  Spectroscopic ellipsometry is a sensitive and optical model-supported
quantitative tool to monitor interfaces. In this work, solid-liquid interfaces
are studied using the Kretschmann-Raether configuration for biosensing
applications. The interface layers support two purposes simultaneously: (i)
chemical suitability for the adsorption of molecules to be detected and (ii)
the optical enhancement of the signal to increase the sensitivity. Ellipsometry
is not only used as a sensor but also as a quantitative measurement tool to
study and understand the interface phenomena, and to develop the sensing layers
for the largest possible optical sensitivity. Plasmonic and structured layers
are of primary importance in terms of optical sensitivity. Layers structured
both in lateral and vertical directions have been studied. Optical models based
on both the transfer matrix and the finite element method were developed and
used for the structural analysis where the material and geometrical derivatives
are used in the inverse fitting process of the model data to the measurement.
Structures utilizing plasmonic, diffraction, multilayer field enhancement, and
other methods were analyzed as possible candidates for the improvement of the
optical performance of the cell. Combinatorial and periodic plasmonic surface
structures were developed to enhance the sensitivity of in-situ ellipsometry at
solid-liquid interfaces utilizing the Kretschmann-Raether (KR) geometry.
Ag$_x$Al$_{1-x}$ layers with variable compositions and Au layers with changing
periods and critical dimensions were investigated to improve the performance of
sensors based on the KR arrangement.
