Come with Erich P. Ippen into femtosecond time, where molecules vibrate, electrons collide and atoms and ions break apart.
We can't see these things with the naked eye, but before Harold "Doc" Edgerton invented high-speed strobe photography in the 1930s, we couldn't see speeding bullets either.
Ippen, the Elihu Thomson Professor of Electrical Engineering and Professor of Physics, is the recipient of the Killian Award for 2001-2002. Making and using really short flashes of light was the topic of the Killian Award lecture he delivered March 13 in Wong Auditorium.
One of the creators of the field of femtosecond optics, Ippen described how ultrafast laser pulses allow researchers to freeze motion on a microscopic level. Edgerton's electronic strobe created flashes of light less than a millionth-of-a-second long, but "what we're talking about today is generating flashes of light that are one million and 10 million times shorter," Ippen said.
To give an idea of scale, Ippen said there are as many femtoseconds in one second as there are seconds in 30 million years.
With lasers, scientists are able to produce extremely bright pulses of light that last for only a few femtoseconds. Nothing large enough to see with the naked eye moves at that time scale, but microscopic phenomena do. "The importance, of course, is that what goes on in this time scale actually determines fundamental chemical and biological events," Ippen said.
HOW IT'S DONE
One very short laser pulse is focused onto some material. The light energy is absorbed and the atoms or molecules get excited. This in turn changes another variable, such as, in the case of a metal, reflectivity.
"If we come in with another short pulse a short time after [the first pulse], we can see that change," Ippen said. "And now, simply by moving a mirror, we delay this pulse in time and we scan out what's happening to the material and how it's recovering and what's going on. This can be done with pulses of the same color or pulses of a different color, so we can even map out a full spectrum of the event."
A look of this nature at copper and gold produced the first direct measure of the speeds of electrons inside metals, which play an important role in superconductivity.
SLICING UP TIME
When you slice up time this finely, you can see molecules store photon energy for vision. You can watch cells divide and take part in photosynthesis in real time. The phenomena that limit electronic devices occur on this time scale as well.
Among the applications is a medical tool that diagnoses retinal diseases. James G. Fugimoto, professor of electrical engineering and computer science, used femtosecond pulses to develop optical coherence tomography for performing high-resolution cross-section imaging of biological tissues on a micron scale. Other applications of the future include optical circuits written over each other in glass, and fiber optics communication that makes use of both very high bandwidth and the ultra-high speed of light.
Femtosecond optics also can be used to create the largest electrical fields being achieved in the world. In government labs, Ippen said, high-power femtosecond pulses are being used to accelerate electrons to extremely high velocities. These devices might be the next-generation accelerators.
"The number of applications just seems to expand every year," Ippen said. "We really are having a lot of fun."
KILLIAN AWARD WINNER
The James R. Killian Jr. Faculty Achievement Award was established in 1971 to recognize extraordinary professional accomplishments by full-time members of the MIT faculty. A faculty committee chooses the recipient from candidates nominated by their peers for outstanding contributions to their fields, to MIT and to society.
The son of an MIT civil engineering professor, Ippen earned a bachelor's degree from MIT in electrical engineering. He attended Berkeley for graduate degrees and then worked at Bell Labs until 1980, when he returned to MIT.
A version of this article appeared in MIT Tech Talk on March 20, 2002.