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Description:
We demonstrate that Freeman resonances have a strong impact on the nonlinear optical response in femtosecond filaments. These resonances decrease the transient refractive index within a narrow intensity window and strongly affect the filamentation dynamics. In particular, we demonstrate that the peak intensity of the filament can be clamped at these resonances, hinting at the existence of new regimes of filamentation with electron densities considerably lower than predicted by the standard model. This sheds further light on the phenomenon of filamentary intensity clamping and the nearly plasmaless filaments predicted by the controversial higher-order Kerr model.