Single-photon emitters (SPEs) are quantum systems that can produce individual photons when excited. These photons can be manipulated in their polarization states to encode quantum bits, which are the quantum-mechanical analogs of classical bits. SPEs are critical to the development of quantum information technology applications, including quantum communication, computing, and sensing. Despite their importance, there are currently no solid-state SPEs that meet the requirements for large-scale applications. Researchers have explored various materials hosts, including quantum dots, carbon nanotubes, and bulk semiconductors, but many challenges remain. For example, producing scalable and integrated SPEs with tunable wavelengths, high clocking rates (brightness), and single-photon purity at room temperature is still an ongoing research goal. In recent years, there has been significant research interest in single-photon emitters (SPEs) in two-dimensional (2D) Van der Waals (VdW) materials. Most research in this area has focused on SPEs in multilayer insulating hexagonal boron nitride (hBN), which can be operated at room temperature, and monolayer tungsten diselenide (WSe2), which is a direct bandgap semiconductor. The SPEs in hBN are derived from defect emission, while those in monolayer WSe2 stem from either defect or strain-bound excitons. Despite this promising research, there are critical challenges that impede the development of these SPEs. For example, hBN is an insulator with a band gap of 6.0 eV, which limits electrical control, and controlling defects is difficult. Additionally, the photo-stability of monolayer WSe2 is vulnerable to environmental fluctuations, such as surface contaminants. Multilayer gallium selenide (GaSe) is another 2D Van der Waals (VdW) SPE host, and the initial experimental observation of GaSe SPEs was reported by Tonndorf. et al. in 2017.2,3 However, GaSe SPEs have received less attention compared to hBN and WSe2 for several reasons. Firstly, early reports2,3 show that GaSe SPEs arising from defects are less brighter than SPEs in WSe24 and hBN.5 Secondly, increasing the laser power for brighter GaSe SPEs would cause the formation of biexcitons, which degrades the single photon purity.2 Since 2017, to the best of our knowledge, there have been no further experimental studies conducted on overcoming those challenges to improve the performances of GaSe SPEs. In this dissertation, I present three research projects focused on addressing the challenges of developing single-photon emitters (SPEs) in multilayer gallium selenide (GaSe). First, I achieved localized bright and stable GaSe SPEs in multilayer GaSe through the manipulation of nanoscale strain. Second, I performed below-diffraction limit hyperspectral imaging of strain-localized GaSe SPEs through cathodoluminescence and demonstrated the wide spectral range tunability, significant enhancement of emission intensities controlled by nanoscale strain, as well as the robust spectral stability of GaSe SPEs. In the last project, I demonstrated a 30%-50% improvement in emission intensities of GaSe, converted non-SPEs to SPEs, and increased operating temperatures from 23 K up to 85K above cryogenic temperature through electrostatic doping. The research works in this dissertation lays a crucial foundation for future fundamental studies and the development of GaSe SPEs and their analogues.

We report on the systematic characterization of the optical properties of diamond color centers based on Pb impurities. An ensemble photoluminescence analysis of their spectral emission was performed at different excitation wavelengths in the 405–520 nm range and at different temperatures in the 4–300 K range. The series of observed spectral features consist of different emission lines associated with Pb-related defects. Finally, a room-temperature investigation of single-photon emitters under 490.5 nm laser excitation is reported, revealing different spectral signatures with respect to those already reported under 514 nm excitation. This work represents a substantial progress with respect to previous studies on Pb-related color centers, both in the attribution of an articulated series of spectral features and in the understanding of the formation process of this type of defect, thus clarifying the potential of this system for high-impact applications in quantum technologies.

Dans les dernières années nombre de matériaux lamellaires à dimensions réduites ont démontré des propriétés optiques remarquables. Cependant, la plupart des études ont porté sur le système parfait et le rôle des défauts en tant que centres optiques actifs restent encore largement inexploré. Le nitrure de bore hexagonal (h-BN) est l'un des candidats les plus prometteurs pour les dispositifs émetteurs de lumière dans la région de l’UV lointain, présentant une forte émission excitonique à 5,8 eV. Cependant, émission n’apparaît uniquement que dans des monocristaux très purs qui peuvent difficilement être obtenus que par des procédés de synthèse complexes. Les échantillons ordinaires de h-BN présentent des spectres d'émission plus complexes qui ont été généralement été attribuée à la présence de défauts structuraux. Malgré un grand nombre d'études expérimentales jusqu'à présent il n'a pas été possible d'attribuer cette émission additionnelle à des défauts structuraux bien définis. Nous abordons ici cette question fondamentale en adoptant une approche théorique et expérimentale combinant une technique de cathodoluminescence nanométriquement résolu avec une caractérisation structural résolu atomiquement par microscopie électronique a transmission et de l'état de l'art de simulations excitoniques. Très récemment, l'équipe d'Orsay a mis au point un système de détection de cathodoluminescence intégré au sein d'un microscope électronique à transmission à balayage. Cette expérience unique est maintenant en mesure de fournir des spectres d'émission complet avec une résolution aussi faible que quelques dizaines de MeV associés à une taille de sonde électronique du nanomètre. Une image hyper-spectrale cathodoluminescence peut donc être enregistrée en parallèle avec une image HAADF. La cathodoluminescence résolu au nanomètre sur quelques-couche chimiquement exfoliée de h-BN a montré que les spectres d'émission sont fortement inhomogènes dans les feuillets individuels. Les pics d'émission à proximité de l'exciton libre apparaissent dans des régions étendues. Les examens complémentaires par microscopie électronique à transmission à haute résolution permettent d'associer ces raies d'émission avec des défauts étendue dans le cristal tels que les défauts d'empilement et les plis des facetter. Au moyen de calculs ab-initio dans le cadre de la « Many Body perturbation theory » (GW) et l'équation de Bethe-Salpeter nous fournissons une description détaillée de la structure électronique et la réponse spectroscopique du nitrure de bore hexagonal en présence de défaut d’empilements. En particulier, nous montrons un bon accord avec les résultats expérimentaux, les excitons supplémentaires sont associées à des changements de symétrie locaux qui se produisent par des fautes d'empilement dans le cristal. Ce résultat sera ensuite étendu à des nanotubes de BN à parois multiples. Des émissions supplémentaires qui apparaissent à l'intérieur du gap présentent une localisation spatiale élevée, typiquement inférieure à 100 nm, et par conséquent ils peuvent être liés à des défauts ponctuels individuels. Lorsqu’ils sont adressés individuellement à travers une sonde électronique très ciblé, ils pourraient avoir un caractère d’émetteur de photon unique. Cette hypothèse a été récemment confirmée par des expériences combinant notre système de cathodoluminescence avec un interféromètre Handburry-Brown et Twiss (HBT). / Within the latest years number of layered materials at reduced dimensions have demonstrated remarkable optical properties. However most studies focused on perfect system and the role of defects as optical active centers remain still largely unexplored. Hexagonal boron nitride (h-BN) is one of the most promising candidates for light emitting devices in the far UV region, presenting a single strong excitonic emission at 5.8 eV. However, a single line appears only in extremely pure mono-crystals that can hardly be obtained only though complex synthesis processes. Common h-BN samples present more complex emission spectra that have been generally attributed to the presence of structural defects. Despite a large number of experimental studies up to now it was not possible to attribute specific emission features to well identify defective structures. Here we address this fundamental question by adopting a theoretical and experimental approach combining few nanometer resolved cathodoluminescence techniques with high resolution transmission electron microscopy images and state of the art quantum mechanical simulations. Very recently, the Orsay team has developed a cathodoluminescence detection system integrated within a scanning transmission electron microscope. This unique experimental set up is now able to provide full emission spectra with a resolution as low as few tens of meV associated with an electron probe size of one nanometer. A cathodoluminescence hyper-spectral image can thus be recorded in parallel with an HAADF image. Nanometric resolved cathodoluminescence on few-layer chemically exfoliated h-BN crystals have shown that emission spectra are strongly inhomogeneous within individual flakes. Emission peaks close to the free exciton appear in extended regions. Complementary investigations through high resolution transmission electron microscopy allow to associate these emission lines with extended crystal deformation such as stacking faults and folds of the planes. By means of ab-initio calculations in the framework of Many Body Perturbation Theory (GW) approximation and Bethe-Salpeter equation) we provide an in-depth description of the electronic structure and spectroscopic response of bulk hexagonal boron nitride in the presence of extended morphological modifications. In particular we show that, in a good agreement with the experimental results, additional excitons are associated to local symmetry changes occurring at crystal stacking faults. These result will then be extended to faceted multiwalled BN nanotubes, they display additional emission at the same energy as characterized within the flakes.

<p dir="ltr">This thesis is about the exciting discovery of a new kind of single photon emitter that<br>is suspected to occur at the interface of silicon nitride SixNy and silicon dioxide SiO2 after<br>being rapidly annealed. Since SixNy is one of the most developed platforms for integrated<br>photonics the discovery of a native emitter in this platform opened up the possibility for<br>seamless integration of these single photon emitters with photonic circuitry for the first<br>time. This seamless integration was demonstrated as is shown in Chapter 3 by creating the<br>emitters and then patterning the SixNy layer into a waveguide. This work demonstrated for<br>the first time the coupling of such single photon emitters with on-chip integrated photonics.<br>However, the integration approach demonstrated was based on the stochastic integration of<br>emitters which limits the efficiency of the devices and the possible types of devices that can<br>be designed. This is why the next stage of research focused on the development of a site-<br>controlled process for creating these single photon emitters. Remarkably, it was found that<br>if the SixNy and SiO2 are nanostructured into nanopillars and then annealed then a single<br>photon emitter forms over 65% of the time within the nanopillar! Due to the lithography<br>defined nature of this process for creating the single photon emitters the first multi-mask<br>integration process was also developed and demonstrated. This fabrication process was used<br>to demonstrate the integration of several thousand single photon emitters with complex<br>integrated photonic structures such as topology optimized couplers. These developments<br>has generated a great deal of excitement due to the inherent scalability of the approach and<br>it’s obvious applications for the development of very large scale integrated (VLSI) on-chip<br>quantum photonic systems.</p>