Ultra‐High Temperature Calcination of Crystalline α‐Fe₂O₃ and Its Nonlinear Optical Properties for Ultrafast Photonics

As a typical transition metal oxide, α-Fe₂O₃ has garnered significant attention due to its advantages in nonlinear optical applications, such as strong third-order nonlinearity and fast carrier recovery time. To delve into the nonlinear optical properties of α-Fe₂O₃, crystalline α-Fe₂O₃ materials with different microstructures are prepared. The nonlinear optical features of α-Fe₂O₃ calcined at the previously unexplored ultra-high temperature of >1100°C are emphasized. It is found that α-Fe₂O₃ exposed to ultra-high temperatures undergoes the phase transition, leading to the formation of Fe₃O₄. Subsequently, the nonlinear absorption coefficient is measured as -0.6280 cm GW^-1 at 1.5 µm. The modulation depth and saturation intensity for the Fe₂O₃-based saturable absorber at 1.5 µm are 4.20% and 13.94 MW cm^-2, respectively. Ultimately, the incorporation of the Fe₂O₃-based saturable absorber into an Er-doped fiber laser cavity resulted in the achievement of both conventional soliton mode-locking operation with a central wavelength of 1560.3 nm and a pulse duration of 1.13 ps, as well as the dissipative soliton resonance mode-locking operation with a central wavelength near 1564.0 nm. Overall, the phase transition and the nonlinear optical features in iron oxides under ultra-high temperatures are revealed, indicating the great potential in advanced ultrafast photonic applications.