Plasmonics could allow manipulation of smallest particles yet

Optical tweezers could become extremely dexterous at manipulating incredibly tiny particles if a new theoretical design based on plasmonics becomes a reality.

Optical tweezers have been an invaluable tool for the biological and physical sciences. They allow researchers to trap and move nanoscale objects – like viruses and bacteria – just like the tweezers found in most bathrooms. However, to successfully trap and manipulate something with light, the object must be around the same size as the wavelength of the light being used, usually on the order of 400 nm.

The aperture design. The device is made from two layers of silver separated by a layer of silicon dioxide. Courtesy of Amr Saleh, Stanford University.

Stanford University doctoral student Amr Saleh and Jennifer Dionne, assistant professor of materials science and engineering, have developed a novel theoretical design for an aperture that could stably trap objects as small as 2 nm across – about the size of a protein.

Traditionally, optical tweezers are hindered by light’s diffraction limit. An ideal light beam should terminate in a point; instead, light diffuses into a blur. The light’s energy is distributed over a disk, the diameter of which is proportional to the wavelength of the light and the f-stop of the aperture. Because of this, the wavelength of the light being used is effectively halved.

Power also is a problem. “If you want to trap something very small,” Saleh said, “you need a tremendous amount of power, which will burn your specimen before you can trap it.” With too little power, you will not be able to trap it.

The new design uses surface plasmon polaritons to focus light into narrow beams without a significant loss of energy. The design allows for the trapping and manipulation of 2-nm particles. No other optical tweezers design has been able to trap particles this small. Courtesy of Amr Saleh, Stanford University.

Saleh and Dionne’s design takes advantage of plasmons, the electron oscillations that occur in the interface between a dielectric and a metal when excited by photons.

The aperture is built like a coaxial cable, with a layer of silicon dioxide sandwiched between two layers of silver. When incident light excites the electrons in the cable, a wave of coupled electron oscillations – called surface plasmon polaritons – propagates through the aperture. These oscillations have a much smaller wavelength than the light going in, so the light becomes focused into a much narrower beam.

“We have been working on the fabrication of these structures and are very close to a fully working prototype at the particle sizes we theoretically predict,” Saleh said. “The main challenge is to fabricate coaxial apertures with channel thicknesses smaller than 25 nm.”

The new technology would benefit many fields, including biology, pharmacology and genomics. As a materials scientist, Dionne had thought of the device as a tool to help precisely move molecular building blocks into new configurations. “Optical tweezers seemed like a really cool way of assembling new materials,” she said.

Saleh and Dionne will present their most recent experimental results at the April Materials Research Society meeting in San Francisco. Their paper, “Toward efficient optical trapping of sub-10-nm particles with coaxial plasmonic apertures,” was published in the Nov. 14, 2012, issue of Nano Letters (doi: 10.1021/nl302627c).