Sir John Pendry: Uncloaking transformation optics
It is often the case in science that awesome new technological capabilities begin with pioneering efforts in basic science.
For example, today’s burgeoning interest in metamaterials—materials whose properties depend on their internal microstructure as much as on their chemical composition—has largely been the brainchild and life’s work of condensed matter physicist Sir John B. Pendry, chair in theoretical solid-state physics at Imperial College London (ICL).
As a theoretician, Pendry has devoted his career to new ways of thinking about nature and its rules. Among many other achievements, he has probed theory and mathematics to discover such optics and photonics mega-hits as the perfect lens, the perfect magnifying glass, and the invisibility cloak.
Those innovations and more stem from his concepts of transformation optics (TO), which is based on Maxwell’s Equations, and is now an established method for understanding and designing electromagnetic (EM) systems and metamaterials.
In essence, TO allows optics designers, using metamaterials, to create devices that will steer EM waves, for example, in a manner to hide or disguise objects like planes, boats, cars, and more. Pendry’s concepts have inspired ideas beyond invisibility cloaks. For example, researchers have proposed materials that would steer sound and water waves, or even surface waves from earthquakes in hopes of protecting buildings and other infrastructure by deflecting these destructive forces.
Asked why he chose physics to be his life’s work, Pendry says, “I was born a physicist. There wasn’t a time when I wasn’t interested in some sort of science.”
Born in 1943 in the UK, Pendry grew up at a time when “science was the solution to everything. We know now that it isn’t, but it was the place to be, the thing to do. And I was very happy with the situation.”
After completing an undergraduate degree in physics from Downing College at the University of Cambridge in 1962, he stayed on to study for a PhD. He worked out the theory of low-energy electron diffraction with Volker Heine.
In a brief memoir for the Kavli Foundation—Pendry is the 2014 Kavli Award in Nanoscience laureate—he writes, “Cambridge at the time was an immensely exciting place to be a scientist: Crick and Watson had a few years previously, working in what looked like a bicycle shed just below my office, solved the structure of DNA. Josephson in the neighboring Mond Laboratory had astonished the world with his predictions about superconducting junctions, and I remember the susurrations of excitement that preceded the announcement of the first quasi-stellar object or quasar by Tony Hewish and Jocelyn Bell. Stephen Hawking was one of our classmates in Dirac’s lectures on quantum mechanics.”
After finishing his doctoral work, Pendry spent a year at Bell Laboratories (1972-73), where he wrote a book on electron diffraction and several papers on electron states at surfaces. “This year was also my introduction to the American research scene in general, which has since been an important part of my research collaborations,” Pendry writes.
From Bell Labs, he returned to the UK and Cambridge, followed by a stint as head of the theory group at Daresbury Synchrotron Radiation Laboratory. Along the way he wrote a paper with Patrick Lee on extended X-ray absorption spectroscopy, “my first brush with light-matter interaction.” In 1981, he took his present position at ICL.
Pendry’s work on TO began in earnest in the 1990s.
“I wrote some of the earliest papers [on TO],” he says, noting that while the invariance under transformation of Maxwell’s Equations had been known since the time of Einstein, this was not generally true for people interested in materials science. “Its relationship to making structures and so on wasn’t exploited.” The key insight was thinking of an EM transformation as deforming space, and that the lines of force were actually embedded in the material in question.
“And that was what enabled us to design a cloak,” Pendry says. “If you think you can use a transformation to move the lines of force to where you want them to be—or in this case, where you don’t want them to be—then that is very powerful.”
In 2000, Pendry published a paper in Physical Review Letters, “Negative Refraction Makes a Perfect Lens,” that would cause a sensation in optics and photonics.
Pendry explains that with a conventional lens image, sharpness is always limited by the wavelength of light. He proposed an alternative: A slab of negative refractive index material with the power to focus all Fourier components of a 2D image, even those that do not propagate in a radiative manner. Simulations showed that a version of this superlens, operating at the frequency of visible light, could be made with a thin slab of silver and that it would resolve objects only a few nanometers across. Russian physicist Victor Veselago, who first explored negative refraction in the mid-20th century, showed that negative refraction could focus light but was not aware that the focus could be perfect.
After some heated back and forth with reviewers, the paper was eventually published. “And it’s now quite heavily cited,” Pendry says. “The last time I looked, it was about 12,000 citations.”
Pendry’s work laid the foundation for ongoing research to create superlenses. These might one day be used to image living cells at the nanoscale, enabling, for example, better understanding of how proteins operate as well as cell structure and function.
Asked how he feels about popular treatment of some of his work, notably the invisibility cloaks that turned up in the Harry Potter series, Pendry says, “I think it’s one of the duties of the scientist to try and explain him or herself to the public. They’re paying most salaries after all. We do have a duty to say, ‘See what we’re doing?’ And to tell that science particularly to children and younger people. They’ve all heard of Harry Potter’s cloak, and it’s a gift, because they come to a lecture wanting to know how the cloak works!”
William G. Schulz is Managing Editor of Photonics Focus.
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