Photon Manipulation of Two-Dimensional Plasmons in Metal Oxide Nanosheets for Surface-Enhanced Spectroscopy and Ultrafast Optical Switching

Two-dimensional (2D) confinement of plasmons is of particular interest for photonic applications, while material platforms supporting 2D plasmons remain largely limited to graphene and metallic nanostructures. Here, we demonstrate the synthesis of 2D MoO3 nanostructures, including nanosheets (NS) and nanobelts (NB), with sonication-assisted exfoliation and the manipulation of plasmonic responses through photon-induced doping by ultraviolet irradiation. The resultant 2D plasmonic MoO3 exhibits strong localized surface plasmon resonance (LSPR) in the visible-near-infrared (NIR) regions. Benefiting from the strong field enhancement localized along the sharp edges, the 2D plasmonic NSs are employed as substrates for the detection of molecules by surface-enhanced Raman scattering, exhibiting a detection limit down to the part-per-billion (ppb) level. We further show that the strong field localization in the plasmonic NSs leads to a dramatic enhancement of the nonlinear optical response manifested by ultrafast transient saturation absorption, which enables the development of an ultrafast optical switch for pulsed laser generation in the NIR region. Our results suggest that MoO3 could be a versatile 2D platform that can be exploited for applications in photonics and sensing.