Eickhoff constructed a complex experiment consisting of a laser system with widely tunable photon energies and an ultra-high vacuum system with two-dimensional (2D) photoelectron detection. With a time resolution of a few tens of femtoseconds (1 fs = 1015 sec), he followed the dynamics of optically excited electrons in the surface states and in the conduction band of silicon and investigated the influence of elastic and inelastic scattering processes on energy relaxation and surface recombination.
He was able to explain how the optical excitation of the charge carriers leads to complex two-dimensional interference phenomena that can be described by initial and intermediate Fano resonances. Secondly, he showed in his work why the elevated temperature of hot electrons in the conduction band persists for an unusually long period of many picoseconds (1 ps = 1012 sec). The extraction of hot electrons is discussed, for example, for increasing the efficiency of third-generation solar cells.
In his dissertation, Dr. Wilhelm Kühn has developed a new method of nonlinear spectroscopy in the terahertz range (1 THz = 1012 Hz) and used it in solid-state physics. The nonlinear interaction between light and matter is measured in two independent time dimensions in order to derive 2D spectra in the frequency domain. These spectra shed light on the coupling of different excitations of the investigated system and their temporal evolution.
THz waves oscillate very "slowly" compared to visible light, one oscillation period lasts 250 femtoseconds. By focusing high electric fields (about 300 kV / cm) can be achieved and used to accelerate charge carriers in solids. Kühn was therefore able to use the new method to study the transport properties of electrons in the semiconductor material gallium arsenide (GaAs).
He found that strongly accelerated electrons move almost without friction ("ballistic") through the GaAs crystal. At very high kinetic energies, however, they are decelerated and even accelerated in the opposite direction because their effective mass becomes negative. The resulting orbital electron movements, so-called Bloch oscillations, were thus detected for the first time in a volumetric crystal.
In the next step, Kühn has investigated a semiconductor model system with an extremely strong coupling of the electrons to the crystal lattice. He was able to show by 2D spectroscopy that a new particle, the polaron, is formed from electrons and lattice vibrations. His results, which were confirmed by theoretical calculations, also show how and in which channels the energy of the electron flows into the crystal lattice.
