Analysis of linear and quadratic optical response of mixed Langmuir-Blodgett films of stearic acid and 5-CT

Published experimental optical second-harmonic generation (SHG) results for mixed films formed between stearic acid and the mesogen 5-CT (4″-n-pentyl-4-cyano-p-terphenyl) are reanalyzed. Experimental refractive index results for the pure films are used to deduce molecular polarizabilities in order to calculate local fields. At low and high fractions of 5-CT, a consistent interpretation of the SHG is obtained using a mean molecular response based on a fixed axial hyperpolarizabilityβLLL for the mesogen, but at high mesogen fractions molecular tilt makes small off-diagonal componentsβLLM≈−βLLL/40 essential. At intermediate fractions the treatment can interpret the SHG only by requiring molecular tilt not found in the experiments, implying that a more detailed structural model is required.