U.S. patent application number 10/395824 was filed with the patent office on 2004-01-15 for holographically-formed polymer dispersed liquid crystals with multiple gratings.
Invention is credited to Bowley, Christopher C., Crawford, Gregory P., Faris, Sadeg M., Fontecchio, Adam K., Li, Le, Lin, Jaujeng.
Application Number | 20040008391 10/395824 |
Document ID | / |
Family ID | 23577544 |
Filed Date | 2004-01-15 |
United States Patent
Application |
20040008391 |
Kind Code |
A1 |
Bowley, Christopher C. ; et
al. |
January 15, 2004 |
Holographically-formed polymer dispersed liquid crystals with
multiple gratings
Abstract
A multicolored reflection liquid crystal display device includes
a pair of substrates having a reflective holographic polymer
dispersed liquid crystal (H-PDLC) film disposed therebetween. The
H-PDLC film contains at least two different reflection gratings
capable of reflecting two different wavelengths of light. A
multicolored reflection H-PDLC is obtained by simultaneously
illuminating a plurality of regions of a film comprised of a
mixture of a liquid crystal and a photo-polymerizable monomer with
a plurality of holographic light patterns capable of providing
liquid crystal layers of different spacings so as to obtain
different reflection gratings in each of the regions. A mask is
placed between each of the laser light beams and the film to form a
pattern of light and dark regions on the film. Each mask is
positioned such that at least one light region of a first beam pair
coincides with at least one dark region of a second beam pair
within the film. A multiple grating liquid crystal display device
including an H-PDLC film having a first region comprising liquid
crystal and matrix polymer layers forming a transmission grating
and a second region comprising liquid crystal and matrix polymer
layers forming a reflection grating capable of reflecting a
preselected wavelength of light also is described.
Inventors: |
Bowley, Christopher C.;
(Woodbury, MN) ; Fontecchio, Adam K.; (Medfield,
MA) ; Lin, Jaujeng; (Yorktown Heights, NY) ;
Crawford, Gregory P.; (Providence, RI) ; Faris, Sadeg
M.; (Pleasantville, NY) ; Li, Le; (Yorktown
Heights, NY) |
Correspondence
Address: |
Reveo, Inc.
85 Executive Boulevard
Elmsford
NY
10523
US
|
Family ID: |
23577544 |
Appl. No.: |
10/395824 |
Filed: |
March 24, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10395824 |
Mar 24, 2003 |
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09398964 |
Sep 16, 1999 |
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6538775 |
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Current U.S.
Class: |
359/3 |
Current CPC
Class: |
G03H 1/0248 20130101;
G03H 2223/12 20130101; G03H 1/0236 20130101; G02B 5/32 20130101;
G02B 5/203 20130101; G03H 2260/12 20130101; G03H 2223/18 20130101;
G03H 2260/33 20130101; G03H 1/04 20130101; G03H 1/28 20130101; G02F
1/13342 20130101; G03H 2001/2231 20130101; G03H 1/265 20130101 |
Class at
Publication: |
359/3 |
International
Class: |
G03H 001/02 |
Claims
What is claimed is:
1. A single layered multispectral reflection liquid crystal display
device, comprising: a pair of substrates having a reflective
holographic polymer dispersed liquid crystal (H-PDLC) film disposed
therebetween, the H-PDLC film comprised of liquid crystal and
matrix polymer layers forming a plurality of different reflection
gratings capable of reflecting and/or diffracting a plurality of
different wavelengths of energy.
2. The display of claim 1, wherein the substrate comprises
ITO-coated glass or plastic.
3. The display of claim 1, wherein the H-PDLC comprises three
different reflection gratings capable of reflecting three different
wavelengths of energy.
4. The display of claim 1 or 3, wherein the different reflection
gratings spatially overlap a region of the film.
5. The display of claim 1 or 3, wherein the different reflection
gratings are spatially non-overlapping.
6. The display of claim 1, wherein the different reflection
gratings are arranged in a preselected pattern.
7. The display of claim 1 or 3, wherein the film comprises
spectrally non-overlapping reflection gratings.
8. The display of claim 7, wherein the spectrally non-overlapping
reflection gratings are selected to reflect wavelengths of light
from the group consisting of red-green-blue and
cyan-magenta-yellow.
9. The display of claim 7, wherein the reflection gratings further
are spatially non-overlapping.
10. The display of claim 1, wherein the reflection gratings are
selected to provide a broadband reflection.
11. The display of claim 10, wherein the reflection gratings are
spectrally overlapping.
12. The display of claim 1, wherein different wavelengths of energy
are any desired wavelength of the visible spectra.
13. The display of claim 1, wherein different wavelengths of energy
are any desired wavelength of the IR spectrum.
14. A method of making a multicolored reflective liquid crystal
display, comprising: simultaneously illuminating a film comprised
of a mixture of a liquid crystal and a photo-polymerizable monomer
with a plurality of holographic light patterns capable of providing
liquid crystal layers of different spacings so as to obtain
different reflection gratings in said film.
15. The method of claim 14, wherein said holographic light pattern
is obtained by providing at least two pairs of laser light beams,
each said beam pair incident on said film at a different angle to
form an optical interference pattern associated with reflection of
a different wavelength of energy.
16. The method of claim 14, wherein said holographic light pattern
is obtained by providing laser light of a different wavelength,
each said laser light forming an optical interference pattern
associated with reflection of a different wavelength of energy.
17. The method of claim 15, wherein the step of illuminating the
film comprises illuminating a plurality of regions of the film,
each said region illuminated with laser beam pairs with different
angles of incidence.
18. The method of claim 17, further comprising: placing a mask
between each of said laser light beams and said film, each said
mask forming a pattern of light and dark regions on said film and
each said mask positioned such that at least one light region of
said first beam pair coincides with at least one dark region of
said second beam pair within the film; and illuminating the film
whereby photo-polymerization of the monomer takes place and
formation of polymer and liquid crystal layers occurs.
19. The method of claim 14, wherein at least two different gratings
are introduced into the film in a single illumination step.
20. The method of claim 14, wherein power of said light beams is
substantially equal.
21. The method of claim 17, wherein said mask is of a grid pattern
having transparent and opaque grid squares.
22. The method of claim 17, wherein two beam pairs are used and a
film comprising reflection gratings of two different wavelengths is
obtained.
23. The display of claim 17, wherein three beam pairs are used and
a film comprising reflection gratings of three different
wavelengths is obtained.
24. The method of claim 14 or 17, wherein the method provides
spectrally non-overlapping holographic elements.
25. The method of claim 14 or 17, wherein the method provides
spectrally overlapping holographic elements.
26. The method of claim 14 or 17, wherein the method provides
spatially non-overlapping holographic elements.
27. The method of claim 14, wherein the method provides spatially
overlapping holographic elements.
28. The method of claim 14, wherein illumination of the film by a
selected beam pair is blocked for a portion of the illumination
time.
29. An apparatus for preparation of a multicolored reflective
liquid crystal display, comprising: means for supporting a film
comprised of a mixture of liquid crystal and a photo-polymerizable
monomer; a laser source; means for producing at least two pairs of
laser light beams from said laser source, each said beam pair
capable of directing light onto a film housed in the supporting
means at a different angle to form an optical interference pattern
within a film associated with reflection of a different wavelength
of energy; and a mask disposed between each of said laser light
beams and said supporting means, each said mask forming a pattern
of light and dark regions on a film housed in the supporting means
and each said mask positioned such that at least one light region
of said first beam pair coincides with at least one dark region of
said second beam pair within a film.
30. The apparatus of claim 29, further comprising: switchable
shutters disposed between a laser beam pair and the film to block
light of a beam pair from illuminating said film.
31. A method of making a holographic polymer dispersed liquid
crystal having multiple gratings, comprising: providing a film
comprised of a mixture of liquid crystal and a photo-polymerizable
monomer and having a first and second opposing surfaces; and
illuminating the film with at least three beams of laser light,
wherein at least one beam is incident on the first opposing surface
of the film and at least one beam is incident of the second
opposing surface of the film, and having at least one region in
which three laser beams overlap to form a transmission grating the
three-beam overlapping region upon photopolymerization of the
monomer.
32. The method of claim 31, wherein multiple reflection gratings
further are formed in the three-beam overlapping region.
33. The method of claim 31, wherein the intensity of light incident
on the first opposing surface of the film is approximately equal to
the intensity of light incident on the second opposing surface.
34. The method of claim 31, wherein each laser beam is incident on
the film at a different angle.
35. A multiple grating polymer dispersed liquid crystal display
device, comprising: a pair of substrates having a holographic
polymer dispersed liquid crystal (H-PDLC) film disposed
therebetween, the H-PDLC film comprised of a first region
comprising liquid crystal and matrix polymer layers forming a
transmission grating capable of transmitting a fixed wavelength of
light and a second region comprising liquid crystal and matrix
polymer layers forming multiple reflection gratings capable of
reflecting a fixed wavelength of energy.
36. The display of claim 35, wherein the device further includes a
plurality of spatially overlapping reflection gratings in said
first region.
37. A method of making a holographic polymer dispersed liquid
crystal having reflection in the infrared band, comprising:
providing a film comprised of a mixture of liquid crystal and a
photo-polymerizable monomer and having a first and second opposing
surfaces; optically coupling the first and second opposing surfaces
of the film with a pair of prisms which bend light at an angle to
shift its wavelength into the infrared; and illuminating the
coupled film/prism arrangement with a holographic light pattern
capable of providing liquid crystal layers of spacings on the order
of infrared band so as to obtain an infrared reflecting reflection
grating in said film.
38. A method of making a holographic polymer dispersed liquid
crystal having reflection in the infrared band, comprising:
providing a film comprised of a mixture of liquid crystal and a
photo-polymerizable monomer and having first and second opposing
surfaces; optically coupling the first opposing surface of the film
with a prism which bends light at an angle to shift its wavelength
into the infrared; optically coupling the second opposing surface
of the film with a mirror, said mirror positioned at an angle with
respect to the second opposing surface of the film; and
illuminating the coupled film/prism arrangement with a laser beam,
wherein the transmitted light reflects back out at a preselected
angle from its original incident path to create an interference
pattern on the sample and thereby obtain an infrared reflecting
reflection grating.
39. The method of claim 37 or 38, wherein said film is
simultaneously illuminated with a plurality of holographic light
patterns, each said pattern capable of producing a reflection
grating associated with a different wavelength.
40. The method of claim 37 or 38, wherein the prism is selected
from the group consisting of a 45.degree.-right angle prism and a
30.degree.-60.degree.-90.degree. prism.
41. An apparatus for preparation of an infrared reflecting
holographic polymer dispersed liquid crystal display, comprising:
means for supporting a film comprised of a mixture of liquid
crystal and a photo-polymerizable monomer; a laser source; means
for producing a pair of laser light beams from said laser source,
said beam pair capable of directing light onto a film housed in the
supporting means at an angle to form an optical interference
pattern within a film associated with reflection of a wavelength of
light; and a prism optically coupled to said film supporting means
for bending light from said laser source into the angles necessary
to shift the laser wavelength into the near infrared.
42. The apparatus of claim 41, further comprising: a mask disposed
between each of said laser light beams and said supporting means,
each said mask forming a pattern of light and dark regions on a
film housed in the supporting means and each said mask positioned
such that at least one light region of said first beam pair
coincides
43. The apparatus of claim 41, further comprising a second prism
optically coupled to the opposing surface of the supporting
means.
44. The apparatus of claim 41, further comprising a mirror
positioned at the opposing surface of the supporting means to
reflect recreate the holographic pattern.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to holographically-formed polymer
dispersed liquid crystals (H-PDLCs). In particular, the invention
relates to multiple grating reflective displays using H-PDLC
technology. The invention also relates to H-PDLCs having multiple
reflection and transmission gratings.
[0002] Polymer dispersed liquid crystals (PDLCs) in their
conventional form consist of micrometer-sized liquid crystal (LC)
droplets dispersed in a rigid polymer matrix. PDLCs are typically
formed using phase-separation or emulsification methods.
Photo-polymerization induced phase separation utilizes a mixture of
a low molecular weight liquid crystal and a photo-curable monomer.
Irradiation of the homogeneous pre-polymer mixture initiates
polymerization, which in turn induces a phase separation between
the polymer and liquid crystal. The result is a liquid crystal
phase separated into droplets and immobilized in a rigid polymer
matrix.
[0003] FIG. 1A illustrates a conventional PDLC formed by phase
separation of a liquid crystal phase from a matrix polymer phase.
The entire LC-monomer film is photopolymerized and phase separation
occurs randomly throughout the film and results in LC droplets on
the order of microns. In the zero-voltage state, the symmetry axis
of the droplets is randomly oriented and there is a mismatch of the
index of refraction between the matrix polymer and the LC droplets.
This condition results in a strongly light scattering (opaque)
appearance. By matching the ordinary refractive index of the liquid
crystal with that of the matrix polymer, a transparent appearance
is achieved when sufficient voltage is applied to reorient the LC
droplets. Thus, conventional PDLC displays are capable of switching
between an opaque off-state and a transparent on-state, but do not
have inherent ability to display color.
[0004] Reflective liquid crystal displays have been developed which
rely on PDLC materials and, more recently, holographic or optical
interference preparative techniques have been used to carry out
polymerization to selectively positioned regions of liquid crystal
and polymer. Planes of liquid crystal droplets are formed within
the sample to modulate the LC droplet density on the order of the
wavelength of light. On exposure to an optical interference
pattern, typically formed by two coherent lasers, polymerization is
initiated in the light fringes. A monomer diffusion gradient is
established as the monomer is depleted in the dark fringes, causing
migration of liquid crystal to the dark fringes. The result is
LC-rich areas where the dark fringes were located and essentially
pure polymer regions where the light fringes were located.
[0005] The resulting optical interference pattern reflects at the
Bragg wavelength, .lambda.=2nd sin .theta., where n is the average
index of refraction, .theta. is the angle between the substrate and
viewing direction, and d is the Bragg layer spacing. The
interference pattern can be selected to form Bragg gratings which
can reflect any visible light. In the "off state", that is, with no
applied voltage, the LC directors are misaligned and light of the
Bragg wavelength is reflected back to the observer. Upon
application of an applied voltage, the "on state", the device
becomes transparent. The reflection intensity is electrically
controlled by changing the effective refractive index of the LC
droplet planes with an applied voltage. If the refractive index of
the LC droplet planes (n.sub.LC) is different from that of the
polymer planes (n.sub.p), light of a specific wavelength is
reflected by the periodic modulation in the refractive index. If
n.sub.LC is equal to n.sub.p, the periodic refractive index
modulation disappears and the incident light is transmitted. If the
LC has a positive dielectric anisotropy and the ordinary refractive
index n.sub.o is approximately equal to n.sub.p, the reflection
intensity decreases with increasing applied voltage. This results
in a material transparent at all wavelengths and all incident light
is transmitted.
[0006] Displays incorporating these materials have been reported in
"Holographically formed liquid crystal/polymer device for
reflective color displays" by Tanaka et al. in Journal of the
Society for Informational Display (SID), Volume 2, No. 1, 1994,
pages 37-40; and also in "Optimization of Holographic PDLC of
Reflective Color Display Applications" in SID '95 Digest, pages
267-270 (1995). In each of the reported H-PDLC displays, however,
reflection gratings capable of reflecting only a single wavelength
of light were created. See, Tanaka et al. in U.S. Pat. No.
5,748,272.
[0007] A major interest in the display industry is the creation of
full color reflective displays. U.S. Pat. No. 5,875,012 to Crawford
et al. describes a full-color liquid crystal device incorporating
three single-color stacked reflective H-PDLCs, which can be
activated alone or in combination to provide a broad spectrum of
color. Although this configuration results in high reflection
efficiencies, it is complicated to fabricate and requires
sophisticated electrical drive schemes.
[0008] Date et al. in "Three-Primary-Color Holographic Polymer
Dispersed Liquid Crystal (H-PDLC) Devices for Reflective Displays"
(Proceedings of the 15.sup.th International Display Research
Conference, Hamamatsu, Japan, 1995; p. 603) report single exposure
films of different color. A red, a green and a blue reflecting
H-PDLC are reported formed using a single laser source, in which
the different reflection gratings were obtained at different
incident angles from different H-PDLC layers. Date also reported
the use of prisms to obtain the appropriate cross angles for longer
wavelengths of light. Using this technique, a full color reflective
display can only be built by stacking three H-PDLC layers that
individually reflect at red, green or blue wavelengths. There were
no multiple grating films made from a single layer H-PDLC to
reflect multiple colors.
[0009] There is a need to provide a single layer H-PDLC with
multiple reflective gratings for constructing a reflective display
device that can have a range of colors. Such displays are desirable
due to their simplified configuration and because they are
sufficiently reflective at low power and in normal operating
environments.
[0010] Lastly, multiple Bragg gratings in display panels and other
devices are desired because specular reflections off of multiple
gratings within the layer would increase the operative viewing
angle and improve the quality of the reflected image. There is
currently no method which provides such capability in the prior
art.
[0011] Creating a near infrared reflecting H-PDLC is a difficult
task to accomplish due to the large wavelength shift required to
create Bragg gratings in the near infrared band (.about.1000 nm)
using light in the visible range. The use of visible light lasers
to fabricate IR H-PDLCs is attractive for a variety of reasons. The
beam is visible with the unaided eye which simplifies alignment and
fabrication; and IR photoinitiators, needed for the
polymer-initiated phase separation of the H-PDLC, are not readily
available or are not developed to a point sufficient for use in
this application.
[0012] Unfortunately, at the incident angles required to form the
infrared interference pattern, the glass surface is highly
reflective and very little of the light passes through the
supporting glass into the LC-monomer layer. Furthermore, some of
the light that does enter the layer is in the form of multiple
reflections which wash out the interference pattern.
[0013] There is a need to provide an infrared reflective modulating
device and a method for obtaining an infrared reflective device
that addresses the problems and limitations of the prior art.
SUMMARY OF THE INVENTION
[0014] The present invention provides advances and improvements in
the manufacture of H-PDLC compositions. The use of simultaneous,
coherent multiple laser beam exposure has been exploited to provide
multiple grating liquid crystal devices from a single layer
H-PDLC.
[0015] In one aspect of the invention, a multicolored reflection
liquid crystal display device is provided from a single layer
configuration having a pair of substrates having a reflective
holographic polymer dispersed liquid crystal (H-PDLC) film disposed
therebetween. The H-PDLC film includes liquid crystal and matrix
polymer layers which form a reflection grating capable of
reflecting a wavelength of light, wherein the H-PDLC film includes
at least two different reflection gratings capable of reflecting
two different wavelengths of light. The substrate may be made up of
ITO-coated glass or plastic.
[0016] In preferred embodiments, the H-PDLC comprises three or more
different reflection gratings capable of reflecting three different
wavelengths of light. The reflection gratings may be superimposed
on the same area of the film, or they may be located in specific
regions of the film, so as to form holographic elements patterns,
i.e., spatial overlap or non-overlap, respectively.
[0017] In preferred embodiments, the display may provide
holographic elements having non-overlapping reflectance spectra,
i.e. spectral non-overlap. The display may be capable of reflecting
three primary wavelengths of light, i.e., red, blue and green,
cyan-magenta-yellow, or any other combination.
[0018] In other preferred embodiments, the reflection gratings are
selected to provide a broadband reflection in which the holographic
elements have overlapping reflectance spectra. The reflected
wavelengths of light are any desired wavelength, and in particular
are of the visible energy range and IR energy range.
[0019] In other preferred embodiments, the reflected light is of
substantially equal intensity. Regions of different reflection
gratings may be arranged in an array or may be arranged to produce
a preselected pattern.
[0020] In another aspect of the invention, a method of making a
multicolored reflective liquid crystal display is provided, in
which a film comprised of a mixture of a liquid crystal and a
photo-polymerizable monomer are simultaneously illuminated with a
plurality of holographic light patterns capable of providing liquid
crystal layers of different spacings so as to obtain different
reflection gratings in each of the regions. A plurality of regions
may be illuminated with different holographic light patterns.
[0021] In preferred embodiments, the holographic light pattern is
obtained by providing at least two pairs of laser light beams, each
beam pair incident on the film at a different angle to form an
optical interference pattern associated with reflection of a
different wavelength of light. Additional beam pairs, e.g., three
or more are contemplated.
[0022] In another preferred embodiment of the invention, the
holographic light pattern is obtained by providing laser light of a
different wavelength, each laser light forming an optical
interference pattern associated with reflection of a different
wavelength of light.
[0023] In other preferred embodiments, a mask is placed between
each of the laser light beams and the film, and each mask forms a
pattern of light and dark regions on the film. Each mask is
positioned such that at least one light region of the first beam
pair coincides with at least one dark region of the second beam
pair within the film, and the film is illuminated whereby
photo-polymerization of the monomer takes place and formation of
polymer and liquid crystal layers occurs. This gives rise to
spatially non-overlapping holographic elements.
[0024] In other embodiments, at least two different gratings are
introduced into the film in a single illumination step, or the
power of the light beams is substantially equal. In other
embodiments, the mask is of a grid pattern having alternating
transparent and opaque grid squares, or other patterns of
transparent and opaque regions. In some embodiments, the grid
squares are on the order of about 25 mm.sup.2 or less, and
preferably about 9 mm.sup.2 or less; however, much smaller sizes
are contemplated as within the scope of the invention. In addition,
shapes other than grid squares may be used, such as, rectangular or
circular shapes and the like.
[0025] In one embodiment, two beam pairs are used and a two-color
display is obtained; and in other embodiments, three beam pairs are
used and a three-color display is obtained. The method may provide
films having a plurality of spectrally non-overlapping
reflectances, such as red, blue and green, or cyan, magenta and
yellow. Alternatively, the method may provide films having a
plurality of spectrally overlapping reflectances, which gives rise
to broadband reflectance.
[0026] In yet another aspect of the invention, illumination of the
film by a selected beam pair or pairs is blocked by a shutter for a
portion of the exposure time of the film. Shuttering may be used to
shorten or lengthen the exposure of one beam pair with respect to
the other beam pairs.
[0027] In another aspect of the invention, an apparatus for
preparation of a multicolored reflective liquid crystal display
includes means for supporting a film comprised of a mixture of
liquid crystal and a photo-polymerizable monomer; a laser source;
means for producing at least two pairs of laser light beams, each
beam pair capable of directing light onto a film housed in the
supporting means at a different angle to form an optical
interference pattern within a film associated with reflection of a
different wavelength of light; and a mask disposed between each of
the laser light beams and the supporting means, each mask forming a
pattern of light and dark regions on a film housed in the
supporting means and each mask positioned such that at least one
light region of the first beam pair coincides with at least one
dark region of second beam pair within a film. The apparatus may
further include shutters disposed between the laser source and the
film for preferentially blocking illumination from one or more beam
pairs.
[0028] In still another aspect of the invention, a method of making
a holographic phase dispersed liquid crystal having multiple
gratings, includes providing a film comprised of a mixture of
liquid crystal and a photo-polymerizable monomer capable of phase
separation of the liquid crystal upon polymerization and having
first and second opposing surfaces; and illuminating the film with
at least three beams of laser light, wherein at least one beam is
incident on the first opposing surface of the film and at least one
beam is incident on the second opposing surface of the film, and
having at least one region in which three laser beams overlap,
whereby upon photo-polymerization of the monomer and phase
separation of the liquid crystal/polymer, a transmission grating
and two reflection gratings are formed in the three-beam
overlapping region. Beam arrangements employing a greater number of
beams and resulting in a greater number of gratings also are
contemplated.
[0029] In a preferred embodiment, the film further has at least one
region in which two beams overlap and a reflection grating is
formed in the two-beam overlapping region. Beam arrangements
employing a greater number of beams and resulting in a greater
number of gratings also are contemplated.
[0030] The intensity of light incident on the first opposing
surface of the film may be unequal or may be approximately equal to
the intensity of light incident on the second opposing surface,
depending upon the desired outcome.
[0031] In other preferred embodiments, the laser beam is incident
on the film at a different angle.
[0032] In another aspect of the invention, a multiple grating
liquid crystal display device includes a pair of substrates having
a holographic polymer dispersed liquid crystal (H-PDLC) film
disposed therebetween. The H-PDLC film includes a first region
comprising liquid crystal and matrix polymer layers forming a
transmission grating and a plurality of reflection gratings and at
least one second region comprising liquid crystal and matrix
polymer layers forming a reflection grating capable of reflecting a
preselected wavelength of light. The transmission grating typically
exhibits Bragg diffraction (when the spacing between the LC droplet
layers is on the order of the wavelength of incident light), but
may also exhibit other behavior, such as for example, Raman Nath
diffraction (when the spacing between the LC droplet layers is
greater than the wavelength of incident light).
[0033] It is also within the scope of the invention to combine a
plurality of aforementioned single layer H-PDLC films having
multiple spectral gratings into a single display device.
Definitions
[0034] "Bragg grating" means periodically repeating layers of a
polymer and liquid crystal (LC) which form LC planes having a
spacing that satisfy the grating equation, 1 = 2 n sin ( / 2 ) ( 1
)
[0035] where .lambda. is the wavelength of the incident laser
light, n is the average index of refraction of the holographic
medium, and .psi. is the angle between the interfering beams. When
the light source and the observer are on the same side of the
holographic film, the grating is known as a reflection grating.
When the light source and the observer are on opposite sides of the
holographic film, light is diffracted upon transmission through the
holographic film and the grating is known as a transmission
grating.
[0036] "Holographic technique", "holography", "holographic light",
as those terms are used herein refer to the formation of
interfering light patterns in a three dimensional space.
[0037] "Holographic element" refers to the smallest spectrally
distinct element of a display, i.e., the smallest region having a
homogenous grating. The holographic element may be defined by one
or more electrodes, e.g., multiple gratings may be homogeneously
superimposed over a region of the film, however, each of the
gratings may switch between on- and off-states at different
potentials. In those instances where the holographic element is
defined by a single electrode, the holographic element is also a
"pixel", i.e., the smallest switchable element of the device.
[0038] "Spatially overlapping" and "spatially non-overlapping"
refer to the location of the grating on the film. When the gratings
are non-overlapping, a single grating occupies a defined region of
the film and does not share the region with other gratings (other
than minor and unintentional overlap due to improper
alignment).
[0039] "Spectrally overlapping" and "spectrally non-overlapping"
refer to the separation between two reflectance peaks. Reflectance
peaks are considered non-overlapping if two adjacent spectra do not
overlap at full width at half maximum (FWHM).
[0040] When referring to spectral reflectance and wavelength, it is
understood that the peak wavelength represents the peak centered
around a peak maximum. Width of the fill peak may vary, but
typically is in he range of 20 nm (FWHM) for single grating
peaks.
BRIEF DESCRIPTION OF THE DRAWING
[0041] The invention is described with reference to the following
drawings, which are provided for the purpose of illustration only
and which are in no way limiting of the invention, and in
which:
[0042] FIGS. 1A-C are schematic views illustrating a conventional
PDLC (1A); a reflective H-PDLC (1B); and a transmission H-PDLC
(1C);
[0043] FIG. 2A is a cross-sectional view of a multicolored
reflective H-PDLC display of the invention in which the holographic
elements are spatially separated;
[0044] FIG. 2B is a cross-sectional view of a multicolored
reflective H-PDLC display of the invention in which the holographic
elements are spatially superimposed to reflect multiple wavelengths
from a common region;
[0045] FIG. 2C is a cross-sectional view of a multicolored
reflective H-PDLC display of the invention in which the holographic
elements are angularly multiplexed;
[0046] FIG. 3 is a schematic view illustrating the method and
apparatus used in the production of a multicolor reflective
H-PDLC;
[0047] FIG. 4 is a schematic view illustrating beam arrangement in
the method used in the production of a multicolor reflective
H-PDLC;
[0048] FIG. 5 is an illustration of a shadow mask used to create a
two-color pixellated H-PDLC;
[0049] FIG. 6A is a top view of a multicolor reflective H-PDLC
display with periodic and repeating holographic elements, and 6B is
a top view of a multicolor reflective H-PDLC display with
holographic elements arranged into a preselected design;
[0050] FIG. 7 is a schematic view illustrating one method and
apparatus used in the production of an infrared reflecting
H-PDLC;
[0051] FIG. 8 is a schematic view illustrating one method and
apparatus used in the production of an infrared reflecting
H-PDLC;
[0052] FIG. 9 is a schematic view illustrating the method and
apparatus used in the production of a multiple grating H-PDLC;
[0053] FIG. 10A is a diagram shown the regions formed using a
three-beam apparatus, and 10B the resultant gratings;
[0054] FIG. 11A is a schematic illustration of a four-beam overlay
arrangement; and 11B is an illustration of another alternative
arrangement of laser beams in practice of the method of the
invention;
[0055] FIG. 12 is a color photograph of a blue-green holographic
element reflective H-PDLC;
[0056] FIG. 13 is a voltage response curve for the blue-green
holographic element H-PDLC of Example 1;
[0057] FIG. 14 is a color photograph of a three-color RGB
holographic element reflective H-PDLC;
[0058] FIG. 15 is a graph of reflectance vs. wavelength which
illustrates spectral reflectance of substantially equal intensity
from a three holographic element sample;
[0059] FIG. 16 is a graph of reflection efficiency vs. wavelength
which illustrates peak broadening for a temporally exposed
blue-green sample;
[0060] FIG. 17 is a graph of reflection efficiency vs. wavelength
which illustrates peak broadening for a temporally exposed
blue-green sample; and
[0061] FIG. 18 are reflectance spectra for different regions of a
multigrating film of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0062] The invention centers on techniques for creating H-PDLCs
having new and useful properties. FIG. 1 shows three different
configurations of polymer dispersed liquid crystals (PDLC) which
capitalize on the phase separation of liquid crystal and evolving
polymer during polymerization. FIG. 1A, as previously discussed,
illustrates a conventional PDLC.
[0063] H-PDLCs are phase separated compositions formed under
holographic conditions. Instead of random arrangement of LC
droplets, the holographic exposure induces a periodic array of LC
droplets and matrix polymer planes, as shown in FIGS. 1B and 1C.
Upon illumination with holographic light, the monomer diffuses to
high light intensity regions where it polymerizes. The liquid
crystal remains in the dark regions and phase separates into small
droplets on the order of nanometers, e.g., 10-200 nm, in ordered,
stratified layers. The actual phase-separated morphology varies
dependent upon the particular liquid crystal and the relative
composition of the liquid crystal and matrix polymer used. For
lower liquid crystal concentrations, spherical or ellipsoidal LC
droplets are localized in stratified layers and are completely
surrounded by matrix polymer. At higher liquid crystal
concentrations, connectivity between the LC droplets may be
observed.
[0064] The devices can reflect as shown in FIG. 1B or diffract as
shown in FIG. 1C for various wavelengths depending upon the layer
spacing. The coherent scattering occurs as either a reflected or a
diffracted wavefront depending on the orientation of the grating.
This, in turn, is dependent upon the beam geometry during phase
separation.
[0065] Reflective H-PDLCs are attractive candidates for reflective
display applications. They exhibit narrow wavelength bands of high
reflection efficiency, which can be controlled by electric fields.
Due to the random nematic director alignment within the LC-rich
plane of the "off-state", these materials are not polarization
dependent and hence all polarization states can be used for a
brighter display.
[0066] It is particularly attractive to include multiple reflection
gratings in a single film. Multiple reflection grating H-PDLC may
be used, by way of example, for full color displays and broadband
spectral reflectance devices. The intristic Bragg peaks from a
conventional H-PDLC are typically very narrow (FWHM <20 nm),
making them excellent candidates for full color displays. By
incorporating multiple reflection gratings capable of reflecting
light of particular wavelengths, for example, the primaries red,
green and blue, full-color displays are obtained. Alternatively,
multiple reflection gratings centered at wavelengths very close to
one another in the spectral range results in a broadening of the
bandwidth and a broadband reflector is obtained. Finally, such
multiple reflection gratings can increase the viewing angle of the
display.
[0067] An exemplary reflective H-PDLC 20 is shown in FIG. 2A, in
which the reflection gratings are spatially separated. An H-PDLC
layer 22 is supported by opposing glass or plastic substrates 24,
26. The substrates are preferably coated with a conductive layer
28, such as indium tin oxide (ITO). A region 30 has a reflection
grating (i.e., a periodic layering of LC droplet layers 31 in a
polymer matrix 32) with a spacing d.sub.1. Adjacent to this is a
region 33 having a different reflection grating with a spacing
d.sub.2. Adjacent to this is a region 34 having a different
reflection grating with a spacing d.sub.3. All light incident on
the surface of the layer will be transmitted with the exception of
the wavelength which satisfies the Bragg equation for the
particular d-spacing of the reflection grating. Thus, region 30
reflects light of wavelength .lambda..sub.1, whereas region 33
reflects light having a wavelength .lambda..sub.2 and region 34
reflects light having a wavelength .lambda..sub.3.
[0068] The reflection gratings may be located in discreet regions
in the layer with little or no overlap of the individual reflection
gratings. These domains within the H-PDLC are hereafter referred to
as "holographic elements". In one embodiment, the holographic
elements are located adjacent to one another, without significant
spatial overlap, so as to maximize color density and variety in the
layer, while maintaining color purity. When each of the holographic
elements reflects only a single wavelength of light and relies on a
single electrode to switch the grating, such elements may be
considered pixels in the conventional meaning of the term.
[0069] The reflection gratings may be selected to reflect any
wavelength of light. For example, region 30 may be reflective of
light having a center wavelength in the range of 610-650 nm (e.g.,
red light) and region 33 may be reflective of light having a center
wavelength in the range of 520-560 nm (e.g., green light).
Additional regions 34 may be included which are reflective of light
in other ranges, such as light having a center wavelength in the
range of 440-480 nm (e.g., blue light). Such films are considered
to contain spectrally, as well as spatially, non-overlapping
gratings.
[0070] The reflection gratings may be selected to reflect light
centered at similar wavelengths of light. For example, region 30
may be reflective of light having a center wavelength in the range
of 610 nm; region 33 may be reflective of light having a center
wavelength in the range of 630 nm; and regions 34 may be reflective
of light having a center wavelength in the range of 650 nm. The
result is a broadband reflectance centered around light in the red
range of the visible spectrum. Such films contain spectrally
overlapping gratings.
[0071] Alternatively, the reflection gratings may be spatially
overlapped, that is, two or more reflection gratings occupy a
common region of the film, to reflect a broad bandwidth light. Such
an exemplary reflective H-PDLC 40 is shown in FIG. 2B, in which
similar elements are similarly labeled. The grating associated with
region 30 may, for example, reflect at 610 nm, while those
associated with regions 33 and 34 reflect at 630 nm and 650 nm,
respectively. All three gratings are superimposed within a region
42 to reflect a broad band light from 610 to 650 nm. If more
gratings are overlapped, a broader reflection bandwidth is
generated. Such a configuration also helps in increasing the
viewing angle of the display device, in that multiple gratings have
different angles of observation with respect to the surface of the
film may be incorporated into a single holographic element, yet
still reflect substantially the same color (as detected by the
human eye).
[0072] Alternatively, the reflection gratings may be spatially
overlapping in that they occupy a common region of the film, and
yet not be superimposed upon one another. Such an exemplary
reflective H-PDLC 50 is shown in FIG. 2C, in which similar elements
are similarly labeled. Both gratings occupy the same region 52 of
the film; however, different reflection gratings are located at
different depths within the film, as measured as the distance from
either surface towards the center of the film. The grating
associated with region 54 at the outermost layers of the film
(indicated as d.sub.54) may, for example, reflect at 610 nm, while
that associated with region 56 (indicated as d.sub.56) may reflect
at 480 nm. A film having this novel arrangement of reflection
gratings is believed to be made in practicing the temporal
multiplexing method of the invention, as is discussed in greater
detail below.
[0073] According to a method of the invention, a multiple grating
reflective H-PDLC layer advantageously is prepared in a single
step, thereby greatly simplifying the method of manufacture.
Production of multiple gratings may be accomplished by
simultaneously illuminating a precursor layer containing a
photocurable monomer and a liquid crystal with two or more
holographic light patterns capable of producing LC layers of
different d-spacings.
[0074] The light source used in producing different reflection
gratings may be light of the same wavelength which illuminate the
sample surface at different angles of incidence (varying the value
of .theta. in the grating eq 1). Alternatively, different
reflection gratings may be obtained illuminating the prepolymer
layer with light of different wavelengths (varying the value of
.lambda. in the grating eq 1). While effective, the latter approach
is less desirable due to the added cost and complexity of using
multiple lasers.
[0075] In a preferred embodiment, a single laser source is used.
The beam is split into the appropriate number of beams, which are
directed so that pairs of light beams interfere so as to produce
the holographic light patterns used to create different reflection
gratings within the sample. The crossing point of each laser beam
pair is positioned and arranged so that a monomer-LC layer may be
exposed to multiple holographic patterns in a single exposure.
Multiple reflection gratings in a single layer are obtained
thereby.
[0076] The method and apparatus is described with reference to FIG.
3. A laser light source 100 generates light of a predetermined
wavelength and optionally is then passed through a beam expander
and spatial filter 102. The resultant laser beam 104 is split into
the number of beam pairs required for the particular application.
Shown in FIG. 3, beam 104 is split first using a beam splitter 106
into beams 108, 110, which is further split at beam splitters 112,
114 into beams 116, 118 and 120, 122, respectively. With the
additional use of mirrors 124, 126, the laser beams are crossed to
create a holographic light pattern. A sample 128 is located at the
crossover points of beam pairs.
[0077] Beam pairs are aligned so that a first beam pair 118, 120 is
incident at one angle, .theta..sub.1 and the second beam pair 116,
122 is incident at a second angle, .theta..sub.2, relative to the
plane of the sample surface, as shown in FIG. 4. Additional laser
beams are used to create as many additional holographic patterns as
are desired for a particular display application. If it is desired
to produce three reflection gratings, the beam is split into six
beams. In preferred embodiments, the beams incident on the sample
are of equal intensity, that is having {fraction (1/4)}th or
{fraction (1/6)}th the power of the original laser in preparing
samples having two or three colors, respectively. It is observed
that light of equal intensity forms holographic light of higher
grating contrast leading to more efficient reflection gratings.
[0078] The sample is exposed to light for a short time, typically
in the range of 20-60 seconds. The exposure time strongly depends
on laser power (intensity), the choice of monomer, dye and liquid
crystal, as well as the relative concentrations of the materials.
The area of the sample illuminated by the holographic light formed
by the first beam pair will produce a reflection grating different
from that of the area of the sample illuminated by the holographic
light formed by the second beam pair, thereby simultaneously
producing areas having different reflection gratings in a single
film. Spectral reflectance of each holographic element may be
adjusted to produce whatever ratio of color mixing is desired.
[0079] In the above description, the dimensions of the reflection
gratings produced upon exposure are dependent on the beam area
incident on the film and is preferentially used when preparing
spatially overlapping reflection gratings. The area may be varied
with the use of beam expanders and condensers to increase or
decease the area of light incidence. However, control over the
shape of the reflection grating (e.g., circular vs. rectangular)
and the relative positions of different reflection grating regions
(holographic elements) is difficult. In a preferred embodiment when
preparing spatially non-overlapping, holographic elements, a shadow
mask 130 is used to select the dimensions and positions of
holographic elements. Shadow mask 130 is inserted between the beams
and the sample. The mask permits only light of a preselected
pattern (size and shape) to pass through and illuminate the
film.
[0080] An exemplary mask 130 is shown in FIG. 5, in which dark
regions 142 represent gaps in the mask through which light passes.
The masks are arranged so that light from beams 118, 120 pass
through gaps 142 and coincide at the sample surface. Light from the
other two beams 116, 122 also form overlapping light patterns at
the sample surface. Adjacent regions of different reflection
gratings may be produced by positioning the two sets of masks such
that the light of a second mask set illuminates the dark regions
produced by the first mask set.
[0081] The masks may provide any desired pattern. In one embodiment
shown in FIG. 6A, the holographic elements are arranged in a
predetermined order. In the top view of a reflective display,
regions 30, 33 and 34 are arranged in a periodic and repeating
array. Holographic elements of a particular reflection grating may
be evenly distributed throughout the film. This is particularly
desirable when a full-color display is desired having reflected
primary light. The holographic element arrays may be selectively
activated to provide any color combination and thereby obtain a
full color display.
[0082] Another embodiment is shown in FIG. 6B in which holographic
elements of different reflection gratings may be grouped together
to generate a particular design or geometry. In the top view of a
reflective display, regions 160 shown in dark hatching may be of
one reflection grating while open regions 162 are of another. In
still other preferred embodiments, the reflection grating may be
located in the film in combination with transmission gratings. In
this embodiment, therefore, regions of transparence may be combined
with regions of reflection to provide a desired display appearance.
These and other variations will be apparent to one familiar with
the display art.
[0083] The masks may provide any desired size of holographic
element. Holographic elements may be prepared having dimensions in
the range of those required of high resolution displays. Technology
for mask design and alignment sufficient to produce high resolution
displays is available in the art.
[0084] The angle of light incidence as described in FIG. 4 is
adjusted to provide reflected light of the desired wavelength. In
those instances where light of significantly different wavelengths
is desired, the angles of incidence vary greatly. For example, the
angle of incidence to produce reflected red, green and blue light
may vary by about 30.degree., e.g., .theta..sub.1, .theta..sub.2
and .theta..sub.3 are 90.degree., 60.degree., and 30.degree.,
respectively. In contrast, where light of similar wavelengths is
desired, the angles of incidence vary only slightly. For example,
the angle of incidence to produce a broadband reflector may vary by
as little as 5.degree. or less.
[0085] It has been observed that ideal exposure times differ for
different incident angles of laser light. In particular, it has
been observed that red angle gratings are of a poor quality due to
the low angle of incident light and waveguiding in the glass mask.
This difference in spectral reflectance quality may be minimized
varying the amount of time each laser beam is incident on the film
surface. The quality of red angle gratings is improved by delaying
exposure of the film to red angle laser light.
[0086] By way of example for a red-green-blue display, a shutter is
disposed between the laser beam pairs and the film of the "red
angle" laser beam, that is the laser beam incident on the film at
an angle to produce red reflected light. The beam is shuttered for
a short time, e.g., 1-2 seconds, while the remaining laser beam
pairs illuminate the film. This reduces the amount of time the film
is exposed to the red angle laser light. After 1-2 seconds the
shutter is opened and the film is fully illuminated by all three
beam pairs. Such a methodology is referred to as "temporal
multiplexing" because it provides the ability for multiple time
exposures of the film for different laser light. Temporal
multiplexing permits one to blanket expose the red angle grating
regions, so no there is no extra diffraction from the mask. The
resultant film demonstrates equal intensity spectral reflectance
for all three reflection gratings. Temporal multiplexing also
permits extended exposure of the film to red angle laser after blue
and green angle regions are developed, since blue and green are
already set and extended illumination in the red angle grating
region does not affect these gratings.
[0087] When it is desirable to produce H-PDLC devices that reflect
in the IR range, it is desirable to modify the procedure so that a
visible light laser can write the significantly larger infrared LC
droplet planes. Near IR bands require spacings of about 1000 nm,
yet they may be produced using a green laser (514 nm) according to
the method of the invention. At the incident angles necessary to
form an infrared interference pattern, the glass is highly
reflective and very little of the light passes through the glass
into film. The light that does enter is in the form of multiple
reflections which wash out the interference pattern. These
drawbacks may be overcome by use of a prism which bends the laser
light into the angles necessary to shift the laser wavelength into
the near infrared band and which is optically coupled to the
film.
[0088] In preferred embodiments, a 45.degree. right angle prism or
a 30.degree.-60.degree.-90.degree. prism (or prism of another angle
set) may be used. One way of bending the light to an angle suitable
for infrared reflectance is to use two equal prisms with the film
sandwiched in between, as is shown in FIG. 7. A film 60 is placed
between prisms 62, 64 with an index matching fluid 66 placed
between the prism and the film. An exemplary index matching fluid
is glycerin.
[0089] A laser light source (not shown) is used to generate light
of a predetermined wavelength and optionally is then passed through
a beam expander and spatial filter (not shown). The resultant laser
beam is split into the number of beam pairs, shown as beams 67, 68,
required for the particular application. The beams are reflected so
that they enter the prism from the same, nearly parallel angle. The
beams are bent by the prisms and form an interference pattern on
the sample at the angles necessary to shift the laser wavelength
into the near infrared band.
[0090] In another embodiment of the invention, a single prism may
be used in conjunction with a mirrored surface to obtain an
infrared reflection grating. With reference to FIG. 8, a single
prism 70 is placed in optical contact with a top surface of a film
60. The prism/film is positioned onto a prism/mirror/prism stack
72. Prisms 74, 76 are selected to provide a selected angle of
reflection off of mirror 78, while maintaining a horizontal surface
onto which the film is positioned. This offsets the resulting
grating by about 30.degree. from parallel, so that the viewing
angle is not in the specular reflection. The various components of
the set-up are optically coupled using an index matching fluid,
such as glycerine. During exposure, the beam enters the prism, is
bent down through the sample and reflected back out at an angle
(set by the arrangement of the prism/mirror/prism stack) from its
original incident path. The reflected beam thereby combines with
the incident beam to create an interference pattern on the sample
to obtain the infrared Bragg reflector. The entire apparatus may be
mounted on a movable track 79 so that is can be moved in one
dimension (designated by arrow 80) with respect to the mirror
reflecting the laser beam. In this way, the incident beam angle can
be adjusted simply by moving the apparatus along one dimension.
Using fixed positions along the track, one can predict the
resulting wavelength of the Bragg grating to be created during an
exposure.
[0091] Although the method is described using a single laser beam
to write a single reflection grating, the methods described herein
for the preparation of multiple grating films may be used to
produce films having multiple reflection gratings in the infrared
spectrum. These multiple gratings can be spatially and/or
spectrally overlapped or non-overlapped to reflect broadband IR
spectrum or multiple IR wavelengths, respectively.
[0092] The H-PDLC includes conventional materials used in the
preparation of PDLC compositions. Thus, the polymer may be any
suitable photocurable composition. Exemplary polymers include
acrylics and urethanes having multiple functionalities to provide
varying degrees of crosslinking. See, Fontecchio et al.
"Improvement in Holographically-formed Polymer Dispersed Liquid
Crystal Performance through Acrylated Monomer Functionality
Studies" in Proc. SPIE Vol. 3800, '99. The polymer should have a
refractive index similar to that of the ordinary refractive index
of the liquid crystal. Exemplary materials include
dipentahexacrythritolpentaacrylate (DPHPA) (Sigma-Aldrich), SR399
available from Sartomer, SAMI 114 available from EM Industries, and
Ebecryl 8301, 4883 and 4866, available from UCB Radcure.
[0093] The LC may be any suitable liquid crystal. Suitable LCs
include low molecular weight liquid crystals having high
birefringence and an ordinary refractive index substantially
similar to that of the matrix polymer. Exemplary LCs are those
available from EM Industries, such as BL038, E7, E44 and TL205.
[0094] The single illumination process provides decided advantages
over multiple step processes. A comparative, multistep process in
which a mask was used to sequentially expose the sample to
different incident angles of light, thus adding holographic
elements of a single color in steps, was considered unsuccessful.
Poor results were obtained due to polymerization beyond the exposed
area, presumably due to free radical diffusion and light scattering
into the unexposed regions. The unexposed areas are then partially
cured and depleted of materials and produce an inferior grating
when ultimately exposed to light.
[0095] The methodology which has been used hereinabove to prepare
multiple reflection grating LC devices may be used to create
multiple gratings of reflection and transmission in a single
sample. The multiple gratings may be used to broaden the viewing
angle of a reflective display due to the appearance of specular
peaks at multiple viewing angles, or to obtain switchable multiple
beam splitters.
[0096] The multiple beam exposure technique for the formation of
multiple grating devices consists of superimposing multiple laser
beams on a single H-PDLC sample. The present embodiment selectively
overlaps incident light beams to create multiple reflection
gratings and transmission gratings in a single exposure.
[0097] FIG. 9 shows the experimental set up of the apparatus used
to create multiple grating samples. A laser light source 200
generates light of a predetermined wavelength and is then passed
through a beam expander and spatial filter 202. The resultant laser
beam 204 is split first using a 50/50 beam splitter 206 into beams
208, 210, of approximately equal intensity, e.g. 50% of original
beam power. The first beam 208 is further split at 50/50 beam
splitter 212 into beams 216, 218, of equal intensity, e.g. 25% of
original beam power. With the additional use of mirrors 224, 226,
the laser beams are directed onto the surface of a sample 228. The
particular beam intensity configuration (50, 25, 25) is merely
exemplary. Other combinations are within the scope of the
invention, although in preferred embodiments the power is
substantially equal on both sides of the film.
[0098] The 50% power beam 210 is used to illuminate the entire
surface. A beam expander may be used for this purpose. Beams 216
and 218 are directed onto the surface within the area illuminated
by beam 210. At least a portion of beams 216 and 218 overlap with
each other on the surface to form the pattern shown in FIG. 10A.
This alignment allows for the formation of two distinct double beam
exposures from interfering beams, thereby creating two different
Bragg reflection gratings 220, 222, shown in FIG. 10B. In the area
where all three beams overlap, a transmission grating, as well as
two reflection gratings are created in the three-beam area.
[0099] It is contemplated that more than three beams may be used in
the method of the invention, although other configurations are
contemplated as within the scope of the invention. For example,
different numbers of incident beams, laser beams of different
intensities, different wavelength and angles of incidence all are
within the scope of the invention. Simultaneous illumination of the
sample with more than three beams could form more complex grating
structures, which may be useful in a variety of applications.
[0100] By way of further example and with reference to FIG. 11A,
four beams may be directed onto the same area of the film surface.
The laser power is desirably equal on both sides of the film. The
beams may be of equal power, e.g., % power 25, 25, 25 and 25, for
beams 1, 2, 3 and 4, respectively. Alternatively, they may be of
unequal power, e.g., % power 10, 40, 10 and 40, for beams 1, 2, 3
and 4, respectively. Such beam configurations may be used to
produce two transmission gratings and four reflection gratings in
the four-beam overlap area.
[0101] In another alternative embodiment, in order to obtain a
broad viewing angle, multiple overlapping light beams of different
incident angles may illuminate one side of the film. An additional
expanded beam illuminates the film from the opposite side and is
positioned to overlay the multiple overlapping light beams of
different incident angles from the opposite side of the film (see,
FIG. 11B). As in previous examples, the laser light power
preferably is equal on both sides of the film. When the incident
angles of the multiple overlapping laser beams differ only
slightly, one may view the reflected image over a broad viewing
angle. This method produces a spectrally and spatially overlapping
display, such as that shown in FIG. 2B. Multiple non-overlapping
beams used in conjunction with a single opposing beam also are
within the scope of the invention.
[0102] This method produces films having both transmission and
reflectance gratings superimposed in a common region. While this
may be desirable for some applications, in other embodiments of the
invention it may be desirable to write a plurality of reflection
gratings onto the film without formation of a transmission grating.
This may be accomplished by use of the previously described
shuttering methodology, as follows.
[0103] Thus, in preferred embodiments one of the laser beam pairs
may be shuttered to allow exposure and photopolymerization of the
remaining beam pairs. After a short time, i.e., several seconds,
the shutter is opened and the film is fully illuminated to complete
photocuring. The resultant film possesses two reflection gratings,
but does not exhibit any diffraction of light, as would be expected
with the presence of a transmission grating.
[0104] While not bound by any mode of operation, it is hypothesized
that the resultant film possesses reflection gratings arranged as
is shown in FIG. 2C. It is hypothesized that the longer exposure
time of the first exposed color angle laser beam pairs allow the
photopolymerization to take place immediately at the surface of the
film. The late arriving (shuttered) color angle laser finds the
surface already depleted of monomer and it is capable of
photopolymerization only deeper within the body of the film. This
results in an H-PDLC device having the structure shown in FIG.
2C.
[0105] The devices of the present invention have many uses.
Multiple color reflectance devices may be used in display
applications. Broadband reflectance permits their use in switchable
reflective mirrors, which may be used in windows, for example, for
selective transmission and reflectance of the full visible spectrum
and/or near infrared or UV spectral region. Multiple grating
devices may also find applications as wave guides and beam shapers
and in applications where multiple functionality of a single film
is needed.
[0106] The invention is described in the following examples which
are presented for the purpose of illustration and which are not
limiting of the invention.
EXAMPLE 1
[0107] This example describes the preparation of a two-holographic
element reflective H-PDLC display.
[0108] The pre-polymer used in the H-PDLC formation was prepared
from commercially available constituents. The monomer was a mixture
of multi-functional acrylates, such as Ebecryl 8301, 4883 and 4866
(UCB Radcure). Nematic liquid crystals, such as those available
under the tradename E7 or BL038 from EM Industries may be used. A
photoinitiator sensitive to the laser wavelength used in
photopolymerization was used to sensitize the monomer to light.
Relative proportions of materials were 50 wt % monomer, 35 wr % LC
and 15 wt % initiator. An argon laser (.lambda.=514 nm) with an
Etalon adapter was used. For an argon laser, Rose Bengal is a
suitable photoinitiator. N-Phenylglycine was used as a co-initiator
and 1-vinyl-2-pyrrolidone was also included to improve the optical
properties of the device. All materials are available from
Sigma-Aldrich, Inc. The prepolymer was prepared under darkroom
conditions, as exposure to ambient light may result in unwanted
polymerization. Sample cells were prepared by drop-filling the
prepolymer between two 2".times.2" (5 cm.times.5 cm) ITO-coated
glass substrates. Glass fiber spacers (EM Industries, 5 .mu.m) were
used to control the cell gap.
[0109] The mask was a 30 mm.times.30 mm aluminized glass grid
having 5 mm square holes. Illumination angles were selected for the
desired reflectance wavelength. Generally, blue light is obtained
at an angle of 0.degree. and green light is obtained at an angle of
5-10.degree., all relative to surface normal. The above
illumination angles reflect anticipated changes in the resultant
grating due to polymer shrinkage and the increase in the average
refractive index of the polymer over time. Exposure time was about
20-30 seconds.
[0110] The resultant H-PDLC is shown in FIG. 12 having blue
holographic elements 300 and green holographic elements 302. A
small transmission grating area 304 also developed in the regions
between adjacent holographic elements, which is due to poor mask
alignment. Transmission area appeared to be of the similar type of
grating created in the three-beam procedure described above.
Careful mask alignment eliminated the transmission region. See,
Example 2.
[0111] Reflectance spectra showed sharply defined reflectance of
less than 20 nm FWHM. In addition, the spectral intensity of the
two peaks were comparable indicating that reflection gratings were
of comparable quality. This is in comparison to multiple reflection
grating samples prepared by sequential exposure to light of
different incident angles where subsequently exposed regions
produced poor quality gratings.
[0112] FIG. 13 shows the voltage response curves for each of the
blue and green reflecting regions of the film. The voltage response
is comparable to a non-pixelated sample, i.e., a sample in which
film regions contain more than one reflection grating, indicating
that the color of reflection and size of reflection domains does
not effect the switching field. In addition, the device
demonstrated a gradual decrease in reflection intensity, thus
demonstrating gray-scale ability.
EXAMPLE 2
[0113] This example describes the preparation of a
three-holographic element reflective H-PDLC display.
[0114] This procedure was carried out as described in Example 1,
except that the laser beam was split into six beams, with each beam
containing one-sixth the power of the original laser. Three beam
pairs were used to illuminate the sample to obtain three different
color holographic elements in a single exposure. Holographic
element size for this example was reduced to 3 mm.times.3 mm.
Illumination angles were selected for the desired reflectance
wavelength. Generally, blue light is obtained at an angle of
0.degree.; green light is obtained at an angle of 5-10.degree.; and
red light is obtained at an angle of 60.degree. (when polymer
shrinkage and increase in the average refractive index of the
polymer over time is taken into account), all relative to surface
normal. Exposure time was about 20-30 seconds.
[0115] The resultant three-color RGB holographic element H-PDLC is
shown in FIG. 14. Careful mask alignment eliminated the
transmission region observed in the sample of Example 1.
EXAMPLE 3
[0116] This example describes the use of shuttering (temporal
multiplexing) to improve the quality of a pixillated sample, i.e.,
each holographic element includes a single reflection grating.
[0117] This procedure was carried out using the apparatus and
material described in Example 2, except that a switchable shutter
was placed between the laser source and the film to switchably
block red angle laser beam, i.e., the laser beam incident at an
angle that produces a red reflection grating. Exposure time was
about 20-30 seconds; however, during the first 2 seconds of
exposure, the laser beam pair used to write the red reflectance
grating into the film was covered. After two seconds, the shutters
were opened and exposure was completed with all three laser beam
pairs.
[0118] In Example 2, the red reflecting regions were less intense
than the blue and green reflecting regions. Use of a shutter to
delay exposure of the film to the beams writing the red grating
resulted in a sample having more equal reflectance from all three
gratings. The reflectance spectrum from the resultant three color
sample is shown in FIG. 15. All three colors are resolved
(non-overlapping) and have normal peak shape and relatively equal
reflectance levels.
EXAMPLE 4
[0119] This example describes the use of shuttering (temporal
multiplexing) to produce a spatially overlapping reflectance
pattern in a film, i.e., each holographic element includes more
than one reflection grating, without the formation of a
transmission grating.
[0120] This procedure was carried out using the materials and
apparatus described in Example 1, except that no shadow masks were
used. An argon laser (.lambda.=514 nm) with an Etalon adapter was
used. The apparatus was arranged to produce two pairs of beams
incident on the sample. One beam pair was set for a "blue" angle
and the other was set for a "green" angle. The two colors were
chosen to provide reflectance spectra which were fairly close to
one another so that they produced overlapping peaks.
[0121] The sample was first quickly exposed to a single beam pair
(e.g., 1-2 sec), followed by exposure to both beam pairs for the
duration of the exposure (30 sec). Experiments were conducted in
which either the green or the blue angle beams were exposed first.
Exposure to the green angle beams for one second followed by 29
seconds of combined blue and green angle beam exposure produced
equally reflecting blue and green reflection gratings into the same
overlapping region of the film, without the formation of a
transmission grating.
[0122] The resulting spectra, normalized to a mirror reflection,
are shown in FIG. 16. The two coincident peaks can be plainly
observed.
[0123] In an alternative experiment, the beam pairs used were of
much closer color angles which produced reflectance gratings having
much closer reflectance peaks, resulting in a widening of the
reflectance peaks. This phenomenon is shown in FIG. 17 in which the
two peaks have overlapped to form a single wide peak centered at
486 nm. Widening of the spectral reflectance peak is desired for
applications requiring either broadband reflectance or wider
viewing angles.
EXAMPLE 5
[0124] This example describes the preparation of a multigrating
H-PDLC having spatially overlapping reflection and transmission
gratings.
[0125] This procedure was carried out using the apparatus and
material described in Example 1, except that the laser beam was
split into three beams, with one beam containing one-half the power
of the total incident power and the other two at one-fourth the
power of the original beam and no shadow masks were employed. Three
beams were used to illuminate the sample in a set-up similar to
that shown in FIG. 9. Exposure time was about 20-30 seconds. A
multiple grating structure having both reflection and transmission
gratings was obtained.
[0126] The reflection characteristics of the sample were explored
by spectrophotometry. FIG. 18 shows the reflection spectra from
different regions of the film. FIGS. 18A and 18B show strong
reflectance in the regions illuminated by two beam holographic
light. As expected, reflection is maximized at different
wavelengths due to the different angles of incident light in the
two regions. Interestingly, the resultant multigrating structure
possessed multiple reflection gratings as well as a transmission
grating in the area of three-beam illumination. The three beam area
showed reflectance maximized at two different wavelengths, as is
shown in FIGS. 18C and 18D, indicating that it is possible to write
two different reflection gratings onto the same region of a film.
See, FIG. 2B.
EXAMPLE 6
[0127] This example describes the preparation of a infrared
reflecting grating in a polymer film.
[0128] The pre-polymer used in the H-PDLC formation was prepared
from commercially available constituents, as described in Example
1. An argon laser (.lambda.=514 nm) with an Etalon adapter was
used. For an argon laser, Rose Bengal is a suitable
photoinitiator.
[0129] The apparatus was set up as shown in FIG. 8. A 45.degree.
right angle prism was used as the top prism. The optical
prism/mirror/prism stack was selected to provide a 14.degree. angle
of reflection off the mirror. The sample was exposed for 20-30
seconds, during which time the beam entered the prism, was bent
down through the sample and reflected back out at 14.degree. from
its original incident path. The reflected beam thereby combines
with the incident beam to create an interference pattern on the
sample to obtain the infrared Bragg reflector.
[0130] An H-PDLC prepolymer film was positioned at 4 cm from the
fixed Bragg mirror and illuminated for 20-30 seconds. The resultant
film possessed Bragg reflection planes and reflected at about 500
nm.
[0131] A second H-PDLC prepolymer film was positioned at 10 cm from
the fixed Bragg mirror and illuminated for 20-30 seconds. The
resultant film possessed Bragg reflection planes and reflected at
about 900 nm. Observation of the films by microscopy clearly showed
Bragg planes which are spaced twice as far apart as those for the
Bragg reflector to 500 nm. This demonstrates the viability of this
technique for producing H-PDLC Bragg reflectors deep into the
infrared spectrum.
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