Laser-based optical tweezers are routinely used to trap and move nanoscale objects. An important goal in this field is to constrain the random Brownian motion of the trapped objects, which can be done by increasing the power of the laser beam, but this approach can damage biological samples. Researchers at Manchester University have now shown that it is possible to improve the performance of optical tweezers without increasing the laser power (Nature Photonics doi: 10.1038/nphoton.2008.78; 2008).

Sasha Grigorenko and co-workers used electron-beam lithography to arrange gold nanoparticles in pairs on a glass substrate, as can be seen in these two electron micrographs (the righthand micrograph is shown in false colour). When a laser is directed onto the surface it excites electronic resonances in the pairs, which leads to the production of strong electromagnetic fields. Grigorenko and co-workers used these fields to trap tiny polystyrene beads, which showed almost ten times less Brownian motion than similar beads in a conventional optical-tweezers set-up.

When the laser was moved sideways, the beads jumped to another discrete trapping site above a different pair of nanoparticles. The trapping properties can be tuned by varying the separation of the nanoparticles.