Electric field-resolved nonlinear optical spectroscopy in hexagonal boron nitride

We present measurements of the femtosecond electric field emitted from a third-order nonlinear optical interaction in hexagonal boron nitride (hBN). 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 free-standing multi-layer hBN mounted on a 300-micron diameter aperture from which a nonlinear signal pulse is emitted. Two of the pulses are time delayed with respect to the third which allows us to extract dynamical information about the nonlinear interaction. Our work shows that both the amplitude and phase of the signal provide insights into nonlinear interactions occurring on ultrafast time scales. We then extend these measurements to strong-field ionized molecules in the gas-phase.