Phys. Rev. Lett. 110, 203002 (2013)
Using a strong laser field to probe an atom often causes the atom to become photo-ionized. Ulrich Eichmann and colleagues in Germany have now shown that atoms in Rydberg states are exceptionally stable in strong laser fields. Rydberg atoms — excited states that are described as being 'hydrogen-like', with a valence electron in a large orbit far from the ion core — have a few peculiar characteristics, particularly with respect to their electromagnetic response. Using laser pulses with intensities of the order of 1015 W cm−2, the researchers observed lower-than-expected rates of ionization for helium atoms, which may allow atoms to be probed at much higher intensities than would otherwise be possible. A Rydberg wavepacket was produced using a linearly polarized laser pulse and (after a time delay) an elliptically polarized laser field accelerated any Rydberg atoms through field gradient effects. This acceleration (deflection) of the atoms confirms the interaction of the Rydberg atoms with the strong field. One of the most interesting outcomes of this experiment, apart from the lower-than-expected rate of ionization, is that each atom became 'tagged' with the laser intensity with which it was probed. This intensity can be read-out by detecting the momentum transfer of the laser field to the surviving atom. The researchers hope that their work will allow coherent manipulation using short-pulse lasers and the development of an efficient cascaded acceleration scheme.
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