Laser inscription of holographic diffraction gratings in thin films of azo polymers.

摘要

A series of amorphous azobenzene-containing polymers were cast as thin films and shown to be suitable for laser inscription of reversible volume birefringence diffraction gratings, high-efficiency surface relief diffraction gratings, and photorefractive holographic gratings.; The inscription of surface relief gratings is a novel process which involves substantial mass transport of the polymer chains, as atomic force microscopy reveals induced surface modulations near that of the original film thickness. A mechanism is proposed for this phenomenon which involves pressure gradients as a driving force, resulting from light-induced isomerizations of the azobenzene chromophores varying periodically throughout the light interference pattern. As the geometrical isomerization requires a free volume in excess of that initially available in the cast films, the photochemical conversion in these areas produces a laser-induced internal pressure above the yield point of the material. It is proposed that the resulting viscoelastic flow from these high pressure areas to lower pressure areas leads to the formation of the regularly-spaced sinusoidal surface relief gratings observed by a number of research groups, but previously unexplained.; Based on Newtonian fluid dynamic relations, a model is constructed to describe this laser-induced mass transport in thin films of polymers containing isomerizable azobenzene chromophores. The Navier-Stokes equations for laminar flow of a viscous fluid are used to relate velocity components in the film to pressure gradients resulting from laser-induced isomerization of the bulky chromophores, and this equation is solved directly by application of boundary layer conditions. Predictions from the model were compared with the results of experiments to determine the influence of light and polymer properties, such as the intensity and polarization of the incident laser irradiation, the free volume requirement of the chromophores, the molecular weight of the polymer, and the initial film thickness. Results are shown to be in good agreement with theory.; The process of recording reversible high-efficiency holographic gratings by the nonlinear optical photorefractive effect was demonstrated in a series of azobenzene polymers incorporating a photoconductive carbazole group, and displaying a range of chromophore mobilities. These materials are novel, since they incorporate all of the necessary components for the multi-step photorefractive effect in a single multi-functional polymeric unit. Electric field poling, orientational dynamics, and photoconductivity were measured across the polymer series, in order to determine the influence of chromophore mobility on these component processes, and a two-beam optical gain of 0.015 {dollar}rm mu msp{lcub}-1{rcub}{dollar} was demonstrated to confirm asymmetric beam coupling and hence the inscription of a photorefractive grating as a net result. This gain was exceeded by absorption losses of the polymer however, so that no net gain could be produced.