AGGREGATION IN HEMICYANINE DYE LANGMUIR-BLODGETT-FILMS - ULTRAVIOLET VISIBLE ABSORPTION AND 2ND HARMONIC-GENERATION STUDIES

Absorption spectroscopy and second harmonic generation have been used to examine the effect of systematic dilution on the aggregation of a hemicyanine dye in Langmuir-Blodgett films. As reported previously, the second-order nonlinear activity initially increases with decreasing dye concentration. Our present results show that the second-order nonlinear susceptibility correlates with the residual amount of the nonaggregated dye as a function of total dye concentration. This result, coupled with the observation that the frequency dispersion of the second harmonic generation shows only one nonlinearly active species, leads us to conclude that the dye aggregates evident in the absorption at high concentration do not exhibit a significant second-order nonlinearity.