DONLL  Dinàmica no lineal, òptica no lineal i làsers
http://hdl.handle.net/2117/3214
20170326T01:57:19Z

Axisymmetric photonic structures with PTsymmetry
http://hdl.handle.net/2117/102467
Axisymmetric photonic structures with PTsymmetry
Ahmed Waseem, Waqas Waseem; Herrero Simon, Ramon; Botey Cumella, Muriel; Staliunas, Kestutis
PTsymmetric structures in photonic crystals, combining refractive index and gainloss modulations is becoming a research field with increasing interest due to the light directionality induced by these particular potentials. Here, we consider PTsymmetric potentials with axial symmetry to direct light to the crystal central point obtaining a localization effect. The axial and PTsymmetric potential intrinsically generates an exceptional central point in the photonic crystal by the merge of both symmetries. This particular point in the crystal lattice causes field amplitude gradients with exponential slopes around the crystal center. The field localization strongly depends on the phase of the central point and on the complex amplitude of the PTpotential.
The presented work analyzes in a first stage 1D linear PTaxisymmetric crystals and the role of the central point phase that determines the defect character, i.e. refractive index defect, gainloss defect or a combination of both. The interplay of the directional light effect induced by the PTsymmetry and the light localization around the central point through the axial symmetry enhances localization and allows higher field concentration for certain phases. The linearity of the studied crystals introduces an exponential growth of the field that mainly depends on the complex amplitude of the potential. The work is completed by the analysis of 2D PTaxisymmetric potentials showing different spatial slopes and growth rates caused by symmetry reasons.
Copyright 2016 Society of PhotoOptical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.
20170314T17:19:20Z
Ahmed Waseem, Waqas Waseem
Herrero Simon, Ramon
Botey Cumella, Muriel
Staliunas, Kestutis
PTsymmetric structures in photonic crystals, combining refractive index and gainloss modulations is becoming a research field with increasing interest due to the light directionality induced by these particular potentials. Here, we consider PTsymmetric potentials with axial symmetry to direct light to the crystal central point obtaining a localization effect. The axial and PTsymmetric potential intrinsically generates an exceptional central point in the photonic crystal by the merge of both symmetries. This particular point in the crystal lattice causes field amplitude gradients with exponential slopes around the crystal center. The field localization strongly depends on the phase of the central point and on the complex amplitude of the PTpotential.
The presented work analyzes in a first stage 1D linear PTaxisymmetric crystals and the role of the central point phase that determines the defect character, i.e. refractive index defect, gainloss defect or a combination of both. The interplay of the directional light effect induced by the PTsymmetry and the light localization around the central point through the axial symmetry enhances localization and allows higher field concentration for certain phases. The linearity of the studied crystals introduces an exponential growth of the field that mainly depends on the complex amplitude of the potential. The work is completed by the analysis of 2D PTaxisymmetric potentials showing different spatial slopes and growth rates caused by symmetry reasons.

Modelocking via dissipative Faraday instability
http://hdl.handle.net/2117/102229
Modelocking 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 wellstudied classical modulation instabilities, such as
Benjamin–Feir, Turing and Faraday instability, which play a critical role in the selforganization
of energy and matter in nonequilibrium physical, chemical and biological systems. Here
we experimentally demonstrate the dissipative Faraday instability induced by spatially
periodic zigzag modulation of a dissipative parameter of the system—spectrally dependent
losses—achieving generation of temporal patterns and highharmonic modelocking 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 modelocked lasers and can be extended to other fields of physics and engineering.
20170309T15:22:54Z
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 wellstudied classical modulation instabilities, such as
Benjamin–Feir, Turing and Faraday instability, which play a critical role in the selforganization
of energy and matter in nonequilibrium physical, chemical and biological systems. Here
we experimentally demonstrate the dissipative Faraday instability induced by spatially
periodic zigzag modulation of a dissipative parameter of the system—spectrally dependent
losses—achieving generation of temporal patterns and highharmonic modelocking 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 modelocked lasers and can be extended to other fields of physics and engineering.

Study of the charge profile of thermally poled electrets
http://hdl.handle.net/2117/101794
Study of the charge profile of thermally poled electrets
Parsa, Seyedeh Elaheh; Trull Silvestre, José Francisco; Colom Fajula, Xavier; Sellarès González, Jordi
The charge profile of thermally poled electrets has been studied using two different methods, laser induced pressure pulse (LIPP) and pulsed electroacoustic (PEA), to gain insight into the mechanisms that are activated and assess which is the most appropriate method to study the charge profile. Disc–shaped PET samples have been conventionally poled to activate both the a and the ¿ relaxation and, right after, partially discharged up to a temperature Tpd . In this way, samples with a different combination of dipolar and space charge polarization have been obtained. Both LIPP and PEA reveal asymmetric profiles for Tpd below the glass transition temperature, that progressively become antisymmetric for higher temperatures. The shape and evolution of the charge profiles can be explained assuming injection of negative carriers from the anode that enhances the trapping of positive carriers near this electrode. It can be observed that PEA is able to detect a wider variety of polarization mechanisms in the system while LIPP gives a simpler picture of the charge profile.
Preprint
20170301T12:55:42Z
Parsa, Seyedeh Elaheh
Trull Silvestre, José Francisco
Colom Fajula, Xavier
Sellarès González, Jordi
The charge profile of thermally poled electrets has been studied using two different methods, laser induced pressure pulse (LIPP) and pulsed electroacoustic (PEA), to gain insight into the mechanisms that are activated and assess which is the most appropriate method to study the charge profile. Disc–shaped PET samples have been conventionally poled to activate both the a and the ¿ relaxation and, right after, partially discharged up to a temperature Tpd . In this way, samples with a different combination of dipolar and space charge polarization have been obtained. Both LIPP and PEA reveal asymmetric profiles for Tpd below the glass transition temperature, that progressively become antisymmetric for higher temperatures. The shape and evolution of the charge profiles can be explained assuming injection of negative carriers from the anode that enhances the trapping of positive carriers near this electrode. It can be observed that PEA is able to detect a wider variety of polarization mechanisms in the system while LIPP gives a simpler picture of the charge profile.

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
20170301T12:11:41Z
Reinoso, Jose A.
Torrent Serra, Maria del Carmen
Masoller Alonso, Cristina

Predictability of optical rogue waves in optically injected semiconductor lasers
http://hdl.handle.net/2117/101604
Predictability of optical rogue waves in optically injected semiconductor lasers
Martinez Alvarez, Nuria; Reinoso, Jose A.; Masoller Alonso, Cristina
The predictability of ultrahigh pulses emitted by optically injected semiconductor lasers is studied by using symbolic timeseries analysis. Pulse patterns that are likely to occur before the rogue wave are identified.
20170227T10:14:31Z
Martinez Alvarez, Nuria
Reinoso, Jose A.
Masoller Alonso, Cristina
The predictability of ultrahigh pulses emitted by optically injected semiconductor lasers is studied by using symbolic timeseries analysis. Pulse patterns that are likely to occur before the rogue wave are identified.

Global 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 wellknown 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 dailyaveraged 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 welldefined largescale structures: in the extratropics, the average frequency in general corresponds to the expected oneyear 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, largescale structures are also found, which tend to be located over regions of strong annual precipitation. Our results demonstrate that Hilbert analysis of SAT timeseries uncovers meaningful information, and is therefore a promising tool for the study of other climatological variables.
20170222T19:10:46Z
Zappala, Dario
Barreiro, Marcelo
Masoller Alonso, Cristina
The Hilbert transform is a wellknown 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 dailyaveraged 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 welldefined largescale structures: in the extratropics, the average frequency in general corresponds to the expected oneyear 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, largescale structures are also found, which tend to be located over regions of strong annual precipitation. Our results demonstrate that Hilbert analysis of SAT timeseries 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 timeseries analysis
http://hdl.handle.net/2117/101373
Unravelling the community structure of the climate system by using lags and symbolic timeseries 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 timeseries recorded at different grid points, and by applying symbolic timeseries analysis, we are able to extract meaningful regional communities, which can be interpreted in terms of largescale 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 timeseries 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/
20170222T12:06:34Z
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 timeseries recorded at different grid points, and by applying symbolic timeseries analysis, we are able to extract meaningful regional communities, which can be interpreted in terms of largescale 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 timeseries 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 timeseries 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 welldefined 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.
20170221T13:06:11Z
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 timeseries 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 welldefined 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 highorder acoustic Bessel beams by spiral diffraction gratings
http://hdl.handle.net/2117/101154
Formation of highorder 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 highorder 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 propagationinvariant 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 lowcost acoustic tweezers and acoustic radiation force devices.
20170216T17:50:42Z
Jimenez, Noe
Pico Vila, Rubén
Sánchez Morcillo, Victor José
Romero García, Vicenç
Garcia Raffi, Luis Miguel
Staliunas, Kestutis
The formation of highorder 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 propagationinvariant 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 lowcost acoustic tweezers and acoustic radiation force devices.

Slow light enabled wavelength demultiplexing
http://hdl.handle.net/2117/100768
Slow light enabled wavelength demultiplexing
Hayran, Zeki; Turduev, M.; Botey Cumella, Muriel; Herrero Simon, Ramon; Staliunas, Kestutis; Kurt, H.
Photonic crystal waveguides supporting band gap guided modes hold great potential to tailor the group
velocity of propagating light. We propose and explore different wavelength demultiplexer design approaches that utilize
slow light concept. By altering the dielectric filling factors of each waveguide segment, one can show that different
frequencies can be separated and extracted at different locations along the cascaded waveguide. Furthermore, to
eliminate the inherent reflection loss of such a design, a composite structure involving a tapered waveguide with a
sidecoupled resonator is also presented. Such a structure features not only a forward propagating wave but also a
backward propagating wave acting as a feedback mechanism for the drop channels. We show that by careful design of
the waveguide and the resonator, the destructive and instructive interference of these waves can effectively eliminate
the reflection loss and increase the coupling efficiency, respectively. Numerical and experimental verification of the
proposed structures show that the targeted frequencies can be coupled out with low crosstalks and moderate quality
factors, while maintaining a compact size. © 2016 IEEE.
20170209T13:54:56Z
Hayran, Zeki
Turduev, M.
Botey Cumella, Muriel
Herrero Simon, Ramon
Staliunas, Kestutis
Kurt, H.
Photonic crystal waveguides supporting band gap guided modes hold great potential to tailor the group
velocity of propagating light. We propose and explore different wavelength demultiplexer design approaches that utilize
slow light concept. By altering the dielectric filling factors of each waveguide segment, one can show that different
frequencies can be separated and extracted at different locations along the cascaded waveguide. Furthermore, to
eliminate the inherent reflection loss of such a design, a composite structure involving a tapered waveguide with a
sidecoupled resonator is also presented. Such a structure features not only a forward propagating wave but also a
backward propagating wave acting as a feedback mechanism for the drop channels. We show that by careful design of
the waveguide and the resonator, the destructive and instructive interference of these waves can effectively eliminate
the reflection loss and increase the coupling efficiency, respectively. Numerical and experimental verification of the
proposed structures show that the targeted frequencies can be coupled out with low crosstalks and moderate quality
factors, while maintaining a compact size. © 2016 IEEE.