Controllable broadband nonlinear optical response of graphene dispersions by tuning vacuum pressure.

Nonlinear scattering, originating from laser induced solvent micro-bubbles and/or micro-plasmas, is regarded as the principal mechanism for nonlinear optical (NLO) response of graphene dispersions at ns timescale. In this work, we report the significant enhancement of NLO response of graphene dispersions by decreasing the atmospheric pressure, which has strong influence on the formation and growth of micro-bubbles and/or micro-plasmas. A modified open-aperture Z-scan apparatus in combination with a vacuum system was used to study the effect of vacuum pressure on the NLO property of graphene dispersions prepared by liquid-phase exfoliation technique. We show that the atmospheric pressure can be utilized to control and tune the nonlinear responses of the graphene dispersions for ns laser pulses at both 532 nm and 1064 nm. The lower the vacuum pressure was, the larger the NLO response was. In contrast, the NLO property of fullerene was found to be independent of the pressure change, due to its nature of nonlinear absorption. This work affords a simple method to distinguish the nonlinear scattering and absorption mechanisms for NLO nanomaterials.