U.S. patent application number 14/845924 was filed with the patent office on 2016-04-21 for methods and apparatus for liquid crystal photoalignment.
The applicant listed for this patent is Shaun R. Berry, Philip J. Bos, Carl O. Bozler, Douglas R. Bryant, Harry R. Clark, Valerie A. Finnemeyer, Robert K. Reich. Invention is credited to Shaun R. Berry, Philip J. Bos, Carl O. Bozler, Douglas R. Bryant, Harry R. Clark, Valerie A. Finnemeyer, Robert K. Reich.
Application Number | 20160109760 14/845924 |
Document ID | / |
Family ID | 55440396 |
Filed Date | 2016-04-21 |
United States Patent
Application |
20160109760 |
Kind Code |
A1 |
Finnemeyer; Valerie A. ; et
al. |
April 21, 2016 |
METHODS AND APPARATUS FOR LIQUID CRYSTAL PHOTOALIGNMENT
Abstract
Liquid crystal photonic devices and microcavities filled with
liquid crystal materials are becoming increasingly popular. These
devices often present a challenge when it comes to creating a
robust alignment layer in pre-assembled cells. Previous research on
photo-definable alignment layers has shown that they have limited
stability, particularly against subsequent light exposure. A method
of infusing a dye into a microcavity to produce an effective
photo-definable alignment layer is described, along with a method
of utilizing a pre-polymer infused into the microcavity mixed with
the liquid crystal to provide photostability. In this method, the
polymer layer, formed under optical irradiation of liquid crystal
cells, is effectively localized to a thin region near the substrate
surface and thus provides a significant improvement in the
photostability of the liquid crystal alignment. This versatile
alignment layer method, which can be used in microcavities to
displays, offers significant promise for new photonics
applications.
Inventors: |
Finnemeyer; Valerie A.;
(Kent, OH) ; Reich; Robert K.; (Tyngsborough,
MA) ; Clark; Harry R.; (Townsend, MA) ;
Bozler; Carl O.; (Waltham, MA) ; Berry; Shaun R.;
(Chelmsford, MA) ; Bos; Philip J.; (Hudson,
OH) ; Bryant; Douglas R.; (Aurora, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Finnemeyer; Valerie A.
Reich; Robert K.
Clark; Harry R.
Bozler; Carl O.
Berry; Shaun R.
Bos; Philip J.
Bryant; Douglas R. |
Kent
Tyngsborough
Townsend
Waltham
Chelmsford
Hudson
Aurora |
OH
MA
MA
MA
MA
OH
OH |
US
US
US
US
US
US
US |
|
|
Family ID: |
55440396 |
Appl. No.: |
14/845924 |
Filed: |
September 4, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62046706 |
Sep 5, 2014 |
|
|
|
Current U.S.
Class: |
349/123 ;
445/3 |
Current CPC
Class: |
C09K 19/56 20130101;
G02F 1/133377 20130101; G02F 2202/043 20130101; G02F 2001/133726
20130101; G02F 1/133711 20130101; G02F 1/133788 20130101 |
International
Class: |
G02F 1/1337 20060101
G02F001/1337; G02F 1/1333 20060101 G02F001/1333 |
Goverment Interests
GOVERNMENT SUPPORT
[0002] This invention was made with government support under
Contract No. FA8721-05-C-0002 awarded by the U.S. Air Force. The
government has certain rights in the invention.
Claims
1. A liquid crystal (LC) cell, comprising: a structure defining a
microcavity; LC material disposed within the microcavity; a
dichroic dye layer disposed on an inner surface of the microcavity;
and a polymerized reactive mesogen layer, disposed on and aligned
with the dichroic dye layer, to align the LC material with respect
to the dichroic dye layer.
2. The LC cell of claim 1, wherein the structure comprises a
substrate.
3. The LC cell of claim 1, wherein the dichroic dye layer has a
thickness of up to about 10 nm.
4. The LC cell of claim 1, wherein the dichroic dye layer comprises
Brilliant Yellow azo dye.
5. The LC cell of claim 1, wherein the polymerized layer has a
thickness of up to about 100 nm.
6. The LC cell of claim 1, wherein the polymerized layer comprises
reactive mesogen.
7. A method of aligning liquid crystal material to an inner surface
of a microcavity, the method comprising: infusing anisotropic dye
into the microcavity so as to coat the interior surface of the
microcavity with the anisotropic dye; illuminating the anisotropic
dye with polarized light so as to form an anisotropic dye layer
aligned with respect to the inner surface of the microcavity;
infusing reactive mesogen and liquid crystal material into the
microcavity; and illuminating the reactive mesogen at a wavelength
selected to cause polymerization of the layer of reactive mesogen
material so as to form a polymerized reactive mesogen layer
aligning the liquid crystal material with respect to the
anisotropic dye layer.
8. The method of claim 7, wherein infusing the anisotropic dye
comprises infusing at least one of an azo dye or a dye
substantially similar to an azo compound.
9. The method of claim 7, wherein infusing the anisotropic dye
comprises: disposing the microcavity in a dye solution comprising
the anisotropic dye and a solvent; and heating the microcavity so
as to evaporate the solvent.
10. The method of claim 7, wherein infusing the reactive mesogen
and the liquid crystal material comprises infusing RM257.
11. The method of claim 7, wherein infusing the reactive mesogen
and the liquid crystal material comprises: infusing a mixture of
the reactive mesogen, the liquid crystal material, and a
photoinitiator into the microcavity.
12. The method of claim 11, wherein the mixture of the reactive
mesogen, the liquid crystal material, and the photoinitiator has a
weight ratio of reactive mesogen to liquid crystal material to
photoinitiator of about 1.35 to 98.50 to 0.15.
13. The method of claim 11, further comprising: heating and mixing
the mixture of the reactive mesogen, the liquid crystal material,
and the photoinitiator prior to infusing the mixture into the
microcavity.
14. The method of claim 13, wherein infusing the reactive mesogen
and the liquid crystal material further comprises: allowing the
reactive mesogen to separate from the liquid crystal material
before illuminating the reactive mesogen.
15. The method of claim 7, wherein illuminating the reactive
mesogen further comprises: applying at least one voltage across at
least a portion of the microcavity while illuminating the reactive
mesogen so as to lock in alignment of the polymerized reactive
mesogen layer with respect to the anisotropic dye layer.
16. The method of claim 7, wherein applying the at least one
voltage comprises: applying a first voltage across a first portion
of the microcavity and a second voltage across a second portion of
the microcavity so as to create spatially varying alignment of the
anisotropic dye to the liquid crystal material.
17. The method of claim 7, wherein the polymerized reactive mesogen
layer has a thickness of less than approximately 100
nanometers.
18. The method of claim 7, further comprising: infusing a
photoinitiator into the microcavity before illuminating the
reactive mesogen with ultraviolet light.
19. The method of claim 18, wherein infusing the photoinitiator
comprises infusing Irgacure 651.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application claims the priority benefit under 35 U.S.C.
.sctn.119(e) of U.S. Application No. 62/046,706, entitled "Methods
and Apparatus for Liquid Crystal Photoalignment," and filed on Sep.
5, 2014, which application is hereby incorporated by reference
herein.
BACKGROUND
[0003] Liquid crystals (LCs) are materials that flow like liquid
with crystalline solid-like ordered molecules that align and orient
along a particular direction in the presence (or absence) of an
electric field. These materials are widely used to manipulate the
polarization and transmission of light, including in liquid crystal
displays (LCDs). In an LCD, an LC layer is usually formed by
aligning the LC material with respect to a pair of substrates and
sandwiching the substrates between a pair of crossed polarizers.
Applying an electric field to the LC layer causes the LC to align
or twist, thereby allowing or blocking the incident light.
[0004] Typically, the LC material is aligned to the substrate with
an alignment layer. The alignment layer is typically applied
through a standard spin-coating method with a layer thickness on
the order of several hundred nanometers. This layer orients the LC
molecules, which often have an oblong shape, along a surface of the
substrate, which is typically transparent glass or plastic. This
type of alignment causes most or all of the LC material to form a
"single crystal" that can be re-oriented using an electric field.
Absent this alignment layer, the liquid crystals would behave as a
"polycrystalline" material; that is, the LC layer would form
smaller LC domains, each containing molecules aligning in an
orientation different from those of other LC domains. Light passing
through a polycrystalline LC layer undergoes non-uniform scattering
and random variation in light transmission, producing diffused,
low-intensity lighting.
[0005] One of the most common ways to provide liquid crystal
alignment is by first coating the surface with a thin film of
polymer, such as polyimide, and then rubbing the surface with a
cloth. The cloth aligns the polymer molecules on the surface in the
rubbing direction; the liquid crystal in contact with the surface
aligns to the polymer molecules. This approach has been quite
effective, and thus widely used in the LCD industry. Although this
rubbing alignment technique is generally applicable to display
technologies that work with large, flat display platforms and
substrates, it may not be applicable to particular LC applications
that utilize non-planar, non-standard, and/or smaller cavities to
hold LC. In addition, the rubbing an alignment layer with a cloth
tends to generate particles, making it suitable for certain
applications.
[0006] Other techniques for aligning LC materials include a
photoalignment technique, which utilizes polarized light to form an
alignment layer for LC materials. Photoalignment case be used in a
variety of non-standard geometries. For instance, photoalignment
has been utilized in the creation of a tunable microresonator in
which the alignment layer is applied through a standard
spin-coating method. Great success has also been shown in the use
of photoalignment for tunable photonic crystal fibers (PCFs). In
this case, the application of the photoalignment layer via
spin-coating is not possible; instead, the fiber is filled with the
photoalignment solution through capillary action into the fibers,
then excess solution is removed through a pressure gradient.
[0007] There are a number of different photoalignment techniques
which can be categorized by the way in which the polarized
irradiation causes surface anisotropy: the polarized light can
result in polymerization with cross-linking along one direction
(photo-polymerization), it can result in degradation of molecules
aligned along one direction (photo-degradation), it can result in a
conformational change of molecules along one direction
(photo-isomerization), or it can excite molecules preferentially
along one direction (photo-reorientation). The last two of these
are most commonly accomplished using azo dyes which often absorb
well in the ultraviolet (UV) or visible range. While
photo-isomerization is frequently criticized as having poor
lifetime due to the gradual relaxation of molecules from the cis-
to the trans-state, photo-reorientation, depending on the relaxed
molecular conformation, is a much more attractive choice because it
can be excited preferentially along the polarization. In addition
to the lower irradiation energies compared to both
photo-polymerization and photo-degradation, photo-reorientation of
azo dyes results in an alignment layer with an order parameter
which can be even higher than the liquid crystalline order
parameter.
[0008] In photo-reorientation, a dichroic dye, most often one
containing azo groups, is irradiated with polarized light of an
appropriate wavelength (i.e., one which is well absorbed by the
dye). The probability that a given dye molecule will absorb this
incident irradiation is proportional to cos.sup.2 .theta. where
.theta. is the angle between the incident polarization axis and the
long axis of the dye molecule. Over time, this absorption increases
the population of dye molecules aligned perpendicular to the
incident polarization, where the probability of absorption is at or
near zero. After a sufficient exposure dose, the order parameter,
which is determined by the absorption spectra of the dye both
parallel and perpendicular to the polarization axis of the
irradiating light, can exceed even that of the liquid crystals it
is being used to align.
[0009] Anchoring energies of these layers have also been measured
to be on the same order of magnitude as the anchoring achieved
through rubbed polyimide alignment. This is particularly important
in photonic devices where light scattering from director
fluctuations can degrade device performance. Anchoring energies on
the range of that observed from polyimide and also for azo-dye
alignment layers suppress these fluctuations to an acceptable level
in some devices. It should be noted that director fluctuations are
not a large concern for display devices.
[0010] Unfortunately, conventional photo-aligned layers tend to
degrade when exposed to light or heat, making them unsuitable for
many applications, including displays and thermal sensing. Of
particular importance for photonic applications is stability under
exposure to light of random polarization states. Also, in the case
of photonic devices, the light intensity which the device is
subjected to can be quite high, enhancing the probability of device
failure if the stability is low. It should be noted that for many
applications of azodye alignment layers, the "rewriteability" of
these materials is emphasized as a positive attribute. However, in
the case of photonic devices where the azodyes are desired for
their high anchoring energy, rewriteability is problematic.
SUMMARY
[0011] The inventors have recognized that materials called reactive
mesogens can be used to address the stability issues that plague
conventional photoalignment layers. These materials can be applied
as monomers and subsequently exposed to UV light to become
polymers. Further, in their monomeric state, reactive mesogens can
exist in the liquid crystalline state of matter, but then, after
alignment by or to a photoalignment layer, can be polymerized to
lock-in their order. Reactive mesogens have been applied by spin
coating and have been shown to be effective in stabilizing azo dye
materials to thermal stress. They have also been applied as an
additive to liquid crystals and shown to be effect in stabilizing
against electro-optic stress.
[0012] The inventors have also recognized that spin coating
reactive mesogen is not applicable to preformed cavity photonic
devices and that using reactive mesogen to stabilize liquid
crystals against electro-optic stress does not necessarily apply to
azo dye materials or the stability of alignment under optical
stress. In contrast, the embodiments disclosed herein include
photonic devices with preformed cavities containing azo dye
materials that are stable to exposure to light of relatively high
intensity. Unlike conventional liquid crystal devices, which are
formed by assembling two substrates coated with respective
alignment layers using spin coating, roller coating, meniscus
coating, etc., inventive photonic devices may include "preformed
cavities," which often cannot be coated with alignment layers using
conventional coating techniques.
[0013] More specifically, embodiments of the present technology
include liquid crystal (LC) cells and methods of making and using
LC cells. An example LC cell includes a structure, such as a
substrate, that defines a microcavity; LC material disposed within
the microcavity; a dichroic dye layer (e.g., a layer of azo or
anisotropic dye) disposed on an inner surface of the microcavity;
and a polymerized layer (e.g., polymerized reactive mesogen),
disposed on and aligned with the dichroic dye layer, to align the
LC material with respect to the dichroic dye layer. The dichroic
dye layer may have a thickness of up to about 10 nm and may
comprise Brilliant Yellow azo dye. The polymerized layer may have a
thickness of up to about 100 nm and may comprise RM257 or another
suitable reactive mesogen.
[0014] Another embodiment includes a method of aligning liquid
crystal material to an inner surface of a microcavity. The method
includes infusing anisotropic dye, such as an azo dye or a dye
substantially similar to an azo compound, into the microcavity so
as to coat the interior surface of the microcavity with the
anisotropic dye. The anisotropic dye is illuminated with polarized
light so as to form an anisotropic dye layer aligned with respect
to the inner surface of the microcavity. Reactive mesogen, such as
RM257, and liquid crystal material are infused into the
microcavity. The reactive mesogen is illuminated at a wavelength
selected to cause polymerization of the layer of reactive mesogen
material so as to form a polymerized reactive mesogen layer, which
may be <100 nm thick, that aligns the liquid crystal material
with respect to the anisotropic dye layer.
[0015] In some cases, infusing the anisotropic dye comprises
disposing the microcavity in a dye solution comprising the
anisotropic dye and a solvent. Once the dye solution has wicked
into the microcavity, the microcavity can be heated so as to
evaporate the solvent.
[0016] Similarly, infusing the reactive mesogen and the liquid
crystal material may comprise infusing a mixture of the reactive
mesogen, the liquid crystal material, and a photoinitiator into the
microcavity. The mixture of the reactive mesogen, the liquid
crystal material, and the photoinitiator can have a weight ratio of
reactive mesogen to liquid crystal material to photoinitiator of
about 1.35 to 98.50 to 0.15. If desired, the mixture of the
reactive mesogen, the liquid crystal material, and the
photoinitiator may be mixed and/or heated prior to being infused
into the microcavity. And the reactive mesogen may be allowed to
separate from the liquid crystal material before being
illuminated.
[0017] In some examples, illuminating the reactive mesogen
comprises applying at least one voltage across at least a portion
of the microcavity so as to lock in (fix, freeze, hold) alignment
of the polymerized reactive mesogen layer with respect to the
anisotropic dye layer. If desired, a photoinitiator, such as
Irgacure 651, may be infused into the microcavity before
illuminating the reactive mesogen with ultraviolet light.
[0018] The alignment may be varied by applying different voltages
across the microcavity. For example, a first voltage can be applied
across a first portion of the microcavity and a second voltage can
be applied across a second portion of the microcavity so as to
create spatially varying alignment of the anisotropic dye to the
liquid crystal material. For example, the spatially varying
alignment can be achieved by masking the exposure irradiation
during the polymerization step, with one voltage applied and a
region of the cavity exposed. Next, a different voltage can be
applied and a different region exposed and so on in order to
produce spatially varying alignment.
[0019] It should be appreciated that all combinations of the
foregoing concepts and additional concepts discussed in greater
detail below (provided such concepts are not mutually inconsistent)
are contemplated as being part of the inventive subject matter
disclosed herein. In particular, all combinations of claimed
subject matter appearing at the end of this disclosure are
contemplated as being part of the inventive subject matter
disclosed herein. It should also be appreciated that terminology
explicitly employed herein that also may appear in any disclosure
incorporated by reference should be accorded a meaning most
consistent with the particular concepts disclosed herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The skilled artisan will understand that the drawings
primarily are for illustrative purposes and are not intended to
limit the scope of the inventive subject matter described herein.
The drawings are not necessarily to scale; in some instances,
various aspects of the inventive subject matter disclosed herein
may be shown exaggerated or enlarged in the drawings to facilitate
an understanding of different features. In the drawings, like
reference characters generally refer to like features (e.g.,
functionally similar and/or structurally similar elements).
[0021] FIG. 1A is a cross-sectional view of a microcavity disposed
within a substrate to hold liquid crystal material.
[0022] FIG. 1B is an illustration of the microcavity of FIG. 1A
filled with a mixture of materials in preparation for photoaligning
a liquid crystal material.
[0023] FIG. 1C is an illustration of the liquid crystal microcavity
of FIGS. 1A and 1B with photoalignment in place.
[0024] FIG. 1D is a top-view of the microcavity of FIG. 1C.
[0025] FIG. 1E is an exemplary illustration of a microcavity
disposed on an elevated platform filled with liquid crystal and
photoaligning materials according to another embodiment.
[0026] FIG. 2A is an exemplary fabrication process flow diagram for
creating a versatile alignment layer in a liquid crystal device via
mixing reactive mesogen and liquid crystal prior to infusing into a
microcavity.
[0027] FIG. 2B is an exemplary fabrication process flow diagram for
creating a versatile alignment layer in a liquid crystal device via
infusing reactive mesogen and then infusing liquid crystal into a
microcavity.
[0028] FIG. 2C is an exemplary fabrication process flow diagram for
forming an Azo dye layer in a microcavity.
[0029] FIG. 2D is a graph of the absorption spectrum of Brilliant
Yellow dye.
[0030] FIG. 2E is a set of images showing the bright and dark
states of photoaligned liquid crystal microcavities between crossed
polarizers which operate in either reflective or transmissive
mode.
[0031] FIG. 2F is an exemplary fabrication process flow diagram for
form a reactive mesogen layer within a microcavity.
[0032] FIG. 2G is a set of images showing the degrees of scattering
through planar liquid crystal cells (ii, iii, and iv) with
polymer-stabilization layers as compared to a liquid crystal cell
without a polymerized layer (i).
[0033] FIG. 2H is a set of images showing the degradation of
twisted cells prepared with one photoaligned substrate and one
rubbed-polyimide substrate, using pure BL006 (top).
[0034] FIG. 2I is a set of images showing twist cells prepared with
one photoaligned substrate and one rubbed-polyimide substrate,
filled with RM257 in BL006 at 0.9% wt (top), 1.2% wt (middle), or
1.5% wt (bottom).
[0035] FIG. 3A shows a set of confocal micrographs of monomers'
distribution along the cell gap direction under different mixing
conditions: Sonication, Vortex and Heat, and None.
[0036] FIGS. 3B-I, 3B-II, and 3B-III show a set of intensity
profiles measured along a random vertical cross-section of each
image shown in FIG. 3A.
[0037] FIG. 4A is an illustration of a filled microcavity
containing a mixture of liquid crystal materials and reactive
mesogen monomers, which preferentially localizes near the
microcavity surfaces.
[0038] FIG. 4B is an illustration of the filled microcavity in
which the liquid crystal is re-oriented under applied voltage.
[0039] FIG. 4C is an illustration of the filled microcavity in
which monomers are crosslinked under ultraviolet (UV) illumination
to lock in the orientation of liquid crystal.
[0040] FIG. 4D is an illustration of the filled microcavity with
"oriented" liquid crystal (without an applied voltage).
[0041] FIG. 5A shows simulated dielectric data using surface
concentration X.sub.0=0.08 and decay length .xi.=0.2d and
indicating that the polymer network is evenly distributed
throughout the cell.
[0042] FIG. 5B shows simulated dielectric data using surface
concentration X.sub.0=0.8 and decay length .xi.=0.02d and
indicating that the polymer is more concentrated near the surface
inside the cell.
[0043] FIG. 5C shows simulated dielectric data in which the polymer
network is assumed to be infinitesimally thin (i.e., highly
localized), indicating its suitability as an alignment layer.
[0044] FIG. 6 shows a diagram of a microcavity prepared for testing
surface localization of reactive mesogen. Polyimide alignment
layers on each substrate were rubbed in opposite directions (as
indicated).
[0045] FIG. 7 shows phase profiles for 5 .mu.m planar cells filled
with pure liquid crystal (LC) or LC with 1.5% wt RM257, which was
polymerized at either 0V or 20V.
[0046] FIG. 8A shows images of samples filled with pure BL006 (a)
before exposure and after (b) 50 minutes, (c) 100 minutes, (d) 150
minutes, and (e) 200 minutes of exposure to about 20 mW/cm.sup.2
polarized blue light oriented at 45 degrees to the photoalignment
axis.
[0047] FIG. 8B shows images of samples filled with 1.5% wt RM257 in
BL006 shown between parallel (left) or crossed (right) polarizers
(a) before and (b) after exposure to polarized blue light of about
20 mW/cm.sup.2 for 21 days and (c) before and (d) after exposure to
unpolarized blue light of about 120 mW/cm.sup.2 for 21 days.
[0048] FIG. 9 shows transmission v. voltage (TV) curves of 5 .mu.m
samples with rubbed polyimide alignment on one substrate and BY
photoalignment on the other substrate, aligned in a 90-degree
twisted configuration.
[0049] FIG. 10 shows images of bright (a) and dark (b) state of
reactive mesogen (RM)-stabilized photoaligned LC in 20-.mu.m
diameter microcavities on transmissive substrate, between crossed
polarizers, with image magnification of 50.times..
[0050] FIG. 11A is a graph showing transmission vs. voltage for
various cells polymerized under low and high voltages, and under
low and high UV exposures for 0.9% wt RM in BL006.
[0051] FIG. 11B is a graph showing transmission vs. voltage for
various cells polymerized under low and high voltages, and under
low and high UV exposures for 1.2% wt RM in BL006.
[0052] FIG. 11C is a graph showing transmission vs. voltage for
various cells polymerized under low and high voltages, and under
low and high UV exposures for 1.5% wt RM in BL006.
[0053] FIG. 12A is a set of images showing planar cells with 0.9%
wt (left pair), 1.2% wt (center pair), or 1.5% wt (right pair)
RM257 in BL006 polymerized at 60 Hz 100V.
[0054] FIG. 12B is a pair of images showing hybrid twist cells
filled with BL006 baked in a vacuum oven at 100 C for 45 days
(left) or 7 days (right).
[0055] FIGS. 13A and 13B show RM-stabilized planar cells between
polarized crossed at +45 degrees and -45 degrees, respectively,
after an additional exposure, in the liquid crystal state, to
blue-light polarized at 45 degrees.
DETAILED DESCRIPTION
[0056] As discussed above, conventional photoalignment involves
forming a layer of photo-alignable material, such as a dichroic dye
(a dye that absorbs light anisotropically, such as Brilliant yellow
or another azo dye), on the substrate surface. A thin coating of
the azo dye is placed on the glass or electrode surface, and then
blue polarized light is shined upon it. The polarized light aligns
the azo dye molecules, which tend to be oblong, perpendicular to
the polarization in a semi-permanent position. Unfortunately, azo
dye layers are not stable enough for most applications as they tend
to degrade when exposed to visible light.
[0057] Forming a layer of polymerized reactive mesogen (RM) or
another suitable material over the azo dye layer effectively
increases the photostability of the azo dye layer to create a more
stable alignment layer. This material (e.g., the RM) forms a
polymerized layer which, when polymerized, enforces the existing
liquid crystalline alignment rather than disrupting it. In other
words, acting as an intermediary, the RM aligns with the azo dye
layer, and polymerizing of the RM subsequent fixes this alignment.
The polymerized and aligned RM, in turn, aligns itself with the
liquid crystal material. In other words, polymerizing the RM after
the photoalignment material (azo dye) has been properly aligned
"locks-in" the imposed alignment direction and protects the azo dye
from heat and light exposure. This alignment approach can be
applied after almost all fabrication processing steps and can be
utilized in any application involving cell geometry with minimal
fill-port access.
[0058] Using reactive mesogen in photoaligning the azo dye can be
applied to non-planar surfaces, such as the inner wall surfaces
inside microcavities. Reactive mesogen itself dissolves in liquid
crystal materials at low concentrations, but it can become slightly
immiscible in the base liquid crystal when the RM polymerizes. In
some cases, the process for mixing the reactive mesogen with the
liquid crystal can be controlled such that the reactive mesogen
deposits out of solution onto the microcavity surface(s). When the
RM polymerizes, the polymer network usually agglomerates at the
surface because it is much more concentrated than the bulk liquid
crystal/reactive mesogen mixture; RM, however, has limited
polymerization in the bulk liquid crystal/reactive mesogen mixture
because the mixture is usually diluted. Moreover, photostability
tests (details of which will be described in later sections) have
shown the reactive mesogen on the photoalignment dye layer is very
stable over temperature and exposure compared to samples without
the reactive mesogen.
[0059] RM-stabilized photo-alignment layers can be used in a
variety of emerging photonics applications and devices, including
but not limited to ring resonators, lenses, and photonic crystal
fibers, and uncooled thermal imagers. These imagers comprise high
performance, large format, arrays of thermal imaging pixels to
detect long wavelength infrared (LWIR) light. In this particular
application, aligning the LC material inside micron-sized thermal
imaging pixels can no longer be applicable using conventional
rubbing technique, as it will be exceeding difficult to apply
rubbing alignment technique to any miniature platforms at the
micron scale. Other applications include curved displays, planar
displays, etc. For example, in large-area applications, the azo dye
and RM could be sprayed onto the substrate and illuminated as
described below to align the azo dye and polymerize the RM.
[0060] The following sections describe techniques for creating
photoalignment layers by infiltrating a dissolved photo-definable
dye into microcavities through a single micron-sized opening. Also
presented is a process to stabilize the photoalignment layer by
infiltration into the microcavity of a RM that has been pre-mixed
into host LC materials. The layers generated by the process
disclosed in this application are relatively thin (e.g., <100 nm
thick) and do not exhibit a large degree of light scattering.
I. Stable Photoalignment of Liquid Crystals in Confined
Microcavities
[0061] A technique is described herein for introducing a stable azo
dye photoalignment to confined microcavities with a single
entry/exit port. In this method, the azo dye photoalignment layer
is introduced to the cell and illuminated with polarized light to
form a first alignment layer. A polymer network is then introduced
into the cell in the form of a reactive mesogen. In some cases, the
reactive mesogen is mixed at low concentration with the liquid
crystal, then phase separated to the surfaces and polymerized to
form a layer of polymerized reactive mesogen that aligns the liquid
crystal to the azo dye layer. This simple method offers high
stability against subsequent exposure to both heat and light.
Beneficially, this method also avoids the requirements of strict
process control; both the photoalignment dye and the photoinitiator
for the polymerization process may absorb in the same wavelength
range, in some cases without degradation of the process or decrease
in yield.
[0062] Previously, the infiltration of reactive mesogen (RM) into
the cell along with the liquid crystal has been proposed for
creating customizable pretilt which can be patterned throughout the
cell. However, the RM used to create the pretilt modified a
well-known stable alignment layer (polyimide), not an azo dye
layer, so the RM was not expected to stabilize or improve the
quality of a weak or easily degraded or poor quality alignment
layer.
[0063] Stable alignment has many advantages over previous alignment
methods. These advantages include low cost, simple manufacturing
without the need for expensive and difficult-to-control rubbing
processes, no high temperature bakes that limit substrate material
selection, and the ability to photopattern the alignment axis and
pretilt.
[0064] The process of creating a stable azo dye photoalignment
layer in confined microcavities may begin with the application of
the azo dye layer. A dye solution is prepared in which the azo or
other dichroic dye is mixed into an appropriate solvent at low
concentrations. The microcavities may be fully submerged in this
solution and allowed to soak; this soaking process may provide
sufficient time for the dye solution to fully infiltrate the
cavities, which will depend on both cavity volume and the area of
the entry/exit port. Vacuum-filling of the cavities could also be
used if there is no concern about evaporation of the solvent in
vacuum.
[0065] Next, the microcavity sample is removed from the solution
and residue on outer surface removed. The sample should then be
immediately placed in an oven or on a heat stage at or near the
boiling point of the solvent to force quick evaporation of all
solvent and deposition of a uniform dye layer through the
microcavities. From this point, processing of the photoalignment
layer should continue in the typical fashion; the sample is
irradiated with polarized light of an appropriate wavelength to
effectively align the dye layer.
[0066] A liquid crystal mixture is also prepared containing a low
concentration of reactive mesogen along with a photoinitiator. If
preferred, a thermal initiator may also be used. Appropriate
selection of liquid crystal and reactive mesogen may ensure that
the reactive mesogen in the liquid crystal will phase separate as
desired.
[0067] The mixture is then heated to above the isotropic transition
temperature of the liquid crystal and mixed using either vortex
mixing or sonication. Once mixed, the solution can be introduced
into the cell in any desired manner. The mixture may then be phase
separated, allowing the reactive mesogen to aggregate on the cell
surfaces. This can be done by, e.g., by simply allowing the mixture
time to separate. However, if desired, a low frequency, high
voltage can be used to assist in driving the reactive mesogen to
the cell surfaces. In this case, the liquid crystalline and
reactive mesogen materials may be chosen such that ions in the
solution will preferentially associate with the reactive mesogen
rather than the liquid crystal; the current will assist in driving
those molecules associated with ions to the surface.
[0068] After phase separation, the cell is exposed to an
appropriate wavelength to activate the photoinitiator (or
temperature to activate the thermal initiator). The use of low
intensity for this exposure is recommended to allow slow migration
of the reactive mesogen as the polymer network begins to form and
to avoid any negative effects on the underlying alignment layer.
This polymerization can occur either with or without applied
voltage; the application of voltage results in a liquid crystal
pretilt.
[0069] With a sufficient polymer network formed on the substrate
surfaces, the alignment originally imposed by the photoalignment
layer (the azo dye layer) is locked in by the polymer network (the
polymerized RM layer) with or without additional pretilt. Any
condition which would cause degradation of the photoalignment layer
will now not cause degradation of the liquid crystal alignment in
the cell or microcavities.
II. Photoalignment in Microcavities
Microcavity
[0070] FIG. 1A shows an exemplary microcavity structure 100
disposed in a substrate 110, with inner surfaces 114 and a single
entry/exit port 112. (Other embodiments of the microcavity may have
two or more ports for use as separate entry and ports). The
substrate 110 in FIG. 1A can be any materials, including but not
limited to silicon, silicon oxide, silicon nitride, etc. Depending
on the materials of the substrate 110, the microcavity 100 within
the substrate 110 can be produced using conventional
photolithography techniques including, but not limited to
wet-etching; dry-etching; sputter etching; reactive ion etching
(RIE), including plasma, radio frequency, and deep RIE; vapor-phase
etching; etc. In some embodiments, the shapes of the microcavity
100 can be as followed: the cross-sectional shape of the
microcavity 100 can include circle, oval, triangle, square,
rectangle, trapezium, diamond, rhombus, parallelogram, pentagon,
hexagon, heptagon, octagon, or any other polygonal or 2-dimensional
shapes. Possible volumetric shapes of the microcavity 100 include,
but not limited to rectangular prism (like a match-box type aspect
ratio), triangular prism, pentagonal prism, hexagonal prism, and
any polygonal prism, pyramid, tetrahedron, wedge, cube, sphere,
cone, cylinder, torus, and any possible aspect ratio of ellipsoids
and ellipsoidal dimensions.
[0071] The dimensions of the microcavity 100 can range from about
10 .mu.m to about 1 mm (e.g., about 10 .mu.m, about 15 .mu.m, about
20 .mu.m, about 25 .mu.m, about 30 .mu.m, about 35 .mu.m, about 40
.mu.m, about 45 .mu.m, about 50 .mu.m, about 55 .mu.m, about 60
.mu.m, about 65 .mu.m, about 70 .mu.m, about 75 .mu.m, about 80
.mu.m, about 85 .mu.m, about 90 .mu.m, about 95 .mu.m, about 100
.mu.m, about, 120 .mu.m, about 140 .mu.m, about 160 .mu.m, about
180 .mu.m, about 200 .mu.m, about 250 .mu.m, about 300 .mu.m, about
350 .mu.m, about 400 .mu.m, about 450 .mu.m, about 500 .mu.m, about
550 .mu.m, about 600 .mu.m, about 650 .mu.m, about 700 .mu.m, about
750 .mu.m, about 800 .mu.m, about 850 .mu.m, about 900 .mu.m, about
950 .mu.m, and about 1000 .mu.m).
[0072] Similarly, the size of the port 112 can range from about 1
.mu.m to about 500 .mu.m, depending on the size of the microcavity
100 (e.g., about 1 .mu.m, about 2 .mu.m, about 3 .mu.m, about 4
.mu.m, about 5 .mu.m, about 6 .mu.m, about 7 .mu.m, about 8 .mu.m,
about 9 .mu.m, about 10 .mu.m, about 15 .mu.m, about 20 .mu.m,
about 25 .mu.m, about 30 .mu.m, about 35 .mu.m, about 40 .mu.m,
about 45 .mu.m, about 50 .mu.m, about 55 .mu.m, about 60 .mu.m,
about 65 .mu.m, about 70 .mu.m, about 75 .mu.m, about 80 .mu.m,
about 85 .mu.m, about 90 .mu.m, about 95 .mu.m, about 100 .mu.m,
about, 120 .mu.m, about 140 .mu.m, about 160 .mu.m, about 180
.mu.m, about 200 .mu.m, about 250 .mu.m, about 300 .mu.m, about 350
.mu.m, about 400 .mu.m, about 450 .mu.m, about 500 .mu.m). The
shape of the opening of port 112 (2-dimensional shape) can include
circle, oval, triangle, square, rectangle, trapezium, diamond,
rhombus, parallelogram, pentagon, hexagon, heptagon, octagon, or
any other 2-dimensional shape.
[0073] Since the microcavity 100 is disposed in the substrate 110,
the port 112 of the microcavity 100 can be disposed just about
anywhere on or within the substrate 110, depending on the position
of other layers or components. The port 112 extends between an
inner surface 114 of the microcavity 100 and an outer surface of
the microcavity 100, such as the top surface, the side-wall, or
even the bottom surface (if accessible) of the microcavity 100. The
port 112 can be positioned at the center or off-centered on any of
the surfaces 114. The port 112 can extend perpendicular to the
inner surface 114 or possibly be tilted with respect to inner
surface 114. If the microcavity 100 includes an optional second
port, it can be also located and positioned as described above.
[0074] A microcavity can be etched in a substrate (e.g., silicon,
fused silica, etc.) as follows. A first dielectric material (e.g.
silicon dioxide, silicon nitride, etc.) is deposited on the
substrate to form a layer that is about 50 nm to 300 nm thick.
Next, a sacrificial layer (e.g., molybdenum) with a thickness of
0.5 to 3 microns is deposited on the dielectric layer. A second
dielectric layer (e.g. silicon dioxide, silicon nitride) with a
thickness of about 50 nm to 300 nm is deposited on the sacrificial
layer. A fill hole (e.g., 0.5 to 2 microns square) or array of fill
holes is defined photolithographically in the second dielectric
layer. The second dielectric layer is etched (e.g., with a dry
etch), and the molybdenum sacrificial layer is removed via the fill
hole(s), e.g., with hydrogen peroxide etch, to form one or more
cavities. Then the cavity or cavities are filled with liquid
crystal materials.
Microcavity Filled with Liquid Crystal and Photoalignment
Materials
[0075] FIGS. 1B and 1C show the microcavity 100 in two different
stages of a process for creating a photo-alignment layer for liquid
crystal materials in the microcavity 100. The first stage of the
alignment as shown in FIG. 1B is the microcavity 100 filled with a
mixture 130 of reactive mesogen 140 and liquid crystal material
160. At this stage, the inner surface 114 is at least partially
coated with an azo dye layer 120 and photoaligned prior to the
introducing of the mixture 130 into the microcavity 100. The single
entry/exit port 112 is shown capped with a capping layer 190, which
may be formed by spinning on CYTP (perofluoropolymer) or defined
through photolithography.
[0076] The Azo dye layer 120 includes oblong azo dye molecules
aligned in a particular direction (e.g., into and out of the page).
Suitable materials for the azo dye layer 120 include, but are not
limited to Brilliant Yellow. Without being restrictive, sukphonic
azo dyes are particularly suited for this type of photoalignment.
Other suitable dyes include SD1 and Chrysophenine.
[0077] The Azo dye layer 120 was first photoaligned and the
thickness obtained after alignment ranges from about 1 nm to about
10 nm (e.g., about 1 nm, about 2 nm, about 3 nm, about 4 nm, about
5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, or about 10
nm).
[0078] Likewise, the reactive mesogen 140 can be any reactive
mesogen, including but not limited to RM257, RM84, etc. Similarly,
the liquid crystal 160 used in this experiment is an exemplary
material and it can be any other liquid crystal material including,
but not limited to liquid crystal materials for which the RM is
sufficiently insoluble so as to separate at the substrate surface
(e.g., when not applying a voltage). In this stage, the RM 140 and
the LC 160 are mixed to form the mixture 130, then infiltrated into
the entire microcavity 100. The capping layer 190 can include, but
is not limited to cytop, silicon dioxide, etc.
[0079] The second stage of the photoalignment process as shown in
FIG. 1C is the microcavity 100 filled with the materials shown in
FIG. 1B. In FIG. 1C, however, the RM 140 has been "photo-processed"
to achieve the desired materials properties after certain
processes, and the details of these fabrication processes will be
further described in the following section. More specifically, in
the process stage as shown in FIG. 1C, the RM 140 has been
separated to localize near the interface of the Azo dye layer 120
and polymerized to form a polymerized RM layer 142, which is
aligned to the azo dye layer 120. The thickness of the polymerized
RM layer 142 can range from about 1 nm to about 100 nm (e.g., about
1 nm, about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm,
about 7 nm, about 8 nm, about 9 nm, about 10 nm, about 15 nm, about
20 nm, about 25 nm, about 30 nm, about 35 nm, about 40 nm, about 45
nm, about 50 nm, about 55 nm, about 60 nm, about 65 nm, about 70
nm, about 75 nm, about 80 nm, about 85 nm, about 90 nm, about 100
nm,). The remaining LC material 160 now occupies the rest of the
microcavity 110 and is aligned to the polymerized RM layer 142.
[0080] FIG. 1D is a top view of the microcavity 100, which shows a
top view of the entirety of the microcavity 100 disposed inside the
substrate 110 with the inner most circle representing the single
entry/exit port 112.
[0081] FIG. 1E is another exemplary embodiment of the microcavity
100 in a different environment. Whereas FIGS. 1A-1D show the
microcavity 100 disposed within the substrate 110, FIG. 1E shows an
exemplary embodiment in which the microcavity 100 is supported
above a substrate 115 by several thermal legs 117. The thermal legs
117 provide thermal and electrical isolation of the microcavity 100
from the substrate 115.
Fabrication Process Flow for RM-Stabilized Photoalignment in
Microcavities
[0082] FIG. 2A shows an exemplary fabrication process flow for
using reactive mesogen to stabilize photoalignment in a
microcavity. The first step 200a in the fabrication process is the
creation of a microcavity with a single entry/exit port. Once the
microcavity with a single entry/exit port is obtained, an
anisotropic dye, such as an Azo dye, can be infused into the
microcavity so as to coat the interior surface of the microcavity
with the anisotropic dye in process step 222a. In step 224a, the
anisotropic dye is illuminated with linearly or elliptically
polarized light so as to align the anisotropic dye with respect to
the interior surface of the microcavity. In step 256a, reactive
mesogen and liquid crystal materials are infused into the
microcavity. In step 258a, the reactive mesogen is allowed to
separate from the liquid crystal material. This can be accomplished
by storing the microcavity in the dark (to prevent photodegradation
of the azo dye layer) until the reactive mesogen has accumulated on
the azo dye layer. Step 280a in this fabrication process is to
illuminate the layer of reactive mesogen at a wavelength selected
to cause polymerization of the layer of reactive mesogen material
so as to form a layer of polymerized reactive mesogen between, and
aligned with, the anisotropic dye layer and the liquid crystal
material. In other words, the polymerized reactive mesogen aligns
the liquid crystal material to the anisotropic dye layer.
[0083] FIG. 2B shows another exemplary fabrication process flow for
using reactive mesogen to stabilize photoalignment in a
microcavity. In this process, the first step 200b also starts with
creation of a microcavity with a single entry/exit port. In step
222b, which is similar to step 222a of the process shown in FIG.
2A, once the microcavity with a single entry/exit port is obtained,
an anisotropic dye, such as an Azo dye, is infused into the
microcavity so as to coat the interior surface of the microcavity
with the anisotropic dye. In step 224b, the anisotropic dye is then
illuminated with a polarized light so as to form a layer of
anisotropic dye aligned with respect to the interior surface of the
microcavity. In step 246b, the reactive mesogen is infused into the
microcavity. This step is different from step 256a (FIG. 2A) in
that it includes infusion of the reactive mesogen without any
liquid crystal material whereas step 256a instructs to infuse both
the reactive mesogen and the liquid crystal material. Following
step 246b is step 268b, which involves infusing liquid crystal
material into the microcavity after infusing the reactive mesogen
in the step 246b. Note that since infusing reactive mesogen
separately into the microcavity allows direct localization of RM
onto the underlying anisotropic dye, there is no need to allow the
RM to separate from the LC material. After all the materials have
been infused into the microcavity, the microcavity is illuminated
in step 280b so that the reactive mesogen polymerizes to form an
alignment layer that aligns the liquid crystal to the azo dye
layer.
Infusing Anisotropic Azo Dye Materials in Confined
Microcavities
[0084] FIG. 2C illustrates a process of creating a stable azo dye
photoalignment layer in confined microcavities in greater detail.
The first step 200c in the fabrication process is the creation of a
microcavity with a single entry/exit port. Brilliant Yellow (BY)
dye is mixed into anhydrous N,N-dimethylformamide at 0.5% by
weight. For example, the mixture may be vortexed for one minute to
create a uniform dye solution. The microcavity is then submerged in
this dye solution and allowed to soak, e.g., for 15 minutes (step
222c). Once removed from the dye solution, the top surfaces of the
microcavity are cleaned and immediately baked at 150.degree. C. for
at least 15 minutes to evaporate solvent out of the microcavity
(step 223c). In step 224c, the azo dye is illuminated with blue or
UV light (e.g., with a Royal Blue LED with a central wavelength of
447 nm). The intensity of this light at the sample surface may be
about 50 mW/cm.sup.2 and the irradiation time may be at least 5
minutes.
[0085] FIG. 2D is a plot of the absorption spectrum of Brilliant
Yellow azo dye. Brilliant Yellow has a somewhat wide absorption
spectrum which allows for reorientation utilizing wavelengths
ranging from high UV (such as 365 nm) or blue light, as shown in
FIG. 2D. As a result, the azo dye absorbs relatively strongly in
step 224c of the process shown in FIG. 2C.
[0086] One measure of alignment quality is the order parameter of
the azo dye layer. The order parameter is determined by using a
spectrophotometer to measure the absorption spectra of the dye both
parallel and perpendicular to the polarization axis of the
irradiating light. The maximum absorbances from these spectra,
A.sub..mu. and A.sub..perp. respectively, are then utilized to
calculate the two-dimensional order parameter S.sub.2D according to
EQN. 1:
S 2 D = A || - A .perp. A || - A .perp. . ( EQN . 1 )
##EQU00001##
The absolute value of this order parameter represents the degree to
which the dye is aligned, with values ranging from 0 to 1.0, with
1.0 representing perfect order and 0 representing complete
disorder.
[0087] It may be difficult or impractical to measure absorption
spectra in confined microcavities. However, measuring similar
photoalignment layers prepared utilizing 1% wt Brilliant Yellow in
dimethylformamide (DMF) applied to glass via spin-coating gives
order parameters in the range of 0.8, indicating very strong order
of the dye molecules. And liquid crystals vacuum-filled (in the
isotropic state) into the microcavities prepared with azo dye
layers as shown in FIG. 2C exhibit very strong, uniform dark and
bright states, as shown in FIG. 2E, which suggests that the azo dye
layers in the cavities also have relatively high order parameters
(e.g., about 0.8 or higher). More specifically, FIG. 2E shows
bright (201e and 202e) and dark (203e and 204e) states of
photoaligned LC microcavities displayed between crossed polarizers
which operate in either reflective (201e and 203e) or transmissive
(202e and 204e) mode. Cavity diameter is .about.12 .mu.m for 201e
and 203e (top row), and .about.20 .mu.m for 202e and 204e (bottom
row). Bright and dark images for each in FIG. 2E are taken with
equal exposure.
Infusing Reactive Mesogen in Confined Microcavities
[0088] The fabrication process described in FIG. 2F is an exemplary
processing method for infusing reactive mesogen into microcavities.
Step 256f of the process begins with creating a mixture of reactive
mesogen (e.g., RM257 mixed with 10% wt of photoinitiator, such as
Irgacure 651) in liquid crystal BL006. The RM/photoinitiator
mixture was either 0.9% wt, 1.2% wt, or 1.5% wt in the LC BL006. In
step 257f, this mixture is heated to 125.degree. C., then vortexed
for 3 minutes to create a somewhat uniform mixture. Note that a
1.5% wt of mixture can be used for a 2-.mu.m thick microcavity and
a 3.0% wt of mixture can be used for a 5-.mu.m thick microcavity
(258f). Generally, the percentage of RM should be low enough to
avoid undesired light scattering and high enough so as to stabilize
the surface. After vortexing, the mixture is infused into the
microcavity and allowed to cool. The microcavity is then stored
(e.g., in a dark, airtight container overnight) to allow phase
separation of the RM to the cell surfaces in step 259f of FIG. 2F.
In step 280f, the cells are polymerized by exposure to an
unpolarized Mightex high power UV LED source (.lamda.=365 nm) at
.about.3.5 mW/cm.sup.2. This results in a polymerized RM layer on
the substrate surfaces that is thin enough not to scatter incident
light.
Experimental Assessment of Polymerized Reactive Mesogen Layers
[0089] FIG. 2G shows macroscale liquid crystal cells that include
polymerized RM layers prepared using the process shown in FIG. 2F.
These cells either had a rubbed-polyimide layer on both substrates
and were configured in an untwisted planar orientation or had
rubbed-polyimide on one substrate and a spun 2% wt Brilliant Yellow
photoalignment layer on the other and were configured with a
55-degree twist. This low twist angle is utilized to assure the
handedness of the twist was identical in all cells. Planar cells
are utilized to observe scattering while twist cells are utilized
for stability testing (described below).
[0090] The images shown in FIG. 2G were acquired by placing the
samples directly in front of a camera viewing text on a screen 2
feet away. A picture through the cell of this screen was taken for
each sample and compared to a planar sample filled with pure BL006
liquid crystal material. The degree of scattering in each is
characterized as either major, minor, or none.
TABLE-US-00001 TABLE 1 Scattering grade for each prepared cell. RM
Concentration (%) Scattering? (A/I/N) 0.9 I 0.9 N 0.9 A 1.2 N 1.2 I
1.2 A 1.5 N 1.5 N 1.5 N
[0091] Table 1 shows a number of cells prepared and the degree of
scattering present, with A=major, I=minor, N=none. The evidence of
scattering is mixed in samples prepared with either 0.9% wt or 1.2%
wt RM/LC solution, but there is no scattering in cells prepared
with 1.5% wt RM/LC solution.
[0092] Stability testing of the twisted cells prepared with one
substrate coated with a rubbed polyimide alignment layer and the
other with a spun-on Brilliant Yellow (BY) photoalignment layer was
performed. The BY was applied by mixing the dye at 2.0% wt in DMF,
then vortexing for 30 seconds to create a uniform solution. The
glass was cleaned via ultrasonic and UV/O.sub.3 cleaning just prior
to application of the dye solution, which was passed through a 1
.mu.m filter as it was applied. The entire substrate was coated and
the sample spun at 1500 rpm for 30 s to create an even dye layer
coating. The substrate was then baked at 120 C for 40 minutes to
evaporate any remaining solvent. These substrates were then exposed
to polarized blue light at .about.50 mW/cm.sup.2 for 7 minutes
using the same exposure setup used to expose the microcavity
samples.
[0093] Samples were then assembled in twisted configuration at 55
degrees and filled with BL006 liquid crystal which was either pure
or mixed with RM257 and a photoinitiator then polymerized as
described above.
[0094] For photostability testing, samples were exposed to the same
blue LED setup .lamda..sub.max=447 nm) used to align them, except
they were now irradiated with unpolarized light at either 3
mW/cm.sup.2 or 15 mW/cm.sup.2 (10 mW/cm.sup.2 for pure BL006
samples). Samples containing pure BL006 were exposed for a total of
24 hours while samples containing a polymer-stabilization layer
were exposed for a total of 48 hours. Table 2 shows the total
irradiation dose for each of these exposure conditions.
TABLE-US-00002 TABLE 2 Total irradiation dose for the various
exposure conditions used for photostability trials. Type Intensity
(mW/cm.sup.2) Time (hr) Dose (J/cm.sup.2) Pure BL006 3 24 259.2
Pure BL006 10 24 864 RM/BL006 3 48 518.4 RM/BL006 15 48 2592
[0095] The twisted cell configuration was utilized to provide fast
visual determination of the degradation of photoalignment layers.
When the cell was initially fabricated, the anchoring energies of
the rubbed-polyimide and of the photoalignment layer were similar,
so the twisted configuration was as designed. As the photoalignment
layer degraded, rubbed-polyimide alignment direction dominated in
the cell and the alignment became planar instead of twisted.
[0096] Samples containing pure BL006 showed complete degradation
after the 24-hour exposure period; the cells when viewed between
parallel polarizers exhibit a planar rather than twisted alignment,
as shown in FIG. 2H; this is regardless of the intensity of the
irradiation.
[0097] A total of 46 samples across all RM concentrations were
tested. FIG. 2I shows a representative selection of these samples
after 48 hours of exposure to either 3 mW/cm.sup.2 or 15
mW/cm.sup.2 (shown between parallel polarizers). The cells prepared
with 0.9% wt RM showed degradation after 48 hours, though it is a
significant improvement from the pure BL006 liquid crystals
samples. All samples prepared using either 1.2% wt or 1.5% wt RM
solution remain in a highly uniform twisted configuration; the
polymer layers in these samples are sufficient to offer strong
stabilization of the alignment against subsequent photoexposure.
The twist angles of all samples were measured after this
photoexposure; all cells which remained in a twisted configuration
showed no loss of twist angle, within experimental error.
[0098] For thermal stability testing, similar twist cells were
utilized with rubbed-polyimide on one substrate and BY
photoalignment on the other, prepared alongside the photostability
twist samples. These samples were filled with pure BL006. The
samples were then baked in a vacuum oven at 100.degree. C. for a
total of 2 weeks (about 340 hours). No visible degradation in
alignment or loss in twist angle was observed in these cells after
this baking. The thermal stability of the photoalignment layer on
its own is sufficiently strong.
III. Surface Localization of Reactive Mesogen
Illustrating Surface-Localization
[0099] To illustrate surface localization of reactive mesogen, bulk
cells (on the order of inches) were fabricated with reactive
mesogen (RM84) and measured to estimate the conditions for creating
a thin and stable alignment layer in microcavities. Confocal
microscope images were taken using 0.08% weight concentration of
Fluorescein Dimethacrylate, a dye which selectively associates with
the RM in the test cells (the wavelength used was 460 nm, which is
well absorbed by the dye). This method was used to assess the
effect of mixing on the diffusion of RM to the cell surfaces.
[0100] FIG. 3A shows confocal images of the bulk cells fabricated
with RM monomers and their distribution and concentration of the
dye (and therefore, the RM) along the cell gap direction under
different mixing conditions. For all of the images, the bright
areas represent the more concentrated areas of dye. The top image
shows the monomer distribution only using sonication. The middle
image shows the distribution of monomers after vortexing and
heating, and the bottom image shows the distribution of monomers
without using any particular mixing approach.
[0101] FIGS. 3B-I, 3B-II, and 3B-III show plots of the intensity
variation along a vertical cross-section of each image in FIG. 3A,
indicating the distribution and concentration of the dye in the LC
cell show that, for very weak mixing, RM diffuses out of the LC and
is localized at the surface rather symmetrically. However, both
sonication and no mixing resulted in non-symmetric distribution of
RM to the substrates surfaces (worst in the case of no mixing) and
a lower surface concentration of RM overall (reduction in the
maximum measured dye intensity near the surfaces). Through control
of mixing of the RM into the LC, the RM layer is concentrated on
the surface to provide stabilization of the photo-definable
layer.
Simulated Surface-Localization
[0102] Simulated studies show that the polymer network density
gradient normal to the plane of the cell can affect the
surface-localized polymer layer, and thus affect photoalignment.
The LC director configuration in the cell, given a particular
applied voltage, was determined numerically by utilizing the free
energy density of the system, given by EQN. 2,
f d = k 11 2 ( .gradient. n ^ ) 2 + k 22 2 ( n ^ .gradient. .times.
n ^ ) 2 + k 33 2 ( n ^ .times. .gradient. .times. n ^ ) 2 - W 2 ( n
^ n ^ o ) 2 - 1 2 ( D E ) 2 , ( EQN . 2 ) ##EQU00002##
where k.sub.11, k.sub.22, and k.sub.33 are the splay, twist, and
bend elastic constants of the LC, respectively, D is the electric
displacement, E is the electric field, n is the LC director at a
particular point, n.sub.o is the preferred direction of the
director (at points along the polymer network), and W is the
effective anchoring strength of the LC director in contact with the
polymer (W=0 in regions without polymer). The preferred director,
n.sub.o, is determined by the director orientation at the time of
polymerization, where the orientation is imprinted onto the polymer
network, illustrated in FIGS. 4A-4D and explained below. If the
sample is polymerized (e.g., by exposure to UV light) with no
applied voltage, then the polymer network will lock in a planar
orientation. However, if a voltage is applied during the
polymerization process, then the tilted director configuration will
be locked in, even after the voltage has been turned off.
[0103] The effect of the polymer distribution through the cell was
modeled by making the anchoring parameter, W, effectively
proportional to a polymer distribution given by EQN. 3,
X(z)=X.sub.0(e.sup.-z/.xi.+e.sup.-(d-z)/.xi.), (EQN. 3)
where X.sub.0 is considered as the polymer concentration at the
substrate surface, d is the cell thickness, and .xi. is the length
scale for the decay of the concentration going away from the
surface.
[0104] FIG. 4A shows a microcavity 400 within the substrate 410
filled with photoaligned azo dye (not shown) and a mixture 430 of
RM 440 and LC 460. As shown in FIG. 4A, a thin layer of RM 440 has
localized closer to the inner surfaces 414 of the microcavity 400,
leaving the LC 460 in the bulk of the microcavity 400.
[0105] FIG. 4B shows the microcavity 400 under an applied electric
field 470. In this stage, the LC 460 molecules in the bulk (center)
portion of the microcavity 400 align with the applied polarizing
electric field 470, although the orientation of the LC 460
molecules close to or intermixed with the RM 440 concentrated near
the inner surfaces 414 may remain unchanged. The RM 440 (and
possibly some LC 460) closer to the inner surfaces 414 remains
aligned with the photoaligned azo dye (not shown).
[0106] FIG. 4C shows the microcavity 400 under UV illumination 480.
In this stage, the UV illumination 480 causes the RM 440 molecules
polymerize, forming a polymerized RM layer 442 that locks-in the
orientation of the LC 460 molecules intermixed within its
network.
[0107] FIG. 4D shows the microcavity 400 after the applied electric
field 470 and UV illumination 480 are removed. It shows microcavity
400 with LC 460 aligned to the polymerized RM layer 442.
[0108] FIGS. 5A-5C show simulated dielectric data versus voltage
for different surface concentrations and decay lengths. In FIG. 5A,
at a surface concentration of X.sub.0=0.08 and decay length of
=0.2d, the plot indicates that the polymer network is evenly
distributed throughout the cell. On the other hand, FIG. 5B shows
that for a surface concentration X.sub.0=0.8 and decay length
.xi.=0.02d, the polymer is more concentrated closer to the surface
inside the cell. These plots show the effect of the values of
X.sub.0 and .xi. where the polymer orientation, also equal to
n.sub.o, is determined by the director distribution in the cell
with 10 V RMS applied. Note that these plots show dielectric
constant versus voltage--the dielectric constant was calculated
directly based on the simulated director configuration. It can be
seen that, if the polymer is quite evenly distributed through the
cell, the main effect is to see a shift in the threshold voltage of
the device (the voltage below which no change in dielectric
constant has occurred). However, if the polymer is more
concentrated on the surface, one effect is a shift in the
saturation voltage of the device (the voltage above which the
change in dielectric constant has saturated).
[0109] On the other hand, when a simplified model was used, it was
assumed that the effect of the polymer was restricted to an
infinitesimally thin layer (effectively a monolayer). This thin
layer at the surface that acts as an alignment layer with a pretilt
(no polymer network exists in the liquid-crystal-filled region of
the cell and the polymer interaction term is dropped), the effect
on the dielectric constant vs. voltage curve is simulated as shown
in FIG. 5C. Here, there is little effect on the curve where the
polymer is cured at 0 V. One effect is to lower the zero-volt value
of the capacitance for the case where the cell is cured at high
voltage. The zero-volt value of the capacitance will be related to
the induced pretilt that results from the given value of the
applied voltage, with higher voltages and/or higher polymer
concentrations yielding values of the zero volt capacitance that
are higher, approaching the saturation value with the effective
pretilt of 90 degrees.
Measured Surface-Localization
[0110] Transmission vs. voltage (TV) curves on cells prepared
similarly to the referenced simulations discussed in the previous
section are utilized in the measurements. Phase vs. voltage for
each sample shows similar behavior to the dielectric constant vs.
voltage curves; a change in the zero volt phase indicates an
increase in the pretilt of the sample while a change in the
threshold voltage indicates that the polymer network is not surface
localized and exists in the bulk.
[0111] FIG. 6 shows a liquid crystal cell 600 used to measure the
surface localization of the polymerized RM. For the experiments, 5
.mu.m thick cells with at least a 1 cm.times.1 cm active area were
prepared with rubbed polyimide alignment layers on the substrate
surfaces (each substrate was rubbed in the opposite direction).
Before infiltrating the cells, RM257 was mixed with a
photoinitiator Irgacure 651 in which the photoinitiator was at 10%
concentration by weight. This was then added to the LC BL006 such
that the RM257/photoinitiator was at 1.5% concentration by weight.
This mixture was then heated to 125.degree. C. and vortexed for 3
minutes to produce a uniform mixture. Cells were then infused with
either the 1.5% wt RM257/BL006 mixture or pure BL006. The RM/LC
cells were then stored overnight in dark conditions to allow for
separation of the RM to the substrate surfaces. Next, these samples
were polymerized using 20 minutes of exposure to about 3.5
mW/cm.sup.2 UV light (.lamda.=365 nm) provided by a Mightex
collimated UV LED light source. The cell was either polymerized at
0V or 20V (1 kHz AC).
[0112] Once cells were polymerized, TV curves were obtained by
placing the sample between two polarizers. TV curves were taken
with both crossed and parallel polarizers; in both cases, the cell
was oriented with the alignment at 45 degrees to the input
polarizer. A broadband Oriel fiber optic illuminator was used as a
light source and an interference filter (2\, =633 nm) was utilized
to produce monochromatic light. To neglect transmission losses for
phase calculations, the TV curve was adjusted so that the maximum
and minimum transmission through the cell (i.e., detected voltage)
were taken to be equivalent to a transmission of 1 or 0,
respectively.
[0113] To produce a plot more comparable to the CV curves, each set
of TV curves was further converted into a phase retardation vs.
voltage profile. This utilizes the fact that the transmitted
intensity between crossed polarizers is given by EQN. 4,
I .perp. = I o ( sin .delta. 2 ) 2 , ( EQN . 4 ) ##EQU00003##
with .delta. being the phase retardation of the LC sample. This
transmitted intensity between parallel polarizers is similarly
given by EQN. 5,
I || = I o ( cos .delta. 2 ) 2 . ( EQN . 5 ) ##EQU00004##
The phase retardation of the sample at a particular voltage, then,
is given by the transmission ratio in these two plots, as
|.delta.|=N.pi.+2 tan.sup.-1 {square root over
(I.sub..perp./I.sub..parallel.)}, N=0,2,4, . . . , (EQN. 6)
or
|.delta.|=(N+1).pi.-2 tan.sup.-1 {square root over
(I.sub..perp./I.sub..parallel.)}, N=1,3,5, . . . , (EQN. 7)
where N is the peak number in the TV curve (counted up from the
high-voltage end of the curve). When the cell is almost completely
switched, N=0.
[0114] FIG. 7 shows the phase retardation vs. voltage for the
prepared cells. In this case, the sample polymerized at 0 V shows
no significant differences from the cell filled with pure
LC--neither the threshold voltage nor the saturation voltage is
noticeably different. This indicates that the RM layer is
sufficiently thin, e.g., on the order of several hundred nanometers
or less, so as to have no effect on the bulk LC. In the case of the
sample polymerized at 20 V, though, the zero volt retardation has
dropped and the threshold voltage has also decreased, indicating
that the pretilt of the cell has increased. These results are very
similar to the case of an infinitesimally thin RM layer, shown in
FIG. 5C, indicating that the RM layer is quite thin.
IV. Photostability Testing
[0115] To test the ability of the polymer layer to stabilize the
alignment generated using a photoalignment layer, additional 7
.mu.m cells were constructed in which one substrate was coated with
a rubbed polyimide alignment layer and the other was coated with a
spun-on BY photoalignment layer. The BY was applied to the glass by
mixing the dye at 2% concentration by weight into DMF, then
vortexing for 1 minute to create a uniform solution. The glass was
cleaned via ultrasonic and UV/O3 cleaning just prior to the
application of the dye solution, which was passed through a 1 .mu.m
filter as it was applied. The entire substrate was coated and the
sample was spun at 1500 rpm for 30 seconds to create an even dye
layer coating. The substrate was then baked for 120.degree. C. for
40 minutes to evaporate any remaining solvent.
[0116] Test samples, once assembled, were exposed to about 50
mW/cm.sup.2 polarized blue light for 7 minutes using the same
exposure setup described above for microcavities. This exposure was
incident on the back of the photoaligned substrate and with the
polarization direction aligned with the rubbed polyimide alignment
direction. This exposure results in an approximately 90-degree
twist with the photoalignment direction perpendicular to the rubbed
alignment.
[0117] This twisted cell configuration provides for fast visual
determination of the degradation of alignment. When the cell is
initially fabricated, the anchoring energies of the
rubbed-polyimide and of the photoalignment layer are both
sufficiently strong, so the twisted LC director configuration is
observed. If the photoalignment layer is rewritten to a new angle,
then the twist angle through the cell will change. If the
photoalignment layer is degraded, the rubbed polyimide alignment
direction will dominate and the cell will lose its twisted director
configuration completely. The director field in the cell will then
be co-planar and aligned with the axis determined by the polyimide.
When viewed between crossed polarizers, twisted regions will appear
bright while non-twisted planar regions will appear dark. When
viewed between parallel polarizers, non-twisted planar regions will
appear bright while twisted regions will appear dark.
[0118] The samples were filled with either pure BL006 liquid
crystal or the same 1.5% wt RM257/BL006 mixture as described above,
with storage and polymerization at 0V occurring as previously
described. For photostability testing, samples were exposed to the
same blue LED used to align them. In one case, samples were exposed
to 20 mW/cm.sup.2 polarized light at 45 degrees to the original
photoexposure direction; this approximates the highest level of
illumination expected on the thermal pixels. In another case,
samples were exposed to unpolarized light of 120 mW/cm.sup.2. In
this case, unpolarized light was used so as to simulate flux five
times higher than utilized in the thermal pixel application.
[0119] FIG. 8A shows that samples containing pure BL006 liquid
crystal showed a complete loss of their original photoalignment
direction in the low intensity polarized exposure. As shown in FIG.
8A, samples are filled with pure BL006 (a) before exposure or after
(b) 50 minutes, (c) 100 minutes, (d) 150 minutes, and (e) 200
minutes of exposure to about 20 mW/cm.sup.2 polarized blue light
oriented at 45 degrees to the photoalignment axis. Before images
are shown between crossed (left) and parallel (right) polarizers.
After images are shown between parallel (left) and crossed (center)
polarizers as well as polarizers oriented at 45 degrees (right).
For all samples, the alignment was rewritten within the first 50
minutes of exposure. Between polarizers oriented at 45 degrees (the
newly written twist angle of the cell), the sample exhibits a
somewhat dark twist state. However, as the sample is exposed for
longer, even this alignment is lost, with the sample failing to
twist light at all. The photoalignment layer has been completely
degraded and the intended alignment of the sample has been
lost.
[0120] Samples with pure BL006 liquid crystal also showed a rapid
degradation of their photoalignment layer in the high intensity
unpolarized case. These samples exhibited planar alignment after
less than 20 minutes of exposure to this condition. On the other
hand, samples which contained the RM-stabilization layer exhibited
a high degree of stability. In both the low intensity polarized
condition and the high intensity unpolarized condition, samples
maintained their 90-degree twisted alignment for 3 weeks with no
sign of degradation in their alignment, as shown in FIG. 8B.
[0121] The electro-optic response of these samples was also
considered. In this case, a sample filled with pure BL006 which had
not been exposed, as well as the sample filled with 1.5% wt RM257
in BL006 which had been exposed to about 120 mW/cm.sup.2
unpolarized light for 3 weeks, were utilized. Samples were placed
between crossed polarizers with the entrance and exit LC director
aligned with the entrance and exit polarizer, respectively. TV
curves for the samples taken in this configuration are shown in
FIG. 9. There is no significant difference in the electro-optic
response of these two cells indicating that, not only does the RM
layer have little to no effect on this response, but also that this
TV response remains stable against extended light exposure.
V. Demonstrating RM-Stabilized Photoalignment in Microcavities
[0122] In this section, the previous results are confirmed by
demonstrating RM-stabilized photoalignment in microcavities. A
microcavity sample on transmissive substrate was prepared as
described above, with the LC mixed with 3% wt RM257 (with 10% wt
Irgacure 651) in BL006 liquid crystal which had been vortexed for 3
minutes just prior to filling the microcavity. Note that the
concentration of the RM was increased to 3% wt in this case because
the microcavity samples had a thickness of 2-.mu.m rather than the
5-.mu.m thick cells utilized in Section III. Samples were then
stored and polymerized at 0 V as described in Section III. FIG. 10
shows this sample on the microscope between crossed polarizers at
fifty times in both the bright state and the dark state. Again,
this sample exhibits uniform alignment. Using these images to
calculate the intensity of a cavity in both its bright and dark
states results in a contrast ratio of 24:1, which could be
increased.
Experimental Verification of RM Polymerization and RM
Photostability
[0123] A number of liquid crystal cells were prepared to verify
polymerization of the RM layer or to test the photostability of
cells with this polymerization. The cells were fabricated
simultaneously; from the cleaning and cutting of the glass through
to the fabrication of individual cells, all steps were conducted in
a single day to keep substrates as clean as possible. Cells were
either planar cells with rubbed-polyimide on both substrates or
twist cells with rubbed-polyimide on one side and photoalignment on
the other. The range of thicknesses for the planar cells were from
10-20 .mu.m. For the twist cells, the thicknesses range from 6-15
.mu.m.
[0124] Some cells were filled with pure BL006 liquid crystal. The
rest were filled with mixtures of RM257 reactive mesogen (with
.about.10% photoinitiator Irgacure 651) with BL006 liquid crystal
with the reactive mesogen mixed at 0.9% wt, 1.2% wt, or 1.5% wt.
This results in a range of RM layer thicknesses on the substrate
surfaces. After filling, all cells were stored in the dark for at
least 24 hours to allow for phase separation of the RM. Then, cells
were polymerized by UV light for 10 minutes; in the case of twist
cells, this exposure occurred from the photoaligned side of the
cell.
[0125] For this investigation, the planar cells were polymerized
under a number of different conditions. Cells were either
polymerized at 0 V or with a 60 Hz 100 V AC voltage applied. In the
case of the high voltage, the liquid crystals in the bulk of the
cell should be homeotropic (normal to the cell surface). Molecules
directly next to the substrate should remain planar; the planar
alignment should decay rapidly from the cell surface. In the region
with polymer, this decaying alignment will be locked in by the
polymerization. Once the voltage is turned off, the liquid crystal
orientation on the boundary between the polymerized region and the
bulk liquid crystal will become the effective pretilt of the liquid
crystal cell and will be carried through the bulk. By comparing
cells polymerized at 100V with those cured at 0V, one should gain a
general understanding of the thickness and uniformity of the
polymer layer within the cell. Additionally, two different exposure
intensities were used, controlled by varying the distance of the
sample from the UV source. These intensities are referred to below
as "low" or "high" intensity.
[0126] After the cells were polymerized, each was placed between
parallel polarizers with the principle axis of the cell oriented at
45 degrees from the polarizer transmission axis. The transmission
through the system (of 439.5 nm light) was measured at applied
voltages from 0 to 5V. The plots of transmission versus voltage for
a number of these cells are shown in FIGS. 11A-11C. The cell was
slightly tilted during the measurement; this results in curves of
small amplitude, with the thickness through the cell lacking in
uniformity across the measurement region.
[0127] These curves provide an indication of the quality of the
polymer layer. In particular, a lower threshold voltage indicates a
higher pretilt angle, which should appear in any cell exposed at
100 V. In all but one cell exposed at 0 V, the threshold appears to
be somewhere between 1.5 V and 2.0 V. The threshold is somewhat
similar for all of the 100 V cells with 0.9% wt RM and for the 100V
cell with 1.2% wt RM which was exposed to "low" intensity UV.
Without being bound to any particular theory, this suggests that
the polymer network in the 0.9% wt cells is either too thin or
insufficient to create any sort of pretilt. The lack of pretilt in
the "low" intensity cell with 1.2% wt RM suggests that the network
in these cells is now sufficiently strong, but the "low" intensity
exposure condition does not completely polymerize the layer
(perhaps due to a lack of absorption by the lower concentration of
photoinitiator). In the cells with 1.5% wt RM, the 100 V cells
exhibit minimal threshold, indicating a high pretilt angle as is
expected; this concentration produces a sufficiently thick and
uniform layer.
[0128] The differences among these cells can also be seen through
visual inspection. In this case, the cell is placed between
parallel polarizers, again with principle axis at 45 degrees to the
transmission axis of the polarizers. Cells are viewed normal or
tipped up to almost normal between the polarizers, as shown in
FIGS. 12A-12B. An example cell from each RM concentration is shown;
where all cells shown were polymerized at 100V. The brighter areas
in the tipped-up orientation indicate a pretilt. The 0.9% wt cell
shows no significant change in coloration, suggesting little to no
pretilt. The 1.2% wt cell shows a slight increase in coloration
while the 1.5% wt cell shows a significant increase in coloration.
These results are all in agreement with the results shown in the TV
curves.
[0129] The concentration of RM affects the ability to create a
uniform polymer layer on the surfaces. While a polymer layer may
exist in the cells with 0.9% wt, it is not sufficient to create a
pretilt when polymerized at high voltage. The polymer layer begins
to become sufficient at 1.2% wt and is well established at 1.5% wt.
Based on these results, cells utilized in the photostability
investigation were polymerized with the "high" intensity UV to
create the strongest possible polymer layer at each of the given
concentrations.
[0130] For another investigation, the hybrid twist cells were used,
with photoalignment on one substrate and rubbed-polyimide on the
other. Cells were polymerized at the "high" intensity UV discussed
in the previous section. To test the photostability of these cells,
they were exposed to unpolarized blue light (447 nm) at either 3
mW/cm.sup.2 or 15 mW/cm.sup.2 (10 mW/cm.sup.2 for the case of cells
with pure BL006). Three different exposure setups were used; two of
these utilized a Tri-Star LED with a foil diffuser while the third
utilized a single LED with collimator/diffuser. Two cells were
exposed to each LED at a time (one at the high intensity, one at
the low intensity). To maintain reliability of results, cells for
each RM concentration were exposed using each of these three
exposure stations.
[0131] FIGS. 13A and 13B show RM-stabilized planar cells between
polarized crossed at +45 degrees and -45 degrees, respectively,
after an additional exposure, in the liquid crystal state, to
blue-light polarized at 45 degrees. The images show that the cells
transmit light, which indicate that the upper and lower cell
surfaces align the liquid crystal material in different directions.
To create these cells, each RM-stabilized cell was placed between
crossed polarizers in the liquid-crystalline state (this may not
work if the liquid crystal is isotropic). Each cell was then
exposed to additional blue light polarized at 45 degrees with
respect to the original photoalignment direction.
CONCLUSION
[0132] A technique to generate a stable alignment utilizing a
photodefinable dye and a surface-localized polymer layer has been
described herein. This alignment technique is especially useful for
LC applications in uniquely challenging geometry, including
microcavities in photonic devices like LC thermal imagers. It has
been successfully shown that a non-degrading photoalignment layer
can be infused into these fully fabricated microcavities.
[0133] A low cost, robust liquid crystal alignment layer whose
alignment direction and stabilization can be done after a cell or
cavity is created, is demonstrated. The method can be used even if
the only one entry point to the cavity is available. The procedure
does not require any special coating processes such as spin
coating, and does not require a high temperature bake or the
difficult rub process needed for the common polyimide alignment
layers.
[0134] One aspect of the disclosed methods is the stabilization of
a photoaligned azo dye layer with an ultrathin reactive mesogen
that layer that forms without special process steps. Surprisingly,
a very small amount of reactive mesogen, mixed with the liquid
crystal, may have a very significant effect on the stability of the
azo dye layer. It has been demonstrated that this
surface-polymer-stabilized photoalignment layer exhibits incredibly
high resilience to light exposure (and is thermally stable even
without the polymer-stabilization layer).
[0135] The methods described herein have a number of benefits.
Stable photoalignment layers may be prepared exclusively using
commercially available materials, without complicated or expensive
process steps. Additionally, the robust photoalignment layer
created with polarized light exposure is able to survive subsequent
photoexposure for the polymerization of the reactive mesogen layer.
Thus, the methods reduce the necessity for strict process control
and can even allow for the use of the same exposure setup for both
the patterning of the alignment layer, and the polymer
stabilization of it.
[0136] The polymer-stabilization layer can be introduced into the
microcavities by mixing it with the LCs at low weight
concentration. A polymer layer introduced into a cell in this
manner is able to naturally localize in a thin region near the
substrate surfaces. This layer significantly improves the
robustness of the alignment against subsequent light exposure,
regardless of any degradation of the underlying photoalignment
layer. The alignment process described in this here offers
versatile ways to expand the field of LC photonic devices.
[0137] While various inventive embodiments have been described and
illustrated herein, those of ordinary skill in the art will readily
envision a variety of other means and/or structures for performing
the function and/or obtaining the results and/or one or more of the
advantages described herein, and each of such variations and/or
modifications is deemed to be within the scope of the inventive
embodiments described herein. More generally, those skilled in the
art will readily appreciate that all parameters, dimensions,
materials, and configurations described herein are meant to be
exemplary and that the actual parameters, dimensions, materials,
and/or configurations will depend upon the specific application or
applications for which the inventive teachings is/are used. Those
skilled in the art will recognize, or be able to ascertain using no
more than routine experimentation, many equivalents to the specific
inventive embodiments described herein. It is, therefore, to be
understood that the foregoing embodiments are presented by way of
example only and that, within the scope of the appended claims and
equivalents thereto, inventive embodiments may be practiced
otherwise than as specifically described and claimed. Inventive
embodiments of the present disclosure are directed to each
individual feature, system, article, material, kit, and/or method
described herein. In addition, any combination of two or more such
features, systems, articles, materials, kits, and/or methods, if
such features, systems, articles, materials, kits, and/or methods
are not mutually inconsistent, is included within the inventive
scope of the present disclosure.
[0138] The above-described embodiments can be implemented in any of
numerous ways. For example, embodiments of designing and making the
technology disclosed herein may be implemented using hardware,
software or a combination thereof. When implemented in software,
the software code can be executed on any suitable processor or
collection of processors, whether provided in a single computer or
distributed among multiple computers.
[0139] Further, it should be appreciated that a computer may be
embodied in any of a number of forms, such as a rack-mounted
computer, a desktop computer, a laptop computer, or a tablet
computer. Additionally, a computer may be embedded in a device not
generally regarded as a computer but with suitable processing
capabilities, including a Personal Digital Assistant (PDA), a smart
phone or any other suitable portable or fixed electronic
device.
[0140] Also, a computer may have one or more input and output
devices. These devices can be used, among other things, to present
a user interface. Examples of output devices that can be used to
provide a user interface include printers or display screens for
visual presentation of output and speakers or other sound
generating devices for audible presentation of output. Examples of
input devices that can be used for a user interface include
keyboards, and pointing devices, such as mice, touch pads, and
digitizing tablets. As another example, a computer may receive
input information through speech recognition or in other audible
format.
[0141] Such computers may be interconnected by one or more networks
in any suitable form, including a local area network or a wide area
network, such as an enterprise network, and intelligent network
(IN) or the Internet. Such networks may be based on any suitable
technology and may operate according to any suitable protocol and
may include wireless networks, wired networks or fiber optic
networks.
[0142] The various methods or processes (e.g., of designing and
making the technology disclosed above) outlined herein may be coded
as software that is executable on one or more processors that
employ any one of a variety of operating systems or platforms.
Additionally, such software may be written using any of a number of
suitable programming languages and/or programming or scripting
tools, and also may be compiled as executable machine language code
or intermediate code that is executed on a framework or virtual
machine.
[0143] In this respect, various inventive concepts may be embodied
as a computer readable storage medium (or multiple computer
readable storage media) (e.g., a computer memory, one or more
floppy discs, compact discs, optical discs, magnetic tapes, flash
memories, circuit configurations in Field Programmable Gate Arrays
or other semiconductor devices, or other non-transitory medium or
tangible computer storage medium) encoded with one or more programs
that, when executed on one or more computers or other processors,
perform methods that implement the various embodiments of the
invention discussed above. The computer readable medium or media
can be transportable, such that the program or programs stored
thereon can be loaded onto one or more different computers or other
processors to implement various aspects of the present invention as
discussed above.
[0144] The terms "program" or "software" are used herein in a
generic sense to refer to any type of computer code or set of
computer-executable instructions that can be employed to program a
computer or other processor to implement various aspects of
embodiments as discussed above. Additionally, it should be
appreciated that according to one aspect, one or more computer
programs that when executed perform methods of the present
invention need not reside on a single computer or processor, but
may be distributed in a modular fashion amongst a number of
different computers or processors to implement various aspects of
the present invention.
[0145] Computer-executable instructions may be in many forms, such
as program modules, executed by one or more computers or other
devices. Generally, program modules include routines, programs,
objects, components, data structures, etc. that perform particular
tasks or implement particular abstract data types. Typically the
functionality of the program modules may be combined or distributed
as desired in various embodiments.
[0146] Also, data structures may be stored in computer-readable
media in any suitable form. For simplicity of illustration, data
structures may be shown to have fields that are related through
location in the data structure. Such relationships may likewise be
achieved by assigning storage for the fields with locations in a
computer-readable medium that convey relationship between the
fields. However, any suitable mechanism may be used to establish a
relationship between information in fields of a data structure,
including through the use of pointers, tags or other mechanisms
that establish relationship between data elements.
[0147] Also, various inventive concepts may be embodied as one or
more methods, of which an example has been provided. The acts
performed as part of the method may be ordered in any suitable way.
Accordingly, embodiments may be constructed in which acts are
performed in an order different than illustrated, which may include
performing some acts simultaneously, even though shown as
sequential acts in illustrative embodiments.
[0148] All definitions, as defined and used herein, should be
understood to control over dictionary definitions, definitions in
documents incorporated by reference, and/or ordinary meanings of
the defined terms.
[0149] The indefinite articles "a" and "an," as used herein in the
specification and in the claims, unless clearly indicated to the
contrary, should be understood to mean "at least one."
[0150] The phrase "and/or," as used herein in the specification and
in the claims, should be understood to mean "either or both" of the
elements so conjoined, i.e., elements that are conjunctively
present in some cases and disjunctively present in other cases.
Multiple elements listed with "and/or" should be construed in the
same fashion, i.e., "one or more" of the elements so conjoined.
Other elements may optionally be present other than the elements
specifically identified by the "and/or" clause, whether related or
unrelated to those elements specifically identified. Thus, as a
non-limiting example, a reference to "A and/or B", when used in
conjunction with open-ended language such as "comprising" can
refer, in one embodiment, to A only (optionally including elements
other than B); in another embodiment, to B only (optionally
including elements other than A); in yet another embodiment, to
both A and B (optionally including other elements); etc.
[0151] As used herein in the specification and in the claims, "or"
should be understood to have the same meaning as "and/or" as
defined above. For example, when separating items in a list, "or"
or "and/or" shall be interpreted as being inclusive, i.e., the
inclusion of at least one, but also including more than one, of a
number or list of elements, and, optionally, additional unlisted
items. Only terms clearly indicated to the contrary, such as "only
one of" or "exactly one of," or, when used in the claims,
"consisting of," will refer to the inclusion of exactly one element
of a number or list of elements. In general, the term "or" as used
herein shall only be interpreted as indicating exclusive
alternatives (i.e. "one or the other but not both") when preceded
by terms of exclusivity, such as "either," "one of," "only one of,"
or "exactly one of" "Consisting essentially of," when used in the
claims, shall have its ordinary meaning as used in the field of
patent law.
[0152] As used herein in the specification and in the claims, the
phrase "at least one," in reference to a list of one or more
elements, should be understood to mean at least one element
selected from any one or more of the elements in the list of
elements, but not necessarily including at least one of each and
every element specifically listed within the list of elements and
not excluding any combinations of elements in the list of elements.
This definition also allows that elements may optionally be present
other than the elements specifically identified within the list of
elements to which the phrase "at least one" refers, whether related
or unrelated to those elements specifically identified. Thus, as a
non-limiting example, "at least one of A and B" (or, equivalently,
"at least one of A or B," or, equivalently "at least one of A
and/or B") can refer, in one embodiment, to at least one,
optionally including more than one, A, with no B present (and
optionally including elements other than B); in another embodiment,
to at least one, optionally including more than one, B, with no A
present (and optionally including elements other than A); in yet
another embodiment, to at least one, optionally including more than
one, A, and at least one, optionally including more than one, B
(and optionally including other elements); etc.
[0153] In the claims, as well as in the specification above, all
transitional phrases such as "comprising," "including," "carrying,"
"having," "containing," "involving," "holding," "composed of," and
the like are to be understood to be open-ended, i.e., to mean
including but not limited to. Only the transitional phrases
"consisting of" and "consisting essentially of" shall be closed or
semi-closed transitional phrases, respectively, as set forth in the
United States Patent Office Manual of Patent Examining Procedures,
Section 2111.03.
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