A thin electrowetting controlled optical system with pan/tilt and variable focus functions
Introduction
A liquid droplet can be actuated at high speed [1] and high frequency [2] using a low voltage [3] through the use of electrowetting. Due to the transparency and refractive index of the liquid, electrowetting has been used for MEMS optical devices, such as tunable lenses [4] and as pixels for color displays [5]. The droplet works as an optical component, and the sizes of optical devices driven by electrowetting can be kept small. Previously, the curvature of the liquid interface was tuned by electrowetting, and a tunable oil lens was achieved [4]. The electrowetting lens has the potential to achieve other optical functions when combined with a solid apparatus. Li et al. achieved an optical switch by integrating an electrowetting lens and a nontransparent cap with a pin hole [6]. As reviewed in a paper by Syms and Whitesides [7], liquid has also been used for assembling microstructures due to the dominance of the capillary forces at the microscale. In recent work [8], [9], a capillary force exerted on a microstructure was controlled by electrowetting, and a tilting motion in the microstructure was achieved. In previous experiments [8], we achieved an angle-tunable prism driven by electrowetting. A water droplet is sandwiched between two plates and forms a prism-like shape by changing the angle of the two plates. Here, the light path through the prism changes direction according to the tilting angle. In this prism driven by electrowetting, the transparent water droplet works as an optical component, and operation at resonant frequencies, such as would be required with an electrostatic actuator, In this research, we report on a newly discovered advantage of optical devices that use electrowetting, which is that they can be integrated while maintaining the original thickness. Electrowetting can be used in both the water/air phase and the oil/water phase [10]. If two different devices both work in the water/air and the oil/water phase, then the devices can share the water phase. In this paper, we integrated an oil lens inside a water prism without changing the overall size of the prism. The optical device has a variable focus function and a view shifting function. Our device does not require three-dimensional fabrication such as side wall patterning. Only by patterning electrodes on a single plate can the two functions be activated individually or simultaneously.
The efficiency of the variable focus and view shifting functions can be estimated by considering the liquid shape. Liquid droplets actuated by electrowetting have been well studied [11]. The capillary forces exerted on the microstructures in the lateral [12], vertical and rotational [13] directions have also been studied. By contrast, the tilting motion of the microstructure by electrowetting is comparatively less well studied. In this paper, the order of the tilting angle of the plate is captured through simulation, and experimental results correspond to the results from the simulation.
This paper examines a multifunctional liquid optical device driven by electrowetting. Additionally, the simulation and experimental results from this work are useful for designing future MEMS devices that have liquid in contact with microstructures.
Section snippets
Principle
Fig. 1(a) presents the principles of the angle tunable function. The angle tunable function is achieved by forming a prism shape with a water droplet, an upper plate and a lower plate. The upper plate is supported by a beam and can be tilted around it. The lower plate is made of a glass substrate with the surface electrodes and insulating layer patterned onto it. The view through the prism changes direction according to the wedge angle of the prism. The droplet is electrically connected to the
Fabrication
Our device is composed of the upper plate, lower plate and the water and oil droplets. The upper plate was composed of a silicon on insulator (SOI) wafer. On the top layer, the beam of the upper plate was formed by deep reactive-ion etching (DRIE). The center part was then patterned with a transparent material, SU-8. The handling layer and glass layer were removed by DRIE and hydrofluoric (HF) acid, respectively. The width w, length l and thickness of the beam t are 30 μm, 2 mm and 5 μm,
Experiment
First, we performed experiments manipulating the water and oil droplets on the lower plate. As shown in Fig. 4, we placed the water droplet on the lower plate and injected an oil droplet in it. The water droplet was manipulated by applying voltage to the outer electrode, while the oil droplet was manipulated by applying the voltage to the inner electrode. Throughout the experiment, the state of the oil lens can be switched using 60 V (Fig. 4(b)). The water droplet is moved to the right side by
Discussion
In this paper, the oil drop lens can only switch between two states. As reported previously [5], the contact angle is tunable by electrowetting, and the multifocus lens is determined to be feasible. In the previous work, the lens was self-centered by a tube wall. However, the use of a tube wall is not adequate for self-centering our device because a three-dimensional structure is difficult to integrate into the water prism. Vasudev et al. reported that the self-centering alignment of the
Conclusion
We have proposed a thin optical device with pan/tilt and variable focus functions driven by electrowetting. The thickness of the optical device is 2.5 mm. With the optical device, the view shifted a maximum of 6.3 degrees with an applied voltage of 70 V. By changing the curvature of the oil droplet, the focal length is changed from 33.0 mm to 15.4 mm. In the experiment, the oil lens and water prism can be controlled individually. The electrodes to control the liquids are patterned onto the same
Acknowledgements
The photolithography masks were fabricated using the EB lithography apparatus of VLSI Design and Education Center (VDEC) of the University of Tokyo.
Atsushi Takei received B.E., M.E., and Ph.D. degrees in mechanical engineering from The University of Tokyo, Tokyo, Japan, in 2005, 2007 and 2010, respectively. In 2011, he joined the faculty of the University of Tokyo and is now a research fellow in the Institute of Industrial Science. His current interests are micro-systems using capillary forces and the instability of thin films.
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Atsushi Takei received B.E., M.E., and Ph.D. degrees in mechanical engineering from The University of Tokyo, Tokyo, Japan, in 2005, 2007 and 2010, respectively. In 2011, he joined the faculty of the University of Tokyo and is now a research fellow in the Institute of Industrial Science. His current interests are micro-systems using capillary forces and the instability of thin films.
Kiyoshi Matsumoto received B.E., M.E., and Ph.D. degrees in mechanical engineering from The University of Tokyo, Tokyo, Japan, in 1985, 1987, and 1999, respectively. In 1987, he joined the Central Research Laboratory of Hitachi Co. Ltd., where he researched and developed optical disk systems. In 1995, he joined the University of Tokyo and is now an Associate Professor in the Department of Mechano-Informatics, School of Information Science and Technology. His research interest is micro systems.
Isao Shimoyama received B.E., M.E., and Ph.D. degrees in mechanical engineering from The University of Tokyo, Japan, in 1977, 1979, and 1982, respectively. He joined The University of Tokyo in 1982 and is presently a Professor in the Department of Mechano-Informatics, School of Information Science and Technology. He has been a MEMS conference committee member since 1996, and his current research interest is microsystems, including MEMS structures and functions based on insects.