  The exact quantum time-dependent optical response of hydrogen under strong
field near infrared excitation is investigated and compared to the perturbative
model widely used for describing the effective atomic polarization induced by
intense laser fields. By solving the full 3D time-dependent Schr\"{o}dinger
equation, we exhibit a supplementary, quasi-instantaneous defocusing
contribution missing in the weak-field model of polarization. We show that this
effect is far from being negligible in particular when closures of ionization
channels occur and stems from the interaction of electrons with their parent
ions. It provides an interpretation to higher-order Kerr effect recently
observed in various gases.

Optics;Atomic Physics;Quantum Physics