Dielectric and hybrid metal-dielectric nanoantennas for fluorescence enhancement

• Main Author: Dmitriev, Pavel
• Other Authors: Hilmi Volkan Demir
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics; Engineering::Electrical and electronic engineering::Semiconductors
• Online Access: https://hdl.handle.net/10356/163085
• Institution: Nanyang Technological University

Fluorescence enhancement of localized light sources is important for both fundamental science, e.g. for study of quantum emitters and strong coupling effects, and for practical applications, e.g. for detection systems in genome sequencing or bio-imaging. Traditionally, plasmonic nanostructures have been used for fluorescence enhancement applications. Plasmonic nanoantennas demonstrate very strong field confinement, but suffer from large dissipation losses. Plasmonic structures are typically limited to electric resonances, which makes designing nanoantennas for specific radiation patterns a very difficult task. Newer dielectric nanophotonic structures nearly eliminate dissipative losses and have greater flexibility in resonance tuning including both electric and magnetic resonances, but are unable to match the field confinement of their plasmonic counterparts. In this thesis I experimentally demonstrate two approaches to combining plasmonic and dielectric nanostructures, harnessing the strengths of each to create a nanoantenna capable of providing both high enhancement factors and high out-of-plane directivity. One approach is based on coupling two standard plasmonic and dielectric nanostructures and optimizing their interaction, while the second approach is a proof-of-concept of a novel design of hybrid plasmonic-dielectric nanoantenna. In the first approach I took two canonical antenna designs, a gold bowtie nanoantenna and a silicon ring nanoantenna and developed techniques to optimize their geometries to work as single efficient hybrid nanoantenna. Since the resulting structure geometry has many degrees of freedom, brute force scanning of parameters was not a feasible way of achieving an optimal design, so more elaborate iterative approaches were developed. Having achieved an optimal design, fabrication was the next hurdle to overcome, because of the complexity of integrating several distinct nanostructures on the same substrate. After fabrication, expected antenna performance was characterized by optical measurements. In parallel, several techniques for the integration of my hybrid antenna with quantum emitters were being explored, from simple spin coating of quantum dots, to precise fabrication of single quantum emitters via FIB. Ultimately, the full process of fabricating the hybrid nanoantenna and integration with quantum emitters was experimentally demonstrated, though the overall complexity was deemed too high. Therefore focus shifted to a different approach, using a concept that drastically simplified the antenna fabrication process – replacing the gold bowtie nanoantenna integrated into the hybrid structure by a gold mirror placed under the silicon ring portion of the antenna. This also had the additional effect of simplifying the integration of quantum emitters with the hybrid structure, giving me more freedom to choose from all of the explored techniques. Finally, the resulting silicon ring on gold mirror antenna structure was fabricated and coupled to quantum dots positioned beneath the silicon ring, and the structures high directional enhancement factor, as well as capabilities to tune the nanoantenna’s directivity were demonstrated.