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Back in 1871, James Clerk Maxwell predicted that light exerts a force on any surface it hits. This radiation pressure was experimentally discovered some 30 years later and has since emerged as a hugely important force that is now exploited in systems such as solar sails and laser cooling.

Today, John Zhang and buddies at the University of Southampton in the UK go one better. These guys predict that a far more powerful optical force can exist between a metal or dielectric plate and a metamaterial, a substance with optical properties that have been engineered to control light in specific ways.

Metamaterials can be designed so they allow tiny oscillations of electrons called plasmons to exist on their surfaces. The oscillations are tiny–measured in nanometres, that’s about the same as the wavelength of visible light.

Zhang and co point out that if plasmons are set up when the metamaterial is close to a metal or dielectric surface, the plasmon ought to interact with electrons in the other material, setting up a kind of resonant effect and pulling the two surfaces together.

Like other oscillations, plasmons have a resonant frequency. So when zapped with light of just the right frequency, this attractive force can be particularly strong. In fact, the strength depends on the frequency and intensity of incident light.

They calculate that not only is this near field force stronger than other short range effects such as the Casimir force, it ought to be more powerful than gravity too. “This near-field force can exceed radiation pressure and Casimir forces to provide an optically controlled adhesion mechanism mimicking the gecko toe: at illumination intensities of just a few tens of nW/µm^2 it is sufficient to overcome the Earth’s gravitational pull,” they say.

That’s a highly significant prediction. This is an entirely new force that can be turned on and off with the flick of a switch and made to apply to some materials but not others.


Zhang and co point out some applications. The say, for example, that an optical scanning tip covered with a metamaterial could be used to pick up and place nano-objects or that metmaterials suspended near the surface of a metal could be tuned with incident light to dramatically change the broadband reflectance of the material.

In fact, there’s no end of interesting applications. These guys even tease their readers with images of a gecko’s toes implying that some kind of spiderman-like adhesion may be possible too.

The only outstanding problem is to find this force. Zhang and co clearly outline the conditions under which it ought to be measurable and say: “The near-field electromagnetic force described here should be easy to detect.”

So it should only be a matter of weeks or months before they reveal the first evidence of it. Maxwell would have been amazed.

Ref: arxiv.org/abs/1201.2801: Metamaterial ‘Gecko Toe’: Optically-Controlled Adhesion to Any Surface

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Tagged: Materials

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