Electrical and Computer Engineering ETDs

Publication Date

Summer 7-13-2018

Abstract

Interaction of optical waves with nanostructures made of various material systems has been the subject of intensive research for many years. These researches have been mainly driven by the need to make smaller optical devices and exploiting the functionalities offered by light-matter interaction in nanoscale. Majority of the nanostructures are fabricated using electron beam (e-beam) lithography that is slow and expensive. As such alternative methods have been developed to enable nanoscale fabrication faster and less expensive. Among these interferometric lithography (IL) is a relatively simple method for quick fabrication of nanostructures. As IL method generates periodic patterns, exploring the potential applications of the nanostructures that can be fabricated using it, is of primary importance. This dissertation is focused on two applications of silicon nanostructures fabricated by IL method: nanostructured anti-reflection layers (NALs) and plasmonic nanostructures based on arrays of silicon nanopillars (SiNPs) for surface enhanced Raman spectroscopy (SERS). Silicon has been chosen as the structural material due to its extensive usage as the substrate for monolithic electronic circuits and many optical devices.

NALs offer several advantages over traditional antireflection coatings made by multilayer deposition. NALs are created by fabricating a nanostructured surface on the substrate material without the need for deposition of different materials and therefore can tolerate large thermal gradients in high power laser applications. We have developed a mathematical model and calculated the optimal profile for the unit cell for a silicon NAL and examined its performance using rigorous coupled-wave analysis (RCWA). The impact of different geometrical parameters on the performance of NALs have been carefully studied. In particular we have evaluated the impact of these geometrical parameters on the transmitted optical power and suppression of higher spatial modes generation. Next using the theoretical outcomes as a guide, we have fabricated several silicon NALs using IL patterning followed by dry etching and measured and computed their performance in mid-IR spectral region.

The second category of silicon nanostructures studied here consist of flat top silicon nanopillar (SiNP) arrays with one or a stack of metallic nanodisks on top (with silica nanodisk as spacer) used for Raman enhancement applications. These structures, fabricated using IL, are designed to enhance Raman emission from the adsorbed molecules using surface plasmons. This is achieved by high electric field enhancement, through localization of plasmons at the edges. In order to understand the enhancement mechanism, resonance of these nanostructures along with the E-field enhancements are carefully studied using numerical simulations. Regarding possible role of nanopillars in field enhancement, simulation results have revealed hybridization between SiNP and plasmonic nanodisk stacks. This indicates possibility of transfer of energy of incident laser into plasmonic structure through nanopillar, further amplifying the E-field enhancement. We have also studied the role of geometrical and structural parameters on the field enhancement of these nanostructures. This provides a guide for designing nanostructures with optimal field enhancement for SERS. Next, we have fabricated several samples of SiNPs caped with gold nanodisks and gold-silica-gold nanodisk (stacks) and tested their performance as SERS substrates by measuring the spectrum of the Raman signal (using Thionine and Methylene Blue as target molecules). Our experimental studies have revealed the impact of geometrical parameters of the SiNP and gold nanodisks on the Raman signal. We have also fabricated and tested gold nanodisk performance using silica nanopillar. Finally we have fabricated and tested SiNPs caped with selected non-metallic nanodisks obtained by post processing of nanodisks made of Ge, and TiN. This preliminary study paves the road for a new category of SiNP based SERS substrates that may have advantages over those that use metallic caps in certain applications.

Keywords

Anti-Reflection-Coatings, SERS, Field-Enhancement, Germanium, Plasmon

Document Type

Dissertation

Language

English

Degree Name

Electrical Engineering

Level of Degree

Doctoral

Department Name

Electrical and Computer Engineering

First Committee Member (Chair)

Mani Hossein-Zadeh

Second Committee Member

Terefe Habteyes

Third Committee Member

Tito Busani

Fourth Committee Member

Payman Zarkesh ha

Fifth Committee Member

Steven R.J. Brueck

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