Rydberg Excitons in Cu ₂ O at Millikelvin Temperatures

Rydberg excitons in the semiconductor CuO have been observed in absorption experiments up to a principal quantum number of at millikelvin temperatures. Here, we extend the experimental parameter space by varying both temperature and excitation power. In particular, we show that excitons react more sensitively to an increase of the excitation power than states of the associated exciton multiplet, even though the latter are located at comparatively higher energy. This finding is similar to the one observed when applying an external electric field, suggesting that the observed behavior arises from internal electric fields created by charged impurities that are optically ionized. At laser intensities below 1 W/cm, absorption lines of excitons with are observed.