Broadband Visible Nonlinear Absorption and Ultrafast Dynamics of the Ti₃C₂ Nanosheet

The Ti₃C₂ nanosheet, as a new two-dimensional (2D) group, has been found to have attractive characteristics as material for electromagnetic shielding and energy storage. In this study, the nonlinear broadband absorption and ultrafast dynamics of the Ti₃C₂ nanosheet were investigated using nanosecond open-aperture Z-scan and transient absorption techniques. The mechanism of two-photon absorption (TPA) was revealed in the visible region (475-700 nm). At lower incident energies, nonlinear absorption could not happen. When the laser energy increased to 0.64 GW/cm², electrons in the valence band could absorb two photons and jump to the conduction band, with TPA occurring, which meant that the sample exhibited reverse saturable absorption (RSA). In addition, when transient absorption was used to investigate the ultrafast carrier dynamics of the sample, it demonstrated that the relaxation contains a fast decay component and a slow one, which are obtained from electron-phonon and phonon-phonon interactions, respectively. Moreover, with the increasing pump fluence, the fast decay lifetime τ₁ increased from 3.9 to 4.5 ps, and the slow one τ₂ increased from 11.1 to 13.2 ps. These results show that the Ti₃C₂ nanosheet has potential applications in broadband optical limiters.