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 British physicist J. J. Thomson, who identified
the electron in 1897, showed in 1906 that light could cause electrons to oscillate up and down and reemit at the same frequency in a dipole pattern; this phenomenon was subsequently termed Thomson scattering.
Nearly a century later, researchers have demonstrated a new phenomena -- relativistic Thomson scattering -- in which electrons oscillate in a more complex figure-8 pattern and emit light at both the exciting
laser frequency and multiples of that frequency, each emitted in a different direction. The more complex pattern results from the electron interacting simultaneously
with both the electric and magnetic fields of the laser light. To observe this phenomena, the research team had to build a tabletop neodymium-glass laser
and compress its billionth-of-a-second pulses by a factor of about 1,000, boosting their power to 4 trillion watts of very high-quality beam. This experiment is an important milestone in the study of nonlinear optics with
electrons unbound to atoms. Furthermore, this work may lead to new laboratory tabletop x-ray sources producing very short x-ray pulses useful, for example, for probing molecular motion during reactions.
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Terawatt-peak-power laser pulses are focused onto a Helium gas jet with a peak laser intensity of >1018 W/cm2. The foot of the laser pulse ionizes the gas to
create a plasma. Plasma electrons then oscillate in the intense laser field nonlinearly and radiate photons at the harmonics of the laser frequency. Each harmonic has a unique angular radiation pattern which can be measured by
imaging with a camera. |
