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.

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.

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.

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 . 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 produce commercially.

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.

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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