DETERMINATION OF THE 2 1ST NONTRIVIAL ORIENTATIONAL ORDER PARAMETERS IN LB FILMS OF ROD-LIKE MOLECULES BY 3RD-ORDER SUM FREQUENCY MIXING

We demonstrate a new method for investigating orientational order in Langmuir-Blodgett films built from rod-like poly[bis(m-butoxyphenyl)silane] molecules. Two laser beams with different input frequencies omega1 and omega2 were collinearly overlapped to generate mixed light at the frequency 2omega1 + omega2 using all possible permutations of the beam polarizations. From the independent tensor components of chi(3)(-omega3; omega1, omega1, omega2) the first non-trivial in-plane order parameters C2 and C4 could be deduced with high accuracy. The orientational order could be well described by a two-dimensional diffusion model.