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B3: Optical studies of the dielectric function and its dynamics on oxidic heterostructures

The general aim of this project is to study the interaction of pyroelectric, ferroelectric and ferromagnetic materials like ZnO, ZnCoO or BaTiO₃, and similar substances in thin film heterostructures. In particular, the dynamics of electronic and lattice excitations in such systems is being investigated by several linear and nonlinear optical methods.

The linear optical methods applied comprise conventional absorption spectroscopy, ellipsometry over a huge spectral range (0,04 – 9,5 eV) and Raman spectroscopy using a special setup using UV excitation. Based on the results from these experiments, the optical, opto-electronic and opto-magnetic properties off epitaxial films and heterostructures are studied resulting in information about, e.g., band structure, excitons, lattice vibrations and free electrons. The latter are then typically evaluated to give an equilibrium dielectric function of the individual system studied.

Based in this dielectric function, nonlinear optical experiments addressing the ultrafast dynamics of the same samples are being performed. In particular, femtosecond pump-probe experiments with broadband (‘supercontinuum’) probe pulses yield rich information about phenomena like charge carrier dynamics or exciton lifetimes on the femto- and picosecond time scale. However, the very complicated transient spectral data can only be evaluated by help of the dielectric function(s) extracted from the linear optical investigations. Together, linear and nonlinear optics can provide a self-consistent picture of the physical processes (and their ultrafast behavior) in a special sample expressed in terms of a confirmed or refined dielectric function. In particular, the mutual influence on the observed dynamics of different oxidic materials sharing an interface will be studied in this project.

A further point in the latter context to be addressed in the project is the coupling of fixed and ferroelectric polarization in neighboring materials. This will be done by linear electro-optical investigations complemented by electric studies on heterostructures in project A2; the correlation between of electrical conduction mechanisms and optical properties is required to understand the physics behind possible new effects. Concerning the switchable, ferroelectric polarization, two color pump-probe experiments with comprehensive polarization analysis will be conducted to study the switching dynamics. It is intended, for instance, to observe the breakdown of an existing polarization by laser-induced temperature jump above the Curie temperature.

As all the experimental methods mentioned are not limited to special material classes, it is also planned on the medium term to extend the investigations to materials and layer system being studied in other projects.