Palmieri, V., Barba, M., Pietro, L.D., Gentilini, S., Braidotti, M.C., Ciancico, C., Bugli, F., Ciasca, G., Larciprete, R., Lattanzi, W., Sanguinetti, M., Spirito, M.D., Conti, C., Papi, M.(2018)“Reduction and shaping of graphene-oxide by laser-printing for controlled bone tissue regeneration and bacterial killing”, 2D Materials, 5(1), 015027, available: http://stacks.iop.org/2053-1583/5/i=1/a=015027.
Graphene and graphene oxide (GO) are capable of inducing stem cells differentiation into bone tissue with variable efficacy depending on reductive state of the material. Thus, modulation of osteogenic process and of bone mineral density distribution is theoretically possible by controlling the GO oxidative state. In this study, we laser-printed GO surfaces in order to obtain both a local photo-thermal GO reduction and the formation of nano-wrinkles along precise geometric pattern. Initially, after cells adhered on the surface, stem cells migrated and accumulated on the reduced and wrinkled surface. When the local density of the stem cells on the reduced stripes was high, cells started to proliferate and occupy the oxidized/flat area. The designed surfaces morphology guided stem cell orientation and the reduction accelerated differentiation. Furthermore the reduced sharp nano-wrinkles were able to enhance the GO antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA), a common cause of prosthetic joints infections. This strategy can offer a revolution in present and future trends of scaffolds design for regenerative medicine.
Marcucci, G., Conti, C., Materassi, M.(2018)“Metriplectic Structure of a Radiation-Matter Interaction Toy Model”, ArXiv e-prints.
Topological concepts open many new horizons for photonic devices, from integrated optics to lasers. The complexity of large scale topological devices asks for an effective solution of the inverse problem: how best to engineer the topology for a specific application? We introduce a novel machine learning approach to the topological inverse problem. We train a neural network system with the band structure of the Aubry-Andre-Harper model and then adopt the network for solving the inverse problem. Our application is able to identify the parameters of a complex topological insulator in order to obtain protected edge states at target frequencies. One challenging aspect is handling the multivalued branches of the direct problem and discarding unphysical solutions. We overcome this problem by adopting a self-consistent method to only select physically relevant solutions. We demonstrate our technique in a realistic topological laser design and by resorting to the widely available open-source TensorFlow library. Our results are general and scalable to thousands of topological components. This new inverse design technique based on machine learning potentially extends the applications of topological photonics, for example, to frequency combs, quantum sources, neuromorphic computing and metrology.
Braidotti, M.C., Conti, C., Faizal, M., Dey, S., Alasfar, L., Alsaleh, S., Ashour, A.(2018)“Path Integral for Non-Paraxial Optics”, ArXiv e-prints.
The intriguing connection between Black holes' evaporation and the physics of solitons is opening novel roads to finding observable phenomena. In particular, due to the recent observation of gravitational waves, Hawking radiation of moving black holes is one of the first candidates to investigate. However, a theoretical context for the description of this phenomenon is still lacking. Here, we adopt a soliton geometrization technique to study the quantum emission of moving black holes in a one-dimensional model. Representing a black hole by the one soliton solution of sine-Gordon equation, we consider Hawking emission spectra of quantized massless scalar fields on the soliton-induced metric. We study the relation between the soliton velocity and the black hole temperature. Our results address a new scenario in the detection of new physics in the quantum gravity panorama.
Standing out as the new wonder bidimensional material, graphene oxide (GO) has aroused an exceptional interest in biomedical research by holding promise for being the antibacterial of future. First, GO possesses a specific interaction with microorganisms combined with a mild toxicity for human cells. Additionally, its antibacterial action seems to be directed to multiple targets in pathogens, causing both membranes mechanical injury and oxidative stress. Lastly, compared to other carbon materials, GO has easy and low-cost processing and is environment-friendly. This remarkable specificity and multi-targeting antibacterial activity come at a time when antibiotic resistance represents the major health challenge. Unfortunately, a comprehensive framework to understand how to effectively utilize this material against microorganisms is still lacking. In the last decade, several groups tried to define the mechanisms of interaction between GO flakes and pathogens but conflicting results have been reported. This review is focused on all the contradictions of GO antimicrobial properties in solution. Flake size, incubation protocol, time of exposure and species considered are examples of factors influencing results. These parameters will be summarized and analyzed with the aim of defining the causes of contradictions, to allow fast GO clinical application.
Palmieri, V., Bugli, F., Lauriola, M.C., Cacaci, M., Torelli, R., Ciasca, G., Conti, C., Sanguinetti, M., Papi, M., De Spirito, M.(2017)“Bacteria Meet Graphene: Modulation of Graphene Oxide Nanosheet Interaction with Human Pathogens for Effective Antimicrobial Therapy”, ACS Biomaterials Science & Engineering, available: http://dx.doi.org/10.1021/acsbiomaterials.6b00812.
The development of new pharmacological strategies that evade bacterial resistance has become a compelling worldwide challenge. Graphene oxide (GO) can represent the nanotechnology answer being economical and easy to produce and to degrade and having multitarget specificity against bacteria. Several groups tried to define the interaction between GO sheets and human pathogens. Unfortunately, controversial results from inhibition to bacterial growth enhancement have been reported. The main difference among all experimental evidence relies on the environmental conditions adopted to study the bacteria–GO interaction. Indeed GO, stable in deionized water, undergoes a rapid and salt-specific DLVO-like aggregation that influences antimicrobial effects. Considering this phenomenon, the interaction of bacteria with GO aggregates having different sizes, morphologies, and surface potential can create a complex scenario that explains the contrasting results reported so far. In this article, we demonstrate that by modulating the GO stability in solution, the antibacterial or growth enhancement effect can be controlled on S. aureus and E. coli. GO at low concentration cuts microorganism membranes and at high concentration forms complexes with pathogens and inhibits or enhances bacterial growth in a surface potential-dependent manner. With the framework defined in this study, the clinical application of GO gets closer, and controversial results in literature can be explained.
Ornigotti, M., Conti, C., Szameit, A.(2017)“Quantum X Waves with Orbital Angular Momentum in Nonlinear Dispersive
Media”, available: http://arxiv.org/abs/1704.07258.
We present a complete and consistent quantum theory of generalised X waves
with orbital angular momentum (OAM) in dispersive media. We show that the
resulting quantised light pulses are affected by neither dispersion nor
diffraction and are therefore resilient against external perturbations. The
nonlinear interaction of quantised X waves in quadratic and Kerr nonlinear
media is also presented and studied in detail.
The propagation of decelerating Airy pulses in non-instantaneous cubic medium is investigated both theoretically and numerically. In a Debye model, at variance with the case of accelerating Airy and Gaussian pulses, a decelerating Airy pulse evolves into a single soliton for weak and general non-instantaneous response. Airy pulses can hence be used to control soliton generation by temporal shaping. The effect is critically dependent on the response time, and could be used as a way to measure the Debye type response function. For highly non-instantaneous response, we theoretically find a decelerating Airy pulse is still transformed into Airy wave packet with deceleration. The theoretical predictions are confirmed by numerical simulations.
Braidotti, M.C., Musslimani, Z.H., Conti, C.(2017)“Generalized uncertainty principle and analogue of quantum gravity in optics”, Physica D: Nonlinear Phenomena, 338, 34 - 41, available: http://www.sciencedirect.com/science/article/pii/S0167278916301671.
Abstract The design of optical systems capable of processing and manipulating ultra-short pulses and ultra-focused beams is highly challenging with far reaching fundamental technological applications. One key obstacle routinely encountered while implementing sub-wavelength optical schemes is how to overcome the limitations set by standard Fourier optics. A strategy to overcome these difficulties is to utilize the concept of a generalized uncertainty principle (G-UP) which has been originally developed to study quantum gravity. In this paper we propose to use the concept of G-UP within the framework of optics to show that the generalized Schrödinger equation describing short pulses and ultra-focused beams predicts the existence of a minimal spatial or temporal scale which in turn implies the existence of maximally localized states. Using a Gaussian wavepacket with complex phase, we derive the corresponding generalized uncertainty relation and its maximally localized states. Furthermore, we numerically show that the presence of nonlinearity helps the system to reach its maximal localization. Our results may trigger further theoretical and experimental tests for practical applications and analogues of fundamental physical theories.
Ornigotti, M., Villari, L.D.M., Szameit, A., Conti, C.(2017)“Squeezing of X-waves with orbital angular momentum”, Phys. Rev. A, 95(1), 011802, available: http://link.aps.org/doi/10.1103/PhysRevA.95.011802.
Multilevel quantum protocols may potentially supersede standard quantum optical polarization-encoded protocols in terms of amount of information transmission and security. However, for free-space telecommunications, we do not have tools for limiting loss due to diffraction and perturbations, as, for example, turbulence in air. Here we study propagation invariant quantum
waves with angular momentum; this representation expresses the electromagnetic field as a quantum gas of weakly interacting bosons. The resulting spatiotemporal quantized light pulses are not subject to diffraction and dispersion, and are intrinsically resilient to disturbances in propagation. We show that spontaneous down-conversion generates squeezed
waves useful for quantum protocols. Surprisingly, the orbital angular momentum affects the squeezing angle, and we predict the existence of a characteristic axicon aperture for maximal squeezing. These results may boost the applications in free space of quantum optical transmission and multilevel quantum protocols, and may also be relevant for novel kinds of interferometers, such as satellite-based gravitational wave detectors.
Marcucci, G., Braidotti, M.C., Gentilini, S., Conti, C.(2017)“Time Asymmetric Quantum Mechanics and Shock Waves: Exploring the Irreversibility in Nonlinear Optics”, Annalen der Physik, 1600349--n/a, available: http://dx.doi.org/10.1002/andp.201600349.
Abstract Fractional calculus is entering the field of nonlinear optics to describe unconventional regimes, as disorder biological media and soft-matter. Here we investigate spatiotemporal modulational instability (MI) in a fractional nonlinear Schrödinger equation. We derive the \MI\ gain spectrum in terms of the Lévy indexes and a varying number of spatial dimensions. We show theoretically and numerically that the Lévy indexes affect fastest growth frequencies and \MI\ bandwidth and gain. Our results unveil a very rich scenario that may occur in the propagation of ultrashort pulses in random media and metamaterials, and may sustain novel kinds of propagation invariant optical bullets.
Saleh, M.F., Conti, C., Biancalana, F.(2017)“Anderson localisation and optical-event horizons in rogue-soliton generation”, Opt. Express, 25(5), 5457--5465, available: http://www.opticsexpress.org/abstract.cfm?URI=oe-25-5-5457.
We unveil the relation between the linear Anderson localisation process and nonlinear modulation instability. Anderson localised modes are formed in certain temporal intervals due to the random background noise. Such localised modes seed the formation of solitary waves that will appear during the modulation instability process at those preferred intervals. Afterwards, optical-event horizon effects between dispersive waves and solitons produce an artificial collective acceleration that favours the collision of solitons, which could eventually lead to a rogue-soliton generation.
Gentilini, S., Conti, C.(2017)“Nonlocal Disordered Media and Experiments in Disordered Fibers”, in Boscolo, S. and Finot, C., eds., Shaping Light In Nonlinear Optical Fibers, John Wiley & Sons, 395, available: http://eu.wiley.com/WileyCDA/WileyTitle/productCd-1119088127.html.
The use of geometrical constraints exposes many new perspectives in photonics and in fundamental studies of nonlinear waves. By implementing surface structures in vertical cavity surface emitting lasers as manifolds for curved space, we experimentally study the impacts of geometrical constraints on nonlinear wave localization. We observe localized waves pinned to the maximal curvature in an elliptical-ring, and confirm the reduction in the localization length of waves by measuring near and far field patterns, as well as the corresponding energy-angle dispersion relation. Theoretically, analyses based on a dissipative model with a parabola curve give good agreement remarkably to experimental measurement on the reduction in the localization length. The introduction of curved geometry allows to control and design lasing modes in the nonlinear regime.
Braidotti, M.C., Marcucci, G., Gentillini, S., Conti, C.(2017)“Irreversibility and squeezing of shock waves”, in Nonlinear Guided Wave Optics, 2053-2563, IOP Publishing, 3-1 to 3-18, available: http://dx.doi.org/10.1088/978-0-7503-1460-2ch3.
Are shock waves reversible? Despite the wide range of investigation in nonlinear fiber optics and the general theory of nonlinear waves, until now we could not answer to this intriguing and fundamental question. In this chapter, the time asymmetric behavior of optical shock waves in nonlinear nonlocal media is presented. The introduction of exponentially decaying eigenstates, called Gamow vectors, allows answering to this irreversibility problem. Our novel theoretical approach unveils new features as decay rates quantization and non-classical states generation. Furthermore, we report on squeezing of light during shock excitation and present the expected experimental signatures of this squeezing dynamics.
Pierangeli, D., Conti, C., DelRe, E.(2017)“Rogue waves in photorefractive media”, in Nonlinear Guided Wave Optics, 2053-2563, IOP Publishing, 8-1 to 8-21, available: http://dx.doi.org/10.1088/978-0-7503-1460-2ch8.
In this chapter, we discuss rogue waves emerging as light beams that undergo highly-nonlinear regimes of propagation in photorefractive ferroelectric crystals. The interplay between unstable nonlinear wave dynamics and hosting disorder gives rise to a strong turbulent scenario characterized by localized spatial waves of extreme intensity populating long-tailed statistics. We describe basic experiments, with emphasis on the direct study of the role played by saturable nonlinearity, spatial solitons, system dimensionality, and incoherence of the optical field in the appearance and properties of extreme wave events.
Braidotti, M.C., Mecozzi, A., Conti, C.(2017)“Squeezing in a nonlocal photon fluid”, Phys. Rev. A, 96(4), 043823, available: https://link.aps.org/doi/10.1103/PhysRevA.96.043823.
A landmark of statistical mechanics, spin-glass theory describes critical phenomena in disordered systems that range from condensed matter to biophysics and social dynamics. The most fascinating concept is the breaking of replica symmetry: identical copies of the randomly interacting system that manifest completely different dynamics. Replica symmetry breaking has been predicted in nonlinear wave propagation, including Bose-Einstein condensates and optics, but it has never been observed. Here, we report the experimental evidence of replica symmetry breaking in optical wave propagation, a phenomenon that emerges from the interplay of disorder and nonlinearity. When mode interaction dominates light dynamics in a disordered optical waveguide, different experimental realizations are found to have an anomalous overlap intensity distribution that signals a transition to an optical glassy phase. The findings demonstrate that nonlinear propagation can manifest features typical of spin-glasses and provide a novel platform for testing so-far unexplored fundamental physical theories for complex systems.
Braidotti, M.C., Conti, C.(2017)“Quantum simulation of rainbow gravity by nonlocal nonlinearity”, ArXiv e-prints.
Fusaro, A., Garnier, J., Xu, G., Conti, C., Faccio, D., Trillo, S., Picozzi, A.(2017)“Emergence of long-range phase coherence in nonlocal fluids of light”, Phys. Rev. A, 95(6), 063818, available: https://link.aps.org/doi/10.1103/PhysRevA.95.063818.
Abstract We measured the optical absorption spectra of thermally treated, gamma irradiated LiF crystals, as a function of temperature in the range 16–300 K. The temperature dependence of intensity, peak position and bandwidth of F and M absorption bands were obtained.
The design of optical systems capable of processing and manipulating ultra-short pulses and ultra-focused beams is highly challenging with far reaching fundamental technological applications. One key obstacle routinely encountered while implementing sub-wavelength optical schemes is how to overcome the limitations set by standard Fourier optics. A strategy to overcome these difficulties is to utilize the concept of generalized uncertainty principle (G-UP) that has been originally developed to study quantum gravity. In this paper we propose to use the concept of G-UP within the framework of optics to show that the generalized Schrodinger equation describing short pulses and ultra-focused beams predicts the existence of a minimal spatial or temporal scale which in turn implies the existence of maximally localized states. Using a Gaussian wavepacket with complex phase, we derive the corresponding generalized uncertainty relation and its maximally localized states. We numerically show that the presence of nonlinearity helps the system to reach its maximal localization. Our results may trigger further theoretical and experimental tests for practical applications and analogues of fundamental physical theories.
Conti, C.(2016)“The Enlightened Game of Life in Designing Beauty: The Art of Cellular Automata”, in Andrew Adamatzky, G.J.M., ed., Emergence, Complexity And Computation, Springer International Publishing, 83-86, available: http://link.springer.com/book/10.1007/978-3-319-27270-2.
The investigation of the interplay between geometry and nonlinearity may open the road to the control of extreme waves. We study three-dimensional localization and dispersive shocks in a bent cigar shaped potential by the nonlinear Schrödinger equation. At high bending and high nonlinearity, topological trapping is frustrated by the generation of curved wave-breaking. Four-dimensional parallel simulations confirm the theoretical model. This work may contribute to novel devices based on geometrically constrained highly nonlinear dynamics and tests and analogs of fundamental physical theories in curved space.
Leonetti, M., Karbasi, S., Mafi, A., DelRe, E., Conti, C.(2016)“Secure information transport by transverse localization of light”, Scientific Reports, 6, 29918, available: http://dx.doi.org/10.1038/srep29918.
A single-photon beating with itself can produce even the most elaborate optical fringe pattern. However, the large amount of information enclosed in such a pattern is typically inaccessible, since the complete distribution can be visualized only after many detections. In fact this limitation is only true for delocalized patterns. Here we demonstrate how reconfigurable localized optical patterns allow to encode up to 6 bits of information in disorder-induced high transmission channels, even using a small number of photon counts. We developed a quantum key distribution scheme for fiber communication in which high information capacity is achieved through position and momentum complementarity.
We have very little experience of the quantum dynamics of the ubiquitous nonlinear waves. Observed phenomena in high energy physics are perturbations to linear waves, and classical nonlinear waves, like solitons, are barely affected by quantum effects. We know that solitons, immutable in classical physics, exhibit collapse and revivals according to quantum mechanics. However this effect is very weak and has never been observed experimentally. By predicting black hole evaporation Hawking first introduced a distinctly quantum effect in nonlinear gravitational physics.Here we show the existence of a general and universal quantum process whereby a soliton emits quantum radiation with a specific frequency content, and a temperature given by the number of quanta, the soliton Schwarzschild radius, and the amount of nonlinearity, in a precise and surprisingly simple way. This result may ultimately lead to the first experimental evidence of genuine quantum black hole evaporation. In addition, our results show that black hole radiation occurs in a fully quantised theory, at variance with the common approach based on quantum field theory in a curved background; this may provide insights into quantum gravity theories. Our findings also have relevance to the entire field of nonlinear waves, including cold atomic gases and extreme phenomena such as shocks and rogue-waves. Finally, the predicted effect may potentially be exploited for novel tunable quantum light sources.
Ornigotti, M., Conti, C., Szameit, A.(2016)“Cylindrically polarized nondiffracting optical pulses”, Journal of Optics, 18(7), 075605, available: http://stacks.iop.org/2040-8986/18/i=7/a=075605.
We extend the concept of radially and azimuthally polarized optical beams to the polychromatic domain by introducing cylindrically polarized nondiffracting optical pulses. In particular, we discuss in detail the case of cylindrically polarized X-waves, both in the paraxial and nonparaxial regime. The explicit expressions for the electric and magnetic fields of cylindrically polarized X-waves are also reported.
The description of shock waves beyond the shock point is a challenge in nonlinear physics. Finding solutions to the global dynamics of dispersive shock waves is not always possible due to the lack of integrability. Here we propose a new method based on the eigenstates (Gamow vectors) of a reversed harmonic oscillator in a rigged Hilbert space. These vectors allow analytical formulation for the development of undular bores of shock waves in a nonlinear nonlocal medium. Experiments by a photothermal induced nonlinearity confirm theoretical predictions: as the undulation period as a function of power and the characteristic quantized decays of Gamow vectors. Our results demonstrate that Gamow vector are a novel and effective paradigm for describing extreme nonlinear phenomena.
Marcucci, G., Conti, C.(2016)“Irreversible evolution of a wave packet in the rigged-Hilbert-space quantum mechanics”, Phys. Rev. A, 94(5), 052136, available: http://link.aps.org/doi/10.1103/PhysRevA.94.052136.
It is well known that a state with complex energy cannot be the eigenstate of a self-adjoint operator, such as the Hamiltonian. Resonances, i.e., states with exponentially decaying observables, are not vectors belonging to the conventional Hilbert space. One can describe these resonances in an unusual mathematical formalism based on the so-called rigged Hilbert space (RHS). In the RHS, the states with complex energy are denoted as Gamow vectors (GVs), and they model decay processes. We study the GVs of the reversed harmonic oscillator, and we analytically and numerically investigate the unstable evolution of wave packets. We introduce the background function to study initial data that are not composed only by a summation of GVs, and we analyze different wave packets belonging to specific function spaces. Our work furnishes support for the idea that irreversible wave propagation can be investigated using rigged-Hilbert-space quantum mechanics and provides insight for the experimental investigation of irreversible dynamics.
Braidotti, M.C., Gentilini, S., Marcucci, G., DelRe, E., Conti, C.(2016)“Nonlinear Gamow vectors in nonlocal optical propagation”, Il Nuovo Cimento C, 39, 281, available: https://www.sif.it/riviste/ncc/econtents/2016/039/02/article/9.
Shock waves dominate in a wide variety of fields in physics dealing
with nonlinear phenomena, nevertheless the description of their evolution is not
resolved for the entire dynamics. Here we propose an analytical method based on
Gamow vectors, which belong to irreversible quantum mechanics. We theoretically
and experimentally show the appearance of these decaying states during shock evolution
allowing to describe the whole wave propagation. These results open new ways
to the control of extreme nonlinear regimes such as supercontinuum generation or
in the analogies of fundamental physical theories.
Palmieri, V., Papi, M., Conti, C., Ciasca, G., Maulucci, G., Spirito, M.D.(2016)“The future development of bacteria fighting medical devices: the role of graphene oxide”, Expert Review of Medical Devices, available: http://dx.doi.org/10.1080/17434440.2016.1245612.
AbstractThe clinical challenge that research on antibacterial coatings faces nowadays is the need of reduction of resistant bacterial infections, major source of implant rejection and repeated surgery.In order to avoid microorganisms attachment and biofilm formation, coating materials on medical devices have been developed with shortcomings represented by short-term durability and induction of new mechanisms of bacterial resistance.Graphene-based films and hydrogel could represent the next generation protective coatings due to their excellent mechanical, chemical and thermal properties, high nanoparticle adsorption and antibacterial action. In this short commentary, we will report the recent developments of graphene oxide based coatings. Graphene oxide is a water-soluble derivative of graphene that allows high drug loading and miscibility with polymers, making it mouldable in any desired shape. Recent applications in wound healing and tissue engineering will be discussed as well as critical issues prior to clinical use of graphene oxide coatings.
Papi, M., Palmieri, V., Bugli, F., Spirito, M.D., Sanguinetti, M., Ciancico, C., Braidotti, M.C., Gentilini, S., Angelani, L., Conti, C.(2016)“Biomimetic antimicrobial cloak by graphene-oxide agar hydrogel”, Scientific Reports, 6(1), available: http://dx.doi.org/10.1038/s41598-016-0010-7.
We report the direct observation of the onset of turbulence in propagating one-dimensional optical waves. The transition occurs as the disordered hosting material passes from being linear to one with extreme nonlinearity. As the response grows, increased wave interaction causes a modulational unstable quasihomogeneous flow to be superseded by a chaotic and spatially incoherent one. Statistical analysis of high-resolution wave behavior in the turbulent regime unveils the emergence of concomitant rogue waves. The transition, observed in a photorefractive ferroelectric crystal, introduces a new and rich experimental setting for the study of optical wave turbulence and information transport in conditions dominated by large fluctuations and extreme nonlinearity.
Pierangeli, D., Musarra, G., Di Mei, F., Di Domenico, G., Agranat, A.J., Conti, C., DelRe, E.(2016)“Enhancing optical extreme events through input wave disorder”, Phys. Rev. A, 94(6), 063833, available: http://link.aps.org/doi/10.1103/PhysRevA.94.063833.
Ghofraniha, N., La Volpe, L., Van Opdenbosch, D., Zollfrank, C., Conti, C.(2016)“Biomimetic Random Lasers with Tunable Spatial and Temporal Coherence”, Advanced Optical Materials, available: http://dx.doi.org/10.1002/adom.201600649.
Dispersive shock waves dominate wave-breaking phenomena in Hamiltonian systems. In the absence of loss, these highly irregular and disordered waves are potentially reversible. However, no experimental evidence has been given about the possibility of inverting the dynamics of a dispersive shock wave and turn it into a regular wave-front. Nevertheless, the opposite scenario, i.e., a smooth wave generating turbulent dynamics is well studied and observed in experiments. Here we introduce a new theoretical formulation for the dynamics in a highly nonlocal and defocusing medium described by the nonlinear Schroedinger equation. Our theory unveils a mechanism that enhances the degree of irreversibility. This mechanism explains why a dispersive shock cannot be reversed in evolution even for an arbitrarirly small amount of loss. Our theory is based on the concept of nonlinear Gamow vectors, i.e., power dependent generalizations of the counter-intuitive and hereto elusive exponentially decaying states in Hamiltonian systems. We theoretically show that nonlinear Gamow vectors play a fundamental role in nonlinear Schroedinger models: they may be used as a generalized basis for describing the dynamics of the shock waves, and affect the degree of irreversibility of wave-breaking phenomena. Gamow vectors allow to analytically calculate the amount of breaking of time-reversal with a quantitative agreement with numerical solutions. We also show that a nonlocal nonlinear optical medium may act as a simulator for the experimental investigation of quantum irreversible models, as the reversed harmonic oscillator.
Antenucci, F., Conti, C., Crisanti, A., Leuzzi, L.(2015)“General Phase Diagram of Multimodal Ordered and Disordered Lasers in Closed and Open Cavities”, Phys. Rev. Lett., 114, 043901, available: http://link.aps.org/doi/10.1103/PhysRevLett.114.043901.
Ornigotti, M., Conti, C., Szameit, A.(2015)“Universal form of the carrier frequency of scalar and vector paraxial $X$ waves with orbital angular momentum and arbitrary frequency spectrum”, Phys. Rev. A, 92, 043801, available: http://link.aps.org/doi/10.1103/PhysRevA.92.043801.
Leonetti, M., Conti, C.(2015)“Observation of three dimensional optical rogue waves through obstacles”, Applied Physics Letters, 106(25), -, available: http://scitation.aip.org/content/aip/journal/apl/106/25/10.1063/1.4922552.
Armaroli, A., Conti, C., Biancalana, F.(2015)“Rogue solitons in optical fibers: a dynamical process in a complex energy landscape?”, Optica, 2(5), 497--504, available: http://www.osapublishing.org/optica/abstract.cfm?URI=optica-2-5-497.
Nondeterministic giant waves, denoted as rogue, killer, monster, or freak waves, have been reported in many different branches of physics. Their physical interpretation is however still debated: despite massive numerical and experimental evidence, a solid explanation for their spontaneous formation has not been identified yet. Here we propose that rogue waves \more precisely, rogue solitons (RSs)\ in optical fibers may actually result from a complex dynamical process very similar to well-known mechanisms such as glass transitions and protein folding. We describe how the interaction among optical solitons produces an energy landscape in a highly dimensional parameter space with multiple quasi-equilibrium points. These configurations have the same statistical distribution of the observed rogue events and are explored during the light dynamics due to soliton collisions, with inelastic mechanisms enhancing the process. Slightly different initial conditions lead to very different dynamics in this complex geometry; a RS turns out to stem from one particularly deep quasi-equilibrium point of the energy landscape in which the system may be transiently trapped during evolution. This explanation will prove to be fruitful to the vast community interested in freak waves.
Ghofraniha, N., Viola, I., Di Maria, F., Barbarella, I., Gigli, G., Leuzzi, L., Conti, C.(2015)“Experimental evidence of replica symmetry breaking in random lasers”, Nat Commun, 6, 6058, available: http://www.nature.com/ncomms/2015/150114/ncomms7058/full/ncomms7058.html.
Spin-glass theory is one of the leading paradigms of complex physics and describes condensed matter, neural networks and biological systems, ultracold atoms, random photonics and many other research fields. According to this theory, identical systems under identical conditions may reach different states. This effect is known as replica symmetry breaking and is revealed by the shape of the probability distribution function of an order parameter named the Parisi overlap. However, a direct experimental evidence in any field of research is still missing. Here we investigate pulse-to-pulse fluctuations in random lasers, we introduce and measure the analogue of the Parisi overlap in independent experimental realizations of the same disordered sample, and we find that the distribution function yields evidence of a transition to a glassy light phase compatible with a replica symmetry breaking.
Mei, F.D., Pierangeli, D., Parravicini, J., Conti, C., Agranat, A.J., DelRe, E.(2015)“Observation of diffraction cancellation for nonparaxial beams in the scale-free-optics regime”, Phys. Rev. A, 92, 013835, available: http://link.aps.org/doi/10.1103/PhysRevA.92.013835.
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.
Ornigotti, M., Aiello, A., Conti, C.(2015)“Goos Hnchen and Imbert Fedorov shifts for paraxial X-waves”, Opt. Lett., 40(4), 558--561, available: http://ol.osa.org/abstract.cfm?URI=ol-40-4-558.
We present a theoretical analysis for the Goos&\#x2013;H&\#xE4;nchen and Imbert&\#x2013;Fedorov shifts experienced by an X-wave upon reflection from a dielectric interface. We show that the temporal chirp, as well as the bandwidth of the X-wave, directly affect the spatial shifts in a way that can be experimentally observed, while the angular shifts do not depend on the spectral features of the X-wave. A dependence of the spatial shifts on the spatial structure of the X-wave is also discussed.
Gentilini, S., Chiara, B.M., Marcucci, G., DelRe, E., Conti, C.(2015)“Physical realization of the Glauber quantum oscillator”, Scientific Reports, 5, 15816, available: http://www.nature.com/articles/srep15816.
Abstract We investigate the effect of non-paraxiality in the dynamics of spatial dispersive shock waves in the defocusing nonlinear Schrödinger equation. We find that the lowest order correction in the degree on non-paraxiality enhances the wave-breaking and imposes a limit to the highest achievable spatial spectral content generated by the shocks.
Mei, F.D., Parravicini, J., Pierangeli, D., Conti, C., Agranat, A.J., DelRe, E.(2014)“Anti-diffracting beams through the diffusive optical nonlinearity”, Opt. Express, 22(25), 31434--31439, available: http://www.opticsexpress.org/abstract.cfm?URI=oe-22-25-31434.
Anti-diffraction is a theoretically predicted nonlinear optical phenomenon that occurs when a light beam spontaneously focalizes independently of its intensity. We observe anti-diffracting beams supported by the peak-intensity-independent diffusive nonlinearity that are able to shrink below their diffraction-limited size in photorefractive lithium-enriched potassium-tantalate-niobate (KTN:Li).
Pasquazi, A., Peccianti, M., Clerici, M., Conti, C., Morandotti, R.(2014)“Collapse Arrest in Instantaneous Kerr Media via Parametric Interactions”, Phys. Rev. Lett., 113(13), 133901, available: http://link.aps.org/doi/10.1103/PhysRevLett.113.133901.
Leonetti, M., Karbasi, S., Mafi, A., Conti, C.(2014)“Observation of Migrating Transverse Anderson Localizations of Light in Nonlocal Media”, Phys. Rev. Lett., 112, 193902, available: http://link.aps.org/doi/10.1103/PhysRevLett.112.193902.
Silica aerogels are materials well suited for high power nonlinear optical applications. In such regime, the non-trivial thermal properties may give rise to the generation of optical shock waves, which are also affected by the structural disorder due to the porous solid-state gel. Here we report on an experimental investigation in terms of beam waist and input power, and identify various regimes of the generation of wave-breaking phenomena in silica aerogels.
We propose an interpretation of the pronounced &\#x201C;M&\#x201D; spectral shape that is a recurrent feature in all-normal-dispersion mode-locked fiber laser dynamics. Our interpretation involves shock wave formation regularized by dissipation, modeled by a modified Burgers equation. The large fringes appearing at the edges of the spectrum result from discontinuities in the spectral phase.
Conti, C.(2014)“Quantum gravity simulation by nonparaxial nonlinear optics”, Phys. Rev. A, 89, 061801, available: http://link.aps.org/doi/10.1103/PhysRevA.89.061801.
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 fibres
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 fibres allow a more efficient focusing action with respect
to standard fibres in a way independent of their length, because
of the propagation-invariant features and cooperative action of transverse
Leonetti, M., Karbasi, S., Mafi, A., Conti, C.(2014)“Experimental observation of disorder induced self-focusing in optical fibers”, Applied Physics Letters, 105(17), -, available: http://scitation.aip.org/content/aip/journal/apl/105/17/10.1063/1.4900781.
We demonstrate an electro-optic response that is linear in the amplitude but independent of the sign of the applied electric field. The symmetry-preserving linear electro-optic effect emerges at low applied electric fields in freezing nanodisordered KNTN above the dielectric peak temperature, deep into the nominal paraelectric phase. Strong temperature dependence allows us to attribute the phenomenon to an anomalously reduced thermal agitation in the reorientational response of the underlying polar-nanoregions.
Conti, C.(2014)“Quantum X-waves and Applications in Nonlinear Optics”, in Hernandez-Figueroa, H.E., Recami, E. and Zamboni-Rached, M., eds., Non-Diffracting Waves, Wiley-VCH Verlag GmbH & Co. KGaA, 231--245.
A transparent material exhibits ultrafast optical nonlinearity and is subject to optical pressure if irradiated by a laser beam. However, the effect of nonlinearity on optical pressure is often overlooked, even if a nonlinear optical pressure may be potentially employed in many applications, such as optical manipulation, biophysics, cavity optomechanics, quantum optics, and optical tractors, and is relevant in fundamental problems such as the Abraham-Minkoswky dilemma or the Casimir effect. Here, we show that an ultrafast nonlinear polarization gives indeed a contribution to the optical pressure that also is negative in certain spectral ranges; the theoretical analysis is confirmed by first-principles simulations. An order-of-magnitude estimate shows that the effect can be observable by measuring the deflection of a membrane made by graphene.
Strinati, M.C., Conti, C.(2014)“Bose-Einstein condensation of photons with nonlocal nonlinearity in a dye-doped graded-index microcavity”, Phys. Rev. A, 90, 043853, available: http://link.aps.org/doi/10.1103/PhysRevA.90.043853.
We study the random walk of solitons and characteristic lines of shock fronts in the presence of disorder for the one-dimensional nonlinear Schrödinger equation in Kerr-like media. We analyze the interplay of nonlocality and randomness, and the way their competition affects strongly coherent nonlinear waves is theoretically and numerically investigated.
We study experimentally the aging of optical spatial solitons in a dipolar glass hosted by a nanodisordered sample of photorefractive potassium-sodium-tantalate-niobate (KNTN). As the system ages, the waves erratically explore varying strengths of the nonlinear response, causing them to break up and scatter. We show that this process can still lead to solitons, but in a generalized form for which the changing response is compensated by changing the normalized wave size and intensity so as to maintain fixed the optical waveform.
Conti, C.(2013)“Localization and shock waves in curved manifolds for the Gross-Pitaevskii equation”, arXiv:1302.3806, available: http://arxiv.org/abs/1302.3806.
We investigate the dynamics of a Bose-Einstein condensate in a progressively bended three dimensional cigar shaped potential. The interplay between geometry and nonlinearity is considered. At high curvature, topological localization occurs and becomes frustrated by the generation of curved dispersive shock-waves when the strength of nonlinearity is increased. The analysis is supported by four-dimensional parallel simulations.
Ghofraniha, N., Viola, I., Zacheo, A., Arima, V., Gigli, G., Conti, C.(2013)“Transition from nonresonant to resonant random lasers by the geometrical confinement of disorder”, Opt. Lett., 38(23), 5043--5046, available: http://ol.osa.org/abstract.cfm?URI=ol-38-23-5043.
We report on a transition in random lasers that is induced by the geometrical confinement of the emitting material. Different dye doped paper devices with controlled geometry are fabricated by soft lithography and show two distinguished behaviors in the stimulated emission: in the absence of boundary constraints, the energy threshold decreases for larger laser volumes showing the typical trend of diffusive nonresonant random lasers, while when the same material is lithographed into channels, the walls act as cavity and the resonant behavior typical of standard lasers is observed. The experimental results are consistent with the general theories of random and standard lasers and a clear phase diagram of the transition is reported.
Viola, I., Ghofraniha, N., Zacheo, A., Arima, V., Conti, C., Gigli, G.(2013)“Random laser emission from a paper-based device”, J. Mater. Chem. C, 1, 8128, available: http://dx.doi.org/10.1039/C3TC31860E.
Random laser emission is obtained from a fluidic paper-based device realized by conventional soft-lithography techniques on common, flexible, renewable and biocompatible commercial paper. The device is realized exclusively on paper by creating microfluidic porous channels on the cellulose fibres, in which a laser dye (Rhodamine B) can flow by capillarity. The modulation of the random lasing characteristics, in terms of threshold and spectral position, can be tailored by acting on the confinement induced by the lithographic process as well as on the shape and functionalization at the interface of the emitting regions.
Gentilini, S., Ghofraniha, N., DelRe, E., Conti, C.(2013)“Shock waves in thermal lensing”, Phys. Rev. A, 87, 053811, available: http://link.aps.org/doi/10.1103/PhysRevA.87.053811.
DÁguanno, G., Mattiucci, N., Conti, C., Bloemer, M.J.(2013)“Field localization and enhancement near the Dirac point of a finite defectless photonic crystal”, Phys. Rev. B, 87(8), 085135, available: http://link.aps.org/doi/10.1103/PhysRevB.87.085135.
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 that determines the amplification rate. The results also apply to cubic nonlinearities.
Leonetti, M., Conti, C., Lopez, C.(2013)“Non-locality and collective emission in disordered lasing resonators”, Light Sci Appl, 2, e88--, available: http://dx.doi.org/10.1038/lsa.2013.44.
The random laser emission from the functionalized thienyl-S,S-dioxide quinquethiophene (T5OCx) in confined patterns with different shapes is demonstrated. Functional patterning of the light emitter organic material in well defined features is obtained by spontaneous molecular self-assembly guided by surface tension driven (STD) lithography. Such controlled supramolecular nano-aggregates act as scattering centers allowing the fabrication of one-component organic lasers with no external resonator and with desired shape and efficiency. Atomic force microscopy shows that different geometric pattern with different supramolecular organization obtained by the lithographic process tailors the coherent emission properties by controlling the distribution and the size of the random scatterers.
Dong, H., Conti, C., Marini, A., Biancalana, F.(2013)“Terahertz relativistic spatial solitons in doped graphene metamaterials”, Journal of Physics B: Atomic, Molecular and Optical Physics, 46(15), 155401, available: http://stacks.iop.org/0953-4075/46/i=15/a=155401.
We propose an electrically tunable graphene-based metamaterial that shows a large nonlinear optical response at THz frequencies. The responsible nonlinearity comes from the intraband current, which we are able to calculate analytically. We demonstrate that the proposed metamaterial supports stable 2D spatial solitary waves. Our theoretical approach is not restricted to graphene, but can be applied to all materials exhibiting a conical dispersion supporting massless Dirac fermions.
Leonetti, M., Conti, C., Lopez, C.(2013)“Dynamics of phase-locking random lasers”, Phys. Rev. A, 88(4), 043834, available: http://link.aps.org/doi/10.1103/PhysRevA.88.043834.
Laser modes may coalesce into a mode-locked state that enables femtosecond pulse compression. The nature of the interaction and the interaction time play fundamental roles in the onset of this collective state, but the investigation of the transition dynamics is technically challenging because phases are not always experimentally accessible. This is even more difficult for random lasers, a kind of disordered laser in which energies in play are much smaller than in the ordered macroscopic case. Here we investigate experimentally and numerically the dynamics of the phase-locking transition in a random laser. We developed an experimental setup able to pump individual modes with different pulse durations and found that the mode-locked regime builds only for quasicontinuous pumping, resulting in an emission linewidth dependent on the pump duration. Numerical simulation confirms experimental data.
Leonetti, M., Conti, C., Lopez, C.(2013)“Switching and amplification in disordered lasing resonators”, Nat Commun, 4, 1740, available: http://dx.doi.org/10.1038/ncomms2777.
Using the history dependence of a dipolar glass hosted in a compositionally disordered lithium-enriched potassium tantalate niobate (KTN:Li) crystal, we demonstrate scale-free optical propagation at tunable temperatures. The operating equilibration temperature is determined by previous crystal spiralling in the temperature/cooling-rate phase space.
Gentilini, S., Ghofraniha, N., DelRe, E., Conti, C.(2012)“Shock wave far-field in ordered and disordered nonlocal media”, Opt. Express, 20(24), 27369--27375, available: http://www.opticsexpress.org/abstract.cfm?URI=oe-20-24-27369.
We investigate the far field of a spatial dispersive shock wave generated
from a Gaussian beam propagating in nonlinear nonlocal colloidal
disordered media. The interplay between nonlinearity and structural
randomness is quantified in terms of the threshold power for the
occurrence of the shock wave.
We demonstrate rejuvenation in scale-free optical propagation. The
phenomenon is caused by the non-ergodic relaxation of the dipolar
glass that mediates the photorefractive nonlinearity in compositionally-disordered
lithium-enriched potassium-tantalate-niobate (KTN:Li). We implement
rejuvenation to halt aging in the dipolar glass and extend the duration
of beam diffraction cancellation.
Butsch, A., Conti, C., Biancalana, F., Russell, P.S.J.(2012)“Optomechanical Self-Channeling of Light in a Suspended Planar Dual-Nanoweb Waveguide”, Phys. Rev. Lett., 108, 093903, available: http://link.aps.org/doi/10.1103/PhysRevLett.108.093903.
We theoretically and numerically investigate the effect of focusing and defocusing nonlinearities on Anderson localization in highly nonlocal media. A perturbative approach is developed to solve the nonlocal nonlinear Schroedinger equation in the presence of a random potential, showing that nonlocality stabilizes Anderson states.
Leonetti, M., Conti, C., López, C.(2012)“Tunable degree of localization in random lasers with controlled interaction”, Applied Physics Letters, 101(5), 051104, available: http://link.aip.org/link/?APL/101/051104/1.
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.
Folli, V., Conti, C.(2012)“Two-level laser by the interaction of self-induced transparency pulses and surface Anderson localizations of light”, J. Opt. Soc. Am. B, 29(8), 2080--2089, available: http://josab.osa.org/abstract.cfm?URI=josab-29-8-2080.
Self-induced transparency (SIT) pulses induce a traveling population
inversion in two-level atoms. As a rule, the active medium in which
the soliton travels has to be homogeneous. Here, we study the effect
of a spatially disordered modulation in the refractive index profile
that may lead to Anderson localizations. The interplay between the
ultrashort SIT pulse, a nonlinear effect, and this kind of disorder-induced
mode exhibits intriguing features. Once the SIT pulse is confined
in the spatially confined regions, they act as closed cavities for
the SIT population inversion. A positive optical feedback mechanism
can be thus activated and, as a result, a two-level laserlike emission
can be obtained.
We show how the cross-over effect of dipolar glasses can be used to observe diffraction cancellation in composite ferroelectric samples independently of composition. We are able to selectively frustrate the dielectric anomaly of different compositionally disordered photorefractive ferroelectrics to achieve scale-free optical propagation at one same temperature.
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.
Conti, C.(2012)“Solitonization of the Anderson localization”, Phys. Rev. A, 86, 061801(R), available: http://link.aps.org/doi/10.1103/PhysRevA.86.061801.
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.
Granular materials have been studied for decades, driven by industrial and technological applications. These very simple systems, composed of 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) to the arrangement of grains; they are not substantially affected by thermal phenomena, but can be controlled by mechanical solicitations. Laser emission from shaken granular matter is so far unexplored. Here we provide experimental evidence that laser emission can be affected and controlled by the status of the motion of the granular material; 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.
Ghofraniha, N., Santamaria Amato, L., Folli, V., Trillo, S., DelRe, E., Conti, C.(2012)“Measurement of scaling laws for shock waves in thermal nonlocal media”, Optics Letters, 37(12), 2325--2327, available: http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-37-12-2325.
We are able to detect the details of spatial optical collisionless wave-breaking through the high aperture imaging of a beam suffering shock in a fluorescent nonlinear nonlocal thermal medium. This allows us to directly measure how nonlocality and nonlinearity affect the point of shock formation and compare results with numerical simulations.
Ghofraniha, N., Andr`e, P., Di falco, A., Conti, C.(2012)“Multimode SPASER in nano-colloidal plasmonic systems”, ArXiv e-prints arXiv:1208.5321, available: http://adsabs.harvard.edu/abs/2012arXiv1208.5321G.
Nano-Sized light emitting devices have a wide range of potential applications, from medicine to all-optical computing. Surface plasmon amplification by stimulated emission of radiation (SPASER) have recently attracted considerable attention providing the smallest reliable lasers. Plasmonic assisted lasing has been demonstrated in a variety of geometries, embedded in or embedding a gain media. Despite these efforts, the physics underlying a realistic SPASER remains to date only drafted, because of the controversial and the limited number of experimental results. Here we demonstrate unambiguous coherent emission from polyhedral silver nano-particles dispersed in liquid gain media evidenced clearly by the narrowing of the spectral linewidth up to 3-5 nm, the several competing SPASER modes and the nonlinear effects as emission saturation and energy dependent spectral shifts. These novel results open the way to a variety of applications, as the emission of ultrashort optical pulses from nano-size lasers.
Conti, C., DelRe, E.(2012)“Scale-Free Optics”, in Chen, Z. and Morandotti, R., eds., Springer Series In Optical Science, Springer, 207-230.
The most fascinating images and patterns emerge when light diffracts from minute structures. Even the image of an otherwise featureless hole produces enthralling ripples that spread out to invest space and form what is known as an Airy pattern. It is a basic fact that diffraction becomes dominant when the size of the feature becomes micrometric, and the transmitted wave has an angular spread that depends on the size of the aperture d measured in units of the optical wavelength. From a practical perspective, diffraction represents a major obstacle to imaging of finer details, and a great research effort is continuously exerted to overcome it. In fact, diffraction spreads the optical wave and blurs the spatial information encoded in the optical beam. Consider an image composed of separate pixels of characteristic size d and spacing l.
Leonetti, M., Conti, C., López, C.(2012)“Random laser tailored by directional stimulated emission”, Phys. Rev. A, 85(4), 043841, available: http://link.aps.org/doi/10.1103/PhysRevA.85.043841.
The spectrum of a random laser (RL) may appear either as a set of sharp resonances or as a smooth line superimposed on the fluorescence. Recently, Leonetti, Conti, and Lopez Nat. Photonics 5 615 (2011) accounted for this duality with the onset of a mode-locked regime that is triggered by the increase of both the number of activated modes and of the intermode interaction and is accompanied by a pulse shortening. Here we report an extensive review of the experimental approach used by Leonetti et al., including the sample preparation and the particulars of the setup. Here we describe also the way in which our approach allows us either to set the degree of interaction between modes or to have a certain degree of control over the effective set of resonances brought to lasing. Moreover we report an investigation on the spatial properties of the RL which brings further confirmation of the synchronization picture, whose physical origin is deepened.
We theoretically investigate the stability and dynamics of self-channeled beams that form via nonlocal optomechanical interactions in dual-nanoweb microstructured fibers. These beams represent a class of spatial soliton.
Wong, G.K.L., Kang, M.S., Lee, H.W., Biancalana, F., Conti, C., Weiss, T., Russell, P.S.J.(2012)“Excitation of Orbital Angular Momentum Resonances in Helically Twisted Photonic Crystal Fiber”, Science, 337(6093), 446-449, available: http://www.sciencemag.org/content/337/6093/446.abstract.
Spiral twisting offers additional opportunities for controlling the loss, dispersion, and polarization state of light in optical fibers with noncircular guiding cores. Here, we report an effect that appears in continuously twisted photonic crystal fiber. Guided by the helical lattice of hollow channels, cladding light is forced to follow a spiral path. This diverts a fraction of the axial momentum flow into the azimuthal direction, leading to the formation of discrete orbital angular momentum states at wavelengths that scale linearly with the twist rate. Core-guided light phase-matches topologically to these leaky states, causing a series of dips in the transmitted spectrum. Twisted photonic crystal fiber has potential applications in, for example, band-rejection filters and dispersion control.
Sapienza, R., Leonetti, M., Froufe-Pérez, L.S., Galisteo-López, J.F., Conti, C., López, C.(2011)“Optical amplification enhancement in photonic crystals”, Phys. Rev. A, 83(2), 023801, available: http://pra.aps.org/abstract/PRA/v83/i2/e023801.
We predict the existence of a class of multidimensional light localizations in out-of-equilibrium ferroelectric crystals. In two dimensions, the nondiffracting beams form at an arbitrary low-power level and propagate even when their width is well below the optical wavelength. In three dimensions, a subwavelength light bullet is found. The effects emerge when compositionally disordered crystals are brought to their metastable glassy state, and leading to the suppression of evanescent waves, they can have a profound impact on super-resolved imaging and ultradense optical storage, resembling metamaterials in many ways.
DelRe, E., Spinozzi, E., Agranat, A.J., Conti, C.(2011)“Scale-free optics and diffractionless waves in nanodisordered ferroelectrics”, Nature Photonics, 5(1), 39--42.
The statistical properties of the phases of several modes nonlinearly coupled in a random system are investigated by means of a Hamiltonian model with disordered couplings. The regime in which the modes have a stationary distribution of their energies and in which the phases are coupled is studied for arbitrary degrees of randomness and energy. The complexity versus temperature and strength of nonlinearity is calculated. A phase diagram is derived in terms of the stored energy and amount of disorder. Implications in random lasing, nonlinear wave propagation, and finite-temperature Bose-Einstein condensation are discussed.
Sapienza, R., Leonetti, M., Froufe-Perez, L.S., Galisteo Lopez, J., Conti, C., Lopez, C.(2011)“Optical amplification enhancement in photonic crystals”, Phys. Rev. A, 83, 023801, available: http://link.aps.org/doi/10.1103/PhysRevA.83.023801.
Improving and controlling the efficiency of a gain medium is one of the most challenging problems of laser research. By measuring the gain length in an opal based photonic crystal doped with laser dye, we demonstrate that optical amplification is more than twenty-fold enhanced along the Gamma-K symmetry directions of the face centered cubic photonic crystal. These results are theoretically explained by directional variations of the density of states, providing a quantitative connection between density of the states and light amplification.
Leonetti, M., Conti, C., Lopez, C.(2011)“The mode-locking transition of random lasers”, Nature Photonics, 5(10), 615-617, available: http://dx.doi.org/10.1038/nphoton.2011.217.
We report the simultaneous diffraction cancellation for beams of different wavelengths in out-of-equilibrium dipolar glass. The effect is supported by the photorefractive diffusive nonlinearity and scale-free optics, and can find application in imaging and microscopy.
Folli, V., Conti, C.(2011)“Self-Induced Transparency and the Anderson Localization of Light”, Optics Letters, 36(15), 2830, available: http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-36-15-2830.
The nonlinear propagation of pulses in liquid-filled photonic crystal fibers is considered. Because of the slow reorientational nonlinearity of some molecular liquids, the nonlinear modes propagating inside such structures can be approximated, for pulse durations much shorter than the molecular relaxation time, by temporally highly nonlocal solitons, analytical solutions of a linear Schrödinger equation. The physical relevance of these novel solitons is discussed.
Leonetti, M., Conti, C.(2010)“Haus/Gross--Pitaevskii equation for random lasers”, J. Opt. Soc. Am. B, 27(7), 1446--1451, available: http://josab.osa.org/abstract.cfm?URI=josab-27-7-1446.
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.
Conti, C.(2010)“The Enlightened Game of Life”, in Adamatzky, A., ed., Arxiv:0810.3179, Springer, available: http://adsabs.harvard.edu/abs/2008arXiv0810.3179C.
We investigate a special class of cellular automata (CA) evolving in a environment filled by an electromagnetic wave. The rules of the Conway Game of Life are modified to account for the ability to retrieve life-sustenance from the field energy. Light-induced self-structuring and self-healing abilities and various dynamic phases are displayed by the CA. Photo-driven genetic selection and the nonlinear feedback of the CA on the electromagnetic field are included in the model, and there are evidences of self-organized light-localization processes. The evolution of the electromagnetic field is based on the Finite Difference Time Domain (FDTD) approach. Applications are envisaged in evolutionary biology, artificial life, DNA replication, swarming, optical tweezing and field-driven soft-matter.
Conti, C., D'Asaro, E., Stivala, S., Busacca, A., Assanto, G.(2010)“Parametric self-trapping in the presence of randomized quasi phase matching”, Opt. Lett., 35(22), 3760--3762, available: http://ol.osa.org/abstract.cfm?URI=ol-35-22-3760.
We investigate theoretically, numerically and experimentally nonlinear optical waves in an absorbing out-of-equilibrium colloidal material at the gelification transition. At sufficiently high optical intensity, absorption is frustrated and light propagates into the medium. The process is mediated by the formation of a matter-shock wave due to optically induced thermodiffusion, and largely resembles the mechanism of hydrodynamical supercavitation, as it is accompanied by a dynamic phase-transition region between the beam and the absorbing material.
Gentilini, S., Fratalocchi, A., Conti, C.(2010)“Signatures of Anderson localization excited by an optical frequency comb”, Phys. Rev. B, 81(1), 014209.
We investigate a special class of cellular automata (CA) evolving in a environment filled by an electromagnetic wave. The rules of the Conway Game of Life are modified to account for the ability to retrieve life-sustenance from the field energy. Light-induced self-structuring and self-healing abilities and various dynamic phases are displayed by the CA. Photo-driven genetic selection and the nonlinear feedback of the CA on the electromagnetic field are included in the model, and there are evidences of self-organized light-localization processes. The evolution of the electromagnetic field is based on the Finite Difference Time Domain (FDTD) approach. Applications are envisaged in evolutionary biology, artificial life, DNA replication, swarming, optical tweezing and field-driven soft-matter.
We investigate the evolution of solitary waves in a nonlocal medium in the presence of disorder. By using a perturbational approach, we show that an increasing degree of nonlocality may largely hamper the Brownian motion of self-trapped wave-packets. The result is valid for any kind of nonlocality and in the presence of non-paraxial effects. Analytical predictions are compared with numerical simulations based on stochastic partial differential equation.
Gentilini, S., Fratalocchi, A., Angelani, L., Ruocco, G., Conti, C.(2009)“Ultrashort pulse propagation and the Anderson localization”, Opt. Lett., 34(2), 130--132, available: http://ol.osa.org/abstract.cfm?URI=ol-34-2-130.
Leonetti, M., Sapienza, R., Ibisate, M., Conti, C., López, C.(2009)“Optical gain in DNA-DCM for lasing in photonic materials”, Opt. Lett., 34(24), 3764--3766, available: http://ol.osa.org/abstract.cfm?URI=ol-34-24-3764.
Leonetti, M., Capuani, S., Peccianti, M., Ruocco, G., Conti, C.(2009)“Characterization of archeological human bone tissue by enhanced backscattering of light”, Applied Physics Letters, 94, 101101, available: http://link.aip.org/link/?APPLAB/94/101101/1.
Enhanced backscattering of light is used to detect microarchitectural changes in human archeological bones. Measurements on tibia cortical and trabecular tissue demonstrate the high sensitivity of the cone width and enhancement factor to the different morphologies. The approach allows to unveil the presence of periostitic lesions, thus addressing its feasibility for anthropological studies.
Conti, C., Fratalocchi, A., Peccianti, M., Ruocco, G., Trillo, S.(2009)“Observation of a gradient catastrophe generating solitons”, Physical Review Letters, 102, 083902, available: /brokenurl# http://link.aps.org/doi/10.1103/PhysRevLett.102.083902.
We investigate the propagation of a dark beam in a defocusing medium in the strong nonlinear regime. We observe for the first time a shock fan filled with noninteracting one-dimensional gray solitons that emanates from a gradient catastrophe developing around a null of the optical intensity. This scenario turns out to be very robust, persisting also when the material nonlocal response averages the nonlinearity over dimensions much larger than the emerging soliton filaments.
Gentilini, S., Fratalocchi, A., Angelani, L., Ruocco, G., Conti, C.(2009)“Ultrashort pulse propagation and the Anderson localization”, Optics Letters, 34, 130, available: http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-34-2-130.
We investigate the dynamics of a 10 fs light pulse propagating in a random medium by the direct solution of the 3D Maxwell equations. Our approach employs molecular dynamics to generate a distribution of spherical scatterers and a parallel finite-difference time-domain code for the vectorial wave propagation. We calculate the disorder-averaged energy velocity and the decay time of the transmitted pulse Versus the localization length for an increasing refractive index.
Ghofraniha, N., Ruocco, G., Conti, C.(2009)“Collective Thermal Diffusion of Silica Colloids Studied by Nonlinear Optics”, Langmuir, 25(21), 12495--12500.
We investigate the collective thermal diffusion of silica charged spheres in Sulpho-Rhodamine B/water solution at different concentrations by measuring time-dependent thermal and Soret lensings. We show a significant concentration-dependence of the thermal diffusion coefficient D-T, not previously reported. Moreover, the results clearly show that both mass diffusion and Soret coefficient are collective quantities being strongly dependent on the particles' packing fraction. Our Z-scan setup allows us to investigate the dynamics of the system at low wave vector, which addresses the influence of the interparticle interactions oil the thermal diffusion of the colloids.
We investigate the time-dependent nonlinear optical absorption of a clay dispersion (Laponite) in an organic dye (rhodamine B) water solution displaying liquid-arrested state transition. Specifically, we determine the characteristic time D of the nonlinear susceptibility buildup due to the Soret effect. By comparing D with the relaxation time provided by standard dynamic light scattering measurements we report on the decoupling of the two collective diffusion times at the two very different length scales during the aging of the out-of-equilibrium system. With this demonstration experiment we also show the potentiality of nonlinear optics measurements in the study of the late stage of arrest in soft materials.
Perra, N., Zlatić, V., Chessa, A., Conti, C., Donato, D., Caldarelli, G.(2009)“PageRank equation and localization in the WWW”, EPL (Europhysics Letters), 88(4), 48002, available: http://stacks.iop.org/0295-5075/88/i=4/a=48002.
We show that the PageRank in a network can be represented as the solution of a differential equation discretized over a directed graph. By exploiting a formal relationship with the time-independent Schrödinger equation it is possible to interpret hub formation and related phenomena as a wave-like localization process in the presence of disorder and trapping potentials. The result opens new perspectives in the physics of networks with interdisciplinary connections and opens the way to the employment of various mathematical techniques to the analysis of self-organization in structured systems. Applications are envisaged in the World-Wide Web, traffic, social and biological networks.
Leuzzi, L., Conti, C., Folli, V., Angelani, L., Ruocco, G.(2009)“Phase diagram and complexity of mode-locked lasers: from order to disorder”, Physical Review Letters, 102, 083901, available: /brokenurl# http://link.aps.org/doi/10.1103/PhysRevLett.102.083901.
We investigate mode-locking processes in lasers displaying a variable degree of structural randomness. By a spin-glass theoretic approach, we analyze the mean-field Hamiltonian and derive a phase diagram in terms of pumping rate and degree of disorder. Paramagnetic (noisy continuous wave emission), ferromagnetic (standard passive mode locking), and spin-glass phases with an exponentially large number of configurations are identified. The results are also relevant for other physical systems displaying a random Hamiltonian, such as Bose-condensed gases and nonlinear optics.
Fratalocchi, A., Conti, C., Ruocco, G.(2008)“Three-dimensional ab initio investigation of light-matter interaction in Mie lasers”, Phys. Rev. A, 78(1), 013806.
Photons propagate in photonic crystals in the same way as electrons propagate in solids. The periodical refractive index induces forbidden frequency bands, which nurture a variety of novel integrated devices and several fundamental studies ranging from threshold-less lasers to quantum computing. However, these investigations have to face the unavoidable disorder of real-world structures: if on one hand it largely hampers experiments, on the other hand it opens the possibility to study three-dimensional (3D) photon strong localization. We report on 3D+1 Maxwell-Bloch simulations of light dynamics in inverted opals exhibiting a complete photonic bandgap. We show that the disorder-induced localized states strongly alter the photonic crystal's response to femtosecond optical pulses, drastically reducing the diffusion constant and trapping light. We find that an optimal amount of randomness favours the strongest localization; correspondingly, self-starting laser processes are mediated by Anderson states that prevail over spatially extended Bloch modes.
Fratalocchi, A., Conti, C., Ruocco, G., Sette, F.(2008)“Nonlinear refraction of hard x rays”, Phys. Rev. B, 77(24), 245132.
Fratalocchi, A., Conti, C., Ruocco, G.(2008)“Three-dimensional ab initio investigation of light-matter interaction in Mie lasers”, Physical Review A, 78(1), 013806, available: http://link.aps.org/abstract/PRA/v78/e013806.
Fratalocchi, A., Conti, C., Ruocco, G., Trillo, S.(2008)“Free-Energy Transition in a Gas of Noninteracting Nonlinear Wave Particles”, Physical Review Letters, 101(4), 044101, available: http://link.aps.org/abstract/PRL/v101/e044101.
We investigate the dynamics of a gas of noninteracting particlelike soliton waves, demonstrating that phase transitions originate from their collective behavior. This is predicted by solving exactly the nonlinear equations and by employing methods of the statistical mechanics of chaos. In particular, we show that a suitable free energy undergoes a metamorphosis as the input excitation is increased, thereby developing a first-order phase transition whose measurable manifestation is the formation of shock waves. This demonstrates that even the simplest phase-space dynamics, involving independent (uncoupled) degrees of freedom, can sustain critical phenomena.
Di Falco, A., Conti, C., Trillo, S.(2008)“Tunneling Mediated by 2D + 1 Conical Waves in a 1D Lattice”, Physical Review Letters, 101(1), 013601, available: http://link.aps.org/abstract/PRL/v101/e013601.
The propagation of 2D+1 wave packets in 1D band gap systems shows that the interplay of periodicity and nonlinearity leads to the spontaneous formation of fast and slow conical localized waves. Such nonlinear tunneling has features that differ on the two edges of the band gap and it is characterized by the competition of bullets and nonlinear X waves.
Fratalocchi, A., Conti, C., Ruocco, G.(2008)“Mode competitions and dynamical frequency pulling in Mie nanolasers: 3D ab-initio Maxwell-Bloch computations”, Opt. Express, 16(12), 8342--8349, available: http://www.opticsexpress.org/abstract.cfm?URI=oe-16-12-8342.
Using a 3D Finite-Difference Time-Domain parallel code, we report on the linear and nonlinear propagation of light pulses in a disordered assembly of scatterers, whose spatial distribution is generated by a Molecular Dynamics code; refractive index dispersion is also taken into account. We calculate the static and dynamical diffusion constant of light, while considering a pulsed excitation. Our results are in quantitative agreement with reported experiments, also furnishing evidence of a non-exponential decay of the transmitted pulse in the linear regime and in the presence of localized modes. By using an high power excitation, we numerically demonstrate the ``modulational instability random laser\\\'\\\': at high peak input powers energy is transferred to localized states from the input pulse, via third-order nonlinearity and optical parametric amplification, and this process is signed by a power-dependent non-exponential time-decay of the transmitted pulse.
Ghofraniha, N., Conti, C., Ruocco, G.(2007)“Aging of the nonlinear optical susceptibility in doped colloidal suspensions”, Physical Review B, 75(22), 224203, available: http://link.aps.org/abstract/PRB/v75/e224203.
We investigate the nonlinear optics response of a colloidal dispersion undergoing dynamics slowing down with age, by using Z -scan and dynamic light scattering measurements. We study the high optical nonlinearity of an organic dye (rhodamine B) dispersed in a water--clay (laponite) suspension. We consider different clay concentrations (2.0--2.6 wt%) experiencing dynamics arrest. We find that (i) the concentration dependent exponential growth of both mean relaxation time and nonlinear absorption coefficient can be individually scaled to a master curve and (ii) the scaling times are the same for the two physical quantities. These findings indicate that the optical nonlinear susceptibility exhibits the same ageing universal scaling behaviour, typical of disordered out of equilibrium systems.
We report oil a recent model introduced to describe the behaviour Of light in a nonlinear active random medium, i.e., laser. The emitted radiation is described by I disordered mean-field Hamiltonian for the phases of the complex amplitudes of the electromagnetic field modes. Using the replica trick the thermodynamics can be exactly solved. A replica-symmetry breaking phase transition is predicted, corresponding to mode-locking processes ill random lasers. The expected random laser phase diagram varying the pumping rate is reported, together with some perspectives opened by the present study.
Conti, C.(2005)“Complex light: Dynamic phase transitions of a light beam in a nonlinear nonlocal disordered medium”, Physical Review E, 72(6), 066620, available: http://link.aps.org/abstract/PRE/v72/e066620.
Dynamic responses of photonic crystal microcavities in nonlinear media are analyzed via both a finite-difference code and coupled-mode theory in the time domain. Optical frequency generation in both second- and third-order nonlinear materials is demonstrated based on the transient evolution of cavity modes. Terahertz waves can be generated in quadratically nonlinear crystals by optical rectification, whereas state generation inside the band gap can be linked to a Rabi-like splitting in cubic media. An all-optical ultra-fast wavelength shifter is proposed.
Conti, C.(2005)“Complex light: Dynamic phase transitions of a light beam in a nonlinear nonlocal disordered medium”, Phys. Rev. E, 72(6), 066620.
Peccianti, M., Conti, C., Assanto, G.(2005)“Interplay between nonlocality and nonlinearity in nematic liquid crystals”, Opt. Lett., 30(4), 415--417, available: http://ol.osa.org/abstract.cfm?URI=ol-30-4-415.
We study one-dimensional transverse modulational instability in a non local medium excited by a spatially incoherent source. Employing undoped nematic liquid crystals in a planar pre-tilted configuration, we investigate the role of the spectral broadening induced by incoherence in conjunction with spatially non local molecular reorientation. The phenomenon is modeled using the Wigner transform..
Conti, C., Peccianti, M., Assanto, G.(2005)“Spatial solitons and modulational instability in the presence of large birefringence: The case of highly nonlocal liquid crystals”, Phys. Rev. E, 72(6), 066614.
Spatial solitons in nematic liquid crystals, ornematicons, can be generated in bulk through a reorientational nonlinearity. We investigate nematicon interactions, showing their behavior related to the inherent nonlocal response of the medium. Solving for both the mechanics and the optics of molecular reorientation in nematic liquid crystals with a nonlinear paraxial version of a Beam Propagator, we demonstrate both in plane and out of plane interactions, disclosing soliton crossing and spiraling.
Crosignani, B., Porto, P., Conti, C.(2004)“The adiabatic piston: a perpetuum mobile in the mesoscopic realm”, Entropy, 6(1), 50--56, available: http://www.mdpi.com/1099-4300/6/1/50/.
A detailed analysis of the adiabatic-piston problem reveals, for a finely-tuned choice of the spatial dimensions of the system, peculiar dynamical features that challenge the statement that an isolated system necessarily reaches a time-independent equilibrium state. In particular, the piston behaves like a perpetuum mobile, i.e., it never comes to a stop but keeps wandering, undergoing sizeable oscillations around the position corresponding to maximum entropy; this has remarkable implications on the entropy changes of a mesoscopic isolated system and on the limits of validity of the second law of thermodynamics in the mesoscopic realm.
Ciattoni, A., Conti, C., Porto, P.D.(2004)“Universal space--time properties of X waves”, J. Opt. Soc. Am. A, 21(3), 451--455, available: http://josaa.osa.org/abstract.cfm?URI=josaa-21-3-451.
We investigate 2-dimensional spatial optical solitons in media exhibiting a large nonlocal response coupled with a self-focusing nonlinearity. To this extent, with reference to a specific system in undoped nematic liquid crystals, we develop a general theory of spatial solitons in media with an arbitrary degree of nonlocality and carry out experimental observations to validate the model. The remarkable agreement between predictions and data yields evidence of narrow-waist solitons, revealing an important connection between nonparaxiality and nonlocality and emphasizing the role of nonlocality.
Conti, C., Di Falco, A., Assanto, G.(2004)“Controlled transmission in the forbidden photonic bandgap via transientnonlinear states”, Opt. Lett., 29(24), 2902--2904, available: http://ol.osa.org/abstract.cfm?URI=ol-29-24-2902.
We investigate four wave mixing in photonic crystal wire microresonators realized in an isotropic medium. One-dimensional optical parametric oscillators are numerically analyzed by solving Maxwell’s equations in all dimensions and including material dispersion as well as nonlinear polarization.
Conti, C., Peccianti, M., Assanto, G.(2004)“Observation of Optical Spatial Solitons in a Highly Nonlocal Medium”, Phys. Rev. Lett., 92(11), 113902.
Assanto, G., Peccianti, M., Conti, C.(2004)“One-dimensional transverse modulational instability in nonlocal media with a reorientational nonlinearity”, IEEE J. Sel. Top. Quantum Electron., 10(5), 862--869.
Transverse modulational instability is investigated in undoped nematic liquid crystals, a highly nonlocal material system encompassing a reorientational nonlinear response. Using an elliptic Gaussian excitation, we observe the one-dimensional development of transverse patterns eventually leading to beam breakup, filamentation, and spatial solitons, both in the cases of spatially coherent and partially incoherent excitations.
Conti, C., Trillo, S.(2004)“Nonspreading Wave Packets in Three Dimensions Formed by an Ultracold Bose Gas in an Optical Lattice”, Phys. Rev. Lett., 92(12), 120404.
Slab waveguides with a two-dimensional periodic distribution of the refractive index are proposed and investigated as optical microcavities for efficient parametric oscillations through four-wave mixing in isotropic materials. We carry out a case study based on the complete solution of three-dimensional Maxwell equations, including material dispersion and cubic nonlinear response.
Di Falco, A., Conti, C., Assanto, G.(2004)“Three-Dimensional Superprism Effect in Photonic-Crystal Slabs”, J. Lightwave Technol., 22(7), 1748, available: http://jlt.osa.org/abstract.cfm?URI=JLT-22-7-1748.
We present an overview of our recent experimental results on two-dimensional optical spatial solitons in voltage biased planar cells with nematic liquid crystals. Excitation, induced waveguiding, and interactions are illustrated and interpreted in terms of the inherent re-orientational, non-resonant and nonlocal nonlinear response.
Peccianti, M., Conti, C., Assanto, G.(2003)“Optical multisoliton generation in nematic liquid crystals”, Opt. Lett., 28(22), 2231--2233, available: http://ol.osa.org/abstract.cfm?URI=ol-28-22-2231.
Optical propagation in nematic liquid crystals is characterized by a large and highly-nonlocal Kerr-like nonlinearity. We investigate the fundamental role played by spatial nonlocality in nonlinear optical propagation, and develop a model able to predict the main features of spatial solitons and modulational instability in nematic liquid crystals. The model unifies solitons in physical systems exhibiting different degrees of nonlocality, disclosing a connection between nonlocal solitons and parametric solitons in quadratic media. Finally, soliton breathing as well as other characteristics of nonlocal propagation are experimentally demonstrated in a specifically-engineered liquid crystal cell.
Optical nonlinearity and feedback through Bragg periodicity are the basic ingredients for light localization into gap solitons. We review the basic concepts and model equations for gap solitons in Kerr and quadratic nonlinear media encompassing a one-dimensional Bragg resonance. With specific regard to frequency doubling media, we generalize the concept of a photonic crystal to band-gaps of a nonlinear origin, and finally address the slow character of quadratic gap-solitons with reference to their excitation.
Trillo, S., Conti, C., Trapani, P.D., Jedrkiewicz, O., Trull, J., Valiulis, G., Bellanca, G.(2002)“Colored conical emission by means of second-harmonic generation”, Opt. Lett., 27(16), 1451--1453, available: http://ol.osa.org/abstract.cfm?URI=ol-27-16-1451.
Boscolo, S., Conti, C., Midrio, M., Someda, C.G.(2002)“Numerical Analysis of Propagation and Impedance Matching in 2-D Photonic Crystal Waveguides With Finite Length”, J. Lightwave Technol., 20(2), 304--310, available: http://jlt.osa.org/abstract.cfm?URI=JLT-20-2-304.
Energy localization and Bragg solitons can be sustained by the interplay of distributed feedback and quadratic nonlinearity. With reference to second harmonic generation, we review the main features of slowly-moving and stationary two-color solitons or simultons both in singly and doubly resonant structures, and discuss their experimental feasibility and potential applications. Finally, with reference to the case of purely nonlinear feedback through short-period quasi-phase-matching, we show that cavityless parametric oscillations can occur in the presence of gain.
di Lallo, A., Cino, A., Conti, C., Assanto, G.(2001)“Second harmonic generation in reverse proton exchanged Lithium Niobate waveguides”, Opt. Express, 8(4), 232--237, available: http://www.opticsexpress.org/abstract.cfm?URI=oe-8-4-232.
Di Trapani, P., Bramati, A., Minardi, S., Chinaglia, W., Conti, C., Trillo, S., Kilius, J., Valiulis, G.(2001)“Focusing versus Defocusing Nonlinearities due to Parametric Wave Mixing”, Phys. Rev. Lett., 87(18), 183902.
We propose a novel approach for efficient frequency doubling of near infrared light, using the coupled system of buried and surface waveguides obtainable by reverse proton exchange in z-cut lithium niobate. In such guides, supporting TM and TE polarizations, respectively, in spite of the use of the d31 nonlinear element, we predict conversion efficiencies as high as 90 or 14% mu;m/W cm in planar structures excited at 1.32 mu;m
Conti, C., Trillo, S., Assanto, G.(2000)“Energy Localization in Photonic Crystals of a Purely Nonlinear Origin”, Phys. Rev. Lett., 85(12), 2502--2505.
Quadratically nonlinear waveguides are studied in the presence of a Bragg grating resonant with one of the involved frequencies. In the case of second-harmonic generation and distributed feedback at the fundamental, we investigate the existence and features of two-color gap-solitary waves, focusing on controlling their propagation speed. This leads to the conception of optically controlled delay lines and retiming schemes for pulse streams in communication systems.
Assanto, G., Conti, C., De Sario, M., Trillo, S.(2000)“Parametric Optical Solitons in Bragg Resonant Media”, Journal of Nonlinear Optical Physics & Materials (JNOPM), 9(1), 69--78.
Temporal solitary waves in material systems yielding a quadratic nonlinear response and in the presence of a Bragg grating are theoretically identified and numerically investigated for the specific case of second-harmonic generation. Their peculiar and intriguing features are reviewed and discussed in view of potential applications.
Trillo, S., Conti, C., Assanto, G., Buryak, A.V.(2000)“From parametric gap solitons to chaos by means of second-harmonic generation in Bragg gratings”, Chaos: An Interdisciplinary Journal of Nonlinear Science, 10(3), 590--599, available: http://link.aip.org/link/?CHA/10/590/1.
In a waveguide for second-harmonic generation, a linear corrugation able to couple counterpropagating waves at the second-harmonic and/or at the fundamental frequency can induce localization effects through the formation of gap-simultons, i.e. bi-color gap-solitons. These can move slowly or be stationary, collide and merge. All-optical memories are envisaged.
Conti, C., De Rossi, A., Trillo, S.(1998)“Existence, bistability, and instability of Kerr-like parametric gap solitons in quadratic media”, Opt. Lett., 23(16), 1265--1267, available: http://ol.osa.org/abstract.cfm?URI=ol-23-16-1265.
Self-guided multi-frequency waves or simultons can form and propagate in media exhibiting a quadratic nonlinear susceptibility and, specifically, able to generate second harmonic. Some of their relevant properties are discussed, illustrating beam cleanup, instability, switching, steering, collisional interactions and other features in the spatial and temporal domains, including energy localization and gap solitons in periodic structures.
Conti, C., Assanto, G., Trillo, S.(1998)“Parametric gap solitons in quadratic media”, Opt. Express, 3(11), 389--404, available: http://www.opticsexpress.org/abstract.cfm?URI=oe-3-11-389.
The effects of the nonlinear interplay between modes of a lithium niobate channel waveguide are numerically investigated. Based on the nonlinear phase-shift obtainable via up- and down-conversion of each mode at the input frequency into and from the second-harmonic modes generated through Type I and Type II interactions, the waveguide can operate as an all-optical transistor with large small-signal-gain, and as a mode-mixing device capable of spatial switching and routing. It exhibits good switching contrast at 1.55 µm with low driving powers.