The first “frequency comb” in the extreme- ultraviolet (EUV) band of the spectrum has the potential to advance nuclear clocks and to measure previously unexplored behavior in atoms and molecules. Laser-generated frequency combs are the most accurate method available for precisely measuring frequencies of light. Physicists from JILA — a joint venture of the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder — performed experiments demonstrating a very fine mini-comb-like structure within each subunit, or harmonic, of the larger comb, drastically sharpening the measurement tool. Described in the Feb. 2 issue of Nature, the discovery confirms and expands upon the scientists’ 2005 claim that it could generate EUV frequencies for making precise measurements in that part of the electromagnetic spectrum. The new tool could help develop nuclear clocks based on changes in energy levels of an atom’s nucleus, instead of on electronic structure, as in today’s atomic clocks. An artist’s conception of JILA’s extreme-ultraviolet frequency comb. The original light source is a pulsed infrared laser, which is used to create a train of attosecond-long pulse bursts at EUV wavelengths, indicated by the bright white spot in the distance. Each of the resulting “harmonics” has its own set of “teeth” marking individual frequencies (series of adjacent white lines in the foreground), creating a frequency comb within each harmonic. To prove the new structure exists, JILA scientists observed a tooth interacting with argon atoms, indicated by the glowing atom symbol in the center foreground. (Image: Baxley/JILA) “Nobody doubted that the EUV frequency comb was there; it’s just that nobody had seen it with real experimental proof,” said Jun Ye, NIST/JILA Fellow and group leader. “The new work provides the first experimental proof, and also really shows that one can now do science with it.” Frequency combs are created with ultrafast pulsed lasers and produce a span of very fine, evenly spaced “teeth,” each a specific frequency, which can be used like a ruler to measure light. Frequency combs are best known for measuring visible and near-infrared light at wavelengths of about 400 to 1500 nm. Over the past few years, researchers at JILA, NIST and other laboratories have pushed comb boundaries toward other regions of the electromagnetic spectrum. To create the world’s first EUV frequency comb, JILA physicists used a high-power laser to generate infrared light pulses that bounce back and forth and overlap in an optical cavity 154 million times per second. When xenon gas is injected into the cavity, the laser field temporarily drives an electron out of each atom of gas. When the electron snaps back into the atom, it generates a train of light pulses with a duration of several hundred attoseconds. The process generates “harmonics” — strong signals at regular fractions of the original infrared wavelength. As a result of the high repetition frequency of the laser, each harmonic has its own set of “teeth” marking individual frequencies — a mini frequency comb within the big comb. The EUV comb is the first system for high-accuracy laser spectroscopy at wavelengths below 200 nm, a frequency of more than 1 petahertz (quadrillion cycles per second). The comb is the culmination of several technical advances, including improved high-power ytterbium fiber lasers, an optical cavity formed by five mirrors in which light pulses overlap perfectly and build on each other in a stable way, and better understanding of the plasma required to generate EUV light inside the cavity. Researchers finally achieved an ideal balance of high power and stability in the cavity. Besides more advanced nuclear clocks, potential applications include studies of plasmas such as those in outer space; and searches for any changes in the fundamental “constants” of nature, values crucial to many scientific calculations. Ye hopes to continue extending combs toward shorter wavelengths to create an x-ray frequency comb. This research is the result of a five-year collaboration between JILA and IMRA America Inc. of Ann Arbor, Mich., which designed and built the high-power precision ytterbium fiber laser specifically for this project. The research was funded in part by the Defense Advanced Research Projects Agency, the Air Force Office of Scientific Research, NIST and the National Science Foundation. For more information, visit: www.nist.gov