1. Liquid-Crystal Fabry-Perot filters
Wing-Kit Choi (蔡永傑)
PKU-NTU Joint Workshop on Silicon Photonics,
at Peking University
7/12/2013

2. Outline Introduction to LC Introduction to PDLC/nano-PDLC
Introduction to LCFP filters

3. Liquid Crystal (LC)
A mainstream technology for today’s displays

4. 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

5. 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
Crystalline
Solid
Amorphous
Liquid
Liquid Crystal
temperature

6. Intermediate properties
Fluid properties of liquids + Optical properties of solids
Crystalline Solid
Amorphous Liquid
Liquid Crystal
Highly ordered
Cannot flow
Optically anisotropic
Highly disordered
Can flow easily
Optically isotropic
Some degree of order
Can flow

7. 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 !

8. 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

9. 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

10. Introduction to PDLC LC droplets dispersed in a solid polymer matrix
Most common method to produce PDLC:
Polymerization-Induced Phase Separation (PIPS)
Mix LC with monomers
Cure the mixture with UV light
Polymerization occurs
LC droplets form
2013/6/26

11. Operation principle neff. = np V > Vth neff. > np V = 0
Scattering
(Dark state)
Transmission
(Bright state)

12. 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)

13. Electrically Switchable Windows
Applications
Electrically Switchable Windows
Other possible applications:
Variable Optical Attenuators (VOAs)
Project Displays
Reflective/ Flexible Displays
Tunable lens, etc

14. Transmission vs cell gap
Thicker LC cell  more scattering  CR , V 

15. Nano PDLC @ High Polymer concentration
~ 50% polymer  Max. scattering  highest CR (droplet size ~ 1m )
Polymer % , Scattering , CR  (droplet size  )
Polymer > ~ 70%, ~ no scattering
(droplet size  ~100nm)
Known as nano-PDLC
70%
60%
50%

16. Nano PDLC  Fast Response (<1ms) possible
Polymer % , Response time  (droplet size  )
Polymer interaction with LC stronger (more surface/volume ratio)
Polymer > ~ 70% (nano-PDLC), fast response < 1ms possible
2013/6/26

17. Liquid Crystal Fabry Perot (LCFP) filters

18. Fabry-Perot cavity Air ( or e.g. Liquid Crystal) Incident light
Highly reflective mirror
(with glass substrate)
Air ( or e.g. Liquid Crystal)
Incident light
Transmitted light
T : transmittance
R : reflectance
n’ : the refractive index of the material
d’ : the thickness of the etalon

19. Wavelength tuning
Tuning =~ 50nm

20. 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

21. 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

22. LCFP (using PA-LC) Spectrum with Pol. And without Pol.
Wavelength tuning ne no
(Bellcore ,1990)

23. LCFP (Polarization Independent)
Split into 2 components by Calcite
Spectrum vs V without Pol.
(Bellcore, 1991)

24. LCFP (Polarization Independent)
Spectrum vs V without Pol.
At > ~2.5V , ne and no modes merge
(Bellcore,1991)

25. High speed LCFP using FLC
High speed (<100s)
Binary
Bistable
(Bellcore,1993)

26. High speed LCFP using DHFLC
High speed (< ~ 100 s)
Low Voltage
May have hysteresis effect
(Cambridge, 1996)

27. High speed LCFP using Sm*A LC
High speed (<10 s)
High Voltage
Elevated temp.
Tilted alignment (complicated)
(Colorado, 1996)

28. Liquid Crystal Display & Photonics Laboratory
Thank You!!
Liquid Crystal Display & Photonics Laboratory
Wing-Kit Choi (蔡永傑)
National Taiwan University
Tel: