Steering of Circular Dichroism in Biharmonic Ionization of Atoms

Circular dichroism in photoelectron angular distributions of general biharmonic (i.e., n omega + m omega) atomic ionization is analyzed theoretically and given along with "experimental guidelines" on how it can be steered towards its maximum. It is shown that such a maximum circular dichroism can always be achieved for the ionization of an arbitrary atom and for any incident fundamental photon energy by fine control of only two experimental parameters: the relative flux and phase difference of the two components of the radiation field. In this Letter, we provide a simple analytical description of the circular dichroism as well as a set of rules which enables full control over its magnitude. Our findings are demonstrated explicitly for the ionization of helium with a two-color field composing a fundamental beam and its second harmonic.