307 research outputs found
Giant ultrafast Kerr effect in type-II superconductors
We study the ultrafast Kerr effect and high-harmonic generation in type-II
superconductors by formulating a new model for a time-varying electromagnetic
pulse normally incident on a thin-film superconductor. It is found that type-II
superconductors exhibit exceptionally large due to the progressive
destruction of Cooper pairs, and display high-harmonic generation at low
incident intensities, and the highest nonlinear susceptibility of all known
materials in the THz regime. Our theory opens up new avenues for accessible
analytical and numerical studies of the ultrafast dynamics of superconductors
On the Topological Nature of the Hawking Temperature of Black Holes
In this work we determine that the Hawking temperature of black holes
possesses a purely topological nature. We find a very simple but powerful
formula, based on a topological invariant known as the Euler characteristic,
which is able to provide the exact Hawking temperature of any two-dimensional
black hole -- and in fact of any metric that can be dimensionally reduced to
two dimensions -- in any given coordinate system, introducing a covariant way
to determine the temperature only using virtually trivial computations. We
apply the topological temperature formula to several known black hole systems
as well as to the Hawking emission of solitons of integrable equations.Comment: Updated version with more relevant reference
Universal quantum Hawking evaporation of integrable two-dimensional solitons
We show that any soliton solution of an arbitrary two-dimensional integrable
equation has the potential to eventually evaporate and emit the exact analogue
of Hawking radiation from black holes. From the AKNS matrix formulation of
integrability, we show that it is possible to associate a real spacetime metric
tensor which defines a curved surface, perceived by the classical and quantum
fluctuations propagating on the soliton. By defining proper scalar invariants
of the associated Riemannian geometry, and introducing the conformal anomaly,
we are able to determine the Hawking temperatures and entropies of the
fundamental solitons of the nonlinear Schroedinger, KdV and sine-Gordon
equations. The mechanism advanced here is simple, completely universal and can
be applied to all integrable equations in two dimensions, and is easily
applicable to a large class of black holes of any dimensionality, opening up
totally new windows on the quantum mechanics of solitons and their deep
connections with black hole physics
Interacting ring-Airy beams in nonlinear media
The interactions between copropagating ring-Airy beams in a (2+1)-dimensional
Kerr medium are numerically investigated for the first time. It is shown that
two overlapping ring-Airy beams in such a medium produce controllable regions
of very low intensity during propagation, the geometry of which can be
manipulated by the tuning of initial beam parameters. This may prove useful for
future optical tweezing applications in the nonlinear regime
Do we listen to what we are told? An empirical study on human behaviour during the COVID-19 pandemic: neural networks vs. regression analysis
In this work, we contribute the first visual open-source empirical study on
human behaviour during the COVID-19 pandemic, in order to investigate how
compliant a general population is to mask-wearing-related public-health policy.
Object-detection-based convolutional neural networks, regression analysis and
multilayer perceptrons are combined to analyse visual data of the Viennese
public during 2020. We find that mask-wearing-related government regulations
and public-transport announcements encouraged correct mask-wearing-behaviours
during the COVID-19 pandemic. Importantly, changes in announcement and
regulation contents led to heterogeneous effects on people's behaviour.
Comparing the predictive power of regression analysis and neural networks, we
demonstrate that the latter produces more accurate predictions of population
reactions during the COVID-19 pandemic. Our use of regression modelling also
allows us to unearth possible causal pathways underlying societal behaviour.
Since our findings highlight the importance of appropriate communication
contents, our results will facilitate more effective non-pharmaceutical
interventions to be developed in future. Adding to the literature, we
demonstrate that regression modelling and neural networks are not mutually
exclusive but instead complement each other
Localized waves carrying orbital angular momentum in optical fibers
We consider the effect of orbital angular momentum (OAM) on localized waves
in optical fibers using theory and numerical simulations, focusing on splash
pulses and focus wave modes. For splash pulses, our results show that they may
carry OAM only up to a certain maximal value. We also examine how one can
optically excite these OAM-carrying modes, and discuss potential applications
in communications, sensing, and signal filtering
Path Integrals: From Quantum Mechanics to Photonics
The path integral formulation of quantum mechanics, i.e., the idea that the
evolution of a quantum system is determined as a sum over all the possible
trajectories that would take the system from the initial to its final state of
its dynamical evolution, is perhaps the most elegant and universal framework
developed in theoretical physics, second only to the Standard Model of particle
physics. In this tutorial, we retrace the steps that led to the creation of
such a remarkable framework, discuss its foundations, and present some of the
classical examples of problems that can be solved using the path integral
formalism, as a way to introduce the readers to the topic, and help them get
familiar with the formalism. Then, we focus our attention on the use of path
integrals in optics and photonics, and discuss in detail how they have been
used in the past to approach several problems, ranging from the propagation of
light in inhomogeneous media, to parametric amplification, and quantum
nonlinear optics in arbitrary media. To complement this, we also briefly
present the Path Integral Monte Carlo (PIMC) method, as a valuable
computational resource for condensed matter physics, and discuss its potential
applications and advantages if used in photonics
- …