Thickness- and Wavelength-Dependent Nonlinear Optical Absorption in 2D Layered MXene Films

As a rapidly expanding family of 2D materials, MXenes have recently gained considerable attention. Herein, by developing a coating method that enables transfer-free and layer-by-layer film coating, the nonlinear optical absorption (NOA) of Ti₃C₂T MXene films is investigated. Using the Z-scan technique, the NOA of the MXene films is characterized at ≈800 nm. The results show that there is a strong and layer-dependent NOA behavior, transitioning from reverse saturable absorption (RSA) to saturable absorption (SA) as the layer number increases from 5 to 30. Notably, the nonlinear absorption coefficient β changes significantly from ≈7.13 × 10² cm GW^-1 to ≈-2.69 × 10² cm GW^-1 within this range. The power-dependent NOA of the MXene films is also characterized, and a decreasing trend in β is observed for increasing laser intensity. Finally, the NOA of 2D MXene films at ≈1550 nm is characterized by integrating them onto silicon nitride waveguides, where an SA behavior is observed for the films including 5 and 10 layers of MXene, in contrast to the RSA observed at ≈800 nm. These results reveal intriguing nonlinear optical properties of 2D MXene films, highlighting their versatility and potential for implementing high-performance nonlinear photonic devices.