Nanophotonic Phenomena in Dielectric Photonic Crystals

Citation

Ng, Ryan Cecil (2020) Nanophotonic Phenomena in Dielectric Photonic Crystals. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ZP30-F550. https://resolver.caltech.edu/CaltechTHESIS:02122020-151048251

Abstract

Photonic crystals are periodic optical nanostructures with varying dielectric constant that allow light flow to be controlled and manipulated much in a similar way to electrons within a semiconductor crystal. These nanostructures tend to have a spatially varying refractive index on the order of the wavelength of light to be manipulated. 1D and 2D photonic crystals have already garnered significant attention in the realm of thin-film optics, while 3D photonic crystals have been thus far limited in application, due to difficulties in fabrication and a lack of available materials for fabrication.

In this work, we first explore 1D and 2D photonic crystals based on the concept of a guided mode resonance, which manifests as a narrow near-unity resonance in reflection or transmission that arise from the coupling of an incident wave into a leaky waveguide mode via a grating vector that is subsequently re-radiated. Such a resonance is well-suited for multi- and hyper- spectral filtering applications in the infrared. We designed a platform consisting of amorphous Si arrays embedded in SiO₂ in simulation and experiment for application as narrow stopband filters. We present the tunability of the spectral characteristics of the resonance in these arrays through variation of array geometric parameters in simulation and experiment. Guided mode resonance designs often consider only the case of an infinite array, where the leaky waveguide mode can propagate laterally for hundreds of periods, allowing for this mode to eventually scatter out of the array giving rise to the characteristic narrow near-unity rapid spectral variations of a GMR. With an insufficient number of periods, the quality factor and thus the optical filtering performance is greatly diminished. Thus, we further extend our analysis to compact periodic arrays of finite size, which are required for high spatial resolution snapshot imaging, and introduce array designs that operate under finite size limitations in the near-infrared.

We then transition to 3D photonic crystals, exploring the use of an additive manufacturing process to directly fabricate nanocrystalline rutile TiO₂ with ~100 nm resolution. Though TiO₂ was chosen as the model material, the key to this work is that a similar process can be used to print many different materials, enabling future applications of 3D photonic crystals. The focus here is the additive manufacturing of high index materials such as TiO₂, and its potential for photonic applications is demonstrated by characterizing the optical band gap of 3D PhC TiO₂ structures printed with this method. We present a system where the ability to print high refractive index 3D photonic crystals would be useful, by studying 3D polymer-germanium core-shell structures that should exhibit all-angle negative refraction in the mid-infrared regime.

Thesis Files