Coexistence of excitons and free carriers in Cd_1-xMn_xTe/ZnTe multiple-quantum wells

Optical pump-probe experiments were carried out in Cd_1-xMn_xTe/ZnTe multiple-quantum wells to study the carrier dynamics and the exciton formation in these materials. Starting from 15 ps after the pump excitation, the bleaching of the absorption of the heavy-hole excitons in the wells yields a strong negative pump-probe signal. But before this occurs, the pump-probe signal has a completely different lineshape resembling a lying 'S'. The time evolution of the pump-probe signal is understood to have arisen from a coexistence of carriers and excitons. We treated the excitons in the wells as Mahan excitons and obtained a good fit to the experimental data. The cooling of the carriers after the pump excitation can be monitored as an increasing Fermi energy of the 'cold' carriers at the band edge. The Fermi sea is then depleted starting from 15 ps after the pump excitation due to the formation of excitons.