Phd in PHYSICS

Thesis title: Two-dimensional materials acting as single-photon sources and their integration with photonic cavities

Photonic quantum technologies exploit the generation of single photons from solid-state systems for a variety of uses and applications, e.g. quantum computation, quantum communication, quantum cryptography and metrology. This field, therefore, is always on the lookout for new sources from which to extract quantum light. The photons emitted by these materials can reliably act as qubits, and be used for the aforementioned applications, only if they are able to satisfy stringent conditions. Indeed, photonic quantum sources are required to meet certain thresholds in terms of deterministic generation, purity, indistinguishability, efficiency and, if need be, entanglement. Among the newest materials proposed as efficient sources of single photons, two-dimensional (2D) crystals have gathered the attention of many researchers, due to their peculiar structural, optical and electronic properties. The research field of 2D materials traces its origins back to the first successful isolation of graphene by Geim and Novoselov in 2004, but in the last twenty years the scope of possible crystals and of their applications has broadened more than anyone could have envisioned. In particular, semiconducting Transition Metal Dichalocogenides (TMDCs) are ideal candidates for flexible optoelectronic devices, as well as efficient sources of quantum light at cryogenic temperatures. The layered structure of these crystals allows for the mechanical exfoliation of single crystalline planes, accompanied by an abrupt jump in radiative efficiency due to the indirect-to-direct bandgap transition of TMDCs, when thinned down to the monolayer (ML) limit. Indeed, in their 2D ML form, TMDCs are efficient light emitters, whose all-surface nature allows for the tuning of their electronic and optical properties via external perturbations. One of the most straightforward strategies to gain control over the optoelectronic properties of TMDCs crystals is to induce a mechanical deformation (strain) of their lattice structure, an approach particularly effective due to the 2D geometry of these systems. In particular, the introduction of strain in the lattice has been widely regarded as the key ingredient for the activation of single-photon emitters (SPEs) in low-temperature TMDC MLs, ever since the first experimental reports of these emitters (in 2015). In this thesis, strain in TMDC MLs is achieved by irradiating bulk crystals with low-energy hydrogen ions. Hydrogen irradiation leads to the formation of one-layer thick bubbles on the surface of the crystal; these bubbles are filled with molecular hydrogen at high pressure, so that the ML experiences high values of strain, changing with continuity along the bubble's surface. The first part of the thesis deals with a fabrication strategy to avoid the bubbles' deflation at low temperature, due to solid-to-liquid phase transition of molecular hydrogen for temperatures below 32 K. It is found that the deposition of few-layer hexagonal boron nitride (hBN) on top of the bubbles (capping) ensures that they remain in shape at all temperatures, regardless of the hydrogen phase within. hBN-capped WS2 bubbles show the appearance, at cryogenic temperatures, of intense narrow lines in their photoluminescence (PL) spectra. That these lines are the optical signature of strain-activated quantum emitters is confirmed by autocorrelation measurements, assessing the purity of the emitted single photons. A full optical characterization of these emitters is provided via power-dependent PL studies, time-resolved PL spectroscopy, and magneto-PL spectroscopy. The performances of these newfound quantum emitters are the object of the second section of this thesis, which details experiments aimed at the enhancement of the emitters' brightness and radiative rate, via their integration with optical cavities i.e., with Circular Bragg Gratings (CBGs, commonly known as "Bullseye" cavities). The use of cavities to improve the efficiency of quantum emitters is a widespread approach, which is based on the Purcell effect, i.e. the modification of an emitter radiative transition due to the surrounding environment. These structures etched on the surface of SiN layers are optically characterized in reflectivity to identify the correct geometrical parameters, linked to the desired resonances. The use of hydrogen-filled bubbles serves, here, a double purpose: in a first experiment bubbles are formed on the surface of hBN flakes, therefore acting as mere stressors for a WSe2 ML deposited on top of them; in a second experiment, on the other hand, the bubbles are formed directly on the surface of thin WS2 flakes deposited on top of the BE cavities, whose wavelength resonance is modified accordingly. The last section of this thesis explores alloying as a technique to broaden the tunability range of SPEs in TMDC crystals. The investigated alloy crystal is WSSe, in which the lattice sites reserved to the chalcogen atoms are randomly occupied by a S or Se atom with equal probability. A study of the different optical properties of WSSe MLs deposited on SiO2 or hBN substrates is carried out, showing the appearance of strain-activated narrow lines at low temperature and proving their purity as SPEs. The formation of hydrogen-filled bubbles on the surface of WSSe is studied extensively, with the purpose of identifying possible interplays caused by the combined effects of strain and alloying. The elastic parameters, adhesion energy and optical properties of WSSe bubbles are shown to be linear combinations of the ones of the "parent" crystals WS2 and WSe2, as shown by AFM and PL measurements performed on several bubbles. Finally, the effects of hBN capping on WSSe bubbles are explored by monitoring the PL spectra of bare and capped domes for different temperatures of the samples.