FREQUENCY VARIATION OF CUBIC SUSCEPTIBILITY IN THE NEW CONJUGATED POLYMERS PTT AND PDTB

Third-order nonlinear optical properties of two thiophene-based conjugated polymers (polythieno(3,2-b)thiophene (PTT) and poly[1,4-di-(2-thienyl)benzene] (PDTB)) have been studied by third-harmonic generation in the fundamental wavelength range 1.064 to 1.907 μm. In both cases a three-photon resonance enhancement of the average cubic susceptibility «χ(3)(-3ω; ω, ω, ω)' in the 1.25 to 1.45 μm fundamental wavelength range is observed. Whereas in PDTB the three-photon resonance is narrow (Γ {all equal to} 780 cm-1), in PTT it is large and shows fine structure. The last observation is interpreted in termas of a resonance enhancement with collective CC vibrations. The observed differences in resonant values of «χ(3)(-3ω; ω, ω, ω)' are equal to 2±0.2 × 10-11 and 1.1 ± 0.1 × 10-11 e.s.u. in PTT and PDTB, respectively, and may be well accounted for by the difference in the conjugation length. © 1990.