Ultrafast Dynamics and Nonlinear Optical Responses from sp²- and sp³-Hybridized Domains in Graphene Oxide

The ultrafast relaxation dynamics and nonlinear optical response in single- and few-layered graphene oxide (GO) were studied by ultrafast optical differential transmission spectroscopy and Z-scan technique using various pump intensities. It was found that charge carriers with subpicosecond-to-picosecond dynamics from sp²-hybridized domains dominate the ultrafast response at low pump intensities, like graphene. Surprisingly, the influence of two-photon absorption from sp³-hybridized domains on the transient absorption signal becomes increasingly strong with pump intensities. On the basis of heterogeneous ultrafast dynamics of GO with saturable absorption in sp² domains and two-photon absorption in sp³ domains, the nonlinear optical response can be tailored by manipulation of the degree and location of oxidation on GO sheets; this unravels the important role of sp³ domains in graphene optics and will facilitate the potential applications of GO in optoelectronics.