Optical properties of composite polymer materials

Polymer composites are materials in which nano- and/or micro-sized particles of inorganic materials (e.g. semiconductors, metal, air) are dispersed in a polymer host. Such materials are of interest to combine the optical properties of the inorganic materials with the easy processing of polymers, hence extending the functionality and properties of polymer-based optical devices. The main challenge with polymer composites is their optical loss, resulting from scattering caused by the inclusions and often also absorption due to the host and/or particles. Polymer composite materials can be either weakly or highly scattering media, and can be useful from the optical to the terahertz regimes, depending upon the size and the refractive index of inclusions and the host polymer. Although scattering is usually considered an unwanted property, it is possible to overcome and/or make use of the scattering in polymer composite materials designed for specific applications. For example, it is shown through numerical simulation and experiments that weakly scattering nanocomposites with optical gain are feasible, and highly scattering media may be useful as a THz diffuser and/or depolariser. Modern optical telecommunication systems operate around 1.5 microns, however the loss in the polymer at these wavelengths is significantly larger than in glass-based devices. The question therefore arises how one might reduce the optical losses in polymers at telecommunication wavelengths, e.g. by introducing optical gain. A detailed model of optical gain in polymers containing highly doped glass nanoparticles was developed, taking into account the effects of scattering at both pump and signal wavelengths,and used to optimise the material parameters. It is shown that optical gain is feasible at 1.55 μm in a poly(methyl methacrylate) host containing 10% by volume of erbium-ytterbium-doped phosphate glass nanoparticles 100 nm in diameter when pumped at 980 nm with intensity 1 mW/μm². It is often necessary in communication and sensing systems to collect or radiate energy over a wide range of directions, though most coherent sources are highly directive. A numerical model was developed based on a Monte-Carlo algorithm which was extended to include the effects of both losses and boundaries on the propagation of electromagnetic (EM) waves through composite materials. Moreover, we fabricated a variety of composite materials, such as air-polymer dielectric composites and two dimensional (2-D) polymer microstructures, and investigated numerically and experimentally the scattering and diffusion of THz radiation in such materials, e.g. for use as THz diffusers. The results showed that THz polymer microstructures with an inhomogeneous structure can redistribute the incident THz radiation over a wide range of angles off-axis from the incident beam. The numerical model was further extended to determine the effects of multiple scattering on the polarisation of EM waves escaping from the composite materials, especially at large scattering angles. The propagation of THz pulses through polyethylene containing gold-coated silica microspheres was investigated experimentally, to determine the angle- and frequency-dependent depolarisation in a medium with high scattering and low absorption loss. The results showed that depolarisation increases with angle off-axis from the incident beam at high frequencies. The prediction of Monte-Carlo modelling was found to be in good agreement with the experimental data.