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Liquid-Crystal Fabry -Perot filters

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).

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Liquid-Crystal Fabry -Perot filters

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  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. Liquid Crystal Crystalline Solid Amorphous Liquid temperature 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

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

  7. Liquids Fluid properties Crystals Optical 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 ! What so attractive about LCs ?

  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

  11. Operation principle V > Vth V = 0 neff. > np neff. = np 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. 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

  17. Liquid Crystal Fabry Perot (LCFP) filters

  18. 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)

  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 Spectrumvs 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. 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

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