Expanded viewing-angle reflection from diffuse holographic-polymer
dispersed liquid crystal films
M. J. Escuti, P. Kossyrev, and G. P. Crawford
a)
Division of Engineering and Department of Physics, Brown University, Providence, Rhode Island 02912
T. G. Fiske
b)
and J. Colegrove
c)
dpiX LLC, Palo Alto, California 94304
L. D. Silverstein
VCD Sciences, Incorporated, Scottsdale, Arizona 85260
͑Received 9 October 2000; accepted for publication 30 October 2000͒
A switchable diffuse reflective film with high color purity is demonstrated using
holographic-polymer dispersed liquid crystals ͑HPDLC͒. By recording a diffuse hologram directly
into the LC/polymer film, the diffuse mode HPDLC exhibit viewing angles an order of magnitude
larger than the conventional mode. A simple phenomenological model based on coupled-wave
theory is developed to describe our observations. © 2000 American Institute of Physics.
͓S0003-6951͑01͒00301-1͔
The holographic-polymer dispersed liquid crystal
͑HPDLC͒ configuration
1,2
is a candidate for the portable re-
flective display application due to its superb color purity,
strong peak reflection efficiency, and ability to form stacked
color pixels. A descendant of PDLCs,
3
studies have been
conducted to maximize photopic reflectance,
4
to minimize
switching voltages,
5
and to understand the polymer
morphology
6
and the kinetics of the phase separation
7
on the
fundamental level. The conventional HPDLC mode operates
as an electrically switchable Bragg reflector, and exhibits a
strong, but angularly narrow, specular reflection with a view-
ing cone on the order of ϳ2°. The diffuse mode presented
here, however, exhibits an expanded viewing angle that en-
ables a practical HPDLC display.
HPDLC fabrication begins with a miscible mixture of
LC, prepolymer, and photoinitiator that is exposed with vis-
ible or UV holography.
8
The result is a periodic array of
planes alternately rich in LC droplets and polymer, as shown
in Fig. 1͑a͒. The interference fringe spacing ͑⌳͒ of the grat-
ing is described by the Bragg equation
9
⌳ϭ/(2n
͉
sin
͉
),
where is the wavelength of light, n is the average index of
the material prior to polymerization, and
is half the angle
between the exposing beams.
A diffuse HPDLC is formed with a diffusing film ͑holo-
graphic diffuser, ground glass, reflective metallic foil͒ placed
in the object beam in order to create a diverging wave front
with a random relative phase.
10
This complex pattern is then
captured by the LC/polymer film just as before. When this
hologram is replayed, a collimated incident wave front re-
constructs the diffuse object beam—the result is an expanded
viewing angle whose angular width can be controlled by the
diffuse film during formation.
In both conventional and diffuse modes, materials are
chosen such that a net index of refraction mismatch exists
between the LC droplet layers and the polymer binder, so
that Bragg reflection will occur. If a sufficient electric field is
applied and the ordinary index of the LC matches that of the
polymer, a transparent condition is achieved due to the reori-
entation of the LC droplets ͓see Fig. 1͑a͔͒. Initial switching
experiments on diffuse HPDLCs indicate that the electro-
optic response curve is ‘‘flatter’’ than the conventional mode
͓see Fig. 2͑a͔͒. We attribute this to a wider distribution of
droplet sizes, which is consistent with theory.
11
Our diffuse and conventional HPDLC samples were fab-
ricated with a urethane based LC/polymer system with an
average index of n
0
ϭϳ1.5 and an index modulation of n
1
ϭϳ0.01. A 50:50 wt% blend of trifunctional and hexafunc-
tional oligomers ͑Ebecryl resins, UCB-Radcure͒ was com-
bined with a nematic LC ͑BL038, EM Industries͒. The ini-
tiator was Rose Bengal and n-Phenylglycine in
1-vinyl-2-pyrrolidone.
12
The ratio of oligomer:LC:initiator
solution was 50:36:14 by weight. Indium–tin–oxide coated
substrates were utilized and the cell gap was 4
m. One
sample was formed in the conventional way ͑labeled as ‘‘0°
diffuse’’͒, and two with diffuse films ͑labeled as ‘‘10° and
20° diffuse’’͒ in the object beam. The grating period of all
the samples was ⌳ ϭϳ187 nm and reflected yellow–green
͑ϳ562 nm͒ with normally incident light. Passive holographic
circular diffusing films ͑Edmund Industrial Optics͒ were
used.
Our experimental setup ͓Fig. 2͑b͔͒ included a collimated
white-light source and a PhotoResearch SpectraScan705 po-
sitioned about 30 cm from the sample. We examined two
cases: first, the situation where the observer’s orientation
varied ͑vector rˆ in Fig. 2͒, and second, where the sample was
rotated ͑vector nˆ͒ and the angle between the source and ob-
server was fixed.
First we consider the reflection performance as the ori-
entation of observation deviated from specular by Ϯ20°. The
illumination source was positioned at
0
ϭϪ33°. The reflec-
tion efficiency is shown in Fig. 3͑a͒. The broadening of the
viewing width is evident and the full width at half maximum
(FWHM)ϭ1.8Ϯ0.5°, 10.8Ϯ1.2°, and 28.1Ϯ2.0° for the
0°-, 10°-, and 20°-diffuse samples, respectively. The specu-
a͒
Electronic mail: Gregory
–
Crawford@Brown.edu
b͒
Current address: Philips Flat Panel Display Systems, Sunnyvale,
CA 94085.
c͒
Current address: DigiLens, Inc., Sunnyvale, CA 94085.
APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 26 25 DECEMBER 2000
42620003-6951/2000/77(26)/4262/3/$17.00 © 2000 American Institute of Physics