Berlin 2001 – scientific programme

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AMPD: EPS AMPD

AMPD 3: Sitzung 3

AMPD 3.3: Talk

Tuesday, April 3, 2001, 11:30–11:55, H105

Intense laser–atom interactions: Beyond the electric dipole approximation — •Niels J. Kylstra — Department of Physics Science Laboratories University of Durham DH1 3LE Durham, United Kingdom

Table-top laser systems capable of delivering short pulses of very intense radiation have become available in recent years. Electric fields generated by such lasers are strong enough to compete with the Coulomb field experienced by an atomic electron. Under these conditions, a number of fascinating phenomena can occur [1,2]. Two examples include the emission by the atom of very high harmonics of the driving laser and the stabilization of atoms against ionization in intense, high–frequency fields. For sufficiently high intensities, the electron dynamics in the laser field becomes relativistic, and magnetic field, retardation and spin effects can no longer be neglected. In this contribution, we will give an overview of progress made in describing intense laser–atom interactions in regimes where approaches that go beyond non–relativistic treatments in the dipole approximation are required. Particular emphasis will be placed on discussing how the breakdown of the dipole approximation influences harmonic generation and atomic stabilization.

Harmonic generation has been actively studied in recent years, especially with respect to the possibility of providing a source for sub–femtosecond pulses of coherent, tunable X–rays [2]. This process can be understood qualitatively in terms of a three-step model whereby the active electron of the atom or ion is first detached by tunnelling through an effective potential barrier formed by the instantaneous electric field of the laser pulse and the potential that binds the electron. The ejected electron is then assumed to move like a free particle in the electric field of the incident laser pulse. As the field reverses, the electron can re–collide with the core and recombine radiatively. A quantum mechanical model incorporating these ideas has been developed by Lewenstein and co–workers [2].

Insight into the surprising phenomenon of stabilization in the high-intensity, high-frequency regime relies on first transforming to the rest frame of a classical electron in the laser field [1]. In this Kramers–Henneberger (K–H) frame, the electron experiences an effective time–dependent Coulomb potential that oscillates in space. Gavrila and co–workers have shown that in a stationary field, to first approximation, the dynamics of the atom in the field are governed by the time-average of this potential [1].

At high laser intensities, these relatively simple descriptions of harmonic generation and stabilization, respectively, breakdown since non–dipole and relativistic effects must be accounted for. In particular, it has been realized that a moderately high intensity regime exists where the dynamics is essentially non–relativistic, however the magnetic field component of the laser pulse cannot be neglected. This, and other recent developments will be discussed.

[1] Protopapas M., Keitel C. H. and Knight P. L., Rep. Progr. Phys. 60, 389 (1997); Joachain C. J., Dörr M. and Kylstra N. J., Adv. At. Mol. Opt. Phys. 42, 225 (2000).

[2] Salières P., L’Huillier A., Antoine P. and Lewenstein M., Adv. At. Mol. Phys. 41, 83 (1998); Brabec T. and Krausz F., Rev. Mod. Phys. 72, 545 (2000).