OBSERVATION OF THE LOWEST LYING ELECTRIC-DIPOLE-ALLOWED 2-PHOTON RESONANCE IN C-60

We use time-resolved degenerate four-wavemixing (DFWM) with femtosecon d pulses in the wavelength range 0.74-1.7 mu m to measure both phase a nd amplitude of all nonvanishing elements of the electronic third-orde r nonlinear optical susceptibility tensor c(ijkl)(-omega,omega,omega,- omega) of a 10 mu m amorphous C-60 film on a CaF2 substrate. Linear ab sorption is found to be less than 1% in this range. We find a single r esonance 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 absorp tion coefficient is 0.02 cm/MW, essentially the same peak value as for bulk gallium arsenide, one of the strongest and mast widely studied o f the two-photon absorbers. Our results show there is only one two-pho ton allowed transition below 3.4 eV (as well as below the first one-ph oton transition), an unambiguous signature which is expected from theo ry, Theory assigns the symmetry H-g to this lowest lying two-photon st ate, We see a clearly nonresonant long-wavelength limit for the third- order optical susceptibility tensor which is 250 +/- 70 times the know n long-wavelength limit for fused quartz (our nonlinear standard). Thi s result is at least an order-of-magnitude larger than any of several theoretical predictions.