Second harmonic generation from alternate-layer LB films: quadratic enhancement with film thickness

The second harmonic generation (SHG) from alternate-layer Langmuir-Blodgett (LB) films of stilbazolium dyes and 4,4$^{\prime}$-dioctadecyl-3,5,3$^{\prime}$,5$^{\prime}$-tetramethyldipyrryl-methene hydrobromide increases as I$_{2\omega (N)}$ = I$_{2\omega (1)}$N$^{2}$ for N $\geq $ 2 bilayers, but the equivalent monolayer signal (I$_{2\omega (1)}$) is significantly less than the intensity from LB monolayers of the corresponding dye on glass. This is attributed, in part, to the different molecular tilts and, in part, to the position of the anion which influences the charge distribution within the chromophore. For alternate-layer films of E-N-octadecyl-4-[2-(4-dimethylaminophenyl)ethenyl]pyridinium octadecylsulphate (dye I) the second-order susceptibility and chromophore tilt angle are $\chi _{zzz}^{(2)}$ = 45 pm V$^{-1}$ and $\phi $ = 30 degrees respectively, whereas the methoxy congener, E-N-octadecyl-4-[2-(4-methoxyphenyl)ethenyl]pyridinium octadecylsulphate (dye II), has $\chi _{zzz}^{(2)}$ = 11 pm V$^{-1}$ and $\phi $ = 35 degrees. The nonlinear optical properties of such films are dependent upon the compatibility of the dye and spacer.