Title

Design, fabrication, and characterization of subwavelength metallic structures

Author

Zhi Wu

Date of Award

2011

Degree Name

Ph.D. in Electro-Optics

Department

Department of Electro-Optics and Photonics

Advisor/Chair

Advisor: Qiwen Zhan and Andrew M. Sarangan

Abstract

Subwavelength metallic structures are drawing increasing attention due to the interaction of free electrons in the metal with light waves. By fabricating different metallic structures, electrons will be confined in different ways, which enables the manipulation of light-matter interactions and light wave properties. In this dissertation, several types of subwavelength metallic structures are studied for different applications, including metallic wiregrid linear polarizers and spiral plasmonic lenses for real time full Stokes vector imaging polarimetry, and LRSP devices for bio-chemical sensing, spectral notch filters, and electro-optic (EO) modulators. Surface plasmon polaritons (SPPs) are electromagnetic surface waves propagating along metallic and dielectric interfaces caused by electron density oscillations. Long range surface plasmon (LRSP) is one eigen mode due to the coupling of SPPs excited on both surfaces of an extremely thin metallic film immersed in a symmetric dielectric environment. The uniqueness of LRSP is that its electric field distribution is antisymmetric, which pushes the electric field out of the metal region, resulting in lower propagation loss. A lower propagation loss further yields a surface plasmon resonance (SPR) with smaller full width at half maximum (FWHM), leading to the ultra high sensitivity of LRSP SPR on the change of the dielectric environment around the metallic thin film. This ultra high sensitivity can be utilized for various applications, such as bio-chemical sensors, EO modulators, and spectral notch filters. I use subwavelength metallic grating structures to replace the traditionally applied continuous metallic thin film. Due to the coupling effect between the metallic wire grids, LRSP can still be supported to propagate along the structure, but with lower damping loss due to the reduction of metal volume. An EO modulator with a modulation index of 100% and reflectivity bias of 47% is designed with a voltage change of only 9 V. A spectral notch filter working at 808 nm is designed to have a FWHM down to 3 nm. Moreover, a passive tunability up to 45 nm of the working wavelength of the spectral notch filter can be obtained by simply adjusting the incident angle. Preliminary experiment measurements of SPR curves for both gold films and gold gratings verify the narrowing effect as predicted. The polarimetric content of an image provides an extra degree of information that can be used to enhance and identify objects within a scene, which have important applications in remote sensing, surveillance and target tracking. However, portable devices for real time full Stokes vector imaging polarimetry has been a challenge due to the lack of circular polarizer analyzer arrays compatible to modern nanofabrication technique, which are used for fabricating pixel-sized linear polarizers. A novel spiral plasmonic lens design solves this problem. The handedness dependence of the focusing properties for spiral plasmonic lenses is experimentally characterized. For an array of spiral plasmonic lenses fabricated with focused ion beam (FIB) milling, the transmitted intensity distributions at the surface of the array under both right-hand circular polarized (RHC) and left-hand circular polarized (LHC) illuminations are measured at the far field with a simple two-photo fluorescence (TPF) microscopy setup. The focusing property of the spiral plasmonic lens exhibits strong dependence on the handedness of illumination, enabling the use of these spiral plasmonic lenses as miniature circular polarization analyzers. Circular polarization ER of the TPF signal higher than 200 is obtainable for a spiral device with a lateral size as small as 4λspp and a thickness of only 200 nm. This finding will push the spiral plasmonic lens one step closer towards its desired applications in real time full Stokes parameter polarimetric imaging.

Keywords

Nanostructured materials Optical properties, Metallic films Optical properties, Surface plasmon resonance, Polarizers (Light)

Rights Statement

Copyright 2011, author

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