This contribution consists of several parts. In a first part we discuss transport measurements of excitons in Cu2O, their detection by spatially and temporally resolved pump-and-probe beam spectroscopy and by field ionization in a Schottky barrier; in the next one we report on transitions from the 1s para to the 2p para exciton state observed in visible pump-IR probe spectroscopy and finally we discuss qualitatively a phase diagram for bulk semiconductors including excitonic Bose–Einstein condensation and the transition to an electron–hole plasma.

Differential absorption spectroscopy shows a characteristic dependence on the density of 1s excitons in Cu2O at low temperature. The relation between absorption change and 1s exciton density is established and calibrated using one- and two-photon excitation. The calibrated density evaluation is applied to exciton transport measurements in a quasi-one-dimensional sample geometry. A numerical simulation of the transport yields extremely long exciton lifetimes of up to 3 ms. The Auger recombination of excitons is almost negligible, contrary to previous estimates. A deviation of the transport characteristics from the diffusive regime was not observed.

The photoluminescence quantum efficiency of the yellow series 1s orthoexciton in Cu2O, including its phonon sidebands, was measured in an Ulbricht sphere. The obtained efficiency values between 10(-4) and 10(-6) are remarkably low. The nonmonotonous temperature dependence is analyzed.

We report on the first observation of the 1s to 2p exciton transition in a direct gap semiconductor. Cuprous oxide is well known for its excitonic features which provide a model case for the theory of Wannier excitons. In our pump-probe experiment we measured the infrared transmission of cuprous oxide with the without Ar + -laser illumination. The observed photo-induced absorption line in the differential transmission spectra at 126 meV is assigned to the excitonic transition from 1s to 2p exciton levels i.e. to the analog of the Lyman series in atomic hydrogen. From the dependence of the integrated photo-induced absorption on the pump laser intensity we determine the lifetime of the paraexciton τ p ≈0.3 ms. We also studied the influence of the pump laser on the phonon absorption band in the mid-infrared. Its observed shift to lower energies with increasing pump laser intensity can be explained by heating of the samples.

A high density of cold 1s orthoexcitons was created in Cu2O using two counterpropagating high-power femtosecond infrared (IR) laser beams tuned to half the 1s exciton resonance energy. The initial 1s exciton gas density was estimated to reach up to 1020 cm—3. The resulting differential absorption spectrum revealed a pronounced screening of the yellow np absorption lines, up to complete disappearance of the 2p exciton for the highest pump intensity. On the other hand, no change in the n = 1 phonon-assisted absorption continuum was detected, showing that the Mott condition for dissociation into a two-component plasma was not reached for the n = 1 excitons.

We report on the IR spectroscopic studies in both reflection (50-900 cm^{-1}) and transmission (900-3000 cm^{-1}) mode of the vibration spectrum of the cuprous oxide. A detailed analysis based on a comparison of the temperature dependences of the absorption band at 1125 cm^{-1} and of IR and Raman active fundamental vibrations results in assignment of the former to a biphonon.

A complete analysis of the line shape of the excitonic absorption and photoluminescence lines of the yellow series of Cu2O is given. A detailed fit to the absorption lines up to the n = 5 exciton according to Toyozawa's theory gives precise values for the excitonic resonance energies, allowing to calculate the excitonic Rydberg. The fit to the photoluminescence lines shows very good agreement with experimental data at different temperatures. At high excitation, the fit to the excitonic absorption lines reveals the fundamental mechanism for bleaching of exciton absorption lines in Cu2O. Based on these findings, a new all-optical method for the observation of exciton transport in Cu2O is proposed.

The classical system of two coupled ideal harmonic oscillators is solved and its exact solution is compared to the quantum mechanical equivalent of two coupled states. In both cases, the coupling leads to a non-crossing behavior with a "level repulsion" depending on the coupling parameter. A special Hamiltonian is presented that gives identical results in the classical and in the quantum mechanical case.