Dec. 18, 1998 ANN ARBOR---In a recent discovery---the ramifications of which run the gamut of physics, from the subatomic scale to the astrophysical---scientists at the University of Michigan College of Engineering observed and recorded the relativistic motion of free electrons in the electromagnetic fields of light.
The results, to be published as the Dec. 17 cover story of the journal Nature, are important for several reasons:
¤ The interaction of matter (including electrons) and energy (such as light) is the heart of all physics.
¤ The instantaneous effect of both light's electric and magnetic fields upon electrons that are isolated from the overwhelming forces in atoms had never before been observed.
¤ The results confirm several predictions, based on Einstein's theory of relativity, about how electrons behave in extremely high-strength electromagnetic fields, such as those produced by powerful lasers or supernova explosions.
¤ In such strong fields, a quantity commonly used as a fundamental physical constant in physics theories---known as the Thomson cross section---has now been measured not to be constant.
¤ The discovery marks the dawn of a new field of study---"relativistic nonlinear optics"---that may result in such new technologies as X-rays that could take snapshots on an atomic scale of ultra-fast chemical, physical, and biological processes such as photosynthesis, or make holograms of living cells.
The research team was led by Donald Umstadter, associate professor in the U-M College of Engineering who coordinates the High-Field Science program of the Center for Ultrafast Optical Science, where the work was performed. Szu-yuan Chen, an engineering graduate student, conducted the experiment as part of his Ph.D. thesis. Anatoly Maksimchuk, an assistant research scientist, built the high-power laser system that made the experiment possible.
The National Science Foundation and the Department of Energy (Division of Chemical Sciences, Office of Energy Research) funded participants in the project.
In the experiment, which was designed to test basic aspects of electrodynamic theory formulated over the last century, the researchers focused one of the world's most powerful lasers onto a supersonic jet of helium. The laser---with a power of more than a trillion watts---ionized the atoms of the gas, creating a plasma composed only of free electrons and ions.
The U-M scientists then discovered that the electrons scattered the laser light into colors (frequencies) that differ from, but are harmonically related to, the original beam. Furthermore, the angular direction of the scattered light was observed to be unique to each harmonic. This is the definitive signature of electrons moving in figure-eight patterns due to the combined forces of the light's electric and magnetic fields. The observations were recorded with a digital electronic camera and various filters.
In all previous experiments concerning the scattering of light by free electrons that are initially stationary (from the first experiment, analyzed a century ago by J. J. Thomson, the discoverer of the electron, until now), the effect of light's magnetic field was negligible, and so the electrons had been observed to move in straight lines. The difference in the U-M experiment was the ultra-high field strength of the laser light.
Thomson's classic theory of electrodynamics explains that, as light moves past a free electron that is initially at rest, the electron is accelerated by the light's electric field. Although Thomson was aware that light is composed of both electric and magnetic fields, he supposed the magnetic field should have no effect on the electron's motion. Instead, the electron should move back and forth along a straight line in the same direction as the electric field. He also reasoned that the accelerated electron would radiate (scatter) new light waves with the same frequency as the original wave, in a direction at right angles to the light's electric field.
The Thomson cross section---which gives the probability that light will be scattered by a free electron---was assumed to be independent of the strength of the original light, and therefore a fundamental physical constant.
However, contrary to previous studies, the U-M experiment demonstrates that the probability of scattering actually depends on the strength of light.
Thomson was unaware of relativity, the theory of which was formulated by Einstein several years later. Moreover, the strongest fields of light available in Thomson's day were only equivalent to what can be obtained by focusing a 100-watt light bulb with a magnifying glass.
For light with extreme parameters, modifications to electrodynamic theory would be required. In the 1920s, Compton showed that the Thomson cross section was not constant for high frequency light (X-rays) because of quantum mechanical effects.
Later, other theorists asserted that the effects of relativity would need to be incorporated to account for light with low-optical frequency but extremely high-field strength (as produced in supernova explosions and recently in the laboratory by extremely powerful lasers).
It was predicted that the electrons accelerated by such light would themselves reach nearly the speed of light, causing them to increase in mass, and subjecting them to the influence of light's magnetic field as well as its electric field. Therefore, the instantaneous motion of these electrons should resemble a figure eight, rather than a straight line, due to the combined forces of the light's electric and magnetic fields.
The observations of Umstadter's U-M research team confirm this figure-eight (nonlinear) motion predicted by relativistic theory.
The scientists also showed conclusively that, because the speed of the electrons becomes irregular, they broadcast light not at the same frequency (color) as the original light wave (infrared), but at integer multiples of the original beam (green, blue, and so on, called harmonics). And while the first harmonic (infrared light) radiates principally at right angles to the laser's electric field, the second (green) and subsequent harmonics do not.
Until now, in the absence of a sufficiently powerful laser, the relativistic predictions could not be tested experimentally. The high-power table-top-size laser technology that made the experiments feasible was invented at the University of Rochester in 1985 by Prof. Gerard A. Mourou, who has been director of U-M's Center for Ultrafast Optical Science since 1988.
It is fitting that this new discovery was made at U-M, where the field of nonlinear optics was born in 1961, shortly after the invention of the laser. Prof. Peter Frankin and his colleagues observed for the first time harmonic generation from electrons that are bound to atoms.
Thousands of studies in the intervening 40 years have been done on the subject of nonlinear interactions of light with electrons bound to atoms. They have produced numerous commercial technologies such as fiber-optic communication.
The findings of Umstadter's team usher in a new research field, which might be called "relativistic nonlinear optics"---the study of light scattering from electrons that are free (not bound to atoms), but that move in nonlinear fashion (relativistically) due to the intense light field alone.
Myriad other nonlinear effects are also predicted to occur as lasers continue to produce record-breaking light intensities.
A new set of technologies may flow from this discipline, if the results can be extended to the regime of coherent scattering (as was done with bound electrons). Coherent light results when the scattering from every electron is in phase with that from all the others.
The most promising technologies may arise as coherent harmonics in the X-ray region of the light spectrum are explored. For example, X-rays generated in this way should have extremely short pulse durations (less than a trillionth of a second) and might be used for time-lapse radiography, crystallography, microscopy, and lithography with atomic-scale resolution.
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