JP2839990B2 - Liquid crystal spatial light modulator - Google Patents

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 for 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 modulator is to write a two-dimensional pattern, such as an image, on a liquid crystal spatial light modulator by writing light and read the written two-dimensional pattern by reading light. is there. Thereby, it is possible to perform processing such as amplification of image light, threshold processing, inversion or incoherent-coherent conversion between readout light and write light, and wavelength conversion. Further, by writing an interference pattern of light to the liquid crystal spatial light modulator by writing light and reading an interference pattern of the written light by reading light, the liquid crystal spatial light modulator can be used as a holographic display. . At this time, the resolution required for the holographic display is at least several 100 lp / mm or more.

[0003] As a structure of a conventional liquid crystal spatial light modulator, the orientation state of liquid crystal is controlled by twisted nematic (Applied).
Optics, vol. 26, p. 241, 1987) or Super Twist Nematic (IEICE Technical Report, Vol. 90, N
o.431, p.23, 1990). As a conventional example, FIG. 7 shows a structure of a liquid crystal spatial light modulator in which the alignment state of liquid crystal is super twist nematic. FIG.
, 101a and 101b are glass substrates, 102
a and 102b are transparent electrodes; 103a and 103b are liquid crystal alignment films; 104 is a photoconductive film; 105 is a nematic liquid crystal having a super twist nematic alignment state;
07 is a writing light, 108 is a reading light, and 110 is a dielectric mirror. In the structure of FIG.
Hydrogenated amorphous silicon (hereinafter a-Si: 4)
H), and the nematic liquid crystal 105 is filled via the dielectric mirror 110. Here, the alignment processing is performed so that the alignment state of the nematic liquid crystal 105 becomes super twist nematic. In the portion irradiated with the writing light 107, the specific resistance of the photoconductive film 104 decreases, and the voltage applied to the nematic liquid crystal 105 increases accordingly, exceeding the threshold voltage of the liquid crystal. Therefore, the glass substrate 101a and the glass substrate The liquid crystal axis oriented along the plane of 101b is oriented in the voltage application direction. For this reason,
The rotation of the plane of polarization does not occur in the linearly polarized readout light 108. On the other hand, in the portion where the writing light 107 is not irradiated, the liquid crystal axes are aligned with the glass substrates 101a and 101a.
The polarization plane of the readout light 108 rotates because it remains along the plane b. For this reason, a pattern corresponding to the write pattern can be read by reading through an analyzer (not shown).

As another conventional example, as shown in FIG. 8, a liquid crystal display using a ferroelectric liquid crystal is also known (No. 51).
Annual Meeting of the Japan Society of Applied Physics and Applied Physics Lett
ers, Vol.158, No.8, p.787, 1991). In FIG. 8, 20
1a and 201b are glass substrates, 202a and 202b
Are transparent electrodes, 203a and 203b are liquid crystal alignment films, 20
Reference numeral 4 denotes a photoconductive film, 205 denotes a ferroelectric liquid crystal, 207 denotes a writing light, 208 denotes a reading light, and 210 denotes a dielectric mirror. In the structure of the liquid crystal spatial light modulator of FIG. 8, a ferroelectric liquid crystal 205 is filled as a liquid crystal layer. If a pattern having a spatial frequency of about several hundred lp / mm is written in the liquid crystal spatial light modulator using interference of coherent light or the like, the liquid crystal spatial light modulator behaves as a diffraction grating and can be used as a holographic display. .

[0005]

However, the liquid crystal spatial light modulator according to the above-mentioned prior art has the following problems.

(1) In a liquid crystal spatial light modulator in which the alignment state of the liquid crystal having the structure shown in FIG. 7 is twisted nematic or super twisted mimatic, the polarization of the read light is divided into a portion irradiated with the writing light and a portion not irradiated. Since different planes use different effects, if an interference pattern is written into a liquid crystal spatial light modulator with this structure to form a diffraction grating, this diffraction grating will be of the intensity modulation type, and the diffraction efficiency of the generated diffracted light will be low. Would. Further, in the intensity modulation type diffraction grating using the liquid crystal spatial light modulator, the intensity of the diffracted light is very weak in a high spatial frequency region, and the intensity required for use in a holographic display is several tens.
At present, the diffraction efficiency is not measured even at a high spatial frequency of 0 lp / mm or more.

(2) In the structure of the liquid crystal spatial light modulator using the ferroelectric liquid crystal shown in FIG. 8, when an intensity modulation type diffraction grating is used, it is sufficient in a high spatial frequency region for the same reason as (1). No high diffraction efficiency has been obtained. In addition, a holographic display research society bulletin, 1991, No. 2, p.15 shows a liquid crystal spatial light modulator using a phase modulation type diffraction grating, but it is necessary to use it for a holographic display. Sufficient diffraction efficiency has not been obtained at a high spatial frequency of several hundred lp / mm or more. Further, since the ferroelectric liquid crystal is used, the orientation state of the liquid crystal cannot be changed stepwise due to its bistability, and the intensity of the diffracted light cannot be changed corresponding to the intensity of the writing light. There is also a disadvantage.

Accordingly, the present invention has been made in consideration of the above circumstances, and has been made in consideration of the above-described formula (10) for use in a holographic display.
It is an object of the present invention to provide a liquid crystal spatial light modulator that has a sufficient diffraction efficiency at a high spatial frequency of 0 lp / mm or more and can be applied to a holographic display.

[0009]

SUMMARY OF THE INVENTION Liquid crystal spatial light modulation of the present invention
The element has a first glass substrate and a second glass substrate each having a transparent electrode formed on the surface thereof, and a device in front of the first glass substrate.
Serial is formed on the transparent electrode, a photoconductive layer made of hydrogenated amorphous silicon film, a first liquid crystal alignment film formed on the photoconductive layer, the permeability <br of the second glass substrate /> and the second liquid crystal alignment film formed on the transparent electrode, the being sandwiched between the first liquid crystal alignment layer and the second liquid crystal alignment layer, 0 ° to 90 ° of tilt And a nematic liquid crystal layer in a tilt alignment state having an angle , wherein the photoconductive film has a film thickness of the nematic liquid crystal layer.
It is formed thinner than the liquid crystal layer .

[0010]

[0011]

[0012]

[0013]

[0014]

[Action] As described above, the liquid crystal spatial light modulator element according to the present invention
Is the orientation of Ne Machiikku liquid crystal layer, 0 ° or 90 °
Tomo If you tilt orientation having the following tilt angle
First, a photoconductive film made of hydrogenated amorphous silicon film
Film thickness, it is possible to obtain a sufficiently high diffraction efficiency the Nemachiikku Runode are thinner than liquid crystal layer, the number 100 lp / mm or more high spatial frequency necessary for use in a holographic display. Therefore, if the liquid crystal spatial light modulator of the present invention is used, it can be applied to a holographic display.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing the structure of one embodiment of a liquid crystal spatial light modulator according to the present invention. As shown in FIG. 1, transparent electrodes 2a and 2b made of ITO or the like are provided on glass substrates 1a and 1b, respectively. Further, on the transparent electrode 2a, hydrogenated amorphous silicon (a -Si: H) film is formed. Photoconductive film 4
Liquid crystal alignment films 3a and 3b for tilt-aligning the nematic liquid crystal are formed on the upper and transparent electrodes 2b, respectively. Nematic liquid crystal 5 is interposed between these alignment films 3a and 3b. It is pinched. Spacers 6 are provided on both sides of the nematic liquid crystal 5, respectively. 7 is a writing light and 8 is a reading light.

Next, the tilt alignment, which is the alignment state of the nematic liquid crystal employed in the present invention, will be described with reference to FIG. 2, 11a and 11b are glass substrates, 13a and 13b are liquid crystal alignment films, and 15 is a liquid crystal molecule. As shown in FIG. 2A, the tilt alignment means that the liquid crystal molecules 15 are inclined at a certain angle (tilt angle θ) with respect to the surfaces of the glass substrates 11a and 11b on which the liquid crystal alignment films 13a and 13b are formed. And aligned in the same direction. In particular, as shown in FIG. 2B, the tilt alignment state when the tilt angle θ is 0 °, that is, the alignment state in which the liquid crystal molecules 15 are arranged parallel to the surfaces of the glass substrates 11a and 11b is homogeneous. As shown in FIG. 2C, the tilt alignment state when the tilt angle is 90 °, that is, the alignment state in which the liquid crystal molecules 15 are aligned perpendicular to the surfaces of the glass substrates 11a and 11b is called an alignment. It is called homeotropic alignment, and the tilt alignment in the present invention includes a homogeneous alignment and a homeotropic alignment. Then, even if the arrangement of the liquid crystal molecules 15 is continuously bent between the glass substrate 11a and the glass substrate 11b, that is, even if the tilt angle θ is continuously changed between the two substrates, the tilt at this time is changed. If the angle is in the range of 0 ° or more and 90 ° or less,
This orientation state is also included in the tilt orientation in the present invention.

Next, a method of manufacturing the liquid crystal spatial light modulator according to the above embodiment will be described with reference to FIG.

The a-Si: H film, which is the photoconductive film 4, is formed by forming a mixed gas of Ar (argon) and H 2 (hydrogen) on the glass substrate 1a on the writing side on which ITO is formed as the transparent electrode 2a. A film is formed by a sputtering method using a Si target in an intrinsic a-Si:
An H film 4 is obtained. The characteristics of the a-Si: H film 4 thus formed are, for example, dark conductivity of about 10 ⁻¹⁰ S / cm, photoconductivity of about 10 ⁻⁷ S / cm, and a film thickness of about 1 ⁻⁷ S / cm. .5μ
m. Here, the a-Si: H film 4, which is a photoconductive film used in the liquid crystal spatial light modulator according to the present embodiment, has a ratio of dark conductivity to photoconductivity of two digits or more, and particularly 100
When driving at a frequency higher than 1 Hz, the photoconductivity is 10
Desirably, it is ^-8 S / cm or more. In this embodiment, the a-Si: H film is formed by a sputtering method, but may be formed by a plasma CVD or the like.

Next, ITO as the transparent electrode 2a and a-
Writing-side glass substrate 1 on which Si: H film 4 is formed
A liquid crystal alignment film 3a and a liquid crystal alignment film 3b for tilt-aligning the nematic liquid crystal 5 are formed on the glass substrate 1b on which ITO is formed as the transparent electrode 2b. The liquid crystal alignment films 3a and 3b are preferably formed by oblique deposition of SiO at an angle of 85 ° with respect to the normal direction of the glass substrate surface. In the present embodiment, an SiO film formed by oblique evaporation is used as the liquid crystal alignment films 3a and 3b. However, a liquid crystal alignment film obtained by rubbing polyimide or polyvinyl alcohol, or another alignment agent is used. Any liquid crystal alignment film, such as a liquid crystal alignment film formed by coating, may be used as long as the liquid crystal alignment film has a tilt angle of 0 ° or more and 90 ° or less.

The liquid crystal alignment film 3a and the liquid crystal alignment film 3b thus formed are opposed to each other, a gap is formed by the spacer 6, and the gap is filled with the nematic liquid crystal 5 to have a thickness of 3 μm.
An m liquid crystal layer is formed. In the present embodiment, it is desirable that the nematic liquid crystal 5 exhibit a large birefringence in order to sufficiently increase the amount of phase shift.
44 may be used. In this example, the positive dielectric anisotropy in which the direction of orientation changes from a state substantially parallel to the glass substrate surface to a state perpendicular to the glass substrate surface according to the intensity of the voltage to which the nematic liquid crystal is applied. Although the nematic liquid crystal having a negative dielectric anisotropy is used in a homogeneous alignment, the nematic liquid crystal having a negative dielectric anisotropy may be used in a homeotropic alignment.

In the structure of the liquid crystal spatial light modulator of this embodiment manufactured as described above, the amount of phase shift at a low spatial frequency is the product of the birefringence of the nematic liquid crystal and the thickness of the liquid crystal layer. The larger the thickness of the liquid crystal layer, the larger the amount of phase shift. However, at a high spatial frequency, the smaller the thickness of the liquid crystal layer, the larger the amount of phase shift. Therefore, the number 100 l required for use in a holographic display
In order to obtain a sufficient diffraction efficiency at a high spatial frequency of p / mm or more, it is desirable that the thickness of the liquid crystal layer is thin so that a sufficient amount of phase shift can be obtained in this high spatial frequency region. In the embodiment, it is about 3 μm. Also, a-Si:
The thickness of the H film 4 is desirably about the same as or larger than that of the liquid crystal layer from the ratio of the electric capacity of the a-Si: H film 4 to the liquid crystal layer. 100 lp / m required for graphic display
m to obtain sufficient diffraction efficiency at high spatial frequencies
It is preferable that the thickness of the a-Si: H film 4 is 1.5 μm, which is thinner than the liquid crystal layer.

Next, the operation characteristics of the liquid crystal spatial light modulator manufactured as described above will be described. FIG. 3 is a configuration diagram illustrating a measurement system of the diffraction efficiency of the liquid crystal spatial light modulator of the above embodiment. In FIG. 3, reference numeral 9 denotes a liquid crystal spatial light modulator, 2
0 is a writing He-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, 25 is a reading He-Ne laser light source, 26 Is an ND filter for adjusting the intensity of the read light, and 27 is an optical power meter. The intensity of the linearly polarized laser light from the writing He-Ne laser light source 20 is attenuated by the ND filter 21, the beam diameter is expanded by the beam expander 22, and separated by the half mirror 23. Then, the laser beam reflected by the half mirror 23 is
The two light flux interference fringes are radiated to the writing side of the liquid crystal spatial light modulator 9 by interference with the laser light reflected by 4 and transmitted through the half mirror 23, and the two light flux interference fringes are written to the liquid crystal spatial light modulator 9. .

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 applied to the reading side of the liquid crystal spatial light modulator 9. At this time, if an AC voltage is applied between the transparent electrode 2 and the transparent electrode 2b in FIG. 1, the birefringence of the nematic liquid crystal is applied to the liquid crystal layer of the liquid crystal spatial light modulator 9 by writing the two-beam interference fringes. A phase modulation type reflection diffraction grating is formed. Here, assuming that λ is the wavelength of the reading light, a first-order diffracted light of the reading light is generated at a diffraction angle θ such that sin θ = nλ with respect to the spatial frequency n of the interference fringe of the writing light.
The diffraction efficiency γ of the first-order diffracted light is defined as follows.

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

FIG. 4 shows the results of measuring the diffraction efficiency of the liquid crystal spatial modulation device manufactured in this example using the above-described measurement system. The measurement conditions at this time were: write light intensity of 6 mW / cm ² , read light intensity of 0.1 mW
/ Cm ² , the applied voltage was 6 V, and the applied frequency was 150 Hz. From this measurement result, a spatial frequency of 100 lp / mm
At a diffraction efficiency of 5.7% and a spatial frequency of 500 lp / m
m, the diffraction efficiency was 0.05%. The result is
100 l required for using the liquid crystal spatial modulation device produced in this embodiment for a holographic display
It shows that a sufficiently high diffraction efficiency can be obtained at a high spatial frequency of p / mm or more, and thus shows that application to a holographic display is sufficiently possible.

Hereinafter, a liquid crystal spatial light modulator having a structure having a dielectric mirror will be described as a second embodiment of the liquid crystal spatial modulator according to the present invention.

FIG. 5 is a sectional view showing the structure of a liquid crystal spatial light modulator manufactured according to the present invention. In FIG.
31a and 31b are glass substrates, 32a and 32b are I
Transparent electrodes such as TO, 33a and 33b are liquid crystal alignment films for tilt-aligning nematic liquid crystal, and 34 is hydrogenated amorphous silicon (a-Si:
H) film, 35 is a nematic liquid crystal, 36 is a spacer,
37 is a writing light, 38 is a reading light, 39 is a liquid crystal spatial light modulator, and 40 is a dielectric mirror.

The dielectric mirror 40 is formed by alternately laminating a high-refractive-index dielectric or a semiconductor having a sufficiently low photo-conductivity and a low-refractive-index dielectric. In this embodiment, the dielectric mirror 40 has a sufficiently high photo-conductivity. A-Si: H was used as a low semiconductor, and silicon dioxide (hereinafter abbreviated as SiO ₂ ) was used as a low refractive index derivative. In each of these formations, a film is formed by a sputtering method, and a transparent electrode 32a is formed as in the first embodiment.
SiO ₂ is formed on the a-Si: H film 34 formed on the glass substrate 31 a on the writing side on which TO is formed using an SiO ₂ target in an argon (Ar) gas, and then argon (Ar) is formed thereon. A-Si: H is formed using a Si target in a mixed gas of Ar) and hydrogen (H ₂ ), and these are alternately formed in the same manner to form a dielectric mirror 40 having five layers. Here, a-Si: H was formed under such conditions that the photodielectric constant was sufficiently low.
In this embodiment, a-Si: H is used as a high refractive index dielectric or a semiconductor having a sufficiently low photoconductivity.
₂ , ZnS or the like may be used, and MgF ₂ , Si ₃ N ₄ or the like may be used as the low refractive index dielectric. In this embodiment, a sputtering method is used as a method for forming the dielectric mirror. However, the dielectric mirror may be formed by a vacuum evaporation method or the like. Further, in this embodiment, the number of laminated dielectric mirrors is five, but the present invention is not limited to this. To sufficiently obtain the effects of the present invention, the number of laminated dielectric mirrors is small and the thickness of the entire dielectric mirror is small. Is preferably thinner.

The liquid crystal spatial light modulator of the second embodiment was manufactured in exactly the same manner as the first embodiment except for the dielectric mirror.

Regarding the operating characteristics of the liquid crystal spatial light modulator according to the second embodiment, the diffraction efficiency was measured by a measuring system having the structure shown in FIG. 3 as in the first embodiment. The result is shown in FIG. The measurement conditions at this time were: write light intensity of 4 mW / cm ² , read light intensity of 10 mW / c.
m ² , applied voltage of 8 V, and applied frequency of 1 kHz. From these measurement results, the diffraction efficiency was 0.4% at a spatial frequency of 200 lp / mm, and the diffraction efficiency was 0.01% at a spatial frequency of 400 lp / mm. As described above, in the structure having the dielectric mirror, the value of the diffraction efficiency is slightly reduced as compared with the first embodiment. However, a strong read light can be used as the read light, and a bright diffracted light can be used. Can be obtained. This result indicates that the liquid crystal spatial modulation device according to the second embodiment can obtain a sufficiently high diffraction efficiency at a high spatial frequency of several hundred lp / mm or more necessary for use in a holographic display, and can produce bright diffracted light. This indicates that the method can be applied to a holographic display.

In the liquid crystal spatial light modulator according to the second embodiment shown in FIG. 5, when the intensity of the reading light 38 is further increased and the reading light 38 reaches the a-Si: H film 34, A light shielding film may be formed between the a-Si: H film 34 and the dielectric mirror 40. As a material of the light-shielding film, a transition metal oxide film, a semiconductor having sufficiently low photoconductivity, or the like is appropriate. When the light-shielding film is formed, if the thickness is large, the effect of the above embodiment may not be sufficiently obtained. Therefore, it is preferable to reduce the thickness of the light-shielding film.

Also in the liquid crystal spatial light modulator according to the first embodiment shown in FIG. 1, when the intensity of the reading light 8 is further increased and the reading light 8 reaches the a-Si: H film 4, A-Si: H film 4 and liquid crystal alignment film 3
a.

[0034]

As described above, the liquid crystal spatial light modulator according to the present invention includes a nematic liquid crystal layer in a tilt alignment state having a tilt angle of 0 ° to 90 °.
Together with the light consisting of hydrogenated amorphous silicon film
The thickness of the conductive film is formed to be thinner than the nematic liquid crystal layer.
Since the, it is possible to obtain a sufficiently high diffraction efficiency by the number 100 lp / mm or more high spatial frequency necessary for use in a holographic display. Therefore, if the liquid crystal spatial light modulator of the present invention is used, application to a holographic display becomes possible, and further, three-dimensional display by a holographic display becomes possible.

[Brief description of the drawings]

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

FIG. 2 is a conceptual diagram showing an alignment state of a nematic liquid crystal in a liquid crystal spatial light modulator according to the present invention.

FIG. 3 is a configuration diagram showing a measurement system of a diffraction efficiency of a liquid crystal spatial light modulation element.

FIG. 4 is a graph showing a measurement result of a diffraction efficiency of the liquid crystal spatial light modulator shown in FIG.

FIG. 5 is a sectional view showing the structure of another embodiment of the liquid crystal spatial light modulator according to the present invention.

6 is a graph showing a measurement result of a diffraction efficiency of the liquid crystal spatial light modulator shown in FIG.

FIG. 7 is a sectional view showing the structure of an example of a conventional liquid crystal spatial light modulator.

FIG. 8 is a sectional view showing the structure of another example of the conventional liquid crystal spatial light modulator.

[Explanation of symbols]

 1a, 1b Glass substrate 2a, 2b Transparent electrode 3a, 3b Liquid crystal alignment film 4 Photoconductive film 5 Nematic liquid crystal 6 Spacer 7 Write light 8 Read light 9 Liquid crystal spatial light modulator

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-20720 (JP, A) JP-A-3-264918 (JP, A) JP-A-5-323123 (JP, A) JP-A-6-320 51340 (JP, A) Applied Optics, Vol. 26, p. 241, 1987 Applied Physics Letters, Vol. 58, No. 8, p. 787, 1991 IEICE Technical Report, Vol. 90, no. 431, p. 23, 1990 (58) Field surveyed (Int. Cl. ⁶ , DB name) G02F 1/135

Claims (1)

(57) [Claims] A first glass substrate and a second glass substrate each having a transparent electrode formed on a surface thereof; and a first glass substrate formed on the transparent electrode of the first glass substrate, the first glass substrate being formed of a hydrogenated amorphous silicon film. A first liquid crystal alignment film formed on the photoconductive film; a second liquid crystal alignment film formed on the transparent electrode of the second glass substrate; A tilt-aligned nematic liquid crystal layer sandwiched between the liquid crystal alignment film and the second liquid crystal alignment film and having a tilt angle of 0 ° or more and 90 ° or less.
A liquid crystal spatial light modulator, wherein the thickness of the photoconductive film is thinner than that of the nematic liquid crystal layer.
A liquid crystal spatial light modulator characterized by being formed well .