Reversible tuning of optical metasurfaces
Creators: name: Zangeneh Kamali, Khosro
Publisher: name: The Australian National University
Year: 2024

description: The synergy between advances in fabrication technology and computing power over the last few decades has led to the discovery of novel light-matter interactions at the nanoscale. Arrays of subwavelength-sized and placed particles constitute surface-type optical systems -- metasurfaces. Light confinement and optical resonances in metasurfaces allow tailoring light attributes, such as phase, intensity, and direction, with subwavelength-sized spatial resolution. As a result, metasurfaces have unlocked the reproduction of many functions of conventional optical or optomechanical components, including lenses and beam deflectors, as well as new types of optics with unprecedented applications, for example, optical equation solvers. Still, metasurfaces suffer from a critical shortcoming that significantly reduces their usability: tunability. Tunability refers to the ability of a system to display various responses as required. The geometry, construction material, and surrounding media of metasurfaces define their resonant features and optical properties. Many of these properties are unmodifiable after fabrication; thus, tuning metasurfaces is inherently challenging. Aside from altering these systems mechanically, methods based on electrical gating, liquid crystal implementation, and chemical modification are among the approaches for tailoring the metasurface optical response. Generally, in the metasurface tuning methods, there is a tussle between receiving a faster response and observing prominent modulation. This thesis covers and evaluates several strategies to tune the optical properties of metasurfaces. It includes seven chapters and is structured as follows. Chapter 1 briefly introduces light-matter interactions, examples of nanoparticles in nature, Maxwell's equations and interface boundary conditions. It contains an introduction to meta-optical concepts, followed by an overview of metasurface tuning mechanisms. Chapter 2 reviews the simulation methods for designing a metasurface. Then, it describes the fabrication tools and methods used to manufacture the metasurface samples considered in this thesis. Lastly, I explain the optical characterisation techniques I use to measure the metasurfaces' optical response. Chapter 3 contains the results I obtained for polarisation tuning of metasurfaces to control the intensity of the third-harmonic signal and thus to construct tunable up-converted images. Although this method lacks the modulation speed and required control over the illumination condition, it shows significant tunability. Chapter 4 describes the thermo-optical effect in high-quality factor metasurfaces to create a thermally tunable image. The metasurface quality factor and resonance features determine the modulation temperature requirements and the modulation depth of the system here. Chapter 5 introduces the application of localised heaters to engineer programmable metasurface-based circuitry, revealing a sub-millisecond switching speed. I fabricated the micro-heaters with indium tin oxide, which exhibits a transparency window at wavelengths below 900 nm. Chapter 6 focuses on the ultrafast tuning of the crystalline silicon metasurface for rapid transient transmission modulation. The main contributor to this ultrafast transient modulation is the free carriers driven by photoexcitation of the crystalline silicon illuminated with energy higher than the bandgap. We demonstrated that our system shows an absolute modulation of nearly 30%. Chapter 7 concludes the thesis and provides an overview of the work done during my PhD studies, as well as my outlook on the topic.