A Perfect Lens Makes Perfect Tweezers

Microwave magic.
These patterned holes diffract the microwaves, bending them in the opposite direction than that of a normal lens.

Zhaolin Lu/University of Delaware

Dragging cells and molecules with tractor beams of light is a standard lab technique. But these optical tweezers, as they're often called, are limited in the types of movements they can make. Now, a team of researchers has made a tweezer with an infinite range of motion, using a negative refraction lens. Beyond being a better optical tweezer, the device hints that the flat, super-resolving lenses could soon start showing up in other practical applications, such as medical imaging and electronics.

Optical tweezers work because light carries momentum. Just as momentum is transferred on a pool table when the cue ball hits a colored ball and sets it rolling, light can transfer momentum to an object it hits. When a beam of light hits an object, the light on the perimeter bends toward the center of the beam, dragging the object with it. But because the lens that focuses the light beam is curved, optical tweezers are restricted to swiveling within a circle. This means they can only put a cell somewhere on that circle. Negative refraction lenses eliminate this problem because they're flat; they can extend to make an infinite stage on which to arrange cells or construct nano-objects.

In the current issue of Optics Express, Dennis Prather, an electrical engineer, and colleagues at the University of Delaware in Newark describe how they patterned a slab of a glassy material with holes to make a negatively refractive lens. Through it they shined a beam of microwaves, which are longer than visible light waves and interact with larger holes that are easier to construct. The holes bent the beam in complicated ways so that when it emerged, it focused perfectly 13 mm away from the lens. They used the focused beam to drag millimeter-sized crumbs of polystyrene. Although the microwave tweezer can't drag around cells (a water-filled cell would heat up and die,) it should be able to manipulate other microsized objects.

"The strength of what they've done is to point out that this so-called negative lens is completely different from an ordinary lens," says John Pendry, a physicist at Imperial College London in the U.K. "But they really cannot do the things people want to do with optical tweezers," such as positioning biological objects with nanoprecision. Prather and his colleagues are now constructing a larger lens, with an array of microwave beams to smoothly move the particles longer distances. They hope their device will inspire other practical applications for flat lenses, such as ultra-thin electronic filters and less claustrophobic MRIs with superior resolution.

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