High-resolution study of the yellow excitons in Cu_2O subject to an electric field

摘要

We have used high-resolution transmission spectroscopy to study the exciton level spectrum in Cu_2O subjectto a longitudinal external electric field, i.e., in the geometry where the transmitted light is propagating along thefield direction. Different experimental configurations given by the field orientation relative to the crystal and thelight polarization have been explored. We focus on the range of small principal quantum numbers n ≤ 7. Thenumber of exciton states belonging to a particular principal quantum number increases with n, leading to anenhanced complexity of the spectra. Still, in particular, for n = 3, . . . ,5, a spectral separation of the differentlines is feasible and identification as well as assignment of the dominant state character are possible. We finda strong dependence of the spectra on the chosen light propagation direction and polarization configuration,reflecting the inadequacy of the hydrogen model for describing the excitons. With increasing the field excitonicstates with different parity become mixed, leading to optical activation of states that are dark in zero field. Ascompared with atoms, due to the reduced Rydberg energy states with different n can be brought into resonancein the accessible electric field strength range. When this occurs, we observe mostly crossing of levels within theexperimental accuracy showing that the electron and hole motion remains regular. The observed features are welldescribed by detailed calculations accounting for the spin-orbit coupling, the cubic anisotropy effects, and thesymmetry-imposed optical selection rules.