A four-wave mixing (FWM) microscopy that is designed to probe third-order nonlinear susceptibilities, of target materials with femtosecond light pulses has been constructed and investigated. Nondegenerate FWM signals (at 1500 and 639 nm) were produced in samples by a femtosecond Ti:sapphire laser (790 nm) and a femtosecond optical parametric oscillator (1035 nm). While the effect of electronic and vibrational molecular resonances on the visible FWM signal has been extensively studied, little attention has been paid to the infrared (IR) signal. This IR signal should exhibit a different dependence on the spectrum of molecular electronic resonances, and thus potentially offers a new mechanism for image contrast in microscopy. We have therefore constructed a FWM microscope to characterize these signals in a focused geometry. In polymeric films and beads containing a solute with a resonant (or near-resonant) optical response, the nonresonant polymer background signal was effectively suppressed using polarization-sensitive detection. Longitudinal scans of the beam foci through films were used to determine relative nonlinear third-order susceptibilities and second hyperpolarizabilities of selected solvents and the Rhodamine 6G dye molecule, for both the visible and IR FWM wavelengths.
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