Obtaining Circularly Polarized Optical Spots Beyond the Diffraction Limit Using Plasmonic Nano-Antennas
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1208-O21-02
Obtaining Circularly Polarized Optical Spots beyond the Diffraction Limit Using Plasmonic Nano-Antennas Erdem Ogut1, Gullu Kiziltas1, and Kursat Sendur1 1 Sabanci University, Faculty of Engineering and Natural Sciences, Istanbul, 34956, Turkey. ABSTRACT With advances in nanotechnology, emerging plasmonic nano-optical applications, such as all-optical magnetic recording, require circularly-polarized electromagnetic radiation beyond the diffraction limit. In this study, a plasmonic cross-dipole nano-antenna is investigated to obtain a circularly polarized near-field optical spot with a size smaller than the diffraction limit of light. The performance of the nano-antenna is investigated through numerical simulations. In the first part of this study, the nano-antenna is illuminated with a diffraction-limited circularly-polarized radiation to obtain circularly polarized optical spots at nanoscale. In the second part, diffraction limited linearly polarized radiation is used. An optimal configuration for the nano-antenna and the polarization angle of the incident light is identified to obtain a circularly polarized optical spot beyond the diffraction limit from a linearly polarized diffraction limited radiation. INTRODUCTION Circular polarization is utilized in various applications at radio frequency and microwave regimes due to its advantages, such as increased efficiency in power transmission [1]. At optical frequencies, circular polarization promises to be a rotary power source for various applications [2-6]. With advances in nanotechnology, circularly-polarized electromagnetic radiation beyond the diffraction limit is desired in emerging plasmonic nano-applications. One of these applications is all-optical magnetic recording [7-8]. Stanciu et al. [7-8] demonstrated that magnetization can be reversed in a reproducible manner by using a circularly polarized optical beam without any externally applied magnetic field. The size of the magnetization reversal in that study was on the order of 20 microns due to the large optical spots that were utilized. To advance the areal density of hard disk drives beyond 1 Tbit/in.2, magnetization reversal areas much smaller than 100 nm are required. To achieve sub-100 nm bits in an all-optical magnetic recording system, a circularly polarized optical spot beyond the diffraction limit is necessary. In this study, two different schemes are investigated to obtain a circularly polarized optical spot with dimensions smaller than the diffraction-limit. In the first part of this study, a cross-dipole nano-antenna is investigated to obtain a circularly-polarized optical spot with a size beyond the diffraction limit when the antenna is illuminated with diffraction-limited circularlypolarized radiation. An optimal antenna geometry is specified to obtain an intense optical spot that satisfies two necessary conditions for circular polarization: a phase difference of 90° and a unit amplitude ratio between the electric field components in the vicinity of the antenna gap. In the second par
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