Nonlinear optical spectra of C70 Original Research Article

We employ a continuously tunable 100 fs pulse source to measure the time-resolved degenerate four-wave-mixing (DFWM) spectrum of an amorphous C70 film. The observed nonlinear optical response is essentially instantaneous and electronic in origin. We deduce the complex third-order nonlinear-optical susceptibility tensor function χ1111(−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 C60. The two-photon absorption coefficient at the resonance maximum is 20±8 cm/GW, as large as that of C60. Our amplitude and phase data indicate that we have reached the long-wavelength limit, χ1111LWL, of χ1111. We find it to be (4.5±0.5)×102 times that of fused silica and 1.9±0.6 times that of C60. This result implies χ1111LWL=(1.8±0.2)×10−12 esu, a value that is consistent with the theoretical prediction of Shuai and Brédas. We use linear-absorption and DFWM data to construct an analytic expression for χ1111[−(ω3+ω2+ω1),ω3,ω2,ω1] which, we believe, predicts any nonlinear optical effect within the range ℏωi>0.6 eV (i=1, 2, or 3) and ℏ(ω3+ω2+ω1)<2.8 eV. We find that for ω near the lowest three-photon resonance, the magnitude and phase of χ1111 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[χ1111(−3ω, ω, ω, ω)] from positive to negative for decreasing ω near ℏω=0.8 eV.