Nonlinear optical spectra of C-70

We employ a continuously tunable 100 fs pulse source to measure the time-re solved degenerate four-wave-mixing (DFWM) spectrum of an amorphous C-70 fil m. The observed nonlinear optical response is essentially instantaneous and electronic in origin. We deduce the complex third-order nonlinear-optical susceptibility tensor function chi(1111)(-omega,omega,omega,-omega) through out the wavelength range 0.74-1.6 mu m. We find two-photon resonances with excited-state energies of 2.41 +/- 0.05 and 2.61 +/- 0.05 eV close to the 2 . 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 amp litude and phase data indicate that we have reached the long-wavelength lim it, chi(1111)(LWL), of chi(1111). We find it to be (4.5 +/- 0.5) x 10(2) ti mes that of fused silica and 1.9 +/- 0.6 times that of C-60. This result im plies chi(1111)(LWL) = (1.8 +/- 0.2) x 10(-12) esu, a value that is consist ent with the theoretical prediction of Shuai and Bredas. We use linear-abso rption and DFWM data to construct an analytic expression for chi(1111)[-(om ega(3) + omega(2) + omega(1)),omega(3),omega(2),omega(1)] which, we believe , predicts any nonlinear optical effect within the range (h) over bar omega (i) > 0.6 eV (i = 1, 2, or 3) and (h) over bar(omega(3) + omega(2) + omega( 1)) < 2.8 eV. We find that for omega near the lowest three-photon resonance , the magnitude and phase of chi(1111) behave like that of a two-level atom with a matrix element obtained from the observed one-photon oscillator str ength. This explains the observed change in sign of Re[chi(1111)(-3 omega, omega, omega, omega)] from positive to negative for decreasing omega near ( h) over bar omega = 0.8 eV. (C) 1999 Elsevier Science B.V. All rights reser ved.