Fabrication and Optical Characterisation of Yagi-Uda-Antennas and Silica/Gold Core-Satellite Structures

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URI: http://hdl.handle.net/10900/158403
http://nbn-resolving.de/urn:nbn:de:bsz:21-dspace-1584032
http://dx.doi.org/10.15496/publikation-99735
Dokumentart: PhDThesis
Date: 2024-10-21
Language: English
Faculty: 7 Mathematisch-Naturwissenschaftliche Fakultät
Department: Physik
Advisor: Fleischer, Monika (Prof. Dr.)
Day of Oral Examination: 2024-06-27
DDC Classifikation: 500 - Natural sciences and mathematics
530 - Physics
540 - Chemistry and allied sciences
Other Keywords:
plasmonic
Yagi-Uda-antenna
Core-satellite
nanoparticle
emission
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Abstract:

In this work, different plasmonic nanostructures were fabricated, investigated using various methods and tested for their sensing abilities. Firstly, nanoscopic Yagi-Udaantennas were fabricated, whose working principle is proved in the radio frequency range for decades [1, 2]. Their operation principles rely on the constructive and destructive interference of electromagnetic waves, which are emitted by an active element and re-emitted by passive elements. This proven principle was converted here amongst other works to the optical regime. To find optimal parameters, Finite Difference Time Domain (FDTD) simulations were carried out. The fabrication mostly relied on electron beam lithography, however focused helium ion beam milling is used for the fabrication as well. With the help of this microscope, either whole antenna structures could be fabricated or existing antennas could be manipulated. Spectra were taken in a Dark Field (DF) microscope, where simple antennas as well as antennas incorporated in a flow cell were investigated. The advantage of a flow cell is the simple manipulation of the local refractive index with the help of different liquids. Furthermore, the emission characteristics of the antenna were probed with the help of a Back Focal Plane (BFP) microscope built by Annika Mildner, where it was tried to manipulate the emission direction. To do so, simple antennas were incorporated into a flow cell, and a higher refractive index medium was used to influence the polar angle of emission. To change the azimuthal angle of emission, each element of an antenna was half covered in silicon dioxide, and measurements in different refractive index media were carried out. Additionally, antennas were fabricated on a silicon substrate to be used by Felix Schneider in second harmonic generation (SHG) experiments in a parabolic mirror setup. In the second part of the thesis, core-satellite-structures consisting of one or two silica cores surrounded by gold nanospheres were investigated. The structures were synthesised by Dr. Yingying Cai from the University of Göttingen, using wet chemistry. FDTD simulations were carried out to determine the expected spectra and obtain the plasmonic modes. DF spectra were taken with unpolarised and linearly polarised light of different orientation and compared to the simulations. Additionally, the Surface Enhanced Raman Spectroscopy (SERS) properties of the structures were determined by covering the structures in Raman active material. Lastly, the sensitivity of the resonance shift of the core-satellite structures depending on the local refractive index was investigated.

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