Z-type Langmuir–Blodgett film structures: surface plasmon resonance, second harmonic generation and fibre optic devices
Second harmonic generation (SHG) from Langmuir–Blodgett (LB) films of the iodide (I) and octadecylsulfate (II) salts of E-N-octadecyl-4-[2-(4-dibutylaminophenyl)ethenyl]quinolinium and from films of a related pyridinium dye (III) increases with the number of layers. The LB film structures are non-centrosymmetric (Z-type) and have high second-order susceptibilities, χ(2)zzz, of 120 pm V–1 for dye I (20 layers), 80 pm V–1 for dye II (100 layers) and 30 pm V–1 for dye III (160 layers) at λω= 1.064 µm. The values are resonantly enhanced but for dye III the absorbance is very weak at the harmonic wavelength. The real and imaginary parts of the dielectric permittivity of dye I, derived from the surface plasmon resonance (SPR), are 3.14 ± 0.06 and 0.66 ± 0.03, respectively, at 532 nm and, with the exception of the monolayer, the thickness is 3.0 ± 0.3 nm layer–1. Waveguiding overlays of dye I, evanescently coupled to side-polished optical fibres, have resulted in useful in-line wavelength-selective elements. The LB technique allows the necessary control of film thickness for such devices.