Exploring optically active defects in wide-bandgap materials using fluorescence microscopy

Defects in solid-state systems can be both detrimental, deteriorating the quality of materials, or desired, thanks to the novel functionality they bring. Optically active point defects, producing fluorescent light, are a great example of the latter. Naturally existing in various materials, of which the so-called wide-bandgap materials constitute a major part, they can be used as sensors, single-photon emitters or even quantum bits. As the defects preferentially absorb only specific wavelengths of light, the whole material can acquire a visible macroscopic color, becoming the more intense the more there are defects in its lattice. Due to this fact such defects are commonly referred to as "color centers". The most famous example of color centers is the nitrogen-vacancy (NV) center in diamond, consisting of a nitrogen atom that substitutes carbon next to a vacancy (a missing carbon atom) in the diamond lattice. From the 1990s NV centers have been at the forefront of the second quantum revolution, enabling countless experimental demonstrations of quantum phenomena, even at room temperature. Being currently widely used everywhere from secured telecommunication networks to living cells, diamond NV centers have sparked a persistent interest into novel fluorescent defects, both in diamond (e.g. silicon-, germanium- and tin-vacancy centers) and in other wide-bandgap materials. Very recently a novel class of material platforms hosting fluorescent defects has emerged -- namely layered van der Waals (vdW) materials, which can be thinned down to an ultimate single-atom thickness, opening the door into the realm of two dimensional (2D) materials. This area of research has virtually exploded after the discovery of graphene in 2004, followed by continuous reports of the superb mechanical, electrical and optical properties of graphene-based devices. The whole family of graphene-like vdW materials was rapidly and continuously expanding with new members (graphene oxide, fluorographene, borophene, transition-metal dichalcogenides (TMDCs), layered perovskites, etc.), each of which was enabling various functionalities. This thesis explores the properties of newly discovered color centers in a layered vdW wide-bandgap semiconductor - hexagonal boron nitride (hBN). These optically-active defects have shown themselves as exceptionally bright single-photon emitters (SPEs) and optically-addressable spin defects that hold a great promise for quantum sensing and quantum information processing. In this work I have shown how optical super-resolution techniques (specifically, the single-molecule localization microscopy, SMLM) can be used to study the properties of emitters in hBN, including their spectra and temporal dynamics. By engineering a specialized waveguide-based imaging platform I managed to overcome certain limitations of SMLM-based imaging and further showed how the very same imaging platform can be used for the nanophotonic on-chip integration of hBN via direct growth. In addition, I have explored the behaviour of hBN defects in aqueous solutions and how there they can be used as nanoscale charge sensors, tracking the diffusion of single protons. Finally, I developed a novel method for the deterministic engineering of optically-active defects in hBN via focused ion beam (FIB) irradiation. All together these findings pave the way for the use of optically-active defects in hBN for applications in nanophotonics, nanofluidics and nanoscale sensin