Alternative descriptions of the light-matter interaction beyond the dipole approximation

Different descriptions of the light-matter interaction which include the full spatial dependence of the electromagnetic field are presented. It is demonstrated how nondipole forms of both the Kramers-Henneberger and the length gauge Hamiltonians may be obtained from the minimum-coupling Hamiltonian in the velocity gauge through the Baker-Hausdorff lemma. Furthermore, the length gauge version of the Dirac equation is presented. The applicability of the different descriptions is discussed