In the 1980s researchers at AT&T Bell Laboratories (now Bell Labs) created “optical tweezers” that could manipulate micron-size objects with focused laser beams, taking advantage of the gentle forces that light exerts on matter. Yet despite advancements made over the past 30 years, a problem has remained: as a result of the law of diffraction, which limits the degree to which light can be focused, most objects smaller than about 100 nanometers have evaded the tweezers.
It turns out that the law has a loophole, according to research recently described in Nature Nanotechnology. (Scientific American is part of Nature Publishing Group.) Diffraction applies to propagating light waves, but on the nanoscale, noble metals such as gold can convert light into evanescent fields, which are nonpropagating waves that quickly fade. Applying this phenomenon to a gold-plated optical cable, physicists at the Institute of Photonic Sciences near Barcelona were able to focus light at a fine enough scale to manipulate particles as small as 50 nanometers.
Previously researchers could work with particles of that size by attaching them to larger ones, but that method restricted movement. With the new tool, the physicists were able to pick up particles on their own and so move them freely in three dimensions.
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“We have something that can be a universal tool of interest to scientists from many different fields—not just physicists,” says photonics researcher Romain Quidant. Potential applications include building medical products with nanoscale exactness, manufacturing nanocrystal geometries for electronic devices and manipulating single molecules such as proteins.
