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Thesis (M.Sc.)--York University, 2007. Graduate Programme in Physics and Astronomy. / Typescript. Includes bibliographical references. Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pqdiss&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&rft_dat=xri:pqdiss:MR32016

The development of new multiphoton absorbing materials has attracted the attention of researchers for the last two decades. The advantages that multiphoton absorbing materials offer, versus their one-photon absorbing counterparts, rely on the nature of the nonlinearity of the absorption process, where two photons are absorbed simultaneously offering increased 3D resolution, deeper penetration, and less photobleaching and photodamage as a result of a more confined excitation. The applications of efficient two-photon absorbing materials have been extensively expanding into the fields of photodynamic therapy, microscopy, and optical data storage. One of the fields where an increased interest in multiphoton absorbing materials has been most evident is in bioimaging, in particular, when different cellular processes and organelles need to be studied by fluorescence microscopy. The goal of this research was to develop efficient two-photon absorption (2PA) compounds to be used in fluorescence bioimaging, meaning that such compounds need to posses good optical properties, such as high fluorescence quantum yield, 2PA cross section, and photostability. In the first chapter of this dissertation, we describe the synthesis and structural characterization of a new series of fluorescent donor–acceptor and acceptor-acceptor molecules based on the fluorenyl ring system that incorporated functionalities such as alkynes and thiophene rings, through efficient Pd-catalyzed Sonogashira and Stille coupling reactions, in order to increase the length of the conjugation in our systems. These new molecules proved to have high two-photon absorption (2PA), and the effect of these functionalities on their 2PA cross section values was evaluated. Finally, their use in two-photon fluorescence microscopy (2PFM) imaging was demonstrated. iii One of the limitations of the compounds described in Chapter 1 was their poor water solubility; this issue was addressed in Chapter 2. The use of micelles in drug delivery has been shown to be an area of increasing interest over the last decade. In the bioimaging field, it is key to have dye molecules with a high degree of water solubility to enable cells to uptake the dye. By enclosing a hydrophobic dye in Pluronic® F-127 micelles, we developed a system that facilitates the use of 2PA molecules (typically hydrophobic) in biological systems for nonlinear biophotonic applications, specifically to image the lysosomes. Furthermore, we report in this chapter the efficient microwave-assisted synthesis of the dye used in this study. In addition, linear photophysical and photochemical parameters, two-photon absorption (2PA), and superfluorescence properties of the dye studied in Chapter 2, were investigated in Chapter 3. The steady-state absorption, fluorescence, and excitation anisotropy spectra of this dye were measured in several organic solvents and aqueous media. In Chapter 4, we describe the preparation and the use of an efficient and novel twophoton absorbing fluorescent probe conjugated to an antibody that confers selectivity towards the vascular endothelial growth factor receptor 2 (VEGFR-2) in porcine aortic endothelial cells that express this receptor (PAE-KDR). It is known that this receptor is overexpressed in certain cancer processes. Thus, targeting of this receptor will be useful to image the tumor vasculature. It was observed that when the dye was incubated with cells that do not express the receptor, no effective binding between the bioconjugate and the cells took place, resulting in very poor, nonspecific fluorescence images by both one and two-photon excitation. On the other hand, when the dye was incubated with cells that expressed VEGFR-2, efficient imaging of the cells was obtained, even at very low concentrations (0.4 μM). Moreover, incubation of the bioconjugate iv with tissue facilitated successful imaging of vasculature in mouse embryonic tissue

Technological progress in commercializing ultrafast lasers and detectors has allowed realization of cooperative processes on an ultrashort time scale, which demand a re-evaluation of the conventional cooperative phenomena with a new insight. Ultrafast cooperative phenomena in coherently prepared media and various applications of superfluorescence and coherent Raman scattering are studied in this dissertation. In particular, a simple theoretical testimony on analogy between a cooperative emission and coherent Raman scattering is presented by offering an opportunity to perform parallel research on these two processes from a unified point of view. On one hand, the superfluorescent pulse with a time duration of a few tens of picoseconds (ps) from alkali metal vapor is observed for the first time, even though cooperative phenomena in atomic vapor have been extensively studied for more than five decades. A dense rubidium vapor pumped by ultrashort (100 femtosecond, fs) pulses allows a realization of the ultrafast superfluorescence while a time-resolved study of superfluorescence is accomplished by using a streak camera with 2 ps time resolution. Experimental research on quantum nature of cooperative emissions has been “frozen” over the years (three decades) possibly because of the technical difficulties. Quantum fluctuations of superfluorescence development are explored experimentally by taking advantage of the ultra fast streak camera. Presumable applications of the superfluorescent pulse in e.g., a remote sensing, and an ultraviolet upconversion of the input infrared laser pulse are presented. The quantum interference due to different excitation pathways is revealed by the temporal coherent control technique while observing interferometric signals from alkali metal vapors. On the other hand, a new spectroscopic technique based on ultrafast coherent Raman scattering is developed. The key advantage of the presented technique is to suppress the non-resonant background noise which usually obscures possible applications of the other conventional coherent Raman techniques in practice. A reduction of the background noise is achieved by shaping and delaying the third pulse which probes the coherence of the medium (i.e., an enhancement of specific vibrations of the target molecules in unison) firstly prepared by two broadband pulses. We demonstrate a robustness and superiority of signal-to-noise ratio of the developed technique by identifying as few as 10000 bacterial spores at a single laser shot level. Finally, several comparative studies between cooperative and uncooperative processes are presented. A picosecond cooperative phenomenon in a three-photon resonant medium induced by a single as well as two-color ultrashort pulses is investigated. A time-resolved study shows that a picosecond cooperative effect is crucial in the well-established fields of resonant-enhanced multiphoton ionizations and harmonic generations. We also present a quantitative analysis for spontaneous versus broadband coherent Raman scattering on pyridine molecules. The spontaneous Raman signal is enhanced by 5 orders as a result of cooperative phenomena.

It is well known that the optical properties of multi-level atomic and molecular system can be controlled and manipulated efficiently using quantum coherence and interference, which has led to many new effects in quantum optics for e.g. lasing action without population inversion, ultraslow light, high resolution nonlinear spectroscopy etc. Recent experimental and theoretical studies have also provided support for the hypothesis that biological systems uses quantum coherence. Nearly perfect excitation energy transfer in photosynthesis is an excellent example of this. In this dissertation we studied quantum coherence and interference effects in the transient and the continuous-wave regimes. This study led to (i) the first experimental demonstration of carrier-envelope phase effects on bound-bound atomic excitation in multi-cycle regime (~15 cycles), (ii) a unique possibility for standoff detection of trace gases using their rotational and vibrational spectroscopic signals and from herein called Coherent Raman Umklappscattering, (iii) several possibilities for frequency up-conversion and generation of short-wavelength radiation using quantum coherence (iv) the measurement of spontaneous emission noise intensity in Yoked-superfluorescence scheme. Applications of the obtained results are development of XUV (X-Ray) lasers, con- trolled superfluorescent (superradiant) emission, carrier-envelope phase effects, coherent Raman scattering in the backward direction, enhancement of efficiency for generating radiation in XUV and X-Ray regime using quantum coherence with and without population inversion and to extend XUV and X-Ray lasing to ~4.023 nm in Helium-like carbon.

<p dir="ltr">Photovoltaics are leading devices in green energy production. Understanding the fundamental physics behind energy transport in candidate materials for future photovoltaic and optoelectronic devices is necessary to both realize material limitations and improve efficiency. Excitons, which are bound electron-hole pairs, are central to determining how energy propagates throughout semiconductors. Exciton transport is greatly influenced by material dimensionality. In highly ordered quantum dot (QD) systems, electronic coupling between individual QDs can lead to coherent exciton transport, whereas in two-dimensional heterostructures, excitons can form at the interface of a heterojunction, creating charge-transfer excitons.</p><p dir="ltr">This dissertation is dedicated to summarizing the studies of exciton transport and behavior in two systems: perovskite QD superlattices and transition metal dichalcogenide (TMDC)/polyacene heterostructures. Chapter 1 provides readers with details on these materials in addition to information on the fundamental concepts (i.e., excitons, phonons, energy transfer) needed to best appreciate further chapters. Chapter 2 summarizes the spectroscopic techniques (photoluminescence and transient absorption spectroscopy and microscopy) used to examine exciton behavior. Next, the effects of disorder and dephasing pathways on the ability of perovskite QDs to coherently couple is investigated through the lens of superradiance in Chapter 3. After this, the temperature-dependent exciton transport within perovskite QD superlattices is imaged with high spatial and temporal resolutions in Chapter 4. The experimental transport data on these superlattices provides evidence for environment-assisted quantum transport, which, until this study, had yet to be realized in solid-state systems. In Chapter 5, attention is switched to verifying the existence and deepening the understanding of the behavior of several spatially separated interlayer excitons in a tungsten disulfide/tetracene heterostructure. Finally, Chapter 6 summarizes the preliminary results obtained through transient absorption spectroscopy on other TMDC/polyacene heterostructures where separation of the triplet pair state is attempted. </p><p dir="ltr">It is this author’s hope that this dissertation will not only summarize their graduate work but will also serve as inspiration for others to continue learning and contribute to the advancement of the energy research field.</p>