Exploring the interactions of light and matter
By David F. Salisbury
Oct. 9, 2001
The 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.
The different 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.
Each color, 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. [FEL HOW IT WORKS] [ANIMATION]
The W.W. 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.1. It operates in the infrared portion of the spectrum. It can produce laser
light in wavelengths ranging from two to nine microns2 and has a peak power of more than 10
megawatts.
Vanderbilt has 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.
1
For a complete list see http://sbfel3.ucsb.edu/www/vl_fel.html
A micron is one millionth of a meter or one 25-thousandth of an inch. Human hair: about 100 microns.
Particles smaller than 35 microns are invisible to the naked eye.
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Exploring the interactions of light and matter
Another center 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 3. Essentially, the infrared photons bounce off the electrons
and pick-up enough energy to transform them into X-rays in the process. The monochromatic Xray beam is similar to a laser beam 4. 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 company5 with Vanderbilt support has developed a prototype
monochromatic X-ray machine that it estimates should cost about $1 million to produce
commercially.
The FEL is also proving its worth in the emerging field of proteomics6. 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 gel7 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.
Laser surgery, 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.
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Velocities approach the speed of light, where effects predicted by Einstein’s theory of relativity become
significant.
4 All but a few percent of the photons are the same wavelength, but they are not coherent.
5 The company is named MXISystems. Its website is http://www.mxisystems.com/#top.
6 The proteome is defined as all the proteins associated with a given genome and proteomics is the effort to
catalog and characterize all these proteins and to compare how they function in different conditions.
Chemical & Engineering News Online has published a good overview of the subject at
http://pubs.acs.org/cen/coverstory/7831/7831scit1.html.
7 Electrophoresis gels are widely used to separate proteins, nucleic acids and other bio-molecules. It relies
on the fact that when electrically charged molecules are added to a gel and an electrical charge is applied,
the molecules begin migrating through the gel. Lighter molecules with more electrical charges move more
rapidly than heavy molecules with fewer charges. So molecules of roughly the same size then to form bands
in a strip of gel.
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