Polarization-Independent Modulation
& Simplified Spectropolarimetry Using LC Polarization Gratings
Michael J. Escuti1, W.M. Jones1, C. Oh1, R. Komanduri1, C. Sanchez2, C. Bastiaansen3, D.J. Broer3
contact: [email protected], 1) NC State Univ, 2) Univ de Zaragoza, 3) Eindhoven Univ of TechnologyOpto-Electronics and Lightwave Engineering Group (OLEG) Electrical & Computer Engineering
Step 2: Hologram Exposure Step 3: Fill w/ LC
+ careful optimization of material choices and
critical thickness designSimilar to method in Eakin et al, Appl Phys Lett 85 (2004)
OBJECTIVE: NEW LIQUID CRYSTAL DEVICE MODES• 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
Summary & Motivation
LCPG Properties
Construction & Properties• Hologram w/ continuous in-plane profile• Splay-bend nematic LC texture• Bulk liquid crystals• Photo-alignment layers capture pattern
Fabrication• Used standard MERCK liquid crystals and ROLIC photo-alignment materials=11μm d=2μm
n=0.2
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n(x) = sin(πx /Λ) cos(πx /Λ) 0[ ]
Side View (0V)
Top View
• Diffraction Efficiency
• Voltage Threshold:
• Switching Times:
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ηm= 0 = cos2 πΔnd
λ
⎛
⎝ ⎜
⎞
⎠ ⎟
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Σηm= ±1 = sin2 πΔnd
λ
⎛
⎝ ⎜
⎞
⎠ ⎟
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τ on =γ1d
2
ε0Δε V 2 −Vth2
( )τ off =
γ1d2
ε0Δε Vth2
( )€
Vth = πK
ε 0Δε1−
d2
dC2
⎛
⎝ ⎜
⎞
⎠ ⎟
(V>Vth)
Transmittance Spectra
Basic Operation (monochromatic light)
Reactive-Mesogen Polarization Gratings
Diffractive Modulator• Directs optical power between 0th and ±1st diffraction orders (Floquet modes)• Polarization independent!• Controlled by applied voltage
• Threshold voltage = ~1.65 V (2 m thickness)• Modest operating voltages• Very low scattering (< 0.3%)• High contrast for monochromatic light
Experiment Specifications• Magnification = 15X, Throw = 0.6m
I+1+ I–1
Iincident
Transmittance vs Voltage
Zero V, Linear Polzn
Full (30) V, All Polarizations
(633 nm laser light)
Switching Times
Wavelength-Parallel Spectropolarimeter
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 entriesManuscripts in preparation/submission (2006): Nat. Photonics, etc.
Modulation of Unpolarized LED Light (first results)
Bright-Field Schlieren (BFS) Projection System
Maximum Contrast Ratios
Color
Red
Green
Blue
Contrast
144:1
73:1
82:1
* Potentially as high as 600:1 given higher voltages & dark-field
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
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S0(λ ) =I+1
PG #1(λ )+ I−1PG #1(λ )
sin2ζ1
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S1(λ ) =I−1
PG #1(λ )− I+1PG #1(λ )
cos2ζ 3 cos2ζ 2 sin2ζ1
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S2(λ ) =I+1
PG #2(λ )− I−1PG #2(λ )
cos2ζ 3 sin2ζ 2
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S3(λ ) =I−1
PG #3(λ )− I+1PG #3(λ )
sin2ζ 3
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!
