Abstract

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) [1] 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) [2] further support the experimental findings.

© 2015 IEEE