Lenses are a part of everyday life—they help us focus words on a page, the light from stars, and the tiniest details of microorganisms. But making a lens for highly energetic light known as gamma rays had been thought impossible. Now, physicists have created such a lens, and they believe it will open up a new field of gamma-ray optics for medical imaging, detecting illicit nuclear material, and getting rid of nuclear waste.
Glass is the material of choice for conventional lenses, and like other materials, it contains atoms which are orbited by electrons. In an opaque material, these electrons would absorb or reflect light. But in glass, the electrons respond to incoming light by shaking about, pushing away the light in a different direction. Physicists describe the amount of bending as the glass’s “refractive index”: A refractive index equal to one results in no bending, while anything more or less results in bending one way or the other.
Refraction works well with visible light, a small part of the electromagnetic spectrum, because the light waves have a frequency that chimes well with the oscillations of orbiting electrons. But for higher energy electromagnetic radiation—ultraviolet and beyond—the frequencies are too high for the electrons to respond, and lenses become less and less effective. It was only toward the end of last century that physicists found they could create lenses for x-rays, the part of the electromagnetic spectrum just beyond the ultraviolet, by stacking together numerous layers of patterned material. Such lenses opened up the field of x-ray optics, which, with x-rays’ short wavelengths, allowed imaging at a nanoscale resolution.
There the story should have ended. Theory says that gamma rays, being even more energetic than x-rays, ought to bypass orbiting electrons altogether; materials should not bend them at all and the refractive index for gamma rays should be almost equal to one. Yet this is not what a team of physicists led by Dietrich Habs at the Ludwig Maximilian University of Munich in Germany and Michael Jentschel at the Institut Laue-Langevin (ILL) in Grenoble, France, has discovered.