(564b) Deposition of Reactive Procion Dyes in Layer-by-Layer Films for Second Order Nonlinear Optical Materials

Organic films of oriented chromophores can be used to
fabricate electro-optic modulators for telecommunications applications. Low
molecular weight chromophores can be integrated into films by a layer-by-layer
deposition process using electrostatic interactions and covalent bonding. The
deposition process involves aqueous solutions of ionic chromophores and a
polycation at ambient conditions with individual immersion times as low as 2
minutes/monolayer. The chromophore used was Procion Brown (PB) while poly(allylamine
hydrochloride) provided both the amine groups for reaction with the triazine
ring of PB and for electrostatic interaction with the sulfonic acid groups of
PB. This results in non-centrosymmetric films that exhibit significant second
harmonic generation (SHG) intensities where the square root of the SHG
intensity scaled linearly with bilayer number. Values of the second order
nonlinear susceptibility c⁽²⁾
from the SHG data as large as 31x10^-9
esu and the electro-optic coefficient r₃₃ = 14 pm/Volt were found.
By comparison, films of LiNbO₃, an inorganic material used in EO
modulators, exhibit values of c⁽²⁾
» 200x10^-9 esu and r₃₃ = 30 pm/Volt. The pH and ionic
strength were critical for achieving a high degree of ordering of PB. The
effects of solution concentration, ionic strength, and convection on the
deposition rates of the PB dye and the polycation were explored using a quartz
crystal microbalance to measure the intrinsic reaction rate of the PB dye with
the polycation.