Infrared absorption by excitons in cuprous oxide

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 and 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 Is 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 tau(p) approximate to 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 ran be explained by heating of the sample.