Coupled Femtosecond Laser Assisted Doping and Fragmentation of MoO₃ Nanosheets Generates Plasmonic QDs with Strong NLO Response

Abstract The ability to modulate the nonlinear optical (NLO) response of low‐dimensional metal oxides is highly desirable for diverse applications. It is shown here that the irradiation of a dispersion of 2D oxide (MoO₃) nanosheets (NSs) by femtosecond laser pulses enables simultaneous scissoring of the NSs to form well‐dispersed MoO₃ quantum dots (QDs) and the modulation of the NLO response in the visible and near‐infrared (NIR) regions. The inversion of the absorptive nonlinearity is observed combined with a larger enhancement of NLO response in the laser‐irradiated MoO_3−x NSs and the as‐generated QDs, which is associated with the steady laser‐deprival of oxygen atoms and the resultant localized surface plasmon resonance due to electron doping. By leveraging the sub‐picosecond NLO response of the plasmonic MoO_3−x QDs, the development of ultrafast optical switches is demonstrated for ultrashort pulse laser generation based on fiber lasers operating in the NIR regions. Furthermore, the strong field enhancement of these non‐noble metal plasmonic QDs is exploited for the fabrication of substrates for surface‐enhanced Raman scattering, which demonstrate a detection limit down to the part‐per‐billion level. This work delineates an efficient strategy for creating plasmonic oxide QDs with strong NLO response that is attractive for photonic applications.