Wide-field Rayleigh scattering imaging and spectroscopy of gold nanoparticles in heavy water under laser trapping

We demonstrated wide-field Rayleigh scattering spectroscopy and imaging of gold nanoparticle trapping upon a focused near-infrared laser irradiation under dark-field illumination. The migration, trapping, and assembling behavior of gold nanoparticles were examined at single nanoparticle level at the focus spot and its surrounding area in heavy water. It is clarified that particle migration to the laser spot within the focal plane is not appreciable while scattering force drives the migration vertically along the direction of light propagation. The gradient force at the focus point and the scattering force are theoretically estimated to be 3.1 and 0.17 pN, which are consistent with the direct observation. Analysis of light scattering intensity fluctuation at the focus shows that three nanoparticles can be trapped at most, which is well supported by light scattering spectral measurement. The particle existence probability at the focus is estimated as a function of lateral distance from the focus, and is compared with numerically obtained photon pressure potential.