Nanosized coatings of ZrO₂ were deposited on silicon substrates using sol-gel and spin-coating techniques. The precursor solutions were prepared from ZrOCl₂.8H₂O with the addition of different percentage (0.5-5%) of rare earth Gd³⁺ ions as dopant. The thin films were homogeneous, with average thickness of 115 nm and refractive index (n) of 1.83. The X-ray diffraction analysis (XRD) revealed the presence of a varying mixture of monoclinic and tetragonal ZrO₂ polycrystalline phases, depending on the dopant concentration, all of which with nanosized crystallites. Scanning electron microscopy (SEM) as well as atomic force microscopy (AFM) methods were deployed to investigate the surface morphology and roughness of the thin films, respectively. They revealed a smooth, well uniform and crack-free surface with average roughness of 0.5 nm. It was established that the dopant concentration affects the photoluminescence (PL) properties of the samples. The undoped films exhibited broad violet PL emission, while the addition of Gd³⁺ ions resulted in new narrow bands in both UV and visible light regions, characteristic of the rare earth metal. These exact emissions can find useful applications in medical lamps or as red phosphors.

Acknowledgement

Research equipment of distributed research infrastructure INFRAMAT (part of Bulgarian National roadmap for research infrastructures) supported by Bulgarian Ministry of Education and Science under contract D01-284/17.12.2019 was used in this investigation.

Lanthanide-doped upconversion nanoparticles (UCNPs) are capable of converting multiple near-infrared (NIR) photons into shorter-wavelength one by utilizing real long-lived, ladder-like energy levels of lanthanide ions. Such unique anti-Stokes emissions are immune to the auto-fluorescence background interference and derive from moderate irradiation compared to conventional multi-photon fluorescence, ideally suitable for imaging applications, coupled with improved imaging depth by NIR excitation light, excellent photostability, multicolor sharp-band emissions and tunable long emission lifetimes. However, the critical concentration quenching on upconversion luminescence generally observed in UCNPs sets barriers to obtain bright high doping UCNPs at low excitation power density, which hampers this promising luminescence probe applying to long-term live cell imaging. We explore the mechanism of Tm³⁺ concentration quenching from ensemble and single-particle levels to design the new generation sub-25-nm ultra-bright highly Tm³⁺-doped UCNPs for high-contrast imaging at limited excitation power density condition. This achievement provides a new strategy for developing small-sized bright highly activator-doped UCNPs and will broaden the applications of UCNPs in microscopic imaging.

The beginnings of Metal-Organic Frameworks (MOFs) chemistry were established by Yaghi et al. in the 90s. They started a new promising field for which, depending on the nature of the organic functionality and metal−ligand coordination chemistry, diversity of MOFs in terms of their structures and chemical properties is virtually endless. Since the 90s it has been reported different synthetic routes (e.g. hydro-solvothermal synthesis, microwave and ultrasound-assisted synthesis, mechanochemistry, microemulsion synthesis, continuous flow production). Nevertheless, no control on the shape and size of the crystal was achieved in a proper way. The results obtained during this work demonstrates that the surfactant plays an important role in the MOF´s synthesis protocol, in particular, in those with Zeolitic Imidazolate Framework (ZIF-8) structure, by changing the former physico-chemical properties without altering their crystalline structure. That is, variations on surfactant´s properties lead to changes both in shape and size of MOFs without altering their intrinsic properties. Thus, this work is focused on the effect of two surfactants: Sodium Dodecyl Sulfate (SDS) and Hexadecyltrimethylammonium bromide (CTAB). In this sense, for each family of surfactant the influence of the surfactant tail chain length and the nature of their head group were investigated. For these studies, dynamic laser-light scattering (DLS), scanning electron microscopy (SEM) and powder X-Ray Diffraction (PXRD) were performed in order to characterize the physicochemical properties and the morphology of the obtained MOFs.

Acknowledgements: This work was supported by Agencia Estatal de Investigación (AEI) through Project MAT2016-80266-R, and Xunta de Galicia (Grupo de Referencia Competitiva ED431C 2018/26; Agrupación Estratégica en Materiales-AEMAT ED431E 2018/08). FEDER funds are also acknowledged.

Bulk waveguides (BWGs) in nanoporous materials are promising to be applied in photonics and sensors industries. Such light guiding components interrogate the internal conditions of nanoporous material and are able to detect chemical or physical reactions occurred inside nanopores especially with small molecules, which represent a separate class for sensing technologies.

Recent years have demonstrated remarkable progress in the design and fabrication of optical porous glass (PG) sensors applied for monitoring and controlling different media and object parameters. Basically, such sensors consist of three main parts: a light source, receiver, and primary transducer. The primary transducer is a PG plate, which stores indicator; it is ready to absorb the target molecules. When the PG sensor is placed in an environment with target molecules, the primary transducer converts the chemical reaction occurring in nanopores into a measurable optical signal, for example, the absorption of radiation from the light source at a certain spectral range.

In this work, we suggest a novel concept of the primary transducer. It is the BWG inside a PG plate fabricated by the laser direct writing technique using a femtosecond laser (220 fs, 1035 nm, 1 MHz). After the writing step, PG plates are impregnated with the indicator - rhodamine 6G, which penetrates through the nanoporous framework of glass to the BWG cladding. The radiation transmitted through BWG interacts with the indicator generating a fluorescence signal at the output. The fluorescence spectrum is sensitive to chemical reactions occurred in the nanoporous framework in the cladding. The sensitivity of the peak shift in the fluorescence spectrum to the refractive index of the solution is quantified as 6250 ± 150 nm/RIU.

The results obtained open an opportunity to build a novel sensing photonics platform for detecting small doses of nanoscale objects captured by porous glass.

Two-dimensional materials allow for extreme light confinement, thus becoming important candidates for all optical application platforms. Monolayers of transition-metal dichalcogenides are direct band gap semiconducting 2D materials featuring bandgaps in the visible and near-IR range, strong excitonic resonances, and high oscillator strengths, among other properties, as well as, supporting exciton polaritons. The optical properties of quantum emitters, such as molecules or quantum dots, near single or multilayer transition-metal dichalcogenides have been investigated, where the relaxation rate of the quantum emitter increases or decreases. The studies on the coupled quantum emitter - transition-metal dichalcogenides remain so far in the weak light-matter coupling regime. In this work, we study the spontaneous emission spectrum of a two-level quantum emitter near a WS₂ layer, in which case the Purcell factor of the QE can take values up to 10⁴. We further study the Rabi splitting in the spontaneous emission spectrum at room temperature for a quantum emitter with free-space decay times in the 10 ps to 500 ps range. We observe that at close distance of the quantum emitter to the WS₂ layer, combined with short decay times, the spectrum can feature several peaks. In such cases, the Rabi splitting lies between 0.25 eV and 0.05 eV, for increasing free-space decay times, indicating strong coupling conditions for the light-matter interaction between the quantum emitter and the WS₂ layer. Moreover, no simple relation between the inverse free-space decay time and the corresponding Rabi splitting value has been found. As the distance between the quantum emitter and the layer increases farther, the light-matter interaction coupling enters the weak coupling regime, which leads to vanishing Rabi splitting in the spontaneous emission spectrum for free-space decay times larger than a few tenths of ps.