Comparison of exciton optical nonlinearities for resonant and non-resonant excitation

The saturation of exciton resonances with increasing carrier density has been measured in the case of resonant and non-resonant excitations by using a pump-probe technique. We observed that the bleaching density for resonantly excited excitons is considerably higher than that for non-resonantly excited excitons and that with increasing excitation energy, the exciton bleaching density converges to the Mott density of free carrier model. This results suggest that resonant excitons show weaker screening than an unbound electron-hole plasma mixed with excitons and that the transient optical nonlinearities associated with screening and bandgap renormalization are governed for the ratio of excitons to free carriers with different interactions. Also, we observed that the non-resonantly excited excitons had longer lifetimes than the resonantly excited excitons, which we attributed to the relaxation of free carriers and to the polariton bottle-neck effect.