1,721,052 research outputs found
Cavity Light Bullets: Three-Dimensional Localized Structures in a Nonlinear Optical Resonator
No full textThe paraxial model for a nonlinear resonator with a saturable absorber beyond the meanfield limit was investigated. Total radiation confinement and the formation of 3D localized bright structures was observed for accessible parametric domains. It was observed that self-organization proceeded from the resonator feedback, combined with diffraction and nonlinearity. Analysis shows that light bullets can be independently excited and erased by appropriate pulses, and after creation they endlessly travel the cavity round-trip
Thermal effects and cavity solitons in passive semiconductor microresonators
No full textWe formulate a model including the thermal dynamics in the time evolution of a passive semiconductor microresonator, containing a bulk medium, driven by a coherent holding beam. Thermal effects are taken into account via a dynamical equation for the lattice temperature, describing heat dissipation toward the environment, heating due to carrier generation, and thermal diffusion. The temperature dynamics is coupled to the carrier and field dynamics via the material susceptibility, a red-shift of the band-gap energy of the semiconductor upon an increase of temperature and a linear shift of the cavity resonance. The presence of thermal effects introduces a Hopf instability which, in certain regions of the parameter space, dominates the dynamics of the system. In this case our numerical simulations show that the output intensity may oscillate for constant holding beam intensity (regenerative oscillations), and if the input intensity grows slowly enough the hysteresis cycle may be inverted (switching point inversion). Oscillatory instabilities can also develop a modulational character, meaning that travelling patterns can be found. These phenomena develop over the slow timescale (microseconds) characterizing thermal effects in these devices. In other parameter regimes, the well-known Turing instability giving rise to stationary modulated patterns prevails, and the system displays the usual scenario of stable patterns and cavity solitons. Thermal effects seem not to play any relevant role in these regimes
3D self-organized patterns in the field profile of a semiconductor resonator
No full textWe cast a suitable model to describe the 3D dynamics of the coherent field in a monolithic multi-quantum-well (MQW) microresonator within and beyond the mean field limit (MFL) and provide a stability analysis and discriminating criteria to predict 3D pattern formation. While for fast media spontaneous self-confinement leads to the formation of 3D dissipative addressable spatial solitons, we show that for carrier dynamics compatible with GaAs/GaAlAs MQW devices longitudinal self-confinement is hindered by carrier 'sleuth'. We discuss turnaround strategies there
Cavity solitons in semiconductor microresonators: Existence, stability, and dynamical properties
No full textWe apply a versatile numerical technique to establishing the existence of cavity solitons (CS) in a semiconductor microresonator with bulk GaAs or multiple quantum well GaAs/AlGaAs as its active layer. Based on a Newton method, our approach implies the evaluation of the linearized operator describing deviations from the exact stationary state. The eigenvalues of this operator determine the dynamical stability of the CS. A typical eigenspectrum contains a zero eigenvalue With which a ''neutral mode" of the CS is associated. Such neutral modes are characteristic of models with translational symmetry. All other eigenvalues typically have negative real parts large enough to cause any excitations to die out in a few medium response times. The neutral mode thus dominates the response to external random or deterministic perturbations, and its excitation induces a simple translation of the CS, which are thus stable and robust. We show how to relate the speed with which a CS moves under external perturbations to the projection of the perturbations on to the neutral mode, and give some examples, including weak gradients on the driving field and interaction with other CS. Finally, we show that the separatrix between two stable coexisting solutions: the homogeneous solution and the CS is the intervening unstable CS solution. Our results are important with a view to future applications of CS to optical information processing
An overview on preseismic anomalies in LF radio signals revealed in Italy by wavelet analysis
Full text linkSince 1996, the electric field strength of the two broadcasting stations MCO (f=216 kHz, southeast France) and CZE (f=270 kHz, Czech Republic) has been sampled every ten minutes by a receiver (AS) located in central Italy. Here, we review the results obtained by a detailed analysis applied to the data recorded from February 1996 up to December 2004. At first, the daytime and nighttime data were extracted and then, in the daytime data, the data collected in winter were separated from those collected in summer. On the second step the wavelet transform was applied. The results of this analysis are radio anomalies detected as earthquake precursors both for MCO and CZE data. In particular, regarding the MCO data, the main result was the appearance of a very clear anomaly during May-August 1998, at daytime and at nighttime. Such an anomaly can be considered as a precursor of a seismic sequence started on August 15, 1998 with 17 earthquakes (M=2.2-4.6) on the Reatini mountains, a seismogenic zone located 30 km far from the AS receiver along the path MCO-AS. As concerns with the CZE data, the first result was obtained from the summer daytime data and it was the appearance of a very clear anomaly during August-September 1997, that can be considered a precursor of the two earthquakes with magnitude M=5.6 and M=5.9 that occurred on September 26 in the Umbria-Marche region (Central Italy). The second result was the appearance of an anomaly during February-March 1998, at daytime and at nighttime, that can be related to the preparatory phase of the strong (M=5.1-6.0) Slovenia seismic sequence that occurred in a zone lying in the middle of the CZE-AS path
The European VLF/LF radio network: current status
No full textFor several years researches about correlation between seismicity and disturbances
in radio broadcasting are being carried out: in particular, the Japanese Pacific VLF radio
network and the European VLF–LF radio network have been developed during the last
years. The European network has been developed starting from two LF receivers located in
central Italy in 1996. Up to now, 11 receivers of a new type, able to sample the VLF and LF
intensity of ten radio signals, are being into operation in different European countries. The
daily updating of data is effective and the data bank is located at the Department of Physics
of the University of Bari (Italy) which is the central node of the network. In order to discover
anomalies, the software able to carry out automatically a daily data analysis by the Wavelet
spectra method has been planned and realized. At the moment, the software operates on four
signals (two LF and two VLF) collected by one of the receiver located in Italy. If the anomaly
is particularly strong a warning system gives an advise on the work station into operation
in the central node of the Network. In any case, before assuming an anomaly as a seismic
anomaly, geomagnetic and meteorological data must be checked as well as any possible
instrumental malfunction. At present these controls are carried out only discontinuously by
the researchers of the Bari Team
Cavity light bullets: 3D self-confined structures in a passive nonlinear resonator
No full textWe consider the paraxial model for a nonlinear resonator with a saturable absorber beyond the mean-field limit. We introduce a general stability analysis to evidence modulational-instabilities leading to the destabilization of a homogeneous field profile, eventually causing the formation of 3D structures. Further on, for accessible parametric domains, we show in simulations the phenomenon of total radiation confinement leading to the formation of 3D localized bright structures. Such structures are a direct generalization of 2D Cavity Solitons, recently observed in broad-area VCSELs, but they are confined also in the propagation dimension. At difference from freely propagating light bullets, here the self-organization proceeds from the resonator feedback/dissipation, combined with diffraction and nonlinearity. We show that such cavity light bullets can be independently excited and erased by appropriate pulses. They can be addressed to form arrays in the transverse field profile as well as serial trains in the longitudinal direction of the resonator thus combining serial and parallel encoding in the same device. Once created, they endlessly travel the cavity roundtrip
Cavity solitons in semiconductor microcavities: Fundamental and applicative aspects
No full textCavity solitons appear as stationary, isolated peaks of light superimposed onto a homogeneous background field in the transverse profile of the coherent field transmitted or reflected by a non-linear resonator. These self-organised structures are theoretically predicted and simulated in a broad area multi-quantum-well vertical microresonator. We develop models suited to describe the macroscopic properties of the medium and the nonlinear interaction with the coherent field. Parametric domains and operational regimes for stable solitons are investigated along with some quantitative appreciation of their characteristics. Intrinsic stability properties of solitons are investigated by means of semi-analytical techniques and this allows to describe the destabilising mechanisms for solitons, mutual interaction properties, their response to perturbations and some of their dynamical features
Electric field strength analysis of 216 and 270 kHz broadcast signals recorded during 9 years
No full text[ 1] The electric field strength analysis of Czech Republic (CZE) ( 270 kHz) and Monte Carlo (MCO) ( 216 kHz) broadcast signals, collected with a 10 min sampling frequency by a receiver located in central Italy from 1996 to 2004, is presented. The distance from transmitter to receiver ranges from 515 km ( MCO) to 818 km ( CZE). The daytime data and the nighttime data were extracted and then in the daytime data the data collected in winter ( 21 December to 21 March) were separated from the data collected in summer ( 21 June to 21 September). Under the hypothesis that the simple addition of the ground wave and sky wave contributions holds, at first, the analysis was focused on the comparison between the experimental and theoretical values of these waves. The theoretical values were calculated by the ground wave (GRWAVE) algorithm and the wave hop theory, respectively. Ratios between the experimental and theoretical values ranging from a few tenths of decibels to some decibels were obtained. Then the analysis of the sunspots' influence on the sky wave propagation mode was performed, and the electric field strength of the two radio signals reveals a reduction of some decibels in sunspot maximum years with respect to the values during the sunspot minimum years. In addition, an influence of the sunspots also was recognized for the ground wave propagation mode
- …
