Transient Optical Spectra of a Dense Exciton Gas in a Direct-Gap Semiconductor

The observation is reported of a remarkable persistence of the $1s$ exciton resonance in transient picosecond absorption spectra of GaAs under conditions of resonant optical excitation up to pair densities two orders of magnitude above the Mott density. The stability (excess linewidth 0.8 meV at n=6×10¹⁶ cm^-3) of this dense exciton gas against decay into the energetically more favorable continuum of electron-hole pairs is due to the relatively weak self-screening of discrete excitons.