Optical trapping of non-spherical plasmonic nanoparticles

Laser manipulation with plasmonic nano-particles is a rapidly growing field with various practical applications stretching beyond physics towards biology and chemistry. For example gold nano-particles can be employed as local heat source, probes for surface enhanced Raman spectroscopy with a sensitivity going down to a single molecule or contact-less probe in scanning near-field optical microscope. A single tightly focused laser beam optical tweezers was also employed to three-dimensional trapping of gold and silver nano-particles with diameters between 20 to 250 nm. However, theoretical models assuming the spherical shape of a nano-particle predict spatial confinement only for particles with diameter lower than 100 nm. Our results indicate this discrepancy is caused by ignoring particles shape which is very important for understanding of light-matter interaction.