Ultrafast relaxation and absorption saturation at near exciton resonance in a thin ZnO epilayer

We observed ultrafast free exciton thermalization time of 700-900 fs and obtained the magnitude of maximal differential absorption to be 1.8 x 10(4) cm(-1) with the pumping fluence of 10 mu J/cm(2) 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. (C) 2011 American Institute of Physics. [doi:10.1063/1.3525993]