Neil R. Smith

Agile wide-angle beam steering with electrowetting microprisms

A novel basis for beam steering with electrowetting microprisms (EMPs) is reported. EMPs utilize electrowetting modulation of liquid contact angle in order to mimic the refractive behavior for various classical prism geometries. Continuous beam steering through an angle of 14 degrees (+/-7 degrees ) has been demonstrated with a liquid index of n=1.359. Experimental results are well-matched to theoretical behavior up to the point of electrowetting contact-angle saturation. Projections show that use of higher index liquids (n~1.6) will result in steering through ~30 degrees (+/-15 degrees ). Fundamental factors defining achievable deflection range, and issues for Ladar use, are reviewed. This approach is capable of good switching speed (~ms), polarization independent operation, modulation of beam field-of-view (lensing), and high steering efficiency that is independent of deflection angle.

Ion and Liquid Dependent Dielectric Failure in Electrowetting Systems

Electrowetting devices often utilize aqueous solutions with ionic surfactants and inorganic salts to modify the electrowetting response. It has been observed in low-voltage electrowetting devices (thin dielectric, <12 V) that a frequent onset of dielectric failure (electrolysis) occurs with use of ionic solutes such as potassium chloride (KCl) or sodium dodecyl sulfate. More detailed current-voltage investigations reveal less dielectric failure for the larger size ions. Specifically, improved resistance to failure is seen for surfactant ions carrying a long alkane chain. Therefore, a catanionic surfactant (in which both ions are amphiphilic) was custom synthesized, and elimination of dielectric failure was observed in both negative and positive voltage. Because water is a small molecule that easily penetrates dielectrics, further experiments were performed to show that dielectric failure can also be eliminated by use of larger size polar molecules such as propylene glycol. In addition to these results, important parameters such as conductivity and interfacial tensions are reported.

Enhanced electro-optical lithium niobate photonic crystal wire waveguide on a smart-cut thin film.

We report an electro-optically tunable photonic crystal linear cavity etched on a 200 nm lithium niobate waveguide ridge. The photonic crystal cavity and the ridge are both fabricated on a 1 μm thin film of lithium niobate obtained by smart-cut technology. The photonic crystal, of area 4x0.8 μm2, has been engineered to work in a slow light configuration so that the electro-optic effect is 20 times more important than in bulk material.

Electrowetting manipulation of any optical film

Electrowetting manipulation of any optical film is reported. A square channel was constructed with four sidewall electrodes, coated with a hydrophobic dielectric, and filled with saline and oil. In a first experiment a dielectric mirror film was suspended between the oil/saline meniscus. Electrowetting at each sidewall produced a saline contact angle change of 35°<θ<170°. This change in contact angle tilted the mirror and ±105° of laser beam deflection was achieved. A second experiment utilized a Mylar film imprinted with a diffraction grating (625lines∕mm). Electrowetting tilting of the grating was shown to alter the diffraction of the laser beam.

Fabrication and Demonstration of Electrowetting Liquid Lens Arrays

Reported is the fabrication and demonstration of an array of >12 000 switchable liquid microlenses, each ~300 mum in diameter and switchable through plano-concave to plano-convex. Electrowetting is used to modulate the contact angle of an aqueous/oil liquid system over a range of 100deg, resulting in a switchable dioptric range of -360 m-1 to 230 m-1. Compared to previous reports of single 2-6 mm electrowetting lenses, the fabrication process reported herein reduces the individual lenslet size by ~10times. To dose liquids into large arrays of these small liquid lenslets, a scalable self-assembled dosing process was developed. The completed liquid lens array has a fill factor of 50% which can be extended to >80%.

Recent Progress in Arrayed Electrowetting Optics

Electrowetting devices can now be formed in arrays covering thousands of square centimeters of glass. New research is pointing the way toward exciting applications for laser radar, 3D displays, adaptive camouflage, electronic paper, retroreflector communication and lab-on-a-chip.

Flat electrowetting optics and displays

Flat electrowetting optics currently include pixel arrays for displays and prism arrays for beam steering. Electrowetting display pixels utilize a colored oil layer that provides high efficiency control of light transmission or light reflection. Electrowetting microprisms tilt the angle of the meniscus between liquids with different refractive index and thereby cause refraction of a light beam passing through the meniscus. Both of these technologies are projected to provide an order of magnitude increase in raw performance compared to liquid-crystal and other technologies. For example, transmissive electrowetting displays are expected to achieve >80% transmission, which far exceeds the ~8% transmission of a commercial liquid crystal display. Electrowetting microprisms have a clear roadmap leading to greater than +/- 45° of continuous beam steering, which surpasses the few degrees of beam steering achieved with electro-optic phased arrays. However, before widespread commercial application can be achieved, a variety of other challenges, such as low-voltage operation, must be solved. Many of these challenges are engineering problems, not fundamental scientific discoveries, and significant technological progress is expected for flat electrowetting optics.

Composite Dielectrics and Surfactants for Low Voltage Electrowetting Devices

In this work electrowetting operation at <12 V has been achieved through implementation of composite dielectrics and surfactants. The composite dielectrics consist of A12O3 and Si3N4 covered by a thin film of hydrophobic Cytop fluoropolymer. These composite dielectrics exhibit higher capacitance and therefore lower voltage electrowetting operation. Atomic layer deposition of A12O3 and plasma-enhanced chemical vapor deposition of Si3N4 were chosen because they can be deposited at low temperature, are pin-hole free, and because they can be conformably coated on 3D surfaces. Surfactants were also explored to lower the water/oil interfacial surface tension and thereby further reduce the voltages required for electrowetting. These results are important for continued development of arrayed electrowetting optics for displays and beam steering applications.

Experimental Validation of >1 kHz Electrowetting Modulation

Presented here is an experimental measurement of the switching speed one could expect for electrowetting optics. The switching capabilities of various sized elements are studied through basic electrowetting droplet modulation. The relative size of the droplets can be directly related to the dimensions of pixels or prisms used in arrayed electrowetting optics. Experimental results reported herein confirm that scaling down the liquid volume can potentially provide switching speeds of >1 kHz for elements that are <100 mum in size. The factors that determine switching speed are contact line velocity and damping time for the liquid meniscus oscillation.

Electrowetting Micro-prisms and Micro-mirrors

Electrowetting beam steering using prism refraction and mirror reflection is presented. Electrowetting at the sidewalls of a square liquid channel is able to tilt prism or mirror apex angles over +/- 55deg.