All-Optical Active Plasmonics Based on Ordered Au Nanodisk Array Embedded in Photoresponsive Liquid Crystals
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All-Optical Active Plasmonics Based on Ordered Au Nanodisk Array Embedded in Photoresponsive Liquid Crystals Yue Bing Zheng, Vincent K. S. Hsiao, and Tony Jun Huang Department of Engineering Science and Mechanics, Pennsylvania State University, 212 EarthEngineering Sciences Building, University Park, PA, 16802 E-mail: [email protected] (T. J. Huang) ABSTRACT We propose a new approach towards all-optical active plasmonics based on ordered Au nanodisk arrays embedded in azobenzene-doped liquid crystals (LCs). Upon photoirradiation, the doped LCs went through phase transition induced by trans-cis photoisomerization of the azobenzene molecules. The phase transition led to the change in the refractive index of the LCs experienced by incident light, and enabled reversible tuning of the localized surface plasmon resonance (LSPR) of the embedded Au nanodisks. The tuning utilized the sensitivity of the LSPR of the Au nanodisks to the change in the surroundings’ refractive index. Experimental observations on both peak shift and intensity change of the LSPR matched those from discrete dipole approximation (DDA) calculations. INTRODUCTION Plasmonics has both the capacity of photonics and the miniaturization of electronics, offering the potential to merge photonics and electronics at nanoscale dimensions [1]. Plasmonic waveguide, a passive component that acts as interconnector in the integrated circuits, has been intensely studied and shown optimistic for practical applications [2]. However, active plasmonic devices such as switches and modulators have to be developed before the full potential of plasmonics can be realized [3]. For achieving most of the active plasmonic devices, the capability of reversible tuning of the surface plasmon resonance (SPR) of metal nanostructures with control signals is a prerequisite. Chau et al. demonstrated that terahertz light transport in spintronic-plasmonic media, a dense ensemble of subwavelength bimetallic ferromagnetic/nonmagnetic microparticles, can be actively controlled by magnetic field [4]. Leroux et al. showed that the shape and peak wavelength of LSPR of Au nanoparticles embedded in polyaniline thin films can be reversely tuned by electrochemically changing refractive index of the polyaniline thin films [5]. Electrochemical tuning of LSPR of Ag nanoparticle arrays modified by WO3 sol-gel was realized by Wang et al. [6]. The birefringence in LCs, a large change in refractive index obtained by changing the alignment of LCs, made LCs an ideal functional material for active plasmonics. Active tuning of SPR of different metal nanostructures has been achieved by electrically controlling the alignment of LCs near the surfaces of the nanostructures [7]. Light exhibits several advantages as control signals, including fast response speeds, and simultaneous reading and writing. So does all-optical active plasmonics in which the active tuning of SPR is controlled by light. In this paper, we demonstrate all-optical active plasmonics based on photoresponsive LCs. The photoresponsive L
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