▎ 摘　　要

Ultrafast pump-probe spectroscopy on as-grown graphene oxide single sheet thin layers was employed in order to study their response following ultrafast excitation of electrons in the sp(3) hybridized domains. The study of the transient spectra showed the existence of two wavelength regions with distinct responses: a saturable absorption region and a reverse saturable absorption region. The study of the competing responses on these two regions revealed for the first time that these wavelength regions are not stable following excitation but instead they present a dynamic redshift resulting in a crossover wavelength. This marks the transition from saturable to reverse saturable absorption and is dynamic in nature. The ultrafast dynamics of this reported effect may be of crucial importance to the application of graphene-oxide based elements in optoelectronics.