title: Linear and Non-Linear Polarising Meta-Optics Enabled by Photonic Inverse-Design
Creators: name: Li, Neuton
Publisher: name: The Australian National University
Year: 2025

description: Optical devices are ubiquitous in many of the most advanced technologies that we have developed, from polarisation cameras, to space-faring telescopes, to nanostructured displays in augmented reality headsets. Metasurfaces have the potential to disrupt and revolutionise all of these systems, by offering a compact solution with unmatched novel functionality. Whatever the promise of metasurfaces, it is still a challenging effort to design efficient and robust devices. In this thesis, we formulate and develop a novel and general inverse design optimisation scheme that enables the freeform metasurface topologies. This massively expands the utility of our optimisation algorithm, which we then employ to several different applications covered within this thesis. Our primary focus are applications which involve manipulating multiple properties of light simultaneously. These aim to address the most compelling research goals, from high efficiency polarised diffraction, to polarised phase derivative imaging. We first explore the intersection of interactions between polarisation, diffraction, dispersion, and incident angle. These understandings lead us to optimise for different scenarios that would have the highest benefit using metasurfaces. We report the first single metasurface that has beam splitting into the same polarisation states, demonstrating high conversion efficiency of unpolarised light. We also report a novel idea of continuously tunable polarisation dichrosim by varying the tilt of the metasurface, allowing the metasurface to act as fundamentally different polarisers simultaneously. In both examples, we fabricate and experimentally validate our simulated performance with optical characterisation. Next, we explore more complex systems involving multi-objective optimisations. We propose dispersion engineered beam splitting metamaterials, which is achieved by optimisation over the entire metamaterial volume. Our design pushes the boundary on what is possible for broadband devices. Another application is analogue optical processing using a single piece of metasurface. We achieve the largest asymmetrical NA linear OTF for phase derivative imaging, as well as single-shot quantitative imaging using polarised sensing. We believe that our advancements in these metasurface technologies will lead to future novel optical devices with practical utility. Finally we extend our inverse design optimisation algorithm for non-linear metasurfaces, especially the case of sum frequency generation (SFG) for this thesis. Non-linear metasurfaces are an attractive option for creating entangled photon sources, as well as compact up-conversion devices. We employ our optimisation algorithm to design freeform metasurfaces with SFG that have purposeful diffraction shaping and polarisation sensitivity, all the while maintaining high generation efficiency. We anticipate that such inverse algorithms will be crucial in designing metasurfaces for complex applications, such as non-linear quantum photon sources.