Wing-Kit Choi (蔡永傑)
PKU-NTU Joint Workshop on Silicon Photonics,
at Peking University
7/12/2013
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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Outline
1. Introduction to LC
2. Introduction to PDLC/nano-PDLC
3. Introduction to LCFP filters
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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Liquid Crystal (LC)
 A mainstream technology for today’s displays
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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Advantages of LC technology
 Low voltage / low power consumption
 Large electro-optic effects/Large birefringence
 No moving part / Reliable
 Long life
 Robust
 Compact
 Easily scaled to large area / large number of pixels,
etc
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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What are Liquid Crystals ?
 Intermediate between crystalline solid and amorphous liquid
 Usually found in organic molecules with:
 Highly anisotropic shapes, e.g. rod or disc shape
 Intermolecular forces: Crystals > Liquid crystals > Liquid
Liquid Crystal
Crystalline
Solid
Amorphous
Liquid
temperature
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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Intermediate properties
 Fluid properties of liquids + Optical properties of solids
Crystalline Solid
• Highly ordered
• Cannot flow
• Optically anisotropic
Liquid Crystal
• Some degree of order
• Can flow
• Optically anisotropic
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Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
Amorphous Liquid
• Highly disordered
• Can flow easily
• Optically isotropic
What so attractive about LCs ?
Crystals
Optical properties
+
Liquids
Fluid properties
Liquid Crystals
Optical anisotropy
+
Molecules can be re-arranged easily by electric
fields
Large electro-optic effects are possible with
only small applied voltages !
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Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
Slow response of LC (nematic)
 Turn-ON is Fast (can be < 1ms)
Electric field driven
 Turn-OFF is Slow (e.g. tens of ms)
Non-electric field driven
weak restoring force of LC molecules
 A major limitation of LCs
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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How to achieve a faster LC
response time ?
 Use of different LC Phase
 Modified Electrode Design
 Different LC Mode
 Polymer/LC e-o effects
 Thinner cell gap
 Over-Drive schemes
 Dual frequency, etc
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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Introduction to PDLC
• LC droplets dispersed in a solid polymer matrix
• Most common method to produce PDLC:
 Polymerization-Induced Phase Separation (PIPS)
1.
2.
3.
4.
Mix LC with monomers
Cure the mixture with UV light
Polymerization occurs
LC droplets form
2013/6/26
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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Operation principle
V=0
neff. > np
Scattering
(Dark state)
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
V > Vth
neff. = np
Transmission
(Bright state)
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Advantages
High Optical efficiency (No polarizers)
Ease of Fabrication (No Alignment layers)
Potentially Lower cost (No Alignment layers)
Compatible with plastic substrates to form
Flexible Displays
Polarization Independent (in normal direction)
Fast Response time possible (esp. nano-PDLC)
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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Applications
Electrically Switchable Windows
Other possible
applications:
 Variable Optical
Attenuators (VOAs)
 Project Displays
 Reflective/
Flexible Displays
 Tunable lens, etc
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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Transmission vs cell gap
Thicker LC cell  more scattering  CR , V 
0.9
0.8
Transmittance (%)
0.7
0.6
5um
0.5
7.5um
12um
0.4
15um
0.3
19um
0.2
23um
0.1
0
0
10
20
Voltage (V)
30
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
40
50
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Nano PDLC @ High Polymer
concentration
0.9
Polymer % , Scattering ,
CR  (droplet size  )
Polymer > ~ 70%, ~ no
scattering
(droplet size  ~100nm)
Known as nano-PDLC
0.8
0.7
Transmittance (%)
~ 50% polymer  Max.
scattering  highest CR
(droplet size ~ 1m )
70%
0.6
0.5
50%
0.4
60%
60%
70%
0.3
0.2
50%
0.1
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
0
0
20
40
Voltage (V)
60
80
100
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Nano PDLC  Fast Response (<1ms) possible
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Polymer % , Response
time  (droplet size  )
Polymer > ~ 70% (nanoPDLC), fast response < 1ms
possible
response time (ms)
Polymer interaction with LC
stronger (more
surface/volume ratio)
25
20
15
10
5
0
50
55
60
65
70
75
concentration of polymer (wt%)
2013/6/26
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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Fabry-Perot cavity
Air ( or e.g. Liquid Crystal)
Incident light
T : transmittance
R : reflectance
n’ : the refractive index of the material
d’ : the thickness of the etalon
Transmitted light
Highly reflective mirror
(with glass substrate)
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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Wavelength tuning
Tuning
=~ 50nm
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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LCFP filters
 First Proposed by a group at Rockwell Int. Science
Centre, US, 1981 (Gunning et al)
 To employ large n of LC:
 Highly efficient wavelength tunable filters
 Visible and Infrared Applications
 Lower Voltage
 Wider tuning range compared to other solid e-o
materials
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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LCFP filters
 Since 1990, further improved by groups at e.g. :
1) Bell Core , NJ ( Patel et al)
2) NTT Optoelectronics, Japan (Hirabayashi et al)
for WDM in telecommunications
 with Lower Loss, narrow bandwidth (<1-2nm),
wide tunable range
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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LCFP (using PA-LC)
Spectrum with Pol. And without Pol.
Wavelength tuning
ne
no
(Bellcore ,1990)
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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LCFP (Polarization Independent)
Split into 2 components by Calcite
Spectrum vs V without Pol.
(Bellcore, 1991)
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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LCFP (Polarization Independent)
At > ~2.5V , ne and no modes
merge
Spectrum vs V without Pol.
POLARIZER
GLASS
LC
GLASS
POLARIZER
(Bellcore,1991)
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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High speed LCFP using FLC
 High speed (<100s)
 Binary
 Bistable
(Bellcore,1993)
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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High speed LCFP using DHFLC
 High speed (< ~ 100 s)
 Low Voltage
 May have hysteresis effect
(Cambridge, 1996)
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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High speed LCFP using Sm*A LC
 High speed (<10 s)
 High Voltage
 Elevated temp.
 Tilted alignment (complicated)
(Colorado, 1996)
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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Thank You!!
Wing-Kit Choi (蔡永傑)
National Taiwan University
wkchoi@cc.ee.ntu.edu.tw
Tel: +886-2-3366-3669
Prof. WK Choi (蔡永傑) GIPO/EE, National Taiwan University
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