Broadband nonlinear photonics in few-layer MXene Ti₃C₂T_x (T = F, O, or OH)

Studies of the nonlinear optical phenomena that describe the light-matter interactions in 2D crystalline materials have promoted a diverse range of photonic applications. MXene, as a recently developed new 2D material, has attracted considerable attention because of its graphene-like but highly tunable and tailorable electronic/optical properties. In this study, we systematically characterize the nonlinear optical response of MXene Ti₃C₂T_x nanosheets over the spectral range of 800 nm to 1800 nm. A large effective nonlinear absorption coefficient (β_eff∼-10⁻²¹ m²/V²) due to saturable absorption is observed for all of the testing wavelengths. The contribution of saturable absorption is two orders of magnitude higher than other lossy nonlinear absorption processes, and the amplitude of β_eff strongly depends on the light bleaching level. A negative nonlinear refractive index (n₂∼-10⁻²⁰ m²/W) with value comparable to that of the intensively studied graphene was demonstrated for the first time. These results demonstrate the efficient broadband light signal manipulating capabilities of Ti₃C₂T_x, which is only one member of the large MXene family. The capability of an efficient broadband optical switch is strongly confirmed using Ti₃C₂T_x as saturable absorbers for mode-locking operation at 1066 nm and 1555 nm, respectively. A highly stable femtosecond laser with pulse duration as short as 159 fs in the telecommunication window is readily obtained. Considering the diversity of the MXene family, this study may open a new avenue to advanced photonic devices.