Abstract
We discuss the connection between strong-field ionization, saturation of the Kerr response and the formation of the Kramers–Henneberger (KH) atom and long-living excitations in intense infrared (IR) external fields. We present a generalized model for the intensity-dependent response of atoms in strong IR laser fields, describing deviations in the nonlinear response at the frequency of the driving field from the standard model. We show that shaping the driving laser pulse allows one to reveal signatures of the excited KH states in the Kerr response of an individual atom.
Highlights
For standard laser pulses, separating the contribution of ionization from other possible mechanisms leading to the saturation of the Kerr effect is very challenging
The complexity in the analysis of the contribution of the nearly free states is confirmed by results of Kano et al [30], who find that bound states alone can provide saturation of the Kerr response
We show that shaped laser pulses offer additional opportunities for addressing this difficult question: they emphasize the role of the restructured spectrum of the dressed atom compared to the field-free system and allow us to identify the origin of resonance structures appearing in the Kerr response
Summary
Due to the large gap between the ground and the excited states in noble gas atoms, or in air molecules, it is useful to break the full Hilbert space of the field-free system into two subspaces, one containing the lone ground state |g , another all other states |n , both excited bound and continuum. At the intensities of interest, ak(inst) ≪ 1 since F zkg ≪ Ek − Eg. At the intensities of interest, ak(inst) ≪ 1 since F zkg ≪ Ek − Eg Substituting this expression into (6) for the leading term D12(t), we obtain the contribution to the ground-state atom response from the instantaneous polarization following the driving laser field: D1(i2nst)(t ) + c.c. There are several obvious restrictions on the applicability of this expression It is only valid in the low-frequency field, when the photon energy is small compared to the energy gap between the ground and the excited states. As the application of the first-order perturbation theory to the excited states becomes insufficient, one should not be surprised to see deviations from the linear intensity dependence From this perspective, the recent experimental results for near-IR laser fields [39] showing that this does not happen until close to the onset of ionization, are counter-intuitive. For states with binding energies of a few eV this implies I 1013 W cm−2
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