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

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.