chi.(3)Dispersion measurements of the conjugation length dependence of carotenoid derivatives

The dependence of the nonlinear susceptibility, χ (3) , on conjugation length has been investigated by third harmonic generation measurements of four astaxanthene molecules (carotenoid derivatives). These polyenes contain 5-17 double bonds and were synthesized and prepared as thin films by spin coating. Measurements, from 1 to 2.5 μm, revealed χ (3) to be linearly dependent on conjugation length, reaching a maximum of 1.2×10 -10 esu for the longest chain astaxanthene. In contrast to β-carotene, the three-photon resonance extends over a greater wavelength range to yield an enhanced figure of merit χ (3) /α≃10 -12 esu-cm. Extrapolation of these results to polyacetylene is analyzed