Douglas R. Robello

Second harmonic generation and thermally stimulated current measurements: A study of some novel polymers for nonlinear optics

The relaxation of electric field‐induced polar orientation in novel side and main chain polymers was measured by simultaneous second harmonic generation and thermally stimulated discharge current techniques. Using this combination of experiments, individual microscopic relaxation mechanisms could be discriminated and identified. The relaxation of a polymer bearing polar chromophores on a side chain polymer was found to occur through local reorientation. For polymers containing the polar chromophore within the main chain, both a local rotation of the chromophores and the reorientation of the end‐to‐end vectors of the chains could be detected. Physical aging near the Tg of the polymers greatly increased the stability of the poling‐induced orientation below Tg. Nonlinear dependencies on the poling field were observed for low fields and are discussed in terms of screening due to space charges. Kirkwood–Frohlich correlation factors were found to be very close to unity.

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.

Quantum amplified isomerization in polymers based on triplet chain reactions.

Photoinitiated triplet quantum amplified isomerizations (QAI) of substituted Dewar benzene derivatives in polymeric media are reported. The quantum efficiencies and the ultimate extents of reactant-to-product conversions increase significantly with the incorporation of appropriate co-sensitizers; compounds whose triplet energies are similar to or lower than that of the sensitizer and close to that of the reactant. These co-sensitizers serve to promote chain-propagating energy transfer processes and thereby increase the action sphere of photosensitization. Isomerization quantum yields increase, as predicted, with increasing concentrations of the reactants and the co-sensitizers. Chain amplifications as large as approximately 16 and extents of conversion that approach 100% have been achieved. Mechanistic schemes are proposed to account for the dynamics of the inherent energy transfer processes and provide a predictively useful model for the design of a new class of photoresponsive polymers based on changes in the refractive index of the materials.

Revisions to the microstructural assignments of poly(ethyl cyanoacrylate)

The tacticity assignments from the literature for the 13C-NMR signals of the side-chain methylene group of poly(ethyl cyanoacrylate) are reversed, and new assignments of the main-chain methylene group are proposed. The assignments were made possible by a combination of DEPT and HETCOR NMR experiments on samples of varying microstructure distributions. The polymer tends toward syndiotacticity, not isotacticity as previously reported. The distribution of stereoisomers fits well to a Bernoulian statistical model. The stereochemical assignments were corroborated by similar analyses of a model compound for two cyanoacrylate repeat units.

Refractive index imaging via a chemically amplified process in a solid polymeric medium

A new refractive index imaging media system is described, based on the photoinitiated isomerization of dewarbenzene derivatives in a solid polymer medium. The system can exhibit a refractive index contrast as high as 0.02, with low dimensional changes on recording. Furthermore, the medium is highly sensitive because the recording process is chemically amplified, that is, many product molecules are formed per photon absorbed. The change in refractive index occurs spontaneously under blue or UV irradiation; no subsequent processing is required. This system may find applications in holographic recording and for integrated optical devices. For example, many spatially overlapping holographic diffraction gratings were recorded (angular multiplexing) in the material in the same volume.

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.

Linear and nonlinear optical properties of polymeric Langmuir-Blodgett films

We have prepared Langmuir-Blodgett monolayer and multilayer films of a polymeric amphiphile with a side chain containing a chromophore of high molecular hyperpolarizability for second order nonlinear optics. Using optical techniques including polarized transmission and attenuated total reflectance spectroscopies, second harmonic generation, and ellipsometry, we have characterized the orientational order of the chromophore and other polymer groups in the films. The high degree of chromophoric order seen in these films indicates that such polymers are good candidates as materials to fabricate Langmuir-Blodgett films for nonlinear optical applications.