Fig. 1. Observation of extremely fast ballistic movements of Au NPs.

a Schematic of the experimental system to monitor the dynamics of Au NPs dispersed in water. The red solid lines after the 20× lens indicate the Gaussian beam envelope. Au NPs scatter the incident light (red dots), which is captured by a high-speed camera with the field of view depicted by the blue solid lines. The blue and magenta arrows depict the Au NPs moving in the +z and −z directions, respectively referred to as the “positive motion” and the “negative motion”. b, c Dark-field optical images (scale bars –50 μm) of ultra-fast ballistic Au NPs with b the positive motion and c the negative motion. The cyan triangle in b and the magenta triangle in c indicate the positions of the selected Au NPs at t = 0. The cyan dotted line in b and the magenta dotted line in c are guide for the eye. The power of the laser is 690 mW, corresponding to 12 mW μm⁻² at the focal plane. d, e Representative d positive and e negative motion trajectories of the ballistic Au NPs projected onto the x − z plane. Each color corresponds to a ballistic Au NPs. The time interval between two adjacent symbols is 400 μs. The black arrows at the top of the figures indicate the focal plane location. In a–d, k_z is the wavevector of the incident laser light. f, g Images from the pump-probe optical scattering imaging experiment for f without pump beam and g with pump beam. Each green spot corresponds to the diffraction-limited scattered probe light from the Au NPs with nanobubbles.