Note: Descriptions are shown in the official language in which they were submitted.
A NEMATIC LIQUID CRYSTAL STORAGE DISPLAY DEVICE
This invention relates to display devices and,
more particularly, to bistable liquid crystal devices.
Bistable nematic liquid crystal display devices
generally require large AC electric potentials to initiate
interstate switching between bistable states. One
important reason for such large AC electric switching
potentials is that sufficient electric energy must be
supplied to each display cell for detaching and moving
disclinations from pinning sites.
One embodiment of a nematic liquid crystal
display device exhibits configurational bistability between
two states. See ~. S. Patent 4,333,708. The two states,
which exist separately in the absence of a holding
potential, are topologically inequivalent and derive
stability from disclination pinning. Interstate switching
is accomplished by detaching and moving disclinations from
a pinning site in response to an applied AC switching
potential which exceeds a large, sharp switching threshold.
Reduction of the switching threshold level for
this type of liquid crystal display device is achieved by
prebiasing selected cells in the display with a small AC
priming potential prior to applying the larger switching
potential.
It should be noted with respect to the display
devices described above that AC switching potentials are
employed to effect interstate switching. The signals which
generate these AC switching potentials are generally from
the family of constant envelope signals and, more
particularly, of substantially constant envelope, gated, AC
pulse signals. Constant envelope AC signals are preferred
to constant amplitude or DC signals because the latter
signals give rise to space charge polarization effects
which reduce the amplitude of the applied electric field.
,
For above mentioned display devices, the problems
of relatively large AC switching potentials and switching
by disclination motion still exist.
In accordance with one aspect of the invention
~here is provided a liquid crystal display cell capable of
being switched to either a first state or a second state,
comprising first and second substrates disposed in parallel
relationship to each other, and a nematic liquid crystal
material having orientational directors disposed between
both substrates, characterized by each substrate having a
topographically textured inner surface exhibiting a uniform
tilt at an acute surface tilt angle from a respective
surface normal, means for generating a first polarity DC
potential across the liquid crystal material to initiate a
first change in orientational director configuration by
breaking s~mmetry of the liquid crystal display cell in
order to favor an orientational director configuration
wherein an inversion layer of orientational directors is
substantially adjacent and parallel ~o the first substrate,
and means connected to each substrate for generating an
electric potential through the liquid crystal material to
complete and maintain the first change in orientational
director configuration so that the orientational directors
are configured in the first state.
In accordance with another aspect of the
invention there is provided method of switching a liquid
crystal display cell having a nematic crystal material
with orientational directors disposed between parallel
disposed first and second substrates, each substrate
having a topographically textured inner surface exhibiting
a uniform tilt at an acute surface tilt angle from a
respective surface normal, generating a first polarity DC
potential across the liquid crystal material to initiate a
first change in orientational director configuration by
breaking symmetry of the li~uid crystal display cell in
=
~l2~ 8~
- 2a -
order to favor an orientational director configuration
wherein an inversion layer of orientational directors is
substantially adjacent and parallel to the ~irst substrate,
and generating an electric potential through the liquid
crystal material to complete and maintain the first change
in orientational director configuration so that the
orientational directors are configured in the first state,
inversion layer of orientation directors is substantially
adjacent and parallel to the first substrate, and
generating an electric potential through the liquid
crystal material to complete and maintain the first change
in orientational director configuration so that the
orientational directors are configured in the first state.
In accordance with the present invention, a small
DC electric potential is applied to the nematic liquid
crystal display cell to initiate bistable interstate
switching between two topologically equivalent horizontal
orientational director configurations. The polarity of
the DC potential determines the configuration toward
which an interstate switching cycle is initiated~l After
switching commences, a small AC electric potentia~, for
example, less than ten volts, is applied to the clll to
complete the switching cycle and to maintain the drient-
ational director configuration in one of the two horizontal
states. The bistable nematic liquid crystal display cell
comprises upper and lower substrates, nematic liquid
crystal material disposed between both substrates and a
combination of elements integrally connected to the
substrates capable of preferentially orienting directors
of the liquid crystal material into an asymmetric
horizontal state having an inversion layer substantially
adjacent and parallel to a predetermined substrate in the
presence of a symmetry breaking DC electric field followed
in sequence by a particular applied AC electric potential.
~,
~Z~L8~
- 2b -
In one embodiment of the invention, the liquid
crystal display cell comprises upper and lower parallel
substrates having electrically conductive strips and
topographically textured tilt alignment surfaces disposed
thereon, a nematic liquid crystal material disposed
between opposing textured surfaces and a source of variable
potential connected to the conductive strips for generating
electric switching fields through the liquid cyrstal
material. A cell is divided into an active region and an
isolation region which surrounds the active region. In
the active region of the cell, the opposing textured tilt
alignment surfaces exhibit an equal reverse tilt boundary
condition and a twist or angular di~ference between
A' .~
~2~ 8~
azimuthal orientations of the opposing textured tilt
alignment surfaces for optical differentiation of the
states. On each textured tilt alignment surface, the
isolation region is characterized by a parallel boundary
condition. Interstate switching is performed by applying a
first DC potential to the liquid crystal material to
initiate alignment of th~ orientational directors in a
first asymmetric horizontal stateO A small AC holding
potential greater than a critical potential is applied
normal to the substrates to complete switching to the first
sta~e. Transitions to the second state are accomplished by
applying a second DC potential to the liquid crystal
material in order to initiate proper alignment of the
orientational directors in a second asymmetric horizontal
state. Again, the small AC holding potential is applied to
complete switching to the second stateO
In another embodiment of the invention, the
liquid crystal display cell similarly comprises upper and
lower parallel substrates having electrically conductive
strips and topographically textured tilt alignment surfaces
disposed thereon, the nematic liquid crystal material
disposed between opposing textured surfaces, and the source
of variable potential connected to the conductive strips
for generating electric switching fields through the liquid
crystal materialO This embodiment differs from the
embodiment previously described in that topographically
textured tilt alignment surfaces on opposing substrates
exhibit a reverse tilt boundary condition throughout.
Active regions of the latter embodiment are not defined by
the texture of the tilt alignment surfaces but, rather, the
active regions are defined by the overlap region between
conductive strips on opposing substrates.
A more complete understanding of the invention
may be obtained by reading the following description of a
specific illustrative embodiment of the invention in
conjunction with the appended drawings in which:
~2~ 8~
FIG. 1 shows a three-dimensional view of a liquid
crystal display cell;
FIG. 2 shows a conceptual rendering of upper
topographically textured tilt alignment surface 20 as
viewed from line 2-2 in FIG. l;
FIG. 3 shows a conceptual rendering of lower
topographically textured tilt alignment surface 21 as
viewed ~rom line 3-3 in FIG. l; and
FIGS. 4 through 7 illustrate various horizontal
orientational director alignments within the active region
in the display cell of FIG. 1 in accordance with the
principles of the invention.
A new bistability effect is shown for nematic
liquid crystals wherein interstate switching between two
topologically equivalent states is initiated by application
of a symmetry breaking DC electric potential. A state is
maintained in its proper configuration by subsequent
application of a small AC holding potential. Each state
exhibits a boundary inversion layer containing
substantially horizontally aligned orientational directors
adjacent to a corresponding boundary. Switching from one
state to another requires no disclination motion because of
the topological equivalence of the states.
A liquid crystal display cell is shown in FIG. 1.
This display cell is one exemplary embodiment of the
invention. The cell in FIG~ 1 is only one of a plurality
of such cells which are included in an entire liquid
crystal display. As shown in FIG. 1, the liquid crystal
display cell includes upper substrate 10, lower
substrate 11, upper topographically textured tilt alignment
surface 20, lower topographically textured tilt alignment
surface 21, nematic liquid crystal material 30, upper
conductor 40, and lower conductor 41. Switching and
holding potentials are supplied to the cell from variable
potential source 50 connected to upper conductor 40 and
lower conductor 41. A set of reference basis vectors
(x,y,z) is shown in the Figures to assist in orienting
FIG. 1 with respect to FIGS. 4 through 7.
Substrates 10 and 11 support conductors 40 and
41, respectively, as well as provide a means for containing
liquid crystal material 30. Each substrate is composed
primarily of a transparent dielectric material such as
silicon dioxide or glass or the like.
Conductors 40 and 41 are disposed on an inner
opposing surface of each respective substrate in order to
permit an AC or DC electric field to be imposed
substantially perpendicular to each substrate. Both
interdigital electrodes and continuous uniform strip
electrodes are arrangements suitable for use as
conductors 40 and 41.
As shown in FIG. 1 for illustrative purposes
only, conductors 40 and 41 are continuous uniform strip
electrodes orthogonally disposed with respect to each
other. Conductor 40 is formed on an inner sur~ace of upper
; substrate 10, while conductor ~1 is similarly formed on an
inner surface of lower substrate 11 in a direction
orthogonal to the direction of conductor 40. Each
conductor is deposited or etched by conventional
photolithographic techniques as a thin film on the inner
surface of the respective substrateO Transparent films
such as indium tin oxide are used as conductors on both
substrates of transmission mode display cells, whereas
opaque films comprised of aluminum, for example, are used
for conductors on one substrate in reflection mode display
cells.
Topographically textured tilt alignment
surfaces 20 and 21 are utilized to induce a known tilt
alignment on the liquid crystal molecules adjacent to each
surface. These surfaces have also been called tilt
alignment surfaces. Surfaces 20 and 21 are transparent
non-conducting layers on the exposed inner surfaces of the
substrates and conductors for defining surface alignment of
the orientational directors of liquid crystal material 30.
Surfaces 20 and 21 are integrally connected to each
8~.
respective substrate by oblique electron beam deposition or
thermal evaporation of a material such as titanium oxide or
silicon oxide, both of which act as insulators. This
results in a uniformly tilted columnar topography for each
tilt align~ent surface. The topography on each of
surfaces 20 and 21 defines a surface tilt angle 00 measured
from each substrate normal (inner surface) in the range 0
degrees to 90 degrees. Surface tilt angles greater than 45
degrees are preferred in order to ensure dominance of the
horizontal orientational director configuration. Tilt
alignment surfaces 20 and 21 are more completely described
below in reference to FIGS. 2 and 3.
Liquid crystal material 30 is a liquid crystal
substance in the nematic mesophase having positive
dielectric anisotropy at least in some frequency range. In
an exemplary display cell, material 30 is comprised of
cyanobiphenyl samples of E7 from Merck Chemical Company.
; Liquid crystal material 30 is disposed between opposite,
parallel substrates wherein the surface to surface
separation of the substrates is less than 20 ~m and,
typically, is about 10 pmO
Each display cell is part-tioned into an active
region and an inactive region. The active region includes
a volume of liquid crystal material 30 which is capable of
interstate switching in response to appropriately applied
electric fields. In general, for the type of cell shown in
FIG. 1, the active region is defined as that region between
the overlap of conductors 40 and 41. In FIG. 1, a lower
boundary of the active region is shown as the crosshatched
area on surface 21.
The inactive region surrounding each active
region is a volume of liquid crystal material which
maintains a fixed orientational director configuration
regardless of the configurations in adjacent active
regions. Each inactive region, also known as a neutral
isolation region, separates, isolates and stabilizes the
surrounded active region of a corresponding cell in the
.,
~LZ~ 8~
liquid crystal display. A theory of neutral isolation
regions is explained by J. Cheng in "Surface Pinning of
Disclinations and the Stability of Bistable Nematic Storage
Displays," J. Appl. Phys. 5-~, pp. 72~-727 (1981).
Additional information concerning physical
aspects and construction of the basic display cell shown in
FIG. 1 is contained in both U. S. Patent 4,333,708.
Variable potential source 50 generates several
electrical signals which are supplied to upper conductor 40
and lower conductor 41 to impose various AC or DC electric
fields through liquid crystal material 30 and substantially
normal to substrates 10 and 11. Depending upon the
characteristics of the electric field imposed in the active
region of the display cell, the orientational director
configuration of liquid crystal material 30 is transformed
through a distorted horizontal configuration (FIG. 5) into
either an upper asymmetric horizontal state (FIG. 6) or a
lower asymmetric horizontal state (FIG. 7). After
switching to an asymmetric state is initiated, source 50
generates an AC holding signal to complete the switching
cycle and to maintain the asymmetric horizontal state in
the display cell with a holding potential.
Signals generated by source 50 are generally from
the families of constant envelope signals and constant
amplitude signals. More particularly, constant envelope
signals are substantially constant envelope, gated, AC
pulse signals whereas constant amplitude signals are gated
DC pulse signals.
` In order to carry out switching in accordance
with the principles of this invention, signals from
source 50 generate potentials referenced to a critical
potential Vc, which is described in more detail below. The
signals are classified into broad categories, namely, a DC
write signal, a DC initialization or DC erase signal, and
an AC holding signal. A write signal from source 50
imposes a DC potential of magnitude Vw across the display
cell to initiate switching of the cell to a first (upper or
8~.
-- 8 --
lower) asymmetric horizontal state, where potential Vr~J is
either above or below critical potential Vc in order to
produce desired switching characteristics. An erase signal
imposes a potential of magnitude VE across the display cell
to initiate switching of the cell to a second (lower or
upper) asymmetric horizontal state, where VE is
substantially equal in magnitude but opposite in polarity
to Vw. An AC holding signal is generated by source 50 to
complete the switching cycle and to maintain orientational
directors in the particular asymmetric horizontal state to
which they have been switched. The holding signal creates
an AC potential of magnitude VH across the cell, wherein VH
is at least greater than the critical potential Vc.
Holding potential magnitude VH can be increased to improve
optical contrast between the first and second asymmetric
horizontal states. It should be noted that potentials VE,
VH, Vw and Vc depend upon the dimensions and other
~ characteristics of the liquid crystal display cell.
; However, by way of example, it is known that, for a thin
cell ~lOIlm intersubstrate separation) containing E7,
preferred potentials are Vc = 1.5 volts, V~ and VE are
between 1.5 volts and 5.0 volts, and VH is less than 10.0
volts. More detailed information concerning variable
potential source 50 and bistable switching of the liquid
crystal display cell is given below with respect to FIGS. 5
through 7.
~ IG. 2 shows a view of upper tilt alignment
surface 20 from a position along line 2-2 in FIG. 1. Tilt
alignment surface 20 includes active region surface 201
(dark outlined ellipses) and isolation region surface 202
(light outlined ellipses). Ellipses have been drawn to
represent tilted molecular columns in the tilted topography
of surface 20. Along the major axis of each of several
ellipses on active region surface 201, a vector has been
drawn as an orthogonal projection of the major axis of each
ellipse, i.e., the molecular axis of a metallic column,
onto the tilt alignment surface. Since the vector
~.2~
g
indicates a direction in which the columns are pointed awa~
from the tilt alignment surface, it can be said that the
vector indicates a direction of surface tilt for the
metallic oxide columns and, hence, a direction of azimuthal
bias for the tilt alignment surface.
Azimuthal bias for an active region surface is
measured as an angular displacement from a reference line.
In the Figures, line 213 is the reference line. Line 203
is parallel with the vectors on surface 201 to indicate the
direction of azimuthal bias for active region surface 201
at angle ~, where ~ is an acute angle between -90 degrees
and +90 degrees. It should be noted that isolation region
surface 202 is aligned parallel with the direction of
azimuthal bias of active region surface 201.
FIG. 3 shows a view of lower tilt alignment
surface 21 from a position along line 3-3 in FIG. 1
Surface 21 includes active region surface 211 (dark
outlined ellipses) and isolation region surface 212 (light
outlined ellipses). Reference line 213 also shows the
direction of azimuthal bias for active region surface 211
so that the azimuthal bias for surface 211 is zero degreesO
The azimuthal bias for surface 212 is parallel with the
direction of bias for surface 211.
In the active region of the display cell,
surfaces 20 and 21 form a reverse tilt boundary condition.
Reverse tilt occurs because the azimuthal bias ~ of
surface 201 is between -90 degrees and +90 degrees and,
when measured as an acute angle from each respective
substrate normal (inner surface), the surface tilt angle
for surface 201 has an opposite polarity to the surface
tilt angle for surface 211. For example, as shown in
FIGS. 2 and 3, the surface tilt angle for surface 201 is
measured counterclockwise from the inner surface normal of
substrate 10 as an acute angle, whereas the tilt angle for
surface 211 is measured clockwise from the inner surface
normal of substrate 11. As stated above, the surface tilt
angles for surfaces 201 and 211 are required to have
¢~
-- 10 --
absolute values in -the range 0 degrees to 90 degrees from
the respective substrate normals and, more preferably,
greater than 45 degrees to favor a horizontal orientational
director configuration. Furthermore, it is important to
the principles of the invention that the reverse tilt be
equal so that the absolute-valued tilt angle of surface 201
is substantially equal to the absolute-valued tilt angle
for surface 211.
In the isolation region, surfaces 20 and 21 form
a uniformly parallel boundary condition aligned parallel
with the azimuthal bias of the corresponding active region
surfaces. That is, isolation region surfaces 202 and 212
have columns exhibiting surface tilt angles of
approximately 90 degrees from the substrate normal (see
FIGS. 2 and 3). It has been found that, for ease in
fabrication, the parallel boundary condition of
surfaces 201 and 202 be made by oblique evaporation of SiOx
with the plane of incidence in a direction perpendicular to
the preferred azimuthal bias direction from an angle of
approximately 65 degrees from the substrate normal.
Upper and lower tilt alignment surfaces are
important, individually and in combination, to bistable
switching of the liquid crystal display cell. Upper and
lower tilt alignment surfaces are fabricated in a manner
for eliminating a preference of one asymmetric horizontal
state over the other in the absence of a particular
switching electric field and for providing optical
differentiation of the asymmetric states. Particularly,
the difference between the azimuthal biases of the upper
and lower active region surfaces provides optical
differentiation between the bistable states. Symmetry of
the surfaces in the display cell eliminates a preference
; for establishment of an asymmetric horizontal state near a
particular surface in the absence of the symmetry breaking
field. These features will become more apparent with
reference to the description of FI~S. 4 through 7 below.
831
-- 11
FIG~ 4 shows a three-dimensional view of the
volume of liquid crystal material in the active region of
the display cell depicted with the orientational directors
in an undistorted horizontal configuration. This is the
quiescent configuration of the cell because the
orientational directors of the liquid crystal material
assume this configuration in the absence of an electric
field. Planar section ~01 of a boundary layer contain
directors of the liquid crystal material oriented
substantially at the surface tilt angle of surface 211,
while planar sections 403 of a boundary layer contain
directors oriented at the surface tilt angle of
surface 201. Planar section 402 of an inversion layer
contains orientational directors which are substantially
horizontal (substantially parallel) with respect to each
substrate surface.
For simplicity, FIG. 4 shows only enough detail
to see planar section 402 as a single section of coplanar
orientational directors in the inversion layerO Clearly,
there are a plurality of identical planar sections parallel
to planar section 402 which comprise the entire inversion
layer. Similarly, there are corresponding pluralities of
identical planar sections parallel to each of planar
sections 401 and 403 which comprise boundary layers at
surfaces 20 and 21, respectively. This simplification of
detail has also been applied to FIGS. 5, 6 and 7.
Orientational director alignment is not changed
from the undistorted horizontal configuration until a
symmetry breaking field is applied to the c~ll.
Furthermore, this change is maintainable provided that a
holding potential equal to or greater than the critical
potential is subsequently applied to the display cell.
Critical potential Vc is defined as the potential above
which liquid crystal material 30 behaves in a bistable
manner with respect to horizontal configurations. The
critical potential is described as follows. Assume that
the boundary and inversion layers are completely separated
~ ~2~4~
- 12 -
and exhibit uniform splay bend distortion energy UO per
unit volume where
U0 = k ~2~ 2, and
_ _ 1
E a~ 2
where ~ is the electric coherence length defined as the
characteristic distance in which liquid crystal molecules
with means splay-bend modulus ~ and dielectric anisotropy
~ rotate from perpendicular to parallel with respect to an
: applied electric field E. The energy density per unit area
of each boundary layer is proportional to the thickness of
: the particular layer as shown in the table below:
Layer Type Thickness Energy Density
(Reference Numerals) Per Unit Area
Boundary ~/2 Uo~/2
20(501, 503)
Inversion 2~ 2U
(502)
Boundary Inversion
(504, 505~
: 25 From the table above, it is clear that the distorted
horizontal configuration shown in FIG. 5 has a total energy
per unit area of 3Uo~, whereas the asymmetric horizontal
states of FIGS. 6 and 7 each have total energy per unit
area of 2Uo~. However, the argument presented is not valid
for an applied field for which the boundary and inversion
layers merge across the total thickness, d, of the display
cell. Therefore, the cell thickness d is at least equal to
~Z~8~
- 13 -
3 ~ and the critical potential is given by the relation,
Vc = dEC = 3 ~c
For a sample of cyanobiphenyl E7 and absolute-valued
surface tilt angles of approximately 53 degrees, the
critical potential Vc is approximately 1.3 to 1.7 volts.
When a potential greater than critical potential
Vc is applied to the display cell, the orientational
directors of the active region are briefly transformed into
a distorted horizontal configuration as shown in FIG. 5.
The distorted horizontal configuration contains planar
sections 501 of the lower boundary layer, planar
section 502 of the inversion layer and planar section 503
of the upper boundary layer, hereinafter referred to,
respectively, as lower boundary layer 501, inversion
layer 502, and upper boundary layer 503. This state is
unstable because the overall elastic and dielectric energy
of the orientational director configuration can be lowered
when inversion layer 502 merges with either upper boundary
layer 503 (FIG. 6) to form the upper asymmetric horizontal
state or with lower boundary layer 501 (FIG. 7) to form the
lower asymmetric horizontal state. Both resulting
asymmetric horizontal states havP equal energy, are
topologically equivalent, and are separated by an energy
barrier represented by the distorted horizontal
configuration.
If the DC potential applied to the cell in the
distorted horizontal configuration is Vw corresponding to
the write signal from source 50, an orientational director
transformation is initiated from the distorted horizontal
configuration (FIG. 5) toward ~he upper asymmetric
horizontal state shown in FIG. 6. The transformation
occurs by direct vertical movement of inversion layer 502
toward boundary layer 503. This results in the formation
of boundary inversion layer 504 adjacent to active region
surface 201 of surface 20. When AC holding potential VH is
,,
t?~
subsequently applied to the cell via the holding signal
from source 50, the switching cycle is completed and the
orientational directors are maintained in the upper
asymmetric horizontal state. Orientational directors in
boundary inversion layer 504 reside in the plane which
includes both the substrate normal and the azimuthal bias
line for active region surface 201, i.e., line 203.
On the other hand, if the DC potential applied to
the cell in the distorted horizontal configuration is VE
corresponding to the erase signal from source 50 an
orientational director transformation is initiated from the
distorted horizontal configuration toward the lower
asymmetric horizontal state shown in FIG. 7. The
transformation occurs by downward vertical movement of
inversion layer 502 toward lower boundary layer 501. When
AC holding potential VH is subsequently applied to the cell
via the holding signal from source 50, the switching cycle
is completed and the orientational director configuration
is maintained in the lower asymmetric horizontal state.
Orientational directors in boundary inversion layer 505
reside in a plane which includes both the substrate normal
and the azimuthal bias line for active region surface 211,
i.e., reference line 213.
Interstate switching between asymmetric
~5 horizontal states, for example, upper-to-lower or lower-
to-upper, is accomplished by extinguishing the AC holding
signal to the cell and allowing li~uid crystal material 30
to relax momentarily into the distorted horizontal
configuration (FIG 5) or the undistorted horizontal
configuration (FIG. 4). After a short relaxation period, a
DC write signal or a DC erase signal is supplied to the
cell to initiate the switching appropriately.
It should be noted that the cell will relax into
a substantially undistorted horizontal configuration in the
presence of any potential less than or even slightly above
the critical potential Vc. Hence, interstate switching may
also be performed by lowering the potential on the cell
8~
- 15 -
from the holding potential level to a level slightly above
or below the critical potential.
It is advantageous to the operation of the
display cell in either asymmetric state for the
orientational directors to be inhibited from switching to a
vertical configuration. Vertical configuration switching
is capable of being prevented by operating variable
potential source 50 below the threshold level at which
detachment of disclinations occurs. This threshold level
is generally found to be on the order of 60 volts.
In a second embodiment of the display cell, the
essential elements and methods of operations are as
previously described above in relation to the display cell
shown in FIGS. 1 through 7. Additionally, isolation
regions 202 and 212 exhibit a tilted columnar topography
similar to the respective active region which each
isolation region surrounds.
Although not shown in the Figures, an appropriate
combination of linear polarizers and perhaps a fixed
retarder plate can be employed to enhance the optical
contrast between the asymmetric states.
One application for this type of nematic liquid
crystal display cell is in high speed, matrix addressable,
storage display devices. Although addressing speeds are
presently found to be on the order of 30 msec, it is clear
that certain modifications of the display cell
characteristics are capable of improving the performance of
the display cell. Particularly, these modifications
include reducing the intersubstrate spacing and reducing
the viscosity of the nematic liquid crystal material.
