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Claims  |
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What is claimed is:
1. An electro-optic device comprising:
light modulating means for modulating light to have a first optical state
when an electric field of a first polarity is applied and to have a second
optical state different from said first optical state when an electric
field of a second polarity opposite said first polarity is applied, a
characteristic response time of said light modulating means for a reversal
in applied electric field polarity being substantially shorter than its
characteristic response time due to the removal of an applied electric
field,
photoresponsive means, and
electrical driving means for applying a driving voltage to said light
modulating means and photoresponsive means, said light modulating means,
photoresponsive means, and electric driving means being connected
electrically in series and said electrical driving means having a first
state for allowing said light modulating means to assume said first
optical state and a second state for allowing said light modulating means
to be switched from said first optical state to said second optical state
according to an intensity of a writing light illuminating said
photoresponsive means,
said light modulating means, photoresponsive means, and electrical driving
means being arranged so that when the state of said electrical driving
means is changed from said first state to said second state, charge
sufficient to switch said light modulating means from said first optical
state to said second optical state is prevented from accumulating on said
light modulating means unless said intensity of writing light received by
said photoresponsive means is sufficient to place said photoresponsive
means in a predetermined conductivity state,
wherein said electrical driving means applies a first voltage in said first
state and a second voltage in said second state, a magnitude of said first
voltage being substantially larger than a magnitude of said second
voltage.
2. An electro-optic device comprising:
light modulating means for modulating light to have a first optical state
when an electric field of a first polarity is applied and to have a second
optical state different from said first optical state when an electric
field of a second polarity opposite said first polarity is applied, a
characteristic response time of said light modulating means for a reversal
in applied electric field polarity being substantially shorter than its
characteristic response time due to the removal of an applied electric
field,
photoresponsive means, and
electrical driving means for applying a driving voltage to said light
modulating means and photoresponsive means, said light modulating means,
photoresponsive means, and electric driving means being connected
electrically in series and said electrical driving means having a first
state for allowing said light modulating means to assume said first
optical state and a second state for allowing said light modulating means
to be switched from said first optical state to said second optical state
according to an intensity of a writing light illuminating said
photoresponsive means,
said light modulating means, photoresponsive means, and electrical driving
means being arranged so that when the state of said electrical driving
means is changed from said first state to said second state, charge
sufficient to switch said light modulating means from said first optical
state to said second optical state is prevented from accumulating on said
light modulating means unless said intensity of writing light received by
said photoresponsive means is sufficient to place said photoresponsive
means in a predetermined conductivity state,
wherein said light modulating means is conductive and said electrical
driving means changes states more slowly than the characteristic
self-discharge of said conductive light modulating means.
3. An electro-optic device comprising:
light modulating means, comprising a layer of ferroelectric liquid crystal,
for modulating light to have a first optical state when an electric field
of a first polarity is applied and to have a second optical state
different from said first optical state when an electric field of a second
polarity opposite said first polarity is applied, a characteristic
response time of said light modulating means for a reversal in applied
electric field polarity being substantially shorter than its
characteristic response time due to the removal of an applied electric
field,
photoresponsive means, and
electrical driving means for applying a driving voltage to said light
modulating means and photoresponsive means, said light modulating means,
photoresponsive means, and electric driving means being connected
electrically in series and said electrical driving means having a first
state for allowing said light modulating means to assume said first
optical state and a second state for allowing said light modulating means
to be switched from said first optical state to said second optical state
according to an intensity of a writing light illuminating said
photoresponsive means,
said light modulating means, photoresponsive means, and electrical driving
means being arranged so that when the state of said electrical driving
means is changed from said first state to said second state, charge
sufficient to switch said light modulating means from said first optical
state to said second optical state is prevented from accumulating on said
light modulating means unless said intensity of writing light received by
said photoresponsive means is sufficient to place said photoresponsive
means in a predetermined conductivity state, wherein said ferroelectric
liquid crystal has a high spontaneous polarization, greater than or equal
to 33.6 nC/cm.sup.2.
4. An electro-optic device comprising:
light modulating means for modulating light to have a first optical state
when an electric field of a first polarity is applied and to have a second
optical state different from said first optical state when an electric
field of a second polarity opposite said first polarity is applied,
photoresponsive means, and
electrical driving means for applying a driving voltage to said light
modulating means and photoresponsive means, said light modulating means,
photoresponsive means, and electric driving means being connected
electrically in series and said electrical driving means applying a first
voltage V1 for allowing said light modulating means to assume said first
optical state and a second voltage V2 for allowing said light modulating
means to be switched from said first optical state to said second optical
state according to an intensity of a writing light illuminating said
photoresponsive means,
said light modulating means, photoresponsive means, and electrical driving
means being arranged so that when the state of said electrical driving
means is changed from said first state to said second state charge
sufficient to switch said light modulating means from said first optical
state to said second optical state is prevented from accumulating on said
light modulating means, unless said intensity of writing light received by
said photoresponsive means is sufficient to place said photoresponsive
means in a predetermined conductivity state,
wherein capacitances of said photoresponsive means and of said light
modulating means are such that
##EQU4##
wherein C.sub.D is a capacitance per unit area of said photoresponsive
means, and C.sub.FLC is a capacitance per unit area of said light
modulating means.
5. An electro-optic device as in claim 4 wherein a magnitude of said first
voltage is substantially larger than a magnitude of said second voltage.
6. A method of forming and using an electro-optic device comprising the
steps of:
forming a light modulating means for modulating light to have a first
optical state when an electric field of a first polarity is applied and to
have a second optical state different from said first optical state when
an electric field of a second polarity opposite said first polarity is
applied, said light modulating means having a capacitance per unit area
C.sub.D ;
forming a photoresponsive means, having a capacitance per unit area
C.sub.FLC ;
connecting said light modulating means and photoresponsive means
electrically in series to form a series circuit and applying a first
voltage V1 to said series circuit for allowing said light modulating means
to assume said first optical state and a second voltage V2 to said series
circuit for allowing said light modulating means to be switched from said
first optical state to said second optical state according to the
intensity of a writing light illuminating said photoresponsive means;
arranging said light modulating means, photoresponsive means, and
electrical driving means so that when the state of said electrical driving
means is changed from said first state to said second state charge
sufficient to switch said light modulating means from said first optical
state to said second optical state is prevented from accumulating on said
light modulating means unless said intensity of writing light received by
said photoresponsive means is sufficient to place said photoresponsive
means in a predetermined conductivity state; and
forming said capacitances of said photoresponsive means and of said light
modulating means such that
##EQU5## |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
(a) Field of the Invention
This invention relates to optically addressed spatial light modulators,
which are electro-optical devices whereby a writing light controls the
modulation of a reading light according to the image pattern existing in
the writing light beam. Such devices have many uses such as, for example,
allowing an image illuminated with light of weak intensity to be projected
with light of strong intensity, converting an image in light of one
wavelength to an image in light of another wavelength, or converting an
image in incoherent light to an image in coherent light.
(b) Description of the Prior Art
Examples of electro-optic devices which allow the modulation of light to be
controlled by light, and which specifically make use of spatial variation
or the image content of the controlling light, are generally known in the
prior art. The type most similar to that of the present invention is
frequently called a liquid crystal light valve. These devices comprise a
liquid crystal light modulating layer in contact with a photoresponsive
layer, typically a photoconductor Changes in the light illuminating the
photoresponsive layer (write light) cause corresponding changes in the
electric fields applied to the liquid crystal layer, further causing
corresponding changes in light passing through the liquid crystal (read
light).
Typical liquid crystal light valves, such as the one disclosed in U.S. Pat.
No. 3,824,002, use nematic liquid crystals as the light modulator. Nematic
liquid crystals have several advantages for use in these devices in that
they exhibit large modulations for relatively small changes in applied
voltage and their operation consumes very little electrical power.
However, nematic liquid crystals respond only to changes in the magnitude
but not in the polarity of applied electric fields. Therefore, although
they can be switched in one direction increasingly fast by increasingly
large applied fields, switching them in the opposite direction can only be
accomplished by removing the electric field. The switching then proceeds
under the drive of comparatively small surface elastic forces. The
combination of the small surface elastic forces with the nematic liquid
crystal's viscosity limits the response speed of all such devices to the
millisecond range, as is well known in the art. Also, nematic liquid
crystal layers used are typically 5-20 .mu.m thick, which limits the
spatial resolution as well, for it is not possible to switch regions of
the liquid crystal that are much smaller in extent than the thickness of
the film.
The liquid crystal light valves of the prior art also typically use
photo-conductive materials such as the CdS layer taught by U.S. Pat. No.
3,824,002 for their photoresponsive layers. While such photoconductors are
usually very sensitive, producing changes in the charge of many electrons
for every absorbed photon, they are also usually slow, taking several
milliseconds to respond to changes in their illumination level.
Furthermore, many of the easiest to use photoconductive materials exhibit
significant responses only to light of relatively short wavelengths as
found in the blue end of the visible spectrum.
Alternately, other disclosures in the prior art such as that by Efron et
al., "The silicon liquid-crystal light valve," Journal of Applied Physics,
volume 57, p. 1356 (1985), that by Armitage et al., "Gallium arsenide
photoaddressed liquid-crystal spatial light modulator," Advances in
Optical Information Processing III, Dennis R. Pape, editor, Proceedings
SPIE, volume 936, pp. 56-67 (1988) and those of U.S. Pat. Nos. 4,191,452,
4,239,348, 4,619,501, and 4,655,554 teach the use of crystalline silicon
or gallium arsenide as the photoresponsive layer. These materials have
favorable response times and are sensitive to longer wavelengths, but they
have their own disadvantages. For example, they must be fabricated in
single crystal form, which is difficult and expensive. Also, since a wafer
of these semiconductors is obtained by sawing from a large single crystal,
it is not possible to obtain thin, optically flat layers as desired for
the liquid crystal light valve. Moreover, the only way to obtain
reasonable spatial resolution with such a thick layer is to add further
patterns of doping and metal deposition. Such additional steps are
difficult and produce a light valve whose resolution is limited by the
resolution of the patterning steps.
In the operation of any liquid crystal light valve, care must be taken that
the reading or projection light does not also write the device. Devices of
the prior art have typically used one of two methods to accomplish this
isolation. The first method involves placing a dielectric mirror of high
reflectivity at the interface between the photoresponsive layer and the
liquid crystal, as taught in U.S. Pat. No. 3,824,002. By reflecting or
blocking substantially all of the light incident from the liquid crystal
side of the device from reaching the photoresponsive layer, the reading
and writing functions can be separated. However, such a dielectric mirror,
in order to be efficient, must be at least as thick as several wavelengths
of the light it is to reflect, and thus it comprises many layers. Also, no
matter how fine a pattern of electric charge can be generated at the
surface of the photoresponsive layer, the electric fields resulting across
the liquid crystal layer can change substantially over distances
comparable to the thickness of the mirror layer, thereby limiting the
resolution of the device. The second method involves placing a metal
mirror at the same interface. However, since metals conduct electricity,
the mirror must be patterned if the device is to be able to respond to
images. Methods of fabricating such a device are taught in U.S. Pat. No.
4,538,884. Again, the maximum resolution achievable is equal to the
resolution with which the metal mirror can be patterned.
Most of the devices of the prior art are constituted such that continuing
modulation of the read light requires continuing illumination by the
writing light. Such a requirement prevents the desirable function of
integration, whereby further increments of exposure to the write light
produce more or less irreversible increments in the modulation of the read
light. Photographic film provides such an integration function whose
advantages are readily apparent to those skilled in the art. However, film
requires the separate and time consuming extra step of development.
Both fast liquid crystal light modulators and fast photoresponsive layers
are separately known in the prior art. For example, U.S. Pat. Nos.
4,367,924 and 4,563,059 teach the use of chiral tilted smectic liquid
crystals which are ferroelectric. These ferroelectric liquid crystals
(hereafter "FLCs") have the property, unlike nematics, of being sensitive
to the sign or polarity of an applied electric field. This allows them to
have two states, into either of which they can be driven by externally
applied electric fields, thereby obviating the need to rely on the much
weaker internal surface elastic forces. FLCs can be switched in
microseconds, which is quite fast compared to the nematics' millisecond
switching time. However, FLCs cannot simply be substituted for nematics in
an optically addressed spatial light modulator. As pointed out by Armitage
et al in "Photoaddressed ferroelectric liquid crystal devices," Optical
Society of America Annual Meeting, 1988, Technical Digest Series, Volume
11 (Optical Society of America, Washington, D.C., 1988), p. 118,
photoaddressing structures that are effective in addressing nematic liquid
crystal cells often prove ineffective for addressing ferroelectric liquid
crystal cells.
Photoresponsive layers of hydrogenated amorphous silicon (hereafter a-Si:H)
have been investigated extensively over the past two decades, with much
work directed toward their use in photovoltaic solar cells. As is known in
the art, a-Si:H photoresponsive layers are used in vidicons and photocopy
drums. The material may be used either with ohmic contacts, as a
photoconductor, or with rectifying layers, as a photodiode, as in the
solar cells. The photodiodes are known to have microsecond response times
to changes in illumination. Use of a-Si:H combined with nematic liquid
crystals in optically addressed spatial light modulators is taught in U.S.
Pat. No. 4,538,884 to Masaki and U.S. Pat .No. 4,693,561 to Davis and in
Ashley et al, "Amorphous silicon photoconductor in a liquid crystal
spatial light modulator," Applied Optics, vol. 26, pp. 241-246 (1987) and
"Liquid crystal spatial light modulator with a transmissive amorphous
silicon photoconductor," Applied Optics, vol. 27, pp. 1797-1802 (1988).
Finally, Moddel et al. disclose in "Photoaddressing of High Speed Liquid
Crystal Spatial Light Modulators," Optical and Digital Pattern
Recognition, Hua-Kuang Liu and Paul Schenker, editors, Proceedings SPIE,
vol. 754, pp. 207-213 (1986); "Design and Performance of High-speed
Optically-addressed Spatial Light Modulators," Advances in Optical
Information Processing III, Dennis R. Pape, editor, Proceedings SPIE, vol.
936, pp. 48-55 (1988); and "Optical Addressing of High-speed Spatial Light
Modulators with Hydrogenated Amorphous Silicon," Materials Research
Society Proceedings, vol. 118, pp. 405-410 (1988) and Williams et al.
disclose in "An Amorphous Silicon/Chiral Smectic Spatial Light Modulator,"
Journal of Physics D: Applied Physics, vol. 21, pp. 156-159 (1988), the
use of liquid crystal light valves having photoresponsive layers made of
a-Si:H and having light modulating layers made of FLCs. Such devices
appear to have the potential for overcoming many of the short-comings of
the prior art devices recited above. Namely, both the photoresponsive
layer and the light modulating layer are thin so as to support a high
spatial resolution and fast switching, thereby allowing response times in
the microsecond range.
However, those skilled in the art will appreciate that the devices
disclosed in the above publications have their own difficulties.
Specifically, the a-Si:H layer has a large capacitance, which makes it
difficult to take such a device from an erased state to a light sensitive
state. The necessary changes in the applied voltages feed through the
layer's capacitance and write the liquid crystal layer even in the absence
of writing illumination. Moddel et al teach that this problem may be
solved by reducing the amplitude of the driving voltage waveform.
Unfortunately, this also increases the optically addressed spatial light
modulator's response time since the FLC responds more slowly to a smaller
applied voltage.
SUMMARY OF THE INVENTION
The principal object of the present invention is to provide an optically
addressed spatial light modulator with high spatial resolution and a fast
response.
The desire for high resolution in a device that is easy to fabricate
requires that the photoresponsive and light modulating layers be thin. The
desire for high speed dictates that both layers also have short response
times. These requirements are met in the present invention by
photoresponsive layers of amorphous silicon (a-Si:H) and light modulating
layers of ferroelectric liquid crystals (FLCs). Further, in the present
invention the semiconducting a-Si:H is made into rectifying diodes, which
gives rise to advantageous embodiments when the a-Si:H layer is coupled to
a light modulator whose action depends on the sign of the voltage applied
to it, as is the case for FLCs. The a-Si:H can also be deposited on a
variety of substrates such as optically flat windows.
Amorphous silicon is responsive to visible light but is transparent and
insensitive to near infrared light. This allows the isolation of the read
and write light in devices of the present invention by having the read
light at a comparatively long wavelength in the near infrared such as is
readily obtained from a semiconductor LED or laser diode and by having the
write light at a comparatively short wavelength in the visible spectrum,
which may be obtained from any of the multitude of sources known in the
art. These devices can then be read in reflection in the usual way, or,
since the photoresponsive layer is transparent, in transmission.
The devices of the present invention are divided into types according to
whether or not the photoresponsive layer is rectifying. Devices made with
ohmic or nonrectifying layers are generally more sensitive to write light,
but somewhat slower. Devices with rectifying layers are generally faster.
Rectifying photoresponsive layers also allow devices whose total
modulation of the read light is determined by the total integrated
exposure of the device to write light.
The differences between the way FLCs of the present invention and prior art
light modulators respond to electrical driving signals causes devices in
accordance with the present invention to differ significantly from light
valves in the prior art. Within certain limitations, the FLC light
modulating layer may be characterized as having a switching threshold
defined by the integrated product of the applied voltage multiplied by
elapsed time. Thus, it switches rapidly in response to high voltages, and
more slowly in response to low voltages. Further, once the FLC has
switched, the return of the applied voltage to zero does not induce the
FLC to switch back to its original slate, i.e., the FLC has memory. In the
case of the surface-stabilized FLC (SSFLC), as taught by Clark and
Lagerwall in U.S. Pat. Nos. 4,367,924 and 4,563,059, the memory is
indefinitely long. Even without surface stabilization, the forces tending
to drive the FLC to any given state are usually small compared to the
force produced by the applied voltage; therefore, the FLC's relaxation
away from the voltage-selected state is slow compared to its switching
speed. Thus, under any condition where the FLC has a fast switching time,
it has a long memory time compared to the switching time.
By contrast, nematics have a definite threshold in r.m.s. applied voltage
below which little or no optical response is produced even if the voltage
is applied for a long time. Their response depends only on the magnitude
and is thus independent of the sign of the applied voltage. Thus,
practical devices are configured so that with zero voltage applied they
spontaneously turn off, and the spontaneous turn-off time is usually about
equal to the turn-on time. Other light modulators including solid-state
electrooptic materials have very fast turn-on times and equally fast
turn-off times.
However, FLC has the disadvantage that it is much more sensitive to
spurious or undesired electrical signals than other light modulators. With
other light modulators, if some small amplitude or short duration spurious
signal should produce some unwanted light modulation, the magnitude of the
unwanted light modulation will typically be small, since the response will
either be small or of short duration. However, with FLCs, their fast
response time coupled with their long memory time and their low switching
threshold for signals of long duration can result in even small or short
spurious signals producing a large unwanted optical modulation.
Therefore, the main obstacle in making optically addressed spatial light
modulators from thin layers in accordance with the present invention is
that the large capacitance of the layers tends to result in spurious
signals being applied to the light modulator whenever the applied voltage
is changed. For light modulators that respond to the sign of the applied
voltage, changes are necessary to allow light modulation. The present
invention solves these problems by preventing excessive spurious signals
from being applied to the light modulating layer. This is accomplished by
judicious combination of thin light modulating and thick photoresponsive
layers, use of asymmetric driving waveforms, use of slow driving waveforms
combined with conductive light modulators, and use of suitable materials
constants, such as high polarization or high dielectric constant FLCs.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 represents a generalized structure of the optically addressed
spatial light modulator of the present invention.
FIG. 2 shows the structure of the FLC light modulating layer with a gap
between the surfaces of the photoresponsive layer and the transparent
conductively coated substrate filled with FLC having smectic layers.
FIG. 3 represents the structure of an embodiment of an optically-addressed
spatial light modulator with a rectifying photosensitive layer.
FIG. 4 is an equivalent circuit of the rectifying spatial light modulator
of FIG. 3.
FIG. 5 including parts (a), (b), (c), and (d), shows voltage versus time
waveform diagrams of the voltage provided by the electrical driver to the
rectifying spatial light modulator as well as the resulting voltage across
the FLC light modulator and the transmitted read light intensity for the
shown write light intensity waveform.
FIG. 6 shows a pattern of the metal layer for reflective pixels in
accordance with the invention.
FIG. 7 represents the structure of an embodiment of an optically-addressed
spatial light modulator with a nonrectifying photoresponsive layer.
FIG. 8 shows an equivalent circuit of the nonrectifying spatial light
modulator of FIG. 7.
FIG. 9 including parts (a), (b), (c), and (d), shows voltage versus time
waveform diagrams of the voltage provided by the electrical driver to the
nonrectifying spatial light modulator as well as the resulting voltage
across the FLC light modulator and the transmitted read light intensity
for the shown write light intensity waveform.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will now be described with reference to the drawings.
FIG. 1 shows the basic structure of the optically addressed spatial light
modulator of the invention. It comprises two transparent substrates 1 and
2, typically glass, each coated with a partially transparent electrode (3
and 4), and faced together with a light modulating layer 5 and a
photoresponsive layer 6 therebetween. Leads connect each plate's electrode
to a terminal of an electrical driving means 7. The light to be modulated
can either be directed through both the photoresponsive layer 6 and the
light modulating layer 5 for operation in transmission, or it can be
directed once through the light modulating layer 5, reflected at the
interface between the light modulating layer 5 and the photoresponsive
layer 6, and then back through the light modulating layer 5 again for
operation in reflection. The device's performance in reflection operation
may be enhanced by the addition of a light reflecting layer 8 at the
interface between the photoresponsive layer 6 and light modulating layer
5. Each layer will be described in turn below, as well as how they may be
combined into useful devices.
TRANSPARENT ELECTRODES 3 AND 4
The required transparent electrodes may be deposited on the substrate
plates by a variety of methods known in the art. For example, they may be
made of tin oxide or of indium-tin oxide deposited by vacuum evaporation,
sputtering, or spray pyrolysis. They may also be made of a metal,
deposited by vacuum evaporation or sputtering, provided the metal is
deposited in a layer thin enough to allow significant optical
transmission. The metal chromium is particularly useful for this function
as it adheres well to glass and does not diffuse readily into the a-Si:H.
Consideration in the choice of the transparent electrode on the plate
having the photoresponsive layer must be given to the properties of the
interface between the a-Si:H and the electrode, specifically if an ohmic
or rectifying contact is formed. Generally, high work function metals,
e.g., platinum, will form a rectifying Schottky barrier interface. The art
for the formation of ohmic contacts depends upon both the a-Si:H
deposition conditions as well as on the properties of the contact
material.
LIGHT MODULATING LAYER 5
A useful light modulating layer would have the properties of fast response
to low drive voltage and complete optical modulation in a thin layer so as
to give high spatial resolution. Ferroelectric liquid crystal (FLC) layers
have these properties, so the preferred embodiments of the present
invention will be described with reference to these materials, although
other materials may be found that could equally well be used to make
devices employing the teachings of the present invention.
FIG. 2 shows in more detail a typical FLC light modulating layer. This
layer comprises a film of FLC material 11 in the gap between the
photoresponsive layer 6 and the transparent conductively coated glass
substrate 2. The edge of the gap may be sealed with a gasket 12 that also
helps define the gap spacing. Distributed spacer particles 13 or deposited
spacer pads 14 within the gap may be used to further define the gap
spacing. The FLC-facing surfaces of the photoresponsive layer 6 and of the
transparent conductor 4 are further coated with "alignment" layers 15 and
16 to orient the intersections of the FLC's smectic layers with the
bounding layers parallel to a common direction. The discussion below
describes each of these elements in more detail; however, the discussion
is largely a summary of the prior art as taught in U.S. Pat. Nos.
4,367,924 and 4,563,059 and in U.S. patent application Ser. No. 108,799,
for example, and numerous other documents and publications well known to
those skilled in the art.
FLC Materials. Many suitable FLC materials are available commercially. For
example, the material SCE-9 sold by the British Drug House may be used as
the FLC material of the invention which has a spontaneous polarization
value of 33.6 nC/cm.sup.2. This material, like most commercially available
ones, has a non-tilted smectic A phase at temperatures above its
ferroelectric tilted smectic C phase, followed by a nematic phase, and
eventually an isotropic liquid phase (I) at still higher temperatures. The
presence of the smectic A and nematic (N) phases are generally regarded to
ease the FLC's alignment, although materials with the I-N-C and I-A-C
phase sequence are known and have been successfully used as light
modulators. The strength of the ferroelectricity of these materials is
characterized by the magnitude of their spontaneous polarization P, with
FLC materials having any value of P between 0 and 300 nC/cm.sup.2 being
readily available. The switching time .tau. of the material is greatly
influenced by the value of P, having the approximate dependence r
.tau..apprxeq..eta./PE, where .eta. is the FLC's orientation viscosity and
E is the magnitude of the applied electric field. FLCs have strongly
anisotropic dielectric properties, which are characterized by a low
frequency anisotropy .DELTA..epsilon. of the dielectric constants and a
refractive index anisotropy or birefringence .DELTA.n. The torques
produced by applied electric fields on the spontaneous polarization P
reorient the anisotropy axes of the FLC, and hence change the refractive
indices, which is the mechanism of the optical modulation used in the
preferred embodiments of the present invention.
The anisotropy axis of the FLC is tilted an angle .psi. from the smectic
layering direction; therefore, the application of oppositely directed
electric fields in the plane of the smectic layers changes the preferred
orientation of this axis from one position on the .psi.-defined cone to
the diametrically opposite one. In the absence of any applied field, the
axis prefers to helix about the layering direction, meaning that a thick
FLC film returns to a helixed configuration upon the removal of the
switching electric field. U.S. Pat. Nos. 4,367,924 and 4,563,059 teach
that this helical structure can be permanently unwound by making the FLC
film thin enough and that devices made in this manner have the useful
property of memory, i.e., that once an electric field switches the
anisotropy axis to a given orientation, it will not switch back either to
the helixed state or to the state selected by the oppositely directed
field upon removal of the switching field. This property can be useful in
devices of the present invention as described further below.
Spacing. Being partly fluid in nature, the FLC material 11 will fill the
gap allotted to it between the surfaces of the photoresponsive layer and
the glass substrate 2, as shown in FIG. 2. Since the FLC film's optical
and electrical properties depend strongly on its thickness, maintaining a
gap of a uniform and desired thickness is essential to the operation of
the device of the present invention. The gap between the confining
surfaces can be defined by placing a spacing gasket 12 of the desired
thickness around the edge of the gap and/or distributing spacers 13 and 14
throughout the gap. The spacing gasket 12 may also be formed of a sealing
material and used to prevent the entrance of unwanted materials into the
gap after it is filled with the FLC. The distributed spacers 13 can be
formed by a number of techniques. For example, they could be formed by
evaporating a patterned layer of oxide (e.g. SiO, SiO.sub.2, etc.) or by
spinning and patterning a layer of a polymer such as polyimide onto one of
the confining surfaces. Alternately, the spacers 13 may be comprised of
particles such as glass fibers or polymer spheres of the desired size
which are deposited onto the surfaces from a liquid or gas suspension.
Once the gap is formed, the FLC material may be introduced into it from
its edge by relying on capillary forces. This filling technique is aided
by heating the FLC to its isotropic liquid phase and by evacuating the air
from the gap prior to filling.
Alignment. Modulation of light with an FLC film is most conveniently
accomplished if the projection of the smectic layering direction onto the
plane of the film does not vary much in direction throughout the
modulator. This alignment may be achieved by a variety of different
methods. The most practical methods rely on the application of an
anisotropic coating to the FLC-confining surfaces, as shown in FIG. 2.
Many of the similar coatings used for aligning nematic liquid crystals, as
reviewed by Jaques Cognard in "Alignment of Nematic Liquid Crystals and
Their Mixtures", (Gordon and Breach, New York, 1982), can also be used for
FLCs. In particular, rubbed polymer layers and obliquely evaporated oxides
may be used.
PHOTOSENSITIVE LAYER 6
Unpatterned photoresponsive layers are generally simpler to make than
patterned ones, with high spatial resolutions being achieved with thin
unpatterned layers. Efficient photoresponse then dictates that the layer
have a high optical absorption so that substantially all incident writing
light is absorbed in spite of the layer's thinness. For this reason, many
materials such as indirect bandgap semiconductors cannot be used in the
invention. The photoresponsive layer should also be susceptible to a
fabrication method which allows it to be easily made in an optically flat
form. This has been difficult to accomplish with most semiconductors grown
in single-crystal form, but it is practical with materials that can be
deposited in a non-single-crystal form on an already flat substrate.
Amorphous silicon (a-Si:H) exhibits these desirable properties; therefore,
the further features of the invention will be | | |
