Saturable Absorption in 2D Ti₃ C₂ MXene Thin Films for Passive Photonic Diodes

MXenes comprise a new class of 2D transition metal carbides, nitrides, and carbonitrides that exhibit unique light-matter interactions. Recently, 2D Ti₃ CNT_x (T_x represents functional groups such as OH and F) was found to exhibit nonlinear saturable absorption (SA) or increased transmittance at higher light fluences, which is useful for mode locking in fiber-based femtosecond lasers. However, the fundamental origin and thickness dependence of SA behavior in MXenes remain to be understood. 2D Ti₃ C₂ T_x thin films of different thicknesses are fabricated using an interfacial film formation technique to systematically study their nonlinear optical properties. Using the open aperture Z-scan method, it is found that the SA behavior in Ti₃ C₂ T_x MXene arises from plasmon-induced increase in the ground state absorption at photon energies above the threshold for free carrier oscillations. The saturation fluence and modulation depth of Ti₃ C₂ T_x MXene is observed to be dependent on the film thickness. Unlike other 2D materials, Ti₃ C₂ T_x is found to show higher threshold for light-induced damage with up to 50% increase in nonlinear transmittance. Lastly, building on the SA behavior of Ti₃ C₂ T_x MXenes, a Ti₃ C₂ T_x MXene-based photonic diode that breaks time-reversal symmetry to achieve nonreciprocal transmission of nanosecond laser pulses is demonstrated.