Real-time determination of molecular orientation in Langmuir-Blodgett monolayers on integrated-optical structures

Waveguide linear-dichroism measurements were used in determining the molecular orientation of N-(sulfopropyl)-4-(p-dioctylaminostyryl) pyridinium in Langmuir-Blodgett monolayers upon 150-microm-thick glass (Ti:Zn) substrates in real time. Acquisition of complete spatial-decay curves in t - 1 s with P(laser) = 100 microW was made possible by combining propagation in integrated-optical structures with direct imaging of monolayer or glass fluorescence onto the active area of a charge-coupled device camera. Multichannel detection permitted measurement of optical waveguide loss coefficients and ultimately of molecular orientations in approximately 10(-3) of the amount of time required by previous methodology. The effects of sample preparation and materials processing on the accuracy and precision of dichroic ratios are discussed. Observed dichroic ratios for a hemicyanine dye revealed no measurable change in the orientation of the electronic transition-dipole moment when Langmuir-Blodgett films were deposited at different surface pressures. These results are in good agreement with the results from singly resonant second-harmonic generation experiments.