Scaling laws of second-order hyperpolarizabilities in molecular wires

Summary form only given. Whereas principles for the optimization of first-order hyperpolarizabilities /spl beta/ are well established, rules for the second-order hyperpolarizability /spl gamma/ are less well documented, owing to the significantly more complex physics. We have shown by DFWM, THG measurements, and by quantum-chemical calculations that the chain-length dependence of the second-order hyperpolarizability /spl gamma/ of poly(triacetylene) molecular wires follows a power law for short oligomers. This power law saturates smoothly around 60 carbon-carbon bonds, which corresponds to an effective conjugation length of 7.5 nm. A comparison with various other systems with electrons delocalized over a one-dimensional path shows that the exponent a tends to be around 2.5 for various polymers in the transparency range. In the presence of two- or three-photon resonances, the measured exponent a increases. Moreover, the values presented here are of similar magnitude as for other organic material systems.