Credit: © 2009 ACS

Gold nanoparticles are useful in biophysics and biotechnology, and can be held in place by optical tweezers. Standard optical tweezers, however, have focal spots at least half a wavelength in size, which places an upper bound on the force and a lower bound on the size of the trapping region they are able to produce. Optical manipulation using surface plasmons, which have subwavelength features, can surpass these bounds.

Now, Kenneth Crozier and colleagues at Harvard University have used surface plasmons to propel, rather than trap, gold nanoparticles1. The particles are driven to speeds as high as 50 μm s−1 above a gold film in which plasmons are induced. The particles are kept close to the gold film, and thus in reach of the plasmons, by a strong attractive force. A thin layer of silicon dioxide on top of the film prevents the particles and film from sticking to each other.

By exploiting the electric-field enhancement associated with plasmons, this approach reduces the optical power required to propel nanoparticles. This work may also provide a platform for the study of optically coupled metallic particles.