Einstein and Lasers

A wave and a particle were walking side by side.
Each said to the other which one of us am I?

There's this funny thing about light. A real head-scratcher of a riddle that over the years has intrigued some of the best minds in physics. Is it particles or waves?

For Newton, who gave much attention to the question, light was a stream of particles, and his view held the field until about 1800. In the 19th century, however, physicists found that light behaved like water and sound waves, and thinking shifted. Then in 1905 along came Einstein's paper on the photoelectric effect. Light, he proposed, should be understood as discrete lumps of energy -- photons, as they were later christened -- and it takes only a single photon with high enough energy to knock an electron loose from the atom it's bound to. This revelation, which opened the door to quantum mechanics and won a Nobel Prize, led to the modern view that light -- contrary to common sense -- is not simply either particles or waves; it's both.

Einstein's explanation of the photoelectric effect has withstood the test of time. "It's remarkable," says University of Rochester physicist Joseph H. Eberly, "that such a simple concept, although revolutionary, held up for 75 years without any modification or experimental contradiction."

Beginning around 1980, however, atomic physics experiments with high-intensity lasers showed electrons escaping from atoms with more energy than Einstein's theory predicts. To better understand these experiments, Eberly and his Rochester colleagues Juha Javanainen (now at the University of Connecticut) and Qichang Su, and the research team headed by K. C. Kulander at Lawrence Livermore National Laboratory, developed numerical methods to simulate them. In recent work, the researchers used their computational approach to go beyond the experiments, and what they found turns Einstein's theory on its head. "The expected tendency," says Eberly, "is that as you make the laser more intense, it's easier to strip the electron out of the atom. What we found is that beyond a certain point the opposite happens -- the electron is more likely to stay in the atom."

Eberly's computations -- using the CRAY C90 at Pittsburgh Supercomputing Center -- predict that when a super-intense laser irradiates a hydrogen atom it can, in effect, trap the electron. The laser-zapped electron moves around in the atom with much higher energy than should be required to break free, but it's not free. The laser field is strong enough to hold the electron under its own influence, and thus the atom -- rather than losing the electron and becoming ionized -- is stabilized.

Death Valley

This 3-D contour plot shows "Death Valley" and the surrounding landscape. The elevations represent electron probability distribution at varying laser intensity. The green dotted line at x = 0 marks the position of the nucleus. At weak intensities (orange region), the electron stays at the nucleus, with negligible ionization. At strong intensities (red regions), the electron is pushed away, and the flat central region (yellow) of total ionization is Death Valley. At super-strong intensities (purple region), where the laser stabilizes the atom, the electron is once again at the nucleus.

Researcher: Joseph Eberly, University of Rochester
Hardware: CRAY Y-MP C90
Software: User developed code.
Keywords: atomic physics, photon, photoelectric effect, ionization, Einstein, strong-field effects, multiphoton physics, superintense lasers.

Related Material on the Web:
Projects in Scientific Computing, PSC's annual research report.

References, Acknowledgements & Credits