Second-order nonlinear optical properties and relaxation characteristics of poled linear epoxy polymers with tolane chromophores

A new linear epoxy polymer containing 4-amino-4'-nitrotolane chromophores attached to the chain backbone was synthesized in an attempt to enhance both the poling-induced nonlinear optical susceptibility and its long-term stability. The polymer films poled by corona discharge showed very large nonlinear optical doubling coefficients of d₃₃ ≅ 89 pm/V and d₃₁≅ 25 pm/V for incident light of 1.06 μm wavelength, and a linear electro-optic coefficient r₁₃≅8 pm/V at 633 nm wavelength. Poling-induced alignment of the nonlinear optical moieties showed no detectable decay in two weeks at ambient temperature, as studied by the birefringence. Even at 100 °C the relaxation time estimated from the birefringence decay was ca. 450 h, consistent with roughly 16% decrease in d_ij coefficients in 20 h at this temperature and also the extrapolation from the dielectric α relaxation characteristics. The dielectric α relaxation temperatures of the poled polymer samples exhibited higher values than that of the unpoled counterpart. This unusual behavior may be due to a significant contribution of Maier-Saupe thermotropic interactions among the tolane moieties that favor parallel alignment.