Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

Wing-Kit Choi ( 蔡永傑 )PKU-NTU Joint Workshop on Silicon

Photonics,at Peking University

7/12/2013

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Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

Outline

1. Introduction to LC2. Introduction to PDLC/nano-PDLC3. Introduction to LCFP filters

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Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

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

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Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

Advantages of LC technologyLow voltage / low power consumptionLarge electro-optic effects/Large birefringenceNo moving part / ReliableLong lifeRobustCompactEasily scaled to large area / large number of pixels,

etc

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Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

What are Liquid Crystals ?Intermediate between crystalline solid and amorphous liquidUsually found in organic molecules with:

Highly anisotropic shapes, e.g. rod or disc shapeIntermolecular forces: Crystals > Liquid crystals > Liquid

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Crystalline Solid

Amorphous Liquid

Liquid Crystal

temperature

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

Intermediate properties

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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• Optically anisotropic

Fluid properties of liquids + Optical properties of solids

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

What so attractive about LCs ?

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CrystalsOptical properties

Liquids Fluid properties +

Liquid CrystalsOptical anisotropy

+Molecules can be re-arranged easily by electric

fields

Large electro-optic effects are possible with only small applied voltages !

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 drivenweak restoring force of LC molecules

A major limitation of LCs

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Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

How to achieve a faster LC response time ? Use of different LC PhaseModified Electrode DesignDifferent LC ModePolymer/LC e-o effects Thinner cell gapOver-Drive schemesDual frequency, etc

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Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

Introduction to PDLC• LC droplets dispersed in a solid polymer matrix• Most common method to produce PDLC: Polymerization-Induced Phase Separation (PIPS)

1.Mix LC with monomers 2.Cure the mixture with UV light 3.Polymerization occurs4.LC droplets form

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Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

Operation principle

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Scattering(Dark state)

Transmission (Bright state)

neff. > np

V = 0neff. = np

V > Vth

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

Advantages

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High Optical efficiency (No polarizers)Ease of Fabrication (No Alignment layers) Potentially Lower cost (No Alignment layers) Compatible with plastic substrates to form

Flexible DisplaysPolarization Independent (in normal direction)Fast Response time possible (esp. nano-PDLC)

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

Electrically Switchable Windows

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Applications

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

Transmission vs cell gap

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Thicker LC cell more scattering CR , V

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

Nano PDLC @ High Polymer concentration

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~ 50% polymer Max. scattering highest CR (droplet size ~ 1m )

Polymer % , Scattering , CR (droplet size )

Polymer > ~ 70%, ~ no scattering

(droplet size ~100nm)Known as nano-PDLC

50%

60%

70%

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

Nano PDLC Fast Response (<1ms) possible

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Polymer % , Response time (droplet size )

Polymer interaction with LC stronger (more surface/volume ratio)

Polymer > ~ 70% (nano-PDLC), fast response < 1ms possible

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University 17

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

Fabry-Perot cavity

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T : transmittanceR : reflectancen’ : the refractive index of the material d’ : the thickness of the etalon

Highly reflective mirror (with glass substrate)

Air ( or e.g. Liquid Crystal) Incident light

Transmitted light

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

Wavelength tuning

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Tuning =~ 50nm

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

LCFP filtersFirst 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

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Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

LCFP filtersSince 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

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Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

LCFP (using PA-LC)

Spectrum with Pol. And without Pol.

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Wavelength tuning

ne

no

(Bellcore ,1990)

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

LCFP (Polarization Independent)

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Split into 2 components by Calcite Spectrum vs V without Pol.

(Bellcore, 1991)

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

LCFP (Polarization Independent)

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POLARIZER

POLARIZER

GLASS

LC

GLASS

Spectrum vs V without Pol.At > ~2.5V , ne and no modes merge

(Bellcore,1991)

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

High speed LCFP using FLC

High speed (<100s)BinaryBistable

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(Bellcore,1993)

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

High speed LCFP using DHFLC

High speed (< ~ 100 s)Low Voltage May have hysteresis effect

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(Cambridge, 1996)

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

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

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(Colorado, 1996)

Prof. WK Choi (蔡永傑 ) GIPO/EE, National Taiwan University

Wing-Kit Choi ( 蔡永傑 )National Taiwan University

wkchoi@cc.ee.ntu.edu.twTel: +886-2-3366-3669

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