Non-Linear Effects in Thin Film Fullerene Structures
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75 Mat. Res. Soc. Symp. Proc. Vol. 479 01997 Materials Research Society
The non-linear properties of thin film fullerene material have been examined by Kafafi [10]. Most of the results described related to 1lirm thick C60 fim and to degenerate four wave mixing measurements at 1.06imr using a 30 psec source. The temporal response of the conjugate signal followed the incident laser pulse, but with a weak tail extending to times >500 psec implying some energy transfer to the triplet state. Values of X(3 )/a were given as 12 x 10-13 esu cm, almost an order of magnitude greater than those found in phthalocyanine-related materials (H2Pc). Optical limiting measurements of C70 films showed a reduction in transmittance by about 30% at energy densities of 9GW/cm 2 . Values of non-linear absorption coefficient for C70 (100cm/GW) were close to 2.5 times greater than those of C60. ht294 i4 ___
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Figure 1 Molecular orbitaldiagram illustratingoptical excitation process in C60 and C60-. Others have examined the incorporation of the materials into solution and in glass and polymer hosts. McBranch [11] compared the optical limiting and excited state properties of fullerene derivatives (C60, C70, C76, and C84) and glasses doped with these materials. The fullerenes were incorporated into the glasses as they were being produced. Specific cases studied included 5,6 phenyl C61 butyric acid cholesteric ester and C60 - Ferrocene. Optical limiting properties were measured in the visible and near IIR and excited state absorption spectra measured from 500 to 1000nm with psec resolution. It was found that functionalised variants of the fullerenes optimised for solubility retained their broadband optical limiting properties. The excited state spectra Of C78 showed little detail, with most of the absorption process being broadband in nature. In a sol-gel matrix, the damage threshold of the host was found to be >400J/cm2. This was attributed to the role of the fullerene limiting fluence levels at the beam focus. Nevertheless, saturation levels were about 20J/cm2, which for well-focussed 10lir spots, implied limiting at the 101gJ level. No nanosecond limiting was observed in thin film materials, possibly as a result of exciton-exciton annihilation leading to zero population of the triplet state. The role of energy exchange processes has been highlighted by several workers. Brandelik et al. [12] found that charge transfer complexes form in solutions with electron donors e.g., amines, and the non-linear absorption behaviour correlated inversely with the concentration of the complex. Indeed at 694nm, diethyl aniline was very effective [131 in reducing the ratio of
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excited state to ground state absorption with a resulting quenching of any optical limiting effect. It was clear that the effects on the upper energy levels were different to those on the lower states. In exploring the way forward towards producing all-solid state devices exploiting C60, different groups have examined the use of polymer and
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