Ultraviolet Frequency Comb

Physicists at JILA, the joint institute of NIST and the University of Colorado, have created a new optical process to extend the production of coherent radiation into the extreme ultraviolet region of the electromagnetic spectrum. This process takes advantage of the fact that ultrafast laser pulses of femtosecond widths, separated by nanoseconds, manifest themselves as a superposition of light at different frequencies over a wide spectral band.

The Fourier transform of these short pulses is long series of evenly spaced spikes; that look like the tines of a comb (for background, see Physics Today, June 2000). What's new is that the JILA researchers have pushed the coverage of the frequency comb into the extreme ultraviolet by generating a series of high harmonics of the original, near-infrared laser frequency comb. (A comparable result has also been achieved by Ted Hansch's group in Munich, a result to be published elsewhere.)

In the JILA experiment, 50-femtosecond-long pulses, spaced 10 nanoseconds apart, are sent into a coherent storage device---an optical buildup cavity. The cavity length is determined so that each tine of the incoming frequency comb is matched to a respective cavity resonance mode. In other words, the pulse train is matched exactly into the cavity such that a pulse running around inside the cavity is reinforced by a steady stream of incoming pulses.

After a thousand roundtrips through the cavity, the infrared laser light becomes sufficiently energized to directly ionize xenon atoms inside the cavity. The quick repatriation of the xenon electrons to their home atoms is what produces light pulses of high frequency harmonics. Coherent high harmonic generation has been achieved with other techniques, typically involving single, actively amplified, ultrashort laser pulses.

The new approach demonstrated in the JILA work has drastically improved the spectral resolution of these high harmonic generated light sources by many orders of magnitude and will also permit an important increase of the efficiency of the harmonic generation process. Moreover, the buildup of intense UV happened without the need for expensive or bulky amplifying equipment.

Optical frequency combs have led to demonstrations of optical atomic clocks and are furthering research in extreme nonlinear optics, precision spectroscopy, and laser pulse manipulation and control. Jun Ye (ye@jila.colorado.edu, 303-735-3171) and his colleagues believe that the new ultraviolet frequency comb promises to provide an important tool for ultrahigh resolution spectroscopy and precision measurement in that spectral domain.

It will open the door to unprecedented spectral resolution, making it possible for scientists to study the fine structure of atoms and molecules with coherent XUV light. (Jones et al., Physical Review Letters, 20 May 2005, Cover Figure article; http://jilawww.colorado.edu/YeLabs/ )