University of Cambridge
Hybrid metal-dielectric metasurfaces for enhanced optical performance
Abstract
dc:description.abstractMetasurfaces are periodic arrays of light scatterers or resonators. The dimensions of these resonators and the spacing between them are smaller than the incident light. Metasurfaces can manipulate the amplitude, phase, and polarisation of the incident light by controlling the resonators’ shape, size, arrangement, type of material, and the surrounding environment. This allows for the creation of novel devices with unprecedented performance. Metasurfaces can be fabricated from metals (plasmonic metasurfaces) or dielectrics (all-dielectric metasurfaces). The work outlined in this thesis focuses on the effect of the combination of metals and dielectrics on the performance of metasurfaces. The thesis investigated the performances of multiple hybrid metal-dielectric metasurface devices across the visible and mid-infrared wavelength ranges. For each device, different parametric sweeps for all the geometrical parameters were done to figure out the parameters yielding the optimum performance and to show the effect of varying the geometrical parameters on the response. Additionally, the physical mechanisms of the responses were explained by calculating the modal decomposition and the scattering cross-section. Moreover, the transmission responses of the metallic-only, the dielectric-only, and the hybrid metasurfaces were calculated to show the effect of using hybrid metasurfaces on the performance of the metasurfaces. Hybrid metal-dielectric metasurfaces have been difficult to realize experimentally. However, this thesis demonstrates how they can be fabricated using two approaches, namely one mask and two masks approaches. Firstly, the one mask approach is discussed in detail, highlighting its associated challenges. For example, the two-masks approach required two lithography steps, which increased the fabrication time and cost. There were also problems related to the LaserWriter, such as performance inconsistency due to the instability of its output energy and the stitching error, which affected the alignment between the two lithography steps. After exploring various options, electron beam lithography was used to pattern the metasurfaces due to its consistent performance and accurate alignment. However, there were still problems with etching high thicknesses of germanium. To address these issues, a simple fabrication process was developed. The developed fabrication process is based on one lithography step followed by the deposition of metals/dielectrics and, finally, the typical lift-off process to define the metasurface structures. This method simplified the fabrication process by dispensing three main steps and eliminated misalignment problems. These steps include the second lithography step, depositing an etchant mask, and the challenging etching process. The key point was using an electron beam lithography resist called SML resist, optimized for patterning high-resolution and high aspect ratio devices. The thesis explored the optimum conditions for the developed fabrication process in detail, including SML resist preparation (resist spinning speed, soft bake temperature, and development time), electron beam lithography exposure dose, and depositing accurate thicknesses of metals/dielectrics. The fabricated hybrid metasurface was used for CO2 detection as a proof of concept. The developed process can be used to fabricate complex structures.
Degree
thesis:*- Name dc:type.qualificationname
- Doctor of Philosophy (PhD)
- Level dc:type.qualificationlevel
- Doctoral
- Grantor dc:publisher.institution
- University of Cambridge
- Year dc:date.issued
- 2023
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Soliman, Amr
- Advisor dc:contributor.advisor
-
- Wilkinson, Timothy
Subjects
dc:subject × 3Rights
dc:rightsIdentifiers
dc:identifier.*- DOI dc:identifier.doi
- https://doi.org/10.17863/CAM.106595
- OAI identifier oai:identifier
- oai:www.repository.cam.ac.uk:1810/365207