European Theoretical Spectroscopy Facility

We studied the intersystem crossing (ISC) of neutral, monoanion, and dianion of a series of phenyl, pyrazole, and triazole fused perylene bisimides (PBI) derivatives. The neutral compounds exhibit high fluorescence quantum yields (ΦF = 66.5%-91.9%) and low singlet oxygen quantum yields (ΦΔ < 7.8%), with no triplet state signals detected via femtosecond transient absorption (fs-TA) or nanosecond transient absorption (ns-TA) spectral measurements. Time-resolved electron paramagnetic resonance (TREPR) revealed an e,e,e,a,a,a electron spin polarization (ESP) for the T1 state of these PBIs. Calculations of spin-orbit coupling matrix elements (SOCMEs) and g-tensor anisotropy both confirm the weak spin-orbit coupling (SOC) of the molecules, accounting for their low ISC efficiency. Pulse electron paramagnetic resonance (pulse EPR) measurements of radical anions showed that PBI derivatives lacking pyrazole/triazole moieties have much longer transverse relaxation times (Tm = 2.5-2.6 µs) than the other compounds. Additionally, ns-TA spectra confirmed ISC in PBI dianions, which possess long-lived triplet states (213-558 µs). These species can be used as super strong reductants in photocatalytic redox organic reactions.

Cobalt sulfide (Co–S) phases are of significant interest for energy conversion and sensing technologies due to their combined catalytic activity and electronic conductivity. In particular, Co9S8 is a promising precatalyst for oxygen evolution reaction, but its synthesis as high-quality thin films has been scarcely reported. Here, we demonstrate the synthesis of highly crystalline, coalescent, mixed-phase Co–S thin films using plasma-assisted molecular beam epitaxy (PAMBE). The epitaxial film exhibits substantially improved crystallinity of Co9S8 compared to conventional deposition techniques and reveals the formation of epitaxial, tetragonal CoS2, which has not been previously observed in this nonmagnetic form. Thus, this work provides crucial diagnostic tools supporting the fabrication of phase-pure, single-crystal Co–S thin films and, with the discovery of the nonmagnetic CoS2 phase, a new platform for investigating Co–S catalysts in energy and sensing applications.

Photonic devices greatly benefit from the ability to reconfigure their optical properties. To achieve this with disordered optical materials, it is possible to make use of polymer dispersed liquid crystals (PDLCs) in which the scattering originates from randomly distributed liquid crystal droplets that respond to external stimuli, such as light, temperature, and magnetic or electric fields. In this work, we present a speckle-based analysis to investigate the reconfigurability of the dynamic and static scattering properties of dye-doped PDLCs, as a function of dye concentration and light intensity. Our findings allow us to distinguish between reversible and non-reversible molecular rearrangements after a light-induced phase transition, occurring on millisecond timescale. The degree of reversibility is quantified using cross-correlation analysis based on the Pearson correlation coefficient. As a proof-of-concept application, we exploit their non-reversible reconfiguration properties to create a cryptographic optical physical unclonable function whose behavior can progressively be erased in case of malicious attacks. We demonstrate a non-reversibility, assessed through the uniqueness metric, that ranges from 30% to 40% depending on dye content. Leveraging their intensity-dependent scattering properties, these disordered materials emerge as ideal candidates for advanced nonlinear information processing and secret-free security applications.

Stacking defects in noble metal nanoparticles influence their optical, catalytic, and electrical properties, but the mechanisms behind these changes are in many cases not well understood. While some mechanisms behind the formation of stacking defects have been studied, the possibility to deliberately manipulate the nanoparticle bulk morphology remains largely unexplored. Here, we introduce a vacancy-driven twinning mechanism that transforms face-centered cubic (fcc) gold nanoparticles into locally hexagonal close-packed (hcp) structures upon reaching a critical vacancy concentration. Atomistic simulations reveal that this transformation leads to multiply twinned morphologies, including cross-twinning patterns and stable hcp domains spanning 2-6 atomic layers. Experimentally, we validate this mechanism through thermal treatment of Au-Pt alloy nanoparticles, with multidomain X-ray diffraction (MDXRD) confirming controllable ordering and disordering transitions in the bulk structure. Furthermore, we investigate how the nanoparticles' bulk morphology influences their optical properties. Using time-dependent density-functional theory with Hubbard U corrections (TDDFT+U), we analyze the plasmonic response of models with specific defect motifs demonstrating that structural defects influence the absorption profile. Finally, we discuss how these morphology-induced changes in the electronic structure and plasmonic behavior could potentially modulate electrochemical activity in catalytic systems.

ABSTRACT Molybdenum chalcogenides are one of the most studied layered materials. They have hexagonal symmetry in the single layer case as many other van der Waals materials. Thus, they can serve as a model for the analysis of many physical properties, in particular the phonon modes, the appearance of new low wave number modes when going from the single layer to the bulk, the existence of a flexural acoustic mode and the grouping of the optical modes into pairs whose difference comes from the interlayer coupling. In this work, to address some of these points, we have calculated the phonon dispersion of MoS , MoSe , and MoTe in bulk and in a single layer using Density Functional Perturbation Theory and discuss the evolution of the energy branches along the different directions of the Brillouin zone. The calculated values are in very good agreement with Raman data reported in the literature. We have also calculated the static and optical dielectric constants, as well as the reflectance, which has been compared with available infrared reflectance data available in the literature. Finally, we discuss the activity of the optical modes in the bulk and single layer case.

The intensity noise of a laser source represents one of the key factors limiting the ultimate sensitivity in laser-based systems for sensing and telecommunication. For advanced applications based on interferometry, the availability of a shot-noise-limited local oscillator is even more important for the effective feasibility of high-precision measurements. This is particularly crucial in quantum optics applications based on homodyne detection schemes to measure non-classical light states, such as squeezed states. This work deeply investigates and analyzes the intensity noise features of the most widely used mid-infrared semiconductor heterostructured lasers: quantum cascade and interband cascade lasers. For this purpose, a comprehensive comparison of three different continuous-wave lasers operating at room temperature around 4.5 µm wavelength is presented. First, a thorough electro-optical characterization is given, highlighting the differences and the shared common characteristics of the tested devices. Then, a detailed intensity noise analysis is reported, identifying their different noise operations with a particular reference to shot-noise-limited operations. Finally, some perspectives towards advanced applications are discussed.

Free-space quantum key distribution (QKD) represents a groundbreaking advancement in secure communication, enabling secure key exchange over vast distances and offering robust encryption for the future quantum internet. However, the compatibility between fiber and free-space infrastructures continues to pose challenges for QKD protocols. Indeed, free-space and fiber-based networks commonly use different wavelengths and qubits encoding schemes. On the one hand, free-space QKD typically exploits visible light for its beneficial beam divergence compared with longer wavelengths, and polarization encoding for its robustness against turbulence. On the other hand, fiber-based QKD employs infrared light, particularly the C-band, because it shows the minimum losses with silica fibers, and time-bin encoding, due to polarization instability in optical fibers. In our study, we demonstrate the viability of a time-bin encoded QKD protocol operating in the C-band through horizontal turbulent free-space channels. We test the setup into a 50-m- and a 500-m-long link, respectively achieving an average secure key rate of 793 kbps and 40 kbps over several hours of measurements. The results encourage further exploration of the interoperability between free-space and fiber-based infrastructures, opening new possibilities for connecting terminal users with satellites in hybrid infrastructures.

We characterize a continuous buffer gas cooled source using CO molecules. We show results about the source performance considering different parameters, like gas flow rate, nozzle size, and internal cell volume. The beam contains 2.5 × 1014 molecules/(s sr) at about160 m/s. Moreover, for two rotational states we observe an unexpected population distribution that we tentatively attribute to a lower temperature inside the cell. Considering the importance of buffer-gas cooling for experiments on ultracold molecules prepared with direct laser cooling, we believe that our work will improve this key first-stage cooling, accelerating the adoption of molecules in the framework of quantum technologies.