Well-defined Phase Relationship Research Articles

Consequences for predator–prey dynamics caused by the presence of obstacles

In order to understand how a heterogeneous habitat affects the population dynamics of the predator–prey system, a spatially explicit lattice model consisting of predators, prey and obstacles is constructed. The model includes smart pursuit (predators to prey) and evasion (prey from predators). Both species can affect their movement by visual perception within their finite sighting range. Non-conservative processes that change the number of individuals within the population, such as breeding and physiological dying, are implemented in the model. Obstacles are represented by non-overlapping lattice shapes that are randomly placed on the lattice. In the absence of obstacles, numerical simulations reveal regular, coherent oscillations with a nearly constant predator–prey phase difference. Numerical simulations have shown that changing the probabilities for non-conservative processes can increase or decrease the period of coherent oscillations in species abundances and change the relative lag between coherent components. After introducing obstacles into the model, we observe random transitions between coherent and non-coherent oscillating regimes. In the non-coherent regime, predator and prey abundances continue to oscillate, but without a well-defined phase relationship. Our model suggests that stochasticity introduced by density fluctuations of obstacles is responsible for the reversible shift from coherent to non-coherent oscillations.

Synthesis of discrete phase-coherent optical spectra from nonlinear ultrasound.

Nonlinear acoustic interactions in liquids are effectively stronger than nonlinear optical interactions in solids. Thus, harnessing these interactions will offer new possibilities in the design of ultra-compact nonlinear photonic devices. We theoretically demonstrate a new scheme for synthesis of optical spectra from nonlinear ultrasound harmonics using a hybrid liquid-state and nanoplasmonic device compatible with fibre-optic technology. The synthesised spectra consist of a set of equally spaced optical Brillouin light scattering modes having a well-defined phase relationship between each other. We suggest that these spectra may be employed as optical frequency combs whose spectral composition may be tuned by controlling the nonlinear acoustic interactions.

Phase and coherence of optical microresonator frequency combs

We use a combination of theoretical analysis, numerical simulation, and experimental measurement to investigate the near-threshold phase and coherence properties of parametric optical frequency combs generated in low-loss dielectric microresonators. Our analysis reveals that near threshold the phases of the comb lines do not stabilize to a constant value across the spectrum, although well-defined phase relationships relative to the pump laser do exist. Our results are supported by numerical simulations of two different microresonator combs operated under varying conditions of input drive, dispersion, and detuning. These results are also experimentally confirmed through phase measurements of the individual comb lines. We also investigate the processes leading to the breakdown of the equidistant frequency spacing of the modes in a microresonator comb.

Nonlinear Mode Coupling and Synchronization of a Vacuum-Trapped Nanoparticle

We study the dynamics of a laser-trapped nanoparticle in high vacuum. Using parametric coupling to an external excitation source, the linewidth of the nanoparticle's oscillation can be reduced by three orders of magnitude. We show that the oscillation of the nanoparticle and the excitation source are synchronized, exhibiting a well-defined phase relationship. Furthermore, the external source can be used to controllably drive the nanoparticle into the nonlinear regime, thereby generating strong coupling between the different translational modes of the nanoparticle. Our work contributes to the understanding of the nonlinear dynamics of levitated nanoparticles in high vacuum and paves the way for studies of pattern formation, chaos, and stochastic resonance.

Quantum-coherent energy transfer: implications for biology and new energy technologies

The important thing in science is not so much to obtain new facts as to discover new ways of thinking about them. Sir William Henry Bragg Nobel Prize For Physics, 1915 There is a growing body of scientific evidence that light-initiated reactions in the molecular components of photosynthetic

Effects of ovarian hormones and aging on respiratory sinus arrhythmia and breathing patterns in women

We investigated the effect of ovarian hormones and aging on breathing pattern [pulmonary minute ventilation (V(E))], tidal volume (V(T)), breathing frequency (F(b)), and respiratory sinus arrhythmia (RSA) in women. Recordings of V(E) and electrocardiogram (ECG) were obtained from 23 healthy women (10 premenopausal, 13 postmenopausal) under resting, isocapnic hypoxia (IH), and euoxic hypercapnia (EH) conditions. Premenopausal women were tested on three different days, each day corresponding to a specific phase of the menstrual cycle (follicular, mid-cycle, and luteal); postmenopausal women (PMW) were tested on 1 day only. On each test day, subjects were challenged with IH and EH. The order of the two tests was randomized and separated by at least 1 hour. Due to the low F (b) of several PMW, the band limits for RSA analysis had to be adjusted. The spectral coherence between respiratory flow and ECG RR-interval was used to determine the spectral band. Within the spectral band, there was a consistent phase relationship between the two variables where high values of spectral coherence indicate a well-defined phase relationship between respiratory flow and RR-interval variability. The main findings in this study for RSA are fourfold. First, RSA did not change with different levels of ovarian hormones (progesterone, serum 17beta-estradiol) during the menstrual cycle. Second, RSA was not influenced by hormone replacement therapy. Third, RSA did not change with age. Fourth, RSA did not change with IH and EH-induced changes in breathing patterns. Finally, high individual variability of average RR-interval change per breath was found.

Coherent control and phase locking of two-photon processes in the nanosecond domain

We demonstrate phase locking between two pairs of nanosecond laser pulses generated from independent sources. We achieve phase locking experimentally by separately mixing two uncorrelated dye lasers of frequencies ω1(a) and ω1(b), with a common beam of frequency ω0, thereby generating two additional frequencies ω2(b)≡ω1(a)+ω0 and ω2(a)≡ω1(b)+ω0. We demonstrate that there are well-defined phase relationships between any two-photon process using the ω1(a) and the ω2(a) pair of frequencies versus any two-photon process that uses the ω1(b) and the ω2(b) pair. In particular, interference between the two identical sum frequencies ωtotal=ω1(a)+ω2(a) and ωtotal=ω1(b)+ω2(b), which we generate in a separate pair of mixing crystals, yields stable interference fringes with measured modulation depths of ±40%. Well-defined phase relationships are especially useful for two-photon versus two-photon coherent control experiments. In addition, the system can be used to transport, with a high degree of stability, the phase of a given input laser frequency ω0 to higher frequencies ωtotal by use of carrier lasers that need not be correlated.

Interference effects in the two-photon ionization of nitric oxide

An experimental demonstration of the interference between two different resonant two-photon, two-color ionization processes from the A 2Σ+, v′=1 state of nitric oxide is presented. This interference effect does not require a well-defined phase relationship between the two ionizing lasers and should, in principle, allow the control of the overall ionization process by small detunings of either or both of the two resonant ionization pathways. This interference effect is quite general, and the potential of this scheme for the control of photoionization and photodissociation partial cross sections is discussed.

SELF-ORGANIZATION IN BIOLOGICAL TISSUES: ANALYSIS OF ASYNCHRONOUS AND SYNCHRONOUS PERIODICITY, TURBULENCE AND SYNCHRONOUS CHAOS EMERGENT IN COUPLED CHAOTIC ARRAYS

Large discrete arrays of chaotic attractors, coupled by diffusion, organize into asynchronous periodic spiral waves, synchronous periodic bands, turbulent fields or synchronous chaos as a function of coupling strength and array size. Self-organization of periodic spirals in both two and three dimensional arrays of nonexcitable systems appears to require the early establishment of an antipodal phase relationship between the few cells that will form the vortex. Cells within or close to the vortex maintain low z amplitude, near limit cycle trajectories, with stable, well-defined phase relationships. In periodic banding structures, initial antipodal phase seeds evolve to isochrons that form nested periodic trajectories. The likelihood that biological systems are fundamentally oscillatory and chaotic is discussed.

Simulation of ENSO-like Phenomena with a Law-Resolution Coupled GCM of the Global Ocean and Atmosphere

A 140-year simulation of the ocean-atmosphere climate system has been performed by the GFDL Climate Dynamics Project using a low-resolution coupled general circulation model (GCM). The model was subjected to annually averaged insolation throughout the integration. This coupled system exhibits well-defined fluctuations in the tropical Pacific, with a preferred time male of 3–4 years. The characteristics of these recurrent anomalies were examined by applying an extended empirical orthogonal function (EEOF) analysis to selected model variables. These results indicate that the simulated oscillations are accompanied by coherent changes in the atmospheric and oceanic circulation. The spatial patterns associated with the leading EEOF mode indicate that SST anomalies make their first appearance off the Peru-Ecuador coast and then migrate steadily westward, with an average transit time of 12–15 months. The arrival and eventual decay of SST fluctuations in the western Pacific is typically followed by the initiation of anomalies of the opposite polarity along the American coasts. The space–time evolution of various meteorological and oceanographic signals exhibits well-defined phase relationships with the SST perturbations. Some aspect of the model behavior during these warm and cold episodes are reminiscent of observed phenomena associated with the El Niño–Southern Oscillation (ENSO). Analysis of the climatological heat budget for the top ocean layer indicates a near balance between horizontal and vertical temperature advection by the time-mean flow, vertical diffusion, and heat input from the overlying atmosphere Contributions of transient effects to this balance are negligible. The principal mechanisms associated with the simulated ENSO-like cycles were then studied by examining the local heat budget for the SST perturbations. It is shown that the relative importance of various linear advective processes in the heat budget exhibits a notable dependence on geographical location and on the specific phase of the ENSO-like cycle.

Picosecond time-scale phase-related optical pulses: measurement of sodium optical coherence decay by observation of incoherent fluorescence

A technique for producing phase-related picosecond optical pulses is described. A sequence of two phase-related pulses, well separated in time, is applied to the D1 line in Na vapor. When the phase between the two pulses is scanned, oscillations in the incoherent fluorescence are observed. This demonstrates the well-defined phase relationship between the pulses. Measuring the amplitude of the oscillations as a function of pulse separation yields the Na optical coherence decay. A beat arising from the ground-state hyperfine splitting is observed, and the results agree well with calculation.

Coherence in the ac Josephson-Fiske effect

Pair transfer accompanied by photon emission does not demand the existence of a well-defined phase relationship between the two superconductors. This raises certain questions about the role of coherence in the ac Josephson-Fiske effect, which we discuss here.