Nonlinear Optical Probes of Glassy Polymers

Abstract

Theory and experiments show that the second order nonlinear optical susceptibility is a sensitive probe of polar orientational order. By incorporating optically nonlinear molecules in glassy polymers and orienting the dipole moments using an external field, it is possible to deduce the molecular orientational distribution from measurements of the nonlinear optical susceptibility. Presented here are results of such measurements of a poled dye-doped polymer film under external uniaxial stress measured using polarized second harmonic generation. The ratio of the tensor components of the second order susceptibility, χ ⁽²⁾₁₁₃ /χ ⁽²⁾₃₃₃ (≡a), is shown to vary with poling field and stress. Tensor ratios of a =0.33 and a=0.7 are obtained for stresses of zero and 3.71×10⁷ dyne/cm ², respectively, and fall into a regime of order unlike other material classes such as poled liquid crystals and Langmuir-Blodgett films.