Fig. 3.

Effect of χ⁽³⁾ on THG-FWM contrast. (a) z-scan geometry used for estimating χ^(THG) and χ^(FWM) of various liquids relative to glass. The focus of a coverslip-corrected objective is scanned through a water-glass and a glass-liquid interface. (b) Top, THG (left) and FWM (right) z-scan profiles for different liquids (immersion oil, water, plant oil). The displayed profiles are normalized to their values at the first interface. Bottom, estimated values of χ^(THG) (left) and χ^(FWM) (right) relative to glass (see text), for different OPO wavelengths and TiS-OPO frequency differences. Deviation of the water χ^(FWM) curve near 3600cm⁻¹ is due to the presence of CARS at this frequency shift. It can be seen that ${\chi }_{\mathit{water}}^{(3)}<{\chi }_{\mathit{oil}}^{(3)}$ in all the conditions explored here. (c) Simultaneously recorded images of immersion oil droplets in water for λ_OPO = 1140nm and ω_TiS – ω_OPO = 3890cm⁻¹. Water/immersion oil interfaces are visible in the THG images, and immersion oil produces a stronger FWM signal than water. Scale bar: 50μm.