Nonlinear photoluminescence imaging of isotropic and liquid crystalline   dispersions of graphene oxide
  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. 
Bohdan Senyuk, Natnael Behabtu, Benjamin G. Pacheco, Taewoo Lee,   Gabriel Ceriotti, James M. Tour, Matteo Pasquali, Ivan I....
cond-mat.mtrl-sci cond-mat.soft physics.optics