1,721,055 research outputs found
Discrete breathers and negative-temperature states
Full text linkWe explore the statistical behaviour of the discrete nonlinear Schrödinger equation as a test bed for the observation of negative-temperature (i.e. above infinite temperature) states in Bose–Einstein condensates in optical lattices and arrays of optical waveguides. By monitoring the microcanonical temperature, we show that there exists a parameter region where the system evolves towards a state characterized by a finite density of discrete breathers and a negative temperature. Such a state persists over very long (astronomical) times since the convergence to equilibrium becomes increasingly slower as a consequence of a coarsening process. We also discuss two possible mechanisms for the generation of negative-temperature states in experimental setups, namely, the introduction of boundary dissipations and the free expansion of wavepackets initially in equilibrium at a positive temperature
Combined effects of polarization and non-paraxial propagation on pattern formation
No full textWe show analytically that the presence inside an optical cavity of a polarizer and a hard diffracting object, for example, a circular aperture or waveguide, reduces the symmetry of the system from cylindrical to rectangular. As a consequence, patterns with concentric rings of bright or dark spots made up by modes whose azimuthal indexes are not multiples of one another, appear when the zero-field solution becomes unstable. These types of bifurcations are anomalous if the symmetry is cylindrical and therefore indicate the presence of polarization effects induced by diffraction. As an example of this type of system, we investigate numerically the bifurcations of a ring laser with a metallic duct and a polarizer inside the optical cavity
A versatile cryogenic temperature controller
No full textwe present the design and implementation of a versatile cryogenic temperature controller based on a microcontroller, characterized by high-accuracy and stability. It has been used to perform automated measurements on electron devices operating at very low temperatures. Noise measurements oil such devices are particularly sensitive, because random fluctuations of temperature call result in electrical noise superimposed to the quantity being measured. The system described ill this contribution can operate as a stand-alone unit or call be remotely controlled via all RS232 interface. High accuracy a!ld stability (+/-50 mK in the 10-300 K range), low noise, and on overall low cost are the main results that have been achieved
Diffraction-induced polarization effects in optical pattern formation
No full textWe show theoretically that patterns with concentric rings of bright or dark spots made up by modes with coprime azimuthal indexes may arise in codimension one bifurcations in optical systems with a metallic diffractor, a polarizer, and a nonlinear medium. These patterns are a consequence of diffraction-induced polarization in systems without polarization dynamics. As an example of this type of system, we investigate numerically the symmetry-breaking bifurcations of a ring laser with a metallic duct and a polarizer inside the optical cavity
Pulse compression by slow saturable absorber action in an optical parametric oscillator
No full textWe present numerical simulations of a singly-resonant, synchronously pumped optical parametric oscillator with intra-cavity slow saturable absorber. These indicate that compression ratios of about five or more may be achieved for realistic absorber parameters. The behaviour of the parametric oscillator with absorber presents a novel nonlinear resonance which is due specifically to the nature of the parametric three-wave interaction and which would not appear in a laser system
Centre manifold reduction of laser equations with transverse effects: an approach based on modal expansion
No full textCentre Manifold theory is a valuable method for analysing and simplifying partial differential equations that appear in the study of transverse effects in nonlinear optics. In this paper we analyse its application to the Maxwell-Bloch equations for lasers with spherical mirrors and finite size pumps. By taking advantage of the expansion in cavity modes, we successfully compare reduced models with the original one for broad ranges of the parameters
Self-localization of Bose-Einstein condensates in optical lattices via boundary dissipation
Full text linkWe introduce a technique to obtain localization of Bose-Einstein condensates in optical lattices via boundary dissipations. Stationary and traveling localized states are generated by removing atoms at the optical lattice ends. Clear regimes of stretched-exponential decay for the number of atoms trapped in the lattice are identified. The phenomenon is universal and can also be observed in arrays of optical waveguides with mirrors at the system boundaries
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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