Modification of Plasmonic Nano Structures' Absorption and Scattering Under Evanescent Wave Illumination Above Optical Waveguides or With the Presence of Different Material Nano Scale Atomic Force Microscope Tips

Author

Gazi Mostafa Huda, University of KentuckyFollow

Date Available

2-1-2014

Year of Publication

2014

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department/School/Program

Electrical Engineering

First Advisor

Dr. J. Todd Hastings

Abstract

The interaction of an evanescent wave and plasmonic nanostructures are simulated in Finite Element Method. Specifically, the optical absorption cross section (C_abs) of a silver nanoparticle (AgNP) and a gold nanoparticle (AuNP) in the presence of metallic (gold) and dielectric (silicon) atomic force microscope (AFM) probes are numerically calculated in COMSOL. The system was illuminated by a transverse magnetic polarized, total internally reflected (TIR) waves or propagating surface plasmon (SP) wave. Both material nanoscale probes localize and enhance the field between the apex of the tip and the particle. Based on the absorption cross section equation the author was able to demonstrate the increment of absorption cross section when the Si tip was brought closer to the AuNP, or when the Si tip apex was made larger. However, the equation was not enough to predict the absorption modification under metallic tips, especially for a AgNP's C_abs; neither it was possible to estimate the optical absorption based on the localized enhanced field caused by a gold tip. With the help of the driven damped harmonic oscillator equation, the C_abs of nanoparticles was explained. In addition, this model was applicable for both TIR and Surface Plasmon Polaritons illuminations. Fitting the numerical absorption data to a driven damped harmonic oscillator (HO) model revealed that the AFM tip modifies both the driving force (F₀), consisting of the free carrier charge and the driving field, and the overall damping of the oscillator beta. An increased F₀ or a decreased beta will result in an increased C_abs and vice versa. Moreover, these effects of F₀ and beta can be complementary or competing, and they combine to either enhance or suppress absorption. Hence, a significantly higher beta with a small increment in F₀ will result in an absorption suppression. Therefore, under a Si tip, C_abs of a AuNP is enhanced while C_abs of a AgNP is suppressed. In contrast, a Au tip suppresses the C_abs for both Au and Ag NPs. As an extension of this absorption model, further investigation of the guided mode and a close by nanostructure is proposed, where the scattered wave off the structure attenuates the guided mode with destructive interference.

Recommended Citation

Huda, Gazi Mostafa, "Modification of Plasmonic Nano Structures' Absorption and Scattering Under Evanescent Wave Illumination Above Optical Waveguides or With the Presence of Different Material Nano Scale Atomic Force Microscope Tips" (2014). Theses and Dissertations--Electrical and Computer Engineering. 43.
https://uknowledge.uky.edu/ece_etds/43