Saturation effects in the nonlinear optical response of regioregular, highly conjugated poly(3-alkyl-thiophene) thin films

Third harmonic generation measurements on highly conjugated regioregular poly(3-dodecylthiophene) thin films using a fundamental wavelength of 1907 nm give an absolute value of χ(3)(-3ω;ω,ω,ω) = 4×10−11 esu. This value is the nonlinear susceptibility of the most highly conjugated polythiophene film reported to date (λmax= 555 nm). Comparison to less conjugated samples reveals a saturation in the dependence of χ(3) the effective conjugation length.