Neutralization of impurities by cw-excitation of high-$n$ Rydberg-excitons

We investigate the neutralization of charged defects in a Cu${}_2$O crystal experimentally with a cw two-color pump-probe setup. We demonstrate that the excitation of high-lying Rydberg excitons at low densities generates an enhancement of exciton absorption up to a factor of 3. The result is an overall improvement of the absorption spectrum’s quality in terms of increased oscillator strengths and narrower linewidths, referred to as purification. The reduction of the amount of charged impurities in the crystal is further verified by a thorough analysis of impurity photoluminescence signals.