Ultrathin
two-dimensional (2D) metal oxides have recently emerged
as members of the 2D family with broad use in the catalytic field
and energy storage techniques. However, investigations on their optoelectronic
properties, nonlinear optical properties in particular, remain largely
elusive. Defect (site) engineering had been a powerful tool to tailor
semiconductor band gaps for their catalytic use, while, herein, it
was carried out to expand the third-order nonlinear response of ultrathin
2D transition-metal oxide WO3 to the infrared region. By
engineering the oxygen vacancy, WO3 demonstrated an indirect
band gap adjustable from 2.33 eV (WO3) to 1.54 eV (D-WO3) with enhanced nonlinear saturable absorption extending from
the visible to the near-infrared range, which have promising use in
mode-locking, laser beam shaping, and ultrafast photonics. Transient
absorption techniques revealed rapid fs-to-ps carrier dynamics followed
by slower exciton bleaching on the μs time scale for the oxygen
vacancy-rich metal oxides upon photoexcitation, which accounted for
the origin of their strong and broad nonlinear optical responses.
The strategy provided not only insights into the underlying photophysics
of the 2D metal oxides but also fresh ammunition to bring out their
remaining potential into full play, such as the fabrication of optoelectronic
devices for nonlinear optics.