Electric field measurement of four-wave mixing signals in magnesium oxide with attosecond resolution

We present measurements of the electric field emitted from a third order nonlinear interaction in crystalline magnesium oxide (MgO). Using a sensitive interferometric technique known as TADPOLE, we measure the complete femtosecond electric field of the weak nonlinear signal generated using a degenerate four wave mixing (DFWM) scheme. In this scheme, three femtosecond pulses of the same wavelength (800 nm) interact with MgO 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. Using a specific geometry of the incoming beams, the signal pulse is spatially separated from the incoming beams to allow for a background-free measurement. We observe modulations in the temporal phase of the emitted electric field on attosecond timescales. With help from theoretical calculations, the source of such modulations, arising from a modulation in the nonlinear response of the material, will be discussed.