Improved second-harmonic generation from homomolecular Langmuir-Blodgett films of a transparent dye

The optically nonlinear dye [E-N-dodecyl-4-(2-(4-docosyloxyphenyl)ethenyl)quinolinium](+) (octadecylsulfate(-))(x)(bromide(-))(1-x) forms noncentrosymmetric Langmuir-Blodgett films. The second-harmonic intensity increases quadratically with the number of layers in the range investigated, i.e., as I-2 omega(N) = I-2 omega'N-2 for 2 less than or equal to N less than or equal to 100, and shows long-term stability. Films of the dye are almost transparent, with an absorbance of 3 x 10(-4) layer(-1) at the second-harmonic wavelength, but, nonetheless, the equivalent monolayer intensity, I-2 omega(N)IN2, is similar to 30% of the resonantly enhanced signal from monolayer films of the colored hemicyanine dye reported by Girling ct al. [J. Opt. Sec. Am. B 4, 950 (1987)]. The second-order susceptibility, film thickness, and chromophore tilt angle relative to the normal to the substrate are chi(222)((2)) = 26 pm V-1 at 1.064 mu m, l = 5.0 nm layer(-1), and phi = 30 degrees, respectively, for x = 0.5. (C) 1998 Optical Society of America.