Floquet scattering and classical-quantum correspondence in strong time-periodic fields

We study the scattering of an electron from a one-dimensional inverted Gaussian atomic potential in the presence of strong time-periodic electric fields. Using Floquet theory, we construct the Floquet scattering matrix in the Kramers-Henneberger frame. We compute the transmission coefficients as a function of electron incident energy and find that they display asymmetric Fano resonances due to the electron interaction with the driving field. We find that the Fano resonances are associated with zero-pole pairs of the Floquet scattering matrix in the complex energy plane. Another way we "probe'' the complex spectrum of the system is by computing the Wigner-Smith delay times. Finally, we find that the eigenphases of the Floquet scattering matrix undergo a number of "avoided crossings'' as a function of electron Floquet energy, and this number increases with increasing strength of the driving field. These "avoided crossings'' appear to be quantum manifestations of the destruction of the constants of motion and the onset of chaos in classical phase space.