Second-harmonic generation from Z-type Langmuir-Blodgett films of a transparent dye and a comparison of the properties when the layers are interleaved with poly(t-butyl methacrylate)

The Langmuir-Blodgett (LB) film structure of E-4-[(N-octadecyl-6,7,8-trihydro-5-isoquinolylidene)methyl]-N,N-dibutylaniline octadecylsulfate is non-centrosymmetric when the dye is deposited on the up-stroke (Z-type) and when the layers are interleaved with poly(t-butyl methacrylate). The second-harmonic intensity increases as I-(N)(2 omega) = I-(I)N-2 omega(2), where N is the number of active layers, and the intensity is further enhanced when the dye is mixed in a 1:1 ratio with octadecanoic acid. The second-order susceptibility and repeat lattice spacing of the mixed LB films are as follows: chi(zzz)((2)) = 76 pm V-1 at 1.064 mu m and l = 3.15 nm layer(-1) when Z-type; chi(zzz)((2)) = 52 pm V-1 and l = 4.13 nm bilayer(-1) when interleaved. The films are transparent at the fundamental wavelength and have a slight absorbance of ca. 5 x 10(-4) per dye layer at 532 nm. The second-harmonic intensity is the strongest to date from such a weakly absorbing LB film and this is attributed to the close proximity of the charge-transfer band (lambda(max) = 450 nm) and to an optimised packing arrangement.