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New aperture design (left) with two layers of silver separated by another of silicon dioxide  (Source: Stanford School of Engineering)
The new method is a plasmonic trap, which uses the optical/electronic characteristics of metals to move particles with light

A new method of optical tweezing can manipulate particles as small as 2 nanometers with the use of light.

A team of Stanford researchers, led by Amr Saleh and Jennifer Dione, is responsible for the new technique. While optical tweezing itself isn't new, the ability to move particles as small as 2 nanometers (and potentially just a few atoms in size) is.

Optical tweezing involves a beam of light, which is shaped into a narrow point for a strong electromagnetic field. This beam attracts tiny particles, bacteria, etc. and allows them to be moved around as if they were manipulated by tweezers.

The problem is that optical tweezing is that it "breaks down" for objects much smaller than the wavelength of light, meaning it can't pick up super small objects that are only a few nanometers or less. But now, Stanford's new optical tweezing technique has been able to move particles as small a 2 nanometers.

The new method is a plasmonic trap, which uses the optical and electronic characteristics of metals to move particles with light. When light waves meet these mobile electrons, they move and bend the light into plasmon-polaritons, which are electromagnetic waves. With their short wavelength (compared to visible light), they can trap small objects with a much firmer handle. In normal cases involving visible light, the diffraction limit of light makes it so the smallest space for trapping a particle is half the wavelength of the light beam. This means that trying to trap a particle that is 2 nanometers would require a space of 200 nanometers, offering loose control.

On top of the new gripping method, the scientists created an aperture that acts like coaxial cables to effectively focus the light. It's able to do this thanks to a silver coating within a thin layer of silicon dioxide, and both coatings are wrapped in an outer layer of silver. When light enters this ring of silicon dioxide, plasmons are produced where the silicon dioxide and silver meet. The plasmons then move along the aperture and come out of the other end as a more focused beam of light.

This could one day move tiny particles within living cells.

Source: Science Daily





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