Broadband nonlinear optical response and ultrafast carrier dynamics in defect-engineered Fe–Co3O4 for photonics

Abstract

In this paper, Fe-doped Co₃O₄ (Fe–Co₃O₄) with oxygen vacancy defects, derived from Fe-doped zeolitic imidazolate framework-67 (ZIF-67), was successfully synthesized via high-temperature calcination and verified using several special characterization methods. Femtosecond-resolved transient absorption spectroscopy revealed that Fe–Co₃O₄ exhibits ultrafast recovery times, with average fast and slow recovery times as short as 20 fs and 120 fs, respectively, which are faster than those of undoped Co₃O₄. This fast recovery time can be attributed to the numerous lattice defects introduced by Fe doping and oxygen vacancy defects, which result in the formation of a recombination central band state in the forbidden band, thereby accelerating the recombination process of electrons and holes. The nonlinear optical parameters of Fe–Co₃O₄ are evaluated using a twin-balanced detector system and open (closed)-aperture Z-scan techniques. The excellent effective nonlinear absorption coefficient (β_eff) and refractive index (n₂) measured are −(0.84 ± 0.15) cm MW⁻¹, 0.94 cm² GW⁻¹ at 1 μm, and −(0.61 ± 0.08) cm MW⁻¹, −0.6 cm² GW⁻¹ at 1.5 μm, respectively. The broadband and exceptional nonlinear optical responses of Fe–Co₃O₄ are closely associated with the further adjustment of the electronic structure by Fe doping and oxygen vacancy engineering. Using Fe–Co₃O₄ as a saturable absorber, mode-locking pulse outputs with pulse widths of 444 fs, 910 fs, and 820 fs are realized at 1030.6 nm, 1561 nm, and 1899 nm, respectively. Overall, this work demonstrates an effective method for the design of high-performance nonlinear optical materials with a broadband functionality and fast recovery times.