Time resolved study of carrier relaxation dynamics inα-Al₂O₃

The relaxation of excited carriers in α-Al₂O₃is complex, depending for instance on the type of ionizing radiation. Using femtosecond time-resolved absorption spectroscopy, we can induce a controllable excitation density on a wide range, and follow the relaxation dynamics from 30 fs to 7 ns. We show that the excited carrier decay is non-exponential: it is dependent on the pump intensity, i.e. on the initial carrier concentration. We describe the relaxation as a two-steps process, involving the trapping of initially free electron-hole pairs, followed by recombination. A numerical model taking into account the initial electronic excitation by multiphoton absorption and the subsequent relaxation allows to quantitatively reproduce the amplitude of the measured absorption and its temporal evolution.