  As a rapidly expanding family of two dimensional (2D) materials, MXenes have
recently gained considerable attention due to their appealing properties. Here,
by developing a solution based coating method that enables transfer free and
layer by layer film coating, we investigate the layer dependent nonlinear
optical absorption of Ti3C2Tx films, an important member of the MXene family.
By using the Zscan technique, we characterize the nonlinear absorption of the
prepared MXene films consisting of different numbers of monolayers. The results
show that there is a strong and layer dependent nonlinear absorption behavior,
transitioning from revisable saturable absorption (RSA) to saturable absorption
(SA) as the layer number increases from 5 to 30. Notably, the nonlinear
absorption coefficient beta varies significantly within this range, changing
from 7.13 x 100 cm/GW to -2.69 100 cm/GW. We also characterize the power
dependent nonlinear absorption of the MXene films at various incident laser
intensities, and a decreasing trend in beta is observed for increasing laser
intensity. These results reveal the intriguing layer dependent nonlinear
optical properties of 2D MXene films, highlighting their versatility and
potential for implementing high-performance nonlinear photonic devices.
