2,211 research outputs found
Two-Level Laser-like Emission by the Interaction of Self-Induced Transparency Solitons and Surface Anderson Localizations of Light
Self-induced transparency pulses propagating in a random medium embedded in a
two-level system can transfer energy to localized Anderson states. This allows
the onset of two-level laser-like action.Comment: 5 pages, 5 figures, revised versio
Purely nonlinear disorder-induced localizations and their parametric amplification
We investigate spatial localization in a quadratic nonlinear medium in the
presence of randomness. By means of numerical simulations and theoretical
analyses we show that, in the down conversion regime, the transverse random
modulation of the nonlinear susceptibility generates localizations of the
fundamental wave that grow exponentially in propagation. The localization
length is optically controlled by the pump intensity which determines the
amplification rate. The results also apply to cubic nonlinearities.Comment: 5 pages, 5 figure
Laser propulsion of nanobullets by adiabatic compression of surface plasmon polaritons
Laser propulsion and guide of nanosized objects is fundamental for a wide number of applications. These applications are often limited by the fact that the optical forces acting on nanoparticles are almost negligible even in the favorable case of metallic particles and hence large laser powers are needed to accelerate and guide nanosize devices in practical applications. Furthermore, metallic nanoparticles exhibit strong absorption bands and scattering and this makes more difficult controlling nanopropulsion. Thus, finding some mechanism enhancing the optomechanical interaction at the nanoscale controlled by laser is specifically challenging and pivotal. Here, we demonstrate a novel physical effect where the well-known adiabatic localization of the enhanced plasmonic surface field on the apex of metallic nanocones produces a significant optical pressure employable as a propulsive mechanism. The proposed method gives the possibility to develop new photonics devices to accelerate metallic nanobullets over long distances for a variety of applications
Bibliometric indicators: the origin of their log-normal distribution and why they are not a reliable proxy for an individual scholar’s talent
There is now compelling evidence that the statistical distributions of extensive individual bibliometric indicators collected by a scholar, such as the number of publications or the total number of citations, are well represented by a Log-Normal function when homogeneous samples are considered. A Log-Normal distribution function is the normal distribution for the logarithm of the variable. In linear scale it is a highly skewed distribution with a long tail in the high productivity side. We are still lacking a detailed and convincing ab-initio model able to explain observed Log-Normal distributions-this is the gap this paper sets out to fill. Here, we propose a general explanation of the observed evidence by developing a straightforward model based on the following simple assumptions: (1) the materialist principle of the natural equality of human intelligence, (2) the success breeds success effect, also known as Merton effect, which can be traced back to the Gospel parables about the Talents (Matthew) and Minas (Luke), and, (3) the recognition and reputation mechanism. Building on these assumptions we propose a distribution function that, although mathematically not identical to a Log-Normal distribution, shares with it all its main features. Our model well reproduces the empirical distributions, so the hypotheses at the basis of the model are not falsified. Therefore the distributions of the bibliometric parameters observed might be the result of chance and noise (chaos) related to multiplicative phenomena connected to a publish or perish inflationary mechanism, led by scholars' recognition and reputations. In short, being a scholar in the right tail or in the left tail of the distribution could have very little connection to her/his merit and achievements. This interpretation might cast some doubts on the use of the number of papers and/or citations as a measure of scientific achievements. A tricky issue seems to emerge, that is: what then do bibliometric indicators really measure? This issue calls for deeper investigations into the meaning of bibliometric indicators. This is an interesting and intriguing topic for further research to be carried out within a wider interdisciplinary investigation of the science of science, which may include elements and investigation tools from philosophy, psychology and sociology
Shock waves in disordered media
We experimentally investigate the interplay between spatial shock waves and
the degree of disorder during nonlinear optical propagation in a thermal
defocusing medium. We characterize the way the shock point is affected by the
amount of disorder and scales with wave amplitude. Evidence for the existence
of a phase diagram in terms of nonlinearity and amount of randomness is
reported. The results are in quantitative agreement with a theoretical approach
based on the hydrodynamic approximation.Comment: 4 pages, 5 figure
Time-resolved dynamics of granular matter by random laser emission
Because of the huge commercial importance of granular systems, the
second-most used material in industry after water, intersecting the industry in
multiple trades, like pharmacy and agriculture, fundamental research on
grain-like materials has received an increasing amount of attention in the last
decades. In photonics, the applications of granular materials have been only
marginally investigated. We report the first phase-diagram of a granular as
obtained by laser emission. The dynamics of vertically-oscillated granular in a
liquid solution in a three-dimensional container is investigated by employing
its random laser emission. The granular motion is function of the frequency and
amplitude of the mechanical solicitation, we show how the laser emission allows
to distinguish two phases in the granular and analyze its spectral
distribution. This constitutes a fundamental step in the field of granulars and
gives a clear evidence of the possible control on light-matter interaction
achievable in grain-like system.Comment: 16 pages, 7 figure
Beam Instabilities in the Scale Free Regime
The instabilities arising in a one-dimensional beam sustained by the
diffusive photorefractive nonlinearity in out-of-equilibrium ferroelectrics are
theoretically and numerically investigated. In the "scale-free model", in
striking contrast with the well-known spatial modulational instability, two
different beam instabilities dominate: a defocusing and a fragmenting process.
Both are independent of the beam power and are not associated to any specific
periodic pattern.Comment: 4 pages, 3 figure
Phase diagram and complexity of mode-locked lasers: from order to disorder
We investigate mode-locking processes in lasers displaying a variable degree
of structural randomness, from standard optical cavities to multiple-scattering
media. By employing methods mutuated from spin-glass theory, we analyze the
mean-field Hamiltonian and derive a phase-diagram in terms of the pumping rate
and the degree of disorder. Three phases are found: i) paramagnetic,
corresponding to a noisy continuous wave emission, ii) ferromagnetic, that
describes the standard passive mode-locking, and iii) the spin-glass in which
the phases of the electromagnetic field are frozen in a exponentially large
number of configurations. The way the mode-locking threshold is affected by the
amount of disorder is quantified. The results are also relevant for other
physical systems displaying a random Hamiltonian, like Bose-Einstein
condensates and nonlinear optical beams.Comment: 4 pages, 2 figure
On the number of limit cycles in asymmetric neural networks
The comprehension of the mechanisms at the basis of the functioning of
complexly interconnected networks represents one of the main goals of
neuroscience. In this work, we investigate how the structure of recurrent
connectivity influences the ability of a network to have storable patterns and
in particular limit cycles, by modeling a recurrent neural network with
McCulloch-Pitts neurons as a content-addressable memory system.
A key role in such models is played by the connectivity matrix, which, for
neural networks, corresponds to a schematic representation of the "connectome":
the set of chemical synapses and electrical junctions among neurons. The shape
of the recurrent connectivity matrix plays a crucial role in the process of
storing memories. This relation has already been exposed by the work of Tanaka
and Edwards, which presents a theoretical approach to evaluate the mean number
of fixed points in a fully connected model at thermodynamic limit.
Interestingly, further studies on the same kind of model but with a finite
number of nodes have shown how the symmetry parameter influences the types of
attractors featured in the system. Our study extends the work of Tanaka and
Edwards by providing a theoretical evaluation of the mean number of attractors
of any given length for different degrees of symmetry in the connectivity
matrices.Comment: 35 pages, 12 figure
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