Negative Radiative Pressure on Plasmonic Supercavitating Nanoparticles

Driving nano swimmers with light is very useful for many applications like in-situ nanofabrication, targeted-molecular assembly, or biosensor array. Here, we demonstrate the optical pulling of plasmonic nanoparticles (NPs) in water, which moves against the light propagation direction. A plasmonic nanobubble encapsulating the NP (i.e., supercavitation) transforms the incident plane wave into a unique internal mode to exert a negative radiative pressure on the NP. We theoretically study the optical force on an Au NP consisting of a 100-nm SiO$_{\mathrm{2}}$ core and 10-nm-thick Au shell. It is found that the single plane wave at the wavelength ($\lambda )$ of 800 nm can attract the NP when the NP is inside a nanobubble with the size between 100 nm -- 400 nm. The pulling force becomes apparent when the NP is close to the bubble surface directly facing the light incident. We suspend the NP in water and use a loosely focused Gaussian beam, which can create a nanobubble with a size of \textasciitilde O(100 nm) around the NP. We demonstrate that the laser pulls the NP against the photon stream and enable a speed of 10$^{\mathrm{5}}\mu $m/s for a typical travel distance of \textasciitilde 100 $\mu $m. Moreover, we deposit the optically pulled NPs on a quartz/water interface with a spot size of \textasciitilde 10 $\mu $m and successfully create a surface bubble on the interface.