Degenerate four-wave mixing (DFWM) spectroscopy is modified to exploit femtosecond pulses, phase-sensitive-detection, frequency (wavelength) agility, two-color (nearly degenerate multiwave mixing) radiation, and improved signal-to-noise capabilities that can be realized through a combination of new solid state lasers, nonlinear optical components, and novel design concepts. The resulting time-resolved nonlinear optical techniques permit “instantaneous” optical nonlinearities, such as two-photon absorption cross sections, to be accurately measured over the spectral range from 450 to 2500 nm (and with significantly greater effort from 225 to 5000 nm). The power of the new techniques is illustrated by their application to the definition of Hg two-photon resonances of C60 and C70 as well as to the characterization of optical nonlinearities in two linear chromophores of putative utility for sensor protection and electrooptic modulation. Explicitly, these measurements provide accurate determination of both transition energies and transition moments (matrix elements connecting the two photon levels). Results are compared to those previously reported in the literature illustrating the advantages and problems associated with particular measurement techniques. All of the molecules studied are found to exhibit two-photon absorption coefficients comparable to that of GaAs, the most studied putative sensor protection material (based on utilization of electronic optical nonlinearity). Femtosecond pulse techniques are shown, in all cases, to be necessary to avoid complications arising from excited-state absorption and relaxation phenomena. The importance of phase-sensitive detection in identifying complications from overlapping transitions is illustrated.

We use time-resolved degenerate four-wavemixing (DFWM) with femtosecond pulses in the wavelength range 0.74−1.7 μm to measure both phase and amplitude of all nonvanishing elements of the electronic third-order nonlinear optical susceptibility tensor c ijkl (−ω,ω,ω,−ω) of a 10 μm amorphous C60 film on a CaF2 substrate. Linear absorption is found to be less than 1% in this range. We find a single resonance in DFWM, the amplitudes and phases of which are fit well by a Lorentzian model of a two-photon resonance to a level 2.7 ± 0.1 eV above the ground level, with width 0.25 eV. The peak two-photon absorption coefficient is 0.02 cm/MW, essentially the same peak value as for bulk gallium arsenide, one of the strongest and most widely studied of the two-photon absorbers. Our results show there is only one two-photon allowed transition below 3.4 eV (as well as below the first one-photon transition), an unambiguous signature which is expected from theory. Theory assigns the symmetry Hg to this lowest lying two-photon state. We see a clearly nonresonant long-wavelength limit for the third-order optical susceptibility tensor which is 250 ± 70 times the known long-wavelength limit for fused quartz (our nonlinear standard). This result is at least an order-of-magnitude larger than any of several theoretical predictions.

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