  We study the influence of third-harmonic generation (THG) and negative
frequency polarization terms in the self-phase modulation (SPM) of short and
intense pulses in Kerr media. We find that THG induces additional symmetric
lobes in the SPM process. The amplitude of these new sidebands are greatly
enhanced by the contributions of the negative frequency Kerr (NFK) term and the
shock operator. We compare our theoretical predictions based on the analytical
nonlinear phase with simulations carried out by using the full unidirectional
pulse propagation equation (UPPE).
