Linear and nonlinear optical response of metal colloid heterostructures by molecular self-assembly on optical chemical benches

An optical chemical bench is a nanostructured integrated optics structure - usually a waveguide circuit - where chemistry and spectroscopy can be combined for the purpose of studying chemical reactivity in thin films and at interfaces. In this paper, we describe how glass waveguide surfaces can be decorated with Rayleigh-limit (sub-20 nm dia.) silver and gold colloid particles by covalent bonding to an organothiolate adhesion layer attached to the glass. The step-by-step assembly of the heterostructure is monitored by X-ray photoelectron spectroscopy. XPS, transmission electron microscopy and linear extinction confirm selective attachment of gold to silver. The extinction spectrum of the dipolar surface plasmon mode of mutually bound Ag and Au colloid species is perturbed by mutual polarization of the particles. There is little perturbation of the extinction spectrum when the particles are deposited in the same plane, but not chemically linked to one another. Fractal aggregates of particles are of unique interest for enhancing nonlinear optical field responses. Over long time scales, we show that silver particles can be grown in a two-dimensional polycrystal having hexagonal symmetry. At shorter times, the surface population of particles scales as a fractal. The fractal state probably represents the transitional regime appropriate to diffusion limited aggregation in the early stages of colloid deposition. The silver particles occupy a fractal dimension of approximately 1.4, whereas the gold particles attached to silver occupy a dimension of 1.75. The overall structure is the first step in the fabrication of artificial three-dimensional crystals of colloidal metals joined by molecular scale "chemical bonds". MSA is also used to prepare assemblies of self-poled stilbazolium chromophores by acid-base reaction of the pyridyl unit with thioacetic acid grafted to the OCB metal particle surface. The linear extinction spectrum appears to reflect a strong coupling of the optical response of the chromophore with the surface confined dipolar plasma excitation of the conduction electrons of the silver particles. This suggests that surface plasmons might be used to amplify the optical field in the vicinity of dye molecules for enhanced harmonic generation. Second harmonic (532 nm) light was detected from the heterostructure. The second order susceptibility for the heterostructure was measured to be 1.48 x 10(-22) C-3/J(2) (4.0 x 10(-8) esu). This value is a factor of 7 larger than the optimized chi((2)) of an analogous corona poled sample of neutral stilbazole molecules hosted in higher number density in a poly(styrene) film. The nonlinear optical response of the chromophore-metal nanoparticle heterostructure can originate from several sources, including surface plasmon-mediated optical field effects.