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

• Published in: Nano research Vol. 15; no. 1; pp. 326 - 332
• Main Authors: Hou, Ruipeng, Li, Hui, Diao, Mengjuan, Sun, Yanhui, Liang, Ying, Yu, Zhiyang, Huang, Zhipeng, Zhang, Chi
• Format: Journal Article
• Language: English
• Published: Beijing Tsinghua University Press 2022; Springer Nature B.V
• Subjects: Absorption; Absorptivity; Atomic/Molecular Structure and Spectra; Biomedicine; Biotechnology; Broadband; Chemistry and Materials Science; Condensed Matter Physics; Defects; Electrochemical activation; Electrochemistry; Excitation; Lasers; Materials Science; Modulation; Nanotechnology; Nonlinear systems; Nonlinearity; Performance enhancement; Research Article; Thin films; Tungsten; Tungsten oxide; Tungsten oxides; tungsten oxide; nonlinear absorption; in-gap states; electrochemical activation
• ISSN: 1998-0124; 1998-0000
• DOI: 10.1007/s12274-021-3478-9

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.