Articles de revista
http://hdl.handle.net/2117/3215
Mon, 24 Apr 2017 17:17:37 GMT2017-04-24T17:17:37ZLocally parity-time-symmetric and globally parity-symmetric systems
http://hdl.handle.net/2117/103403
Locally parity-time-symmetric and globally parity-symmetric systems
Ahmed Waseem, Waqas Waseem; Herrero Simon, Ramon; Botey Cumella, Muriel; Staliunas, Kestutis
We introduce a class of systems holding parity-time (PT) symmetry locally, whereas being globally P symmetric. The potential, U = U(vertical bar r vertical bar), fulfills PT symmetry with respect to periodically distributed points r(0) : U(vertical bar r(0) + r vertical bar) = U*(vertical bar r(0) - r vertical bar) being r(0) not equal 0. We show that such systems hold unusual properties arising from the merging of the two different symmetries, leading to a strong field localization and enhancement at the double-symmetry center, r = 0, when the coupling of outward to inward propagating waves is favored. We explore such general potentials in one and two dimensions, which could have actual realizations combining gain-loss and index modulations in nanophotonic structures. In particular, we show how to render a broad aperture vertical-cavity surface-emitting laser into a bright and narrow beam source, as a direct application.
Wed, 05 Apr 2017 15:54:50 GMThttp://hdl.handle.net/2117/1034032017-04-05T15:54:50ZAhmed Waseem, Waqas WaseemHerrero Simon, RamonBotey Cumella, MurielStaliunas, KestutisWe introduce a class of systems holding parity-time (PT) symmetry locally, whereas being globally P symmetric. The potential, U = U(vertical bar r vertical bar), fulfills PT symmetry with respect to periodically distributed points r(0) : U(vertical bar r(0) + r vertical bar) = U*(vertical bar r(0) - r vertical bar) being r(0) not equal 0. We show that such systems hold unusual properties arising from the merging of the two different symmetries, leading to a strong field localization and enhancement at the double-symmetry center, r = 0, when the coupling of outward to inward propagating waves is favored. We explore such general potentials in one and two dimensions, which could have actual realizations combining gain-loss and index modulations in nanophotonic structures. In particular, we show how to render a broad aperture vertical-cavity surface-emitting laser into a bright and narrow beam source, as a direct application.Linear and Nonlinear Bullets of the Bogoliubov-de Gennes Excitations
http://hdl.handle.net/2117/103389
Linear and Nonlinear Bullets of the Bogoliubov-de Gennes Excitations
Kumar, Shubham; Perego, A.M.; Staliunas, Kestutis
We report on the focalization of Bogoliubov–de Gennes excitations of the nonlinear Schrödinger equation in the defocusing regime (Gross-Pitaevskii equation for repulsive Bose-Einstein condensates) with a spatially modulated periodic potential. Exploiting the modification of the dispersion relation induced by the modulation, we demonstrate the existence of localized structures of the Bogoliubov–de Gennes excitations, in both the linear and nonlinear regimes (linear and nonlinear “bullets”). These traveling Bogoliubov–de Gennes bullets, localized both spatially and temporally in the comoving reference frame, are robust and propagate remaining stable, without spreading or filamentation. The phenomena reported in this Letter could be observed in atomic Bose-Einstein condensates in the presence of a spatially periodic potential induced by an optical lattice.
Wed, 05 Apr 2017 14:01:12 GMThttp://hdl.handle.net/2117/1033892017-04-05T14:01:12ZKumar, ShubhamPerego, A.M.Staliunas, KestutisWe report on the focalization of Bogoliubov–de Gennes excitations of the nonlinear Schrödinger equation in the defocusing regime (Gross-Pitaevskii equation for repulsive Bose-Einstein condensates) with a spatially modulated periodic potential. Exploiting the modification of the dispersion relation induced by the modulation, we demonstrate the existence of localized structures of the Bogoliubov–de Gennes excitations, in both the linear and nonlinear regimes (linear and nonlinear “bullets”). These traveling Bogoliubov–de Gennes bullets, localized both spatially and temporally in the comoving reference frame, are robust and propagate remaining stable, without spreading or filamentation. The phenomena reported in this Letter could be observed in atomic Bose-Einstein condensates in the presence of a spatially periodic potential induced by an optical lattice.Two-dimensional domain structures in Lithium Niobate via domain inversion with ultrafast light
http://hdl.handle.net/2117/103122
Two-dimensional domain structures in Lithium Niobate via domain inversion with ultrafast light
Chen, Xin; Karpinski, Pawel; Shvedov, Vladlen; Wang, Bingxia; Trull Silvestre, José Francisco; Cojocaru, Crina; Boes, A.; Mitchell, A.; Krolikowski, Wieslaw; Sheng, Yan
Periodic inversion of ferroelectric domains is realized in a lithium niobate crystal by focused femtosecond near-infrared laser beam. One and two-dimensional domain patterns are fabricated. Quasi-phase matched frequency doubling of 815nm light is demonstrated in a channel waveguide with an inscribed periodic domain pattern with conversion efficiency as high as 17.45%.
Thu, 30 Mar 2017 16:32:04 GMThttp://hdl.handle.net/2117/1031222017-03-30T16:32:04ZChen, XinKarpinski, PawelShvedov, VladlenWang, BingxiaTrull Silvestre, José FranciscoCojocaru, CrinaBoes, A.Mitchell, A.Krolikowski, WieslawSheng, YanPeriodic inversion of ferroelectric domains is realized in a lithium niobate crystal by focused femtosecond near-infrared laser beam. One and two-dimensional domain patterns are fabricated. Quasi-phase matched frequency doubling of 815nm light is demonstrated in a channel waveguide with an inscribed periodic domain pattern with conversion efficiency as high as 17.45%.Mode-locking via dissipative Faraday instability
http://hdl.handle.net/2117/102229
Mode-locking via dissipative Faraday instability
Tarasov, N.; Perego, A.M.; Churkin, D.V.; Staliunas, Kestutis; Turitsyn, S.K.
Emergence of coherent structures and patterns at the nonlinear stage of modulation
instability of a uniform state is an inherent feature of many biological, physical and engineering
systems. There are several well-studied classical modulation instabilities, such as
Benjamin–Feir, Turing and Faraday instability, which play a critical role in the self-organization
of energy and matter in non-equilibrium physical, chemical and biological systems. Here
we experimentally demonstrate the dissipative Faraday instability induced by spatially
periodic zig-zag modulation of a dissipative parameter of the system—spectrally dependent
losses—achieving generation of temporal patterns and high-harmonic mode-locking in a fibre
laser. We demonstrate features of this instability that distinguish it from both the Benjamin–
Feir and the purely dispersive Faraday instability. Our results open the possibilities for new
designs of mode-locked lasers and can be extended to other fields of physics and engineering.
Thu, 09 Mar 2017 15:22:54 GMThttp://hdl.handle.net/2117/1022292017-03-09T15:22:54ZTarasov, N.Perego, A.M.Churkin, D.V.Staliunas, KestutisTuritsyn, S.K.Emergence of coherent structures and patterns at the nonlinear stage of modulation
instability of a uniform state is an inherent feature of many biological, physical and engineering
systems. There are several well-studied classical modulation instabilities, such as
Benjamin–Feir, Turing and Faraday instability, which play a critical role in the self-organization
of energy and matter in non-equilibrium physical, chemical and biological systems. Here
we experimentally demonstrate the dissipative Faraday instability induced by spatially
periodic zig-zag modulation of a dissipative parameter of the system—spectrally dependent
losses—achieving generation of temporal patterns and high-harmonic mode-locking in a fibre
laser. We demonstrate features of this instability that distinguish it from both the Benjamin–
Feir and the purely dispersive Faraday instability. Our results open the possibilities for new
designs of mode-locked lasers and can be extended to other fields of physics and engineering.Emergence of spike correlations in periodically forced excitable systems
http://hdl.handle.net/2117/101786
Emergence of spike correlations in periodically forced excitable systems
Reinoso, Jose A.; Torrent Serra, Maria del Carmen; Masoller Alonso, Cristina
Wed, 01 Mar 2017 12:11:41 GMThttp://hdl.handle.net/2117/1017862017-03-01T12:11:41ZReinoso, Jose A.Torrent Serra, Maria del CarmenMasoller Alonso, CristinaGlobal atmospheric dynamics investigated by using Hilbert frequency analysis
http://hdl.handle.net/2117/101413
Global atmospheric dynamics investigated by using Hilbert frequency analysis
Zappala, Dario; Barreiro, Marcelo; Masoller Alonso, Cristina
The Hilbert transform is a well-known tool of time series analysis that has been widely used to investigate oscillatory signals that resemble a noisy periodic oscillation, because it allows instantaneous phase and frequency to be estimated, which in turn uncovers interesting properties of the underlying process that generates the signal. Here we use this tool to analyze atmospheric data: we consider daily-averaged Surface Air Temperature (SAT) time series recorded over a regular grid of locations covering the Earth’s surface. From each SAT time series, we calculate the instantaneous frequency time series by considering the Hilbert analytic signal. The properties of the obtained frequency data set are investigated by plotting the map of the average frequency and the map of the standard deviation of the frequency fluctuations. The average frequency map reveals well-defined large-scale structures: in the extra-tropics, the average frequency in general corresponds to the expected one-year period of solar forcing, while in the tropics, a different behaviour is found, with particular regions having a faster average frequency. In the standard deviation map, large-scale structures are also found, which tend to be located over regions of strong annual precipitation. Our results demonstrate that Hilbert analysis of SAT time-series uncovers meaningful information, and is therefore a promising tool for the study of other climatological variables.
Wed, 22 Feb 2017 19:10:46 GMThttp://hdl.handle.net/2117/1014132017-02-22T19:10:46ZZappala, DarioBarreiro, MarceloMasoller Alonso, CristinaThe Hilbert transform is a well-known tool of time series analysis that has been widely used to investigate oscillatory signals that resemble a noisy periodic oscillation, because it allows instantaneous phase and frequency to be estimated, which in turn uncovers interesting properties of the underlying process that generates the signal. Here we use this tool to analyze atmospheric data: we consider daily-averaged Surface Air Temperature (SAT) time series recorded over a regular grid of locations covering the Earth’s surface. From each SAT time series, we calculate the instantaneous frequency time series by considering the Hilbert analytic signal. The properties of the obtained frequency data set are investigated by plotting the map of the average frequency and the map of the standard deviation of the frequency fluctuations. The average frequency map reveals well-defined large-scale structures: in the extra-tropics, the average frequency in general corresponds to the expected one-year period of solar forcing, while in the tropics, a different behaviour is found, with particular regions having a faster average frequency. In the standard deviation map, large-scale structures are also found, which tend to be located over regions of strong annual precipitation. Our results demonstrate that Hilbert analysis of SAT time-series uncovers meaningful information, and is therefore a promising tool for the study of other climatological variables.Unravelling the community structure of the climate system by using lags and symbolic time-series analysis
http://hdl.handle.net/2117/101373
Unravelling the community structure of the climate system by using lags and symbolic time-series analysis
Tirabassi, Giulio; Masoller Alonso, Cristina
Many natural systems can be represented by complex networks of dynamical units with modular structure in the form of communities of densely interconnected nodes. Unraveling this community structure from observed data requires the development of appropriate tools, particularly when the nodes are embedded in a regular space grid and the datasets are short and noisy. Here we propose two methods to identify communities, and validate them with the analysis of climate datasets recorded at a regular grid of geographical locations covering the Earth surface. By identifying mutual lags among time-series recorded at different grid points, and by applying symbolic time-series analysis, we are able to extract meaningful regional communities, which can be interpreted in terms of large-scale climate phenomena. The methods proposed here are valuable tools for the study of other systems represented by networks of dynamical units, allowing the identification of communities, through time-series analysis of the observed output signals.
This work is licensed under a Creative Commons Attribution 4.0 International License. http://creativecommons.org/licenses/by/4.0/
Wed, 22 Feb 2017 12:06:34 GMThttp://hdl.handle.net/2117/1013732017-02-22T12:06:34ZTirabassi, GiulioMasoller Alonso, CristinaMany natural systems can be represented by complex networks of dynamical units with modular structure in the form of communities of densely interconnected nodes. Unraveling this community structure from observed data requires the development of appropriate tools, particularly when the nodes are embedded in a regular space grid and the datasets are short and noisy. Here we propose two methods to identify communities, and validate them with the analysis of climate datasets recorded at a regular grid of geographical locations covering the Earth surface. By identifying mutual lags among time-series recorded at different grid points, and by applying symbolic time-series analysis, we are able to extract meaningful regional communities, which can be interpreted in terms of large-scale climate phenomena. The methods proposed here are valuable tools for the study of other systems represented by networks of dynamical units, allowing the identification of communities, through time-series analysis of the observed output signals.Unveiling temporal correlations characteristic to phase transition in the output intensity of a fiber laser
http://hdl.handle.net/2117/101317
Unveiling temporal correlations characteristic to phase transition in the output intensity of a fiber laser
Aragoneses, Andrés; Carpi, Laura; Tarasov, N.; Churkin, D.V.; Torrent Serra, Maria del Carmen; Masoller Alonso, Cristina; Turitsyn, S.K.
We use advanced statistical tools of time-series analysis to characterize the dynamical complexity of the transition to optical wave turbulence in a fiber laser. Ordinal analysis and the horizontal visibility graph applied to the experimentally measured laser output intensity reveal the presence of temporal correlations during the transition from the laminar to the turbulent lasing regimes. Both methods unveil coherent structures with well-defined time scales and strong correlations both, in the timing of the laser pulses and in their peak intensities. Our approach is generic and may be used in other complex systems that undergo similar transitions involving the generation of extreme fluctuations.
Tue, 21 Feb 2017 13:06:11 GMThttp://hdl.handle.net/2117/1013172017-02-21T13:06:11ZAragoneses, AndrésCarpi, LauraTarasov, N.Churkin, D.V.Torrent Serra, Maria del CarmenMasoller Alonso, CristinaTuritsyn, S.K.We use advanced statistical tools of time-series analysis to characterize the dynamical complexity of the transition to optical wave turbulence in a fiber laser. Ordinal analysis and the horizontal visibility graph applied to the experimentally measured laser output intensity reveal the presence of temporal correlations during the transition from the laminar to the turbulent lasing regimes. Both methods unveil coherent structures with well-defined time scales and strong correlations both, in the timing of the laser pulses and in their peak intensities. Our approach is generic and may be used in other complex systems that undergo similar transitions involving the generation of extreme fluctuations.Formation of high-order acoustic Bessel beams by spiral diffraction gratings
http://hdl.handle.net/2117/101154
Formation of high-order acoustic Bessel beams by spiral diffraction gratings
Jimenez, Noe; Pico Vila, Rubén; Sánchez Morcillo, Victor José; Romero García, Vicenç; Garcia Raffi, Luis Miguel; Staliunas, Kestutis
The formation of high-order Bessel beams by a passive acoustic device consisting of an Archimedes' spiral diffraction grating is theoretically, numerically, and experimentally reported in this paper. These beams are propagation-invariant solutions of the Helmholtz equation and are characterized by an azimuthal variation of the phase along its annular spectrum producing an acoustic vortex in the near field. In our system, the scattering of plane acoustic waves by the spiral grating leads to the formation of the acoustic vortex with zero pressure on axis and the angular phase dislocations characterized by the spiral geometry. The order of the generated Bessel beam and, as a consequence, the size of the generated vortex can be fixed by the number of arms in the spiral diffraction grating. The obtained results allow for obtaining Bessel beams with controllable vorticity by a passive device, which has potential applications in low-cost acoustic tweezers and acoustic radiation force devices.
Thu, 16 Feb 2017 17:50:42 GMThttp://hdl.handle.net/2117/1011542017-02-16T17:50:42ZJimenez, NoePico Vila, RubénSánchez Morcillo, Victor JoséRomero García, VicençGarcia Raffi, Luis MiguelStaliunas, KestutisThe formation of high-order Bessel beams by a passive acoustic device consisting of an Archimedes' spiral diffraction grating is theoretically, numerically, and experimentally reported in this paper. These beams are propagation-invariant solutions of the Helmholtz equation and are characterized by an azimuthal variation of the phase along its annular spectrum producing an acoustic vortex in the near field. In our system, the scattering of plane acoustic waves by the spiral grating leads to the formation of the acoustic vortex with zero pressure on axis and the angular phase dislocations characterized by the spiral geometry. The order of the generated Bessel beam and, as a consequence, the size of the generated vortex can be fixed by the number of arms in the spiral diffraction grating. The obtained results allow for obtaining Bessel beams with controllable vorticity by a passive device, which has potential applications in low-cost acoustic tweezers and acoustic radiation force devices.Asymmetric light transmission in PT-Symmetric microring resonators
http://hdl.handle.net/2117/100764
Asymmetric light transmission in PT-Symmetric microring resonators
Giden, I.; Dadashi, Kh.; Botey Cumella, Muriel; Herrero Simon, Ramon; Staliunas, Kestutis; Kurt, Hamza
We propose a new type of add-drop microring resonator made of gain and loss materials. Microring resonators are compact, narrow band, and optical channel dropping filters. In such linear systems, light transmission to side-coupled signal waveguides is always symmetric. However, we prove that properly arranging a gain and loss modulation in the microring resonator provides a new functionality: asymmetric transmission; so that different resonant modes can be promoted depending on the input channel. This can be achieved when the resonator holds parity-time (PT-) symmetry, with periodic gain-loss and index modulations. PT-symmetry in optics generally requires that the index and gain-loss modulations are dephased by a quarter of the wavenumber of the modulation. Besides, we show that a simple half-gain half-loss microring also produces analogous results to a periodic PT-symmetric system. The results are numerically proved and also accounted by a simple analytical model. The effect of using complex modulated resonators with smaller periodicities is also analyzed.
Thu, 09 Feb 2017 13:32:20 GMThttp://hdl.handle.net/2117/1007642017-02-09T13:32:20ZGiden, I.Dadashi, Kh.Botey Cumella, MurielHerrero Simon, RamonStaliunas, KestutisKurt, HamzaWe propose a new type of add-drop microring resonator made of gain and loss materials. Microring resonators are compact, narrow band, and optical channel dropping filters. In such linear systems, light transmission to side-coupled signal waveguides is always symmetric. However, we prove that properly arranging a gain and loss modulation in the microring resonator provides a new functionality: asymmetric transmission; so that different resonant modes can be promoted depending on the input channel. This can be achieved when the resonator holds parity-time (PT-) symmetry, with periodic gain-loss and index modulations. PT-symmetry in optics generally requires that the index and gain-loss modulations are dephased by a quarter of the wavenumber of the modulation. Besides, we show that a simple half-gain half-loss microring also produces analogous results to a periodic PT-symmetric system. The results are numerically proved and also accounted by a simple analytical model. The effect of using complex modulated resonators with smaller periodicities is also analyzed.