Second-harmonic generation from alternate-layer Langmuir-Blodgett films ofoptically non-linear materials in which the interleaving species is a complementary dye or poly(t-butyl methacrylate)

Non-centrosymmetric Langmuir-Blodgett (LB) films of optically non-linear dy es have been fabricated by alternating layers of complementary molecules, w here the attachment of the hydrophobic alkyl chain, relative to the donor-( pi -bridge)-acceptor chromophore, is reversed. The second-harmonic intensit y increases quadratically with the number of LB bilayers, but the normalize d intensity, I-2 omega ((N))/N-2, is suppressed because the intramolecular and interlayer dipoles (D-pi -A....D-pi -A) are opposed. The second-order s usceptibility, thickness and chromophore tilt angle, relative to the substr ate normal, are chi ((2))(zzz) = 110 pm V-1 at 1.064 mum, l = 3.4 nm and ph i = 31 degrees for E-N-octadecyl-4-{2-[4-(N, N-dimethylaminophenyl)]ethenyl }pyridinium iodide (D-pi -A- C18H37) and chi ((2))(zzz) = 170 pm V-1, l = 2 .5 nm and phi = 26 degrees for E-N-methyl-4-[2-(4-(N-methyl-N-octadecylamin ophenyl))ethenyl]pyridinium iodide (C18H37-D-pi -A), the data corresponding to monolayer films deposited at 35 mN m(-1). The susceptibility of the alt ernate-layer structure is only modest (chi ((2))(zzz) = 20 pm V-1) but may be increased to c. 70 pm V-1 by interleaving the active layers with passive spacers. The harmonic wavelength overlaps the charge-transfer band, wherea s films of the naphthyl analogue, E-N-octadecyl-4-{2-[4-(N,N-dimethylaminon aphthyl)]ethenyl}pyridinium octadecylsulfate, interleaved with poly(t-butyl methacrylate), have a very slight absorbance of c. 4.7 x 10(-4) bilayer(-1 ) at 532 nm and are transparent at 1.064 mum. The second-order susceptibili ty is 36 pm V-1.