260 research outputs found
Light focusing in the Anderson Regime
Anderson localization is a regime in which diffusion is inhibited and waves
(also electromagnetic waves) get localized. Here we exploit adaptive optics to
achieve focusing in disordered optical fibers in the Anderson regime. By
wavefront shaping and optimization, we observe the generation of a propagation
invariant beam, where light is trapped transversally by disorder, and show that
Anderson localizations can be also excited by extended speckled beams. We
demonstrate that disordered fibers allow a more efficient focusing action with
respect to standard fibers in a way independent of their length, because of the
propagation invariant features and cooperative action of transverse
localizations
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
Observation of migrating transverse Anderson localizations of light in nonlocal media
We report the experimental observation of the interaction and attraction of
many localized modes in a two dimensional (2D) system realized by a disordered
optical fiber supporting transverse Anderson localization. We show that a
nonlocal optically nonlinear response of thermal origin alters the localization
length by an amount determined by the optical power and also induces an action
at a distance between the localized modes and their spatial migration. Evidence
of a collective and strongly interacting regime is given
Hyperuniformity in amorphous speckle patterns
Hyperuniform structures possess the ability to confine and drive light,
although their fabrication is extremely challenging. Here we demonstrate that
speckle patters obtained by a superposition of randomly arranged sources of
Bessel beams can be used to generate hyperunifrom scalar fields. By exploiting
laser light tailored with a spatial filter, we experimentally produce (without
requiring any computational power) a speckle pattern possessing maxima at
locations corresponding to a hyperuniform distribution. By properly filtering
out intensity fluctuation from the same speckle pattern, it is possible to
retrieve an intensity profile satisfying the hyperuniformity requirements. Our
findings are supported by extensive numerical simulations.Comment: 13 pages, 7 figure
The mode-locking transition of random lasers
The discovery of the spontaneous mode-locking of lasers, i.e., the
synchronous oscillation of electromagnetic modes in a cavity, has been a
milestone of photonics allowing the realization of oscillators delivering
ultra-short pulses. This process is so far known to occur only in standard
ordered lasers with meter size length and only in the presence of a specific
device (the saturable absorber). Here we demonstrate that mode-locking can
spontaneously arise also in random lasers composed by micronsized laser
resonances dwelling in intrinsically disordered, self-assembled clusters of
nanometer-sized particles. Moreover by engineering a novel mode-selective
pumping mechanism we show that it is possible to continuously drive the system
from a configuration in which the various excited electromagnetic modes
oscillate in the form of several, weakly interacting, resonances to a
collective strongly interacting regime. By realizing the smallest mode-locking
device ever fabricated, we open the way to novel generation of miniaturized and
all-optically controlled light sources
Hopfield-like open channel retrieval for disordered optical media
The measurement of the optical Transmission Matrix (TM) enables to access
"open channels": input patterns, specific to each scattering structure, capable
to deliver very high transmission. Various approaches, based either on multiple
interferometric measurements or on systematic random testing of incident
wavefronts, enable to estimate the inputs required to excite these open
channels. Here, we provide for the first time an approach enabling the complete
and reference-less retrieval of the open channels. It is based on the full
mapping all the pairwise interference terms resulting from all the input modes
couples. We show that these interference terms are organized into a bi-dyadic
coupling matrix whose eigenvalues enables to access the open channel. A
disordered optical system, is thus behaving exactly like an Hopfield neural
network, where a specific input vector (an eigenvalue of the neurons' coupling
matrix) enables to retrieve a specific memory pattern. The proposed Hopfield
like open-channel-retrieval approach, enables to reach almost 100 of the
theoretically expected value of the Intensity. Moreover employing a digital
micromirror device to modulate light, we demonstrate high speed laser scanning
at the back of a disordered medium.Comment: 6 pages, 5 figure
Switching and amplification in disordered lasing resonators
Controlling the flow of energy in a random medium is a research frontier with
a wide range of applications. As recently demonstrated, the effect of disorder
on the transmission of optical beams, may be partially compensated by wavefront
shaping, but losing control over individual light paths. Here we report on a
novel physical effect whereby energy is spatially and spectrally transferred
inside a disordered active medium by the coupling between individual lasing
modes. We show that is possible to transmit an optical resonance to a remote
point by employing specific control over optical excitations. The observed
nonlinear transport bears some analogies to a field-effect transistor for
light, which acts as a switch and as an amplifier
Tunable degree of localization in random lasers with controlled interaction
We show that the degree of localization for the modes of a random laser (RL)
is affected by the inter mode interaction that is controlled by shaping the
spot of the pump laser. By experimentally investigating the spatial properties
of the lasing emission we infer that strongly localized modes are activated in
the low interacting regime while in the strongly interacting one extended modes
are found lasing. Thus we demonstrate that the degree o localization may be
finely tuned at the micrometer level
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