Tailoring linear and nonlinear optical properties of a side-chain liquid crystalline azo-polymethacrylate

Abstract

The molecular orientation in a side-chain liquid crystalline azopolymer with push–pull chromophores (Pol-PZ-CN) is studied by UV-vis spectroscopy, refractive index and second harmonic generation measurements. Fresh films show homeotropic arrangement, but irradiation with polarized light induces “in-plane” birefringence, which is enhanced by subsequent thermal annealing up to values as high as 0.34. The nonlinear response of Corona poled films with “in-plane” anisotropy is governed by three independent NLO dij coefficients, evidencing symmetry different from the usual (C∞v). For irradiated films, stable values d31 = 1.4, d32 = 0.9 and d33 = 11 pm·V−1 are measured at 1.9 µm. A noticeable higher d31/d32 ratio is obtained for preirradiated and annealed films. The proposed two-step method consisting of light irradiation and heating is an effective strategy to tailor the polar molecular orientation of nonlinear liquid crystalline azopolymers.