The Ye group at JILA has created a “ruler of light” in the extreme ultraviolet (XUV). The new ruler is also known more formally as the XUV frequency comb. The comb consists of hundreds of equally spaced “colors” that function in precision measurement like the tics on an ordinary ruler. The XUV colors of this ruler have such short wavelengths they aren’t even visible to the human eye. The wavelengths of the XUV colors range from about 120 nm to about 50 nm ¾far shorter than the shortest visible light at 400 nm. “Seeing” the colors in the XUV ruler requires special instruments in the laboratory. With these instruments, the new ruler is opening up whole new vistas of research.

The creation of the XUV comb was a major milestone in the goal of designing frequency combs that span the entire electromagnetic spectrum. The invention of the XUV comb’s famous cousin, the optical frequency comb, earned its creators a Nobel Prize. The optical frequency comb is a stable pulsed laser that can create millions of equally spaced colors of visible light.

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