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

Abstract

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 I2ω(N)=I2ω(1)′N2 for 2≤N≤100, and shows long-term stability. Films of the dye are almost transparent, with an absorbance of 3×10-4 layer-1 at the second-harmonic wavelength, but, nonetheless, the equivalent monolayer intensity, I2ω(N)/N2, is ∼30% of the resonantly enhanced signal from monolayer films of the colored hemicyanine dye reported by Girling [J. Opt. Soc. Am. B4, 950 (1987)]. The second-order susceptibility, film thickness, and chromophore tilt angle relative to the normal to the substrate are χzzz(2)=26 pm V-1 at 1.064 μm, l=5.0 nm layer-1, and ϕ=30°, respectively, for x=0.5.