Relative Propagation Directions Research Articles

Goos–Hänchen Shift from Cold and Hot Atomic Media Using Kerr Nonlinearity

We study and examine the Goos–Hanchen shifts of a propagating probe light field in a four-level tripodtype cold and hot atomic medium. The behavior of Goos–Hanchen shifts is studied in reflection and transmission beams in the presence of the coherent Kerr effect and the Doppler broadening effect. We observe that these shifts can be controlled by the relative propagation direction of the control field to that of the probe field.

Two dimensional advective heat flux estimation from velocity measurements

The advective heat flux, ρcpv · ∇T, is an important quantity in edge flames because the sign of v · ∇T characterizes relative propagation direction of the edge flame, and its magnitude gives information regarding strength of the resulting propagation or extinction front. Modern laser techniques can measure the velocity and temperature fields simultaneously within a flame, but accurate measurements of temperature simultaneous with velocity outside the high temperature region of the flame are more difficult and time-resolved measurements of velocity and temperature simultaneously for unsteady flows require complex measurement systems. In this paper, the theoretical framework to show that the flow dilatation can be used to directly estimate a scaled advective heat flux is presented. Numerical simulations of both a positive (propagating) and negative (extinguishing) edge flame were studied to assess the relationship between dilatation and advective heat flux. Experimental measurements of the two-dimensional component of dilatation in positive and negative edge flames were also made and compared to the simulations. The full advective heat flux was well represented by the dilatation in the simulations, and the experimental data demonstrates that the measured dilatation is useful for estimating advective heat flux. Flames with varying positive and negative flame speeds are examined with the technique.

Coincidence of heliospheric current sheet and stream interface: Implications for the origin and evolution of the solar wind

AbstractIn general, the heliospheric current sheet (HCS), which defines the boundary of sunward and antisunward magnetic field, is encased by the slow solar wind. The stream interface (SI) represents the boundary between the solar wind plasmas of different origin and/or characteristics. According to earlier studies using data of low time resolution, the SI and HCS get closer further away from the Sun, and the two structures coincide with each other around 5 AU. In this study, we use STEREO data of a much higher time resolution to reveal an unusual case where the SI and HCS are coincident near 1 AU and separated from the so‐called true sector boundary (TSB) at which the suprathermal electrons change their relative propagation directions. Preliminary analysis suggests that the closed loops in pseudostreamers continually have interchange reconnection with the open‐field lines that lead them, resulting not only in the coincidence of HCS and SI but also in the separation of the TSB from the HCS/SI. We therefore conclude that the interchange reconnection plays an important role in the evolution of slow solar wind.

Gravity wave propagation through a vertically and horizontally inhomogeneous background wind

AbstractA combination of ray theory and 2‐D time‐dependent simulations is used to investigate the linear effects of a time‐dependent, vertically, and horizontally inhomogeneous background horizontal wind field on the propagation, refraction, and reflection of small‐scale gravity wave packets. Interactions between propagating waves of different scales are likely to be numerous and important. We find that a static medium‐scale wave wind field of sufficient amplitude can channel and/or critical‐level filter a small‐scale wave or cause significant reflection, depending upon both waves' parameters. However, the inclusion of a time‐dependent phase progression of the medium‐scale wave can reduce energy loss through critical‐level filtering by up to ∼70% and can also reduce reflection by up to ∼60% for the cases simulated. We also find that the relative direction of propagation between the small‐scale and medium‐scale wave can significantly affect small‐scale wave filtering. When the phases are progressing in the same horizontal direction, the small‐scale wave is far more likely to become trapped and ultimately critical‐level filtered than if the phases are propagating in opposite horizontal directions unless reflection occurs first. Considerations of time‐dependent winds associated with medium‐scale‐propagating waves and their directionality are important for assessing the propagation and dispersion of small‐scale waves over large horizontal distances.

Stimulated Compton Scattering in Two-Color Ionization of Hydrogen with keV Electromagnetic Fields

We present a theoretical study of two-color ionization of hydrogen with keV photons at intensities ranging from 1016 to 1018 W/cm2. We consider the atom in interaction with a superposition of two electromagnetic pulses centered around two frequencies that differ by a few atomic units and we present in detail the case of the frequencies 55 and 50a.u. We present the electron energy spectra, angular distributions, and ionization rates based on nonperturbative and perturbative calculations. Although the ejected electron energy distribution is dominated by one-photon ionization from each pulse, we are able to identify the contribution of stimulated Compton scattering, a process in which one photon is absorbed while the other is emitted, the photon energy difference being transferred to the electron. This leads to low-energy electrons, and we show in particular that it is of crucial importance to consider the retardation effects on the ionization rates and the electron angular distributions. The relative propagation direction of the two fields also plays an important role; in the case of counterpropagating fields, the ionization by stimulated Compton scattering is dominated by A2 and competes with one-photon ionization at high intensities.

Dynamic patterns and their interactions in networks of excitable elements

Formation of localized propagating patterns is a fascinating self-organizing phenomenon that happens in a wide range of spatially extended, excitable systems in which individual elements have resting, activated, and refractory states. Here we study a type of stochastic three-state excitable network model that has been recently developed; this model is able to generate a rich range of pattern dynamics, including localized wandering patterns and localized propagating patterns with crescent shapes and long-range propagation. The collectivedynamicsof these localized patterns have anomalous subdiffusive dynamics before symmetry breaking and anomalous superdiffusive dynamics after that, showing long-range spatiotemporal coherence in the system. In this study, the stability of the localized wandering patterns is analyzed by treating an individual localized pattern as a subpopulation to develop its average response function. This stability analysis indicates that when the average refractory period is greater than a certain value, there are too many elements in the refractory state after being activated to allow the subpopulation to support a self-sustained pattern; this is consistent with symmetry breaking identified by using an order parameter. Furthermore, in a broad parameter space, the simple network model is able to generate a range of interactions between different localized propagating patterns including repulsive collisions and partial and full annihilations, and interactions between localized propagating patterns and the refractory wake behind others; in this study, these interaction dynamics are systematically quantified based on their relative propagation directions and the resultant angles between them before and after their collisions. These results suggest that the model potentially provides a modeling framework to understand the formation of localized propagating patterns in a broad class of systems with excitable properties.

Strong enhancement of Raman-induced nonreciprocity in silicon waveguides by alignment with the crystallographic axes

Raman gain in silicon photonic wires depends on the relative propagation directions of the pump and Stokes waves due to strong longitudinal mode-field components. Here we show that this Raman-induced nonreciprocity can be varied in a wide range by changing the orientation of the waveguide with respect to the crystallographic axes. In a ⟨001⟩ orientation, the nonreciprocity reaches its maximum which is larger by more than two orders of magnitude as compared to the traditional ⟨011⟩ orientation. The waveguide is then practically Raman-inactive for copropagation, while the contradirectional Stokes wave may experience significant Raman gain.

Time‐frequency pattern of the sound intensity fluctuations of midfrequency signals in presence of internal waves in Shallow Water 06 experiment.

In given paper fluctuations of intensity of sound signals, radiated by the midfrequency source (R/V Knorr) during ∼5 h were studied. Broadband signals (2–8 KHz) were received by four single hydrophones fixed at the bottom (SHRUs) placed at different distances from the source (from 4 to 12 km). These four acoustic tracks have different directions relative direction of propagation of the train of intensive internal waves (the corresponding angles in horizontal plane are from ∼5 to ∼15 deg). Time‐frequency diagrams were constructed using frequency filtering of the spectrum of broadband signals with sliding narrow window. Temporal dependences of intensities of received signals within frequency window were constructed for period 14:00 until 19:00 GMT for experiment carried out on 13 August. During this time train of internal solitons was registered, propagating toward the coast. Mentioned time‐frequency diagrams demonstrate specific features of influence of internal waves on the temporal variations of the soun...

Waves propagation in turbulent superfluid helium in presence of combined rotation and counterflow

A complete study of the propagation of waves (namely longitudinal density and temperature waves, longitudinal and transversal velocity waves and heat waves) in turbulent superfluid helium is made in three situations: a rotating frame, a thermal counterflow, and the simultaneous combination of thermal counterflow and rotation. Our analysis aims to obtain as much information as possible on the tangle of quantized vortices from the wave speed and attenuation factor of these different waves, depending on their relative direction of propagation with respect to the rotation vector.

Geometric description of lightlike foliations by an observer in general relativity

We introduce new concepts and properties of lightlike distributions and foliations (of dimension and co-dimension 1) in a space-time manifold of dimension $n$, from a purely geometric point of view. Given an observer and a lightlike distribution $\Omega $ of dimension or co-dimension 1, its lightlike direction is broken down into two vector fields: a timelike vector field $U$ representing the observer and a spacelike vector field $S$ representing the relative direction of propagation of $\Omega $ for this observer. A new distribution $\Omega_U^-$ is defined, with the opposite relative direction of propagation for the observer $U$. If both distributions $\Omega $ and $\Omega _U^-$ are integrable, the pair \Omega ,\Omega_U^- $ represents the wave fronts of a stationary wave for the observer $U$. However, we show in an example that the integrability of $\Omega $ does not imply the integrability of $\Omega_U^-$.

High-gain Tm-doped fluoride fiber amplifier

A diode-pumped Tm-doped fiber-optic amplifier that has a small-signal gain of >30 dB at 1870 nm is reported. Output pulses of up to 3-W peak power at a 1-60-kHz repetition rate can be generated by amplification of 20-40-ns laser diode pulses of up to 2-mW launched peak power. The output signal quality, i.e., the ratio of the output pulse energy and the total amplified spontaneous emission (ASE) output energy between two pulses, depends on the relative propagation direction of pump and signal and can be dramatically increased by choice of the correct propagation scheme. In the optimum pump geometry the pulse energy can be raised to as much as 20 times the ASE energy. This is the first report to the author's knowledge of fiber-optic amplification of short diode laser pulses near 1.9 microm with high repetition rates in Tm-doped fibers.

Effects of concentration on the performance of erbium-doped fiber amplifiers

The dependence of erbium-doped fiber amplifier (EDFA) performance on the erbium ion concentration is studied experimentally and theoretically. The quantum efficiency of the amplifier is found to he strongly dependent on the erbium ion concentration, the signal wavelength, and the relative propagation direction of the pump and signal beams. This dependence is fully explained by the presence of an upconversion mechanism between ions residing in pairs or larger clusters and suggests that other sources of amplifier performance degradation (back-ground loss, excited state absorption, homogenous upconversion) are negligible. The experimental data show that in the present EDFA designs with over 80% quantum/conversion efficiency, the aluminum co-doped fibers with erbium ion concentration less than 20/spl times/10/sup 24/ m/sup -3/ (900 molar ppm Er/sup 3+/) are most suitable.

Nonlinear nonreciprocity of soliton amplification with erbium-doped fibers

In the presence of pump absorption, soliton amplification with erbium-doped fibers is no longer reciprocal with respect to the interchange between the relative direction of propagation of signal and pump. We present a numerical analysis of the influence of pump absorption on the temporal and spectral characteristics and on the energy amplification factor of picosecond and femtosecond signals.

Frequency shift in three-photon resonance.

We calculate the line shift in the three-photon resonance-absorption experiment performed by Ferrel, Payne, and Garrett [Phys. Rev. A 36, 81 (1988)] and find it large, to the violet, and close to experiment. In comparison, the calculated single-photon resonance shift in absorption is a factor of 30 times smaller and to the red. The three-photon resonance shift depends on the relative propagation directions of the excitation beams, and we present a formula for the case where only two excitation beams are present.

Investigation of the forward and backward scattering of spatially noncoherent pump radiation

A new method developed by the authors for the analysis of stimulated scattering of spatially noncoherent radiation is used in a comparative study of the concurrent and opposed interactions of a Stokes field with pump radiation. If there is no correlation between the Stokes and pump waves, the interaction is independent of the relative direction of propagation of the waves. However, if the Stokes and pump waves are correlated, the interaction efficiency depends strongly on the relative directions of the waves. Exact expressions are obtained for the structure of the wave fields and gain increments in the backscattering case and approximate expressions are obtained for the forward scattering. The physical reason for the asymmetry in the amplification is the reproduction by the Stokes signal spectrum of the pump spectrum in the forward scattering case and inversion of this spectrum in the backscattering case. The same reason accounts for a deterioration of the quality of wavefront reversal in the backscattering case compared with the forward scattering.

Interactions Between Elastic Waves in an Isotropic Solid

Interactions between elastic waves in an isotropic solid are studied in the elastic-continuum approximation. The analysis is carried out completely in a wave-packet formalism, i.e., scattering of wave packets by wave packets. The maximum amplitude (or intensity) and width of the scattered wave packet are expressed in terms of the maximum amplitudes, frequencies, widths, polarizations, and relative propagation directions of the primary-wave packets. The polarization relations and frequency ranges for the allowed interaction processes are obtained; these are essentially identical to the ones given by Jones and Kobett. The results are shown to be in good order-of-magnitude agreement with the experiments of Rollins. Possible application of elastic-wave scattering to the determination of third-order elastic constants is discussed.