Femtosecond third-order sum frequency and four-wave mixing imaging

Various contrast mechanisms in imaging applications in biological and material sciences are of great importance for multimodal sample visualization. Nonlinear-optical interactions in the sample provide multitude of possibilities to imaging with different contrasts, including sensitivity to chemically-specific vibrational signatures. Cubic order nonlinearity is present in all materials since it does not require broken inversion symmetry. Cubic non- linearity offers several useful interaction modalities, including vibrationally resonant ones, such as third-order sum-frequency generation (TSFG) and four-wave mixing (FWM), which we explore in this work using femtosecond lasers in a laser-scanning all-reflective microscope. We observe strong dependence of image contrast on delay between interacting pulses and the frequency of the mid-IR laser relative to the CH vibrational mode of the sample. Images of oil-water interfaces demonstrate striking visual contrast and impressive signal-to-noise ratio in our system. Pathways to expand TSFG and FWM imaging onto biological samples are explored.