We propose and demonstrate sensitive interferometry that permits the separation of the real and imaginary parts of the nonlinear susceptibility with a femtosecond time resolution by a single measurement. A special reference interferometer compensates for any fluctuations of the fringe and provides high sensitivity to detect a fringe shift as small as 0.025 rad (λ/250) by averaging only 100 shots with a low-repetition-rate laser. This method can be applied to materials with optical anisotropy and/or absorption with high sensitivity. We apply the method to two materials, CS2 and CdSxSe1−x microcrystallite-doped glass.
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