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the freezing point for operation in cold-environments.Initial experiments use saline liquid only (not saline/oil).Therefore the EMP channel spacing is limited to ~1 mmor less in order to alleviate the effect of gravity on theliquid meniscus curvature.EMP operation requires three electrical connections.For a single EMP, the simplest configuration is two voltagesources attached to the sidewalls and the saline electricallygrounded. As shown in Fig. 4b, 4c, when placedbetween sidewalls at zero-voltage, the saline forms a convexshaped meniscus due to a large saline/fluoropolymercontact angle (θ 0 ~115°). When voltage is applied thesaline electrowets the surface and the contact angle (θ V )decreases. This change in contact angle is directly relatedto change in prism apex angle (ϕ) according to ϕ =|90°θV |. In order to obtain a straight-line meniscus from sidewallto side-wall, the contact angles on both the left andright sides should be inverted about 90° contact, orθ V (right) = 180°-θ V (left). Therefore, some combinationof voltage is always applied to both side-walls. PredictiveChannel Width µm's-mm'sSalineSiAngled-View(a)mm's - cm's0VFluoropolymer(~1 µm)SalineGlass0V 0V 80V 30V60V 60V 50V 70V~1 mm EMP (τ ~ 10 ms)(c)Figure 4: Electrowetting microprisms (EMPS) shown as: (a) angled-view, (b) side view, (c)photos of EMPs in operation. Adapted from [5].Si0V30VSide-View(b)voltage levels for contact angle inversion, and flat menisci,are easily obtained from Eq. 1. In order to illustratepossible steering angles through modulation of EMP apexangle, the angular deviation (δ) of a light beam passingthrough a prism with an index of refraction n>1, apexangle ϕ, and angle of incidence αi (0° in our case) can beexpressed as:δ =α i − φ + sin −1[(n 2 − sin 2 α i ) 1/2 sin φ − cos φ sin α i](3)Eqs. 1 & 3 can be combined to generate theoretically predictedEMP behavior. A theoretical plot of deflection vs.prism apex angle is shown in Fig. 5a. The plot correspondsto a saline refractive index of n=1.36. Experimental resultshave shown up to 14° (~±7°) of total continuous deflectionusing a saline solution of n=1.36. The experimentallyachieved value is marked on the plot. The voltage responsefor deflection is shown as an inset in Fig. 5a. The deflectionrange is limited by the non-ideal phenomenon of electrowettingcontact-angle saturation. Forthese first devices, contact angle saturationlimits the switchable apex angle to~±20° which translates to ~±7° beamdeflection. This steering range is still~7X greater than that typicallyachieved for liquid crystal beam-steeringdevices based on a single-plate optical-phased-array.Also included in Fig.5a is marking of theoretical steeringperformance expected from a saline/oilsystem (instead of saline/air). For thissystem a high index silicone oil is utilized(n>1.55) and low index saline(n90%fill factor should be achievable. Scaledown is also important, since as dis-8 IEEE LEOS NEWSLETTER August 2006φδ80V
Continuous Deflection (2δ,°)6050403020102δ,°161412108642002040 60Voltage (V)80100n saline = 1.36 / n airAchieved ∼14° ( + 7°) −Oil/SalineSystem∼20-40°Salineθ vOil−−−−−Fluoropolymer++++Siδ =10's°SalineOilSiO 2Si SubstrateController00 5 10 15 20 25 30 35 40 45Apex Angle (φ°)(a)Proposed Arrayed EMPs(τ < 100 µs, FF>90%)(b)Figure 5: (a) Theoretical plot of continuous deflection vs. apex angle and measured continuous deflection vs. voltage (inset). For large apertureinfra-red applications, such at those requiring high laser power, (b) proposed is linear arrays of EMPs integrated in SOI wafers witha fill factor of >90%. Adapted from [5].cussed in the introduction, small size electrowettingdevices are able to operate much faster than large sizedevices. For long-channels, the cylindrical meniscusgeometry needs to be stabilized since a spherical meniscusis energetically favored (minimizes system energy).Techniques have been demonstrated at Cincinnati to stabilizelong EMP channels, and will be reported in futurepublications.ConclusionTwo approaches for creating flat transmissive electrowettingoptics have been presented. The first, electrowettinglight valves (ELVs), provides high transmission efficiency(>80%) and allows transmission of light at anypolarization or incidence angle. The second, electrowettingmicroprisms (EMPs) provides wide-angle steeringcapability that can be extended to linear or concentricarrays. Electrowetting optics may one-day extend tonano-scale diffractive optics and to possible use in guided-waveoptics.AcknowledgementsThe authors acknowledge technical support for this workby Dr. Paul F. McManamon and financial support fromthe Air Force Research Laboratory.References[1] T. N. Ruckmongathan and N. V. Madhusudana,Liquid Crystal Displays, Handbook of Luminescence,Display Materials, and Devices, Ed. H. S. Nalwa, andL. S. Rohwer, Ch. 7, ISBN: 1-58883-032-2, 2003.SPECIALISTSfor ENGINEEREDFiber Optic CABLESWe're experts in custom cable solutionsbuilt to your specifications:✦ All Wavelengths✦ 3 to 1,000µm Fibers✦ Singlemode & Multimode✦ Polarization Maintaining✦ High Power✦ Reference & Research Grades✦ Industrial & MIL-SPEC✦ Multi-Fiber Assemblies✦ Made in California, USACoastal Connectionssales@coastalcon.com ✦ Phone: (805) 644-5051August 2006 IEEE LEOS NEWSLETTER 9
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