Contemporary projects to measure anisotropies in the cosmic microwave background (CMB) are now detecting hundreds to thousands of extragalactic radio sources, most of them blazars. As a member of a group of CMB scientists involved in the construction of catalogues of such sources and their analysis, I wish to point out the potential value of CMB surveys to studies of AGN jets and their polarization. Current CMB projects, for instance, reach mJy sensitivity, offer wide sky coverage, are blind and generally of uniform sensitivity across the sky (hence useful statistically), make essentially simultaneous multi-frequency observations at frequencies from 30 to 857 GHz, routinely offer repeated observations of sources with interesting cadences and now generally provide polarization measurements. The aim here is not to analyze in any depth the AGN science already derived from such projects, but rather to heighten awareness of their promise for the AGN community.

Interest in rare-earth-doped crystalline materials, e.g., M(RE)F4 (M – alkali metal, RE – rare-earth metal), featuring unique optical properties such as light upconversion and downshifting is experiencing a surge due to the broad spectrum of applications that these photonic systems are facilitating. The development of reliable synthetic methods that grant rapid access to these materials is therefore of great importance. Microwave-assisted synthesis is appealing in this regard, because microwave radiation enables rapid and uniform heating of the reaction mixture and allows for rigid control of the reaction conditions, factors that facilitate the production of high-quality materials within minutes. Surprisingly, the investigation around microwave-assisted synthesis of M(RE)F4 materials featuring upconversion and downshifting luminescence is limited. Methods that have already been developed predominately target Na-based systems, despite the evidence that the Li-based analogues also display excellent optical properties. In fact, only a single microwave-assisted approach toward a nanoscale Li-based system has been reported to date, while to my knowledge, no report of a microwave-assisted synthesis of a microscale Li-based system existed prior to the commencement of the work presented in this thesis. The challenge lies in the fact that access to Li(RE)F4 is not easily achieved through a simple substitution of the alkali metal source in the established protocols that yield Na(RE)F4; rather, a complete re-optimization of the synthesis method is required. This particular challenge was successfully addressed in this work. Presented and discussed in Chapter 3 of this thesis is a rapid microwave-assisted solvothermal synthesis approach toward both upconverting and downshifting LiYF4:RE3+ microparticle systems. More specifically, it is detailed how the rigorous optimization of the reaction temperature/duration profile, initial reaction mixture pH, and ratio of the metal precursors was necessary in gaining control over the crystalline phase, morphology, and size of the microparticles under microwave-induced solvothermal conditions. Importantly, a materials growth mechanism involving the depletion of a Li-free crystal phase, followed by a particle ripening process is also proposed. Moreover, the versatility of the developed method is highlighted by showcasing how it can be extended toward the synthesis of other relevant Li- and Na-based M(RE)F4 nano- and microscale materials (i.e., LiYbF4, NaYF4, and NaGdF4) featuring upconversion luminescence. Lastly, potential challenges associated with microwave-assisted synthesis are discussed, and appropriate solutions are proposed. The upconversion and downshifting luminescence of the M(RE)F4 materials attained via the developed synthesis approach is investigated in Chapter 4. The first part of the chapter provides a general assessment of the characteristic luminescence generated by the M(RE)F4 materials featuring various RE3+ dopant systems. The second part of the chapter is devoted to a much more thorough single-particle investigation of the anisotropic luminescence behaviour exhibited by the LiYF4:RE3+ microparticles via hyperspectral imaging, polarized emission spectroscopy, and optical trapping. It is my hope that you, the reader, will find the work presented in this thesis stimulating from two vantage points – from the development of the most rapid microwave-assisted solvothermal synthesis of upconverting and downshifting M(RE)F4 nano/microscale materials reported to date, as well as from the utilization of specialized luminescence characterization techniques to provide fundamental insight into a seldom-considered luminescence property of crystalline materials such as LiYF4.

Poly-p-phenylenebenzazoles (PBXs) are heterocyclic aromatic rigid-rod liquid-crystalline polymers with fully conjugated backbone having excellent thermo-oxidative, as well as dimensional stabilities. PBXs are considered to be multifunctional polymers of superior mechanical tenacity, non-linear optical response, and electrical properties. The fully conjugated PBX polymers are deemed to have excellent opto-electronic properties. In the last decade, molecular light emitting diodes (LEDs) have been investigated intensively for having distinct advantages as an advanced opto-electronic technology. This dissertation leads to rigid-rod polymer thin-films and mono-layer devices fabricated from acidic solutions. Photoluminescence (PL) spectra for poly-p-phenylenebenzobisthiazole (PBT) freestanding film were measured over a temperature range of 67 K to 300 K showing distinct electron-phonon interaction. Using an Mg cathode, the mono-layer PBT LEDs displayed a diodic electric response with a threshold voltage as low as 1 V. A blue shift in the maximum emission wavelength of the electroluminescence (EL) spectra was also observed with increasing electrical injection energy. For the multi-layer LEDs based on PBT using the same electrodes, the p-type/n-type bi-layer structure showing the most enhanced EL emission, and the tri-layer heterojunction had the least threshold voltage using the same electrodes. Our results indicated that the heterojunction architecture could be applied to balance charge carriers for increasing EL intensity. Meanwhile, the investigation also revealed the advantage in using the extra PBT layer for increasing both EL emission intensity and injection efficiency by lowering its threshold voltage. Two schemes for making uniaxial freestanding films and LED devices for polarized optical absorption and emission were processed from uniaxial poly-p-phenylenebenzobisoxazole (PBO) fiber. The PL of the uniaxial PBO films demonstrated an emission intensity ratio I¡ü/I¡æas high as 5. Anisotropically processed mono-layered PBO LED showed a markedly decreased threshold voltage from 7 V of the isotropic PBO device to 5 V. The polarization effects in optical absorption, PL and EL emissions were acquired and correlated with the uniaxial orientation of the rigid-rod PBO polymer. The molecular modification investigated the opto-electronic properties of poly-2,2'-m-phenylene-5,5'-bibenzimidazole (Pbi) with PBT physical blends, and monolithic 6F-PBO-OH-co-6F-PBO-di(OC10H21) copolymers. Partially conjugated polymer Pbi and fully conjugated polymer PBT were mixed for luminescence study. Their absorption spectra showed superposition of individual absorption response indicating no inter-molecular energy transfer. However, the PL and the EL emission demonstrated a blue shift with increasing Pbi content. This was attributed to the rigid-rod configuration or the aggregation of PBT perturbed by mixing with coil-like Pbi. It was recognized that the backbone of the fully conjugated rigid-rod PBT was collinear having more charge delocalization than that of not fully conjugated coil-like Pbi. The diode threshold voltage of the physical blends varied from 4 V to 14 V with decreasing PBT content. Another molecular modification was changing the composition of 6F-PBO copolymers. Their PL emission exhibited excellent chromatic tuning range from green to blue emission. The Commission Internationale de l¡¦Eclairage (C. I. E.) coordinates of the copolymer EL emission were from (0.25, 0.53) to (0.24, 0.31) covering a wide visible range and demonstrating a white light emission. Atomic substitution of the rigid-rod polymers was utilized to examine individual atomic contribution for luminescence emission. The hydrogen bond effect for PBO-OH and PBO was evidenced in a major Stoke¡¦s shift to a longer wavelength because of protonic transfer on the excited state. Elemental electronegativities affected the delocalization of the £k electron leading to a blue shift in absorption spectra as shown in case of PBO and PBT. The PBO molecule was more collinear and co-planar, providing more charge delocalization than PBT. However the absorption edge of the PBT was about 30 nm higher than that of PBO. This suggested that the electronegativities affected the molecular delocalization. Using the solid-state physics with pseudofunction (PSF) calculation, there was good match between absorption spectra and calculated excitation energies for the rigid-rod polymer systems.

Cette thèse porte sur la synthèse et la caractérisation photophysique d'une série de composés neutres luminescents de platine (II) contenant un ligand tridentate dianionique chromophore N-donneur et un ligand auxiliaire monodentate. Les composés montrent un changement notable des propriétés de photoluminescence selon l'auto-assemblage en raison de la formation d'interactions intermoléculaires non covalentes faibles telles que metal-metal et π-π. Nous avons démontré comment les complexes de Pt (II) peuvent être auto-assemblés d'une manière contrôlée et précise en jouant sur les facteurs cinétiques et thermodynamiques, ainsi que la morphologie des différents ensembles étudiés. Ces approches ont conduit à des matériaux avec des propriétés améliorées et uniques tels que le mécano-chromisme, ainsi que l'absorption et l'émission de la lumière polarisée. Les composés étudiés et leurs assemblages sont utiles non seulement pour le développement de nouveaux matériaux fonctionnels supramoléculaires en équilibre et hors- équilibre, mais aussi pour des applications en bio-imagerie. / This thesis focuses on the synthesis and the photophysical characterization of a series of luminescent neutral Pt(II) compounds containing a tridentate dianionic N-donor chromophoric ligand and a monodentate ancillary moiety. The compounds exhibited notable change of the photoluminescence properties upon self-assembly due to the establishment of weak non-covalent intermolecular interactions – metal-metal and π-π. We demonstrated how Pt(II) complexes can be self-assembled in a controlled and precise manner by playing with kinetic and thermodynamic factors and the morphology of the different assemblies investigated. Such approaches led to materials with enhanced and unique properties such as mechanochromism and polarized light absorption and emission. The investigated compounds and their assemblies were useful for the development of novel functional supramolecular materials in and out of the equilibrium as well as for bioimaging application.

InGaN based light emitting devices operating in the blue and near UV spectral regions are commercialized and used in many applications. InGaN heterostructures experience compositional inhomogeneity and thus potential fluctuations, such that regions of higher indium composition are formed and correspond to lower potentials. The indium rich regions form localization centers that save carriers from non-radiative recombination at dislocations, thus despite the large defect density, their quantum efficiency are surprisingly large. However, the conventional c-plane InGaN QWs suffer from high internal piezoelectric and spontaneous fields. These fields are detrimental for the performance of such structures as they lead to the quantum confined stark effect causing red-shift of the emission as well as reducing the electrons and holes wavefunctions overlap, thereby reducing the radiative recombination rate. However, growth of InGaN QWs on semipolar and nonpolar planes greatly reduced the polarization fields. Semipolar and nonpolar QWs experience an outstanding property which is polarized luminescence, opening a new frontier for applications for InGaN emitting devices. While nonpolar QWs have larger degree of polarized emission than semipolar QWs, semipolar QWs can emit in longer wavelengths due to their higher indium uptake. In this thesis, semipolar 20¯21 and nonpolar m-plane InGaN/GaN QWs were investigated. Photoluminescence, spectral and polarization dynamics were all studied in order to form a whole picture of the carrier dynamics in the QWs. Time resolved photoluminescence measurements were conducted for following carriers distribution between extended and localized states. Both the semipolar and nonpolar samples showed efficient luminescence through short radiative recombination times, as well as carrier localization in lower potential sites after thermal activation of excitons. Carrier localization was found to be benign as it didn’t degrade the performance of the samples or decrease the polarization ratio of their emission. However, the structures showed modest potential variations with the absence of deep localization centers or quantum dots. High polarization ratios were measured for both samples, which is well-known for nonpolar QWs. The high polarization ratio for the semipolar sample is of great importance, thus semipolar 20¯21 QWs should be considered for longer wavelength emitters with highly polarized spontaneous emission.