Nonlinear light absorption in Ti₃C₂T _x MXene: a theoretical study

Nonlinear absorption of MXenes has been investigated experimentally in several recent publications, and applications of these new 2D materials in broadband ultrafast photonics has already been demonstrated. In particular, MXenes have been used in the generation of highly stable femtosecond pulses in mode-locked fiber lasers. The optical nonlinearities appearing in different MXenes in the vis-NIR wavelength range have been explained theoretically based on the theory of saturation of nonlinear absorption in a two-level atomic system. This approach requires a fitting procedure applied to determine the saturation intensity of the MXene. However, this approximation does not account for the band structure of MXene, and therefore the microscopic character of the saturation of nonlinear absorption is still not understood. In this communication we use published results of calculations of the band structure of titanium carbide MXene and apply the density matrix theory to calculate the nonlinear absorption coefficient for a two-band model, in the resonance approximation. The latest experimental results on nonlinear transmittance at pump wavelengths between 1000 nm and 1500 nm are discussed in this framework and the saturation dynamics of absorption is revealed.