Bendy 'plasmon' beams focus better than light alone

Physics 17 August 2011

BENDY beams of light and matter can now be steered in real time, paving the way for devices with a focus sharper than anything possible using light alone.

From microscopes to optical circuits, tools that rely on beams of light are held back by a fundamental rule: light cannot be focused to a point smaller than half its wavelength. This diffraction limit affects how small an object can be imaged and makes super-fast optical computers – which would use light rather than electrons for processing – impractically large.

Waves of matter can be squeezed smaller, however. Light hitting a metal excites waves of electrons on its surface, called plasmons. These travel along the surface and have wavelengths that can be much smaller than the light’s diffraction limit. But until now, rigid gratings have been needed to direct and steer plasmons to a given point so that they could image something there.

“Waves of electrons on a metal’s surface span a tiny fraction of the wavelength of light”


Peng Zhang at the University of California, Berkeley, and colleagues can now bend beams of plasmons to their whim. They sent laser light through a screen on which a pattern was displayed. This pattern split the light into beams that interfered with each other to create what appeared to be a single beam that arced through space.

This so-called “Airy beam” then hit a gold film, where it excited surface plasmons and steered them along the surface according to the arc of the light (Optics Letters, DOI: 10.1364/OL.36.003191). The new set-up means it is possible to steer the plasmons to a focus simply by changing the pattern on the screen, allowing this focal point to range over an object rather than having to move the object itself.

“If we can focus light much smaller than a wavelength, then we can use it to fabricate much smaller integrated circuits, do much more accurate imaging, all those kinds of things,” Zhang says. “That’s our dream.”

The new scheme is “quite an elegant way of generating ‘Airy plasmons'”, says Alexander Minovich at the Australian National University in Canberra. He says the set-up could also be used to grab and move nano-sized particles.