Polarization control of absorption of virtual dressed states in helium

The extreme ultraviolet absorption spectrum of an atom is strongly modified in the presence of a synchronized intense infrared field. In this work we demonstrate control of the absorption properties of helium atoms dressed by an infrared pulse by changing the relative polarization of the infrared and extreme ultraviolet fields. Light-induced features associated with the dressed 1s2s, 1s3s, and 1s3d states, referred to as 2s+, 3s±, and 3d± light-induced states, are shown to be strongly modified or even eliminated when the relative polarization is rotated. The experimental results agree well with calculations based on the solution of the time-dependent Schrödinger equation using a restricted excitation model that allows efficient treatment of the three-dimensional problem. We also present an analysis of the light-induced states based on Floquet theory, which allows for a simple explanation of their properties. Our results open a new route to creating controllable superpositions of dipole allowed and nondipole allowed states in atoms and molecules.