You can't get much smaller than this: Physicists have fashioned a mirror from a single atom. The advance might lead to an atom-sized transistor for light, and experts say it bodes well for broader efforts to shrink optical elements to the nanometer scale.
"In terms of the basic physics, it's incredibly cute," says Christian Kurtsiefer, an experimental physicist at the National University of Singapore, who was not involved in the work. "It's a very striking effect because you wouldn't necessarily expect that a single atom would exert a lot of influence on the flow of light."
In fact, the atom effectively reflects less than 1% of the light that hits it. So to detect the reflection, Gabriel Hétet, Rainer Blatt, and colleagues at the University of Innsbruck in Austria relied on a wave effect known as interference. They fashioned a device called a Fabry-Pérot interferometer, which ordinarily consists of two mirrors facing each other. Laser light of a fixed wavelength shines on the back of one mirror and some leaks through the mirror, entering the "cavity" between the mirrors. A small amount of light then leaks through the second mirror, while most of it reflects back toward the first. The reflected light can make multiple roundtrips between the mirrors. Each time, a little more light can leak through the second, farther mirror. (A similar effect takes place at the first mirror, too.)
Here's where the interference comes in. If the roundtrip distance between the mirrors equals a multiple of the light's wavelength, then all the light waves leaking through the second mirror will be in sync and reinforce each other, greatly increasing the transmission. If this roundtrip distance is slightly different, all those waves will be out of sync and cancel each other out, reducing the transmission. So the amount of transmitted light goes up and down as the distance between the mirrors increases.
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