Abstract: Anovel cross-linked polyurethane with high second-order nonlinear optical(NLO) activity and high thermal stability was synthesized from disperse Red-19 and the trimer of 2,4-toluene diisocyanate. The electro-optical coefficient(r₃₃) of the polyurethane and the electro-optical modulation efficiency of the spatial modulator were measured at lasers of 632, 650, 832 and 980 nm by the improved attenuated-total-reflection technology after the polyurethane was poled and cured. The results indicate that r₃₃and the modulation efficiency increase in the absorption region of the polyurethane with the descending of the incident laser wavelength. Further theoretical and experimental researches show that the enhancement of the electro-optical effect is resulted from the complex refractive index change. The change increased with the ab-sorbance of the polyurethane due to the resonance of the NLO polyurethane and the incident laser. As a result, for spatial electro-optical(EO) modulator like attenuated-total-reflection EOmodulator, the absorption region can be effectively utilized notonly because the effective optical path length is much shorter that of a waveguide EO modulator and optical losses do not present a serious problem but also because the electro-optical effect will be strengthened by the resonance.

Key words: Second-order nonlinear optics, Cross-linked polyurethane, Electro-optical effect, Resonance enhancement