Picosecond spectroscopy of highly excited CdS

Abstract

Picosecond luminescence and excite-and-probe transmission as well as transient grating measurements are performed for highly excited CdS at a bath temperature of 5 K. The luminescence and the optical gain due to both the electron-hole plasma and the excitonic molecule recombination are observed. The electron-hole plasma decays very fast and disappears within 150–200 ps, whereas the excitonic molecule decays rather slowly. Both bimolecular recombination of the electrons and holes within the plasma and the diffusion of the carrier toward the low-density directions cause the decrease of the electron-hole density. In the case of a small volume of the electron-hole plasma, i.e., for rather-low-excitation levels, the transformation of the plasma into the excitonic system results in an additional shortening of the decay time of the plasma. The possibility of electron-hole liquid formation is definitely excluded.