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First superlens not so super yet

Researchers report on the first lens made from a negative index material. The …

The world of physics was abuzz last week because scientists had reported the first bulk negative index material that works with optical wavelengths. Although most popular press reports were breathlessly enthusing over the possibility of cloaking, our opinion was that the first use of these materials would be for lenses with a resolution that exceeds anything currently available. Well, last week in Physical Review Letters, the first lens constructed from a meta-material with a negative refractive index was reported.

A material with a refractive index less than zero bends light in a manner opposite to the way that natural materials do. One oft-cited result of this is that, very close to the lens surface (called the near field), the light can resolve objects that are much smaller than the wavelength of light. Since optical frequencies are the most compatible with cellular life, there is a great deal of interest in the applicability of these lenses. This optimism, however, rests on the results of theoretical studies that are still equivocal about the existence of the superlens. The only way to put the subject to rest is to put the lens to the test.

The lens in question was manufactured from a two-dimensional photonic crystal. The photonic part means that the crystal structure contains repeats that are separated by a distance that's about the same as the wavelength of light. The crystal structure itself consists of a regular array of holes drilled into a semiconductor material, which explains the two dimensional aspect of it.

In the plane of the crystal, focusing is achieved by the array of holes, while the out-of-plane light is confined by total internal reflection—this is the same mechanism that keeps light in a normal optical fiber—and is not focused. The lens has a focal distance of about four micrometers on one side, and eight micrometers on the other side of the crystal and was designed to work with light with a wavelength of about 1.5 micrometers. Light from one of the focal points would pass through an internal focal point located near the center of the photonic crystal.

Nature, J.Valentine et al

By measuring how intensely light was scattered out of the plane of the lens, the researchers were able to produce maps of how the light propagated through the crystal and out again. They found that these compared very well to numerical models of the lens' performance.

The method used to make these measurements actually involved a sort of superlens as well. The light was emitted and collected by a fiber optic cable that had been tapered so that it was much smaller than the wavelength of light. The outside of the fiber was coated in metal, so as the cable core gets narrower, more of the light impinges on the metal, exciting electron motion. These coherent electron motions carry more and more of the light energy, until eventually, all the light is carried by electron motion to the tip of the fiber, where it is re-emitted in a spot that is just a 100nm in diameter. Emitters and receivers of this nature are exactly symmetric, so if a tip will emit light, it will also collect light.

Ironically, this sub-diffraction-limited imaging allowed the researchers to test the limits of their lens' resolution, which proved to be about 1 micrometer. Considering that the wavelength of the light used was about 1.5 micrometer, this wasn't to bad for a first attempt, but it is still larger than the conventional diffraction limit.

One reason for this is that all the light they detected was that scattered out of plane, and therefore diffracting—improved crystal quality should take care of that in the long term. Another reason is that the light emitter was actually quite a distance from the lens. Near-field imaging makes use of evanescent waves, which are light waves that do not propagate, but instead decay rapidly as they move away from their point of emission. Usually, near-field imaging must be performed within about a wavelength of the object: in this case, 1.5 micrometers.

One question that would naturally spring to mind: why didn't they just image something to test the resolution? The answer lies in the way they mapped the light propagation through the lens. Light scattered out of plane is lost, and these photonic crystals have very high out-of-plane losses. The researchers report that less than one percent of the light injected into the crystal was transmitted through the entire crystal, with the rest being lost to scattering.

This is the big weakness of photonic crystals: they require lots of scattering to work, but each scattering unit (in this case, a hole drilled into the semiconductor) must be identical so that it scatters identically. Unfortunately, this is presently impossible on the scale required for bulk devices. Of course, the transistor started life as a crappy amplifier too, and look where we are today.

Physical Review Letters, 2008, DOI: 10.1103/PhysRevLett.101.073901

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