Strongly interacting Rydberg excitons in Cu2O

Abstract

Excitons are fundamental electronic excitations in a semiconductor. They are bound states consisting of an electron in the conduction band and a positively charged hole in the valence band. They have hydrogen-like properties and are therefore referred to as the hydrogen analogues in solid states. In the semiconductor Cuprous Oxide, Cu2O, highly excited states of excitons can be observed with principal quantum numbers up to n = 25. These states are called Rydberg excitons in analogy to their atomic counterparts, the Rydberg atoms. The intention of this thesis is to gain new insights into fundamental properties of these Rydberg excitons by optical spectroscopy. The focus lies on one-photon absorption spectroscopy with high spectral resolution up to 5 nanoelectronvolts. The physics addressed in this thesis can be divided into three main topics. First, the interaction of Rydberg excitons with high principal quantum numbers n with both external electric and magnetic fields is studied. In the regime of high n, the density of states becomes so large that it becomes unfeasible to use a microscopic theory that explicitly considers every single state. Instead, general n-dependent scaling laws for various fundamental properties of Rydberg excitons are derived theoretically and proven experimentally. These scaling laws provide an efficient description of Rydberg excitons in the high-n regime and give fundamental insights into similarities and differences between Rydberg excitons and Rydberg atoms. Second, the behavior of Rydberg excitons surrounded by an electron-hole plasma is investigated. For this purpose, absorption spectra are presented, recorded at different densities of free carriers injected into the crystal by an off-resonant pump laser. Carrier densities as low as 0.01 µm−3 are found to lead to a lowering of the band gap and the disappearance of the highest exciton lines. A model based on screening of the Coulomb interaction by free charge carriers is presented that allows for a phenomenological description of the data. In this context, the experimental parameter space spanned by excitation power and temperature is investigated to determine the limiting factors for the observation of Rydberg excitons with principal quantum numbers higher than n = 25, the highest Rydberg exciton state observed so far. Indeed, at nanowatt laser powers and millikelvin temperatures, the extension of the observable exciton series to n = 28 is possible. The third part of this thesis addresses mutual interactions between Rydberg excitons. Combining data from pump-probe experiments with a detailed theoretical model for the shape of these spectra for several exciton interaction mechanisms clearly shows that long-range van der Waals-type interactions are the dominant contribution to interactions between Rydberg excitons.