  We report a visible-range nonlinear photoluminescence (PL) from graphene
oxide (GO) flakes excited by near-infrared femtosecond laser light. PL
intensity has nonlinear dependence on the laser power, implying a multiphoton
excitation process, and also strongly depends on a linear polarization
orientation of excitation light, being at maximum when it is parallel to
flakes. We show that PL can be used for a fully three-dimensional label-free
imaging of isotropic, nematic, and lamellar liquid crystalline dispersions of
GO flakes in water. This nonlinear PL is of interest for applications in direct
label-free imaging of composite materials and study of orientational ordering
in mesomorphic phases formed by these flakes, as well as in biomedical and
sensing applications utilizing GO.
