493 research outputs found
Resonant modulational instability and self-induced transmission effects in semiconductors:Maxwell-Bloch formalism
The nonlinear optical properties of semiconductors near an excitonic resonance are investigated theoretically by using the macroscopic J model [Ostreich and Knorr, Phys. Rev. B 48, 17811 (1993); 50, 5717 (1994)] based on the microscopic semiconductor Bloch equations. These nonlinear properties cause modulational instability of long light pulses with large gain and give rise to a self-induced transmission of short light pulses in a semiconductor. By an example of the latter well-studied effect, the validity of the used macroscopic model is demonstrated, and good agreement is found with both existing theoretical and experimental results
Self-Assembly of Geometric Space from Random Graphs
We present a Euclidean quantum gravity model in which random graphs
dynamically self-assemble into discrete manifold structures. Concretely, we
consider a statistical model driven by a discretisation of the Euclidean
Einstein-Hilbert action; contrary to previous approaches based on simplicial
complexes and Regge calculus our discretisation is based on the Ollivier
curvature, a coarse analogue of the manifold Ricci curvature defined for
generic graphs. The Ollivier curvature is generally difficult to evaluate due
to its definition in terms of optimal transport theory, but we present a new
exact expression for the Ollivier curvature in a wide class of relevant graphs
purely in terms of the numbers of short cycles at an edge. This result should
be of independent intrinsic interest to network theorists. Action minimising
configurations prove to be cubic complexes up to defects; there are indications
that such defects are dynamically suppressed in the macroscopic limit. Closer
examination of a defect free model shows that certain classical configurations
have a geometric interpretation and discretely approximate vacuum solutions to
the Euclidean Einstein-Hilbert action. Working in a configuration space where
the geometric configurations are stable vacua of the theory, we obtain direct
numerical evidence for the existence of a continuous phase transition; this
makes the model a UV completion of Euclidean Einstein gravity. Notably, this
phase transition implies an area-law for the entropy of emerging geometric
space. Certain vacua of the theory can be interpreted as baby universes; we
find that these configurations appear as stable vacua in a mean field
approximation of our model, but are excluded dynamically whenever the action is
exact indicating the dynamical stability of geometric space. The model is
intended as a setting for subsequent studies of emergent time mechanisms.Comment: 26 pages, 9 figures, 2 appendice
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
Optical analogue of spontaneous symmetry breaking induced by tachyon condensation in amplifying plasmonic arrays
We study analytically and numerically an optical analogue of tachyon
condensation in amplifying plasmonic arrays. Optical propagation is modeled
through coupled-mode equations, which in the continuous limit can be converted
into a nonlinear one-dimensional Dirac-like equation for fermionic particles
with imaginary mass, i.e. fermionic tachyons. We demonstrate that the vacuum
state is unstable and acquires an expectation value with broken chiral
symmetry, corresponding to the homogeneous nonlinear stationary solution of the
system. The quantum field theory analogue of this process is the condensation
of unstable fermionic tachyons into massive particles. This paves the way for
using amplifying plasmonic arrays as a classical laboratory for spontaneous
symmetry breaking effects in quantum field theory.Comment: 5 pages, 5 figure
A room-temperature alternating current susceptometer - Data analysis, calibration, and test
An AC susceptometer operating in the range of 10 Hz to 100 kHz and at room
temperature is designed, built, calibrated and used to characterize the
magnetic behaviour of coated magnetic nanoparticles. Other weakly magnetic
materials (in amounts of some millilitres) can be analyzed as well. The setup
makes use of a DAQ-based acquisition system in order to determine the amplitude
and the phase of the sample magnetization as a function of the frequency of the
driving magnetic field, which is powered by a digital waveform generator. A
specific acquisition strategy makes the response directly proportional to the
sample susceptibility, taking advantage of the differential nature of the coil
assembly. A calibration method based on conductive samples is developed.Comment: 8 pages, 7 figures, 19 ref
Peer victimization in Primary School. Gender difference
Purpose: The peer victimization is frequent phenomenon in the school. The aim of study was to investigate
the gender distinction in the peer victimization, also considering the role that physical activity inside and
outside school could have in this phenomenon.
Methods: The study included 277 students (142 females; 134 males), between the ages of 8 and 10,
attending the primary school. A questionnaire based on "Multi-dimensional Peer-Victimization Scale
(MPVS)'' (Mynard et al., 2000), Italian version (Scarpa et al, 2010) and adapted to the age of the subjects,
was given, to evaluate four areas of victimization (physical victimization, verbal victimization, social
manipulation and attack on property). In addition, subjects were asked which physical activities they
practiced in and out of school.
Results: The results showed that the phenomenon victimization is present in a content way, and that males
are more involved than females. Physical victimization falls more in the male gender (average value of 2±1.4
and 1.6±0.9 for the males and females, respectively), while the social manipulation in the women gender
(average value of 1.9±1.2 and 1.7±1.1 for the females and males, respectively). The correlation between two
or more macro-areas is there for the 74.32% of the cases and verbal victimization is the most widespread
phenomenon in our sample (9.46%). The extra-school physical activity is performed by the majority of
students (83.57%).
Conclusions: The results confirm existing studies in the literature. The peer victimization includes not only
being the obvious target of physical attacks but also includes more subtle forms of abuse, falling more in
females behavior than in males
Microcavity polariton-like dispersion doublet in resonant Bragg gratings
Periodic structures resonantly coupled to excitonic media allow the existence
of extra intragap modes ('Braggoritons'), due to the coupling between Bragg
photon modes and 3D bulk excitons. This induces unique and unexplored
dispersive features, which can be tailored by properly designing the photonic
bandgap around the exciton resonance. We report that one-dimensional
Braggoritons realized with semiconductor gratings have the ability to mimic the
dispersion of quantum-well microcavity polaritons. This will allow the
observation of new nonlinear phenomena, such as slow-light-enhanced nonlinear
propagation and an efficient parametric scattering at two 'magic frequencies'
Emergence of geometrical optical nonlinearities in photonic crystal fiber nanowires
We demonstrate analytically and numerically that a subwavelength-core
dielectric photonic nanowire embedded in a properly designed photonic crystal
fiber cladding shows evidence of a previously unknown kind of nonlinearity (the
magnitude of which is strongly dependent on the waveguide parameters) which
acts on solitons so as to considerably reduce their Raman self-frequency shift.
An explanation of the phenomenon in terms of indirect pulse negative chirping
and broadening is given by using the moment method. Our conclusions are
supported by detailed numerical simulations.Comment: 5 pages, 3 figure
Nonlinear wavelength conversion in photonic crystal fibers with three zero dispersion points
In this theoretical study, we show that a simple endlessly single-mode
photonic crystal fiber can be designed to yield, not just two, but three
zero-dispersion wavelengths. The presence of a third dispersion zero creates a
rich phase-matching topology, enabling enhanced control over the spectral
locations of the four-wave-mixing and resonant-radiation bands emitted by
solitons and short pulses. The greatly enhanced flexibility in the positioning
of these bands has applications in wavelength conversion, supercontinuum
generation and pair-photon sources for quantum optics
Multiple hydrodynamical shocks induced by Raman effect in photonic crystal fibres
We theoretically predict the occurrence of multiple hydrodynamical-like shock
phenomena in the propagation of ultrashort intense pulses in a suitably
engineered photonic crystal fiber. The shocks are due to the Raman effect,
which acts as a nonlocal term favoring their generation in the focusing regime.
It is shown that the problem is mapped to shock formation in the presence of a
slope and a gravity-like potential. The signature of multiple shocks in XFROG
signals is unveiled
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