In praise of Pendry and plasmons

By Hamish Johnston

Yesterday I had the pleasure of attending day two of “Pendry Fest”, a series of lectures at Imperial College given in honour of the physicist Sir John Pendry, who recently turned 65.

Pendry is probably most famous for his prediction in 2000 of the negative refraction of electromagnetic radiation by new manmade materials called metamaterials. This was confirmed experimentally by Duke University’s David Smith, who spoke yesterday, and Pendry and Smith went on to use this property of metamaterials to create an invisibility cloak in 2006.

Although making such metamaterials is a significant challenge of electrical engineering, Pendry realized that the physics could be described rather elegantly by borrowing ideas from Einstein’s general theory of relativity — a concept he dubbed “Transformation Optics”. Indeed, in his talk Smith credited Pendry with encouraging a generation of electrical engineers to read-up on general relativity.

Also speaking in London yesterday was Eli Yablonovitch of the University of California at Berkeley, who said “John rejuvenated the field of plasmonics”. This is the study of plasmons — waves that travel on the sea of conduction electrons at the surface of metals.

Physicists have known for some time that plasmons interact strongly with light — but from a technological point of view, this was seen as more of a nuisance than an opportunity. Pendry, however, realized that these interactions could be harnessed to create materials with weird and wonderful optical properties.

Perhaps the most useful of these is the ability to obtain optical images of structures that are smaller than the wavelength of the light used. Something that cannot be done using conventional optical systems.

In 2000, Pendry suggested that this could be done by creating a metamaterial “superlens” in which rapidly-decaying sub-wavelength “evanescent” light waves from a tiny object are converted into surface plasmons and then re-emitted as conventional light waves that could form an image.

While such superlenses have since been built, they have one major limitation — they must be very close to the object of interest (tens of nanometres) to capture the evanescent waves. As a result, it would be very difficult to obtain a high-resolution image of larger objects.

That’s why Xiang Zhang of the University of California at Berkeley is working on a “scanning superlens” that will skim about 20 nm above a surface. Zhang told those gathered at Pendry Fest that the tiny spot of light created by such a lens could be scanned over a silicon chip to create tiny features that are usually made using very expensive optical “masks”. He also believes that the tiny spot could be used in laser enhanced magnetic recording in which light is used to heat up a tiny magnetic bit so it can be flipped. The storage density of such devices are currently limited by the wavelength of the light used.

The was no talk yesterday about Pendry retiring, so I’m guessing that his contributions to physics and engineering will keep coming.