Polarization-Independent LC Microdisplays Using Liquid Crystal

NC STATE UNIVERSITY
Opto-electronics &
Lightwave Engineering Group
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Polarization-Independent LC Microdisplays
Using Liquid Crystal Polarization Gratings:
A Viable Solution (?)
Funding Support:
Dr. Michael J. Escuti and Ravi Komanduri
Dept of Electrical & Computer Engineering
01/July/08 @ ILCC ‘08, Korea
Inc.
Opto-electronics & Lightwave Engineering Group
(OLEG)
NC STATE PhD Students (5):
Ravi
Komanduri
Chulwoo Oh
Jihwan Kim
Director:
Dr. Michael Escuti
Assistant Professor
Electrical Engineering
Brandon
Conover
Elena
Nicolescu
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Agenda
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Context for Polarizer-Free LC Devices
General Strategy: Diffractive LC Modulators
Background
Reflective LC Polarization Gratings
Results from latest LCPG Microdisplay
(reflective, 256x256, silicon-backplane)
•  Summary
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Problem: Polarizers Limit LC Display Efficiency
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Indiv.
Cum.
Total (3-chip)
-
34%
Pixel Fill-Factor
93%
34%
Glass
Transmittance
98%
36%
Backplane
Reflectance
92%
37%
Polarizer
Efficiency
48%
40%
Light Management
for Color (3-chips)
85%
85%
•  Therefore, we
aim to remove
polarizers
•  Use of novel
Liquid Crystal
diffraction
gratings
Low Light
Efficiency
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Reality for any LC-based polarization-independent display
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•  Polarization losses
identified as among
remaining weaknesses
of LC technologies
–  …most recently by
Dr. J.H. Souk (Samsung,
ILCC keynote speaker)
•  YES, an LC technology
that modulates
unpolarized light can
offer ~double efficiency
•  BUT… for any real
commercial success, at
least three technical
benchmarks must be
met:
–  Pixel Efficiency
–  Contrast Ratio
–  Aperture (f/#)
Reality for any LC-based polarization-independent display
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NC STATE
•  Polarization losses
identified as among
remaining weaknesses
of LC technologies
–  …most recently by
Dr.Microdisplay
J.H. Souk (Samsung,
Diffractive
ILCC keynote speaker)
•  YES, an LC technology
that modulates
unpolarized light can
offer ~double efficiency
•  BUT… for any real
commercial success, at
least three technical
benchmarks must be
met:
–  Pixel Efficiency
–  Contrast Ratio
–  Aperture (f/#)
Reality for any LC-based polarization-independent display
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NC STATE
•  Polarization losses
identified as among
remaining weaknesses
of LC technologies
–  …most recently by
Dr. J.H. Souk (Samsung,
ILCC keynote speaker)
Diffractive
Microdisplay
•  YES, an LC technology
that modulates
unpolarized light can
offer ~double efficiency
•  BUT… for any real
commercial success, at
least three technical
benchmarks must be
met:
–  Pixel Efficiency
–  Contrast Ratio
–  Aperture (f/#)
Our LC Diffraction Grating Structure
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We are pursuing applications in:
•  Displays (this talk)
•  Achromatic Modulation (ILCC poster)
•  Beam-steering (ILCC poster)
•  Polarimetry (ILCC poster)
•  Tunable Optical Filter (ILCC poster)
•  Telecommunications
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Polarization Grating Behavior
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(Oh & Escuti, 2007, in SPIE Proc 6682 no. 668211, and SID Digest 38 pp. 1401-4)
•  Polarizes incident light
•  Tailorable diffraction
angle
•  Operates on
–  incoherent and coherent
–  white or red, green, blue
–  diverging and collimated
Related LC Grating Technology
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See work of P Bos, C Titus, M Honma, DisplayTech, et al (apologies for brevity here)
•  Prior types
–  Ex: polymer wall gratings, PDLCs,
phase gratings
•  Limitations
–  Discrete  defects
–  Low efficiency, small angles, high
scattering, low contrast
(a) Reverse Twist, (b) Orthogonal Twist Configurations
•  LC Polarization Gratings
–  Continuous structure (no defects)
–  Holographic fabrication
–  In 2006-07 developed for
transmission-mode
Eakin et al, Appl Phys Lett 85, 1671 (2004).
Jones and Escuti, SID Symposium 37, 1443, 2006.
Provenzano et al, Appl Phys Lett 89, 121105, 2006.
Komanduri and Escuti, Phys Rev E 76, 021701, 2007.
Oh and Escuti, Phys Rev A 76, 043815, 2007.
Early difficulties
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•  Crawford, Pelcovits, et al (Brown Univ)
30 µm
–  First To Experiment
–  2004, Appl Phys Lett 85 =>
–  2005, J Appl Phys 98
–  Basic principle
–  Non-ideal alignment,
with severe disclinations
•  Problems arising from defects:
–  Severe incoherent scattering out of Floquet orders
–  No longer act as true polarization gratings
(low max diffraction, non-zero minimum, low contrast)
Transmissive PGs for Displays (prior work)
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Escuti and Jones, Proc SPIE 6332, 63320M, 2006.
Escuti, Broer, et al, Proc SPIE 6302, 630207, 2006.
Komanduri et al, J. SID 15, 589, 2007.
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Basic Fabrication of PGs (transmissive)
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Similar to method in
Eakin et al, Appl Phys Lett 85 (2004)
+ careful optimization of
material choices and
critical thickness design
ITO-substrate
Reflective Substrate
Step 2: Hologram Exposure
Step 3: Fill w/ LC
Typical Parameters:
•  UV Laser (325 nm, ~30 mW)
•  Exposure Dose (0.5-2 J/cm2)
•  Grating periods ( 2 µm to 30 µm)
•  Thickness ( ~2 µm )
•  Photo-Alignment: Rolic ROP-103
•  Nematic: Merck MLC-12100-000
Switching of HeNe Laser
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>49% <0.3%
>49%
Zero V, Linear (& unpol) Polzn
White images as observed
through a
Liquid Crystal Polarization Grating
>99%
Full (30) V, All Polarizations
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PG Structure Desired Here
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•  Diffraction efficiencies:
–  0th order:
⎛ πΔnd ⎞
ηm=0 = cos ⎜
⎟
⎝ λ ⎠
2
–  ±1st order:
1
2 ⎛ πΔnd ⎞
ηm=±1 = {1  S3′ } sin ⎜
⎟
⎝ λ ⎠
2
•  Features:
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NO higher orders!
0th order polarization independent
Sum ±1 orders polzn independent
Maximum ±1 order diffraction when
(@ halfwave retardation)
–  Traded n1 (index modulation from
Bragg holograms) for
Dn (birefringence, potentially >>)
Akin to transmissive, see Nikolova et al, Optica Acta 31 (1984),
–  Wide acceptance (±20°)
Tervo et al, Optics Letters 25 (2000), and
Escuti et al, SID Digest 37, 1443-1446 (2006).
So reflective-mode is desired…
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•  … so we can employ silicon backplanes
•  But there is a problem in the
holography:
(c) UV
absorber
Reflective LCPG
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•  LCPG properties
–  Switchable grating
–  Can be ~100 % efficient
–  Only 3 possible orders
–  First and zero orders are
polarization independent
•  Reflective Mode
–  Sub-ms switching?
–  Grating periods ≤ ~2 µm
(beam separation angles
≥ 15°)
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Fabrication of PGs: Polarization Holography
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Similar to method in
Eakin et al, Appl Phys Lett 85 (2004)
ITO-substrate
+ careful optimization of
material choices and
critical thickness design
Reflective Substrate
Step 2: Hologram Exposure
Nematic LC
MDA-06-177
Step 3: Fill w/ LC
Δn=+0.14, TNI=90°C, Δε =+6.1, AMLCD, Merck
Photo-Alignment ROP103
Rolic Technologies, low-pretilt exposure, ~0.5 J/cm2
Cell Parameters
Thickness ~ 1.5 µm, Grating_Period ≥ 2.2-2.6 µm
Glass/bkplane
Photos
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Reflective LCPG on mirror
Light bulb photographed thru PG
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Definitions
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Im ≤−2
LCPG
Incident light
–1st
+1st
Im ≥2
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I−1
0th
I0
I+1
Im
Efficiency ηm =
∑ Im
Im
Reflectance Rm =
IIN
Single Pixel LCPG Results
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•  ~95% grating efficiency
•  Modulation of unpolarized light from LEDs!
 polarization-independence!
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Potential Projector Designs
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Contrasts~1000:1
Higher Voltages
(Potentially)
(a) Telecentric, Dark-Field
(c) Telecentric, Dark-Field
Contrasts~100:1
Lower Voltages
(b) Non-Telecentric, Bright-Field
(d) Telecentric, Bright-Field
Reflective LCPG Projector
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•  LCPG Properties
–  Switchable grating
–  Can be ~100 % efficient
–  Only 3 possible orders
–  First and zero orders are
polarization independent
•  Reflective Mode
–  Likely ≤ 1 ms switching
(x4 faster than transmissive)
–  Grating periods ≤ ~2 µm
(beam separation angles
≥ 15°)
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Single Pixel LCPG Results
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•  ~800 µs total switching time
•  Contrast ratio reaching 1000:1 above 22V
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Microdisplay (Si backplane) Results
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(a) 256 x 256 LCOS Microdisplay
•  Backplane: SLM from Boulder Nonlinear Systems
–  pixel pitch 24 µm
–  Max voltage 13V
•  Roughly 70% grating efficiency, Best Contrast ~50:1 (so far)
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Grayscale Image
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(Field-Sequential-Color, RGB LED)
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Prototype Projection System
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(b) Sample 256x256 image
•  Prototype Design
–  Simple Optics
–  Field Sequential light source (120 Hz field rate)
•  Photometric data
–  Input ~ 11 lm/W
–  Throughput ~ 1.8 lm/W
–  Optical losses largely due to immature projector implementation !
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Movie
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(from Ice Age 2, © 20th Century Fox)
(note flashing is not seen by eye, but appears due to
slow video capture frame rate of camera)
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Conclusions
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•  Reflective LCPGs can be fabricated w/ high quality
–  On simple mirrors and pixelated backplanes
–  Grating efficiency approaching 90% for LED light
–  Sub-ms switching speed
•  Still work to be done!
–  Good contrast ratio at high voltage (22V),
but currently low 50:1 at 13V in microdisplay sample
–  Current best beam separation is ~15°, need higher!
•  LCoS based LCPG projection proof-of-principle sys
–  System efficacy ~ 2 lm/W
–  15 lm to screen
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Thank you!
NC STATE UNIVERSITY
( http://www.ece.ncsu.edu/oleg )
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Dr. Michael J. Escuti ([email protected])
•  Acknowledgements
–  funding from the
National Science Foundation
(grants 0621906 & 0525830)
–  funding from the
Kenan Institute for Engineering,
Technology, & Science
–  partnership w/ ImagineOptix Corp
Primary Graduate Students on this work:
Ravi Komanduri and Chulwoo Oh