Observation of ortho and para-excitons by time-resolved excitonic Lyman spectroscopy

A review of our recent experiments on the detection of ortho- and para-excitons in Cu2O by the induced absorption associated with an internal excitonic transition (1s to 2p) is presented. In order to evaluate the temporal and spatial dynamics of the excitons, we developed a time-resolved mid-infrared femtosecond pump-probe spectroscopy analysis technique. Because of the large difference in the electron-hole exchange energy between the 1s and 2p excitons (12 and nearly 0 meV, respectively), we can distinguish ortho-excitons from para-excitons in mid-infrared pump probe spectra. Since the transition energy depends on the momenta of excitons because of the difference between the 1s and 2p exciton masses, we can obtain information on the thermal distribution of excitons. We successfully detected a signal associated with para-excitons. Based on the experimental results, we discuss the basic problems to be solved for the observation of exciton BEC, such as para-exciton mass, Auger recombination and ortho to para exciton conversion.