Performing atomic, molecular, and optical measurements at high precision requires very precisely calibrated optical equipment. One method for achieving precision, which won its developers the 2005 Nobel Prize in physics, is known as a frequency comb. These combs create a clear series of evenly-spaced spectral lines that can be put to a variety of uses. Until now, frequency combs have been confined to visible light frequencies, but new developments have extended their usefulness into the extreme ultraviolet portion of the spectrum. The extreme UV corresponds to quantum transitions in many molecules, as well as nuclear oscillations that may power the next generation of nuclear clocks.

Researchers (Arman Cingöz, Dylan C. Yost, Thomas K. Allison, Axel Ruehl, Martin E. Fermann, Ingmar Hartl, and Jun Ye) in Colorado and Michigan built a frequency comb that produces attosecond-range pulses of light. (One attosecond is 10-18 second, or one quintillionth of a second.) Directing this into a special cavity containing xenon gas, they produced a clear coherent spectrum spanning the extreme ultraviolet.

This not only improves on previous experiments in ultraviolet optical combs; it may provide a new test for potential variations in the fine-structure constant, one of the fundamental physical constants.

Lasers are close to being monochromatic: the photons they emit are spread across a very narrow range of wavelengths. Frequency combs are a means of taking that color precision and extend it to a much wider range of frequencies, while only using a single laser as the source. Using a method known as mode locking, the comb produces a series of extremely short bursts of light at a high rate of repetition (154 megahertz in this particular experiment).

Pulses of such short duration are mathematically and physically equivalent to adding together a large number of monochromatic waves of different colors. And we have equipment that allows the colors to be separated. Thus, a single pulse can be divided up into a frequency comb, with light evenly spaced across a chunk of the spectrum. The colors in the comb need not even contain the original laser wavelength, which is highly advantageous for working in spectral regions where lasers may not be available.

The comb frequency, along with the properties of the original laser, determines what colors are present in the output signal. By tuning the wavelengths of light coming out of the frequency comb, researchers can create a coherent spectral line that precisely corresponds to a particular quantum transition in an atom. The well defined frequencies can also be used to calibrate another, less precise instrument such as a spectrometer for astronomical observations.

The researchers designed and built a ytterbium-fiber (Yb:fiber) laser for the heart of the frequency comb, which was then modulated at 154 Megahertz to produce a series of spectral lines running from approximately 40 to 120 nanometers. (For comparison, visible light wavelengths run roughly between 400 and 700 nanometers.) This range encompasses a quantum transition in argon at 82 nanometers, which the experimenters used to calibrate the comb.

Of particular interest to fundamental physics is the two-photon transition in hydrogen, in which two ultraviolet photons are emitted. Other experiments involving this transition provide the strongest limit on any variation in the fine-structure constant, the number that governs the strength of the electromagnetic force. More precise measurements enabled by frequency comb techniques have a chance at constraining possible fluctuations further, or possibly showing that there is variation.

Ultraviolet energies correspond to a wide range of phenomena, from testing complex quantum electrodynamics in multi-electron atoms to the calibration of nuclear clocks, the potential next stage in precision timekeeping. Frequency combs that operate in the ultraviolet regime allow for better spectroscopy than has been possible thus far, opening up a world of applications in atomic, molecular, and optical physics.

Nature, 2012. DOI: 10.1038/nature10711 (About DOIs).