Third-order optical nonlinearities are extensively utilized in both generation and characterization of ultrashort optical pulses. One example is the widely used characterization method of frequency-resolved optical gating (FROG), which can be based on a variety of χ(3) effects. Any variant of FROG relies on the instantaneity of the polarization response, as is usually expected off-resonance. Here we demonstrate that resonant third harmonic excitation in a dielectric medium may exhibit a surprisingly slow polarization decay, with a relaxation time constant that exceeds the duration of the shortest pulses generated to date. For this purpose, a collinear variant of third harmonic FROG (THFROG)  was used to characterize 7.5 fs Ti:sapphire laser pulses using SiO2 or TiO2 as the χ(3) medium. These measurements indicate a polarization decay time of 6.5 fs in TiO2. Numerical solutions of the time-dependent Schrödinger equation (TDSE)  further support the experimental findings.
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