Spatial and temporal characterizations of femtosecond pulses at high-numerical aperture using collinear, background-free, third-harmonic autocorrelation

We show that a simple plane wave analysis can be used even under tight focusing conditions to predict the dependence of third-harmonic generation on the polarization state of the incident beam. Exploiting this fact, we then show that circularly polarized beams may be used to spatially characterize the beam focus and temporally characterize ultrashort pulses in high numerical aperture systems by experimentally demonstrating, for the first time, novel collinear, background-free, third-harmonic intensity autocorrelations in time and space in a high numerical aperture microscope. We also discuss the possibility of using third-harmonic generation with circularly polarized beams for background-free collinear frequency-resolved optical gating.