Exciton-formation time obtained from the spin splitting dynamics

We present a new experimental method to obtain the exciton-formation time based on the detection of the zero-magnetic field energy splitting between excitons with spins +1 and -1 created by circularly polarized light. The splitting is proportional to the product of the exciton circular-polarization degree ρ and density n. The dynamics of the exciton density n can be therefore obtained from the values of splitting and ρ. We investigate the exciton formation dynamics in high purity bulk GaAs and Al0.15Ga0.85As samples at excitation densities 6x1014 - 3x1017 cm-3 and obtain the exciton-formation time, which is found to vary from 70 ps to 360 ps.