release issued by NIST
The world's most accurate "ruler" made with extreme
ultraviolet light has been built and demonstrated
with ultrafast laser pulses by scientists at JILA,
a joint institute of the National Institute of Standards
and Technology (NIST) and the University of Colorado
The new device, which consistently generates pulses of light lasting just femtoseconds
(quadrillionths of a second, or millionths of a billionth of a second) in the
ultraviolet region of the electromagnetic spectrum, is described in the May 20
issue of Physical Review Letters.*
The device is expected to become an important tool for ultraprecise measurements
in many fields of science including chemistry, physics and astronomy. A ruler
made with shorter wavelengths of light makes it possible to "see" more precise
differences than ever before in the energy levels of light emissions that identify
specific atoms, in the timing of chemical reactions, or, if additional applications
are developed, in the dimensions of certain nanometer-scale objects.
The new device also can be compared to a camera with ultrafast shutter speeds
and consistent shot-to-shot frame speed and stability, allowing scientists to
take real-time "pictures" of finer structures and dynamics. By combining many
such pictures at a high speed, scientists can gain a more detailed understanding
of many phenomena.
"This ultraviolet light source has a spectacularly high resolution," says Jun
Ye, a NIST Fellow who leads the JILA research group. "On the technological side,
the system we used to produce this light is simple and low cost, without active
The new laser device generates a "frequency comb," so-called because the frequency
spectrum-a graphical representation of the pattern made by many successive laser
pulses building on each other-looks like the evenly spaced teeth of a hair comb.
(See graphic.) The new comb is a short-wavelength version of the optical frequency
combs that in recent years have enabled demonstrations of optical atomic clocks,
which are expected to be as much as 100 times more accurate than today's microwave-based
atomic clocks. A femtosecond comb, because of its high speed (or repetition rate),
has the finest teeth of any optical ruler.
The JILA device uses a process called "high harmonic generation" to transform
infrared laser pulses with micrometer-sized wavelengths into higher-energy ultraviolet
light with wavelengths on the order of tens of nanometers. The transformation
occurs when intense laser fields irradiate highly stable gases such as xenon.
The gas atoms are ionized, and the liberated electrons are then driven back to
the parent atom by the light field and emit light at frequencies that are multiples
of the original laser frequency. The result is a comb in a different region of
the electromagnetic spectrum that maintains the same consistent distance between
the teeth as the original comb.
In the past, scientists have relied on amplifiers at low repetition rates to
reach sufficiently high light intensity for the harmonic process, but the consistency
of comb structures was lost. Other designs for short-wavelength light sources
have typically been complex and costly. The JILA team overcame these problems
by coupling a low-pulse-energy but high-repetition-rate femtosecond laser to
a high-quality cavity equipped with a xenon gas jet. The laser light bounces
back and forth through the ionized gas between six customized mirrors in a vacuum.
This process enhances the light intensity nearly 1,000-fold while maintaining
the comb structure through hundreds of round trips.
The mirrors are highly reflective and designed to efficiently focus the light.
The JILA team developed the specifications and characterization techniques for
the mirrors. The longer the mirrors can keep the light circulating through the
ionized gas samples without the pulses spreading out and losing their shape,
the shorter the wavelengths that can be generated.
The system is simpler than conventional short-wavelength light sources, using
a standard laser as an oscillator without complicated methods of actively amplifying
pulses. The continuous "recycling" of the laser light by the mirrors also significantly
improves the efficiency of comb production compared with conventional systems.
The work is supported by NIST, the Air Force Office of Scientific Research, Office
of Naval Research, National Aeronautics and Space Administration, and National
Contact: Laura Ost, (301) 975-4034
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