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Experiments about shock waves in thermal lensing

In a manuscript published in the Physical Review A (arXiv:1305.1053), Silvia Gentilini, Neda Ghofraniha, Eugenio Del Re, and Claudio Conti review the work about the generation and experimental characterizations of shock waves in thermal lensing, including the effect of disorder.






In a paper published in Nature Communications (arXiv:1304.8002), Marco Leonetti, Claudio Conti and Cefe Lopez report on the experimental evidence and the theory of an all-optical transistor-like action in a random laser.

It is shown that a signal can be routed inside a disordered resonator by using an all-optical control gate: a specific frequency can be transported, switched and amplified in different positions of the system.

Multiple frequencies can be processed simultaneously. The results are based on a novel spatio-spectral reconstruction technique.

This is the first demonstration of a logical operation in a random laser, and also the measurement of a novel kind of controlled transport in an active random system.

The picture below (by Marco Leonetti) shows the demonstrated transistor, with the gate (G), the source (S), and the drain (D) signals indicated in analogy with an electronic field effect transistor.





Localization and shock waves in curved space

The interplay between geometry and wave-localization has always attracted the attention of researchers, and is continuosly inspiring ideas and new directions.

The effect of topological localization and curvature on shock-waves is investigate by C. Conti in arXiv:1302.3806 with specific reference to Bose-Einstein condensation. 

The figure below shows the 3D+1 time resolved parallel simulations of a dispersive shock wave in a bended cigar shaped potential.



Engineered nano-structures for random lasing

The key advantage of random lasers with respect to conventional ones is their independence on a strict geometry, as that imposed by an optical cavity. This allows to build up lasers with practically any shape, and a frontier is identifying and characterizing different fabrication technologies for these devices. 

N. Ghofraniha, I. Viola, F. Di Maria, G. Barbarella, G. Gigli and C. Conti have reported on random lasing in confined patterns realized by organic materials; evidence is given that the different fabrication methodologies and shapes affect light emission.

The picture below shows an example after ArXiv:1301.7582 (to be published in Laser & Photonics Reviews)



True Anderson Localization of Light


News and Views in Nature Photonics about the observation of

the three-dimensional Anderson localization of light by Sperling and others.

The picture below shows FDTD simulations of the Anderson transition in 3D (courtesy of Silvia Gentilini).



Shock waves in random media and the phase diagram

In arXiv:1210.7604, N. Ghofraniha, S. Gentilini, V. Folli, E. DelRe and C. Conti experimentally and theoretically study the effect of disorder on a strongly nonlinear phenomenon: the formation of spatial optical shock waves (see also Optics Express 20, 27369 (2012)). The disorder is introduced by a three-dimensional distribution of colloidal particles with known concentration and parameters.

The random walk due to wave-scattering delays the shock formation up its total inhibition at strong scattering level, as shown in the following picture.


This effect can be quantified by the hydrodynamic limit of the nonlinear Schroedinger equation, which shows how the wave-breaking phenomenon (left panel below) is modified when introducing disorder (right panel)


It is hence possible to experimentally determine a phase-diagram for the behavior of nonlinear waves in random media, by using the shock point as an order parameter.

This is reported in the figure below and it is, in our knowledge, the first example of a measured phase-diagram in terms of amount of nonlinearity and strength of disorder.





Solitonization of the Anderson Localization

Solitons and Anderson localization have several features in common: exponential localization, negative eigenvalues, criticality in two dimensions, possibility of being located anywhere in space. However, at first glance, they are two completely different forms of wave localization, solitons being due to nonlinearity and Anderson states due to a linear disordered potential.

Hence, even if the mentioned affinities are evident, one could be tempted to exclude any connection. But one also could not.

In a paper on the arXiv, [Phys. Rev. A 86, 016801(R) (2012)] C. Conti reports on a theoretical analysis in one-dimension showing that a properly defined disorder averaged equation allows to derive closed form expressions for the shape of fundamental Anderson state and its features (eigenvalue and localization length) in the presence of nonlinearity. Such an equation is a nonlinear Schroedinger equations, the very same sustaining solitons.

In the picture below, the numerically calculated profiles of the nonlinear Anderson states are shown, and the fact that their shape resembles a bright soliton when increasing nonlinearity (the power P) evidenced.





Multimode and Nonlinear SPASER

A SPASER is a laser sustained by surface plasmons in nanometric optical resonators, commonly realized by metallic nano-particles.

The physics of the SPASER is still not well understood and the literature is controversial.

In a recent manuscript (http://arxiv.org/abs/1208.5321) Neda Ghofraniha, Pascal André, Andrea Di Falco and Claudio Conti, report on experimental results on polyhedral silver nano-particles (fabricated by Pascal André) that reveal a novel form of SPASER action, displaying a seemingly multi-modal oscillation and evidence of pronounced nonlinear effects, as a Kerr induced frequency shift shown in the picture below (by Neda Ghofraniha).




All Optical Control of Localization

It is commonly assumed that the degree of localization of light in a resonant system is given by the geometry and the distribution of refractive index, which determine the supported modes that can be theoretically calculated by the boundary value problem obtained by Maxwell equations. In random systems, however, the modes are not orthogonal because the cavity is open, and this implies that they can exchange energy and oscillate simultaneously at the very same frequency.

In a manuscript published in Applied Physics Letters (arXiv:1207.4181), Marco Leonetti, Claudio Conti and Cefe Lopez show that the specific properties of random lasers allow to control all-optically the degree of localization at a given frequency. This effect is ascribed to the fact that various modes with overlapping resonances can be excited in a controlled way and may oscillate at the same wavelength, correspondingly the distribution of the energy inside the cavity is deterministically affected.

The picture below (courtesy of Marco Leonetti) shows an example of the all opticall control (by changing the aperture angle of the pump beam) of the shape of a resonance in a random laser fabricated by a colloidal system.



More on Optomechanicons

In a paper published in the Physical Review A (arXiv:1204.1682), C. Conti, A. Butsch, F. Biancalana, and P.St.J.Russell report on a theoretical analysis of the optical spatial solitons due to the optomechanical interactions in dual-nanoweb photonic crystals structures. These represent a novel class of nonlocal optical solitons with several potential applications.

The picture below shows a numerically simulated propagation of an optomechanicon. See also our related former article and the Physical Review Letters focus.



SCIENCE paper on orbital angular momentum of light in PCF

Excitation of Orbital Angular Momentum Resonances in Helically Twisted Photonic Crystal Fiber [published in Science 27 July 2012]

authored by the MPL people: G. K. L. Wong, M. S. Kang, H. W. Lee, F. Biancalana, C. Conti, T. Weiss, P.St.J.Russell.




An Enlightened Daemon

Claudio Conti's News and Views in Nature Physics on the experiments about Kinetic Condensation of Classical Waves by Sun, Jia, Barsi, Rica, Picozzi, and Fleisher




Shaken Granular Lasers

The optical and photonic properties of complex systems are in several respects unknown. Difficulties arise in the proper definition of a complex system, and in identifying reliable and experimentally interesting frameworks.

Granular materials have been studied for decades, also driven by industrial and technological applications. These very simple systems, composed by agglomerations of mesoscopic particles, are characterized, in specific regimes, by a large number of metastable states and an extreme sensitivity (e.g., in sound transmission) on the arrangement of grains; they are not substantially affected by thermal phenomena, but can be controlled by mechanical solicitations.

shaken granular matter is so far unexplored; here we provide experimental evidence that it can
be affected and controlled by the status of motion of the granular, we also find that competitive
random lasers can be observed. We hence demonstrate the potentialities of gravity affected moving
disordered materials for optical applications, and open the road to a variety of novel interdisciplinary
investigations, involving modern statistical mechanics and disordered photonics.LLaser emission from
Laser emission from shaken granular matter has been reported by Viola Folli, Andrea Puglisi, Luca Leuzzi and Claudio Conti in arXiv:1205.5977 (Physical Review Letters vol 108 page 248002 (2012)) and exhibits intriguing features, as a mechanically controllable spectrum and the onset of competing random laser processes.
These results demonstrate the potentialities of gravity-affected moving disordered materials for optical applications, and open the road to a variety of novel interdisciplinary investigations, involving modern statistical mechanics and disordered photonics.

The picture below shows a sketch of the experimental setup and a retrieved emission spectrum (after arXiv:1205.5977)

These results have been commented in Physics and NewScientist.



Dynamics of Shaken Granular Lasers

Our activity on shaken granular lasers has been enriched by a new paper published in Scientific Reports by Viola Folli, Andrea PuglisiLuca Leuzzi and Claudio Conti.

We report evidence of the link between the granular time-evolution and the shaken laser emission.




Scaling Laws of Dispersive Shocks

The occurrence of dispersive shock waves in nonlinear propagation is characterized by specific scaling laws, which link the shock point with the amount of nonlinearity, nonlocality, and other parameters of the considered physical systems. So far, however, the number of experimental investigations has been very limited.

In the manuscript ArXiv:1204.5312, [Opt. Lett. 37, 2325(2012)] N.Ghofraniha, L. Santamaria Amato, V. Folli, S. Trillo, E. Del Re and C. Conti report on the characterization of nonlocal dispersive shock waves excited in a thermal medium and on a direct measurement of the relevant scaling laws. The figure below shows the measured shock and the comparison with numerical simulations.