Second-Harmonic Generation from Alternate-Layer Langmuir-Blodgett Films of Optically Non-Linear Materials in which the Interleaving Species is a Complementary Dye or Poly(t-butyl methacrylate)

Abstract

Non-centrosymmetric Langmuir–Blodgett (LB) films of optically non-linear dyes have been fabricated by alternating layers of complementary molecules, where the attachment of the hydrophobic alkyl chain, relative to the donor–(π -bridge)–acceptor chromophore, is reversed. The second-harmonic intensity increases quadratically with the number of LB bilayers, but the normalized intensity, I 2ω(N) /N 2 , is suppressed because the intramolecular and interlayer dipoles (D–π –A....D–π –A) are opposed. The second-order susceptibility, thickness and chromophore tilt angle, relative to the substrate normal, are X(2)zzz = 110 pm V–1 at 1.064 µm, l= 3.4 nm and φ = 31° for E-N-octadecyl-4-{2-[4-(N,N-dimethylaminophenyl)]ethenyl}pyridinium iodide (D–π–A-C 18 H 37 ) and X(2)zzz = 170 pm V –1 ,l = 2.5 nm and φ = 26° for E-N-methyl-4-[2-(4-N-methyl-N-octadecylaminophenyl))ethenyl]pyridinium iodide (C 18 H 37–D–π–A), the data corresponding to monolayer films deposited at 35 mN m –1 . The susceptibility of the alternate-layer structure is only modest (X(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, whereas films of the naphthyl analogue, E-N-octadecyl-4-{2-[4-(N,N-dimethylaminonaphthyl)]ethenyl}pyridinium octadecylsulfate, interleaved with poly(t-butyl methacrylate), have a very slight absorbance of c.4.7 × 10 –4bilayer–1 at 532 nm and are transparent at 1.064 µm. The second-order susceptibility is 36 pm V –1 .