Broadband third-order nonlinear optical responses of black phosphorus nanosheets via spatial self-phase modulation using truncated Gaussian beams

• The coexistence of narrow and wide self-diffraction rings is observed. • The narrow and wide rings originate from OKN and TIN, respectively. • The narrow ring is effectively eliminated by the use of truncated Gaussian beams. • The photophysical parameters of dispersions of BP nanosheets are characterized. Spatial self-phase modulation (SSPM) effect is widely used in the characterization of 2D nanomaterials because of its terse principle and intuitive experimental setup, although the understanding of its physical mechanism is still incomplete so far. Herein, we use the truncated Gaussian beam (TGB) along the horizontal direction to investigate the SSPM effect in solution dispersions of black phosphorus (BP) nanosheets. By means of the gravity-promoted thermal convection effect, we observe the spatial separation of the circular symmetric narrow rings and the distorted wide rings in the upper half of the SSPM patterns at 532 nm and 1064 nm when the laser beam propagates horizontally through the vertically placed sample. Based on the theory of local and nonlocal optical nonlinearities and the experiment of the TGB SSPM, we confirm that the narrow and wide rings originate from the optical Kerr nonlinearity (OKN) and the thermally induced nonlinearity (TIN), respectively. This work provides an efficient approach to distinguish the OKN from the TIN in 2D nanomaterial dispersions, to separate their contributions, and to extract nonlinear optical parameters.