Second-harmonic generation from alternate-layer and Z-type Langmuir-Blodgett films: optimisation of the transparency/efficiency trade-off

Langmuir-Blodgett (LB) films of (E)-4-[(N-octadecyl-5,6,7,8-tetrahydro-5-isoquinolylidene)methyl]-N,N-dialkylaniline octadecyl sulfate, where the donor group is dimethylamino (1a), diethylamino (1b), dibutylamino (1c) and dihexylamino (1d) have optimum susceptibilities when the dyes are co-deposited in a 1 : 1 mole ratio with octadecanoic acid. Alternate-layer structures of the mixed films, interleaved with poly(tert-butyl methacrylate), are non-centrosymmetric and the second-harmonic intensity increases with thickness as I-(N)(2 omega) approximate to I-(I)(2 omega) N-2 to N > 100 bilayers. Furthermore, when the dialkylamino group is sufficiently hydrophobic, i.e. for 1c and 1d, the dyes form non-centrosymmetric Z-type structures without interleaving spacer layers. The optimum susceptibilities, chromophore tilt angles, thicknesses and refractive indices of the interleaved and non-interleaved mixed films are as follows: alternate-layer 1b, chi(zzz)((2)) = 67 pm V-1 at 1.064 mu m for phi = 37 degrees, l = 4.08 nm bilayer(-1), n(2 omega) = 1.58; Z-type (1c), chi(zzz)((2)) = 76 pm V-1 for phi = 33 degrees, l = 3.15 nm layer(-1), n(omega) = 1.52 and n(2 omega) = 1.58. The moderately high susceptibilities arise from an optimised transparency/efficiency trade-off, the films in this series being transparent at the fundamental wavelength (1.064 mu m) and having a very slight absorbance of 5 x 10(-4) per active layer at 532 nm.