Electric field measurement and analysis of four-wave mixing signals in crystalline magnesium oxide with attosecond delay resolution

We present measurements of the electric field emitted from a third-order nonlinear interaction in crystalline magnesium oxide (MgO). Using a sensitive spectral interferometry technique known as TADPOLE, we measure the complete electric field of the weak nonlinear signal generated using a degenerate four wave mixing (DFWM) scheme. In this scheme, three near infrared (NIR) femtosecond pulses of the same wavelength (800 nm) interact with the MgO crystal and emit a signal pulse. Two of the pulses are time delayed with respect to the third which allows us to extract dynamical information about the system. We observe modulations in the temporal phase of the emitted electric field as a function of attosecond time delay between the DFWM pulses. In the second order (chirp) and third order phase parameters of the signal field, we observe time delay dependent modulations which contain harmonic components of the fundamental NIR frequency. We observe that these modulations and harmonics depend on the angle of the crystal with respect to the probe polarization. With help from field-modified band structure calculations, the source of such angle-dependent modulations and harmonics, arising from a modulation in the nonlinear response of the material, will be discussed.