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Invisible Rulers of Light

Published: 06-22-2014

Artist’s conception of a pair of extreme ultraviolet frequency combs (invisible rulers of light) capable of measuring subatomic structures such as atomic nuclei and electrons. Credit: The Ye group and Steve Burrows, JILA

The Ye group has not only made two invisible rulers of extreme ultraviolet (XUV) light, but also figured out how to observe them with ordinary laboratory electronics. With this setup, the researchers were able to prove that the two rulers had extraordinarily long phase-coherence time. This feat is so profound, it is nearly certain to transform the investigation of matter with extreme ultraviolet light, according to Ye’s colleagues in precision measurement and laser science. This research was reported online in Nature Photonics this week.

The invisible rulers of light are a pair of XUV frequency comb lasers that work in tandem. A frequency comb is a light source whose spectrum consists of a series of equally spaced “teeth,” which are like the tics on a ruler, except that they measure frequency and are much closer together. The invention of the optical frequency comb in 2000 has already transformed precision measurement with visible light. The same transformation is now set to happen with XUV wavelengths of light (124–10 nm). It’s not surprising this advance occurred in the Ye labs, where the new work could increase the spectral resolution of any XUV source by 10-millionfold.

“What’s real is what you measure if you’re measuring well,” says Fellow Jun Ye. “And, now what we can measure precisely with visible light, we can do just as well in the extreme ultraviolet. We hope that one day we will be able to shine XUV comb light on nuclear matter and change its states!”

The team responsible for this stellar advance in precision measurement includes graduate student Craig Benko, former research associates Tom Allison (Stony Brook University) and Arman Cingöz (AOSense), research associates Linqiang Hua and François Labaye, former graduate student Dylan Yost (Colorado State University) and Fellow Jun Ye.

The pair of XUV combs will be able to make precision measurements of atomic nuclei, atoms, charged ions, and simple molecules. Such measurements require XUV laser light that is coherent and stable. Until now, it wasn’t clear that there would ever be a ruler of light in the XUV frequencies like there is in the visible.

Since no person or machine can “see” XUV frequency-comb teeth, the XUV comb teeth are detected via “beating” together two combs, resulting in precisely and evenly spaced electronic “beat notes” observable with laboratory electronics. The beat notes occur when the two lasers are slightly offset from one another in frequency, but overlapped in space and time. The lasers are then detected simultaneously, which causes the beat notes to appear.

Because the invisible rulers of light can now be “seen” and precisely measured, new research is on the horizon. Such research includes precision nuclear clocks based on transitions in an atomic nucleus, nuclear isotope selection, precise studies of electron behavior in atoms and molecules subject to intense light fields, and the ability to resolve features of matter that are too small to be seen with an ordinary microscope. And, unlike producing XUV radiation with a synchrotron, which requires many researchers to operate, it only takes a couple of researchers to do an experiment with a pair of XUV lasers in a laboratory! – Julie Phillips

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