Femtosecond Dynamics of Resonantly Excited Excitons in Room-Temperature GaAs Quantum Wells

We investigate the effect of excess excitonic populations created by resonant ultrashort excitation on the optical-absorption properties of GaAs quantum wells. We find that under these conditions at room temperature excitons produce more absorption bleaching than equal densities of free-carrier pairs. This bleaching partly recovers as the excitons ionize to give free carriers. Hence, we directly measure the thermal ionization time of excitons at room temperature for the first time, and find that $\tau \sim 300$ fs.