Nonlinear optical properties of neutral and anionic fullerenes

We have developed coherent anti-Stokes Raman spectroscopy (CARS) lineshape analysis as an accurate and definitive technique for third-order optical nonlinearity measurements of materials. This technique is an interferometric four wave mixing method where the lineshape of a Raman resonance reflects the interference between a sample's nonlinearity and the Raman resonance of a standard. By subjecting the sample and standard to the same experimental conditions, problems associated with separate absolute intensity measurements of the two are avoided. We show that this technique can determine third-order nonlinearities within 10--20%, more accurately than other widely used absolute intensity measurement-based techniques.; Using CARS lineshape analysis, we have determined the third order nonlinearities for the first, second, and third anions of [60]fullerene. Hyperpolarizabilities of 2.4 (+/-1.0) x 10-33 esu, 4.0 (+/-1.0) x 10-33 esu, and 7.6 (+/-0.5) x 10 -33 esu are observed for the first, second, and third anions, respectively, at 452 nm. These values are 65--200 times greater than the corresponding value for neutral [60]fullerene and comparable to values for highly conjugated organic polymers like polydiacetylene. The observed increase in the nonlinearity with the addition of charge to the fullerene cage is explained by symmetry considerations. Despite the increases, the hyperpolarizabilities are not sufficient for use in practical optical devices. However, larger nonlinearities can be obtained through the use of rare-earth endohedrals and wavelengths that have smaller detunings with electronic state resonances in the near-infrared.; We have also measured an upper limit for the hyperpolarizability of neutral [60]fullerene, 3.0 (+/-1.0) x 10-37 esu at 757 nm. This value is three times lower than the previously measured limit. It is consistent with calculations, which predict values that are much lower than other experimental measurements.