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Interest in organic materials for applications in photonics is based mainly on their fast, electronic in origin, and large nonlinear optical response which is due to the large hyperpolarizability of delocalized ii electrons. On the other hand the organic materials are characterized by some interesting physico— chemical properties, such as: processability, versatility and good optical quality. They are also expected to integrate well with semiconductor technolo gy. Quasi one dimensional conjugated polymers like polydiacetylenes, polyacetylene and polythiophenes [1] are indeed characterized by a large nonlinear optical response and fast response time, but on the other hand it is difficult to obtain good optical quality, stable thin films of these materials. Polydiacetylenes are of repute for their excellent crystallinity, which leads to unwanted light scattering, and consequently to large propagation losses. Polyacetylenes are know to be unstable in air.On the other hand these po— lymers are characterized by a residual absorption in the near infrared due to overtones of C-H and 0-H stretching vibrations. Consequently it limits interest of their application in the interesting for practical purposes (telecommunication window) operation wavelength range. For these reasons the fullurenes in general, and C in particular are interesting materials for applica— tions in photonics because of lack of the low energy stretching vibrations. Moreover, the electronic bands in C are narrower [21 than in conjugated po lymers, where they are broadened due to conformations and conjugation length distribution. Consequently one expects the resonances to be narrower too. Large nonlinear optical response in C6 has been reported by third harmo nic generation in thin films [31 and in solution[4] as well as by degenerate four wave mixing [51 in solution, differing by two to four orders of magnitude from the last determinations (cf. Table 1). All these measurements have been performed almost at one wavelength only (mostly 1.064 jim). Also very recently a significant second order nonlinear optical response from C6 thin films has been reported by Wang et al [6] by optical second har monic generation. This result is in contrast with what one could expect for presumably centrosymmetric thin films. In fact, no bulk quadratic susceptibility in these films should be observed, at least at dipolar approximation. A second order activity of C thin films has been also observed by Hoshi et al [3]. There is much less data on a higher fullerene which is C . The nonlinear 70 optical properties of this molecule have been studied almost in solution and at discrete wavelength (cf. Table 1). In this paper we report the results of a systematic study of nonlinear optical properties of C60 thin films by optical third (THG) and second (SHG) harmonic generation techniques. The THG measurements have been done as a function of fundamental wavelength. The tecique allows a precise determination of the electronic cubic susceptibility x (-3w;w,w,w) and to learn more about the electronic structure of these materials. In fact, this technique gives not only the magnitude of the nonlinear susceptibility, but when done as a function of fundamental wavelength, allows also to locate the allowed and forbidden for one photon transition excited states. The SHG measurements have been performed at 1.064 im funcnental wavelength leading to the determination of quadratic susceptibility x (-2w;w,w).
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