Ultra‐High Temperature Calcination of Crystalline α‐Fe 2 O 3 and Its Nonlinear Optical Properties for Ultrafast Photonics

Abstract As a typical transition metal oxide, α‐Fe 2 O 3 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‐Fe 2 O 3 , crystalline α‐Fe 2 O 3 materials with different microstructures are prepared. The nonlinear optical features of α‐Fe 2 O 3 calcined at the previously unexplored ultra‐high temperature of >1100°C are emphasized. It is found that α‐Fe 2 O 3 exposed to ultra‐high temperatures undergoes the phase transition, leading to the formation of Fe 3 O 4 . 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 Fe 2 O 3 ‐based saturable absorber at 1.5 µm are 4.20% and 13.94 MW cm −2 , respectively. Ultimately, the incorporation of the Fe 2 O 3 ‐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.