JPH0750281B2 - Liquid crystal spatial light modulator and driving method thereof - Google Patents

Description

The present invention relates to a liquid crystal spatial light modulator used for optical information processing and a driving method thereof.

[Conventional technology]

Since the liquid crystal spatial light modulator can simultaneously process two-dimensional optical information in parallel, it can be applied to a parallel optical logic element or an image processing element, and may become an important key device in constructing an optical computer in the future. Is hidden.

A conventional liquid crystal spatial light modulator is as shown in FIG.
A glass substrate A8 on which a pixel electrode 801 and a photoconductor layer 802 are formed
A nematic liquid crystal 806 is sandwiched between 03 and a glass substrate B805 on which a counter electrode 804 is formed, and an AC power supply 807 is connected as shown in the figure. Nematic liquid crystal,
Normally, twisted nematic (TN) mode is adopted and twist orientation is applied. In FIG. 8 (a), the layer of the alignment treatment material for obtaining the twist alignment is omitted. This is because it is not directly involved in the operation of the liquid crystal spatial light modulator.

As shown in FIG. 8 (b), the equivalent circuit has a resistor 813 of the photoconductor layer and a resistor 814 of the liquid crystal layer connected in series by an AC power source. The resistance of the photoconductor layer is shown as a variable resistance because its resistance value changes depending on the intensity of input light.

The operation principle and driving method of the liquid crystal spatial light modulator will be described below.

In FIG. 8, assuming that the photoconductor layer resistance 813 when the input light 810 is irradiated and when it is not irradiated is the light resistance R _ph and the dark resistance R _d, and the resistance 814 of the liquid crystal layer is R _LC , R _ph , R _d , R _LC
Are chosen so that the following relationships hold.

R _ph << R _LC << R _d (1) Therefore, when the input light is not irradiated, most of the applied voltage supplied from the AC power source 807 is applied to the photoconductor layer, and the electro-optical effect is applied to the liquid crystal layer. Does not occur. On the contrary, when the input light is irradiated, the resistance of the photoconductor layer decreases and most of the applied voltage is applied to the liquid crystal layer, and the liquid crystal is switched. That is, the liquid crystal can be turned on / off by turning on / off the input light, and an optical input / output system is established.

As shown in FIG. 8 (a), the input light 8810 is the glass substrate A8.
Input from 03 side, output light 812 is read light 811
Similarly to the input light, A808, the liquid crystal spatial light modulator, and the polarizing plate B809 are sequentially read from the glass substrate A side. At this time, the polarizing plates A installed on both sides of the liquid crystal spatial light modulator
The polarizing plate B and the polarizing plate B are set to parallel Nicols or cross Nicols. In the former case, the output light is bright when the input light is illuminated, and it becomes Bright True Logic (BTL). In the latter case, the dark true logic (Dark True Logi
c, DTL).

The element that holds the key in constructing an optical computer is said to be an optical bistable element. An optical bistable element is an element in an optical input / output system in which two stable states exist as output light for a certain input state.

An optical bistable state can be created using a liquid crystal spatial light modulator. FIG. 9 shows a conventional method for realizing an optical bistable device using the liquid crystal spatial light modulator. In this method, a part of the output light 901 is added to the input light 907 as the feedback light 906 using the optical system of the beam splitter A902, the mirror A903, the mirror B904, and the beam splitter B905 to realize the optical bistable state. . This method of positively feeding back the output light to the input light is also the method used when creating an optical bistable state using a semiconductor laser.
This is the most typical method for realizing an optical bistable state in a hybrid device that is formed by mutual exchange of electric energy and light energy. The optical bistable element shown in FIG. 9 can switch the state of the liquid crystal by applying the input light 907 with a pulse, and can be directly used as an optical storage element.

[Problems to be Solved by the Invention]

In the conventional liquid crystal spatial light modulator described above, nematic liquid crystal is used as the liquid crystal material. The electro-optical response time of the nematic liquid crystal is several tens to several hundreds of msec, and the response time of the liquid crystal spatial light modulator is limited by the response time of the nematic liquid crystal. That is, the response time of the conventional liquid crystal spatial light modulator is as slow as several tens to several hundreds of msec, and the above problems become a major bottleneck in performing optical information processing at higher speed using the liquid crystal spatial light modulator. There is.

Further, as described above, the conventional liquid crystal spatial light modulator realizes an optical bistable element by feeding back output light to input light. The above method using the feedback system requires two beam splitters and two mirrors for feedback, a space for installing them, and a space for securing a feedback optical path. This makes the device large and increases the manufacturing cost.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a liquid crystal spatial light modulator and a method for driving the liquid crystal spatial light modulator, which solve the above-mentioned problems of the prior art.

[Means for Solving the Problems]

According to the present invention, one or a plurality of pixel electrodes, a photoconductor layer formed so as to cover an end portion of the pixel electrode, and a photoconductor layer formed so as to cover the photoconductor layer without contacting the pixel electrode are provided. The first conductive light-shielding film, the resistance layer formed so as to cover the end portion of the pixel electrode without overlapping the end portion of the pixel electrode, and the resistance layer without contacting the pixel electrode. A liquid crystal having a structure in which a ferroelectric liquid crystal is sandwiched between a first transparent insulating substrate having a second conductive light-shielding film formed as described above and a second transparent insulating substrate having a counter electrode formed thereon. It is a spatial modulator. Further, in the method of driving the liquid crystal spatial light modulator, a first DC power source is connected between the counter electrode and the first conductive light-shielding film so that the counter electrode has a positive bias, and , A second DC power source is connected between the counter electrode and the second conductive light-shielding film so that the counter electrode has a negative bias, or the positive bias and the negative bias are negative. The first and second DC power supplies are connected to each other so that a bias is applied, and a digital optical signal as writing light is transferred from the side of the first transparent insulating substrate on which the photoconductor layer is formed to the photoconductor layer. Input,
The reading light is emitted from the side of the second transparent insulating substrate on which the counter electrode is formed. Further, another driving method is to connect the power source as described above, and when the predetermined light intensity as bias light is I _B and the light intensity of the writing light is I _W , it is outside the writing time. And the light intensity I _B outside the erase time
The light signal of the light intensity (I _B + I _W ) during the writing time is input to the photoconductor layer from the side of the first transparent insulating substrate on which the photoconductor layer is formed, and the erasing time In
A method for driving a liquid crystal spatial light modulator, characterized in that both the bias light I _B and the write light I _W are blocked, and the read light is irradiated from the side of the second transparent insulating substrate on which a counter electrode is formed. Is.

[Action]

Ferroelectric chiral smectic liquid crystals respond faster to electric fields than nematic liquid crystals. That is, the nematic liquid crystal has a response time of several tens to several hundreds msec, while the ferroelectric liquid crystal shows a high-speed response of several tens to several hundreds μsec which is two to three orders of magnitude faster than that. Therefore, if a ferroelectric liquid crystal is used as the liquid crystal material of the liquid crystal spatial light modulator, it is possible to realize a high-speed liquid crystal spatial light modulator that responds in tens to hundreds of microseconds.

However, while twisted nematic (TN) liquid crystal switches by turning on / off the electric field, ferroelectric liquid crystal switches by positive / negative inversion of the electric field, so that the structure of the conventional liquid crystal spatial light modulator has a ferroelectric property. The liquid crystal cannot be switched.

The liquid crystal spatial light modulator of the present invention has a structure in which the potential of the pixel electrode is inverted when the input light is irradiated and when the potential of the counter electrode is used as a reference, and the switching of the ferroelectric liquid crystal is input. It can be controlled by light. Therefore, a high-speed liquid crystal spatial light modulator can be realized, and optical information can be processed at a higher speed.

Ferroelectric liquid crystal has not only high-speed characteristics but also electro-optical bistability. If this effect is used, an optical bistometer that does not require a feedback system using the liquid crystal spatial light modulator is used. A stable element can be realized. As a result, the size and cost of the optical bistable device using the liquid crystal spatial light modulator can be reduced.

〔Example〕

 Examples of the present invention will be described in detail below.

Example 1 FIG. 1 is a diagram showing an example of a liquid crystal spatial light modulator of the present invention. FIG. 1A shows a sectional structure of the liquid crystal spatial light modulator. In this embodiment, the number of pixels is one. A pixel electrode 101, a photoconductor layer 102 formed so as to cover an end portion of the pixel electrode, a conductive light-shielding film A103 formed so as to cover the photoconductor layer without contacting the pixel electrode, A resistance layer 104 formed so as to cover the end portion of the pixel electrode without overlapping the end portion of the pixel electrode, and a conductive light-shielding film B105 so as to cover the resistance layer without contacting the pixel electrode are formed. Glass substrate A106 and counter electrode
Ferroelectric liquid crystal 109 between glass substrate B 108 on which 107 is formed
It has a structure sandwiching. As for the external power source, the DC power source A110 and the DC power source B111 are connected to the counter electrode so that the conductive light-shielding films A and B are negative and positive, respectively.

The cell gap is controlled to about 2.5 μm by the spacer 112, and the ferroelectric liquid crystal does not exhibit electro-optical bistability in this embodiment.

Here, a plan view and a cross-sectional view of a glass substrate A on which a photoconductor layer, which can be said to be the heart of the liquid crystal spatial light modulator of the present invention, are shown in FIGS. 2 (a) and 2 (b), respectively. In this embodiment, the photoconductor layer 102 and the resistance layer 104 are pixel electrodes.
The conductive light-shielding film A103 and the conductive light-shielding film B105 are formed in the corner portions that share one side of 101, and are formed so as to cover the photoconductor layer and the resistance layer, respectively, as shown in FIG. 2 (a). Has been done.

A cross-sectional view of the glass substrate A106 taken along the line AB is shown in FIG. As the material of the photoconductor layer, aS
i: H is adopted. The reason for adopting a-Si: H is the a-Si:
This is because the H material exhibits high-speed response as a photoconductor material and has high sensitivity in the visible light region. The high sensitivity in the visible light region means a liquid crystal spatial modulator capable of processing a visible image.

In order to make ohmic contact with the pixel electrode and the conductive light-shielding film A, the photoconductor layer has a three-layer structure of n ⁺ layer 207-i layer 208-n ⁺ layer 209, while the resistance layer 104 is n ⁺ -a-. Formed with Si: H. Further, although Cr is used for the conductive light-shielding films A and B, Al can also be applied.

FIG. 1 (b) is a diagram showing an equivalent circuit of the liquid crystal spatial light modulator of the present invention. The resistor 117 of the photoconductor layer 102 turns ON / O of the input light.
Since the resistance value changes depending on FF, it is shown as a variable resistance. Further, it is known that the liquid crystal layer is represented by the parallel connection of the resistor 119 and the capacitor 120, and is connected between the contacts A and B as shown in FIG. 1 (b).

The operation principle of the liquid crystal spatial light modulator of the present invention will be described below with reference to the equivalent circuit of FIG.

The resistance 119 of the liquid crystal layer is R _LC , the resistance 118 of the resistance layer is R _R , the resistance of the photoconductor layer when the input light is irradiated to the photoconductor layer is R _ph ,
Letting R _d be the resistance of the photoconductor layer without irradiation, it is premised that the film thickness and impurity concentration of each layer are selected so that the following relationship is established.

R _ph << R _R << R _LC << R _d … (2) Moreover, the voltage of DC power supply A110 and DC power supply B111 are equal V _B
And

When the input light is not applied, the voltage applied to the liquid crystal layer is A
When the point is taken as a reference, the potential at the point _B becomes + V _B from the equivalent circuit and the equation (2). On the contrary, when the input light is irradiated, the potential at the point _B becomes −V _{B in the} same way. Therefore, the electric field applied to the liquid crystal layer is inverted between when the input light is irradiated and when the input light is not emitted, and the ferroelectric liquid crystal which causes the inversion by switching the electric field can be driven.

Based on the above operation principle, in FIG.
The photoconductor layer 102 is irradiated with the light from the glass substrate A side, and the reading light 113 is irradiated with the polarizing plate A115, the liquid crystal spatial light modulator, and the polarizing plate B11.
If the light is transmitted in order of 6 and is used as the output light 114, the light output controlled by the light input can be obtained. At this time, the polarizing plates A and B installed on both sides of the liquid crystal spatial light modulator are set to be crossed Nicols.

Next, a method of driving the liquid crystal spatial light modulator of this embodiment will be described below with reference to FIGS.

FIG. 3 is a diagram in which the light input / output characteristics of the liquid crystal spatial light modulator of this embodiment are graphically solved. FIGS. 3 (a), (b), (c), and (d) show the cases of the above-mentioned BTL and DTL, respectively, which are the polarizing plate A and the polarizing plate set in crossed Nicols. B can be converted to each other by rotating the tilt angle of the ferroelectric liquid crystal by 45 degrees.

In FIG. 3A, the xy plane shows the dependence of the liquid crystal applied voltage on the input light intensity, and the yz plane shows the dependence of the output light intensity on the liquid crystal applied voltage. The liquid crystal applied voltage changes almost linearly from −V _B to + V _{B as the} input light intensity increases. On the other hand, in the liquid crystal spatial light modulator of this embodiment,
Since the ferroelectric liquid crystal does not show electro-optical bistability, the electro-optical characteristic of the liquid crystal is as shown in the yz plane.

When the xz plane, that is, the light input / output characteristic is obtained from the three-dimensional coordinates of FIG. 3 (a), the result of FIG. 3 (b) is obtained. In the case of DTL, the light input / output characteristics shown in FIG. 3 (d) can be obtained from the three-dimensional coordinates shown in FIG. 3 (c).

FIG. 4 is a diagram showing the light-light response characteristics of the liquid crystal spatial light modulator of this embodiment showing the light input / output characteristics. Figure 4 (a)
When the optical signal shown in Fig. 4 is input to the liquid crystal spatial light modulator of this embodiment, the optical response shown in Fig. 4 (b) in the case of BTL is as shown in Fig. 4 (c) in the case of DTL. FIG. 4 (b) shows an optical response in which the output light is inverted. At this time, the optical response time is equal to the electro-optical response time of the ferroelectric liquid crystal, 100 to 200 μ
The result is about sec, which is two to three orders of magnitude faster than that of the conventional liquid crystal spatial light modulator using the manetic liquid crystal. The fact that the BTL and DTL can be easily obtained by rotating two polarizing plates by 45 degrees is a very advantageous point when the present liquid crystal spatial light modulator is applied to an optical logic element.

[Embodiment 2] FIGS. 5 and 6 are views showing an embodiment of a method for driving a liquid crystal spatial light modulator according to the present invention. The structure of the liquid crystal spatial light modulator of this embodiment is the same as that of the first embodiment, and the number of pixels is one. The driving method is different from that of providing a ferroelectric liquid crystal with electro-optical bistability. In the present embodiment, the cell gap is controlled to be 1.5 μm by the spacer, which is thinner than that of the first embodiment, in order to provide the ferroelectric liquid crystal with electro-optical bistability.

The driving method of the liquid crystal spatial light modulator of this embodiment will be described below with reference to FIGS. 5 and 6.

FIG. 5 is a diagram showing a schematic solution method of light input / output characteristics of the liquid crystal spatial light modulator of the present embodiment. FIGS. 5 (a), (b) and (c), (d) show the cases of the above-mentioned BTL and DTL, respectively, which are the above-mentioned polarizing plates A set in the crossed Nicols. And the polarizing plate B can be mutually converted by rotating the tilt angle of the ferroelectric liquid crystal by 45 degrees.

In FIG. 5A, the xy plane shows the dependence of the liquid crystal applied voltage on the input light intensity, and the yz plane shows the dependence of the output light intensity on the liquid crystal applied voltage. As with the first embodiment, the voltage applied to the liquid crystal linearly changes from −V _B to + V _B as the input light intensity increases. On the other hand, in the liquid crystal spatial light modulator of this embodiment, since the ferroelectric liquid crystal exhibits electro-optical bistability,
The electro-optical characteristic of the liquid crystal has a hysteresis characteristic as shown in the yz plane.

When the xz plane, that is, the light input / output characteristic is obtained from the three-dimensional coordinates shown in FIG. 5A, the result shown in FIG. 5B is obtained, and the optical bistable state is realized. Also in the case of DTL, the optical input / output characteristic showing the optical bistability of FIG. 5 (d) can be obtained by using FIG. 5 (c).

FIG. 6 is a view showing the light-light response characteristics of the liquid crystal spatial light modulator of this embodiment showing the light input / output characteristics shown in FIG. In the present embodiment, the input light intensity I _B 601 corresponding to point B in FIGS. 5B and 5C outside the writing time and the erasing time.
The advance is irradiated to the optical conductor layer as a bias light, the voltage applied to the liquid crystal while the I _B is irradiated to the photoconductive layer is OV, the ferroelectric liquid crystal having the electro-optical bistability previous Holds the state. In writing time, FIG. 6 (a)
As shown in FIG. 7, a write light pulse 602 having a light intensity I _W is superposed on the bias light I _B and input to the photoconductor layer. Even after the optical pulse I _W is turned off, the output light maintains the state at the time of writing until the erase light pulse 603 is input for a period T ₁ as shown in FIG. 6B. In the erase period, to cut off a bias light I _B as shown in FIG. 6 (a) in pulses. Period T ₂ until the next write light pulse I _B + I _W is input to the output light be input to the photoconductive layer is again biased light I _B shown in Figure No. 6 (b), at the time of erasing Hold the state. On the other hand, in the case of DTL, Fig. 6 (a)
When the optical signal of is input, an optical response obtained by inverting the output light of FIG. 6 (b) is obtained as shown in FIG. 6 (c).

As described above, the liquid crystal spatial light modulator of this embodiment exhibits optical bistability in its optical input / output characteristic, and the state of output light can be switched by an optical pulse by the above-mentioned driving method. The optical pulse width of 200 μsec is sufficient for switching the ferroelectric liquid crystal, and high-speed operation can be obtained.

[Embodiment 3] FIG. 7 is a view showing an embodiment of the liquid crystal spatial light modulator of the present invention. Although the number of pixels is 1 in the first and second embodiments, this embodiment shows an example in which the number of pixels is 16 (4 × 4).

FIG. 7A shows the glass substrate A706 on which the pixel electrode 701, the photoconductor layer, the resistance layer, the conductive light-shielding film A704, and the conductive light-shielding film B705 are formed as viewed from the pixel electrode side. It is a top view. Further, FIG. 7 (b) is an enlarged view of the periphery of the pixel electrode,
The photoconductor layer 702 and the resistance layer 703 formed under the conductive light-shielding films A and B are also shown.

In FIG. 7A, the conductive light-shielding film A formed so as to cover the photoconductor layer at the upper left corner of the pixel electrode is the connection terminal A707.
The conductive light-shielding film B formed on the lower left corner of the pixel electrode so as to cover the resistance layer is connected to the connection terminal B708. Further, the connection terminal A and the connection terminal B are connected in the bias direction shown in FIG. 7A via the counter electrode and the DC power supply A709 and the DC power supply B710, respectively.

The sectional view of the glass substrate A taken along the line AB in FIG. 7B is the same as FIG. 2 illustrated in the first embodiment.

In this embodiment, each pixel is independent of the first pixel.
It is represented by the equivalent circuit shown in FIG. 6B, and therefore each pixel can be driven independently. That is, this embodiment is superior to the first and second embodiments in parallel processing capability.

〔The invention's effect〕

As described above, if the liquid crystal spatial light modulator of the present invention is applied, a response speed that is 2-3 orders of magnitude faster than that of the conventional liquid crystal spatial light modulator can be obtained, and faster optical information processing can be performed. Is possible.

Further, by utilizing the electro-optical bistable property of the ferroelectric liquid crystal using the liquid crystal spatial light modulator of the present invention, it is possible to realize an optical bistable device which does not require an optical feed back system. This does not require optical components such as beam splitters and mirrors that have been required in the conventional optical feedback system, and can further eliminate the space of the feedback optical path, thus reducing the size of the optical bistable device using a liquid crystal spatial light modulator. And cost reduction can be achieved.

[Brief description of drawings]

1 and 2 are schematic views of the liquid crystal spatial light modulators of Embodiments 1 and 2 of the present invention, and FIGS. 3 and 4 are diagrams showing the driving method of Embodiment 1 of the present invention, and FIG. FIG. 6 is a diagram showing a driving method of a second embodiment of the present invention, FIG. 7 is a schematic diagram of a liquid crystal spatial light modulator of a third embodiment of the present invention, and FIG. 8 is a conventional liquid crystal spatial light modulator. FIG. 9 is a schematic view of an optical bistable element using a conventional liquid crystal spatial light modulator. 101 ... Pixel electrode, 102 ... Photoconductive layer, 103 ... Conductive light-shielding film A, 104 ... Resistor layer, 105 ... Conductive light-shielding film B, 10
6 ... Glass substrate A, 107 ... Counter electrode, 108 ... Glass substrate B, 109 ... Ferroelectric liquid crystal, 110 ... DC power supply A, 11
1 ... DC power supply B, 112 ... Spacer, 113 ... Read light, 114 ... Output light, 115 ... Polarizing plate A, 116 ... Polarizing plate B, 117 ... Photoconductive layer resistance, 118 ... ... the resistance of the resistance layer,
119 …… Liquid crystal layer resistance, 120 …… Liquid crystal layer capacitance, 207 …… n
⁺ −a−Si: H, 208 …… ia−Si: H, 209 …… n ⁻ −a−S
i: H, 601 …… Bias light, 602 …… Write light pulse, 6
03 ... Erase light pulse, 701 ... Pixel electrode, 702 ... Photoconductor layer, 703 ... Resistive layer, 704 ... Conductive light-shielding film A, 705 ...
... Conductive light-shielding film B, 706 ... Glass substrate A, 707 ... Connection terminal A, 708 ... Connection terminal B, 709 ... DC power supply A, 710
...... DC power supply B, 801 ...... Pixel electrode, 802 ...... Photo conductor layer, 803 ...... Glass substrate A, 804 ...... Counter electrode, 805 ......
Glass substrate B, 806 ... Nematic liquid crystal, 807 ... AC power supply, 808 ... Polarizing plate A, 809 ... Polarizing plate B, 810 ... Input light, 811 ... Readout light, 812 ... Output light, 813 ... ... resistance of photoconductor layer, 814 ... resistance of liquid crystal layer, 901 ... output light, 902 ... beam splitter A, 903 ... mirror A, 90
4 …… Mirror B, 905 …… Beam splitter B, 906 ……
Feedback light, 907 ... Input light.

Claims (3)

[Claims] 1. One or a plurality of pixel electrodes, a photoconductor layer formed so as to cover an end portion of the pixel electrode, and a photoconductor layer which does not contact the pixel electrode and covers the photoconductor layer. The formed first conductive light-shielding film, the resistance layer formed so as to cover the end portion of the pixel electrode without overlapping the end portion of the pixel electrode, and the resistance layer without contacting the pixel electrode. Ferroelectric liquid crystal is sandwiched between a first transparent insulating substrate having a second conductive light-shielding film formed so as to cover it and a second transparent insulating substrate having a counter electrode formed thereon. Liquid crystal spatial light modulator characterized by. 2. The liquid crystal spatial light modulator according to claim 1, wherein a voltage is applied between the counter electrode and the first conductive light-shielding film so that the counter electrode becomes positive, Moreover, a voltage is applied between the counter electrode and the second conductive light-shielding film so that the counter electrode becomes negative, or a voltage is applied so that the positive and negative polarities are opposite to each other, and writing is performed. A digital optical signal as light is input to the photoconductor layer from the side of the first transparent insulating substrate on which the photoconductor layer is formed,
A method of driving a liquid crystal spatial light modulator, characterized in that irradiation is performed from the side of the second transparent insulating substrate on which the counter electrode is formed as readout light. 3. The liquid crystal spatial light modulator according to claim 1, wherein a voltage is applied between the counter electrode and the first conductive light-shielding film so that the counter electrode is positive, Further, a voltage is applied between the counter electrode and the second conductive light-shielding film so that the counter electrode becomes negative, or a voltage is applied so that the positive and negative polarities are reversed. , I _B is a predetermined constant light intensity as the bias light, and I _W is the write light intensity, the light with the light intensity I _B is set to the write time outside the writing time and the erasing time. In that case, an optical signal of light intensity (I _B + I _W ) is input to the photoconductor layer from the side of the first transparent insulating substrate on which the photoconductor layer is formed,
In the erasing time, the bias light I _B and the writing light I _W are both blocked, and the reading light is irradiated from the side of the second transparent insulating substrate on which the counter electrode is formed. Driving method of spatial light modulator.