Spatial coherence of room-temperature monolayer WSe2 exciton-polaritons in a trap

The emergence of spatial and temporal coherence of light emitted from solid-state systems is a fundamental phenomenon intrinsically aligned with the control of light-matter coupling. It is canonical for laser oscillation, emerges in the superradiance of collective emitters, and has been investigated in bosonic condensates of thermalized light, as well as exciton-polaritons. Our room temperature experiments show the strong light-matter coupling between microcavity photons and excitons in atomically thin WSe2. We evidence the density-dependent expansion of spatial and temporal coherence of the emitted light from the spatially confined system ground-state, which is accompanied by a threshold-like response of the emitted light intensity. Additionally, valley-physics is manifested in the presence of an external magnetic field, which allows us to manipulate K and K’ polaritons via the valley-Zeeman-effect. Our findings validate the potential of atomically thin crystals as versatile components of coherent light-sources, and in valleytronic applications at room temperature.

Two-step deswelling in the Volume Phase Transition of thermoresponsive microgels

Thermoresponsive microgels are one of the most investigated types of soft colloids, thanks to their ability to undergo a Volume Phase Transition (VPT) close to ambient temperature. However, this fundamental phenomenon still lacks a detailed microscopic understanding, particularly regarding the presence and the role of charges in the deswelling process. This is particularly important for the widely used poly(N-isopropylacrylamide)–based microgels, where the constituent monomers are neutral but charged groups arise due to the initiator molecules used in the synthesis. Here, we address this point combining experiments with state-of-the-art simulations to show that the microgel collapse does not happen in a homogeneous fashion, but through a two-step mechanism, entirely attributable to electrostatic effects. The signature of this phenomenon is the emergence of a minimum in the ratio between gyration and hydrodynamic radii at the VPT. Thanks to simulations of microgels with different cross-linker concentrations, charge contents, and charge distributions, we provide evidence that peripheral charges arising from the synthesis are responsible for this behavior and we further build a universal master curve able to predict the two-step deswelling. Our results have direct relevance on fundamental soft condensed matter science and on applications where microgels are involved, ranging from materials to biomedical technologies.

Preconditioning, aerosols, and radiation control the temperature of glaciation in Amazonian clouds

Glaciation in clouds is a fundamental phenomenon in determining Earth’s radiation fluxes, sensible and latent heat budgets in the atmosphere, the water cycle, cloud development and lifetime. Nevertheless, the main mechanisms that govern the temperature of glaciation in clouds have not been fully identified. Here we present an analysis of 15 years (2000-2014) of satellite, sunphotometer, and reanalysis datasets over the Amazon. We find that the temperature of glaciation in convective clouds is controlled by preconditioning dynamics, natural and anthropic aerosols, and radiation. In a moist atmospheric column, prone to deep convection, increasing the amount of aerosols leads to a delay in the onset of glaciation, reducing the glaciation temperature. For a dry column, radiative extinction by biomass burning smoke leads to atmospheric stabilization and an increase in the glaciation temperature. Our results offer observational benchmarks that can help a more precise description of glaciation in convective cloud models.

Algorithmization of Power: Digital Metamorphoses of Political Regimes and Sovereignty

The work opens the thematic issue «Digitalization and Novus Ordo Seclorum: International Relations and Geopolitics in Digital». The purpose of the article is to determine the features of the most fundamental phenomenon rooted in the digital metamorphosis of political regimes, sovereignty and geopolitics - the algorithmicization of power. As the main methodological optics, the principles of discourse analysis of scientific literature (including the works of the thematic issue) and the techniques of scriptwriters are used. The work emphasizes that algorithms are becoming a carrier of «structural violence» (an invisible, but real threat of sanctions against those who disagree with the current rules of digital communication). The power of algorithms is revealed in the fact that they begin to determine to the citizen what is best for him and what is not, including in the political sphere. At the same time, the one who owns the management algorithms that build communication processes and determine the interface, communication functionality, he has algorithmic power. Such accompanying phenomena of the algorithmicization of power as sociotechnical reality, predictive analytics, network policy, political interface, filtering bubbles and affordances are considered. It has been determined that in the new socio-technical reality (the phygital world), algorithms occupy the most important place, since it is they that fasten the Social with the Technical. The conclusions emphasize that digital sovereignty presupposes several components - the regime has its own network policy (communication arenas) and a political interface (software serving these communication arenas). The theoretical significance of the article is seen in the author's thesis, according to which algorithms, being constituent elements of software, bring the policy of ensuring digital sovereignty closer to different technologies of political legitimation of the regime - internal and external, top-down and bottom-up. In addition, the paper proposes scenarios for the evolution of algorithmic power. In addition, the practical significance of the study is seen in the fact that the work proposes scenarios for the evolution of algorithmic power.

Understanding the Effects of Exponentially Decaying DC Currents on the Dual dq Control of Power Converters in Systems with High X/R

This paper was accepted for presentation at the IEEE CPE-POWERENG 2021 conference held in Florence, Italy, between the 14th and 16th of July 2021.<div><br></div><div>It investigates the root causes of detrimental oscillations in the dc link voltage of an energy storage system using a dual dq controller, operating at a high-voltage ac grid with high reactance-resistance ratio. Dual dq controllers are recommended in the literature for power converters operating under unbalanced, fault, or reduced voltage conditions. They employ two separated rotating reference frames, one for the positive and one for the negative sequence. The causes of the oscillations are investigated both theoretically and by time-domain computer simulations. As a result of the simulations, the performance of two dual controllers used in the industry is compared. In the presence of exponentially decaying dc currents, the filtering techniques employed by the controllers affect differently the performance of the proportional-integral regulators and disturb the feed forwarding and dq decoupling schemes. Ultimately, this results in undesirable oscillations in the dc-link voltage. This paper sheds light on how a fundamental phenomenon of three-phase ac systems can critically affect the control of power electronic converters. It provides a valuable insight into a possible root cause of oscillations in large electrical system applications with a considerable power converter penetration, such as large industrial plants striving for reducing greenhouse gas emissions.<br></div>

Understanding the Effects of Exponentially Decaying DC Currents on the Dual dq Control of Power Converters in Systems with High X/R

This paper was accepted for presentation at the IEEE CPE-POWERENG 2021 conference held in Florence, Italy, between the 14th and 16th of July 2021.<div><br></div><div>It investigates the root causes of detrimental oscillations in the dc link voltage of an energy storage system using a dual dq controller, operating at a high-voltage ac grid with high reactance-resistance ratio. Dual dq controllers are recommended in the literature for power converters operating under unbalanced, fault, or reduced voltage conditions. They employ two separated rotating reference frames, one for the positive and one for the negative sequence. The causes of the oscillations are investigated both theoretically and by time-domain computer simulations. As a result of the simulations, the performance of two dual controllers used in the industry is compared. In the presence of exponentially decaying dc currents, the filtering techniques employed by the controllers affect differently the performance of the proportional-integral regulators and disturb the feed forwarding and dq decoupling schemes. Ultimately, this results in undesirable oscillations in the dc-link voltage. This paper sheds light on how a fundamental phenomenon of three-phase ac systems can critically affect the control of power electronic converters. It provides a valuable insight into a possible root cause of oscillations in large electrical system applications with a considerable power converter penetration, such as large industrial plants striving for reducing greenhouse gas emissions.<br></div>

Symmetry Violation in Bichromatic Ionization by a Free-Electron Laser: Photoelectron Angular Distribution and Spin Polarization

A fundamental phenomenon of coherent control is investigated theoretically using the example of neon photoionization by the bichromatic field of a free-electron laser. A system exposed to coherent fields with commensurable frequencies loses some symmetry, which manifests itself in the angular distribution and spin polarization of the electron emission. We predict several such effects, for example, the violation of symmetry with respect to the plane perpendicular to the polarization vector of the second harmonic and the appearance of new components of spin polarization. Furthermore, we predict a very efficient control of spin polarization via manipulation of the phase between the harmonics. Experimental observation of these effects is accessible with modern free-electron lasers operating in the extreme ultraviolet wavelength regime.

Crossover from two-frequency pulse compounds to escaping solitons

The nonlinear interaction of copropagating optical solitons enables a large variety of intriguing bound-states of light. We here investigate the interaction dynamics of two initially superimposed fundamental solitons at distinctly different frequencies. Both pulses are located in distinct domains of anomalous dispersion, separated by an interjacent domain of normal dispersion, so that group velocity matching can be achieved despite a vast frequency gap. We demonstrate the existence of two regions with different dynamical behavior. For small velocity mismatch we observe a domain in which a single heteronuclear pulse compound is formed, which is distinct from the usual concept of soliton molecules. The binding mechanism is realized by the mutual cross phase modulation of the interacting pulses. For large velocity mismatch both pulses escape their mutual binding and move away from each other. The crossover phase between these two cases exhibits two localized states with different velocity, consisting of a strong trapping pulse and weak trapped pulse. We detail a simplified theoretical approach which accurately estimates the parameter range in which compound states are formed. This trapping-to-escape transition allows to study the limits of pulse-bonding as a fundamental phenomenon in nonlinear optics, opening up new perspectives for the all-optical manipulation of light by light.

Modelling Wave-Particle Duality of Classical Particles and Waves

Wave-particle duality is the fundamental phenomenon of particles and fields in quantum mechanics. In the past, the trajectory-like (particle-like) behaviour and wave-like behaviour has been considered separately. In this article, a superimposed model is derived to characterise wave-particle duality of classical particles. The superimposed model reflects an invariant mathematical structure (analogous variables and differential equations) from classical mechanics, classical field theories and quantum mechanics. Its analytical solution carries trajectory-like property (phase-independent) and wave-like property (phase-dependent) of particles that is consistent with to Schrodinger’s picture. Subsequently, the presented model is applied to model duality of classical waves in electromagnetism, acoustics and elasticity. The analysis implies the existence of quantum effects of classical waves at macroscopic scale. It predicts quantum picture on energy and momentum exchange between classical particles and waves. As seen in the model, wave-particle duality reflects inherent and indispensable characteristics of classical objects.