Laser-Induced Dynamic Alignment and Nonlinear-Like Optical Transmission in Liquid Suspensions of 2D Atomically Thin Nanomaterials*

Thanks to the development of liquid exfoliation techniques, liquid suspensions of two-dimensional (2D) atomically thin nanomaterials have attracted enormous research interest as platforms to explore nonlinear optical phenomena. However, in most reported studies, the intrinsic geometry and consequential optical anisotropy of these low-dimensional nanomaterials have been overlooked. In this work, we take graphene and graphene oxide as examples and synergize birefringence imaging and computational fluid dynamics to reveal the laser-induced dynamic alignment in these liquid suspensions. Moreover, the influence of the ordered states and birefringence on optical transmission has been systematically scrutinized in Z-scan measurements. We find out that saturable absorption-like and optical limiting-like transmission, observed under both continuous-wave and nanosecond pulsed lasers, should not be misinterpreted as third-order nonlinear effects from electronic or dielectric polarization of nanomaterials. The understanding of 2D atomically thin nanomaterials, solvent fluids, as well as their interactions with laser beams developed here provides valuable guidelines for designing optical transmission measurements and investigating nonlinear optical properties of nanomaterial suspensions.