Higher and Higher
Adopting an optical time standard remains years in the future, however. Metrologists must first carefully evaluate numerous atomic and ionic optical transitions before selecting the one that seems to be the best for a standard. In addition to the many practical applications of combs, fundamental comb research continues apace on many fronts. For example, Ye’s group can use a single comb to detect very sensitively many different transitions of atoms and molecules all at once. Thus, the whole range of energy states of an atom can be analyzed in one measurement. Alternatively, this technique can be applied to detect many trace species in a sample.
Comb technology has already had a large impact on studies of how atoms and molecules respond to the strong electric fields obtainable in intense, ultrashort light pulses. Much of this work has been led by a collaborator of Hänsch’s, Ferenc Krausz, who is now at the Max Planck Institute for Quantum Optics. Among other achievements, his group has used the response of electrons to measure the electric field of a laser’s ultrashort pulses and display the waveform, much like displaying a radio-frequency wave on an oscilloscope. Krausz used optical combs to stabilize the pulses’ phase to have an unchanging waveform from pulse to pulse.
Another very active area of research is the quest to push comb techniques to higher frequencies of the electromagnetic spectrum. (Producing lower-frequency combs, including combs that run from microwaves all the way to visible light, is straightforward.) In 2005 Ye’s group at JILA and Hänsch’s group in Garching generated a precise frequency comb in the extreme ultraviolet (not far below x-rays in frequency). Scientists are using this extended comb to study the fine structure of atoms and molecules with extreme ultraviolet laser light.
In the space of a few short years, optical frequency combs have gone from being a research problem studied by a small number of scientists to being a tool to be used across a broad gamut of applications and fundamental research. We have only begun to explore the full potential of these rulers of light.