Ultrafast Charge Dynamics in Bulk Zinc Oxide under Intense Photoexcitation

The photo-induced charge dynamics of textbook wide-bandgap semiconductor ZnO have been investigated on the picosecond time-scale. We performed optical Pump-THz Probe experiments in order to measure the dielectric constant of the material after high-fluence photo-excitation of charge carriers. The technique allows access to both carrier lifetime and scattering rates, and it provides direct access to the intrinsic dielectric function changes upon excitation. A complex dynamic is unveiled in the high-fluence pumping regime, where the relaxation time is in the hundreds of picoseconds range and increases with increasing Pump fluence, while the onset of photoconductivity takes place in a few picoseconds. The plasma frequency and the relaxation time dependence on the Pump fluence are discussed.