Engineers at Harvard have created plasmonic nanotweezers that may make it easier to isolate and study tiny particles such as viruses. The device uses light from a laser to trap nanoscale particles, creates strong forces more efficiently than traditional optical tweezers, and eliminates the overheating problems of earlier setups.
Conventional optical tweezers, first invented at Bell Labs in the 1980s, are created by shining a laser through a microscopic lens, which focuses it into a very tight spot. The light, made up of electromagnetic waves, creates a gradient force at that focused spot that can attract a tiny particle and hold it within the beam for a short period of time, until random motion, radiation pressure, or other forces knock it out.
One problem, however, is that the lens cannot focus the beam any smaller than half the wavelength of the light. If the targeted particle is much smaller than the focal spot, the trapping will be imprecise. This newly designed device enhances the trapping field by focusing the laser onto an array of nanoscale gold disks. The light excites the electrons at the surface of the metal, creating rapid waves of electromagnetic charge called plasma oscillations, resulting in "hot spots" of enhanced field around the edges of the disk.
Other designs also used tiny gold disks arrayed on a sheet of glass; the whole setup was submerged in water with the target particles. However, one problem that would arise was that the brightest hotspots were at the base of the pillars, partially inside the glass, where the particles could never be trapped. A bigger problem, the team discovered, was that unless they kept the laser power very low, the water boiled.
The Harvard team solved this issue by replacing the glass with a piece of silicon coated in copper and then gold, with raised gold pillars. These materials are much more thermally conductive than glass, so they act as a heat sink.
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Also: A “fiber optic-based integration system” (FOBIS) includes optical tweezers for the noninvasive confinement of a single cell or other particle.
