Ultrafast Photography

“It's an idea or concept at the moment. If it works, it's revolutionary. But there are many obstacles to be overcome,” says Dr. Brabec, the Canada Research Chair in Ultrafast Photonics at the University of Ottawa, and one of six recipients of a 2004 NSERC Steacie Fellowship.

If anyone can help turn the idea into reality, it's Dr. Brabec. In the mid-1990s, during and shortly after his Ph.D. work at the Vienna University of Technology in Austria, Dr. Brabec developed the theoretical underpinnings for the creation of femtosecond (10^-15 seconds) solid-state laser pulses. This led to the generation of five femtosecond laser pulses (five-millionths of a billionth of a second), the world's fastest at the time.

As ultrafast camera flashes, these femtosecond pulses have become the new standard for watching and better understanding chemical reactions.

But in describing his latest work, Dr. Brabec lets slip that he thinks these femtosecond laser flashes are now electromagnetic slow-pokes. He has his theoretical sights set a thousand times faster, on attosecond (10-¹⁸ seconds) pulses. An athlete who won a race by an attosecond would be ahead by less than the width of an atom.

“The first idea of how to measure attosecond pulses came from Dr. Paul Corkum at the National Research Council in Ottawa, with whom I did postdoctoral research in 1995. The second idea came from my research group in Vienna. I was involved in the first measurement of an attosecond pulse, done by my then-colleague Dr. Ferenc Krausz, at the Vienna University of Technology, and I developed the theory for this measurement method,” says Dr. Brabec, who was recruited to the University of Ottawa's Centre for Research in Photonics in June 2002, in what was a significant coup for Canada's ultrafast science ambitions.

Though still in its embryonic stage, attosecond science holds the promise of resolving a new atomic and molecular horizon by using speeds and wavelengths of energy that can see nuclear and electron-level details.

The technique works by using a high-energy, ultrafast laser to rip electrons from their nuclei, creating ionized (or negatively and positively charged) particles. The laser field has positive and negative cycles, so when the cycle reverses, the now accelerated charged particles crash together, releasing a flash of intense x-ray energy. It's this burst of faster, shorter wavelength x-rays that can be used to image the attosecond-speed movements of electrons or nuclear processes.

“It's like a stroboscope,” says Dr. Brabec. “You have these flashes and one flash freezes the electron at one position, and then comes the next flash and it freezes the electron at a slightly different position. It's like the famous 19th century images of the galloping horse caught in stop-action photography.”

Dr. Paul Corkum's lab, with which Dr. Brabec is collaborating, has already demonstrated that it is possible to resolve details of nuclear motion in hydrogen molecules, the simplest of the molecules. And a November 2003 conference on attophysics – the first ever – at Harvard University's Institute for Theoretical and Molecular and Optical Physics, put this emerging discipline, and Dr. Brabec's work, publicly on the scientific front burner.

Brabec says the research time, and money for graduate students, that the NSERC Steacie Fellowship provides will enable him to accelerate his own work to making attosecond science an applied reality.

His research group, which includes two Ph.D. students who followed him from Austria (“Sometimes I feel like a father,” quips Brabec.), will be exploring how to apply theoretical tools developed for attophysics to such nanophotonics challenges as designing a single molecule transistor.

For this master of speed, though, the most tantalizing goal is the visualization of electron behaviour using attosecond pulses. To achieve this, his group is finessing a unique computer code that could open the door to the attosecond analysis of electron behaviour in complex atoms and molecules, something that's been dubbed the Holy Grail of chemistry.

“So far, when chemists look at molecular reactions such as the making and breaking of nuclear bonds, they just look at the motion of the nuclei, which is usually relatively slow, on a femto-second time scale,” explains Dr. Brabec. “But before the nuclei do something in bonding, the electrons have to do something, and that's a much faster process. And experimentally people don't know much about that. So the hope is that if you can watch the electrons, you will learn more about the trigger for the breaking and making of molecular bonds.”

Contact:

Dr. Thomas Brabec
Tel.: (613) 562-5800, ext. 6756
E-mail: brabec@uottawa.ca
Web site: http://itamp.harvard.edu/attosecond.html