Abstract
Femtosecond filaments are self-guided laser beams with nearly constant intensity over distances exceeding the Rayleigh length. According to the so-called standard model, filamentation stems from a dynamical balance between Kerr self-focusing and plasma defocusing. However, this has recently been challenged by a pump-probe experiment [1], which seemed to imply a saturation of the Kerr nonlinearity due to higher-order terms in the intensity. Thus, the existence of plasma-less filaments, maintained solely by the saturating Kerr nonlinearity, was proposed. We now demonstrate that in pump-probe based measurements of the optical response, Freeman resonances significantly decrease the transient refractive index and may be misinterpreted as a higher-order Kerr response [2].
© 2015 IEEE
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