Measurement of the third-order nonlinear susceptibility in a representative soluble polymer with acetylenic linkages

The third-order Raman and nonresonant electronic susceptibilities of a soluble conjugated rigid-rod polymer, poly(2,5-di-n-dodecyloxy-p-phenyleneethynylene), have been measured by the interferometric four wave mixing method of Levenson and Bloembergen. The magnitude of the second-order electronic hyperpolarizability is large and the real part is negative. The negative hyperpolarizability is attributed to the dominance of coherence pathways involving the ground state. The size of the Raman hyperpolarizability is consistent with suggestions that the Raman cross-section can represent the size of the hyperpolarizability.