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.

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• Light-matter interaction
• Multiphoton or tunneling ionization & excitation
• Photoemission
• Single- and few-photon ionization & excitation
• Atoms
• Dipole approximation
• Perturbative methods