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Diffraction is the bending of waves as they pass around obstacles. Refraction is the bending of waves as they pass from one medium to another with different transmission velocities. Conventional lenses focus light waves by using refraction to either converge or diverge the beam of light as they pass through the lens. Any device which can refract a wave form can be used as a lens. The ability of a lens to resolve details is known as resolution and while the limit of resolution is often determined by characteristics of the lens, it is more generally limited by the wavelength of light being used. This is known as the diffraction limit.

There are techniques that allow resolution to be increased beyond the diffraction limit. The diffraction limit is not valid for distances less than one wavelength of light and sub-wavelength details are available, but only when the lens is extremely close to the object being examined. This close range is known as the “near field”.

New Type of ‘Metalens’ Shatters Diffraction Limit – [technologyreview.com]

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.

11 June 2009
Record-Breaking Superlens Smashes Diffraction Limit – [technologyreview.com]

It must be 10 years since John Pendry at Imperial College London dreamt up the idea of superlenses. Until then, physicists had thought that the resolution of all lenses was limited by a phenomenon called the diffraction limit, which holds that you can’t see anything smaller than about half the wavelength of the illuminating light.

That’s true if you look at the propagating component of light waves. But light also records smaller sub-wavelength details in its evanescent components, which do not propagate. At least not usually. What Pendry showed was that evanescent components can propagate in a material with a negative refractive index, and he pointed out that a thin film of silver ought to have just the right properties.

Since then, the race has been on to build superlenses. In 2005, Nicolas Fang at the University of Illinois at Urbana-Champaign created one that could record details as small as one-sixth of a wavelength. That was a significant improvement over the diffraction limit, but why not better?

27 April, 2005
Superlens overcomes diffraction limit – [optics.org]

Physicists in the US have made an optical superlens from a thin layer of silver. The lens has a negative refractive index and can image structures with a resolution that is about one sixth the wavelength of light — thus overcoming the so-called diffraction limit (N Fang et al. 2005 Science 308 534).

Xiang Zhang and colleagues at the University of California at Berkeley say that the lens could have many applications, such as imaging nano-scale objects with light.

Conventional, positive-refractive-index lenses create images by capturing the light waves emitted by an object and then bending them. However, objects also emit “evanescent” waves that contain a lot of information at very small scales about the object. These waves are much harder to measure because they decay exponentially and never reach the image plane.

Acoustic Hyperlens

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