Noncentrosymmetric Ionically Self-Assembled Thin Films for Second Order Nonlinear Optics

Ionic self-assembled monolayers (ISAMs) are a recently developed [1], revolutionary class of materials that allows detailed structural control at the molecular level combined with ease of manufacturing and low cost. The ISAM method involves the alternate dipping of a charged substrate into an aqueous solution of a cation followed by dipping in an aqueous solution of an anion at room temperature. Since the adsorption is based on the electrostatic attraction of interlayer charges, each layer is self-limiting in thickness and uniform at the molecular level. We have recently demonstrated through second harmonic generation (SHG) measurements with fundamental wavelengths of 1064 and 1200 nm that the ISAM technique can produce a noncentrosymmetric arrangement of NLO chromophores to yield thin films with χ(2) values comparable to that of quartz. The development of ISAM χ(2) thin films provides significant advantages over the production of organic χ(2) thin films by alternative methods. For example, ISAM films can exhibit long-term stability of χ(2) in contrast to electric field poling of glassy polymers[2], can provide thicker films (upwards of 10 μm) than the Langmuir-Blodgett technique, and can be fabricated much more rapidly than covalent self-assembly[3] methods.