JPH0713188A - Spatial optical modulating element and its driving method - Google Patents

Abstract

PURPOSE:To obtain a half-tone display which is high in contrast ratio and good in controllability by the spatial light modulating element used for a projection type display, etc. CONSTITUTION:The spatial light modulating element which has a ferroelectric liquid crystal layer and a photoconductive layer having rectification has one period of a driving voltage waveform 103 composed of an erasing voltage period 101 and a writing voltage period 102. Their voltage values are adjusted to make the instantaneous value of a voltage, applied to the ferroelectric liquid crystal layer right after transition from the erasing voltage period 101 to the writing voltage period 102, larger than the switching threshold voltage 106 of the ferroelectric liquid crystal layer. Further, the minimum value of the voltage applied to the ferroelectric liquid crystal layer is made smaller than the switching threshold voltage 106 of the ferroelectric liquid crystal layer in a writing voltage period 103 under write light irradiation conditions.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a projection display,
The present invention relates to a spatial light modulator used in a holographic television or an optical arithmetic device and a driving method thereof.

[0002]

2. Description of the Related Art An optical writing type spatial light modulator is a TF
It is attracting attention as the core of large-screen projection display devices that replace T liquid crystal panels and CRTs. The spatial light modulation element is an element that performs optical amplification of a two-dimensional image pattern, and has a photoconductive layer and a light modulation layer as main constituent elements.
When low-luminance image information is incident from the photoconductive layer side, the electrical characteristics of the photoconductive layer are modulated according to the intensity pattern thereof, and the optical characteristics of the light modulation layer are thereby modulated. Then, the amplified image information can be output by entering the high-brightness read light from the light modulation layer side.

As the photoconductive layer, CdS, crystalline Si, amorphous Si (a-Si: H), etc. are used. Among them, a-Si: H has a high sensitivity to writing light.
It is widely used because of its excellent properties such as low dark conductivity and flexibility in film formation. Since the written image is erased without special pulsed light for erasing, the photoconductive layer often has a rectifying property.

Electro-optical crystals or liquid crystals are used for the light modulation layer. Among them, the surface-stabilized ferroelectric liquid crystal (SSFLC; hereinafter simply referred to as ferroelectric liquid crystal) is a conventional twisted nematic (TN). Type liquid crystal has a characteristic that a high-speed response can be obtained (about 100 μsec), and thus it is receiving much attention.

It has been conventionally known that ferroelectric liquid crystals have bistability (binary characteristic). That is, the direction of spontaneous polarization of the ferroelectric liquid crystal molecules changes depending on the polarity of the applied electric field, and takes two different optical states of ON (also referred to as UP) and OFF (or DOWN). This is described, for example, in the document "Applied Physics Letters", Vol. 36 (1980), pages 899 to 901 (Appl. Phys. Lett. Vol36 (198).
0) P. 899-901).

Now, in order to actually use the ferroelectric liquid crystal in a display or the like, it is necessary to display not only ON and OFF but also an intermediate state between them. As a method of obtaining such an intermediate state, there is a method of controlling not by an electric field applied from the outside but by an applied charge amount. Ferroelectric liquid crystal has a spontaneous polarization represented by Ps, so it is necessary to inject a charge of approximately 2Ps from the outside in order to cause polarization reversal, but the amount of injected charge is 2Ps.
If control is possible within the following range, a complete transition from OFF to ON does not occur, and an intermediate state is realized. This method
For example, the document “Ferroelectrics”, Vol. 122 (1
991) pp. 89-99 (Ferroelec
trics, vol122 (1991) P.I. 89-9
9) and the document "Journal of Applied Physics" Vol. 66 (1989), pages 1132 to 1
136 pages (J. Appl. Phys. 66 (1989)
P1132-1136). However, in these reports, the domains in the ON state and the domains in the OFF state are distributed in an area and only appear macroscopically in the intermediate state. Microscopically, the ferroelectric liquid crystal has a binary characteristic. Is maintained (area gradation). That is, the ferroelectric liquid crystal is not in the third state in which it is neither ON nor OFF.

When driving a spatial light modulator composed of a ferroelectric liquid crystal and a photoconductive layer having a rectifying property (a-Si having a pin structure, etc.), the drive voltage waveform is as shown in FIG. 2B (comprising an erase pulse 201 and a write voltage period 202), or as shown in FIG. 2B (erase pulse 203, first low voltage period 204, write pulse 205, and The second low voltage period) is common. Figure 2 (a)
Is, for example, in the article “IEE Transactions on Electron Devices”, No. 3
Volume 6 (1989) pp. 2959 to 2964 (I
EEE Transactions on Electr
on Devices. vol36 (1989) P. Two
959-2964).

Since this drive waveform is completely symmetrical with respect to positive and negative signs, it has the advantage of preventing the change over time due to the decomposition of the ferroelectric liquid crystal, but on the other hand, since the write voltage period 202 is a very large negative voltage, the write operation is performed. The problem is that even if there is no light, the ferroelectric liquid crystal causes polarization inversion (electric field switching) to reduce the contrast, and the output light has a duty ratio of at most 1/2, resulting in a loss of brightness. is there. FIG. 2B shows the method proposed by us, which can make the duty ratio almost 1, but there is a problem that it is not possible to prevent the deterioration of the contrast due to the electric field switching when the write pulse 205 is applied.

FIG. 3 shows drive pulse waveforms used to solve the above problems. This is, for example, 25th in the document "S.I.D. Digest" (1991).
Pages 4 to 256 (SID Digest (199
1) P. 254-256) and US Pat. No. 5,178,4.
It is used in the specification No. 45, and consists of a high voltage and a short erase pulse 301 and a low voltage and a long write voltage period 302. In this drive pulse, the absolute value of the voltage in the write voltage period 302 is set to the erase pulse 301.
Since the voltage is smaller than the absolute value of the voltage, switching due to the electric field can be suppressed, and since the writing voltage period 302 is long, the duty ratio of the reading light can be almost 1, so that it can be applied to a projection display or the like. Is a suitable method.

In the latter document, in particular, the conditions of the drive voltage for driving with high contrast are also mentioned. However, in any case, it is ON as a ferroelectric liquid crystal.
Other than the two states of OFF and OFF, a state that cannot be stably obtained is used. In the former, halftone display is realized by using the above area gradation. In the latter case, the binary characteristic of the ferroelectric liquid crystal is used as it is and halftone display is not performed. In the above-mentioned document, pulsed light (for example, CRT light emission having a phosphor whose emission time is shorter than the length of one period for driving the spatial light modulator) is used as the writing light.

When such a spatial light modulator is used in a projection type display or a holographic television, it is necessary to have a high contrast, stable and controllable halftone display.

[0012]

However, since the conventional spatial light modulator uses a light modulating layer which can stably take only two states, area gradation is used for halftone display. , The input image has high resolution (that is, the size of one pixel becomes smaller)
Accordingly, there is a problem that the number of domains included in one pixel decreases, the number of gradations obtained decreases, and halftone display becomes difficult.

Further, when the spatial light modulator is actually driven, the charge control is not performed within the range of the applied charge amount so that the ferroelectric liquid crystal is in an intermediate state (for example, the above-mentioned US Patents). No. 5,178,445, etc.), it is difficult to perform high contrast halftone display.

The present invention solves the above-mentioned problems, and in a spatial light modulator used for a projection display or the like, a spatial light modulator having a high contrast ratio and capable of obtaining a halftone display with good controllability, and the same. An object is to provide a driving method.

[0015]

In order to achieve the above object, the spatial light modulator of the present invention is provided with a light modulation layer and a photoconductive layer between two opposing transparent electrodes, and the light modulation layer comprises:
A light modulation layer that exhibits different optical states depending on the applied charge amount, and exhibits a first optical state when the applied charge amount is equal to or greater than a first threshold charge amount, and the applied charge amount is When the second threshold charge amount or less, a second optical state is shown, and when the applied charge amount is smaller than the first threshold charge amount and larger than the second threshold charge amount,
A configuration is used in which a spatially uniform intermediate state between the first optical state and the second optical state is used in correspondence with the applied charge amount.

Further, it is desirable that the photoconductive layer has a rectifying property and generates a photocurrent having a magnitude corresponding to the intensity of incident writing light in the reverse bias state. Further, it is preferable to use the light modulation layer including a ferroelectric liquid crystal layer sandwiched by two alignment films. The specific resistance of the alignment film is 10 ⁸ Ω.
・ It is desirable to be in the range of 10 to 10 ¹¹ Ω · cm.

Further, the method of driving the spatial light modulator is as follows:
One cycle of the waveform of the drive voltage makes the photoconductive layer in a forward bias state and applies an amount of charge larger than the first threshold charge amount to the light modulation layer, and the photoconductive layer in a reverse bias state. And a writing period for generating a photocurrent having a magnitude corresponding to the intensity of the writing light. In the writing period, when the intensity of the writing light is equal to or lower than a first threshold light intensity, it is applied to the light modulation layer. When the intensity of the write light is equal to or higher than the second threshold light intensity, the amount of charge applied to the light modulation layer is equal to or higher than the second threshold charge. A step of applying the driving voltage to the two transparent electrodes so that the driving voltage is reduced to less than a predetermined amount is included.

The light modulation layer includes a ferroelectric liquid crystal layer sandwiched between two alignment films, the maximum drive voltage during the erase period is Ve, and the minimum drive voltage during the write period is Ve. Vw, the capacitance of the ferroelectric liquid crystal layer without polarization reversal of the ferroelectric liquid crystal layer is Cf, the capacitance of the photoconductive layer is Ca, the diffusion potential of the photoconductive layer is Vd, and the capacitance of the ferroelectric liquid crystal layer is If the threshold voltage is -Vth, these values are -Vth ≤ (CfVe + CaV
It is desirable that w) / (Cf + Ca) −Vd and Vw−Vd ≦ −Vth are satisfied.

The drive voltage Ve in the erase period and the drive voltage Vw in the write period are 1V≤Ve≤40V.
And it is desirable that it is in the range of −20V ≦ Vw ≦ 4V.

Further, it is desirable that the ferroelectric liquid crystal layer and the photoconductive layer are in electrical contact with each other through a metal reflection film separated into minute shapes.

[0021]

The function of obtaining a halftone display will be described. If a medium showing a uniform and continuous intermediate state is used as the light modulation layer, even if the number of domains per pixel is reduced by using a high resolution image as an input image (only one in an extreme case). It is possible to continuously realize the state between ON and OFF, and perform high-resolution halftone display. Also, if the applied voltage is controlled, it turns on and off.
Even if it is a medium that can realize only two states of FF, if it is a medium that can stably realize a uniform state between ON and OFF by the amount of electric charge applied from the outside, not by voltage, then use a charge control type external circuit configuration. It is possible to realize an intermediate state by (for example by connecting a current source instead of a voltage source). In particular, if a current source that generates a different current depending on the intensity of the writing light (a photoconductive layer having a rectifying property corresponds to this) is used, this intermediate state can be controlled by the intensity of the writing light.

Consider a case where this is actually operated with a drive voltage waveform consisting of an erase voltage period and a write voltage period. Now, the charge amounts at the time of transition from the OFF state to the intermediate state and at the time of transition from the intermediate state to the ON state are Q ₁ and Q ₂ , respectively. That is, when the applied charge amount Q is Q ≧ Q ₁ , it is in the OFF state, when Q ₁ >Q> Q ₂ , it is in the intermediate state, and Q ₂ ≧
If it is Q, it means that it is in the ON state. First, when an erasing voltage is applied, the amount of charge Q applied to the light modulation layer becomes Q ₁ or more and is forcibly returned to the OFF state. Next, in the write voltage period, the photoconductive layer generates a photocurrent according to the write light intensity. Here, if the writing light is sufficiently small, the state of Q ≧ Q ₁ is maintained within the writing voltage period, and if the writing light is sufficiently large, a photocurrent sufficient to change the charge amount Q to Q ₂ or less. Is generated, an intermediate state can be realized in a wide range from the OFF state to the ON state, and as a result, a high contrast halftone display can be performed.

[0023]

Embodiments of the present invention will be described below with reference to the drawings. An example of the spatial light modulator according to the present invention is shown in FIG. This is a transparent conductive electrode 602 (eg ITO, Zn) on a transparent substrate 601 (eg glass).
O, SnO _2, etc.) to form a photoconductive layer (or light receiving layer) 606 having a rectifying property, and a metal reflection film 607 (for example, Al, Ti, C, etc.) separated into minute shapes is formed thereon.
an alignment film 608 for forming a metal such as r or Ag or a laminate of two or more kinds of metals) and aligning the liquid crystal.
(For example, a polymer thin film such as polyimide) is formed thereon. The transparent conductive electrode 611 (eg IT) is also provided on the other substrate 612 (eg glass).
O, ZnO, SnO _2, etc.) is formed, an alignment film 610 (for example, a polymer thin film such as polyimide) is applied thereon, and finally these are bonded together with a certain gap,
Ferroelectric liquid crystal 609 is injected into the gap.

The material used for the photoconductive layer 606 is, for example,
CdS, CdTe, CdSe, ZnS, ZnSe, Ga
Compound semiconductors such as As, GaN, GaP, GaAlAs, InP, amorphous semiconductors such as Se, SeTe, AsSe, Si, Ge, Si _1-x C _x , Si _1-x Ge _x , Ge _1-x
C _x (0 <x <1) polycrystalline or amorphous semiconductor,
(1) Phthalocyanine pigment (abbreviated as Pc), for example, metal-free Pc, XPc (X = Cu, Ni, Co, TiO, Mg,
Si (OH) ₂ etc.), AlClPcCl, TiOCl
PcCl, InClPcCl, InClPc, InBr
PcBr and other (2) azo-based dyes such as monoazo dyes and disazo dyes, (3) penylene-based pigments such as penylene acid anhydrides and penenylene imides, (4) indigoid dyes, (5) quinacridone pigments, (6) anthraquinone , Polycyclic quinones such as pyrenequinones, (7) cyanine dyes, (8) xanthene dyes, (9) PVK / TNF
There are organic semiconductors such as charge transfer complexes such as (10), eutectic complexes formed from (10) pyrylium salt dye and polycarbonate resin, and (11) azurenium salt compounds.

Amorphous Si, Ge, Si _1-x C _x ,
_{Si 1-x Ge x, Ge} 1-x C x ( hereinafter, a-Si, a-G
e, a-Si _1-x C _x , a-Si _1-x Ge _x , a-Ge _1-x
Abbreviated as C _x ) for the photoconductive layer 606,
Hydrogen or halogen elements may be included, and oxygen or nitrogen may be included to reduce the dielectric constant and increase the resistivity. To control the resistivity, B and A which are p-type impurities
elements such as l and Ga, or P and A that are n-type impurities
Elements such as s and Sb may be added. By stacking the amorphous materials to which impurities are added in this manner, p / n, p / i, i
/ N, p / i / n, etc. are formed to form a photoconductive layer 606.
The dielectric constant and dark resistance or operating voltage polarity may be controlled by forming a depletion layer therein. Not only such an amorphous material, but also two or more kinds of the above materials may be stacked to form a heterojunction to form a depletion layer in the photoconductive layer 606. The thickness of the photoconductive layer 606 is 0.1 to 10 μm.
m is desirable.

An example of a specific method for producing this element will be described. First, a glass substrate 601 (40 mm × 40
(mm × 0.3 mm), an ITO thin film as a transparent conductive electrode 602 is deposited by a sputtering method. The thickness of the ITO film was 1000Å. Then, as the photoconductive layer 606, amorphous silicon (a-Si: H) having a pin structure is deposited by the plasma CVD method. P layer 60 at this time
3, the thickness of the i layer 604 and the thickness of the n layer 605 are each 1000
Å, 17,000 Å, 2000 Å, so that the total is 2 μm. The p layer 603 contains 400 ppm of B (boron) as an impurity, and the n layer 605 contains 40 p of P (phosphorus).
pm was added. The i layer 604 is not added.

Next, in order to form the metal reflection film 607, Cr was formed on the entire surface by a vacuum evaporation method. After that, it was divided into minute shapes by using photolithography. At this time, the size of the metal reflection film 607 was 20 μm □, and the width between pixels was 5 μm. The number of pixels is 10 ⁶ (1000
× 1000). A polyamic acid was applied thereon by a spin coating method and heat-cured to form a polyimide alignment film 608. The thickness of the polyimide at this time was 100Å. Alignment treatment is made of nylon cloth
8 was rubbed in one direction. Similarly, an ITO transparent conductive electrode 611 was formed on the other substrate 612 (glass), a polyimide alignment film 610 was formed, and alignment treatment was performed.

Next, beads having a diameter of 1 μm were distributed on this substrate 612 and the other substrate 601 was bonded to form a gap of 1 μm between the two substrates. Finally, the spatial light modulator 613 was completed by injecting the ferroelectric liquid crystal 609 into this gap and performing heat treatment.

Another configuration example of the spatial light modulator is shown in FIG. The basic structure of this is the spatial light modulator 613 of FIG.
Same as, but the following are different. (1) An input light-shielding film 702 made of a metal such as Cr, Al, Ti, or Ag is formed between the substrate 701 and the transparent conductive electrode 703, and the resistance of the pixel separation 715 portion is lowered by the writing light to reduce the pixel 716. It is possible to prevent a decrease in resolution due to the occurrence of crosstalk between them. (2) The entire n layer 706 and part of the i layer 705 of the photoconductive layer 707 between the adjacent metal reflection films 710 are removed by etching to form a groove. As a result, the adjacent metal reflection films 710 are not connected by the n layer having a low resistance, and are electrically separated to improve the resolution. Note that the trenching may be performed until the p layer 704 is reached. (3) At the bottom of the groove formed in (2), for example, Al, Cr, T
An output light-shielding film 708 made of a metal such as i or Ag is formed. This eliminates a switching malfunction due to the read output light sneaking into the photoconductive layer 707 side.
The read light intensity can be increased. (4) An organic output light shielding film 709 is put in the groove portion.
As a result, the degree of blocking the read light is further improved.

In FIG. 7, the ferroelectric liquid crystal 71
1, the alignment film 712, and the substrate 714 have the same configurations as in FIG.

In addition to this, for example, there is a spatial light modulator having a structure in which a dielectric reflection film is formed on the entire surface instead of the metal reflection film 607 in FIG. This is shown in FIG. This spatial light modulator 808 is composed of two substrates 801 and 8 facing each other.
A photoconductive layer 803, a dielectric reflection film 804, a ferroelectric liquid crystal layer 805, and an alignment film (not shown) are inserted between 07 (having transparent conductive electrodes 802 and 806, respectively).

Next, the driving method and the operating principle will be described by taking the spatial light modulator 717 of FIG. 7 as an example. As an example of the drive voltage waveform applied between the two transparent conductive electrodes 703 and 713, an erase pulse 301 as shown in FIG.
(Voltage Ve, time Te) and write voltage period 302 (voltage Vw, time Tw) are continuously used.

Now, when one pixel portion of the spatial light modulator 717 of FIG. 7 is taken out, as shown in FIG. 9, the ferroelectric liquid crystal layer 903 and the photoconductive layer 902 (controlled by the writing light 904) are driven. It can be regarded as being connected in series to the voltage source 901. Here, the drive voltage is Vin, the voltage applied to the ferroelectric liquid crystal layer 903 is Vf, and the photoconductive layer 902 is
The voltage applied to is Va (= Vin −Vf). Metal reflective film 7
The potential of 10 becomes Vf.

The photoconductive layer 902 has diode characteristics,
When a forward bias is applied, the resistance is low, and when a reverse bias is applied, the resistance is high and a photocurrent is generated.
For simplicity, ideal diode characteristics (forward resistance is 0,
Assuming that the reverse resistance is infinite) and that the dark current at the time of reverse bias is negligibly smaller than the photocurrent, the dark current-voltage characteristic 1001 of the photoconductive layer and the current voltage at the time of light irradiation. The characteristic 1002 is as shown in FIG. Here, Vd is the diffusion potential of the diode and can be defined as the value of Va at the intersection of the curve of the current-voltage characteristic 1002 during light irradiation and the horizontal axis. Vd is usually 0.1V-5
The value is about V. Further, Iph is a photocurrent, and the relationship with the writing light intensity L is (Equation 1). (Equation 1) Iph = (ηe / hν) L In this equation, e is the charge of the electron, hν is the energy of the photon, and η is the quantum efficiency of carrier generation by the photon.
When dealing with a transient phenomenon in the case where the photoconductive layer 902 is in the reverse bias state, the photoconductive layer 1102 includes the current source Iph and the capacitance Ca (= εaε0) of the photoconductive layer as shown in FIG.
/ Da; εa is the relative permittivity of the photoconductive layer, ε0 is the vacuum permittivity, and da is the parallel circuit of the photoconductive layer). It should be noted that the configuration of the driving voltage source 1101, the ferroelectric liquid crystal layer 1103, and the writing light 1104 in FIG.
Is the same as.

Since the ferroelectric liquid crystal layer 1103 has a very high resistance, it can be regarded as only the capacitance component. When the polarization reversal of the liquid crystal is not accompanied, the capacitance Cf (= εfε0 / df; εf is the dielectric constant of the liquid crystal without the polarization reversal, and df is the liquid crystal layer, which is determined by the geometrical shape of the ferroelectric liquid crystal layer 1103. Thickness). However, when polarization reversal is involved, the hysteresis characteristic of polarization must be taken into consideration. The ferroelectric liquid crystal generally has a hysteresis characteristic as shown in FIG. 12A between the applied voltage Vf and the polarization state P. This is because, for example, when the applied voltage shifts from positive to negative, it follows the route of A → G → B → C → D, but when it shifts from negative to positive, D → C →
Indicates that the route E → G → A is followed. Where Ps
Is the magnitude of spontaneous polarization, and Vth or −Vth is the threshold voltage for switching.

The electric charge Q accumulated in the metal reflection film 710 is represented by (Equation 2) in consideration of polarization reversal, and the transmittance T of the liquid crystal layer is expressed by the polarization state P (Equation 3). ). (Equation 2) Q = CfVf + P (Equation 3) T = (1-P / Ps) / 2 Now, when considering the route of A → B → C → D (Equation 2),
According to (Equation 3), first, in A → B, P = Ps, T
= 0, and Q = CfVf + Ps (however, Vf ≧ −Vth, Q ≧ −
CfVth + Ps). And in B → C, P = Q
+ CfVth, T = [1- (Q + CfVth) / Ps] / 2 (where Vf = -Vth, -CfVth-Ps <Q <-CfVth + Ps). Then, in C → D, P = −Ps, T = 1,
And Q = CfVf−Ps (where Vf ≦ −Vth, Q ≦ −CfVt
h-Ps). FIG. 21 shows the relationship between the charge amount Q and the transmittance T in consideration of these matters. Here, Q ₁ and Q ₂ are threshold charge amounts defined by (Equation 4). The (Equation _{4) Q 1 = -CfVth + Ps} Q 2 = -CfVth-Ps, symbol A~D in this figure corresponding to that of FIG. 12 (a). According to this, it is certainly in the OFF state (between AB; Q ₁
It can be seen that intermediate states (between BC; Q ₂ <Q <Q ₁ ) other than the <Q) and ON states (between CDs; Q <Q ₂ ) occur.
The intermediate state is considered to be a state in which the + Ps polarization state and the −Ps polarization state are distributed in an area, or the molecules of the ferroelectric liquid crystal are uniformly in the middle of polarization inversion.

The characteristic of FIG. 12A is a very ideal case, and in practice, the threshold voltage does not have a clear value as shown in FIG. 12B and FIG. The voltage value in the intermediate state (between BC) may have a range. Even in this case, if the slope dP / dVf is sufficiently larger than Cf in the intermediate state, the following analysis can be approximately applied. At this time, in the curve of A → G → B → C → D, the value of Vf at the point where the polarization reversal progressed by 10% (where T = 0.1, that is, P = (4/5) Ps) was − Vth (Vt
It is possible that h <0. It is possible that the curve itself is not point-symmetric with respect to the origin, but in that case, D → C → E → G → A is ignored, and only A → G → B → C → D is used and -Vth You should ask. In the case of FIGS. 12B and 12C, the Q-
You can draw a T curve. Both Q ₁ and Q ₂ are
It becomes as shown in (Formula 5). (Equation 5) Q ₁ = CfV _B + Ps≈−CfVth + Ps Q ₂ = CfV _C −Ps However, V _B and V _C are voltage values at points B and C.

Based on the above, the operation of the spatial light modulator will be described with reference to the drive pulse waveform diagram of FIG. 1 in the figure
01 is an erase pulse, 102 is a write voltage period, 103
Is a drive voltage (Vin), 104 is a voltage applied to the ferroelectric liquid crystal layer (Vf; at the time of an erase pulse), 105a is a voltage applied to the ferroelectric liquid crystal layer (Vf0> −Vth), and 105b is a ferroelectric liquid crystal. Voltage applied to the layer (Vf0 = −Vth), 105c is a voltage applied to the ferroelectric liquid crystal layer (Vf0 <−Vth), 106 is a threshold voltage of the ferroelectric liquid crystal layer, and 107a is a read light intensity (Vf0> −). V
th), 107b is the read light intensity (Vf0 = -Vth), 10b
Reference numeral 7c is a read light intensity (Vf0 <-Vth).

First, consider the case where the erase pulse 101 is applied (Vin = Ve). This pulse length Te and voltage value Ve
Is sufficient, the photoconductive layer 902 is in the forward direction and the ferroelectric liquid crystal layer 903 is in the OFF state (corresponding to point A in FIG. 12, P = Ps). At this time, the ferroelectric liquid crystal layer 903
The voltage applied to V becomes Vf = Ve−Vd as indicated by the broken line. Further, the applied charge, Q = Cf (Ve-Vd ) + Ps (> Q 1)
Becomes

Next, consider the moment when the drive voltage changes from Vin = Ve to Vin = Vw (time t = 0). At this moment, a current flows, and the charge accumulated in the ferroelectric liquid crystal layer 903 and the photoconductive layer 902 as a capacitance changes. Photoconductive layer 90
The capacitance of 2 is Ca, and since the ferroelectric liquid crystal layer 903 does not follow the polarization inversion due to inertia at this moment, the value of P does not change in (Equation 5) and remains P = Ps. Now
Assuming that the value of Vf at this moment is Vf0, (Equation 6) is established in consideration of the conservation of charges in the metal reflection film portion. (Equation 6) (CfVf0 + Ps) − {Cf (Ve−Vd) + Ps} = Ca {(Vw−Vf0) −Vd} However, the charge in the ferroelectric liquid crystal layer 903 is (Equation 2), and at this moment, I ignored the photocurrent. From (Equation 6), Vf
0 can be expressed as (Equation 7). (Equation 7) Vf0 = (CfVe + CaVw) / (Cf + Ca) −Vd Further, when the applied charge amount at this time is Q ₀ , (Equation 8) is obtained. (Equation _{8) Q 0 = CfVf0 + Ps} = Q 1 + Cf (Vf0 + Vth) Next, consider the change in the potential Vf in the case where the writing voltage period Tw is infinitely long. (1) Now, the case of Vf0 ≧ -Vth, i.e. when the Q ₀ ≧ Q _1, the polarization state at t = 0 is between AB of FIG. 12 (OF
F state). At t> 0, the photocurrent is increased until the point B is reached.
Vf changes depending on Iph. The change in the potential Vf during this period is
The circuit of FIG. 11 is expressed by (Equation 9) and (Equation 10). (Equation 9) Ca (d / dt) (Vw-Vf) -Iph = dQ / dt = Cf dVf / dt (Equation 10) dVf / dt = -Iph / (Cf + Ca) Solving with the initial condition of Vf0, Vf, P,
When the Q and the transmittance T are obtained, (Equation 11) is obtained. (Equation 11) Vf = Vf0 -Ipht / ( Cf + Ca) P = Ps Q = Q 0 - {Cf / (Cf + Ca)} Ipht T = 0 time t = t1 to reach point B, Vf = -Vth (i.e. Q =
When Q ₁ ), that is, (Equation 12). (Equation 12) t1 = (Cf + Ca) (Vf0 + Vth) / Iph After reaching the point B, polarization inversion is performed with Vf = −Vth (between BC in FIG. 12). The change in the polarization state P at this time is (Equation 13) using (Equation 2). (Equation 13) −Iph = dQ / dt = dP / dt Since t = t1 and P = Ps, Vf, P, Q and the transmittance T are
(Equation 14) (Formula 14) Vf = −Vth P = Ps−Iph (t−t1) Q = Q ₁ −Iph (t−t1) T = (Iph / 2Ps) (t−t1) In this period, the polarization inversion state is obtained. Is defined by the charge generated by the photocurrent. The time when the polarization inversion is completed, that is, the time t2 when the polarization state reaches the point C in FIG. 12 (Q = Q ₂ ), is when P = −Ps in (Equation 14), that is, the time shown in (Equation 15). is there. (Equation 15) In t2 = t1 + 2Ps / Iph t> t2, the polarization state is P = −Ps (constant), and thus the change in Vf is expressed as in (Equation 9) or (Equation 10). Solving under the condition of Vf = -Vth at t = t2 gives (Equation 16). (Formula 16) Vf = -Vth-Iph (t-t2) / (Cf + Ca) P = -Ps Q = Q2- {Cf / (Cf + Ca)} Iph (t-t2) T = 1 In summary, the write voltage period The changes in Vf, T, and Q at 302 are as shown in FIG.

[0041] (2) Next, Vf0 <-Vth, Sunawachi Q ₀ <Q ₁
Consider the case. At the moment of t = 0, polarization reversal does not follow due to the inertia of the liquid crystal layer, but polarization reversal occurs within a short time (several tens of microseconds) and settles into a certain polarization state between BC in FIG. And the potential Vf settles down to Vf0. If the polarization state at this time is P = P ₀ , (Equation 17) is obtained due to conservation of electric charge. (Equation _{17) (-CfVth + P 0)} - (CfVf0 + Ps) = Ca (Vw + Vth) -Ca (Vw-Vf0) In other words, the equation (18). (Equation 18) P ₀ = Ps + (Ca + Cf) (Vth + Vf0) Since this transient phenomenon occurs in an instant, t
= 0, the parallel state P = P ₀ is analyzed. The change of the polarization state P at t> 0 is BC in FIG.
It is a route between and changes according to (Equation 13). t = 0
And P = P ₀ , Vf, P, Q and T are expressed by (Equation 19). (Equation 19) Vf = -Vth P = P 0 -Ipht Q = Q 1 + P 0 -Ps-Ipht T = {1- (P 0 -Ipht) / Ps} / 2 12 reach the point C in P = -Ps, and the time t3 at this time is (Equation 20). (Equation 20) Vf at t3 = (P0 + Ps) / Iph t> t3 is expressed as in (Equation 10),
(Equation 21) (Equation 21) Vf = -Vth-Iph ( t-t3) / (Cf + Ca) P = -Ps Q = Q 2 - {Cf / (Cf + Ca)} Iph (t-t3) T = 1 In summary, the write voltage Changes in Vf and T in the period 302 are as shown in FIG.

Next, in each of the above cases (1) and (2),
An analysis will be made as to how the output light intensity Y changes with respect to the input light intensity L. In the above, Tw is assumed to be infinitely large and the progress after the erase pulse 302 is analyzed.
In the actual drive pulse waveform, the reset is applied again at t = Tw, and the same operation is repeated in each subsequent cycle. Therefore, what is observed as the output light intensity is that the write voltage period 302 (0 ≦ 0) with the transmittance T = T (t) as shown in (Equation 1).
It is a time average value in t ≦ Tw).

[0043]

[Equation 1]

Now, when the input light intensity L changes (Equation 1)
The photocurrent Iph changes according to
5), or the value of time t1, t2, or t3 in (Equation 20) changes. If Tw is fixed, then t1, t2, or t3 depending on L
The size relationship of Tw is exchanged, and the region entering the integration interval of (Equation 1) is different and the expression is different. Considering this fact, when calculating Y of (Equation 1), (1) When Vf0 ≧ −Vth, (Equation 22) is obtained, and (Equation 22) Y = 0 (L <L1) Y = (L / 2Ls) (1-L1 / L) ² (L1 ≦ L ≦ L1 + L
s) Y = 1− (2L1 + Ls) / 2L (L1 + Ls <L) (2) When Vf0 <−Vth, (Equation 23) is obtained. (Equation 23) Y = (L−2L1) / 2Ls (L <L1 + Ls) Y = 1− (L1 + Ls) ² / 2LsL (L1 + Ls ≦ L) Here, L1 and Ls are represented by (Equation 24). (Equation 24) L1 = (hν / ηe) (Cf + Ca) (Vf0 + Vth) / Tw Ls = (hν / ηe) (2Ps / Tw) As a graph, the relationship between the input light intensity L and the output light intensity Y is shown in FIG. become that way. In FIG. 15, (a)
Is Vf0> -Vth, (b) is Vf0 = -Vth,
(C) is the case of Vf0 <-Vth. As can be seen from the figure, in the case of (a), the reading light intensity does not rise until the writing light intensity reaches a certain value L1, but when L> L1, the reading light intensity increases with the writing light intensity. At> L1 + Ls, a light output that does not substantially depend on the writing light intensity can be obtained.

In the case of (b), the output light intensity and the read light intensity are approximately proportional to each other in a region where the input light intensity is small. In this case, it indicates that the input image is amplified and output with the gradation as it is.

In the case of (c), even when the input light intensity is 0, the output light intensity is a non-zero finite value, and the contrast of the output image is lowered. From the above results,
In the case of (1), gradation display is possible without deteriorating the contrast.

In each of the above cases (1) and (2), according to (Equation 16) or (Equation 21), Vf becomes infinitely small, which means that the photoconductive layer causes a photocurrent. Is a behavior within a range in which a reverse bias state is maintained. In fact, when Vf = Vw−Vd is reached, no matter how high the writing light intensity is, the photoconductive layer will no longer generate photocurrent, and Vf will not be smaller than Vw−Vd. That is, Q has a lower limit. This lower limit can be expressed by (Equation 25). (Equation 25) Q _lo = Cf (Vw-Vd) + Ps (when Vw-Vd> -Vth) Q _lo = Cf (Vw-Vd) -Ps (when Vw-Vd≤-Vth) To reach the ON state _Has Q _lo ≤Q ₂ , that is, (Equation 2
6) must be satisfied. (Equation 26) Vw−Vd ≦ −Vth From the above, it is derived that (Equation 27) is the condition for obtaining the optimum contrast. (Equation 27) −Vth ≦ (CfVe + CaVw) / (Cf + Ca) −Vd and Vw−Vd ≦ −Vth Note that the above operation principle is based on the spatial light modulation element represented by the circuit of FIG. 9, that is, FIG. This is applicable only to the case where the photoconductive layer and the ferroelectric liquid crystal layer are in contact with each other by the metal reflection film as described above. However, as shown in FIG. 8, even if it has a dielectric reflection film (or a light absorption layer, an overcoat layer, etc.), the combination of these and the liquid crystal layer is regarded as one light modulation layer, If the relationship between the applied voltage Vf and the polarization P (or the transmittance T) can be measured to obtain a curve as shown in FIG. 12, -Vth can be known.

Although the writing light is continuous light in the above description, in the case of discontinuous light such as light emitted from a phosphor of a CRT, light from a pulsed laser, or light passing through a chopper (Equation 22) ) (Equation 23) and (Equation 24)
Although the form of the equation of changes, the condition regarding the charge Q or the condition of (Equation 27) is satisfied as it is.

Here, the above discussion holds true whether it is a uniform state or an area gradation state. However, in order to obtain both sufficient resolution and gradation number, the former state is desirable. Here, the conditions for obtaining such a state will be described. Now, an equivalent circuit of one pixel of the ferroelectric liquid crystal is drawn in consideration of the resistance in the thickness direction of the alignment film, as shown in FIG. This is a plurality of series-connected capacitors C _{F for} one domain of a ferroelectric liquid crystal and a series connection of a resistance R _A in the thickness direction of an alignment film in a portion in contact therewith.

Now, assume that the initial state is the OFF state, that is, the polarization of the ferroelectric liquid crystal is uniformly oriented in the upward direction (↑). Then, it is assumed that electric charges just for reversing the polarization of one domain are injected from the outside. The state at that moment may be an area gradation state as shown in FIG. 5 (b) or a uniform state as shown in FIG. 5 (c). However, if the state shown in FIG. 5B is reached, stress acts between adjacent domains because the orientation states of adjacent domains are different, and a more stable uniform orientation state of FIG. 5C is obtained. Trying to make a transition. However, this transition is rate-limited by the movement of the applied charge (in-plane averaging). Therefore,
In order for the transition to occur quickly, the charge transfer needs to be performed quickly. For that purpose, the resistance R _A (or resistivity) of the alignment film may be made sufficiently small.

(Example 1) First, the intermediate state of the liquid crystal was examined. Two transparent electrodes / glass substrates on which an alignment film was formed were stuck together to form a gap of 1 μm, and a ferroelectric liquid crystal was injected into the gap to form a liquid crystal panel.
This is the same structure as the spatial light modulator of FIG. 6 with the photoconductive layer 606 and the metal reflection film 607 removed.
Call it a simple panel. A simple panel was constructed using various kinds of alignment films.

Now, a charged capacitor (capacity of the capacitor is made sufficiently smaller than that of the simple panel) is connected in parallel to the simple panel, and electric charges are injected into the simple panel,
The alignment state of the ferroelectric liquid crystal at that time was observed with a polarization microscope. The amount of injected electric charge was set to a value smaller than the amount of electric charge required to completely invert the ferroelectric liquid crystal. Then, in each case, the ferroelectric liquid crystal was in an intermediate state, but this intermediate state included an area gradation state and a uniform alignment state. The latter was confirmed by observing with a polarization microscope under a crossed Nicols state and rotating the stage on which the simple panel was placed so that the entire surface became uniformly dark.

Here, the uniform state is taken as an alignment film, for example, in the document “Journal of Photopolymer Science and Technology”, Vol. 3 (19).
90 years, pp. 73 to 81 (J. Photopol
ym. Sci. Technol. , Vol. 3, No.
1 (1990) P.I. 73-81), which has a low resistivity (10 ¹¹ Ω) like a series of conductive polyimides.
.Cm or less). This uniform state was stably realized for about several seconds. The reason why this time is finite is that the applied charge is attenuated by the leakage resistance of the ferroelectric liquid crystal itself. In any case, this is a time much longer than the switching time (about 100 μsec) of the ferroelectric liquid crystal and can be regarded as a stable state. Further, the ferroelectric liquid crystal in this intermediate state showed different alignment directions depending on the amount of injected charges. From this, it was found that it is possible to control the orientation direction, that is, the optical state, by the injected charge amount.

As the alignment film, for example, in addition to the above-mentioned ones, a high resistance alignment film mixed with a conductive material, a low resistance film doped with an appropriate material, or the like may be used. Good. In addition, the resistivity of the alignment film is equivalently reduced by leaving a part where the ferroelectric liquid crystal and the transparent electrode are in direct contact with each other without coating the alignment film on the transparent electrode surface.
Charge transfer between domains may occur quickly. Further, since the liquid crystal panel is normally used by irradiating it with the reading light, a photoconductive film may be used as the alignment film.

In each of the following embodiments, a spatial light modulator using the above-mentioned conductive polyimide will be mainly described.

Example 2 The spatial light modulator of FIG. 7 was actually used for driving with the drive pulse of FIG. 3, and the relationship between the writing light intensity L and the reading light intensity Y was measured. The optical system of FIG. 16 was used for the measurement. On the reading side of the spatial light modulator 1601, the read input light 1607 from the light source 1604 enters the spatial light modulator 1601 via the polarizer 1605 and the beam splitter 1606.
The read output light 1608 at that time is observed by the photodetector 1610 via the analyzer 1609.

The writing side of the spatial light modulator 1601 is illuminated with writing light 1603 from a light source 1602. In addition, the polarizer 1605 and the analyzer 1609 are orthogonal to each other, and the spatial light modulator 1601
Means that the alignment direction of the liquid crystal layer in the OFF state is the polarizer 1605.
Is parallel to the polarization direction of. Output light intensity is photodetector 1
The time average value of the light intensity observed at 610 was used.

The drive pulse of FIG. 3 is used to drive the spatial light modulator 1601, and the erase pulse 301 has a voltage Ve = + 15.
V, time Te = 100 μsec, and the write voltage period 302 was set to Tw = 1100 μsec, and the voltage value Vw was changed. The results are shown in the plot of the actual measurement values in FIG.

In this figure, the write voltage Vw is (a)-
5.40V, (B) -4.05V, (C) -2.70
V, (d) -1.35V, (e) 0V, (f) 1.35
Six cases of V are shown. It can be seen that the whole plot shifts to the right at approximately equal intervals as Vw increases. Further, for any value of Vw in the above ranges (a) to (f), the YL relational expression at that time can be analogized by curve interpolation.

Now, in each of these cases, Vf0 is measured and Vf0-(-Vth) is calculated as follows.

[0061]

[Table 1]

Here, the measurement of Vf0 will be supplemented. Since the area of each pixel of the spatial light modulator of FIG. 7 is very small (400 μm ² ), the impedance is large and it is difficult to directly insert a probe for measurement. Therefore, instead of the spatial light modulator of FIG. 7, a spatial light modulator in which an electrode corresponding to the metal reflection film is formed on the entire surface (10 cm ² ) (the entire surface of the spatial light modulator is one very large pixel. It can be measured). The probe was inserted into the electrode, and the potential at the moment when the erase pulse 301 was shifted to the write voltage period 302 was measured with an oscilloscope.

Even if Ca, Cf, and Vd are independently measured and calculated, and Vf0 is calculated by (Equation 7), it almost agrees with the value of Vf0 in (Table 1).

Regarding Vth, a simple panel (liquid crystal is injected between two substrates with transparent conductive electrodes formed) was prepared and the relationship between applied voltage and transmittance was measured. By doing so, it was confirmed that the voltage was almost 0V.

Now, comparing the value of Vf0-(-Vth) in (Table 1) with the plot of the measured value in FIG. 17, Vf0-(-V
When th) is negative (in the case of (a) and (b)), FIG.
It corresponds to (c), and when it is positive (in the case of (c) to (f)), it corresponds to FIG. This is in good agreement with the analysis described above.

One broken line in FIG. 17 is Vf0-(-V
The characteristics corresponding to th) = 0 are shown by analogy with the above plot. It can be said that a response with good contrast can be obtained within the region below the broken line. That is,
It is concluded that Vf0 ≧ −Vth is a suitable condition.

In the case of (f) in FIG. 17,
The output light intensity does not increase even if the writing light intensity increases, but this is because the output light intensity deviates from the condition of (Expression 26). Actually, when Vd = 0.7V and Vth = 0V, the condition of (Equation 26) is Vw ≦ 0.7V, and (a)
It can be seen that ~ (e) satisfies this, but (e) does not.

Although the spatial light modulator of FIG. 7 is described here, it was confirmed that the spatial light modulator of FIG. 6 is almost the same.

In the case of the spatial light modulator of FIG. 8, the switching threshold voltage becomes unclear due to the dielectric reflection film 804. By actually considering the combination of the liquid crystal layer and the dielectric reflection film as a pseudo light modulation layer, and measuring the voltage transmittance characteristics of the simple panel composed of these, the threshold voltage has a width as shown in FIG. Become. This corresponds to the polarization inversion characteristic of FIG. However, as shown in the figure, when the voltage at the position where the transmittance rises 10% with respect to the maximum value is −Vth, good linearity and contrast ratio were obtained within the range of (Equation 27).

In the experiment of FIG. 17, the value of the erasing voltage was Ve = 15V, but when Ve is larger than 40V, the ferroelectric liquid crystal is decomposed by the electric field and easily deteriorates with the passage of time. Not preferable. On the other hand, if Ve is less than 1 V, sufficient reset cannot be applied within the erase pulse application period, which is not preferable.

When the write voltage Vw becomes lower than -20V, it becomes difficult to satisfy the condition of Vf0≥-Vth regardless of any value of Ve in the range of 1V≤Ve≤40V.
On the other hand, if Vw exceeds 4 V, it becomes difficult to satisfy (Equation 26) no matter what ferroelectric liquid crystal is used.

Note that, for example, as shown in FIGS. 4A and 4B, an erase pulse period 401 and a write voltage period 40.
Although it is acceptable to use a drive pulse waveform in which the voltage value is not constant in 2 in this case, if the maximum value in the erase pulse period 401 is Ve and the minimum value in the write voltage period is Vw, (Equation 27 ) Is a preferable condition.

Example 3 A projection display device was constructed using the spatial light modulator shown in FIG. The system at this time is shown in FIG. This uses the image presented on the CRT 1901 as the writing light 1907 to the spatial light modulator 1902, and the reading side 1908 outputs the reading light 1908 from the metal halide lamp via the polarizer 1905 and the beam splitter 1904. Then, the read output light 1909 is taken out through the analyzer 1906 and the lens 1910 and is projected on the screen 1911. The screen of the spatial light modulation element 1902 is 2.5 cm square, and this is 100 c on the screen.
Expanded to m square. The drive pulse generated by the drive voltage source 1903 is Ve = 15V, Te = 10 as in the second embodiment.
0 μsec, write voltage period Tw = 1100 μ
It was fixed to sec, Vw was set to 6 kinds of values of (Table 1), and it tried driving.

In each case, the image was observed on the screen, but the halftone display of the input CRT 1901 image was reproduced with good contrast (C).
It was the case of the conditions (d) and (e). This is consistent with the results in Example 2 and corresponds to the most ideal case.
In these cases, the halftone display characteristic of the image is non-linear with respect to the writing light intensity, but it was possible to perform faithful halftone display by adjusting the γ characteristic of the written image. The brightness on the screen at this time was 1000 lx. The contrast ratio on the screen is 400:
It was 1. One pixel was enlarged to 1 mm square on the screen, but at this time, crosstalk between adjacent pixels was not observed, and a fine-grained image was obtained. (A), (b)
In the case of, the contrast was poor and the image became white as a whole. In the case of (f), only a dark image was obtained, and the brightness on the screen was 300 lx. Note that C
The same was true when writing was performed using a TFT liquid crystal display instead of RT1901.

Next, three sets of CRTs corresponding to RGB and spatial light modulators were prepared and combined to form a system, and images were synthesized on the screen. As a result, a clear color image was obtained.

Example 4 Next, a holographic television was constructed by using the spatial light modulator shown in FIG.
As the spatial light modulator, the number of pixels is 5000 × 5000,
A pixel pitch of 5 μm was used. The system at this time is shown in FIG. First, on the writing side 2011 of the spatial light modulator 2012, the light from the He—Ne laser-2001 is expanded by the beam expander 2002. Then, the light is split into two by a beam splitter 2003, one light is made a reference light 2009 via a mirror 2004, and the other light is made a subject 20 via mirrors 2005 and 2006.
07 is irradiated, and the scattered light at that time is set as object light 2010. These lights are combined again by the beam splitter 2008 to form an interference fringe on the writing side of the spatial light modulator 2012.

On the other hand, the read side 2022 of the spatial light modulator 2012 similarly expands the light from the He—Ne laser 2014 by the beam expander 2015 to make the read input light 2020, which is incident on the spatial light modulator 2012. Then, the interference fringes are read and read output light 2021
Then, an image is formed on the screen 2018 via the polarization beam splitter 2017. The observer 2019 observes the image on the screen 2018. Note that the filter 201
Reference numeral 6 is for adjusting the brightness of the image on the screen.

The drive pulse generated by the drive power supply 2013 used the five conditions of the second embodiment. As a result, a clear stereoscopic image was obtained on the screen 2006. When the subject 2008 was moved, the output image also moved in real time accordingly. (C), (d), and (e) of Example 2
Since the information of the interference fringes is reproduced on the output side with the highest contrast under the condition of (3), the obtained stereoscopic image is also the clearest and has less noise.

Similar results were obtained when the information of the interference fringes was photographed by a CCD camera, and this information was displayed on the CRT and written in the spatial light modulator.

[0080]

According to the present invention, high contrast and good controllability can be achieved and halftone display can be performed. Further, the projection type display device using the driving method of the present invention can obtain a halftone display with good contrast and faithfulness. Further, the holographic television device using the driving method of the present invention can obtain a clear stereoscopic image with less noise.

[Brief description of drawings]

FIG. 1 is a waveform diagram showing a method for driving a spatial light modulator according to an embodiment of the present invention.

FIG. 2A is a waveform diagram of an example of a conventional drive pulse waveform, and FIG. 2B is a waveform diagram of another example of a conventional drive pulse waveform.

FIG. 3 is a waveform diagram showing a drive pulse in the embodiment of the invention.

FIG. 4A is a waveform diagram of an example in a special case of the embodiment of the drive pulse waveform of the present invention. FIG. 4B is a waveform diagram of another example in a special case of the embodiment of the drive pulse waveform of the present invention.

FIG. 5A is an equivalent circuit diagram of a ferroelectric liquid crystal in which an alignment film is taken into consideration. FIG. 5B shows the same area gradation state. FIG. 5C shows the same state.

FIG. 6 is a cross-sectional view showing an example of a structure of a spatial light modulator having a metal reflection film divided into minute shapes.

FIG. 7 is a cross-sectional view showing another embodiment of the structure of the spatial light modulator having a metal reflection film.

FIG. 8 is a sectional view of yet another embodiment of the structure of the spatial light modulator having the dielectric reflection film.

FIG. 9 is a circuit diagram showing the operation of the spatial light modulator.

FIG. 10 is a current-voltage characteristic diagram of a photoconductive layer having a rectifying property.

FIG. 11 is a circuit diagram in which the photoconductive layer in FIG. 9 is replaced with an equivalent circuit.

12A is a diagram showing polarization inversion characteristics of a ferroelectric liquid crystal layer, FIG. 12B is a diagram showing another example of polarization inversion characteristics of a ferroelectric liquid crystal layer, and FIG. Diagram showing other examples of polarization inversion characteristics

FIG. 13 is a timing chart showing changes with time in voltage, transmittance, and charge of a metal reflection film portion of the spatial light modulator.

FIG. 14 is another timing chart showing changes with time in voltage, transmittance, and charge of the metal reflective film portion of the spatial light modulator.

15A is a theoretical curve diagram showing a relationship between input and output light intensities of a spatial light modulator, and FIG. 15B is a theoretical curve diagram showing another example of the same input and output light intensities. The figure of the theoretical curve which shows another example of the relation of the same input and output light intensity

FIG. 16 is a diagram showing a system for measuring input / output light intensity characteristics of a spatial light modulator.

FIG. 17 is an input / output light intensity characteristic diagram of the spatial light modulator obtained by measurement.

FIG. 18 is a diagram showing the relationship between the voltage and the transmittance of the ferroelectric liquid crystal layer when the switching threshold voltage is unclear.

FIG. 19 is a diagram showing a projection display system.

FIG. 20 is a diagram showing a holographic television system.

FIG. 21 is a characteristic diagram showing an intermediate state by controlling the applied charge amount.

[Explanation of symbols]

101 erase pulse 102 write voltage period 103 drive voltage (Vin) 104 applied voltage to ferroelectric liquid crystal layer (Vf; at erase pulse) 105a applied voltage to ferroelectric liquid crystal layer (Vf0> -Vth) 105b ferroelectric property Applied voltage to liquid crystal layer (Vf0 = -Vth) 105c Applied voltage to ferroelectric liquid crystal layer (Vf0 <-Vth) 106 Threshold voltage of ferroelectric liquid crystal layer 107a Read light intensity (Vf0> -Vth) 107b Read light Intensity (Vf0 = -Vth) 107c Readout light intensity (Vf0 <-Vth)

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yasunori Zutomi, 1006 Kadoma, Kadoma City, Osaka Prefecture, Matsushita Electric Industrial Co., Ltd. (72) Junko Asayama, 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd. 72) Inventor Kuni Ogawa, 1006 Kadoma, Kadoma, Osaka Prefecture, Matsushita Electric Industrial Co., Ltd.

Claims (8)

[Claims] 1. A light modulation layer and a photoconductive layer are provided between two opposing transparent electrodes, wherein the light modulation layer is a light modulation layer that exhibits different optical states depending on the amount of applied charges. When the applied charge amount is greater than or equal to the first threshold charge amount, the first optical state is indicated, and when the applied charge amount is less than or equal to the second threshold charge amount, the second optical state is indicated, and the applied charge amount is When the amount is smaller than the first threshold charge amount and larger than the second threshold charge amount, a difference between the first optical state and the second optical state corresponding to the applied charge amount. A spatial light modulator that exhibits a spatially uniform intermediate state. 2. The spatial light modulator according to claim 1, wherein the photoconductive layer has a rectifying property and generates a photocurrent having a magnitude corresponding to the intensity of the writing light incident upon it in the reverse bias state. . 3. The spatial light modulation element according to claim 2, wherein the light modulation layer includes a ferroelectric liquid crystal layer sandwiched by two alignment films. 4. The specific resistance of the alignment film is 10 ⁸ Ω · cm to 10
The spatial light modulator according to claim 3, which is in the range of ¹¹ Ω · cm. 5. The spatial light modulator according to claim 3, wherein the ferroelectric liquid crystal layer and the photoconductive layer are in electrical contact with each other through a metal reflection film separated into minute shapes. 6. A light modulation layer and a photoconductive layer are provided between two opposing transparent electrodes, wherein the light modulation layer exhibits different optical states depending on the applied charge amount, and the applied charge amount is A first optical state is shown when the threshold charge amount is one or more, and a second optical state is shown when the applied charge amount is a second threshold charge amount or less, and the applied charge amount is the first threshold value. When the charge amount is less than the second threshold charge amount and is greater than the second threshold charge amount, the spatial uniformity between the first optical state and the second optical state is corresponding to the applied charge amount. In the intermediate state, the photoconductive layer has a rectifying property, and when the photoconductive layer is in the reverse bias state, a photocurrent having a magnitude according to the intensity of the writing light incident on the photoconductive layer is generated. This is a method of driving a spatial light modulator in which one cycle of the waveform of the driving voltage is , An erasing period in which the photoconductive layer is in a forward bias state and a charge amount larger than the first threshold charge amount is applied to the light modulation layer, and the intensity of writing light is in a reverse bias state in the photoconductive layer. It consists of a writing period that generates a photocurrent with a magnitude according to
In the writing period, when the intensity of the writing light is equal to or lower than the first threshold light intensity, the amount of charge applied to the light modulation layer is maintained at the first threshold charge amount or more,
When the write light intensity is equal to or higher than the second threshold light intensity, the drive voltage is set to 2 so that the charge amount applied to the light modulation layer can be reduced to the second threshold charge amount or less.
A method for driving a spatial light modulation element, which comprises a step of applying to one transparent electrode. 7. The light modulation layer comprises a ferroelectric liquid crystal layer sandwiched between two alignment films, wherein the maximum drive voltage during the erase period is Ve and the minimum drive voltage during the write period is Vw, Cf of the capacity of the ferroelectric liquid crystal layer without polarization reversal of the ferroelectric liquid crystal layer, Cf of the capacity of the photoconductive layer
Ca, the diffusion potential of the photoconductive layer is Vd, and the threshold voltage of the ferroelectric liquid crystal layer is −Vth, and these values are −Vth ≦ (CfVe + CaVw) / (Cf + Ca) −Vd and Vw−
7. The method for driving a spatial light modulator according to claim 6, wherein the relationship of Vd ≦ −Vth is satisfied. 8. The drive voltage Ve in the erase period and the drive voltage Vw in the write period are 1V ≦ Ve ≦ 40V and −20.
The method for driving a spatial light modulator according to claim 7, wherein V ≦ Vw ≦ 4V.