In two dimensions, non-trivial topology and enhanced correlation lead to amazing physical phenomena. Graphene offers a high-quality, ultra-tunable and integratable two dimensional electron system in the study of interacting and topological quantum fluids. In this thesis we studied in detail various emergent quantum phenomena of electron fluids due to both strong in-plane and out-of-plane interaction between electrons in single and multi-layer graphene systems. Using magnetoresistance measurement in the corbino disk geometry, we manged to quantitatively measure the viscosity of electrons in monolayer and bilayer graphene as a function of carrier density and temperature. We demonstrated a crossover between degenerate Fermi liquid and non-degenerate electron-hole liquid. In the quantum Hall regime, we applied the corbino geometry as a probe of the incompressible sample bulk, improving significantly the resolution of fragile quantum Hall states compared to Hall bar devices. The improved resolution enables quantitative studies over a much broader parameter space in both singlelayer and multi-layer graphene system. In double-layer graphene where two vertically stacked graphene layers are in close proximity but electrically separated by a thin hBN tunnel barrier, we observed sequence of FQHS which can be perfectly described by two-component composite fermion theory. Using a combination of different measurement configuration, we found evidence for a novel type of two-component non-abelian FQHS. At \nu = 1 in double-layer graphene where ground states of indirect excitons occur, we mappped out the entire phase diagram. We realized BEC-BCS crossover in the exciton condensation phase tunable with both magnetic field and electrostatic gating. At small exciton filling fraction, we discovered Wigner crystal of excitons. Lastly, we realized a strongly correlated triple-layer quantum Hall system with independent control of carrier density in each layer and demonstrated three-layer coherent quantum Hall effect at total integer filling fraction and possibly fractional filling fraction.

Singlet fission, the conversion of one photoexcited singlet exciton into two triplet excitons, is a promising mechanism to overcome theoretical efficiency limits in single-junction solar cells. Intramolecular singlet fission materials based on molecular dimers are a powerful platform to study singlet fission since triplet dynamics can be fine-tuned through chemical structure. This thesis describes the critical nature of the molecular bridge between singlet fission chromophores in determining the fate of the triplet pair. We demonstrate how bridge energetics, connectivity, length, and planarity are tunable handles for controlling rates of triplet pair generation and recombination. These rates can even be modulated independent of each other, furnishing materials with desirable properties such as fast triplet generation and long triplet lifetimes. This thesis establishes key design principles to provide greater control over triplet pair formation, dephasing, and decay in intramolecular singlet fission materials. Chapter 1 introduces the process of singlet fission and provides an overview of the progress and challenges in the field. In Chapters 2 and 3, we detail the significance of bridge frontier molecular orbital energies and connectivity patterns in mediating triplet pair formation in bridged pentacene and tetracene dimers. We highlight key observables in the linear absorption spectra to predict relative rates of triplet pair formation, and demonstrate how quantum interference graphical models from single-molecule electronics can successfully be applied to explain triplet pair formation behavior in singlet fission. In Chapter 4, we investigate triplet pair recombination in these materials and propose that electronic coupling alone does not dictate triplet pair dephasing and decay. In Chapter 5, we present a new singlet fission chromophore and identify important triplet population signatures distinguishing singlet fission from intersystem crossing in contiguous dimers. Lastly, in Chapter 6, we explore dendrimers as a controlled macromolecular architecture to study singlet fission.

No description available.

Two-dimensional materials such as graphene, boron nitride and transition metal dichalcogenides have attracted significant research interest due to their unique optoelectronic properties. Transition metal dichalcogenides (TMDCs) are a family of two-dimensional semiconductors which exhibit strong light-matter interaction and show great promise for applications ranging from more efficient LEDs to quantum computing. One of the most intriguing qualities of TMDCs is their ability to be stacked on top of one another to tailor devices with specific properties and exploit interlayer phenomena to develop new characteristics. One such interlayer interaction is the generation of charge transfer excitons which span the interface between two different TMDC monolayers. This work aims to study the intrinsic optical properties of charge transfer excitons in TMDC heterostructures. We must first start by investigating methods to protect and isolate our sample of interest from its chemical and electrostatic environment. We demonstrate that near intrinsic photoluminescence (PL) linewidth and exciton emission homogeneity from monolayer TMDCs can be achieved using a combination of BN encapsulation and passivation of substrate hydroxyl groups. Next, we develop clean stacking techniques and incorporate low defect density source crystals to maintain intrinsic properties and ensure a sufficiently high quality heterostructure interface to study characteristics of charge transfer excitons in 2D TDMCs. Strong photoluminescence emission from charge transfer excitons is realized and is shown to persist to room temperature. Charge transfer exciton lifetime is measured to be two orders of magnitude longer than previously reported. Using these high quality heterostructures, we study the behavior of charge transfer excitons under high excitation density. We observe the dissociation of charge transfer excitons into spatially separated electron-hole plasmas under optical excitation. We then probe properties of charge transfer exciton emission enhancement due resonant coupling to surface plasmon modes of gold nanorods.

Thesis (M.S.)--University of Delaware, 2009. / Principal faculty advisor: Stephen P. Bremner, Dept. of Electrical & Computer Engineering. Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (leaves 107-119).

Semiconductor quantum dots tightly confine excited electron-hole pairs, called excitons, resulting in discrete energy levels similar to those of single atoms. Transition energies in the visible or near-infrared make quantum dots suitable for many applications in quantum optics and quantum information science, but to take advantage of all the properties of quantum dot emission, it is necessary to excite them coherently which has been a great challenge due to background scattering of the excitation laser. This dissertation presents the first coherent control of a single quantum dot with observation of its resonance fluorescence and decoherence phenomena. Strong continuous-wave excitation causes the dot to undergo several Rabi oscillations before emitting. These are visible as oscillations in the first- and second-order correlation functions of the emission, and the quantum dot states are "dressed", resulting in a Mollow triplet in the emission spectrum. Some resonantly excited dots, in addition to resonance fluorescence, also emit light from excited states several meV higher in energy. Such up-conversion fits existing theories of decoherence but has never been directly observed before. The up-conversion intensity is shown to be described well by a fairly simple three-level model with single-phonon absorption. The coherent phenomena of resonance fluorescence and the decoherence due to up-conversion paint a dual picture of single quantum dots wherein they can sometimes be treated as an ideal two-level system, but their interactions with the host crystal can lead to many complex behaviors. / text

Among the many examples of Bose condensation considered in physics, electron-hole-pair (exciton) condensation has maintained special interest because it has been difficult to realize experimentally, and because of controversy about condensate properties. In this thesis, we studied the various aspects of spontaneous symmetry broken state of exciton in bilayer using mean field theory. We calculated the photoluminescence of excitonic condensation created by laser. We developed a one-dimensional toy model of excitonic supercurrent using mean field theory plus non-equilibrium Green’s function (NEGF) which give qualitatively consistent results with experiments. We proposed graphene bilayer as a novel system for excitonic condensation to occur and estimate it to exist even at temperature as high as room temperature. / text

Atomic lifetimes of selected levels in Neon I and Argon I were measured using a method of delayed coincidence. Pulsed rf was used to excite a discharge tube containing the neon and argon gas. The radiation emitted from the excited atoms of the gas was passed through a monochromator so only the desired wavelength would be observed. Each time an excitation pulse ended, the decay of light intensity was detected by a photomultiplier tube. By measuring the decay time of the light intensity using the delayed coincidence technique, the average lifetime of the desired level was determined. The values of lifetimes determined in this study were found to contain considerable error. Several factors contributing to these errors were thought to be the shape of the rf pulses and an effort known as cascading which was caused by the use of rf for excitation of the gas.

Forster theory of energy transfer is applied in diluted systems, and yet it remains unknown if it can be applied to the dense media. We have studied the exciton transfer in one-dimensional (1-D) close-packed pure and mixed quantum dot (QD) array under different models and two-dimensional (2-D) perfect lattice. Our approach is based on the master equation created by treating the exciton relaxation as a stochastic process. The random parameter has been used to describe dot-to-dot distance variations. The master equation has been investigated analytically for 1-D and 2-D perfect lattices and numerically for 1-D disordered systems. The suitability of Forster decay law on the excitation decay of close-packed solid has been discussed. The necessity to consider the effect of the further nearest interdot interactions has been checked. / Department of Physics and Astronomy