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The study of the non-linear optical properties of polymeric systems is a challenging and exciting field of research ranging from device engineering, optical measurements, chemical synthesis to design and theoretical issues. At the present time, most of the basic science needed for the synthesis of molecules and the design of devices utilizing second order optical susceptibilities is in hand, although certain issues remain to be resolved. On the other hand, many important questions regarding the design and use of third order optical susceptibilities are still unanswered. The earliest ideas of the importance of low dimensionality optical band gaps suggests the use of conjugated molecules. At present, there is considerable effort, both experimentally and theoretically, in optimizing the value of gamma for polymers or oligomers with conjugated segments, because such conjugated polymers (like polyacetylene, polythiophenes, and the poly-diacetylenes) have very large gamma. These polymers have also been under intense scrutiny because of their large conductivities when doped. Although we are beginning to understand the theoretical reasons for the various unusual properties of the materials, we do not understand the factors that limit the ultimate value of gamma. For example, what are the important structures and interactions in the molecule the prevent gamma from being as large as possible while still having a small absorption coefficient, and how can we design molecules with these constraints in mind.
Intrinsically conducting polymers forms a category of doped conjugated polymers that can conduct electricity. Since their discovery in the late 1970s, they have been widely applied in many fields, ranging from optoelectronic devices to biosensors. The most common type of conducting polymers is poly(3,4-ethylenedioxythiophene), or PEDOT. PEDOT has been popularly used as electrodes for solar cells or light-emitting diodes, as channels for organic electrochemical transistors, and as p-type legs for organic thermoelectric generators. Although many studies have been dedicated to PEDOT-based materials, there has been a lack of a unified model to describe their optical properties across different spectral ranges. In addition, the interesting optical properties of PEDOT-based materials, benefiting from its semi-metallic character, have only been rarely studied and utilized, and could potentially enable new applications. Plasmonics is a research field focusing on interactions between light and metals, such as the noble metals (gold and silver). It has enabled various opportunities in fundamental photonics as well as practical applications, varying from biosensors to colour displays. This thesis explores highly conducting polymers as alternatives to noble metals and as a new type of active plasmonic materials. Despite high degrees of microstructural disorder, conducting polymers can possess electrical conductivity approaching that of poor metals, with particularly high conductivity for PEDOT deposited via vapour phase polymerization (VPP). In this thesis, we systematically studied the optical and structural properties of VPP PEDOT thin films and their nanostructures for plasmonics and other optical applications. We employed ultra-wide spectral range ellipsometry to characterize thin VPP PEDOT films and proposed an anisotropic Drude-Lorentz model to describe their optical conductivity, covering the ultraviolet, visible, infrared, and terahertz ranges. Based on this model, PEDOT doped with tosylate (PEDOT:Tos) presented negative real permittivity in the near infrared range. While this indicated optical metallic character, the material also showed comparably large imaginary permittivity and associated losses. To better understand the VPP process, we carefully examined films with a collection of microstructural and spectroscopic characterization methods and found a vertical layer stratification in these polymer films. We unveiled the cause as related to unbalanced transport of polymerization precursors. By selection of suitable counterions, e.g., trifluoromethane sulfonate (OTf), and optimization of reaction conditions, we were able to obtain PEDOT films with electrical conductivity exceeding 5000 S/cm. In the near infrared range from 1 to 5 µm, these PEDOT:OTf films provided a well-defined plasmonic regime, characterized by negative real permittivity and lower magnitude imaginary component. Using a colloidal lithography-based approach, we managed to fabricate nanodisks of PEDOT:OTf and showed that they exhibited clear plasmonic absorption features. The experimental results matched theoretical calculations and numerical simulations. Benefiting from their mixed ionic-electronic conducting characters, such organic plasmonic materials possess redox-tunable properties that make them promising as tuneable optical nanoantennas for spatiotemporally dynamic systems. Finally, we presented a low-cost and efficient method to create structural colour surfaces and images based on UV-treated PEDOT films on metallic mirrors. The concept generates beautiful and vivid colours through-out the visible range utilizing a synergistic effect of simultaneously modulating polymer absorption and film thickness. The simplicity of the device structure, facile fabrication process, and tunability make this proof-of-concept device a potential candidate for future low-cost backlight-free displays and labels.
Conjugated polymers have important technological applications including solar cells and light emitting devices. They are active components in many important biological processes. This book describes and explains the electronic and optical properties of conjugated polymers by developing theoretical models to understand the key electronic states.
Lists citations with abstracts for aerospace related reports obtained from world wide sources and announces documents that have recently been entered into the NASA Scientific and Technical Information Database.
Experimental and theoretical studies of the optical properties of large core step index (SI) plastic optical fibers (POF) and graded index (GI) POFs are reported. A set of criteria and analyses of physical parameters are developed in context to the major issues of POF applications in short-distance communication systems. Analyses are presented to show how the measured POF optical attenuation affects the overall performance in wavelength division multiplexing (WDM) and how use of perfluorinated polymers can overcome limitation inherent to current POF materials. Results of POF optical bandwidth measurements by direct picosecond time domain methods are reported and their relationship to refractive index profiles theoretically analyzed by the WKB and finite-element methods. Two high-resolution optical techniques of refracted near-field and transverse interferometric methods are presented and used to measure the index profiles of large core POFs. Results reveal that the index profile of current GI POF is not parabolic, but consists of two markedly different regions. Analysis of the index profile reveals that strong mode coupling increases the GI POF bandwidth from its profile-determined value of 0.43 GHz to its measured value of 3.0 GHz for 100m. We further study mode coupling effect inside POFs through various experimental techniques, including pulse broadening-length dependence measurements and far-field radiation pattern measurements.