Fast electrochemical activation of the broadband saturable absorption of tungsten oxide nanoporous film

The on-demand modulation of defects in materials for the effective modulation of optical nonlinearity is desirable, while it remains a great challenge. In this work, we demonstrate that electrochemical activation is a facile and convenient approach to modulating the broadband third-order nonlinear absorption of nanoporous tungsten oxide (WO_3–x) thin film. The film does not exhibit optical nonlinearity at the initial state, while shows a distinct saturable absorption under an applied voltage of –2.5 V with the excitation of 515, 800, and 1, 030 nm laser. The nonlinear absorption coefficient (β_eff) is –766.38 ± 6.67 cm·GW^–1 for 1, 030 nm laser, –624.24 ± 17.15 cm·GW^–1 for 800 nm laser, and –120.70 ± 11.49 cm·GW^–1 for 515 nm laser, and the performance is competitive among inorganic saturable absorbers. The activation is accomplished in 2 min. The performance enhancement is ascribed to the formation of abundant in-gap defect states because of the reduction of the tungsten atoms, and a Pauli-blocking effect occurs during the excitation of in-gap defect states. The small feature size of WO_3–x (~ 12 nm) enables the effective and fast introduction and removal of the defects in porous film, and accordingly the fast and broadband modulation of optical nonlinearity. Our results suggest a controllable, effective, and convenient approach to tuning the nonlinear absorption of materials.