New Type of ‘Metalens’ Shatters Diffraction Limit
Ten years ago, classical optics was but a sleepy backwater of modern physics, a place to while away the hours in the autumn of your career.
Then all hell broke loose with the development of metamaterials, perfect lenses and invisibility cloaks, prospects that now make optics one of the hottest areas in science.
Today, optics gets another boost with the announcement of yet another entirely new kind of lens that makes perfect images. By perfect, optical physicists mean capable of focusing electromagnetic waves in both the near and far fields in a way that shatters the diffraction limit.
The diffraction limit is the inability of conventional lenses to resolve details much smaller than the wavelength of illuminating light. That makes intuitive sense since its hard to see how light can capture details much smaller than its wavelength.
That intuition is correct at distances of more than a few wavelengths from the object, what physicists call the far field. But at closer distances, em waves do capture sub-wavelength details. However, the contribution of this so-called near field to the em wave is tiny and falls off exponentially with distance.
The bottom line is that the near field, and the sub wavelength details that it contains, is practically undetectable at distances beyond a few wavelengths.
Today, Fabrice Lemoult et amis from the Langevin Institute in Paris reveal a lens that gets around this problem. The lens consists of an ordered array of resonating structures that are smaller than the wavelength of illuminating light.
Here’s how it works. The object under study is bathed in light and the lens placed in the near field. Any subwavelength detail in the em field couples with the sub wavelength resonators, which also have modes that couple with larger details in the em field.
These resonances propagate through the lens until they are radiated again on the other side, reproducing the near field exactly (or as well as the losses within the system and its resolution allow).
Lemoult and co call this device a resonant metalens and have even built one to prove the principle in the microwave region of the spectrum. Their lens consists of a 20 x 20 array of copper wires, each 40 cm long and 3mm wide with a period of 1.2 cm.
To test the device, they create a complex near-field using 16 microwave monopoles and used the resonant metalens to create an image of this field. Their measurements show that the lens resolves details as small as 1/80 of the wavelength of the monopoles.
What’s more it has the potential to be much better and work at other frequencies. “The concept of resonant metalens should be realizable in any part of the electromagnetic spectrum, with any subwavelength resonator, such as split-rings, nanoparticles, resonant wires, and even bubbles in acoustics,” say Lemoult and co. That’s impressive potential.
One obvious question which they do not address is the theoretical link between resonant metalenses and other devices that also beat the diffraction limit, in particular, the superlenses described by John Pendry at Imperial College London, the leading theoretician in this field.
Pendry takes an entirely different approach to deriving the properties of his superlens but the end result is more or less identical. Which means there’s bound to be a formal mathematical link between the two approaches.
Presumably it won’t be long for one or other of the players to tease it out.
Ref: arxiv.org/abs/1006.0799: Resonant Metalenses for Breaking the Diffraction Barrier
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