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FIELD OF THE INVENTION
This invention relates to the field of optical systems and elements, and
more specifically to optically addressable spatial light modulators or
light valves incorporating liquid crystal means.
BACKGROUND OF THE INVENTION
Spatial light modulators (SLM or SLMs), as the term is used herein, are
optical masks having one or more small picture element, PEL or pixel areas
that are individually and selectively switchable by the operation of one
or more writing light beams. SLMs that have been selectively written in
this manner (i.e. data has been stored therein) are then used to modulate
a reading optical wavefront, causing the reading wavefront to be either
transmitted through the SLM (i.e. a transmission mode device), or causing
the reading wavefront to be reflected from the SLM (i.e. a reflection mode
device), the transmitted or reflected reading wavefront having a polarized
pixel portions as is determined by the data stored in the modulator's
corresponding pixel portions.
These optical masks are usually two-dimensional, and may comprise a
plurality of small pixel areas that may be arranged in a two or a three
dimensional matrix of pixel rows and pixel columns.
In an optically addressable SLM, a write beam(s), for example a visible
laser beam(s), programs or activates the individual pixels of the SLM to
subsequently rotate the polarization, change the amplitude, or retard the
phase of a read beam(s), for example an infrared laser beam(s). The write
beam(s) programs the SLM by activating individual photosensitive pixel
areas of the SLM. That is, those modulator areas that are to be programmed
to rotate the polarization, change the amplitude or retard the phase of a
read beam(s) are activated by a write beam(s).
U.S. Pat. No. 4,538,884 is an example of such an SLM. In the device of this
patent, a pair of glass plates 1a and 2a support a pair of transparent
electrodes 2a and 2b having a external source of voltage (not shown)
applied thereto. A photoconductive layer 9, which can be amorphous silicon
is supported on electrode layer 2b. A plurality of aluminum reflectors 8
are incorporated into a transparent insulating layer 7 and are supported
on the surface of the photoconductive layer, with the reflectors directly
adjacent to the photoconductive layer. An apertured shading layer 5 of
carbon or metal is carried on the transparent insulating layer, each
apertures 6 facing one of the reflectors. The space intermediate
transparent insulating layer 7 and transparent electrode 2a is occupied by
a liquid crystal 3.
The types of known liquid crystals include nematic liquid crystals,
cholestic liquid crystals, smectic liquid crystals, and chiral smectic
liquid crystals, of which electroclinic smectic A.sup.* and ferroelectric
smectic C.sup.* are two examples.
A preferred liquid crystal material useful in the practice of the present
invention, but without limitation thereto, is ferroelectric smectic
C.sup.* or H liquid crystal material described in U.S. Pat. No. 4,367,924.
A preferred voltage/current generating light sensitive layer useful in the
practice of the present invention, but without limitation thereto, is a
hydrogenated amorphous silicon (a-Si:H) photovoltaic/photodiode layer.
The use of amorphous silicon photoconductor means in a liquid crystal SLM
is suggested in the article "Amorphous silicon photoconductor in a liquid
crystal spatial light modulator", by Paul R. Ashley and Jack H. Davis,
APPLIED OPTICS, Jan. 15, 1987, Vol. 26, No.2, at pages 241-246. The device
of this article uses an external bias voltage supply.
The use of amorphous silicon photoconductor means and ferroelectric liquid
crystal means in a liquid crystal device is suggested in the article
"High-speed light valve using an amorphous silicon photosensor and
ferroelectric liquid crystals", by N. Takahashi, H. Asada, M. Miyahara and
S. Kurita, APPLIED PHYSICS LETTERS, Vol. 51, No. 16, 19 Oct. 1987. Here
again an external power supply is required.
The device of the present invention differs from prior art devices in that
it is self-powered, i.e. no externally applied electrical power is
required. Rather, the invention provides an internal
photovoltaic/photodiode light sensitive layer and a liquid crystal layer
that are sandwiched between two electrically conductive and light
transparent layers. An electrical short circuit (i.e. a low impedance
circuit) interconnects the two electrically conductive layers.
The use of a ferroelectric liquid crystal in an SLM having reflective mode
photodiode or photoconductive amorphous silicon portions is suggested in
THE PROCEEDINGS OF SPIE--THE INTERNATIONAL SOCIETY FOR OPTICAL
ENGINEERING, Vol. 754, 13-15 Jan. 13-15 1987, at pages 207-212. In the
photodiode embodiment of this article, an external voltage source is
applied to the SLM to reverse bias the photodiode. When the photodiode is
in the dark, the supply voltage is dropped across it, and the
ferroelectric liquid crystal is maintained in its off state. When the
photodiode is illuminated, the photodiode produces a current that charges
the ferroelectric liquid crystal and switches the crystal to its on state.
The use of external solar cells to power liquid crystal devices is taught
by the art. For example U.S. Pat. No. 4,475,031 describes a sun sensitive
window, and U.S. Pat. No. 4,620,322 describes a welder's eyeshield, having
liquid crystal elements wherein a solar cell that is external of the
liquid crystal element is used to activate the device. Note that in each
case, the solar cell is an external source of power, and is not an
integral part of the light modulating device. That is, the incoming light
wavefront does not both activate selected areas of the device, and at the
same time cause an operating voltage/current to be generated.
These patents thus fail to teach the present invention wherein a self
powered SLM, having no external power source is created by using a
photovoltaic film whose selectively illuminated pixel area or areas
generate the power that is required to switch only the corresponding pixel
areas of the liquid crystal.
SUMMARY OF THE INVENTION
This invention relates to a device that modulates a reading light wavefront
that is transmitted through the device, or in an alternate embodiment is
reflected by the device, in response to the device being written by a
writing light source, i.e. this invention relates to spatial light
modulators (SLMs). In optically addressable modulators of this type the
reading light may also comprise the writing light, wherein the light is
partially absorbed in, and thus activates, the photosensitive layer.
The device of this invention differs from prior art devices of this general
class in that no externally applied electrical power is required. Rather,
the entire multi-PEL area of the SLM is covered by an internally located
photovoltaic/photodiode light sensitive layer, such that selective write
activation of a pixel or PEL area within this light sensitive layer causes
a localized voltage to be generated only in these selectively activated
PEL areas, and thus causes only the corresponding PEL areas of an adjacent
liquid crystal layer to be switched.
A wide range of applications exist for the device, including use as an
image amplifier, an incoherent-to-coherent converter, an
infrared-to-visible converter, etc.. Some applications suited to the
self-powered device of the invention are self-darkening window panes,
sunglasses and safety goggles.
More specifically, a SLM in accordance with the invention includes two
transparent glass sheets that are each coated with a layer of transparent
electrically conducting material, a voltage/current generating light
sensitive layer and a liquid crystal layer are sandwiched between the two
conducting layers, and an electrical conductor means providing a short
circuit (i.e. a low impedance circuit) interconnects the two conducting
layers.
The voltage/current generating light sensitive layer is preferably a
hydrogenated amorphous silicon (a-Si:H) photodiode layer.
The liquid crystal layer is preferably a ferroelectric crystal (FLC) layer
operating in its asymmetric and not-bistable mode. The curve that plots
reading light intensity verses voltage applied to the liquid crystal
layer, for such a liquid crystal layer, produces a ferroelectric
hysterisis curve that is offset to negative voltages, rather than being
centered on the zero volts vertical axis. This asymmetry can enhange the
dynamic range of the liquid crystal's response, but it is not essential to
the present invention.
As used herein, the terms photovoltaic light sensitive layer, photodiode
light sensitive layer, voltage/current generating light sensitive layer,
and the like, is intended to mean the class of materials, such as
photocells, photodiodes and solar cells, for example, that exhibit an
actinoelectric effect or property, whereby the material generates an
electrical voltage and/or current on exposure to light of a wavelength to
which the material is sensitive.
As a feature of the invention, a reflector may be provided as a metallic
layer or as a dielectric stack to form a reflection mode SLM. The
reflector is eliminated in an alternate construction, to thereby form a
transmission mode SLM.
When the liquid crystal layer of the invention is selected to be a FLC
layer, a FLC alignment layer is formed to produce a built-in crystal bias,
that is, the crystal layer preferentially aligns itself to one crystal
orientation.
When the photovoltaic layer is illuminated (i.e. data is stored in the SLM,
or the SLM is written) by a write light illumination source, the
photovoltaic layer produces a voltage/current that rotates the crystals of
the FLC into the other orientation, thus switching the FLC from an off to
an on condition, for example In this construction, the FLC has no way to
lose its charge, and thus switch off, when the write illumination is
subsequently terminated.
As a feature of the invention, a effective resistance path shunts the
photovoltaic layer, and provides a path for the charge in the FLC layer to
leak through when write illumination is terminated. The electrical effect
of this shunt resistance path may be formed by producing a photodiode
layer so as to have imperfect current-blocking contacts.
In another embodiment of the invention, the leakage circuit path for the
charge in the FLC layer is provided by an effective resistance path that
shunts the FLC layer. This electrical effect may be accomplished by
including appropriate ionic impurities in the FLC layer.
A variation of the invention provides a device in which a small external
electrical bias is provided in the short circuit path, this bias replacing
the use of a crystal-biased FLC layer. In the absence of writing light,
this low magnitude externally applied bias source switches the FLC layer
off. The magnitude of this external bias source may be nearly as large as
the open circuit voltage of the photodiode layer (i.e. on the order of
0.25 DC volts in magnitude for a single P-I-N layer, or as high as perhaps
2.0 DC volts when multiple P-I-N layers are provided). Write light
produces a photovoltaic effect in the photovoltaic layer on the order of
0.5 DC volts and of the opposite polarity to the external source, thus
switching the FLC on. When the write light source is terminated, the
voltage across the photovoltaic layer decreases, and the voltage polarity
that is then applied to FLC layer is reversed, returning FLC layer to the
off state.
An object of the present invention is to provide a spatial light modulator
having a pair of physically spaced transparent electrically conductive
films in confronting relation to each other, a voltage/current generating
light sensitive film on one of the conductive films in confronting
relation to the other of the conductive films, a liquid crystal layer
confined between the other conductive film and the light sensitive film,
and a shorting circuit element interconnecting the two conductive films.
As a feature of the invention, a pair of transparent glass plates are
provided to support the pair of conductive films.
As a further feature of the invention, light reflector means (i.e. a
plurality of electrically conductive reflectors, or a single dielectric
layer reflector) are located between a photovoltaic/photodiode light
sensitive film and the liquid crystal layer, to thereby provide a
reflection mode spatial light modulator.
As yet a further feature of the invention, a self powered spatial light
modulator (SLM) is provided having a continuous photovoltaic/photodiode
light sensitive film that covers the entire SLM, thus allowing a writing
beam to activate only selected pixel portions of the light sensitive film,
and thus causing a voltage to be generated only in such selected pixel
portions of the light sensitive film, to thereby cause an adjacent liquid
crystal film to switch only in its corresponding pixel portions.
These and other objects and advantages of the invention will be apparent to
those of skill in the art upon reference to the following detailed
description of embodiments of the invention, which description makes
reference to the drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side section view of a portion of a SLM in accordance with the
invention, for example, a multi-pixel portion of a transmission mode SLM
matrix or array.
FIG. 2 is a top plan view of a two dimensional, X-Y coordinate system, SLM
array constructed in accordance with the invention, and showing an
exemplary physical location of the multi-pixel portion of FIG. 1 within
this array.
DETAILED DESCRIPTION OF THE INVENTION
This invention relates to a self-powered spatial light modulator that
operates to modulate a reading electromagnetic radiation or light
wavefront. In accordance with the spirit and scope of the invention, the
self-powered SLM includes a photovoltaic or photodiode voltage/current
generating radiation sensitive layer or film that covers the planar area
of the modulator. A liquid crystal layer or film also covers the planar
area of the modulator, and is placed in electrical contact with a first
surface of the radiation sensitive layer. An electrical short circuit
connects the other surface of the liquid crystal film to a second surface
of the radiation sensitive layer.
In operation, writing radiation operates to activate selected pixel or PEL
portions of the voltage/current generating layer, thus causing a voltage
to be generated in these selected portions of the layer. An electrical
current now flows through a series circuit that comprises those portions
of the liquid crystal film that positionally correspond to the selected
portions of the voltage/current generating layer and the above mentioned
short circuit connection.
As a result of this current flow, the liquid crystal film is charged or
activated in the portions thereof that positionally corresponding to the
selected portions of the voltage/current generating layer, and in this way
data is stored in the liquid crystal film.
This activation of selected portions of the liquid crystal film operates to
cause the SLM to modulate a reading wavefront, i.e. to change the state of
polarization of those portions of the reading wavefront that impinge upon
the activated portions of the liquid crystal film wherein data is stored.
As used herein, the terms photovoltaic layer, photodiode layer and
voltage/current generating layer and film will be used interchangeably.
The terms pixel and PEL will also be used interchangeably. The terms light
and electromagnetic radiation will also be used interchangeably herein.
In the FIG. 1-2 embodiment of the invention, information or data that is
optically stored in transmission mode SLM 21 by the operation of a writing
radiation beam or beams 18, and is optically readout by the use of a
reading electromagnetic radiation wavefront (not shown) that is
transmitted through the SLM. More specifically, FIG. 1 is a side section
view of a portion of a transmission mode SLM 21 in accordance with the
invention. For example, FIG. 1 shows a small multi-pixel portion of the
larger transmission mode SLM array 21 that is shown in the top plan view
of FIG. 2. For example, SLM 21 of FIG. 2 may comprise a 128.times.128 or a
1024.times.1024 pixel matrix.
The above mentioned write and read functions can be performed by the same
beam, a fraction of which beam is absorbed in the photodiode layer, with
the remainder of the beam being transmitted through the SLM.
Assume for the moment that write beam 18 operates to activate one
individual pixel area 19,20 within the multi-pixel area of FIG. 1 (i.e.
pixel 22 of FIG. 2). FIG. 2 is a top plan view of the large two
dimensional, X-Y coordinate system SLM array 21, wherein the physical
location of this individually activated pixel area 19,20 is located at the
intersection 22 of horizontal pixel row A and vertical pixel column B of
the large pixel array 21.
With reference to FIG. 1, the entire multi-pixel planar area of FIGS. 1-2
is covered by a photovoltaic light sensitive layer 14, such that selective
write activation of a pixel area 22 (shown in FIG. 2) by writing light
beam 18 causes a localized voltage/current to be generated in photovoltaic
layer 14 only at the corresponding physical location 19 (FIG. 1) within
layer 14.
FIG. 1 shows photovoltaic layer 14 as being a single P-I-N layer. However,
within the spirit and scope of this invention layer 14 may comprise
multiple P-I-N layers, in which case the voltage/current generated thereby
is correspondingly increased. For example, it may be desirable to select a
multi P-I-N layer construction in order to match the voltage/current
characteristics of photovoltaic layer 14 to that of liquid crystal layer
15.
The entire planar area of SLM 21 is also covered by a liquid crystal layer
or film 15. The localized voltage/current that is generated within
photovoltaic layer 14 now causes an electrical current to flow through
shorting circuit means 16, so as to cause only the corresponding pixel
area 20 of the adjacent liquid crystal layer 15 to be switched.
In an alternate construction, when the SLM of the invention comprises one
large, single pixel, (i.e. when spatial operation within the SLM is not
desired, as in the case of a self darkening window pane, or a self
darkening goggle) a transparent and electrically conductive layer 30 of
FIG. 1 is provided between photovoltaic layer 14 and liquid crystal layer
15. Layer 30 covers the entire planar area of SLM 21. As a result of the
current conducting operation of layer 30, the localized voltage/current
that is generated in any group of pixel portions within photovoltaic layer
14, as was indicated at 19, is transmitted to the entire planar area of
liquid crystal layer 15, thus causing the entire liquid crystal layer to
switch. This is an example of the use of a single light source to both
write the SLM and to read the SLM, due to the fact that the input light
will be attenuated (as the window pane of goggle darkens), but a quantity
of this activating light will also be transmitted through the SLM.
The SLM of FIGS. 1-2 includes two transparent glass sheets or flats 10 and
11 that are each coated with a layer of transparent electrically
conducting oxide material 12 and 13. Light sensitive layer 14 and liquid
crystal layer 15 are sandwiched between these two conducting layers 12 and
13. The surfaces of light sensitive layer 14 and conducting layer 13
against which liquid crystal layer 15 abuts may include a crystal
alignment layer (not shown), as is well known by those skilled in the art.
In accordance with the invention, an internal or an external electrical
conductor shorting means 16 provides a short circuit (i.e. a low impedance
circuit) that interconnects the two conducting layers 12,13.
Without limitation thereto, photovoltaic light sensitive layer 14 is
preferably a hydrogenated amorphous silicon (a-Si:H) photodiode layer
(shown schematically as a photodiode 40 in FIGS. 4-7), and liquid crystal
layer 15 is preferably a ferroelectric liquid crystal (FLC) layer (shown
schematically as a capacitor 41 in FIGS. 4-7) operating in its asymmetric
mode.
When the liquid crystal layer of the invention is selected to be a FLC
layer, the FLC layer is formed to have a built-in crystal bias by the use
of the above mentioned crystal alignment layers on the abutting surface of
light sensitive layer 14 and conductive layer 13. That is, the crystal
layer preferentially aligns itself to one crystal orientation. This
results in a ferroelectric hysterisis curve that is offset to negative
voltages, rather than being centered at zero volts, as above described.
When photovoltaic layer 14 is illuminated (i.e. when the SLM is written) by
write illumination source 18, the photovoltaic layer produces a current
that flows through short circuiting conductor 16 to thereby charge
corresponding portions of liquid crystal layer 15. The charging of these
liquid crystal portions rotates these portions into a different
orientation, thus switching the FLC from off to on, for example In this
construction, the switched pixels of the FLC have no way to lose their
charge, and thus switch off, when the write illumination is terminated.
Thus, nonvolatile data storage is provided.
In the self powered SLM of the invention, a continuous photovoltaic film 14
covers the entire X-Y pixel area of SLM 21, and allows a writing beam such
as 18 to activate only selected pixel areas of the photovoltaic film, thus
causing a voltage to be generated only in those selected pixel areas of
the photovoltaic film, and thereby causing the adjacent liquid crystal
film 15 to switch only in its corresponding pixel portions.
Preferably, but without limitation thereto, FLC layer 15 of FIG. 1 acts as
an addressable half wave plate. When appropriately oriented, layer 15
rotates the polarization of a transmitted (or reflected) reading light
wavefront (not shown) by 90.degree.. Liquid crystal layer 15 thus operates
to modulate the intensity of the reading light wavefront when the SLM is
placed between crossed polarizer elements (not shown), as is well known by
those of skill in the art.
In the transmission mode structure of FIGS. 1-2, no reflector is provided
for the reading wavefront. Photodiode layer 14 is sufficiently thin to
absorb at most only a fraction of the incident reading light. The writing
and reading radiation source may have the same optical characteristics, in
which case some of writing light 18 must be absorbed in photodiode layer
14 in order to activate the photodiode layer. Alternatively, the writing
and reading light sources may be distinct, in which case the reading light
source may be of a wavelength such that virtually none of the reading
light is absorbed in layer 14.
Preferably, but without limitation thereto, photovoltaic layer 14 comprises
an a-Si:H based multi-layer film, comprising a p-i-n layer of the type
shown and described relative to FIG. 3.
FIG. 3 is a side section view, much like FIG. 1, showing an embodiment of a
self powered, reflective mode SLM 121 that is constructed in accordance
with the teachings of the invention.
The embodiment of the invention shown in FIG. 3 is a reflective mode SLM
121 that operates to modulate a reading light wavefront 126, causing this
reading wavefront to be reflected as a data-containing wavefront 127. The
SLM of FIG. 3 may be configured as an X-Y pixel matrix of the general type
shown in FIG. 2. Whatever the shape of SLM 121, each individual pixel of
the SLM matrix is constructed and arranged as shown in FIG. 3, i.e. each
pixel of the matrix is provided with its own individual reflector means
125. Reflector means 125 of FIG. 3 is constructed to reflect read beam
126, and may be provided as a metallic layer, or as a dielectric stack of
a plurality of thin dielectric films. In the reflection mode structure of
FIG. 3, reflector means 125 optically isolates reading light source 126
and writing light source 118.
When reflector means 125 is formed as dielectric stack, it may comprise a
dielectric layer of alternating films having a high index of refraction,
such as titanium dioxide, and a low index of refraction, such as magnesium
fluoride, that are deposited over the individual pixel portions of
voltage/current generating layer 114. In this case, the reflector layer
need not comprise a plurality of individual reflectors 125. Rather, a
single dielectric reflective layer covers the entire area of the SLM. Such
a dielectric reflector may comprise a stack of dielectric films, each
stack being 1/4 wavelength thick relative to the frequency of reading
wavefront 126.
As with previously described embodiments of the invention, the basic
construction of SLM 121 includes a pair of transparent glass plates
100,111 on which a pair of electrically conductive and transparent oxide
films 112,113 are coated. In accordance with the present invention, one or
more internally located shorting electrical conductor bars or lead wires
116 are provided to electrically interconnect conductive films 112,113.
The radiation sensitive layer 114 of FIG. 3 is preferably configured as a
p-i-n photodiode, a-Si:H layer. While not critical to the invention, the
a-Si:H layer 114 may be deposited by plasma enhanced chemical vapor
deposition (PECVD) of silane and other gases onto a heated substrate. A
thin p-type layer, on the order of 200 Angstroms thick, is formed by
adding approximately 0.5 percent diborane to the silane. The i-type layer
contains no dopants and is typically about 3 microns thick, although it
can be thinner. The n-type layer may be formed by adding approximately 0.5
percent phosphene to the silane. The n-type layer is also about 200
Angstroms thick, or perhaps less.
The liquid crystal layer 115 of this embodiment of the invention is
preferably a FLC. Crystal alignment layers (not shown) are also provided,
as is well known.
Note that in the embodiment of FIG. 3 layers 114 and 115 are continuous
layers that cover the entire planar area of SLM 121, whereas SLM 121
includes one reflector means 125 for each of the plurality of pixel
row/column intersections of the SLM, one such row/column intersection
being shown as 22 in FIG. 2. Note that when the reflector means of the SLM
comprises a dielectric stack, as above described, an individual reflector
need not be provided for each SLM pixel.
Within the teachings of the invention, it is also possible to configure
layers 114,115 so that each individual pixel of SLM 121 is provided with
its own individual and isolated photodiode portion 114 and its own
corresponding individual isolated liquid crystal portion 115, both of
which portions 114,115 are then generally configured to correspond to the
shape of the individual reflectors 125 of each modulator pixel. In fact,
when the SLM includes a reflector function, this function is usually
provided by a continuous (i.e. a non-pixelized) dielectric stack layer.
In FIG. 3, an exemplary write beam 118 is shown as activating a small,
generally circular area of photodiode layer 114, thus causing this small
area of layer 114 to generate a voltage/current. In this embodiment of the
invention, the individual pixel reflectors 125 are electrically
conductive. Thus, the current that is generated by photodiode portion 119,
and which then flows through shorting conductor portion(s) 116, is
effective to activate the somewhat larger pixel area 120 of FLC layer 115.
In this way, data is stored in the corresponding pixel portion of the SLM,
as is defined by the shape of reflector 125.
Reflector 125 can be of any desired shape and size, for example in the
range of a less than 10 to as large as a 200 micron square area. Small
size pixel reflectors 125 provide high resolution, whereas the use of
larger size reflectors 125 provides for ease of fabrication. As an example
of another shape for reflectors 125, it may be desirable to form these
reflectors in a generally round shape.
In accordance with the teachings of commonly assigned copending U.S. patent
application Ser. No. 07/318,775, filed Mar. 2, 1989, and incorporated
herein by reference, the reflective mode SLM of FIG. 3 may be configured
to include an apertured film that is mounted on the plane of photosensor
layer 114, each aperture being spaced from a reflector 125 so as to define
an exposed ring-shaped area of photosensor layer 114 surrounding each
reflector 125. A plurality of opaque rings are then carried by conductive
film 113, so as to overlie each ring-shaped area of exposed photosensor
layer 114 that surrounds each reflector 125.
FIG. 4 is a simplified equivalent schematic circuit diagram of SLM devices
constructed in accordance with the teachings of the invention, and FIGS.
5-8 are simplified equivalent circuit diagrams of other embodiments of the
invention.
In FIG. 4, the voltage/current generating layer of FIGS. 1 and 3 is shown
schematically as a photodiode 40, the liquid crystal layer of FIGS. 1 and
3 is shown as a capacitor 41, and the electrical short circuit connection
of FIGS. 1 and 3 is shown as an electrical conductor 42.
When liquid crystal 41 is a FLC, the modulator is formed to have a built-in
crystal bias, that is, the liquid crystal preferentially aligns itself to
one crystal orientation. When photodiode 40 is illuminated by a write
light illumination source (not shown), photodiode 40 produces a current
that rotates the crystals of FLC layer 41 into the other of two
orientations, thus switching FLC 41 from off to on. In this simplified
circuit diagram, FLC 41 has no way to lose its charge, and thus switch
off, when write illumination of the SLM is terminated.
As a feature of the invention, and as is shown in the simplified equivalent
circuit of FIG. 5, a resistance path 43 may be provided to shunt
photodiode 40 (i.e. to shunt the voltage/current generating layer of FIGS.
1 and 3), to thereby provide a current path for the charge in FLC layer 41
to drain or leak off when the writing illumination source is terminated.
This shunt resistance path 43 may be formed by producing a photodiode
layer of FIGS. 1 and 3 having imperfect current-blocking contacts.
In the FIG. 6 embodiment of the invention the current leakage path for the
charge in FLC 41 is provided by a resistance path 44 that shunts FLC layer
41. Electrical circuit function 44 may be accomplished by including
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