Articles de revista
http://hdl.handle.net/2117/3215
Mon, 20 Feb 2017 02:15:00 GMT
20170220T02:15:00Z

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.
Thu, 16 Feb 2017 17:50:42 GMT
http://hdl.handle.net/2117/101154
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.

Asymmetric light transmission in PTSymmetric microring resonators
http://hdl.handle.net/2117/100764
Asymmetric light transmission in PTSymmetric microring resonators
Giden, I.; Dadashi, Kh.; Botey Cumella, Muriel; Herrero Simon, Ramon; Staliunas, Kestutis; Kurt, Hamza
We propose a new type of adddrop 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 sidecoupled 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 paritytime (PT) symmetry, with periodic gainloss and index modulations. PTsymmetry in optics generally requires that the index and gainloss modulations are dephased by a quarter of the wavenumber of the modulation. Besides, we show that a simple halfgain halfloss microring also produces analogous results to a periodic PTsymmetric 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 GMT
http://hdl.handle.net/2117/100764
20170209T13:32:20Z
Giden, I.
Dadashi, Kh.
Botey Cumella, Muriel
Herrero Simon, Ramon
Staliunas, Kestutis
Kurt, Hamza
We propose a new type of adddrop 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 sidecoupled 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 paritytime (PT) symmetry, with periodic gainloss and index modulations. PTsymmetry in optics generally requires that the index and gainloss modulations are dephased by a quarter of the wavenumber of the modulation. Besides, we show that a simple halfgain halfloss microring also produces analogous results to a periodic PTsymmetric 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.

Quantification of network structural dissimilarities
http://hdl.handle.net/2117/100483
Quantification of network structural dissimilarities
Schieber, Tiabo A.; Carpi, Laura; Díaz Guilera, Albert; Pardalos, Panos M.; Masoller Alonso, Cristina; Ravetti, Martin G.
Identifying and quantifying dissimilarities among graphs is a fundamental and challenging problem of practical importance in many fields of science. Current methods of network comparison are limited to extract only partial information or are computationally very demanding. Here we propose an efficient and precise measure for network comparison, which is based on quantifying differences among distance probability distributions extracted from the networks. Extensive experiments on synthetic and realworld networks show that this measure returns nonzero values only when the graphs are nonisomorphic. Most importantly, the measure proposed here can identify and quantify structural topological differences that have a practical impact on the information flow through the network, such as the presence or absence of critical links that connect or disconnect connected components.
© 2017. This version is made available under the CCBYNCND 4.0 license http://creativecommons.org/licenses/byncnd/4.0/
Thu, 02 Feb 2017 11:41:14 GMT
http://hdl.handle.net/2117/100483
20170202T11:41:14Z
Schieber, Tiabo A.
Carpi, Laura
Díaz Guilera, Albert
Pardalos, Panos M.
Masoller Alonso, Cristina
Ravetti, Martin G.
Identifying and quantifying dissimilarities among graphs is a fundamental and challenging problem of practical importance in many fields of science. Current methods of network comparison are limited to extract only partial information or are computationally very demanding. Here we propose an efficient and precise measure for network comparison, which is based on quantifying differences among distance probability distributions extracted from the networks. Extensive experiments on synthetic and realworld networks show that this measure returns nonzero values only when the graphs are nonisomorphic. Most importantly, the measure proposed here can identify and quantify structural topological differences that have a practical impact on the information flow through the network, such as the presence or absence of critical links that connect or disconnect connected components.

Analysis of noiseinduced temporal correlations in neuronal spike sequences
http://hdl.handle.net/2117/99468
Analysis of noiseinduced temporal correlations in neuronal spike sequences
Reinoso, Jose A.; Torrent Serra, Maria del Carmen; Masoller Alonso, Cristina
We investigate temporal correlations in sequences of noiseinduced neuronal spikes, using a symbolic method of timeseries analysis. We focus on the sequence of timeintervals between consecutive spikes (interspikeintervals, ISIs). The analysis method, known as ordinal analysis, transforms the ISI sequence into a sequence of ordinal patterns (OPs), which are defined in terms of the relative ordering of consecutive ISIs. The ISI sequences are obtained from extensive simulations of two neuron models (FitzHughNagumo, FHN, and integrateandfire, IF), with correlated noise. We find that, as the noise strength increases, temporal order gradually emerges, revealed by the existence of more frequent ordinal patterns in the ISI sequence. While in the FHN model the most frequent OP depends on the noise strength, in the IF model it is independent of the noise strength. In both models, the correlation time of the noise affects the OP probabilities but does not modify the most probable pattern.
This is a copy of the author 's final draft version of an article published in the journal European physical journal. Special topics.
The final publication is available at Springer via http://dx.doi.org/10.1140/epjst/e2016600246
Tue, 17 Jan 2017 13:04:24 GMT
http://hdl.handle.net/2117/99468
20170117T13:04:24Z
Reinoso, Jose A.
Torrent Serra, Maria del Carmen
Masoller Alonso, Cristina
We investigate temporal correlations in sequences of noiseinduced neuronal spikes, using a symbolic method of timeseries analysis. We focus on the sequence of timeintervals between consecutive spikes (interspikeintervals, ISIs). The analysis method, known as ordinal analysis, transforms the ISI sequence into a sequence of ordinal patterns (OPs), which are defined in terms of the relative ordering of consecutive ISIs. The ISI sequences are obtained from extensive simulations of two neuron models (FitzHughNagumo, FHN, and integrateandfire, IF), with correlated noise. We find that, as the noise strength increases, temporal order gradually emerges, revealed by the existence of more frequent ordinal patterns in the ISI sequence. While in the FHN model the most frequent OP depends on the noise strength, in the IF model it is independent of the noise strength. In both models, the correlation time of the noise affects the OP probabilities but does not modify the most probable pattern.

Transition between functional regimes in an integrateandfire network model of the thalamus
http://hdl.handle.net/2117/99035
Transition between functional regimes in an integrateandfire network model of the thalamus
Barardi, Alessandro; García Ojalvo, Jordi; Mazzoni, Alberto
The thalamus is a key brain element in the processing of sensory information. During the sleep and awake states, this brain area is characterized by the presence of two distinct dynamical regimes: in the sleep state activity is dominated by spindle oscillations (7  15 Hz) weakly affected by external stimuli, while in the awake state the activity is primarily driven by external stimuli. Here we develop a simple and computationally efficient model of the thalamus that exhibits two dynamical regimes with different informationprocessing capabilities, and study the transition between them. The network model includes glutamatergic thalamocortical (TC) relay neurons and GABAergic reticular (RE) neurons described by adaptive integrateandfire models in which spikes are induced by either depolarization or hyperpolarization rebound. We found a range of connectivity conditions under which the thalamic network composed by these neurons displays the two aforementioned dynamical regimes. Our results show that TCRE loops generate spindlelike oscillations and that a minimum level of clustering (i.e. local connectivity density) in the RERE connections is necessary for the coexistence of the two regimes. We also observe that the transition between the two regimes occurs when the external excitatory input on TC neurons (mimicking sensory stimulation) is large enough to cause a significant fraction of them to switch from hyperpolarizationrebounddriven firing to depolarizationdriven firing. Overall, our model gives a novel and clear description of the role that the two types of neurons and their connectivity play in the dynamical regimes observed in the thalamus, and in the transition between them. These results pave the way for the development of efficient models of the transmission of sensory information from periphery to cortex.
Wed, 11 Jan 2017 14:33:37 GMT
http://hdl.handle.net/2117/99035
20170111T14:33:37Z
Barardi, Alessandro
García Ojalvo, Jordi
Mazzoni, Alberto
The thalamus is a key brain element in the processing of sensory information. During the sleep and awake states, this brain area is characterized by the presence of two distinct dynamical regimes: in the sleep state activity is dominated by spindle oscillations (7  15 Hz) weakly affected by external stimuli, while in the awake state the activity is primarily driven by external stimuli. Here we develop a simple and computationally efficient model of the thalamus that exhibits two dynamical regimes with different informationprocessing capabilities, and study the transition between them. The network model includes glutamatergic thalamocortical (TC) relay neurons and GABAergic reticular (RE) neurons described by adaptive integrateandfire models in which spikes are induced by either depolarization or hyperpolarization rebound. We found a range of connectivity conditions under which the thalamic network composed by these neurons displays the two aforementioned dynamical regimes. Our results show that TCRE loops generate spindlelike oscillations and that a minimum level of clustering (i.e. local connectivity density) in the RERE connections is necessary for the coexistence of the two regimes. We also observe that the transition between the two regimes occurs when the external excitatory input on TC neurons (mimicking sensory stimulation) is large enough to cause a significant fraction of them to switch from hyperpolarizationrebounddriven firing to depolarizationdriven firing. Overall, our model gives a novel and clear description of the role that the two types of neurons and their connectivity play in the dynamical regimes observed in the thalamus, and in the transition between them. These results pave the way for the development of efficient models of the transmission of sensory information from periphery to cortex.

Temporally correlated fluctuations drive epileptiform dynamics
http://hdl.handle.net/2117/98638
Temporally correlated fluctuations drive epileptiform dynamics
Jedynak, Maciej; Pons Rivero, Antonio Javier; García Ojalvo, Jordi; Goodfellow, Marc
Macroscopic models of brain networks typically incorporate assumptions regarding the characteristics of afferent noise, which is used to represent input from distal brain regions or ongoing fluctuations in nonmodelled parts of the brain. Such inputs are often modelled by Gaussian white noise which has a flat power spectrum. In contrast, macroscopic fluctuations in the brain typically follow a 1/fb spectrum. It is therefore important to understand the effect on brain dynamics of deviations from the assumption of white noise. In particular, we wish to understand the role that noise might play in eliciting aberrant rhythms in the epileptic brain.
To address this question we study the response of a neural mass model to driving by stochastic, temporally correlated input. We characterise the model in terms of whether it generates “healthy” or “epileptiform” dynamics and observe which of these dynamics predominate under different choices of temporal correlation and amplitude of an OrnsteinUhlenbeck process. We find that certain temporal correlations are prone to eliciting epileptiform dynamics, and that these correlations produce noise with maximal power in the d and ¿ bands. Crucially, these are rhythms that are found to be enhanced prior to seizures in humans and animal models of epilepsy. In order to understand why these rhythms can generate epileptiform dynamics, we analyse the response of the model to sinusoidal driving and explain how the bifurcation structure of the model gives rise to these findings. Our results provide insight into how ongoing fluctuations in brain dynamics can facilitate the onset and propagation of epileptiform rhythms in brain networks. Furthermore, we highlight the need to combine largescale models with noise of a variety of different types in order to understand brain (dys)function.
Tue, 20 Dec 2016 15:33:36 GMT
http://hdl.handle.net/2117/98638
20161220T15:33:36Z
Jedynak, Maciej
Pons Rivero, Antonio Javier
García Ojalvo, Jordi
Goodfellow, Marc
Macroscopic models of brain networks typically incorporate assumptions regarding the characteristics of afferent noise, which is used to represent input from distal brain regions or ongoing fluctuations in nonmodelled parts of the brain. Such inputs are often modelled by Gaussian white noise which has a flat power spectrum. In contrast, macroscopic fluctuations in the brain typically follow a 1/fb spectrum. It is therefore important to understand the effect on brain dynamics of deviations from the assumption of white noise. In particular, we wish to understand the role that noise might play in eliciting aberrant rhythms in the epileptic brain.
To address this question we study the response of a neural mass model to driving by stochastic, temporally correlated input. We characterise the model in terms of whether it generates “healthy” or “epileptiform” dynamics and observe which of these dynamics predominate under different choices of temporal correlation and amplitude of an OrnsteinUhlenbeck process. We find that certain temporal correlations are prone to eliciting epileptiform dynamics, and that these correlations produce noise with maximal power in the d and ¿ bands. Crucially, these are rhythms that are found to be enhanced prior to seizures in humans and animal models of epilepsy. In order to understand why these rhythms can generate epileptiform dynamics, we analyse the response of the model to sinusoidal driving and explain how the bifurcation structure of the model gives rise to these findings. Our results provide insight into how ongoing fluctuations in brain dynamics can facilitate the onset and propagation of epileptiform rhythms in brain networks. Furthermore, we highlight the need to combine largescale models with noise of a variety of different types in order to understand brain (dys)function.

Quantitative identification of dynamical transitions in a semiconductor laser with optical feedback
http://hdl.handle.net/2117/98226
Quantitative identification of dynamical transitions in a semiconductor laser with optical feedback
Quintero Quiroz, Carlos Alberto; Tiana Alsina, Jordi; Roma, Josep; Torrent Serra, Maria del Carmen; Masoller Alonso, Cristina
Identifying transitions to complex dynamical regimes is a fundamental open problem with many practical applications. Semi conductor lasers with optical feedback are excellent testbeds for studying such transitions, as they can generate a rich variety of output signals. Here we apply three analysis tools to quantify various aspects of the dynamical transitions that occur as the laser pump current increases. These tools allow to quantitatively detect the onset of two different regimes, lowfrequency fluctuations and coherence collapse, and can be used for identifying the operating conditions that result in specific dynamical properties of the laser output. These tools can also be valuable for analyzing regime transitions in other complex systems.
Wed, 14 Dec 2016 14:16:15 GMT
http://hdl.handle.net/2117/98226
20161214T14:16:15Z
Quintero Quiroz, Carlos Alberto
Tiana Alsina, Jordi
Roma, Josep
Torrent Serra, Maria del Carmen
Masoller Alonso, Cristina
Identifying transitions to complex dynamical regimes is a fundamental open problem with many practical applications. Semi conductor lasers with optical feedback are excellent testbeds for studying such transitions, as they can generate a rich variety of output signals. Here we apply three analysis tools to quantify various aspects of the dynamical transitions that occur as the laser pump current increases. These tools allow to quantitatively detect the onset of two different regimes, lowfrequency fluctuations and coherence collapse, and can be used for identifying the operating conditions that result in specific dynamical properties of the laser output. These tools can also be valuable for analyzing regime transitions in other complex systems.

Observation of azimuthal modulation instability and formation of patterns of optical solitons in a quadratic nonlinear crystal
http://hdl.handle.net/2117/98018
Observation of azimuthal modulation instability and formation of patterns of optical solitons in a quadratic nonlinear crystal
Petrov, Dimitri; Torner Sabata, Lluís; Martorell Pena, Jordi; Vilaseca Alavedra, Ramon; Pérez Torres, Juan; Cojocaru, Crina
We report what is believed to be the first experimental demonstration of the azimuthal selfbreaking of intense beams containing a vortex phase dislocation into sets of optical spatial solitons in a quadratic nonlinear material. The observations were performed in a KTP crystal.
© 1998 Optical Society of America
Mon, 12 Dec 2016 13:31:55 GMT
http://hdl.handle.net/2117/98018
20161212T13:31:55Z
Petrov, Dimitri
Torner Sabata, Lluís
Martorell Pena, Jordi
Vilaseca Alavedra, Ramon
Pérez Torres, Juan
Cojocaru, Crina
We report what is believed to be the first experimental demonstration of the azimuthal selfbreaking of intense beams containing a vortex phase dislocation into sets of optical spatial solitons in a quadratic nonlinear material. The observations were performed in a KTP crystal.
© 1998 Optical Society of America

Quantum control of population transfer and vibrational states via chirped pulses in four level density matrix equations
http://hdl.handle.net/2117/96738
Quantum control of population transfer and vibrational states via chirped pulses in four level density matrix equations
Afa, Iduabo John; Serrat Jurado, Carles
We investigate the effect of chirped excitation and the excitation detuning on the coherent control of population transfer and vibrational states in a fourlevel system. Density matrix equations are studied for optimally enhanced processes by considering specific parameters typical of oxazine systems. Our simulations show a strong dependence on the interplay between chirp and excitation detuning and predict enhancement factors up to 3.2 for population transfer and up to
38.5 for vibrational coherences of electronic excited states. The study of the dynamics of the populations and vibrational coherences involved in the fourlevel system allows an interpretation of the different enhancement/suppression processes observed.
Wed, 16 Nov 2016 14:24:06 GMT
http://hdl.handle.net/2117/96738
20161116T14:24:06Z
Afa, Iduabo John
Serrat Jurado, Carles
We investigate the effect of chirped excitation and the excitation detuning on the coherent control of population transfer and vibrational states in a fourlevel system. Density matrix equations are studied for optimally enhanced processes by considering specific parameters typical of oxazine systems. Our simulations show a strong dependence on the interplay between chirp and excitation detuning and predict enhancement factors up to 3.2 for population transfer and up to
38.5 for vibrational coherences of electronic excited states. The study of the dynamics of the populations and vibrational coherences involved in the fourlevel system allows an interpretation of the different enhancement/suppression processes observed.

Photonic Crystal Microchip Laser
http://hdl.handle.net/2117/91434
Photonic Crystal Microchip Laser
Gailevicius, Darius; Koliadenko, Volodymyr; Purlys, Vytautas; Peckus, Martynas; Taranenko, Victor; Staliunas, Kestutis
The microchip lasers, being very compact and efficient sources of coherent light, suffer from one serious drawback: low spatial quality of the beam strongly reducing the brightness of emitted radiation. Attempts to improve the beam quality, such as pumpbeam guiding, external feedback, either strongly reduce the emission power, or drastically increase the size and complexity of the lasers. Here it is proposed that specially designed photonic crystal in the cavity of a microchip laser, can significantly improve the beam quality. Experiments show that a microchip laser, due to spatial filtering functionality of intracavity photonic crystal, improves the beam quality factor M2 reducing it by a factor of 2, and increase the brightness of radiation by a factor of 3. This comprises a new kind of laser, the “photonic crystal microchip laser”, a very compact and efficient light source emitting high spatial quality high brightness radiation.
Thu, 03 Nov 2016 15:01:02 GMT
http://hdl.handle.net/2117/91434
20161103T15:01:02Z
Gailevicius, Darius
Koliadenko, Volodymyr
Purlys, Vytautas
Peckus, Martynas
Taranenko, Victor
Staliunas, Kestutis
The microchip lasers, being very compact and efficient sources of coherent light, suffer from one serious drawback: low spatial quality of the beam strongly reducing the brightness of emitted radiation. Attempts to improve the beam quality, such as pumpbeam guiding, external feedback, either strongly reduce the emission power, or drastically increase the size and complexity of the lasers. Here it is proposed that specially designed photonic crystal in the cavity of a microchip laser, can significantly improve the beam quality. Experiments show that a microchip laser, due to spatial filtering functionality of intracavity photonic crystal, improves the beam quality factor M2 reducing it by a factor of 2, and increase the brightness of radiation by a factor of 3. This comprises a new kind of laser, the “photonic crystal microchip laser”, a very compact and efficient light source emitting high spatial quality high brightness radiation.