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.
plasmonic