Ultrafast reflectivity dynamics in highly excited bulk ZnO

Large bandgap semiconductors like ZnO are currently of interest as detectors and lasing media. Advances in these device technologies rely on a fundamental understanding of carrier dynamics and excitonic effects at high excitation densities. High carrier densities in semiconductors can be induced by excitation with intense, ultrashort laser pulses, and the resulting dynamics can be determined through our broadband time-resolved pump-probe reflectivity technique. We use a Ti:Sapph fs-laser pulse to excite the c-plane of a crystalline ZnO sample just below its damage threshold of 3 kJ/m2, under normal atmosphere and room temperature conditions. Because the photon energy is well below the room-temperature ZnO bandgap (1.55 eV vs. 3.35 eV, respectively), multiphoton processes are required to induce interband transitions. A broadband probe pulse generated by focusing the fs-laser pulse through a CaF2 crystal monitors the reflectivity changes of the excited ZnO at different pump-probe delay times. Our dual-angle reflectivity measurements allow us to reconstruct the time-resolved dielectric function of the highly excited ZnO. From the reflectivity data, we find that the reflectivity at the exciton resonance of ZnO dramatically changes immediately after fs-laser excitation. Also, because the bandgap shifts to lower energy, we observe a strong excitation-induced absorption at 3.2 eV. The reflectivity data show a fast 100-fs component and a slower decay of several picoseconds. We discuss the intensity dependence of the reflectivity dynamics for different angles of incidence and discuss how the observed changes in reflectivity relate to the dynamics of the excited electron-hole pairs.