the interactions of light and matter
David F. Salisbury
Oct. 9, 2001
manifold interactions of light and matter play a critical role in
the drama of life. They provide the energy that makes life possible,
as well as the vision required to appreciate its beauty. In the
last 200 years, scientists have learned a tremendous amount about
the nature of light and of matter. Yet there are still significant
gaps in our understanding of this fundamental interplay which the
Free-Electron Laser (FEL) is helping to bridge.
colors of the rainbow and the spectrum of colors produced by a prism
illustrate one of light's fundamental characteristics: It exists
in a virtually infinite number of shades. Each color is produced
by an electromagnetic wave of a specific wavelength, frequency and
energy. Visible light spans only a tiny sliver of the entire electromagnetic
spectrum. Bluer colors have shorter wavelengths, higher frequencies
and carry more energy. Redder colors have longer wavelengths, lower
frequencies and carry less energy. The lower end of the spectrum
ranges from radio waves that are hundreds of meters long, through
millimeter-sized microwaves, to infrared radiation associated with
radiant heat. The upper end proceeds from the ultraviolet rays that
cause sunburn, through X-rays, up to gamma rays with wavelengths
less than the diameter of an atom and energies three trillion times
greater than typical radio waves.
or wavelength, interacts with matter in a different way. Radio waves
push around free-flowing electrons, generating electrical signals
in metal antennae. Microwaves cause the water molecules to vibrate
inside solid food, producing the heat that cooks it. Ultraviolet
radiation from the sun breaks down DNA. Different materials absorb
and radiate light at different wavelengths. The patterns of absorption
and radiation form universal signatures that allow scientists to
identify the elemental composition of a material, whether it is
in a laboratory on earth or surrounding a star in a distant galaxy.
The FEL is an
ideal instrument for charting the interactions of light and matter
in many of the still unexplored regions of the electromagnetic spectrum.
Unlike most conventional lasers, the FEL can be tuned over a broad
range of the spectrum ranging from the infrared to the ultraviolet.
In addition, the FEL design allows for very high power levels.
The W.M. Keck
Foundation Free-Electron Laser Center at Vanderbilt is one of four
university FEL centers in the United States and one of only nine
such centers worldwide where scientific research is conducted.
It operates in the infrared portion of the spectrum. It can produce
laser light in wavelengths ranging from two to nine microns
and has a peak power of more than 10 megawatts.
the only FEL in the world licensed to use this powerful beam for
surgical operations on human patients. The first human surgery using
a free-electron laser beam was performed successfully on December
17, 1999. The laser beam was used to destroy part of a brain tumor.
This feat was repeated the following September with a second patient.
Two weeks later the FEL was used for eye surgery. It was used to
cut the sheath surrounding the optical nerve of a patient whose
eye was being removed. This and several subsequent surgeries following
the same protocol have shown that the laser is a superior instrument
for such an operation.
development with significant medical potential comes from the X-ray
portion of the spectrum. Experiments with the FEL beam have shown
that it is possible to produce monochromatic X-rays by colliding
the infrared beam head-on with a stream of electrons accelerated
to relativistic velocities .
Essentially, the infrared photons bounce off the electrons and pick-up
enough energy to transform them into X-rays in the process. The
monochromatic X-ray beam is similar to a laser beam.
Monochromatic X-rays are capable of producing sharper, cleaner images
than conventional X-ray machines, but, until now, the only sources
for this kind of radiation have been billion dollar synchrotron
radiation laboratories associated with large particle accelerators.
A private company
with Vanderbilt support has developed a prototype monochromatic
X-ray machine that it estimates should cost about $1 million to
The FEL is also
proving its worth in the emerging field of proteomics.
Now that the human genome has been sequenced, researchers are beginning
the task to characterize all of the millions of proteins that build,
power, regulate and protect living organisms. But to do so new methods
must be developed for rapidly identifying and characterizing these
microscopic machines. One method under development relies on the
FEL beam. By tuning the beam to the right frequency, researchers
have shown that they can identify proteins that have been roughly
separated in an electrophoresis gel.
It works by selectively heating the gel molecules enough to release
the proteins without breaking them. Once the molecules are freed,
an electric field pushes them into a mass spectrometer, a conventional
instrument that provides a precise measurement of the protein's
mass, an important key to its identity.
monochromatic X-rays, and protein characterization are three areas
where research at the Vanderbilt FEL is showing particularly promising
results. There are a number of other worthwhile research projects
also being conducted there. The center currently receives about
$3 million in external support from the Department of Defense, the
National Institutes of Health and several foundations. Center management
has identified four areas for growth: materials science, particularly
the use of the FEL in nanotechnology research; laser surgery; proteomics,
identification of the structure and function of proteins; and, in
vivo imaging, using the FEL and monochromatic X-ray beams to image
individual molecules in living animals.