Personalised Optical Sensors based on Metasurfaces

Abstract

Metasurfaces are arrays of subwavelength particles that strongly interact with electromagnetic radiation. Strong light-matter interactions aided by optical resonances allow precise tailoring of optical responses. Metasurfaces have been employed to realise extraordinary applications such as optical sensing, flat optics, non-linear signal generation, etc. In particular, optical sensing has become a significant research focus. Metasurface-based optical sensors are highly sensitive, label-free, non-invasive or minimally invasive, robust, compact, and cost-effective. These unique features make them a great alternative to bulky lab-based devices. Currently, further developments in optical sensing are limited by a low quality factor (Q-factor) and weak light-matter interaction. In this thesis, we employ high Q-factor dielectric resonators based on bound states in continuum (BIC) to enhance the Q-factor, and induce strong lightmatter interactions. Chapter 1 introduces the theory and recent developments in nanophotonic structures. It covers key concepts such as Mie theory, BIC, and optical sensing, followed by the thesis motivation and outline. Chapter 2 presents details on numerical simulations, nanofabrication, surface functionalisation, surface texturing, and optical setups. It also discusses the end-to-end process involved in optical sensing applications. In Chapter 3, I compare different eigenmode resonances and their impact on optical sensing parameters. I present a novel Quasi-BIC dielectric metasurface design based on structural asymmetry, which maximises key sensing parameters to enhanced biosensing performance. Finally, we experimentally compared the implementation of the Quasi-BIC metasurface design to detect various biomolecules, including primary and secondary antibodies, as well as different types of proteins. In Chapter 4, we propose a novel optical sensing mechanism using metagratings. This approach exploits the principle that diffraction order intensity variations with changes in the refractive index. Using this method, we experimentally demonstrate exceptionally high sensitivity for bulk refractive index sensing. Finally, we apply surface functionalisation to detect antibodies selectively, successfully identifying the target biomarker within a mixture of antibodies. In Chapter 5, we develop a novel volatile organic compound (VOC) sensor using Quasi-BIC metasurfaces decorated with metal-organic frameworks (MOFs). By combining metasurfaces with MOFs, we achieve selective detection of acetone vapour at room temperature. Additionally, we demonstrate the dependence of VOC sensitivity and selectivity on MOF density. This work highlights the sensor's high sensitivity and selectivity for acetone vapour while remaining unresponsive to other similar VOCs. Chapter 6 concludes the thesis by highlighting key research outcomes and providing insight into future research directions.