Nanoscale Light Confinement: Principles, Measurement, and Applications
By confining photons to small volumes for long periods of time, optical nanocavities offer the ability to greatly enhance the interaction between light and matter. This can greatly improve the efficiency of photonic devices as well as lead to novel physical phenomena. While over the past several years resonators have improved to confine photons for longer periods of time, the volume in which light can be confined has remained relatively stagnant on the order of a cubic half-wavelength which has been thought to be the fundamental limit. In this dissertation we demonstrate that the effective mode volume of optical resonant cavities can be reduced below a cubic half-wavelength. We develop novel tools to characterize these highly confined optical modes, and utilize this light confinement to achieve efficient light-matter interaction in photonic devices. Finally we present novel physical phenomena which result from this nanoscale light confinement. The dissertation is organized into six chapters. Chapter 1 gives a brief introduction to photonics and the reasons for pursuing nanoscale light confinement. In Chapter 2 we define the effective mode volume and discuss its theoretical limit. We show with analytical and numerical calculations that contrary to previous assumptions sub-wavelength-sized dielectric structures can enable mode volumes smaller than a cubic half-wavelength. In Chapter 3 we discuss experimental techniques for measuring these ultra-small mode volumes. We introduce a new high-resolution near field measurement technique called Transmission-based Near-field Scanning Optical Microscopy (TraNSOM), and show experimental results verifying nanoscale light confinement in our devices. In Chapter 4 we discuss applications for these small-mode-volume devices. We show analytically and numerically that these devices can be surprisingly efficient for achieving gain and lasing in an electrically-pumped silicon-based platform, and we experimentally demonstrate highly sensitive detection of acetylene gas. In Chapter 5 we discuss new physical phenomena associated with small volume optical resonant cavities. We show that these cavities can behave as an individual radiating dipole and, using the TraNSOM technique, the lifetime of this dipole can be modified at long distances. Chapter 6 gives a brief conclusion and outlook on the future of this field.
Photonics; Nanotechnology; Near-field; Optics; Microscopy; Scanning Probe Microscopy