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We employ a continuously tunable 100 fs pulse source to measure the time-resolved degenerate four-wave-mixing (DFWM) spectrum of an amorphous C 70 film. The observed nonlinear optical response is essentially instantaneous and electronic in origin. We deduce the complex third-order nonlinear-optical susceptibility tensor function χ 1111 (− ω , ω , ω ,− ω ) throughout the wavelength range 0.74–1.6 μm. We find two-photon resonances with excited-state energies of 2.41±0.05 and 2.61±0.05 eV close to the 2.7 eV two-photon resonance of C 60 . The two-photon absorption coefficient at the resonance maximum is 20±8 cm/GW, as large as that of C 60 . Our amplitude and phase data indicate that we have reached the long-wavelength limit, χ 1111 LWL , of χ 1111 . We find it to be (4.5±0.5)×10 2 times that of fused silica and 1.9±0.6 times that of C 60 . This result implies χ 1111 LWL =(1.8±0.2)×10 −12 esu, a value that is consistent with the theoretical prediction of Shuai and Bredas. We use linear-absorption and DFWM data to construct an analytic expression for χ 1111 [−( ω 3 + ω 2 + ω 1 ), ω 3 , ω 2 , ω 1 ] which, we believe, predicts any nonlinear optical effect within the range ℏ ω i >0.6 eV (i=1, 2, or 3) and ℏ( ω 3 + ω 2 + ω 1 ) ω near the lowest three-photon resonance, the magnitude and phase of χ 1111 behave like that of a two-level atom with a matrix element obtained from the observed one-photon oscillator strength. This explains the observed change in sign of Re[ χ 1111 (−3 ω , ω , ω , ω )] from positive to negative for decreasing ω near ℏ ω =0.8 eV. |
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