Light-Sound Interaction in Nanoscale Silicon Waveguides

TL;DR

This thesis investigates photon-phonon interactions in nanoscale silicon waveguides, demonstrating Brillouin scattering, amplification, and enhanced optomechanical coupling, with implications for integrated photonics and microwave signal processing.

Contribution

It provides a comprehensive theoretical and experimental analysis of light-sound interactions in nanoscale silicon waveguides, including new fabrication techniques and coupling mechanisms.

Findings

Observation of Brillouin scattering in nanoscale silicon waveguides

Achieved Brillouin amplification exceeding propagation losses

Enhanced optomechanical coupling in narrow silicon slot waveguides

Abstract

This thesis studies the interaction between near-infrared light and gigahertz sound in nanoscale silicon waveguides. Chapter 2 introduces photon-phonon coupling and its theoretical description, describing basic mechanisms and developing a quantum field theory of the process. Chapter 3 explores the dynamical effects in both waveguides and cavities. It also proves a connection between the Brillouin gain coefficient and the vacuum coupling rate. Chapter 4 deals with the observation of Brillouin scattering in nanoscale silicon waveguides. The waveguides tightly confine $193 \, \text{THz}$ light and $10 \, \text{GHz}$ acoustic vibrations. The acoustic quality factor remains limited to about $300$ because of leakage into silica substrate. These waveguides are optically transparent in a narrow band of frequencies at a pump power of $25 \, \text{mW}$. Besides this amplification, we translate a $10 \, \text{GHz}$ microwave signal across $1 \, \text{THz}$. Chapter 5 extends the experimental work of chapter 4 by fabricating a cascade of fully suspended nanowires held by silica anchors. This enhances the mechanical quality factor from $300$ to $1000$, enabling the observation of Brillouin amplification exceeding the propagation losses in silicon. The amount of amplification is mostly limited by a rapid drop in acoustic quality as the number of suspensions increases. We propose a mechanism to cancel this inhomogeneous broadening. Chapter 6 looks at the potential of narrow silicon slot waveguides to enhance the optomechanical coupling. For certain dimensions, these waveguides support opto-acoustic modes with an interaction efficiency simulated an order of magnitude above those of single-nanobeam systems.