Comparison of the χ(3) values of crystalline and amorphous thin films of 4-butoxy-carbonyl-methyl-urethane polydiacetylene at 1.06 and 1.3 μm

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Abstract
The third‐order optical nonlinear susceptibilities of single‐crystalline and spin‐coated amorphous 4‐butoxy‐carbonyl‐methyl‐urethane polydiacetylene (red form) thin films were measured by third‐harmonic generation. The third‐harmonic intensities were calculated as a function of incident angle by solving the propagation equation of the electromagnetic field for a two‐layer system with one nonlinear media. From the best fit of the calculated and experimental data, the ( χ(3)) values of the spin‐coated amorphous film (χ(3)p) were found to be (9.6±1.0)×10−12 esu and (2.0±0.2)×10−11 esu at 1.064 and 1.319 μm, respectively, while the χ(3) values of the crystalline ( χ(3)pc) were (4.9±0.5)×10−11 esu and (1.0±0.07)×10−10 esu at 1.064 and 1.319 μm, respectively. A factor of five increase in the χ(3) of the crystalline film is attributed to the orientation effect of the polymer chains. The enhancement of the nonlinearity at 1.319 μm may be due to either three‐photon or two‐photon resonant effects.