Irreversible circulation and the undamped spiking in lasers

Our new theory of fluctuation was applied to the undamped spiking in lasers with single mode saturable absorption. At the marginal points, which lead to limit cycles, it was found that the irreversible circulation α and the residual circulation α′ diverge together with the variance σ. The model system of this article has effectively three degrees of freedom; however, it was demonstrated that the dominant motion and the fluctuation have two dimensional character in the neighbourhood of a hard mode instability.     

Irreversible circulation of fluctuation

Irreversible circulation of fluctuation around the steady state, or cyclic balance, is useful in describing coupled degrees of freedom at off-equilibrium situation. When it entails instability, a macroscopic orbital revolution may appear, which is characteristic to far from equilibrium situation.     

The origin of spiking pulses in HCN gas lasers

The time varying plasma refractive index of the gas discharge in a 337 μm HCN laser is shown to be responsible for its spiking output pulses.     

Rate equations for dye lasers: Comments on the spiking phenomenon

The rate equations generally used for dye lasers necessitate a number of assumptions. The assumptions are tested in the case of pulsed dye oscillator. It is shown that the assumptions break down during the laser onset. The main result is this: the spikes computed for the excited state population and for laser photon flux cannot be given a physical meaning and cannot be used to explain the properties of pulsed dye lasers.     

Undamped resonance spiking by internal modulation of pulsed solid state lasers

Cavity loss modulation is investigated in the low-frequency range of pulsed Nd-lasers. The response of the system shows a typical resonant behaviour, whose resonance frequency is equal to the frequency of the relaxation oscillations. The resonance step-up is in the order of 10³, which means that an extremely small driving voltage at the electrooptic cell results in very regular, undamped spikes.Experiments are performed with a pulsed Nd: YAG laser. Pulse power, half-width, and distance are in the range required for controlled material removal in high quality laser material processing.     

Irreversible quantum mechanics in the neutralK-system

The neutral kaon system is used to test the quantum theory of resonance scattering and decay phenomena. The two dimensional Lee-Oehme-Yang theory with complex Hamiltonian is obtained by truncating the complex basis vector expansion of the exact theory in rigged Hilbert space. This can be done forK ₁ andK ₂ as well as forK _S andK _L , depending upon whether one chooses the (self-adjoint, semibounded) Hamiltonian as commuting or noncommuting withCP. As an unexpected curiosity, one can show that the exact theory (without truncation) predicts long-time 2π decays of the neutral kaon system even if the Hamiltonian conservesCP.     

Designing the dynamics of spiking neural networks

Precise timing of spikes and temporal locking are key elements of neural computation. Here we demonstrate how even strongly heterogeneous, deterministic neural networks with delayed interactions and complex topology can exhibit periodic patterns of spikes that are precisely timed. We develop an analytical method to find the set of all networks exhibiting a predefined pattern dynamics. Such patterns may be arbitrarily long and of complicated temporal structure. We point out that the same pattern can exist in very different networks and have different stability properties.     

Irreversible magnetization in nickel nanoparticles

We report magnetic studies on nickel nanoparticle films of average particle size of 10 nm. Magnetization as a function of field and temperature show that the system behaves like a random magnet with a strongly field-dependent irreversible temperature, below which the magnetization relax logarithmically with time. The effective barrier extrapolated increases strongly with temperature for a given field. The time dependence suggests the dominant dipole–dipole interaction in this magnetic nanoparticle system.     

Irreversible evolution in quantum logics

The notion of dynamical semigroup is introduced in the quantum logic scheme on the set of the states. Under suitable nonempty mathematical assumptions it is shown that a Heisenberg picture exists equivalent to the Schrödinger one and having many aspects similar to those of the Hilbert case.     

Dynamical entropy production in spiking neuron networks in the balanced state

We demonstrate deterministic extensive chaos in the dynamics of large sparse networks of theta neurons in the balanced state. The analysis is based on numerically exact calculations of the full spectrum of Lyapunov exponents, the entropy production rate, and the attractor dimension. Extensive chaos is found in inhibitory networks and becomes more intense when an excitatory population is included. We find a strikingly high rate of entropy production that would limit information representation in cortical spike patterns to the immediate stimulus response.     

Irreversible shear-activated aggregation in non-Brownian suspensions

We have studied the effect of shear on the stability of suspensions made of non-Brownian solid particles. We demonstrate the existence of an irreversible transition where the solid particles aggregate at remarkably low volume fractions (phi approximately 0.1). This shear-induced aggregation is dramatic and exhibits a very sudden change in the viscosity, which increases sharply after a shear-dependent induction time. We show that this induction time is related exponentially to the shear rate, reflecting the importance of the hydrodynamic forces in reducing the repulsive energy barrier that prevents the particles from aggregating.     

Double heterojunction lasers and quantum well lasers

The improved technology of compound semiconductor heterojunction preparation has resulted in very reliable CW, room temperature diode lasers for optical information read-out grown on p-type substrates on the one hand and very abrupt double heterojunction diode lasers based on quantum effects on the other hand. The influence of quantization effects on the emission wavelength, the threshold current and its temperature dependence are discussed. A distinction has been made between quantization due to strong magnetic fields giving rise to a one-dimensional electron gas (quantum wire) and quantization resulting from electrostatic and/or compositional changes (quantum well). The double heterojunction as a test structure to study carrier scattering into quantum wells, the phonon participation in the hot carrier relaxation process and optical flux guiding in graded heterojunctions have been emphasized.     

On the Cosmological Implications of Irreversible Thermodynamics

The usual arguments related to the validity ofa thermostatical approach to several cosmologicalproblems are reviewed. It is seen that a nonequilibriumformalism is needed to solve conceptual problems in cosmology, such as the the generalization ofthe second law of thermodynamics. A cosmologicalnon-equilibrium formalism with a positive-semidefiniteentropy production is derived. The main features of a universe formulated under such a basis arestudied.     

Semiconductor lasers and fiber lasers for fiber-optic telecommunications

Be performance characteristics of semiconductor lasers for fiber telecommunication systems will be reviewed. Modulation speed, intensity noise, singlefrequency line width, and tunability are addressed. In addition, recent results concerning the same characteristics in single-frequency, tunable, fiber lasers are reviewed and compared with the semiconductor laser.     

Irreversible energy flow in forced Vlasov dynamics

Molecular dynamics (MD) simulations are used to investigate the thermodynamic properties and structural changes of KCl spherical nanoparticles at various sizes (1064, 1736, 2800, 3648, 4224 and 5832 ions) upon heating. The melting temperature is dependent on both the size and shape of KCl models, and the behaviour of the first order phase transition is also found in the present work. The surface melting found here is different from the melting phenomena of KCl models or other alkali halides studied in the past. In the premelting stage, a mixed phase containing liquid and solid ions covers the surface of nanoparticles. The only peak of heat capacity spreads out a significant segment of temperature, probably exhibiting both heterogeneous melting on the surface and homogeneous melting in the core. The coexistence of two melting mechanisms, homogeneous and heterogeneous ones, in our model is unlike those considered previously. We also found that the critical Lindemann ratio of the KCl nanoparticle becomes much more stable when the size of the nanoparticle is of the order of thousands of ions. A picture of the structural evolution upon heating is studied in more detail via the radial distribution function (RDF) and coordination numbers. Our results are in a good agreement with previous MD simulations and experimental observations.     

Propagation of spiking regularity and double coherence resonance in feedforward networks

We investigate the propagation of spiking regularity in noisy feedforward networks (FFNs) based on FitzHugh-Nagumo neuron model systematically. It is found that noise could modulate the transmission of firing rate and spiking regularity. Noise-induced synchronization and synfire-enhanced coherence resonance are also observed when signals propagate in noisy multilayer networks. It is interesting that double coherence resonance (DCR) with the combination of synaptic input correlation and noise intensity is finally attained after the processing layer by layer in FFNs. Furthermore, inhibitory connections also play essential roles in shaping DCR phenomena. Several properties of the neuronal network such as noise intensity, correlation of synaptic inputs, and inhibitory connections can serve as control parameters in modulating both rate coding and the order of temporal coding.     

Dynamical origin of independent spiking and bursting activity in neural microcircuits

The relationship between spiking and bursting dynamics is a key question in neuroscience, particularly in understanding the origins of different neural coding strategies and the mechanisms of motor command generation and neural circuit coordination. Experiments indicate that spiking and bursting dynamics can be independent. We hypothesize that different mechanisms for spike and burst generation, intrinsic neuron dynamics for spiking and a modulational network instability for bursting, are the origin of this independence. We tested the hypothesis in a detailed dynamical analysis of a minimal inhibitory neural microcircuit (motif) of three reciprocally connected Hodgkin-Huxley neurons. We reduced this high-dimensional dynamical system to a rate model and showed that both systems have identical bifurcations from tonic spiking to burst generation, which, therefore, does not depend on the details of spiking activity.     

Breathing,spiking and chaos in a laser with injected signal

We consider the behavior of a laser driven by an injected cw field detuned from the operating laser frequency. Our analysis covers the entire range of incident power levels from zero to the injection locking threshold. In this domain, the output intensity exhibits regular and chaotic oscillations, a period doubling cascade in reverse order, envelope breathing and spiking.