16.2: Polarization-Independent Modulation Using Standard Liquid

16.2 / C. Oh
16.2: Polarization-Independent Modulation Using Standard Liquid Crystal
Microdisplays and Polymer Polarization Gratings
Chulwoo Oh, Ravi K. Komanduri, Brandon L. Conover, and Michael J. Escuti
Department of Electrical and Computer Engineering
North Carolina State University
2410 Campus Shore Dr, Raleigh, NC 27695-7914
Abstract: We introduce a polarization-independent
microdisplay approach employing standard liquid crystal
(LC) panels and multiple polymer polarization gratings
(PGs).
The PGs replace standard polarizers, and
inherently double efficiency by enabling direct
modulation of unpolarized-light by any commercial LC
microdisplay.
Preliminary results include ~80%
throughput and ~120:1 contrast ratios.
Keywords: polarizer-free; polarization-independent;
liquid crystal microdisplay; polarization grating;
diffraction; polarizing beamsplitter;
Introduction
We introduce a deceptively simple method of doubling
the light efficiency of liquid crystal (LC) microdisplays
by replacing the polarizers in common LCDs with
transparent polymer, thin-film, polarizing beam-splitters,
known as polarization gratings1 (PGs). We employ
achromatic PGs2, reactive mesogen (polymerizable LC)
films shown in Fig. 1a and 2b, in such a way so that they
act as both polarizer and analyzer. The result is that both
orthogonal polarizations (i.e. all of the unpolarized light)
can be directed through the microdisplay simultaneously,
so that the display has ~100% (brightness) efficiency (as
opposed to < 50% efficiency when using polarizers). We
term this a “Polymer-PG Display” system, requiring littleor-no modifications to commercial, off-the-shelf LC
microdisplays. Here we report on experimental results
justifying our approach by showing an overall ~80%
intra-pixel throughput and contrast ratios ~120:1 for
unpolarized LED light with up to ±7° aperture, as well as
clear image projection using a commercial microdisplay.
Background
Increasing light efficiency and reduced power
consumption is one of the most urgent and current
challenges for today’s LCD market. Since the invention
of the twisted-nematic mode in 19713, LCDs have been
the primary source of flat-panel displays. However, the
efficiencies of LCDs are limited due to losses (>50%)
from polarizers. This is because most operate by
controlling the direction of light polarization with the
birefringence and twist of the LC layer, while the most
efficient light sources (i.e. CCFLs and LEDs) produce
unpolarized light. There are a number of different
approaches for polarizer-free displays, including LC gels4,
PDLCs5, H-PDLCs6, and binary LC gratings7. All of
them, however, suffer to some degree or another from low
contrast ratios, very narrow acceptance angles, or limited
peak efficiencies. Recently, many groups proposed using
electrically switchable LCPGs alone as highly efficient,
polarization-independent
light
modulators8-10.
Furthermore, we have demonstrated microdisplay
prototypes for projection systems11,12 using switchable
LCPGs, most recently using an LCOS backplane.12
Figure 1. (a) Achromatic Polarization Grating (PG) behavior with white, unpolarized, input light; (b) Photo
of PG diffracting white LED light; The Polymer-PG Display configuration in (c) Bright- and (d) Dark-states.
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16.2 / C. Oh
Figure 2. Optical characteristics of parallel and anti-parallel PG configurations: (a) Illustration of each
configuration; (b) Measured transmission spectra of each (along with parallel polarizers); (c) Measured
extinction ratio of anti-parallel PG.
Polarization Gratings & Diffractive Modulation
Polarization gratings1,14 have unique diffractive
properties13 due to their spatially variant uniaxial
birefringence. We recently identified a periodic LC
profile, an Achromatic PG, that experimentally has nearly
100% diffraction efficiency over the entire visible
wavelength range,2,15 formed in highly cross-linked
acrylate polymer films, and which has the same
polarization dependent properties of conventional PGs2.
Most importantly, the ±1 diffraction orders are each
circularly polarized, orthogonal, and we have
experimentally shown they can have (in total) 100% of
the incident light propagating in them (Fig. 1b).
Here we propose and demonstrate a highly efficient,
polarization-independent microdisplay (“Polymer-PG LC
Display”) with achromatic PGs acting as polarizing and
analyzing elements instead of polarizers. Figs. 1(c) and
1(d) illustrate the basic geometry and operation principles.
Since the polarizers are replaced with these transparent
polymer thin-films, the light efficiency of the display can
potentially be improved by a factor of two. Note contrast
is obtained using aperture stops and lenses to filter the
diffraction orders. However, no modification of the
microdisplay is needed.
As in conventional LCDs, LC switching leads to light
modulation. The first PG outputs circular polarizations in
the first orders, the handedness of which is affected by LC
switching and the relative orientation between the first
and second PG (see Fig. 1c and 1d). The second PG either
diffracts the light into higher angles or directs it toward
the normal direction based on the polarization handedness
of light from the first PG. We limit our discussion to the
anti-parallel alignment of PGs, bearing in mind that there
would be a number of varied designs possible.
We demonstrate the basic operation and provide
maximum achievable transmittance and contrast ratios
with a single pixel display. In addition, we will present
results from a prototype of a Polymer-PG microdisplay
projector using a commercial microdisplay panel and a
LED light engine in color-sequential operation.
Polarization Grating Fabrication
We fabricated defect-free achromatic PGs (Fig. 1a) with
several grating periods (3 µm and 2.5 µm), formed as a
reactive mesogen (RM) film by polarization holography
and photo-alignment techniques. We utilized a linearphotopolymerizable polymer (LPP) ROP-103/2CP (Rolic)
as the photo-alignment material. The PG pattern was
recorded in the LPP layer by using orthogonal circular
polarized beams from a HeCd laser (325 nm). Then two
orthogonally twisted RM layers were spin-coated on the
LPP-coated substrate. The first twisted RM layer was a
mixture composed of RMS03-001C (Merck, Δn = 0.159
at 589 nm) with small amount (0.25%) of chiral dopant
CB15 (Merck, right-handed), chosen so that the RM layer
reached the half-wave thickness (Δnd = λ/2) and twist
angle Φ = 70°. The second RM layer was deposited
directly on the top of the first, and was composed of
RMS03-001C doped with a small amount (0.34%) of a
different chiral agent, ZLI-811 (Merck, left-handed),
subject to the same thickness and twist angle. Authors
refer readers to Ref.2,15for more fabrication details.
Single Pixel (VA-mode) Results
We first highlight the maximum achievable brightness
and contrast ratios of our current Polymer-PG Displays.
As described above, we prepared a pair of achromatic PG
polymer films (grating period = 3 µm) with high
efficiencies (>95%) for red, green, and blue LED lights
with divergence angle ~±5°. To obtain the upper limits of
light efficiency and contrast ratios, we aligned two PG
samples in parallel and anti-parallel orientations (Fig. 2a),
which are nearly equivalent to the bright-state and darkstate of an ideal display system, respectively. Fig. 2b
presents the true transmittance spectra of both
configurations, for unpolarized input light. Parallel PGs
manifest ~90% transmittance, while anti-parallel PGs leak
~0.5% for visible light (450-650 nm). Fig. 2c shows the
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extinction ratio of anti-parallel PGs (analogous to
crossed-polarizers) is ≥ 100, and has a peak of ~400 in the
red.) as shown in Fig. 2c. Note we expect this can be
improved by optimized PG film-coating processing. We
used anti-reflection coated glass to reduce Fresnel losses.
We demonstrate electro-optical switching with a single
monolithic pixel with a conventional LC cells (with
vertical-alignment (VA) mode, prepared with MLC-6610
Merck LC, SE-1211 Nissan homeotropic polyimide, and
6 µm cell gap). We arranged two achromatic Polymer
PGs in anti-parallel orientations to the LC cell (Fig. 1c
and 1d), which controls the polarization of light passing
through it. Recall that the VA-mode cell with an applied
voltage (~4V) applied will present a half-wave retardation
due to its predominantly planar alignment, and will
therefore reverse the polarizations of the two beams
(simultaneously) passing through it (from the first PG).
This light will then be directed toward the normal
direction by the second PG (Fig. 1c), and subsequently
projected to the screen/viewer. However, when zero
voltage is applied to the VA-mode cell, it presents very
little retardation, and light subsequent to the first PG
entering the LC layer will retain its original polarization
(simultaneously) and be diffracted to even higher angles
by the second PG (Fig. 1d).
The transmittance vs. voltage curves for the Polymer-PG
and polarizer-based VA-mode are compared in Fig. 3a,
for unpolarized light from RGB LEDs. The voltage
applied was a 4 kHz square wave with zero DC bias.
Note that the switching curves are similar and reach a
maximum at around 4V. Most remarkably, as Fig. 3b
shows, the true transmittance to unpolarized light for the
Polymer-PG LCD is ~80%, more than double that of a
conventional polarizer-based display, while still
maintaining modest contrast ratios (114:1 to 140:1, Fig.
3c). Note that retardation compensation for the VA-LC
pixel (not used in these measurements), would likely
enhance the contrast similarly to conventional LCDs.
Commercial Display Results
To confirm the imaging properties of the Polymer-PG
Display, we have built a prototype projector using a
commercial LC microdisplay panel (transmissive, 0.41"
VGA, planar-alignment mode) with our achromatic PG
films (Fig. 4). We used a Golden-eye LED light source
(supporting color-sequential operation) limited to a ±6°
divergence angle. We removed the polarizers from the
commercial microdisplay, and added our own retardation
compensation film (RM-based, +A plate). Here, PolymerPGs (with 2.5 µm period) were aligned in anti-parallel
configuration. Simple lenses were used to achive a
magnification was ~20, with a throw of ~2 ft. The overall
system is shown in Fig. 4a, and a photo in Fig. 4b.
A comparison (Fig. 4c) of projected images from the
original display and our Polymer-PG Display shows
equivalently sharp edges, nearly identical color saturation,
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Figure 3. Polymer-PG LC Display experimental data
with VA-mode LC pixel and for unpolarized LEDs (red
@ 625 nm, green @ 530 nm, blue @ 470 nm): (a)
transmittance vs. voltage curve; (b) Maximum
transmittance (compared to crossed-polarizer VAmode LC pixel); (b) Contrast ratios (bright/dark).
and excellent image focus. The full-on and full-off
transmittance values of the Polymer-PG LCD (R: 24%, G:
45%, B: 33%) are more than double the values of the
unmodified polarizer-based LCD (R: 9.4%, G: 20%, B:
18%) for the unpolarized LED input. This includes in
both cases all losses within the display itself (e.g.
electrode absorption, fill-factor blocking, interfaces) and
the PGs and polarizers. The contrast ratios of PolymerPG LCD (around 20:1 for all colors) are admittedly not
nearly as high as the polarizer-based LCD (around 80:1
for all colors), which we feel is predominantly related to
the sub-optimal retardation compensation film we
16.2 / C. Oh
Figure 4. A prototype projector using a commercial LC microdisplay and our Polymer-PGs: (a) Illustration
of system geometry; (b) Photograph of system; (c) Photograph of projected images.
fabricated for the Polymer-PG Display. Improving this
remains one of our primary continuing efforts.
IV. References
Note we observed parallax in projected images with a
small spatial offset, which we attribute to the distance
(glass thickness) between the imaging plane and the
second PG. To avoid this, we place two additional parallel
PGs (identical to the PGs already chosen) with the same
gap thickness as the glass after the second PG, which
perfectly compensates for the offset, as Fig. 4c shows.
1.
Conclusion
5.
The Polymer-PG Display improves the light efficiency of
LC microdisplays simply by replacing polarizers with
diffractive transparent polymer thin films. Brightness of
(unmodified) commercial LC microdisplays can be
roughly doubled, and standard étendue-limited light
sources may be used.
6.
Acknowledgements
The authors gratefully acknowledge the financial support
of the National Science Foundation (ECCS-0621906) and
ImagineOptix Corp. We also thank colleagues at Jabil
Circuit and Goldeneye Inc. for providing the LCD panel
hardware and the recycling LED source, respectively.
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