Hyperpolarisability third order

The conjugated polymeric structure of the diacetylenes produces a planar structure which exhibits a relatively high electronic conductivity and a high third order hyperpolarisability, X3, and thus they have considerable potential in technical applications. These phenomena, however, fall outside the scope of this book. Furthermore, the polycrystalline nature of films discussed above has so far prevented realisation of these potentialities. 

The fundamental component (aE) is linear in E and represents the linear optical properties discussed above. The second (jfiE-E) third ( yE-E-E) and subsequent harmonic terms are nonlinear in E and give rise to NTO effects. The / and values are referred to, respectively, as the first and second hyperpolarisabilities. The second harmonic term gives rise to second harmonic generation (SHG), the third results in frequency tripling effects, and so on. Importantly, since only the time-averaged asymmetrically induced polarisation leads to second-order NLO effects, the molecule and crystal must be non-centrosymmetric, otherwise the effects will cancel one another. Third-order effects, however, may be observed in both centrosymmetric and non-centrosymmetric materials. 

This process involves the hyperpolarisability, or third-order susceptibility in terms of which the polarisation produced is expressed as... 

For the measurements of the NLO properties of organoftillerenes 1,2 and 3, the Z-scan technique has been employed [151]. Z-scan is arelatively simple experimental technique allowing for the simultaneous determination of the real and imaginary parts of the third-order susceptibility Then, the second-order hyperpolarisability... 

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