Ultra-fast ballistic supercavitating plasmonic nanoparticles driven by optical pulling forces

Here, we report, ballistic plasmonic Au nanoparticles that swim at unprecedented speeds, ~245,000 μm s−1 in a water medium, against the propagation direction of a loosely focused Gaussian beam. The beam waist being ~2 orders larger than the radius of the nanoparticles, negates the effect of the optical gradient forces, and can be approximated as a linearly polarized plane wave. Additionally, in water medium, the optical forces experienced by the Au nanoparticles are ~2 orders of magnitude smaller than what Strokes' Law permits to reach such high speeds. The Au nanoparticle can generate a nanoscale bubble, when excited especially at the plasmon resonance peak and can encapsulate the Au nanoparticle (supercavitation), creating a virtually frictionless environment (water vapor), which can support such speeds. Certain nanoparticle-nanobubble configurations can create unique optical conditions, which leads to the phenomenon known as 'optical pulling' to drive the Au nanoparticles against the propagation direction of the beam. The nanoparticle inside the bubble, can reach temperature (~850 K) much higher than the critical temperature of water, and as the nanoparticle reach the nanobubble surface it can instantly vaporize the water thereby extending the bubble boundary. We believe, this light driven ultra-fast movement of the nanoparticles may benefit a wide of range of nano and bio-applications while providing new insights to the field of optical manipulation.