Hybrid covalent/ionic self-assembly of second order nonlinear optical films

Abstract

Ionically self-assembled monolayer (ISAM) films spontaneously assemble with a noncentrosymmetric ordering that gives rise to a substantial second order nonlinear optical (NLO) response with exceptional temporal and thermal stability. Typically, polar ISAM films are made from two oppositely-charged polyelectrolytes with an ionic, conjugated NLO chromophore attached as a side-chain to one of the polymers. The second order susceptibility of such a system is diminished due to competing dipole alignment at opposing ends of each polyelectrolyte layer and by randomized chromophore orientation within thicker layers. Significant enhancements in NLO response have been achieved by replacing the NLO-active polyelectrolyte with a monomeric chromophore that has reactive functionality and ionic moieties on opposite ends of the molecule, as illustrated in Figure 1 for the case of Procion Red MX-5B [1]. The growth of multilayers through alternating mechanisms of covalent coupling and electrostatic adsorption results in highly polar chromophore ordering with χ(2) values as large as 30× 10−9 esu, fifteen times that of quartz. Variation of the solution pH allows one to turn the reactive coupling on and off, verifying the important role of the alternating adsorption mechanisms in producing a film with bulk polar order. Quadratic growth of the second harmonic generation (SHG) intensity with the number of layers demonstrates that the bulk polar order exists through large (>50) numbers of bilayers. Since the solution cells, aqueous solutions, and glass susbtrates are all amorphous and exhibit negligible SHG, in situ SHG measurements allow real-time measurement of the growth of a single, polar monolayer onto the substrate from the solution. These measurements demonstrate that the covalent formation of the chromophore monolayer is complete in less than two minutes, allowing for relatively rapid buildup of thick multilayer films. In addition, we will discuss the effects of the glass-film and film-air interfaces on the observed SHG signals. Each of these produces a large SHG signal that causes a plot of the square root of the SHG versus the number of bilayers to intercept the ordinate axis well-above the origin but does not affect the slope of this plot, which determines χ(2). However, this does demonstrate that care must be taken in determining the χ(2) values of selfassembled films comprising small numbers of layers.