The ultrafast carrier dynamics of non-polar a-plane ZnO epi-film, with the energy difference between the A- and C-valence bands of about 23 meV, grown on r-plane sapphire were investigated using the reflection type pump-probe technique under two perpendicular polarized pumps. By exciting the electron from A-valence band through pump polarization perpendicular to the c-axis of a-ZnO (Epu ⊥ c), the TDR trace revealed two photon absorption (TPA), band filling (BF) and bandgap renormalization (BGR) effects that can be reasonably explained by the electron dynamics in the conduction band. By exciting the electron from C-valence band through parallel pump polarization (Epu∥c), only the BF effect was observed in the TDR trace owing to the hole dynamics in the valence bands. The occurrence of TPA was determined by the pump efficiency depending on the energy difference between the pump photon and the intermediate exciton resonance state.

Coherent acoustic phonons and carrier dynamics in ZnO/Zn0.8Mg0.2O multiple quantum wells from the perturbation of the built-in electric field are investigated using a degenerate pump–probe measurement technique near the exciton resonance.

We observed ultrafast free exciton thermalization time of 700–900 fs and obtained the magnitude of maximal differential absorption to be 1.8×104 cm−1 with the pumping fluence of 10 μJ/cm2 by measuring transient differential transmission in a thin ZnO epitaxial layer at room temperature. The largest induced transparency occurs near exciton resonance associated with absorption saturation by comparing the excitation from the above band-gap to band-tail states. The pumping dependent transient absorption reveals transition of excitonic relaxation from exciton-phonon scattering to exciton-exciton scattering or to an electron-hole plasma.

Energy-dependent free-carrier dynamics was investigated in 70 nm (thin) and 1 µm (thick) ZnO epifilms using the optical pump-probe technique. The far-above-band-gap dynamics in the thin epifilm reveals the prolonged relaxation and the slow recovery of renormalized band gap. The band-gap renormalization (BGR) effect is affected by the inefficient carrier-phonon scattering. In addition, the loss of excited carrier density via surface trapping results in an energy-dependent BGR buildup time. However, the far-above-band-gap dynamics in the thick epifilm reveals fast relaxation followed by BGR recovery, which is independent of the photon energy. The near-band-gap dynamics shows an ultrafast carrier thermalization both in the thin and the thick epifilms.