The free-electron laser is a conundrum. It is a powerful machine that holds much promise and much fundamental research must go on before its full power can be harnessed.

Pulling at the reins are researchers from across various disciplines and schools at Vanderbilt, as well as from around the world, who are investigating ways in which energy from the FEL interacts with, bonds, modifies and cuts materials with greater control and less thermal damage.

"We are interested in the subtle interaction of a photon of light with the atoms and molecules in a solid. We think that using the FEL we can selectively affect impurity atoms or those associated with defects in materials to cause them to migrate, break bonds or do things that in the past only heat could do," says Norman Tolk, (below), physics professor and director of the Center for Molecular and Atomic Studies at Surfaces, or CMASS.

The emphasis of the research at CMASS is on solid materials used in the electronics industry with an eye toward transferring knowledge and techniques to biological and medical materials.
Lasers, once just laboratory instruments, have become workhorses in the medical and industrial worlds. They have been used since the early 1970s as surgical tools to cut, weld or vaporize tissue; seal leaking blood vessels in the eyes of diabetics; and fade birthmarks, among other uses.

Like many lasers that have proved useful, the FEL is tunable, which means that you can get the exact wavelength and color of light that you need, and it provides short rapid pulses of light, giving the material a brief time to cool before the next assault. But what makes the FEL unique is its ability to deliver enormous powers - up to 10 million watts in short pulses - of light across a portion of the spectrum that had previously been inaccessible to powerful tunable lasers.

"If we can selectively energize only the changes we want without otherwise heating the material, we can avoid unwanted side effects," says Jonathan Gilligan, research assistant professor of physics.

Enhancement of transistors, used to make integrated circuits, is an ongoing goal of material scientists, since integrated circuits are the building blocks of modern electronics. Use of the FEL to enhance chip processing could have a major impact on the microelectronics industry.


In collaboration with scientists at Bell Laboratories and the University of Illinois, the CMASS team is studying the effects of modifying transistors by replacing hydrogen atoms with a heavier isotope, deuterium, a substitution that can make circuits last up to 40 times longer.

CMASS investigators are collaborating with other scientists in the United States and Europe to use the FEL to measure the electrical properties of semiconductor heterojunctions, the building blocks of transistors and integrated circuits. CMASS scientists hope to use the FEL to modify the electronic properties so manufacturers will be able to tailor the characteristics of individual transistors to their exact specifications.

Investigators also are studying the fundamental properties of amorphous silicon, which is used to make solar cells. The FEL provides information about how solar cells wear out when they are exposed to bright light. This work benefits from the expertise in theoretical physics given by Sokrates Pantelides, William A. and Nancy F. McMinn Professor of Physics.

Materials physicist Len Feldman, (left), Stevenson Professor of Physics, is creating new materials for "warm" electronic devices - biosensors - with the help of the FEL. Feldman is combining silicon technology use in computers with organic materials to integrate silicon electronics with biological devices. The FEL will help in understanding how bonds are formed at the interface between the inorganic material (silicon) and organic materials like a living cell or nerve.

"The biosensors will be silicon devices with just the right biological material on them so that they will detect a specific other biological chemical. Eventually we'll put this into an interesting medical solution such as implanting it into a person to detect certain biological chemicals like glucose. One could envision a voltage signal signalling to release a compensating drug," Feldman says.

He is already working with sensors of other materials. Work has begun in understanding the bonding of silicon to organic materials. One application of coupling organics with silicon are LEDs (light emitting diodes). "We have begun looking at LEDs that have a lot of commercial applications to people, like the front panel of laptop computers. The computer industry is looking for a display like a laptop screen that could be folded up and put in the pocket. We would like to make that display on plastic."

Jim Davidson, professor of engineering science, electrical engineering and materials science, is

collaborating with CMASS in examining ways the FEL modifies the properties of diamonds to make them more useful in electronic devices. It appears that under the control of the FEL, particular kinds of atoms in the diamonds can be caused to move around.

In their study of the fundamental processes by which atoms and molecules are driven off surfaces by laser radiation or by beams of electrons or ions, Gilligan says "we have demonstrated that when the FEL is tuned to deposit energy into particular vibrations of a surface, the laser can cut through the surface of a diamond with great ease, in the same way that Caruso could shatter a crystal goblet by singing a pure note. We're looking for that pure note."

"Current technology for modifying the properties of diamonds involves heat, which causes the atoms to move farther or more often than is desirable. With the FEL we can get the atoms where we want them. This means better, faster, cheaper transistors and integrated circuits, which in turn means better medical imagery for things like CAT scans and mammograms," Davidson says.

Developing expertise with orderly crystalline materials will eventually make it possible for scientists to transfer their techniques to biological materials as well, which are not so easily controlled.

"The orderly nature of crystalline solid matter allows us to understand in detail what happens to laser energy from the moment a pulse of laser light hits our sample. It also allows us to work with samples that we can control much more precisely than is possible with biological or medical samples," Gilligan says.

Another area of research is near-field scanning optical techniques. In collaboration with Professor Aaron Lewis of Hebrew University in Jerusalem, CMASS scientists are looking at ways to use the FEL with the instrument Lewis pioneered, the near-field microscope. This microscope can shine the laser light on a tiny spot about 1,000 atoms across.

"Right now the FEL performs like a scalpel, but the near-field microscope takes it down to the level of interaction with one cell or part of a cell at a time," Tolk says.

-Ellen Bourne

[ Back to RESEARCH | News and Public Affairs | Vanderbilt Home Page ]
HTML Translation by Billy Kingsley
This document created November 19, 1996