Strong Ultrafast Saturable Absorption and Nonlinear Refraction of Some Non-van der Waals 2D Hematene and Magnetene Nanoplatelets for Ultrafast Photonic Applications

In the present work, some non-van der Waals (non-vdW) 2D materials, namely, hematene and magnetene nanoplatelets, were synthesized starting from hematite and magnetite ores, respectively, using a green synthesis method, and they were dispersed in water. Then, their ultrafast nonlinear optical (NLO) response was studied under 50 fs, 400 nm laser excitation. Both non-vdW 2D materials revealed strong saturable absorption with NLO absorption coefficient β, saturable intensity, and modulation depth of about -33.2 × 10^-15 m/W, 320 GW/cm², and 19%, respectively, for hematene, and about -21.4 × 10^-15 m/W, 500 GW/cm², and 17% for magnetene. These values are comparable to those of other vdW 2D materials, such as graphene, transition metal dichalcogenides (TMDs) like MoS₂, WS₂, and MoSe₂, black phosphorus (BP), and some MXenes (Ti₃C₂T_x), recently reported as efficient saturable absorbers. In addition, both hematene and magnetene dispersions displayed strong Kerr type NLO refraction with nonlinear refractive index parameters γ' comparable and even larger than those of van der Waals 2D materials. In all cases, hematene was found exhibiting significantly larger optical nonlinearities than magnetene, most probably due to the formation of a more efficient charge transfer system. The results of the present work are strongly suggesting that hematene and magnetene can have applications in a wide range of photonic and optoelectronic applications.