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<p>Due to the rapid development of nanophotonics, microring resonators suspended on a membrane holds promises for a scalable optical circuit with strong light-atom interaction. In this dissertation, I introduce a efficiently-coupled microring circuits for on-chip cavity QED with cold atoms and report my experimental efforts to integrate the optical chip into a ultrahigh-vacuum chamber with a magneto-optical trap for Rb atoms. My attempts to load single atoms into optical tweezers are also discussed.</p> <p>  </p> <p>  Although the loading of atom into optical tweezers above the top surface of resonator remains a challenge in experiment, I propose an alternative of cavity cooling based on cavity QED to facilitate the loading of atom into a two-color evanescent field trap around the waveguide. Assuming that the strong interaction between atoms and resonator modes is realized, I theoretically investigate the synthesis via photoassociation and the direct optical detection of a single ground-state cold molecule, whose corresponding excited-state has multiple decay channels. Similarly to the Purcell effect, the decay in a specific decay channel could be enhanced based on cavity QED, and therefore the synthesis efficiency can approach unity when the interaction between the resonator modes and a single cold molecule becomes stronger. In addition, for a single cold molecule without closed optical transition, the electromagnetically induced transparency is possible to be observed on our nanophotonic platform in the case of strong resonator-molecule coupling.</p>

Some aspects of grain boundary segregation (GBS) are discussed. This paper adds two new sprouts. The first is connected with formation of the atomic complexes in boundary region and their effect on grain boundary diffusion (GBD). The second – with a nonhomogeneity of energy distribution between boundary sites.

<p dir="ltr">Parity non-conversation experiments provide a method to explore the weak interaction. Precision measurements of the weak interaction will lead to more limitations on beyond the standard model theories. Our lab will use a two-color coherent control to help us to extract the small amplitude of the weak interaction between the nucleons of the cesium nucleus. In this dissertation, I will discuss how our lab is investigating the anapole moment in cesium using rf and laser fields, along with future steps to finish the measurement.</p>

<p dir="ltr">In this work we investigate the existence and experimentally measurable properties of Majorana quasiparticles in a few-body number conserving atomic system.</p>

<p>This dissertation discusses theoretical methods for describing photoionization in different systems in the context of time-dependent and time-independent non-relativistic and time-independent relativistic systems. We introduce a multichannel quantum defect theory (MQDT) model for describing photoionization in the context of pump-probe experiments. The basics of MQDT are introduced and specialized to the argon atom. Two energy regimes are studied in detail and compared to the experiment: (i) a perturbative calculation describing the dynamics of an autoionizing wave packet, (ii) a time-resolved calculation describing the two-photon ionization of a deeply bound wave packet. In both cases, the model accurately describes the relative ionization with respect to the two spin-orbit split thresholds of the ion and the oscillations shown in the delay between the pump and probe. We finalize with a brief presentation, which is primarily pedagogical, of how to use MQDT inside a finite box.</p> <p>Next, we use MQDT to describe the ytterbium atom in different energy regimes and varying degrees of approximation. The motivation behind this lies in the context of quantum information science, but our study is only concerned with calculating atomic properties. We start with a minimal MQDT model to describe the data observed in the experiment, followed by the presentation of an ab initio two-electron model. Both models compare very well to the experiment, and the ab initio method compares favorably with older spectroscopic results. In addition, we show unpublished results that incorporate the hyper-fine effects into the approximate model.</p> <p>Finally, we present an implementation of the two-electron variational R-matrix method for the Dirac equation, including the complete derivation of the solution of the Dirac equation in a central potential. We provide explicit analytic forms for the solutions of the Coulomb potential and use them to derive the generalized quantum defect parameters. A discussion of the variational R-matrix method for the Dirac equation in single and multichannel contexts is presented, with sample calculations for the beryllium and radium atoms. A chapter that summarizes and points to future work for each one of the projects concludes the work.</p>

<p dir="ltr">Following the development of laser technology in the 1960s, radiation pressure was soon employed to manipulate particles by Arthur Ashkin in the 1970s. Since then, levitated optomechanics has been widely studied across physics, engineering, chemistry, and biology. In this dissertation, we first experimentally demonstrate GHz rotation and sensing with an optically levitated nanodumbbell near a surface. Then, we propose achieving optical refrigeration below liquid nitrogen temperature using near-field Purcell enhancement.</p><p dir="ltr">The first part of this dissertation focuses on fast rotation and sensing with a non-spherical silica nanoparticle levitated near a surface. Specifically, we optically levitate a nanodumbbell at 430 nm away from a surface in high vacuum and drive it to rotate at 1.6 GHz. This corresponds to a relative linear velocity of 1.4 km/s between the tip of the nanodumbbell and the surface at sub-micrometer separation. The near-surface rotating nanodumbbell demonstrates a superior torque sensitivity of (5.0 +/- 1.1 ) x 10^-26 Nm at room temperature. Our numerical simulation shows that such an ultra-sensitive nanodumbbell levitated near nanostructures can be used to detect fundamental physics, such as Casimir torque and non-Newtonian gravity. </p><p dir="ltr">In the latter part of this dissertation, we propose that optical refrigeration of solid with anti-Stokes fluorescence can be enhanced by Purcell effect. The spontaneous emission rate of high-energy photons is Purcell enhanced by coupling with a near-field cavity. The enhanced emission shifts the mean emission wavelength and enables optical refrigeration with high-absorption cooling laser. We estimate a minimum achievable temperature of 38 K with a Yb³⁺:YLiF₄ nanocrystal near a cavity using our proposed Purcell enhanced optical refrigeration method. This method can be applied to other rare-earth ion doped materials and enable applications that require solid-state cooling below liquid nitrogen temperature.</p>

<p dir="ltr">Quantum sensing and information processing rely increasingly on solid-state spin defects, which offer robust qubit candidates at room temperature. Among these, nitrogen-vacancy (NV) centers in diamond have been extensively studied, but the discovery of spin defects in two-dimensional (2D) van der Waals (vdW) materials, particularly hexagonal boron nitride (hBN), has opened new avenues for compact, scalable quantum devices. The unique 2D structure of hBN enables its integration into nanoscale devices, where spin defects like the negatively charged boron vacancy serve as optically addressable qubits with promising optically detected magnetic resonance (ODMR) properties, making them highly suitable for ambient-condition quantum sensors and information storage.</p><p><br></p><p dir="ltr">The first part of this dissertation investigates the controlled generation, characterization, and functionalization of spin defects in hBN, focusing on boron vacancy defect ensembles. Techniques such as laser writing and ion implantation are used to create these defects, while plasmonic enhancement strategies significantly improve brightness and optical visibility. Pulsed ODMR measurements are used to analyze the spin coherence properties, revealing extended coherence times crucial for high-sensitivity applications.</p><p><br></p><p dir="ltr">In the second part, we explore carbon-related defects within both hBN and boron nitride nanotubes (BNNTs), where single defects exhibit unique hyperfine interactions. By combining experimental studies with density functional theory (DFT) calculations, this work identifies the atomic structures and electronic properties of these carbon-based defects. In BNNTs, carbon-related spin defects are examined for their potential in high-resolution magnetic imaging when used in scanning probe microscopy.</p><p><br></p><p dir="ltr">This research advances our understanding of spin defects in 2D materials, laying essential groundwork for future innovations in quantum information storage, nanoscale magnetic sensing and on-chip quantum technologies.</p>

La génération d'harmoniques d'ordre élevé en milieu gazeux est un phénomène décrit par un modèle à trois étapes: sous l'effet d'un champ laser intense, un atome (ou une molécule) est ionisé par effet tunnel. L'électron éjecté est par la suite accéléré dans le champ laser, avant de se recombiner sur son ion parent en émettant un photon XUV. D'abord utilisée dans le but de développer des sources de rayonnement secondaire dans le domaine XUV, la génération d'harmoniques d'ordre élevé est également un bon candidat pour sonder la structure électronique des atomes ou des molécules, avec une résolution potentielle de l'ordre de l'attoseconde dans le domaine temporel (1 as=10-18 s) et sub-nanométrique dans le domaine spatial.Au cours des travaux réalisés pendant cette thèse, nous avons étudié la sensibilité des caractéristiques du rayonnement harmonique (amplitude, état de polarisation, phase) à la structure électronique du milieu de génération. Ces études ont été menées tout d'abord dans un milieu atomique couramment utilisé en génération d'harmonique, l'argon, puis dans des milieux moléculaires (N2, CO2, O2). La confrontation de ces mesures avec différentes simulations numériques montre la nécessité de modéliser de façon détaillée le processus de génération, dépassant certaines hypothèses généralement admises.Nous avons également montré la possibilité d'utiliser la spectroscopie d'harmoniques d'ordre élevé afin de mesurer des dynamiques moléculaires de systèmes complexes (notamment le dioxyde d'azote NO2), pour lesquelles les mesures harmoniques peuvent obtenir des résultats complémentaires aux autres techniques couramment utilisées. Dans le cas d'excitations moléculaires peu efficaces, nous avons pu adapter des techniques de spectroscopie optique conventionnelle au domaine spectral des harmoniques d'ordre élevé, améliorant de manière significative le rapport signal/bruit. / High harmonic generation is a well known phenomenon explained by a “three step” model: because of the high intensity field generated by an ultrashort laser pulse, an atom or a molecule can be tunnel ionized. The ejected electron is then accelerated by the intense electric field, and eventually can recombine on its parent ion, leading to the emission of a XUV photon. Because of the generating process in itself, this light source is a promising candidate to probe the electronic structure of atoms and molecules, with an attosecond/sub-nanometer potential resolution (1 as=10-18 s).In this work, we have studied the sensitivity of the emitted light (in terms of amplitude, but also phase and polarization) towards the electronic structure of the generating medium. We have first worked on atomic medium, then on molecules (N2, CO2, O2). Comparing the experimental results with numerical simulations shows the necessity to model finely the generation process and to go beyond commonly used approximations.We have also shown the possibility to perform high harmonic spectroscopy in order to measure dynamics of complex molecules, such as Nitrogen Dioxide (NO2). This technic has obtained complementary results compared to classical spectroscopy and has revealed dynamics of the electronic wavepacket along a conical intersection. In this experiment, we have adapted conventionnal optical spectroscopy technics to the XUV spectral area, which significantly improved the signal over noise ratio.

Understanding the complexities of the interior of planets and stars requires the help of analyzing the effects of high pressures on certain elements believed to be found within. The Hartree-Fock method uses the Schr¨odinger equation, Kummer’s differential equations and a confinement potential to simulate an atom being squeezed to high pressures. The Hartree-Fock method was used to calculate the total energies of atoms. After being compared to Gaussian03 and VASP, the results were deemed accurate. It was also observed that the pressure versus density data closely approximated those pairs found in outer space in the interiors of, for example, Jupiter.