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Polarization-Independent Modulation & Simplified Spectropolarimetry Using LC Polarization Gratings 1 Escuti , 1 Jones , 1 Oh , 1 Komanduri , 2 Sanchez , C. 3 Bastiaansen , 3 Broer Michael J. W.M. C. R. C. D.J. contact: [email protected], 1) NC State Univ, 2) Univ de Zaragoza, 3) Eindhoven Univ of Technology Opto-Electronics and Lightwave Engineering Group (OLEG) Electrical & Computer Engineering Basic Operation (monochromatic light) Summary & Motivation OBJECTIVE: NEW LIQUID CRYSTAL DEVICE MODES Diffractive Modulator • POLARIZATION INDEPENDENT liquid crystal microdisplays • HYPERSPECTRAL POLARIMETER designs enabling high-speed detection MOTIVATION: Capitalize on Adv Polarization Control Techniques • Achieve ultra-efficient, portable, projection displays, with low-cost potential LCoS • Simplified spectropolarimeter for easier wavelength-parallel polarization detection APPROACH: Surface-directed Holograms in Bulk Liquid Crystals • These optical elements are technically “Liquid Crystal Polarization Gratings” or (LCPG) • First reported by Crawford and coworkers (Eakin et al, Appl Phys Lett 85, 2004) • We improve on prior work by achieving high quality optical elements for 1st time • Directs optical power between 0th and ±1st diffraction orders (Floquet modes) • Polarization independent! • Controlled by applied voltage Zero V, Linear Polzn • Threshold voltage = ~1.65 V (2 m thickness) • Modest operating voltages • Very low scattering (< 0.3%) • High contrast for monochromatic light Full (30) V, All Polarizations LCPG Properties n(x) sin(x /) cos(x /) 0 Construction & Properties Top View • Hologram w/ continuous in-plane profile • Splay-bend nematic LC texture • Bulk liquid crystals • Photo-alignment layers capture pattern • Diffraction Efficiency Transmittance vs Voltage Switching Times I+1+ I–1 Iincident Side View (0V) nd m 0 cos 2 nd m 1 sin 2 (V>Vth) (633 nm laser light) • Voltage Threshold: K d 2 1 2 Vth dC 0 • Switching Times: on Modulation of Unpolarized LED Light (first results) 1d 2 0V V 2 2 th off Bright-Field Schlieren (BFS) Projection System 1d 2 0Vth2 Maximum Contrast Ratios Transmittance Spectra Color Contrast Red 144:1 Green 73:1 Blue 82:1 * Potentially as high as 600:1 given higher voltages & dark-field Experiment Specifications • Magnification = 15X, Throw = 0.6m Fabrication • Used standard MERCK liquid crystals and ROLIC photoalignment materials =11μm d=2μm n=0.2 Similar to method in Eakin et al, Appl Phys Lett 85 (2004) Step 2: Hologram Exposure + careful optimization of material choices and critical thickness design Step 3: Fill w/ LC Wavelength-Parallel Spectropolarimeter Reactive-Mesogen Polarization Gratings PG#1 I1PG#1( ) I 1 ( ) S0( ) sin 2 1 Features • High efficiency (~100%), low scattering (< 0.3%) • Single substrate, robust polymer films • Grating periods as small as 1 m • Rolic Photo-alignment, Merck reactive mesogens Features • Full spectral polarization information w/o active tuning elements • Simultaneous measurement of six intensities only needed • Simple light-weight construction • Proof-of-concept shows very good accuracy! PG#1 I 1 ( ) I1PG#1( ) S1( ) cos2 3 cos2 2 sin 2 1 PG#2 I1PG#2( ) I 1 ( ) S2( ) cos2 3 sin 2 2 PG#3 I 1 ( ) I1PG#3( ) S3( ) sin 2 3 For More Information: [email protected] @ SID Symposium Digest 2006: paper # 39.4, posters P-209, P-167 @ SPIE Optics & Photonics (San Jose, CA, 2006): three entries Manuscripts in preparation/submission (2006): Nat. Photonics, etc.