Nancy J. Armstrong

Blue and green Čerenkov-type second-harmonic generation in a polymeric Langmuir–Blodgett waveguide

Blue (430-nm) and green (532-nm) wavelengths of Cerenkov radiation were generated in an all-polymeric Langmuir–Blodgett (LB) waveguide. A nonzero second-order nonlinear-optical (NLO) susceptibility was obtained by alternate deposition of an NLO active and an NLO passive polymer. The Cerenkov condition was fulfilled through control of the thickness of the LB film and the modal dispersion relations for waveguides. The second-harmonic light, generated in the waveguide, leaks into the substrate at the Cerenkov angle. The Cerenkov-type guided-mode-to-radiation-mode phase-matching scheme reduces the waveguide attenuation for the second harmonic and provides a convenient way of separating the second harmonic from the fundamental.

Langmuir-Blodgett films for second-order nonlinear optics

Langmuir-Blodgett films have been fabricated of a prepolymerized amphiphile containing a chromophore of high second-order nonlinear optical activity. Thick films, containing as many as 262 layers of noncentrosymmetric structure have been prepared by alternate deposition of the dye polymer and several different materials. In contrast to previous literature reports on Langmuir-Blodgett films of other polymeric dyes, these films show the theoretically expected quadratic dependence of second harmonic generation with film thickness. Polarization studies of the second harmonic generation demonstrate that the chromophores are highly ordered in these films. In addition, by using all-polymeric materials it has been possible to fabricate a waveguide of these films with optical attenuation of about 1 dB cm-1, substantially lower than has been previously reported for Langmuir-Blodgett films even without a noncentrosymmetric structure.

Silicon tetraacetate as a sol-gel precursor

Variations in the starting materials, solvent, catalyst, and water to alkoxide ratio affect gel times and gel properties in sol-gel reactions. Silicon tetraacetate-derived sols gel surprisingly quickly. This paper discusses the acid catalyzed hydrolysis and condensation of silicon tetraacetate in an attempt to understand this rapid gelation. 29Si and 1H NMR spectra show that tetraethoxysilane is an intermediate, and ethyl acetate is produced via a secondary reaction. A correlation between gel times and H+ concentration is demonstrated.

Phase-matched second-harmonic generation in waveguides of polymeric Langmuir-Blodgett films

Langmuir-Blodgett (LB) films have been fabricated from a preformed acrylate amphiphilic copolymer with nonlinear optical (NLO) active chromophores covalently attached to the polymer backbone. By alternate deposition of this NLO active polymer with an NLO passive amphiphilic polymer, high quality multilayer films have been obtained with sufficient thickness to allow waveguiding in the LB films. The observed optical attenuation is between 1 and 3 dB cm-1. The linear and nonlinear optical characteristics of these films have been determined, both in transmission (or reflection) and in waveguide format. The high degree of order in LB films makes this type of supramolecular structure attractive for NLO applications. Based on the observed parameters (dispersion of the refractive indices, birefringence, tilt angle of the chromophore, hyperpolarizability), different phase-matching schemes can result in efficient second harmonic generation.