Study of the characteristics of amplification of Tm and Tm-Ho doped fluorides for passive Q-switching applications

The great interest for diode pumped solids state laser sources (DPSSL), in the mid- and far-infrared is principally related to the absorption spectrum of water. The latter shows strong absorption for wavelength longer then 1.6μm, making these sources suitable for a wide range of applications. Infrared lasers above 1.6μm in general, fall in the so called eye safe region, for this reason pulsed 2μm sources are widely used in communication technologies as well as in remote sensing, e.g. LIDAR systems and atmospheric physics. The short penetration depth in biological tissues, due to the high absorption in water, makes these sources suitable also as laser scalpel in medical applications. Moreover, they are used as laser sources for Optical Parametric Oscillators for down conversion to the mid-infrared by nonlinear crystals. Rare Earths (REs), such as thulium and holmium, are commonly used in solid state gain media thanks to their peculiar spectroscopic proprieties. They show sharp peaks in spectra and metastable excited states, typical of free ions behaviour, also when inserted in bulk hosts. These proprieties, clearly, are strongly related with their electronic configuration and this thesis I give an idea of why REs have this configuration and of how to calculate the free ions energy levels scheme. We describe also the characteristic ion-ion coupling of the REs and the resulting non radiative energy transfer mechanism. Such phenomena, when resonant, allow an efficient conversion of the radiation: halving the wavelength via up-conversion, or doubling it by cross-relaxation. Fluorides are widely used as crystal hosts in gain media REs based systems thanks to their low maximum phonon energy. In this work I focus on BaY2F8(BYF) and YLiF4 (YLF) crystals, I introduce the concepts necessary to characterize laser crystals: ground state absorption cross section, stimulated emission cross section and gain cross section. The main goal is to investigate the characteristics of amplification of thulium and thulium-holmium doped fluorides in the 2μm region for application in passive Q-switching, PQS. This technique, simply obtained by intracavity insertion of a saturable absorber, allow me to obtain short pulses (tens of ns long) and high peak power (up to tens of kW), without the need to use active devices and thus considerably reducing costs and complexity. The parts of a general DPSSL system, and in particular the details of the DPSSL used in this work, are described. The role of the saturable absorber (SA) is to introduce time modulated losses. It is engineered in order to prevent stimulated emission at the begin the pumping phase and to be transparent when the fluence reaches the level of the SA saturation fluence value allowing the pulse to be emitted. The saturable absorbers, used in this work, are large bandgap semiconductors (ZnS and ZnSe) doped with doubly ionized chromium ions. The bandgap ensures a wide transparency window and the chromium doping a smooth absorption band from 1.5μm to 2.1μm. The laser crystals have been tested in different configurations and in combination with different absorbers in order to maximize the peak pulse and minimize the pulse duration. Comparing the Tm:BYF performance obtained with the results in literature achieved with other Tm-doped fluorides, we found out that the performance of BYF are lower. The problem is the lower damaging threshold of BYF that makes it very difficult to work without damage. Changing the setup in order to decrease the energy density on the Tm:BYF crystals the damaging problem was still present. Instead working with- Tm- Ho:YLF crystals was easier, mainly because in the co-doped system the energy densities on the optical elements are several times lower. Comparing the data achieved with the literature we demonstrated with this material for the first time sub μs pulse operation at a room temperature (40ns of pulse duration), consequently improving also the peak power. The improvement could be mainly imputed to the use of the Cr2+:ZnSe in place of the Cr 2+ ZnS that shows a higher absorption cross section in the 2050μm region. As regards Tm:BYF the next step could be the use BaYLuF8 instead of the BYF follow- ing the same philosophy between YLF and LLF. Substituting the yttrium with lutetium the thermo-mechanical proprieties could be improved and consequently the damage prob- lem reduced. For Tm-Ho:YLF the next natural step is to perform the same experiments with LLF that for single doped thulium system shows the best results to date

Full-Bloch beams and ultrafast Rabi-rotating vortices

Strongly-coupled quantum fields, such as multi-component atomic condensates, optical fields and polaritons, are remarkable systems where the simple dynamics of coupled oscillators can meet the intricate phenomenology of quantum fluids. When the coupling between the components is coherent, not only the particles number, but also their phase texture that maps the linear and angular momentum, can be exchanged. Here, on a system of exciton-polaritons, we have realized a so-called full-Bloch beam: a configuration in which all superpositions of the upper and the lower polariton -- all quantum states of the associated Hilbert space -- are simultaneously present at different points of the physical space, evolving in time according to Rabi-oscillatory dynamics. As a result, the light emitted by the cavity displays a peculiar dynamics of spiraling vortices endowed with oscillating linear and angular momentum and exhibiting ultrafast motion of their cores with striking accelerations to arbitrary speeds. This remarkable vortex motion is shown to result from distortions of the trajectories by a homeomorphic mapping between the Rabi rotation of the full wavefunction on the Bloch sphere and Apollonian circles in the real space where the observation is made. Such full-Bloch beams offer new prospects at a fundamental level regarding their topological properties or in the interpretation of quantum mechanics, and the Rabi-rotating vortices they yield should lead to interesting applications such as ultrafast optical tweezers.Comment: Published version, 18 pages, 8 figures, 4 ancillary movie

Interactions and scattering of quantum vortices in a polariton fluid

Quantum vortices, the quantized version of classical vortices, play a prominent role in superfluid and superconductor phase transitions. However, their exploration at a particle level in open quantum systems has gained considerable attention only recently. Here we study vortex pair interactions in a resonant polariton fluid created in a solid-state microcavity. By tracking the vortices on picosecond time scales, we reveal the role of nonlinearity, as well as of density and phase gradients, in driving their rotational dynamics. Such effects are also responsible for the split of composite spin-vortex molecules into elementary half-vortices, when seeding opposite vorticity between the two spinorial components. Remarkably, we also observe that vortices placed in close proximity experience a pull-push scenario leading to unusual scattering-like events that can be described by a tunable effective potential. Understanding vortex interactions can be useful in quantum hydrodynamics and in the development of vortex-based lattices, gyroscopes, and logic devices.Comment: 12 pages, 7 figures, Supplementary Material and 5 movies included in arXi

Multi-mode fiber reservoir computing overcomes shallow neural networks classifiers

In disordered photonics, one typically tries to characterize the optically opaque material in order to be able to deliver light or perform imaging through it. Among others, multi-mode optical fibers are extensively studied because they are cheap and easy-to-handle complex devices. Here, instead, we use the reservoir computing paradigm to turn these optical tools into random projectors capable of introducing a sufficient amount of interaction to perform non-linear classification. We show that training a single logistic regression layer on the data projected by the fiber improves the accuracy with respect to learning it on the raw images. Surprisingly, the classification accuracy performed with physical measurements is higher than the one obtained using the standard transmission matrix model, a widely accepted tool to describe light transmission through disordered devices. Consistently with the current theory of deep neural networks, we also reveal that the classifier lives in a flatter region of the loss landscape when trained on fiber data. These facts suggest that multi-mode fibers exhibit robust generalization properties, thus making them promising tools for optically-aided machine learning

Dynamics of a vortex lattice in a non-equilibrium polariton superfluid

If a quantum fluid is put in motion with enough angular momentum, at equilibrium the ground state of the system is given by an array of quantised vortices. In a driven-dissipative polariton fluid, we demonstrate that the reverse process is also possible. Upon initially imprinting a static and regular vortex array, the quantum fluid starts rotating. By tracking on picosecond time scales many quantized vortices, we present the first measure of rigid-body rotation in a polariton condensate. Such many-body motion agrees with the Feynman quantization of superfluid velocity, which we show to be valid even if our system is expanding and equilibrium is never attained

Interactions and scattering of quantum vortices in a polariton fluid

Quantum vortices, the quantized version of classical vortices, play a prominent role in superfluid and superconductor phase transitions. However, their exploration at a particle level in open quantum systems has gained considerable attention only recently. Here we study vortex pair interactions in a resonant polariton fluid created in a solid-state microcavity. By tracking the vortices on picosecond time scales, we reveal the role of nonlinearity, as well as of density and phase gradients, in driving their rotational dynamics. Such effects are also responsible for the split of composite spin–vortex molecules into elementary half-vortices, when seeding opposite vorticity between the two spinorial components. Remarkably, we also observe that vortices placed in close proximity experience a pull–push scenario leading to unusual scattering-like events that can be described by a tunable effective potential. Understanding vortex interactions can be useful in quantum hydrodynamics and in the development of vortex-based lattices, gyroscopes, and logic devices.MAT2016- 79866-R project (AEI/FEDER, UE)

Optically reconfigurable molecules of topological bound states in the continuum

Symmetry protected optical bound states in the continuum (BICs) are charming wave-mechanical objects that provide new and exciting ways to enhance light-matter interactions in compact photonic devices. These ultrahigh quality factor states have quickly transcended from passive structures, and lasing devices in the weak-coupling regime, towards nonequilibrium Bose-Einstein condensates of BIC polaritons in the strong-coupling regime. Here, we show that the large interaction strength of exciton-polaritons in subwavelength quantum-well waveguide gratings in conjunction with their topologically protected BIC nature opens unexplored opportunities in low-threshold optically reprogrammable quantum fluids. The BIC causes polaritons to -- almost counterintuitively -- condense in the extremum of a negative mass dispersion which leads to strong interaction-induced trapping at their respective pump spot and gain region. We exploit this optical trapping mechanism to demonstrate macroscopic mode-hybridization, the hallmark of coherent quantum systems, enabling construction of never-seen-before artificial BIC molecules with unusual topological charge mutliplicity. We underpin the optical write-in aspect of our technique by constructing, on the same sample, artificial mono-atomic and dimerized BIC chains of polariton fluids displaying non-Hermitian quasicrystalline band formation and gap opening. Our findings open new perspectives on large-scale reprogrammable driven dissipative many-body systems in the strong-coupling regime

Ovarian Cancer Biomarkers in the COVID-19 Era

Ovarian Cancer (OC) diagnosis is entrusted to CA125 and HE4. Since the latter has been found increased in COVID-19 patients, in this study, we aimed to evaluate the influence of SARS-CoV-2 infection on OC biomarkers. HE4 values above the cut-off were observed in 65% of OC patients and in 48% of SARS-CoV-2-positive patients (not oncologic patients), whereas CA125 values above the cut-off were observed in 71% of OC patients and in 11% of SARS-CoV-2 patients. Hence, by dividing the HE4 levels into quartiles, we can state that altered levels of HE4 in COVID-19 patients were mostly detectable in quartile I (151–300 pmol/L), while altered levels in OC patients were mostly clustered in quartile III (>600, pmol/L). In light of these observations, in order to better discriminate women with ovarian cancer versus those with COVID-19, we established a possible HE4 cut-off of 328 pmol/L by means of a ROC curve. These results demonstrate that the reliability of HE4 as a biomarker in ovarian cancer remains unchanged, despite COVID-19 interference; moreover, it is important for a proper diagnosis that whether the patient has a recent history of SARS-CoV-2 infection is determined

ROCK. Co-design workshops and self-built transformation of public space in Bologna with students and professionals. Urban regeneration of Piazza Rossini. Le Cinque Piazze experience

The dataset contains the presentation and preparatory materials presented within the co-design workshop and self-built transformation of public space “Le Cinque Piazze. Un workshop per prendersi cura della zona U” held in March 2019 in Bologna, and the project of urban regeneration of Piazza Rossini, co-developed by students and professionals during the Self-construction workshop workshop "Le Cinque Piazze" held in September 2019. The data represent the design proposals for temporary urban transformations, developed during the workshop Le Cinque Piazze, by the students of the Architecture and Advanced Design students of the University of Bologna. The 24 students involved worked together with the University of Bologna – Architecture Department researchers, the Municipality of Bologna staff, Foundation for Urban Innovation and Fondazione Rusconi staff. Data can be used by both university researchers (as methodology to be replicated in similar workshops) or by city staff to take inspiration for projects to be realized in similar parts of the city