hundred lp required for use in a holographic display even if the photoconductor film is 1.2 μm, which is much thinner than the conventional one. It is shown that a sufficiently high diffraction efficiency and a high reflectance can be obtained at a high spatial frequency of / mm or more, and thus, it is sufficiently applicable to a holographic display.

[0030]

As described above, according to the present invention, in the liquid crystal spatial light modulator, as described above, the high refractive index film of the multilayer mirror uses the photoconductor film having the photoconductive effect. Therefore, even if the thickness of the photoconductor film is reduced,
In the liquid crystal layer, it is possible to secure a sufficient ratio of the voltage applied to the portion irradiated with the writing light and the portion not irradiated with the writing light, so that the photoconductor film can be thinned. Furthermore, according to the present invention, it is possible to prevent the blurring of the writing pattern due to the thick film of the photoconductor film. Therefore, by using the liquid crystal spatial light modulator of the present invention, even if the photoconductor film is thinned, a sufficient amount of read light modulated by the liquid crystal layer can be obtained, and the liquid crystal spatial light modulator is used for a holographic display. It is possible to obtain a sufficiently high diffraction efficiency and a high reflectance at a high spatial frequency of several hundred lp / mm or more required for the above. Therefore, the liquid crystal spatial modulation element of the present invention can be applied to a holographic display, and further, stereoscopic display by a holographic display using the liquid crystal spatial light modulation element of the present invention is possible.

Further, in the above-mentioned embodiment, the example in which the liquid crystal spatial light modulating element according to the present invention is formed with the phase modulation type reflection diffraction grating, but according to the present invention, the intensity modulation type diffraction grating is formed. Even in this case, the resolution of the writing image and the reading light can be similarly increased.

Further, in the above embodiment, the diffraction grating is formed in the liquid crystal spatial light modulating element according to the present invention in consideration of application to the holographic display. However, a two-dimensional image which is not holographic is formed in the liquid crystal spatial light modulating element. Even when writing and reading are performed, according to the present invention, it is possible to increase the resolution of the writing image and the reading image. Therefore, if the liquid crystal spatial light modulator according to the present invention is applied to a projection type liquid crystal display device, the resolution of a projected image can be increased.

[Brief description of drawings]

FIG. 1 is a sectional view showing a structure of a liquid crystal spatial light modulator according to an embodiment of the present invention.

FIG. 2 is a configuration diagram of a diffraction efficiency measurement system of the liquid crystal spatial light modulator of the present invention.

FIG. 3 is a diagram showing a result of measurement of diffraction efficiency of the liquid crystal spatial light modulator of the present invention.

FIG. 4 is a sectional view showing a structure of a conventional liquid crystal spatial light modulator.

[Explanation of symbols]

 1a, 1b Glass substrate 2a, 2b Transparent electrode 3 Photoconductor film 4 Multi-layer film mirror 5a, 5b Liquid crystal alignment film 6 Liquid crystal layer 7 Spacer 8 Write light 9 Read light 10 Liquid crystal spatial light modulator

Claims (5)

[Claims] 1. A first glass substrate and a second glass substrate each having a transparent electrode formed on its surface, a photoconductor film formed on the transparent electrode of the first glass substrate, and the light A multilayer mirror formed on the conductor film, a first liquid crystal alignment film formed on the multilayer mirror, and a second liquid crystal alignment film formed on the transparent electrode of the second glass substrate. A liquid crystal spatial light modulator comprising a liquid crystal layer sandwiched between the first liquid crystal alignment film and the second liquid crystal alignment film, wherein the multilayer mirror has a high refractive index photoconductor film and a low refractive index film. A liquid crystal spatial light modulation element, which is a multi-layered film mirror in which multi-layered dielectric films are alternately laminated. 2. The liquid crystal spatial light modulator according to claim 1, wherein the photoconductor film formed on the transparent electrode of the first glass substrate is a hydrogenated amorphous silicon film. 3. The liquid crystal spatial light modulator according to claim 1, wherein the high refractive index photoconductor film is a hydrogenated amorphous silicon film. 4. The liquid crystal spatial light modulator according to claim 1, wherein the liquid crystal layer is a layer made of nematic liquid crystal. 5. The liquid crystal spatial light modulation element according to claim 1, wherein the liquid crystal layer is a layer made of a ferroelectric liquid crystal.