Quantum Dynamics of Strong Field Light-Matter Interactions

Abstract

This study explores exactly solvable models for the light-matter interactions in the strong-field regime. These investigations provided us with valuable insights into long-debated problems related to quantum tunneling, namely, the quantum tunneling time and momentum i.e., the ongoing debate in the literature about how much time a particle spends in the classically forbidden region during the tunneling process and what is its momentum when it emerges from the classically forbidden region. Answering these problems requires connecting the quantum and classical descriptions of the electrons liberated from atoms, molecules, and nano-structures by strong electric fields. Taking advantage of our exact solutions, we were able to rule out some of the competing theories.We also identified a non-adiabatic contribution to tunneling ionization, an effect neglected in strong field light-matter interaction studies so far. Our calculations strongly suggest that this non-adiabatic correction to the tunneling ionization can be significant even for long-wavelength excitation, and that it could account for a source of seed electrons in the avalanche ionization which is important for optical filamentation experiments with long-wavelength lasers. We also demonstrate that a similar addition to the tunneling ionization rate occurs in the 3D Coulomb-Stark(CR) problem i.e. a Hydrogen atom exposed to an electric field. This is achieved with the help of the first numerically exact solution to a three-dimensional time-dependent quantum tunneling problem, which was made possible by our development of an algorithm to calculate numerically exact continuum-energy eigenstates of the CR Hamiltonian. This method allows accurate evaluation of properly normalized wave functions and is suitable for the state-expansion of arbitrary wave functions. In an application to metallic nano-structures, we have extended the notion of the similarity between the strong-field ionization in atoms and electron emission from metallic nano-tips. Taking advantage of a non-Hermitian formulation of the problem, we revealed that this similarity also extends to a nonlinear polarization response. Specifically, we have shown that whenever electrons emitted from a nano-tip can be detected, an accompanying, strongly nonlinear surface polarization exists. We have identified signatures in the resulting harmonic radiation as one possible way to verify this effect in experiments.