Investigation of resonant effects in core-shell metasurface geometries and design concept of a metasurface for harmonic generation

Abstract

Metasurfaces are two-dimensional arrays of sub-wavelength structures often arranged in a periodic lattice. During recent years, metasurfaces have emerged as one of the most promising platforms to shape electromagnetic waves using compact devices, exhibiting extraordinary control over the light amplitude, polarization and phase. Starting with metallic materials, metasurface research has shifted toward dielectric-based devices to achieve low losses and enable control of light in the visible spectrum, with new functionalities emerging from complex resonant effects within the scatterers. Resonant metasurfaces provide a new platform to engineer the electromagnetic response, and the ease of fabrication allowed researchers to explore a variety of new designs over the years for many different applications, including harmonic generation. While some materials of choice have been identified for specific applications and spectral ranges, new approaches based on hybrid material compositions have been lacking and could offer new degrees of freedom for tuning the resonances without modifying the scatterers geometry. This thesis investigates the resonant response of all-dielectric metasurfaces composed of meta-atoms with a core-shell structure, in which a cylindrical pillar of one material is coated by a shell of a second material. The scattering and resonant response of single meta-atoms and metasurfaces having a core-shell geometry are studied using finite element simulations, investigating how the shell thickness, pillar diameter, height and periodicity affect the resonant behaviour of the devices. Two metasurfaces are fabricated using electron beam lithography and atomic layer deposition and are optically characterized using a white light source. In parallel, a metasurface concept optimised for third harmonic generation is designed to be placed inside a multicell pass to increase the efficiency of the nonlinear process.

Popular Abstract

Controlling and generating light has always been a goal of humanity, long before anyone understood what light actually was. But it is only ”recently”, during the last few centuries, that we ”shed some light” on the origins of colors, rainbows, the blue sky, and how mirrors and lenses can reflect and bend the light. For over 200 years, researchers have been studying the interaction
between light and materials, culminating in the recent developments of metasurfaces. Metasurfaces are a new type of very thin artificially designed materials whose surfaces are composed of tiny structures arranged in a regular array, and that are capable of achieving optical properties that are beyond those seen in materials found in nature. This patterned surface can be seen as a mosaic, in which the structures, called meta-atoms, are the individual ceramic pieces. In a mosaic, the complex arrangement of the pieces is indistinguishable, but when viewed from a distance the composition reveals a coherent and beautiful image. Metasurfaces work in a similar way, even if not visible to the naked eye, the tiny meta-atoms on the surface are what enable researchers to manipulate light with unprecedented control in such a thin device. In metasurfaces however, the design of the meta-atoms is crucial to achieve the correct functionality, and to strongly interact with the light that is visible to the human eye, their
dimensions had to be shrunk down to a size hundreds of times smaller than the width of a human hair. At these scales, each meta-atom can interact with light in a precise and controlled way, but collective resonant effects where the light interacts with the whole patterned surface are also possible. By carefully designing the geometry and the materials in use, researchers were able to create devices that focus light without curved lenses, encode images and generate additional light colors from a laser beam emitting light at only one color. Many different shapes and geometries have been investigated, but this project will focus on one design opportunity that has not been explored yet. Instead of building the meta-atoms from a single material, the meta-atoms studied in this thesis are formed by a core cylindrical pillar of one material wrapped by a shell of another, forming what is called a core-shell geometry. The goal is to investigate how this structure affects the way the metasurface interacts with light and this thesis will show how combining two dielectric materials can offer a new degree of freedom for tuning the optical response. In addition, a second goal is to create a metasurface design that by exploiting the resonances, it would be capable of generating light at a higher energy than the incoming laser beam, in a process called harmonic generation.
To achieve these goals, computers helped in simulating the behaviour of the studied metasurfaces without the need to create and test each design. Two designs were then chosen to be fabricated using modern nanotechnology tools and their response, obtained by illuminating them with light, was measured and compared against the simulations.

• author: Marzanati, Samuele
• supervisor: Matias Kagias; Cord Arnold
• organization: Atomic Physics; Synchrotron Radiation Research; Department of Physics
• course: PHYM03 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• language: English
• id: 9229752
• date added to LUP: 2026-06-02 08:21:09
• date last changed: 2026-06-02 08:21:09