Semiconductor micro- and nano-cavities are excellent platforms for experimental studies of optical cavities, lasing dynamics, and cavity Quantum Electrodynamics (QED). Common materials for such experiments are narrow bandgap semiconductor materials with well-developed epitaxial growth technologies, such as GaAs and InP, among others. Gallium nitride (GaN) and its alloys are industrially viable materials with wide direct bandgaps, low surface re-combination velocities, and large exciton binding energies, offering the possibility of room temperature realization of light-matter interaction. Controlling light-matter interaction is at the heart of nanophotonic research which leads to ultra-low threshold lasing, photonic qubits, and optical strong coupling. Technologically, due to its blue emission, GaN photonic cavities with indium gallium nitride (InGaN) active mediums serve as efficient light sources for the fast growing photonic industry, optical computing and communication networks, display technology, as well as quantum information processing. The main challenges in fabricating high quality GaN cavity are due to its chemical inertness and low material quality as a result of strain-induced defects and threading dislocations. In this dissertation, I examine the designs, novel fabrication processes, and characterizations of high quality factor GaN microdisk and photonic crystal nanobeam cavities with different classes of InGaN active medium, namely quantum dots (QDs), quantum wells (QWs), and fragmented quantum wells (fQWs), for investigating light-matter interaction between cavity and these active media. This dissertation is carefully organized into four chapters. Chapter 1 outlines the background of the research, the materials and growth, and the necessary technique Photoelectrochemical (PEC) etching which is uniquely used to undercut and suspend GaN cavities. Chapter 2 outlines the fabrications, optical experiments, and tuning technique developed for GaN/InGaN microdisks. Microdisks are circular resonant cavities that support whispering gallery modes. Through the use of optimized dry etching and PEC, high quality factor microdisks with relatively small modal volume are fabricated with immediate demonstration of low threshold lasing. On the path to achieving light and matter interactions, irreversible tuning of the cavity mode of p-i-n doped GaN/InGaN microdisks is achieved through photo-excitation in a water environment. Such a technique paves the way for deterministically and spectrally matching the cavity mode to the emitter’s principle emission. Chapter 3 outlines the work done on the high quality GaN photonic crystal nanobeams with InGaN QDs and fQWs. The fragmented nature of the fQW layer has a surprisingly dramatic influence on the lasing threshold. A record low threshold is demonstrated that is an order of magnitude lower in threshold than identical nanobeams with homogeneous QW, and comparable to the best devices in other III-V material systems. As an active medium with greater carrier confinement than quantum wells, and higher carrier capture probability than quantum dots, the fQW active medium, in combination with the nanobeam cavity with ultra-small modal volume and high quality factor, provides an ideal means of probing the limits of light and matter interactions in the nanoscale. Moreover, GaN/InGaN nanopillars are fabricated to isolate a single InGaN QD for understanding its emission properties. Antibunching is observed, demonstrating the quantum nature of the QD emission. Gas tuning is attempted on GaN nanobeams with InGaN QDs to achieve QD-cavity mode coupling and to demonstrate cavity enhanced single photon emission. Last but not least, Chapter 4 concludes the dissertation with summary and future directions. / Engineering and Applied Sciences - Applied Physics

I discuss experimental progress towards developing a material with an isotropic, negative index of refraction at optical frequencies. The simplest way to make such a material is to create a metafluid, or a disordered collection of subwavelength, isotropic electromagnetic resonators. Small clusters of metal particles, such as tetrahedra, serve as these constituents. What is needed are methods for manufacturing these structures with high precision and in sufficient yield that their resonances are identical. Jonathan Fan et al. [Science, 328 (5982), 1135-1138, 2010] demonstrated that colloidal self-assembly is a means of preparing electromagnetic resonators from metal nanoparticles. However, the resonances are sensitive to the separation gaps between particles. Standard synthesis routes for metal nanoparticles yield crystals or nanoshells that are inadequate for metafluids due to polydispersity, faceting, and thermal instabilities. To ensure that the separation gaps and resonances are uniform, more monodisperse spherical particles are needed. An additional challenge is the self-assembly of tetrahedral clusters in high yield from these particles. In self-assembly approaches that others have examined previously, the yield of any particular type of cluster is low. In this dissertation I present solutions to several of these problems, developed in collaboration with my research group and others. We demonstrate that slow chemical etching can transform octahedral gold crystals into ultrasmooth, monodisperse nanospheres. The particles can serve as seeds for the growth of larger octahedra which can in turn be etched. The size of the gold nanospheres can therefore be adjusted as desired. We further show that in colloidal mixtures of two sphere species that strongly bind to one another, the sphere size ratio determines the size distribution of self-assembled clusters. At a critical size ratio, tetrahedral clusters assemble in high yield. We explain the experimentally observed 90% yield with a nonequilibrium “random parking” model based on irreversible binding. Simulations based on this model reveal that 100% yield of tetrahedra is possible in principle. Finally, we combine these results and present methods for the self-assembly and purification of tetrahedral plasmonic nanoclusters, the simplest building blocks for isotropic metafluids. / Physics

Realizing complex three-dimensional structures in a range of material systems is critical to a variety of emerging nanotechnologies. This is particularly true of nanomechanical and nanophotonic systems, both relying on free-standing small-scale components. In the case of nanomechanics, necessary mechanical degrees of freedom require physically isolated structures, such as suspended beams, cantilevers, and membranes. For nanophotonics, elements like waveguides and photonic crystal cavities rely on light confinement provided by total internal reflection or distributed Bragg reflection, both of which require refractive index contrast between the device and surrounding medium (often air). Such suspended nanostructures are typically fabricated in a heterolayer structure, comprising of device (top) and sacrificial (middle) layers supported by a substrate (bottom), using standard surface nanomachining techniques. A selective, isotropic etch is then used to remove the sacrificial layer, resulting in free-standing devices. While high-quality, crystalline, thin film heterolayer structures are readily available for silicon (as silicon-on-insulator (SOI)) or III-V semiconductors (i.e. GaAs/AlGaAs), there remains an extensive list of materials with attractive electro-optic, piezoelectric, quantum optical, and other properties for which high quality single-crystal thin film heterolayer structures are not available. These include complex metal oxides like lithium niobate (LiNbO3), silicon-based compounds such as silicon carbide (SiC), III-V nitrides including gallium nitride (GaN), and inert single-crystals such as diamond. Diamond is especially attractive for a variety of nanoscale technologies due to its exceptional physical and chemical properties, including high mechanical hardness, stiffness, and thermal conductivity. Optically, it is transparent over a wide wavelength range (from 220 nm to the far infrared), has a high refractive index (n ~ 2.4), and is host to a vast inventory of luminescent defect centers (many with direct optical access to highly coherent electron and nuclear spins). Diamond has many potential applications ranging from radio frequency nanoelectromechanical systems (RF-NEMS), to all-optical signal processing and quantum optics. Despite the commercial availability of wafer-scale nanocrystalline diamond thin films on foreign substrates (namely SiO2), this diamond-on-insulator (DOI) platform typically exhibits inferior material properties due to friction, scattering, and absorption losses at grain boundaries, significant surface roughness, and large interfacial stresses. In the absence of suitable heteroepitaxial diamond growth, substantial research and development efforts have focused on novel processing techniques to yield nanoscale single-crystal diamond mechanical and optical elements. In this thesis, we demonstrate a scalable ‘angled-etching’ nanofabrication method for realizing nanomechanical systems and nanophotonic networks starting from bulk single-crystal diamond substrates. Angled-etching employs anisotropic oxygen-based plasma etching at an oblique angle to the substrate surface, resulting in suspended optical structures with triangular cross-sections. Using this approach, we first realize single-crystal diamond nanomechanical resonant structures. These nanoscale diamond resonators exhibit high mechanical quality-factors (approaching Q ~ 10^5) with mechanical resonances up to 10 MHz. Next, we demonstrate engineered nanophotonic structures, specifically racetrack resonators and photonic crystal cavities, in bulk single-crystal diamond. Our devices feature large optical Q-factors, in excess of 10^5, and operate over a wide wavelength range, spanning visible and telecom. These newly developed high-Q diamond optical nanocavities open the door for a wealth of applications, ranging from nonlinear optics and chemical sensing, to quantum information processing and cavity optomechanics. Beyond isolated nanophotonic devices, we also developed free-standing angled-etched diamond waveguides which efficiently route photons between optical nanocavities, realizing true on-chip diamond nanophotonic networks. A high efficiency fiber-optical interface with aforementioned on-chip diamond nanophotonic networks, achieving > 90% power coupling, is also demonstrated. Lastly, we demonstrate a cavity-optomechanical system in single-crystal diamond, which builds upon previously realized diamond nanobeam photonic crystal cavities fabricated by angled-etching. Specifically, we demonstrate diamond optomechanical crystals (OMCs), where the engineered co-localization of photons and phonons in a quasi-periodic diamond nanostructure leads to coupling of an optical cavity field to a mechanical mode via the radiation pressure of light. In contrast to other material systems, diamond OMCs possess large intracavity photon capacity and sufficient optomechanical coupling rates to exceed a cooperativity of ~ 1 at room temperature and realize large amplitude optomechanical self-oscillations. / Engineering and Applied Sciences - Applied Physics

Single molecule studies on CdSe quantum dots functionalized with oligo-phenylene vinylene ligands (CdSe-OPV) provide evidence of strong electronic communication that facilitate charge and energy transport between the OPV ligands and the CdSe quantum dot core. This electronic interaction greatly modify, the photoluminescence properties of both bulk and single CdSe-OPV nanostructure thin film samples. Size-correlated wide-field fluorescence imaging show that blinking suppression in single CdSe-OPV is linked to the degree of OPV coverage (inferred from AFM height scans) on the quantum dot surface. The effect of the complex electronic environment presented by photoexcited OPV ligands on the excited state property of CdSe-OPV is measured with single photon counting and photon-pair correlation spectroscopy techniques. Time-tagged-time-resolved (TTTR) single photon counting measurements from individual CdSe-OPV nanostructures, show excited state lifetimes an order of magnitude shorter relative to conventional ZnS/CdSe quantum dots. Second-order intensity correlation measurements g(2)(τ) from individual CdSe-OPV nanostructures point to a weak multi-excitonic character with a strong wavelength dependent modulation depth. By tuning in and out of the absorption of the OPV ligands we observe changes in modulation depth from g(2) (0) ≈ 0.2 to 0.05 under 405 and 514 nm excitation respectively. Defocused images and polarization anisotropy measurements also reveal a well-defined linear dipole emission pattern in single CdSe-OPV nanostructures. These results provide new insights into to the mechanism behind the electronic interactions in composite quantum dot/conjugated organic composite systems at the single molecule level. The observed intensity flickering , blinking suppression and associated lifetime/count rate and antibunching behaviour is well explained by a Stark interaction model. Charge transfer from photo-excitation of the OPV ligands to the surface of the CdSe quantum dot core, mixes electron/holes states and lifts the degeneracy in the band edge bright exciton state, which induces a well define linear dipole behaviour in single CdSe-OPV nanostructures. The shift in the electron energies also affects Auger assisted hole trapping rates, suppress access to dark states and reduce the excited state lifetime.

In this dissertation, I report the results of my research on twisted moiré photonic crystals which can be formed through multi-beam holographic interference without a physical rotation and later fabricated by electron-beam lithography. Their optical properties, such as photonic bandgaps, multiple resonance modes, and quality factor are presented. Randomized moire photonic crystals in lattice are also studied. The applications of moire photonic crystals in improving light extraction efficiency are simulated and verified in light emitting devices. Furthermore, I simulated the light extraction efficiency in OLED when the Al layer is patterned with a triangular GPSC, square moiré PhC with defects in the uniform area, and random locations of the photonic lattice, and obtain light extraction efficiency of 78.9%, 79.9%, 81.7%, respectively. Also, the ratios of photoluminescence intensity of LED integrated with twisted moiré PhCs and random moiré PhCs over that without moiré PhCs are measured to be (1.3-1.9) and 1.74, respectively, in a good agreement with simulated ratios of 1.69 and 1.8.

No description available.

No description available.

No description available.

We have demonstrated a means of quantum control by all-optical control of valence electron spin polarization in the lithium dimer. The energy levels of interest are two closely spaced rovibrational levels (the G1Πg (12, 21f) and 13Sg- (1, 21f) states separated by 720 MHz). In the absence of any optical fields, the spin-orbit interaction couples the two states, resulting in each state being a mixture of the pure singlet and pure triplet states. The initial mixing coefficients are governed by the strength of the spin-orbit coupling between the two levels. In lithium dimer, this mixing is very weak; the nominal singlet (triplet) state is initially 87% singlet (triplet) and 13% triplet (singlet). When a strong coupling field is applied to the nominally singlet state, an Autler-Townes (AT) split pair is created. Since one of the AT components is pushed closer in energy to the nearby triplet state, the triplet state gains more singlet character. Since the AT splitting is dependent upon the magnitude of the applied coupling field Rabi frequency, the mixing coefficients of the perturbed pair can therefore be coherently controlled. Such control may be useful for applications in cold molecule formation and control of predissociation rates, and may also provide insight into the role that valence electron spin polarization plays in reactive collisions. / Physics

This research provides a proof of concept and background theory for the physics behind the state-of-the-art ultra-fast plasmonic spiking neurons (PSN), which can serve as a primary synaptic device for developing a platform for fast neural computing. Such a plasmonic-powered computing system allows localized AI with ultra-fast operation speed. The designed architecture for a plasmonic spiking neuron (PSN) presented in this thesis is a photonic integrated nanodevice consisting of two electro-optic and optoelectronic active components and works based on their coupling. The electro-optic active structure incorporated a periodic array of seeded quantum nanorods sandwiched between two electrodes and positioned at a near-field distance from the topmost metal layer of a sub-wavelength metal-oxide multilayer metamaterial. Three of the metal layers of the metamaterials form the active optoelectronic component. The device operates based on the coupling of the two active components through optical complex modes supported by the multilayer and switching between two of them. Both action and resting potentials occur through subsequent quantum and extraordinary photonics phenomena. These phenomena include the generation of plasmonic high-k complex modes, switching between the modes by enhanced quantum-confined stark effect, decay of the plasmonic excitations in each metal layer into hot-electrons, and collecting hot-electrons by the optoelectronic component. The underlying principles and functionality of the plasmonic spiking neuron are illustrated using computer simulation.