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

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

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

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

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

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

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 !

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

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

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

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

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)

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

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

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%

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

Liquid Crystal Fabry Perot (LCFP) filters

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

Wavelength tuning Tuning =~ 50nm

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

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

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

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

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

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

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

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

Liquid Crystal Display & Photonics Laboratory Thank You!! Liquid Crystal Display & Photonics Laboratory Wing-Kit Choi (蔡永傑) National Taiwan University wkchoi@cc.ee.ntu.edu.tw Tel: +886-2-3366-3669