▎ 摘　　要

Thin graphene oxide (GO) films have been widely explored for their outstanding optical properties, especially their large optical nonlinear coefficients, and high tunability of optical nonlinear coefficients under photoreduction. In this work, an experimental observation of giant nonlinear optical response in GO films under two different photoreduction mechanisms is reported. After ultraviolet (UV) light-induced photochemical reduction and continuous wave (CW) laser-induced photothermal reduction, the nonlinear optical response of reduced graphene oxide (rGO) via the Z-scan method using a femtosecond laser beam at a wavelength of 800 nm is characterized. The rGO exhibits a giant optical nonlinearity, especially two-photon absorption (TPA). A giant TPA coefficient of rGO on the order of approximate to 10(5) cm GW(-1) is obtained under two photoreduction mechanisms (photochemistry and photothermal). This is the first time that a large enhancement effect of TPA has been observed in graphene-based materials, and the TPA coefficient is an order of magnitude higher than the highest value previously reported. The versatile optical nonlinear properties imply a huge potential of the GO film in advanced photonic and optoelectronic devices such as broadband ultrafast optical switching and optical limiting.