Scientists Confirm First 'Frequency Comb' to
Probe Ultraviolet Wavelengths
ScienceDaily (Feb. 1, 2012) —
Physicists at JILA have created the first "frequency comb" in
the extreme ultraviolet band of the spectrum, high-energy light
less than 100 nanometers (nm) in wavelength. Laser-generated
frequency combs are the most accurate method available for
precisely measuring frequencies, or colors, of light. In
reaching the new band of the spectrum, the JILA experiments
demonstrated for the first time a very fine mini-comb-like
structure within each subunit, or harmonic, of the larger comb,
drastically sharpening the measurement tool.
This is an artist's conception of JILA's extreme ultraviolet
(EUV) 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 (the bright
white spot in the distance). Each of the resulting "harmonics"
-- strong signals at regular fractions of the original infrared
wavelength -- 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. (Credit: Baxley/JILA)
The new comb, described in the Feb. 2 issue of the journal Nature,
confirms and expands on the JILA group's 2005 claim of the
ability to generate extreme ultraviolet (EUV) frequencies for
making precise measurements in that part of the electromagnetic
spectrum. The new tool can aid in the development of "nuclear
clocks" based on ticks in the nuclei of atoms, and measurements
of previously unexplored behavior in atoms and molecules.
JILA is a joint venture of the National Institute of Standards
and Technology (NIST) and the University of Colorado Boulder.
"Nobody doubted that the EUV frequency comb was there, it's just
that nobody had seen it with real experimental proof," says
NIST/JILA Fellow Jun Ye, the 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
(frequencies of about 750 to 200 terahertz, or trillions of
cycles per second), enabling development of next-generation
atomic clocks. In the past few years researchers at JILA, NIST
and many other laboratories have pushed comb boundaries toward
other regions of the electromagnetic spectrum.
To create the world's first extreme ultraviolet (EUV) frequency
comb, JILA scientists 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(a frequency of
154 megahertz, or MHz). When xenon gas is injected into the
cavity, the laser field drives an electron temporarily 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 each (1 attosecond is 0.000 000 000 000 000
001 seconds). 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 (154 MHz), for the first time ever, 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 -- the use of light to probe matter and make
measurements traceable to international standards -- at
wavelengths below 200 nm, a frequency of more than 1 petahertz
(quadrillion cycles per second).
The EUV 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 (a mix of electrons and electrically
charged atoms, or ions) required to generate EUV light inside
the cavity. Researchers finally achieved an ideal balance of
high power and stability in the cavity.
Applications for the new comb include the development of nuclear
clocks, based on changes in energy levels of an atom's nucleus
instead of the electronic structure as in today's atomic clocks.
The nucleus is well isolated from external interference and thus
might make an extremely stable clock. Other 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
This research is a 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
The above story is reprinted from
materials provided byNational
Institute of Standards and Technology (NIST).
Note: Materials may be
edited for content and length. For further information,
please contact the source cited above.
1 2 3 4 5 6 7 8 9 10 Newest