Mechanically Tunable Nanophotonic Devices
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Mechanically Tunable Nanophotonic Devices W. Park1, E. Schonbrun1, M. Tinker2, Q. Wu1 and J.-B. Lee2 1 Department of Electrical & Computer Engineering, University of Colorado, Boulder, CO 80309-0425, U.S.A. 2 Department of Electrical Engineering, University of Texas, Dallas, TX 75083-0688, U.S.A. ABSTRACT We report a novel tunable nanophotonic device concept based on Mechanically Controlled Photonic Crystal (MCPC), which is comprised of a periodic array of high index dielectric material and a low index polymer. Tunability is achieved by applying mechanical force with nano-/micro-electron-mechanical system actuators. The mechanical stress induces changes in the periodicity of the photonic crystal, to which the photonic band structure is extremely sensitive. This consequently produces tunability much greater than that achievable by electrooptic materials such as liquid crystal. Our theoretical investigations revealed that we could achieve dynamic beam steering over a wide range of angles up to 75o with only 10% mechanical stretching. We also predicted tunable sub-wavelength imaging in which we could tune the frequency response and focal length of negative index PC lens. For experimental demonstration, we fabricated the PC structures on Si-on-insulator substrates. Optical characterizations clearly showed the anticipated negative refraction in which the incident beam was refracted back to the side it was incident. The experimental demonstration of negative refraction at optical frequencies in a Si-based photonic crystal structure is a significant step toward the next-generation nanophotonics. INTRODUCTION Photonic crystal (PC) is an artificially-made periodic dielectric material in which light propagation is modulated in a similar way to the electronic motion in a periodic potential produced by a real crystal. PCs provide a new means to control the propagation and generation of light in nanoscale geometry and therefore are an ideal platform to develop next-generation nanophotonic devices. However, we identify dynamic control of the photonic properties as a critical functionality needed to fully realize the potential of PC-based nanophotonic devices. There have been some work on achieving tunability by incorporating liquid crystal (LC) or lead lanthanum zirconium titanate (PLZT) in PC structures. However, this scheme fails to produce large tunability primarily because of the small attainable change in refractive index, ∆n, in these electro-optic materials [1,2,3,4,5]. Recently, Park and Lee proposed a novel device concept based on mechanically controlled PC (MCPC) structures and theoretically predicted that extremely wide tunability is achievable [6]. MCPC is composed of periodic Si pillars embedded in a flexible polymer film, which is then subject to mechanical stress by a pair of micro-machined actuators such as microcomb drive actuators or electro-thermal actuators depending on the required force. This radically different scheme of achieving dynamic tunability takes advantage of the fact that photonic band str
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