Optomechanical manipulation of plasmonic nanoparticles is an area of current interest, both fundamental and applied. However, no experimental method is available to determine the forward-directed scattering force that dominates for incident light of a wavelength close to the plasmon resonance. Here, we demonstrate how the scattering force acting on a single gold nanoparticle in solution can be measured. An optically trapped 80 nm particle was repetitively pushed from the side with laser light resonant to the particle plasmon frequency. A lock-in analysis of the particle movement provides a measured value for the scattering force. We obtain a resolution of less than 3 femtonewtons which is an order of magnitude smaller than any measurement of switchable forces performed on nanoparticles in solution with single beam optical tweezers to date. We compared the results of the force measurement with Mie simulations of the optical scattering force on a gold nanoparticle and found good agreement between experiment and theory within a few fN.

We introduce a new optical technique where a train of short optical pulses is utilized to disturb a trapped microscopic particle. Using fast (250 kHz) and accurate (nm) detection of the position of the particle, accurately synchronized to the repetition rate of the laser pulses, we can coherently superimpose the displacement caused by each individual laser pulse. Thereby we are able to both bypass the influence from the Brownian motion of the trapped particle and to simultaneously increase the ability to localize its average trajectory by √n, where n is the number of repetitive pulses. In the results presented here we utilize a train of 1200 pulses to kick a 5 μm polystyrene sphere and obtain a spatial resolution corresponding to 0.09 nm and a time resolution of 4 μs. The magnitude of the optical force pushing the particle corresponds to ∼ 10(4)g and enables an investigation of both the hydrodynamical drag and the inertial effects caused by the particle and the surrounding liquid. Our results enables a more accurate testing of the existing extended models for the hydrodynamic drag and we discuss the observed agreement between experiments and theory.