The Physics Hidden by the Invisibility Cloak

"John Pendry is a physicist at Imperial College London who laid the theoretical foundations for the invisibility cloak and superlenses capable of producing the sharpest ever images. He talks about the profound physics obscured by his invisibility cloak and how metamaterials could help realize the perfect lens.

Valerie Jamieson: Invisibility cloaks can guide light around objects as if they weren't there. It is awe-inspiring physics. So why the frustration?
John Pendry: It's when I give talks, particularly popular ones. Of all the things I am interested in, I am always asked about invisibility cloaks. I think, "Oh God, not another invisibility cloak lecture." I still enjoy giving them, but there are many other things I'm working on that are more profound; they just don't have that fertile soil which J. K. Rowling prepared for us.

VJ: What topics do you wish were better-known?
JP: The concept of a perfect lens is profound. A lens is a complicated thing that takes every point in an object and reconstructs it in the image—with no loss of detail in the case of a perfect lens.

VJ: An ordinary microscope or telescope can't see detail on a scale less than the wavelength of light. You realized it was possible to break this diffraction limit. How?
JP: I knew that Russian engineer Victor Veselago had theorized a lens made out of material with a negative refractive index. In 1999 I checked whether such a lens could be perfect, expecting the usual answer—that it wasn't perfect. I didn't get it; the theory said it was perfect. I was astonished, and so was everybody else. The mechanism of a perfect lens is very strange. I still get letters saying that it is all rubbish, but this has died down.

VJ: How did the perfect lens go from theoretical possibility to reality?
JP: The concept of metamaterials opened up the field. A metamaterial is a material whose electric and magnetic properties are determined as much by its structure as by its chemical composition, although the structure must be on a scale much smaller than the wavelength of light you're using. The real kick-start came when I got together with a team in San Diego who made the first material that had a negative refractive index, which was something of a Holy Grail for electromagnetism. It had been talked about for many, many years but you just couldn't find any stuff that did that.

VJ: Experimentally, what has been achieved?
JP: A perfect lens is very hard to realize in the lab. People have achieved sub-wavelength resolution that is more than 10 times as good as a normal lens, but it is far from being used as a microscope.

VJ: Can these lenses be used for anything else?
JP: There is a halfway house that my research team in London is working on—a light harvester. It concentrates light on a very small area. Ordinarily the area you can shrink to will be limited by the wavelength of the light you are using, as an ordinary lens is. We are using metamaterials to concentrate light onto an area less than a square nanometer. Once you do that, you have the potential to make sensors for single molecules.

VJ: Will metamaterials win a Nobel Prize?
JP: All I can say is that I hope they will. It is a lottery, isn't it really?

VJ: If you met J. K. Rowling, what would you say?
JP: I'd be in awe. I'd let her speak first, like the Queen."

This article originally appeared in New Scientist.