Simultaneous measurement of continuous spectra of real and imaginary parts of complex nonlinear optical susceptibility was performed for porphyrin J-aggregates with sharp absorption peaks using a femtosecond Sagnac interferometer. A nonresonant component (χNR(3)) due to a solvent and a resonant component (χNR(3)) due to the porphyrin J-aggregates are observed. The real and imaginary parts are compared using the Kramers-Kronig (KK) relations, which are valid for linear optical susceptibility. It is shown that χR(3) satisfies the KK relations at all delay times. On the other hand, (χNR(3)) does not satisfy the KK relations at zero delay time. The validity of the KK relations, which have been reported in previous papers by our group, are verified for the first time experimentally using continuous spectra.
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