APL Photon. 1, 026102 (2016)

The scattering force imparted by green light to a single gold nanoparticle in solution has been measured with femtonewton resolution by a German–Danish team of scientists. The researchers used light from a 1,064-nm infrared laser to trap a single 80-nm-diameter gold nanoparticle in water. They then laterally injected, via an optical fibre, intensity-modulated green (532 nm) laser light (resonant to the particle's plasmon frequency) to perturb the motion of the trapped particle in a periodic manner. A video of the nanoparticle's harmonic motion was recorded by a high-speed digital camera operating at a data acquisition speed of 500 Hz and a frame exposure time of 1.9 ms. Position-tracking software was then used to analyse the nanoparticle's motion and deduce the magnitude of the scattering force, which was determined to be a maximum of 21.1 fN with an uncertainity of ±5.8 fN. The high sensitivity of the technique is expected to be valuable for further studies of optomechanics and nanostructure manipulation.