Two-Photon Absorption in Monolayer MXenes

Two-photon absorption (2PA) is a nonlinear optical (NLO) effect that typically occurs in a conventional semiconductor whose bandgap is larger than the energy of one excited photon but smaller than the energy of two photons. In this work, the experimental observation of strong 2PA in gapless MXene monolayers is reported. This phenomenon is verified by a nonlinear transmission method and ultrafast dynamic studies based on a wavelength-degenerate, non-collinear pump-probe technique. The unconventional 2PA is attributed to the structures of the MXene energy bands near the Fermi level, which favors two-photon transitions and suppresses one-photon transitions due to parallel band absorption effects. These results, as well as the measured NLO parameters including the 2PA coefficient, absorption cross-section, excited carrier lifetimes, and third-order NLO susceptibility, provide a systematic understanding of the unusual NLO effect in MXenes and may be applied in advanced photonic and optoelectronic devices.