Tunable plasmonic devices in gold/dielectric nanostructures

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Tunable plasmonic devices in gold/dielectric nanostructures

 

Author: Ng, Ki Fung
Title: Tunable plasmonic devices in gold/dielectric nanostructures
Degree: M.Phil.
Year: 2015
Subject: Nanostructured materials.
Plasmons (Physics)
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Physics
Pages: xiv, 93 leaves : color illustrations ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2806854
URI: http://theses.lib.polyu.edu.hk/handle/200/7929
Abstract: This thesis presents the studies on the optical properties and applications of three types of periodic plasmonic structures, which were investigated by simulation using the Rigorous Coupled Wave Analysis (RCWA) method, and by experiments through angular-dependent reflectivity measurements. The optical responses such as reflectivity, transmittance and electric field enhancement at the nanometer scale depend on the shape and size of nanostructures, which play pivotal roles in the design of devices based on the surface plasmon resonance (SPR) effect. First of all, two-dimensional plasmonic nano-pillar structures were simulated. In such samples with Au/photoresist as the metal/dielectric materials, a thin gold layer was deposited perpendicularly on photoresist pillars, leading to isolated gold caps on dielectric pillars that are in turn surrounded by a perforated gold film. Propagating surface plasmon resonances (PSPRs) on the perforated film, and localized surface plasmon resonances (LSPRs) at the gold caps, were simultaneously observed in such a structure. For the electric field distribution, the incident wave was confined at the Amonil pillars due to the coupling of resonant modes for the perforated gold film and the isolated gold capping layer. Reflection dips were blue-shifted and became sharper with increasing height of the dielectric pillars. The usage of such a device structure for refractive index sensing was discussed. For the second part of the project, plasmonic Au nanostructures with fractal behavior were investigated by RCWA technique. Fractal patterns that show self-similar behavior and space-filling geometrical arrangement can cause unique optical properties such as transmission enhancement. A type of fractal nanostructure based on Au film/glass substrate system was investigated in this thesis. The fractal pattern was named as center fractal nanostructure in the thesis, as the self-similarity effect occurs at the center of the structure. The effects of three structural parameters (edge length, periodicity and order) on the SPR (transmission dip) were studied. The electric field intensity enhancement factor was boosted with increasing order of the fractal pattern nanostructure. In particular, an electric-field intensity enhancement of ~ 70 times was achieved for the 3rd order fractal pattern studied in this work. As compared with a continuous planar gold film on glass, transmission enhancement peaks were observed in the fractal nanostructure, and the transmission peak positions were controlled by the edge length of the fractal nanostructure. Properly designed nanostructures are known to produce so-called hot spots for locally enhancing the incident wave intensity. Such a structure is of potential for center fractal nanostructure to be used in antennas. Finally, SPR tuning via thermally-induced refractive index change in ferroelectric polymer P(VDF-TrFE) was studied. The transition behavior of the copolymer was found to be dependent on the thermal history. The refractive indices of P(VDF-TrFE) were 1.3886 and 1.3637 at 633 nm and 80°C during heating and cooling processes respectively. One-dimensional gratings with periodicity 750 nm were used to excite the SPR. The resonant wavelength was found to change during the heating and cooling process. The experimental results of reflectivity measurement were compared with the simulation results. The results indicated the feasibility of bistable SPR in solid-state structures.

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