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Four-wave mixing processes are ubiquitous in ultrafast optics and the determination of the coefficients of the χ(3) tensor is thus essential. We introduce a novel time-resolved ultrafast spectroscopic method to characterize the third-order nonlinearity on the femtosecond time-scale. This approach, coined as "nonlinear chirped interferometry," makes use of the variation of the optical group delay of a transmitted probe under the effect of an intense pump pulse in the nonlinear medium of interest. The observable is the spectral interference between the probe and a reference pulse sampled upstream and the metric is the transient swing of the probe group delay. We show that the detected signal is enhanced when the pulses are weakly chirped, and that, although interferometric, the method is intrinsically less sensitive to environmental phase fluctuations and drifts. By chirping adequately the reference pulse, the transient frequency shift of the probe pulses is also detected in the time domain and the detected nonlinear signal is enhanced. Nonlinear phase shifts as low as 10 mrad, corresponding to a frequency shift of 30 GHz, i.e., 0.01% of the carrier frequency, are detected without heterodyne detection or active phase-stabilization. The diagonal and/or non-diagonal terms of reference glasses (SiO2) and crystals (Al2O3, BaF2, CaF2) are characterized. The method is finally applied to measure the soft vibration mode of KTiOAsO4 (KTA). [ABSTRACT FROM AUTHOR]

A cross polarized wave (XPW) generation experiment with double BaF2 crystals in air was demonstrated with 120 µJ input pulse. A circular aperture was used to truncate the near field profile of the input beam and remove the outer part of the beam, and circularly symmetric beam profiles were generated at the BaF2 crystal positions. The measured focal spot size was close to the diffraction limit, and the energy stability of XPW was improved from ∼1.89% (without the aperture) to ∼0.67% (with the aperture). [ABSTRACT FROM AUTHOR]