Modifying Laser Written 3D Photonic Structures by Post-processing

Abstract

Direct laser writing can fabricate arbitrary three-dimensional photonic crystals with a given lattice parameter. Such a photonic crystal has the capability to alter the path of photons because of the interference and diffraction arising from its periodic structure on the scale of wavelengths. This capability forms a photonic band structure and bandgap, leading to the band structure and bandgap for photons. The concept of a photonic bandgap is crucial, as it prevents the propagation of light within a certain frequency range, analogous to how an electronic bandgap in semiconductors restricts electron movement across energy levels. However, achieving a photonic bandgap within the visible spectrum presents a notable challenge due to the precise structural control required at smaller scales, which is a struggle in current fabrication technology.
In this thesis, I use a commercial direct laser writing system to write three-dimensional periodic structures in the polymer as templates, aiming to achieve light confinement or reflection in the visible range. However, the mechanism of direct laser writing restricts its resolution and limits us to infrared and mid-infrared wavelength reflecting structures. Hence, in this study, I aim to modify the three-dimensional photonic crystals using various post-processing methods
after the structures have been written. These methods involve the refractive index change by coating with high refractive index material in Chapter 3, unit cell size scale down by thermal shrinkage in Chapter 4 and structure material replacement by backfilling in Chapter 5. I show that it is possible to control the detailed structure by post-processing in the future to locate the photonic bandgap in the desired spectral region. These outcomes have enhanced the fabrication methodology, with the precise management of the bandgap position, advancing the approach towards applications in sensing displays, wide-angle optical filters, and devices exploiting cavity quantum electrodynamics (QED).

Date of Award	19 Mar 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	John G Rarity (Supervisor) & Daniel Ho (Supervisor)