Alternating Current Liquid Crystal Light Value

Abstract

This invention is directed to improved photoactivated liquid crystal light valves or cells which exhibit extended lifetime. All electrically conductive elements of the light valve are separated from the liquid crystal layer by insulating layers. By thus separating all electrically conductive elements from direct contact with the liquid crystal layer, the life of the liquid crystal is significantly extended. To accomplish photoactivation in the presence of electrical insulating layers, the principle of impedance matching is applied to the photoconductor/liquid crystal combination. Three novel means of impedance matching are taught: 1) photoresponsive heterojunction; 2) low impedance liquid crystal; 3) metal grid.

Description

BACKGROUND OF THE INVENTION

My invention is in the field of liquid crystal light valves. In these light valves, an electro-optical property of a liquid crystal, such a dynamic scattering is varied in accordance with a writing light and is used to modulate a projection light. Such a light valve may be used to amplify the imaging light and/or to effect wavelength conversion. Interest in this area has been intense in the last several years. Such valves have multiple uses; for example, in reflective and transmissive projection systems, and in optical data processing.

A typical light valve of the prior art is disclosed in U.S. Pat. No. 3,592,527. That valve utilizes DC current with the electrode in direct contact with the liquid crystal, which adversely effects the lifetime of the valve. It also uses a mosaic type of mirror, which limits resolution.

A somewhat different embodiment is disclosed in U.S. Pat. No. 2,892,380. This valve incorporates a dielectric structure -- a mirror -- and an alternating drive voltage to pass a current through the dielectric mirror. Although this device incorporates an insulating film -- the dielectric mirror -- over one electrode, it does not teach the value of insulating films for extending lifetime; in fact, it fails to include a second insulating film over the opposite electrode, which exclusion leads to reduced lifetime with liquid crystal electro-optic materials. In addition, it fails to include means for impedance matching the photoconductor to the electro-optic material and therefore would only be operable for those combinations of photoconductor and electro-optic material that are intrinsically impedance matched.

In order to function, a photoconductor activated liquid crystal light valve must be capable of switching the voltage that drives the device from the photoconductor to the liquid crystal layer at the command of the photoactivation signal supplied by an external writing light. Since the photoactivation signal alters the impedance of the photoconductor, the device operates by the principle of impedance match. It must be designed so that the photoactivation signal swings the impedance of the photoconductor from a value that was considerably larger than that of the liquid crystal layer when the photoconductor was dark, to a value that is considerably less than that of the liquid crystal layer in the presence of the photoactivation signal. Thus, to make photoconductor activated light valves operate, it is necessary to match the impedance of the photoconductor to that of the liquid crystal layer. In some systems this can be done directly. An example of such a system is a photoactivated liquid crystal light consisting of a thin film zinc sulfide photoconductor and a nematic liquid crystal such as MBBA and operated by direct current. In this arrangement, the high intrinsic impedance of the zinc sulfide permits direct match of the photoconductor to standard preparations of nematic liquid crystal. A system for which this is not true is one in which the zinc sulfide is replaced by cadmium sulfide, a photoconductor that is more sensitive than zinc sulfide and one that exhibits considerable sensitivity in the visible portion of the spectrum (which zinc sulfide does not, being sensitive primarily in the ultra violet). In this system the bulk impedance of the cadmium sulfide is too low to permit acceptable photoactivation of standard nematic liquid crystals. Fortunately, a photoactivated, diode-like heterojunction forms at the physical interface between the liquid crystal. This heterojunction adds sufficient impedance to the photoconductor to operate the device. Hence, in this device an artifact is required to make the basic device operate. In any photoconductor activated liquid crystal light valve in which the impedance match between the photoconductor and the liquid crystal layer is not appropriate, an impedance matching means must be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged cross-sectional view through a liquid crystal light valve of the present invention.

FIG. 2 is a schematic view of a reflective projection system utilizing a liquid crystal light valve of the present invention.

FIG. 3 is an enlarged cross-sectional view through a second liquid crystal light valve of the present invention.

FIG. 4 is a plan view of a grid which may be used in the embodiment of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

This disclosure describes such means for photoconductor activated, liquid crystal light valves that have been improved to exhibit extended lifetimes.

The direct current liquid crystal light valve of Pat. No. 3,592,527 exhibits shortened lifetime in large part because it conducts a non-zero direct current that promotes electrochemical reactions between the liquid crystal and the material that forms the electrodes of the device. To alleviate these electrochemical reactions, a direct course is to employ two dielectric insulating films, one over each electrode, to separate the liquid crystal from the electrodes. In this case an alternating voltage source is required to couple the current through the dielectric layer and into the liquid crystal. The insulating layers limit device performance by destroying the heterojunction between the liquid crystal and the photoconductor and also by presenting an extra impedance load on the photoconductor modulating element which further reduces the voltage that is available to be modulated across the liquid crystal. By my invention improved impedance match in the presence of the lifetimeextending insulating layers are provided by one of the following methods.

i. Photoresponsive Heterojunction: A heterojunction structure is formed between the photoconductor and the light blocking layer which allows the spatial photomodulation of the capacitive as well as the forward and backward resistive impedance of the photoconductor and the photo-diode heterojunction formed therewith; thus the total AC impedance is also modulated and this provides a change in voltage across the liquid crystal layer in response to the light signal.

ii. Low Impedance Liquid Crystal: By the use of liquid crystal impedances lower than that typically used in the art (10.sup.7 - 18.sup.8 .OMEGA.-cm) an improved impedance match can be obtained between the impedance found at low frequencies (<10.sup.3 Hz) of the photoconductor and the liquid crystal. A typical value for this impedance is 10.sup.5 .OMEGA./cm.sup.2 at 100 Hz whereas typical liquid crystal devices found in the state of the art have impedances > 10.sup.6 .OMEGA./cm.sup.2.

iii. The Addition of a Metal Grid: The addition of a metal grid to the device can also provide improved impedance matching by the method of controlling the AC electric field penetration into the liquid crystal layer. This is accomplished by the use of the photoconductor as a shorting plane in regions where light is incident while still maintaining high resistivity (and thus AC field penetration into the liquid crystal) where no imaging light is incident.

The invention will now be described by reference to the drawings.

FIG. 1 discloses a reflective light valve of the present invention. The drawing is greatly enlarged and not necessarily in proportion. The basic structural support of the light valve is provided by transparent cover plates 1 and 1a which are preferably made of glass. Transparent electrodes 2 and 2a are located on the inner faces of cover plates 1 and 1a, respectively. I have used tin doped indium oxide with from 100 to 1,000 .OMEGA./sq. resistivity for the electrodes 2 and 2a. Said electrodes are electrically connected to alternating current sources through leads 12 and 12a. Insulating films 3 and 3a are placed on either side of liquid crystal 13 to provide electrical and chemical isolation between liquid crystal 13 and electrodes 2 and 2a. Liquid crystal 13 being a fluid, spacers 4 and 4a are employed to maintain a suitable gap between insulating films 3 and 3a and to prevent liquid crystal 13 from escaping. Positioned on the side of liquid crystal 13 from which writing light 10 enters the cell are, respectively, dielectric mirror 5 which in my preferred embodiment is backed by a light blocking layer such as cadmium telluride film 6, and cadmium sulfide photoconductor 7. Dielectric mirror 5 is an insulating multilayer structure providing a reflective element; due to its insulating properties, use of separate insulating film 3a may be avoided. Dielectric mirror 5 may be made from alternating layers of transparent materials of high and low optical index of refraction such as MgF and ZnS. The cadmium telluride film 6 blocks any residual projection light 9 which might otherwise leak through the dielectric mirror. It also forms a so-called heterojunction with the combination of photoconductor 7 and this photo-responsive heterojunction enhances the spatial modulation of the alternating current voltage across the cell by both photoconductive and photocapacitive effects. This heterojunction provides improved impedance match to the liquid crystal.

When the light valve includes dielectric mirror 5, it is a reflective unit. If the light valve does not include dielectric mirror 5 and light blocking layer 6, it is a transmissive unit (not shown). In the latter configuration, use of insulating film 3a is essential in order to separate the liquid crystal from electrode 2a.

I have used as photoconductor 7 cadmium sulfide films ranging in thickness from 2 to 12 microns and have obtained the best results with 2 micron films. Cadmium sulfide is thermally deposited on a heated substrate and subsequently baked in an H.sub.2 S atmosphere to bring about a more nearly stoichiometric film.

Cadmium telluride (CdTe) film 6 is deposited in a vacuum onto a heated substrate that already contains the cadmium sulfide. A CdTe film 2.mu. thick is very opaque to light to which cadmium sulfide is photo-sensitive. It is also sufficiently electrically insulative to preclude image degradation due to electrical field spreading.

Dielectric mirror 5 is deposited in a vacuum, using known deposition techniques. It typically comprises fifteen alternate layers of ZnS and MgF, which results in an average reflectivity greater than 90 percent across the visible spectrum.

The insulating layers 3 and 3a, such as silicon dioxide, are also laid down by commonly used deposition techniques. The thickness ranges from 1,000 to 5,000 A.

I have found it advisable to put an anti-reflective coating on the surface of cover plate 1 facing projection light 9. This coating is typically a 1,000 A film of MgF, and it improves the projected image contrast ratio.

Liquid crystal 13 may be any of a number of nematic liquid crystals including MBBA. I prefer those liquid crystals which have very low resistivity; i.e., 10.sup.7 - 10.sup.8 .OMEGA./cm range. The film formed by liquid crystal 13 is extremely thin, e.g., 6 to 12 microns. The 6 micron films provide the best response time and resolution.

FIG. 2 schematically portrays a projection system utilizing a reflective liquid crystal light valve of the present invention. In this view, cover plates 1 and 1a and reflective coating 11 are shown, as well as electrical leads 12 and 12a. The remaining components of the light valve, such as those shown in FIG. 1, are grouped in assembly 20. Projecting light 9 emanating from light source 19 is condensed through condensing lenses 21 and 22 to reflect off mirror 23. It then passes through projecting lens 24 to form projecting light 9, which is directed against light valve assembly 20. Writing light source 26, meanwhile, is projected through writing lens 25 against the opposite side of light valve assembly 20. The system is driven with two AC voltages. AC voltage source 27 provides from 50 to 450 Hz at 20-100V RMS. This low frequency alternating current provided by source 27 causes dynamic scattering in liquid crystal 13. The second AC voltage source 28 provides from 10 to 30 KHz. This high frequency voltage improves the response time.

FIG. 3 is representative of a second liquid crystal light valve of the present invention. This embodiment illustrates the double function of dielectric mirror 5 as mirror and insulating means adjacent liquid crystal 13. It also illustrates the use of grid 31 on one face of photoconductor 7, said grid 31 being separated from transparent electrode 2a by insulating film 3a. The grid facilitates impedance matching between the photoconductor and the liquid crystal.

FIG. 4 is a plan view of grid 31, showing its relationship to photoconductor 7. Grid 31 is grounded through means 32 to lead 12a. When photoconductor 7 is off, the AC field passes through insulating film 3a and activates liquid crystal 13. By turning on photoconductor 7, the field is short circuited to grid 31, and liquid crystal 13 is turned off.

I have found that the liquid crystal light valves of the present invention are more sensitive to writing light at and below 5,150 A. This is the band edge of the cadmium sulfide film. Useful writing light power is from 50 to 500 .mu. watt/cm.sup.2 at 5,100 A for continuous operation. Projection of up to 200 lumen/cm.sup.2 with these light valves has been achieved without noticeable interference by the projecting light. In addition, response time of the light valve varies with operating levels and the composition of the liquid crystal. Faster response time is achieved by applying more high frequency voltage and more writing light.

A particularly useful liquid crystal composition for use with the AC light valves of the present invention is a mixture of p-methylbenzoic acid p'-n-butylphenyl ester, p-n-butoxybenzoic acid p'-n-butoxyphenyl ester, p-n-hexoxy-benzoic acid p'-n-butoxyphenyl ester, p-n-octoxybenzoic acid p'-n-butoxyphenyl ester in a weight ratio of 4:1:2:2. The mixture was doped with 0.5 percent hexadecyltrimethylammonium stearate. Such doped mixtures are low impedance crystals. The benzoic acid esters used in this mixture are prepared by reacting appropriately substituted benzoic acid chloride with an appropriately substituted phenol. This reaction is disclosed in more detail in co-pending application Ser. No. 290,198, filed on Sept. 18, 1972, said co-pending application having a common assignee with the instant application.

The basic photocapacitive effect noted above in the reference to the description of the heterojunction formed between the photoconductor 7 and the light blocking layer 6 is apparently caused by decreased width of the depletion region, resulting from the trapping at the heterojunction of "holes" (positive charges) that have been created by light excitation within the photoconductor. Obviously, the dark AC impedance of the junction, averaged over forward and backward polarities, when added to that of the photoconductor 7, must be great enough to keep the light crystal below its scattering threshold.

The light valves of the present invention have provided typical response times of 10 msec turn-on and 30 msec turn-off. Resolutions with 6 micron liquid crystal film of 50 lines/mm have been obtained, which equates to 1,250 lines across a typical cell aperture of 2.5 cm.

Claims

What is claimed is:

1. A liquid crystal light valve which is operable by alternating current and comprises:

a. a nematic liquid crystal layer, said layer having a first face and a second face;

b. opposite said first face, a first transparent conductive electrode which is separated from said first face by a transparent insulating means;

c. a first transparent plate adjacent said first electrode, said plate forming a first exterior of said light valve;

d. opposite said second face, a second transparent insulating means, photoresponsive means substantially matching the alternating current impedance of said liquid crystal layer, a second transparent conductive electrode, and a second transparent plate respectively, said second transparent plate forming a second exterior face of said light valve.

2. A liquid crystal light valve of claim 1 wherein said impedance matching is achieved by means of a photoresponsive heterojunction between a layer of a photoconductive semiconductor and a layer of another semiconductor having a different energy band gap.

3. A liquid crystal light valve of claim 1 wherein said impedance matching is achieved through said nematic liquid crystal layer having an impedance at 100 Hz of less than 10.sup.5 .OMEGA./cm.sup.2.

4. A liquid crystal light valve of claim 1 wherein said photoresponsive means comprises a conductive grid in ohmic contact with a layer of photoconductive material, said conductive grid being electrically connected to said first electrode and essentially equipotential therewith.

5. A liquid crystal light valve which is operable by alternating current and comprises:

a. a nematic liquid crystal layer, said layer having a first face and a second face;

b. opposite said first face, a first transparent conductive electrode which is separated from said first face by a transparent insulating means;

c. a first transparent plate adjacent said first electrode, said plate forming a first exterior of said light valve; and

d. opposite said second face, a dielectric mirror, photoresponsive means comprising a photoresponsive heterojunction between a layer of photoconductive semiconductor and a layer of another semiconductor having a different energy band gap, a second transparent conductive electrode, and a second transparent plate respectively, said second transparent plate forming a second exterior face of said light valve.

6. A light valve of claim 5 wherein said first exterior has an anti-reflective coating.

7. A light valve of claim 5 wherein said dielectric mirror is backed by means for blocking residual projecting light.

8. A liquid crystal light valve operable by alternating current for modulating projection light in response to changes in writing light, said light valve comprising in sequential order:

a. a first glass plate having an antireflective coating and forming a first exterior of said light valve;

b. a first transparent conductive electrode of tin-doped indium oxide electrically connected to a first terminal of an alternating current source;

c. a transparent insulating layer of silicon dioxide;

d. a layer of nematic liquid crystal;

e. a dielectric mirror consisting essentially of alternate layers of magnesium fluoride MgF and zinc sulfide ZnS;

f. a light-blocking layer of cadmium telluride;

g. a photoconductive layer of cadmium sulfice which forms a photoresponsive heterojunction with said cadmium telluride light-blocking layer;

h. a second transparent conductive electrode of tin-doped indium oxide electrically connected to a second terminal of said alternating current source;

i. a second transparent plate of glass for forming a second exterior face of said light valve;

wherein said insulating layer and said dielectric mirror serve respectively to block the flow of DC current to said liquid crystal, wherein said dielectric mirror serves to reflect said projection light through said liquid crystal layer and in combination with said light-blocking layer serves to block projection light from said photoconductive layer, and wherein said photoconductive layer in combination with said light-blocking layer sufficiently matches the alternating current impedance of said liquid crystal to permit said projection light to be modulated by said liquid crystal layer in response to changes in the level of said writing light impinging on said photoconductive layer and on said heterojunction.

9. A light valve of claim 8 wherein said first and second transparent conductive electrodes are electrically connected to the respective terminals of an alternating current source having a low frequency component for inducing dynamic scattering in selected portions of said liquid crystal layer and a high frequency component for varying the scattering threshold of said liquid crystal layer.

10. A liquid crystal light valve which is operable by alternating current and comprises:

a. nematic liquid crystal layer, said layer having a first face and a second face;

b. opposite said first face, a first transparent conductive electrode which is separated from said first face by a transparent insulating means;

c. a first transparent plate adjacent said first electrode, said plate forming a first exterior of said light valve;

d. opposite said second face, a dielectric mirror, a layer of photoconductive material having in ohmic contact therewith a conductive grid, a second transparent conductive electrode, and a second transparent plate respectively, said second transparent plate forming a second exterior face of said light valve; and

e. a source of alternating current having a first terminal thereof connected to said first electrode and to said conductive grid and having a second terminal thereof connected to said second electrode.

11. The light valve of claim 10 wherein said alternating current source provides a low frequency component and a high frequency component.