Showing papers on "Group velocity published in 2001"

Storage of light in atomic vapor.

Abstract: We report an experiment in which a light pulse is effectively decelerated and trapped in a vapor of Rb atoms, stored for a controlled period of time, and then released on demand. We accomplish this ``storage of light'' by dynamically reducing the group velocity of the light pulse to zero, so that the coherent excitation of the light is reversibly mapped into a Zeeman (spin) coherence of the Rb vapor.

Nonreciprocal magnetic photonic crystals

Abstract: We study band dispersion relations omega(k-->) of a photonic crystal with at least one of the constitutive components being a magnetically ordered material. It is shown that by proper spatial arrangement of magnetic and dielectric components one can construct a magnetic photonic crystal with strong spectral asymmetry (nonreciprocity) omega(k-->) not equal omega(-k-->). The spectral asymmetry, in turn, results in a number of interesting phenomena, in particular, one-way transparency when the magnetic photonic crystal, being perfectly transparent for a Bloch wave of frequency omega, "freezes" the radiation of the same frequency omega propagating in the opposite direction. The frozen radiation corresponds to a Bloch wave with zero group velocity partial differential omega(k)/ partial differential k=0 and, in addition, with partial differential(2)omega(k)/ partial differential k(2)=0.

Transparent anomalous dispersion and superluminal light-pulse propagation at a negative group velocity

Abstract: Anomalous dispersion cannot occur in a transparent passive medium where electromagnetic radiation is being absorbed at all frequencies, as pointed out by Landau and Lifshitz. Here we show, both theoretically and experimentally, that transparent linear anomalous dispersion can occur when a gain doublet is present. Therefore, a superluminal light-pulse propagation can be observed even at a negative 7group velocity through a transparent medium with almost no pulse distortion. Consequently, a negative transit time is experimentally observed resulting in the peak of the incident light pulse to exit the medium even before entering it. This counterintuitive effect is a direct result of the rephasing process owing to the wave nature of light and is not at odds with either causality or Einstein's theory of special relativity.

Slow and Fast Light

Abstract: Recent research has established that it is possible to exercise extraordinary control of the velocity of propagation of light pulses through a material system. Both extremely slow propagation (much slower than the velocity of light in vacuum) and fast propagation (exceeding the velocity of light in vacuum) have been observed. This article summarizes this recent research, placing special emphasis on the description of the underlying physical processes leading to the modification of the velocity of light. To understand these new results, it is crucial to recall the distinction between the phase velocity and the group velocity of a light field. These concepts will be defined more precisely below; for the present we note that the group velocity gives the velocity with which a pulse of light propagates through a material system. One thus speaks of “fast” or “slow” light depending on the value of the group velocity vg in comparison to the velocity of light c in vacuum.

Crustal and upper mantle structure beneath Antarctica and surrounding oceans

Abstract: We present and discuss a new model of the crust and upper mantle at high southern latitudes that is produced from a large, new data set of fundamental mode surface wave dispersion measurements. The inversion for a 2°×2° shear velocity model breaks into two principal steps: first, surface wave tomography in which dispersion maps are produced for a discrete set of periods for each wave type (Rayleigh group velocity, 18–175 s; Love group velocity, 20–150 s; Rayleigh and Love phase velocity, 40–150 s) and, second, inversion for a shear velocity model. In the first step, we estimate average resolution at high southern latitudes to be about 600 km for Rayleigh waves and 700 km for Love waves. The second step is a multistage process that culminates in a Monte Carlo inversion yielding an ensemble of acceptable models at each spatial node. The middle of the ensemble (median model) together with the half width of the corridor defined by the ensemble summarize the results of the inversion. The median model fits the dispersion maps at about the measurement error (group velocities, 20–25 m/s; phase velocities, 10–15 m/s) and the dispersion data themselves at about twice the measurement error. We refer to the features that appear in every member of the ensemble as “persistent.” Some of persistent features are the following: (1) Crustal thickness averages ∼27 km in West Antarctica and ∼40 km in East Antarctica, with maximum thicknesses approaching 45 km. (2) Although the East Antarctic craton displays variations in both maximum velocity and thickness, it appears to be a more or less average craton. (3) The upper mantle beneath much of West Antarctica is slow and beneath the West Antarctic Rift is nearly indistinguishable from currently dormant extensional regions such as the western Mediterranean and the Sea of Japan. Our model is therefore consistent with evidence of active volcanism underlying the West Antarctic ice sheet, and we hypothesize that the West Antarctic Rift is the remnant of events of lithospheric rejuvenation in the recent past that are now quiescent. (4) The Australian-Antarctic Discordance is characterized by a moderately high velocity lid to a depth of 70–80 km with low velocities wrapping around the discordance to the south. There is a weak trend of relatively high velocities dipping to the west at greater depths that requires further concentrated efforts to resolve. (5) The strength of radial anisotropy (vsh − vsv)/vsv in the uppermost mantle across the Southern Hemisphere averages ∼4%, similar to the Preliminary Reference Earth Model. Radial anisotropy appears to be slightly stronger in West Antarctica than in East Antarctica and in the thinner rather than the thicker regions of the East Antarctic craton.

Stopping Light via Hot Atoms

Abstract: We prove that it is possible to freeze a light pulse (i.e., to bring it to a full stop) or even to make its group velocity negative in a coherently driven Doppler broadened atomic medium via electromagnetically induced transparency (EIT). This remarkable phenomenon of the ultraslow EIT polariton is based on the spatial dispersion of the refraction index n(omega,k), i.e., its wave number dependence, which is due to atomic motion and provides a negative contribution to the group velocity. This is related to, but qualitatively different from, the recently observed light slowing caused by large temporal (frequency) dispersion.

Polarization and dispersive properties of elliptical-hole photonic crystal fibers

Abstract: We survey the properties of photonic crystal fibers with elliptical air holes, examining mode shapes, birefringence, group velocity walkoff and dispersion, and cutoff conditions. We find new types of behavior for each quantity and demonstrate the possibility achieving large birefringence with zero walkoff in the single-mode regime. We show that the dispersive properties of the vector modes are closely tied to those of the so-called fundamental space filling modes, and that at long wavelengths, the fibers exhibit a single-polarization single-mode regime of propagation without the presence of material anisotropy.

THz interconnect with low-loss and low-group velocity dispersion

Abstract: We report the demonstration of a physically flexible, practicable THz interconnect with minimal pulse distortion and loss. The interconnect is a parallel-plate waveguide, with the TEM mode excited, constructed of two thin copper strips. The incoming 0.22 ps THz pulse broadens to 0.39 ps after propagating 250 mm in the waveguide and is also attenuated by a factor of ten. We show that this attenuation is mainly due to the finite conductivity of copper with some additional loss caused by the beam spread in the unguided dimension. The pulse broadening is due to the frequency-dependent loss since the group velocity dispersion is negligible.

Automated Detection, Extraction, and Measurement of Regional Surface Waves

Abstract: Our goal is to develop and test an effective method to detect, identify, extract, and quantify surface wave signals for weak events observed at regional stations. We describe an automated surface wave detector and extractor designed to work on weak surface wave signals across Eurasia at intermediate periods (8 s-40 s). The method is based on phase-matched filters defined by the Rayleigh wave group travel-time predictions from the broadband group velocity maps presented by RITZWOLLER and LEVSHIN (1998) and RITZWOLLER et al. (1998) and proceeds in three steps: Signal compression, signal extraction or cleaning, and measurement. First, the dispersed surface wave signals are compressed in time by applying an anti-dispersion or phase-matched filter defined from the group velocity maps. We refer to this as the ‘compressed signal.’ Second, the surface wave is then extracted by filtering ‘noise’ temporally isolated from the time-compressed signal. This filtered signal is then redispersed by applying the inverse of the phase-matched filter. Finally, we adaptively estimate spectral amplitude as well as group and phase velocity on the filtered signal. The method is naturally used as a detector by allowing origin time to slide along the time axis. We describe preliminary results of the application of this method to a set of nuclear explosions and earthquakes that occurred on or near the Chinese Lop Nor test site from 1992 through 1996 and one explosion on the Indian Rajasthan test site that occurred in May of 1998.

Signal velocity, causality, and quantum noise in superluminal light pulse propagation.

Abstract: We consider pulse propagation in a linear anomalously dispersive medium where the group velocity exceeds the speed of light in vacuum ( c) or even becomes negative. A signal velocity is defined operationally based on the optical signal-to-noise ratio, and is computed for cases appropriate to the recent experiment where such a negative group velocity was observed. It is found that quantum fluctuations limit the signal velocity to values less than c.

Phase control of group velocity: From subluminal to superluminal light propagation

Abstract: We show that the group velocity of a weak pulse can be manipulated by controlling the phases of two weak optical fields applied to a V-shaped three-level system. Such control can even cause the probe propagation to change from subluminal to superluminal. We consider two schemes: in the first, the excited states are coupled by decay-induced coherence, which is an inherent property of the medium, and in the second, quantum coherence is created by coupling the excited states to each other by a strong microwave field. We also discuss the group velocity reduction experienced by a single weak propagating probe due to decay-induced coherence.

Tracking Femtosecond Laser Pulses in Space and Time

Abstract: We show that the propagation of a femtosecond laser pulse inside a photonic structure can be directly visualized and tracked as it propagates using a time-resolved photon scanning tunneling microscope. From the time-dependent and phase- sensitive measurements, both the group velocity and the phase velocity are unambiguously and simultaneously determined. It is expected that this technique wilt find applications in the investigation of the local dynamic behavior of photonic crystals and integrated optical circuits.

Guided waves energy velocity in absorbing and non-absorbing plates

Abstract: This paper presents a study of the velocity of the propagation of energy in guided waves in plates. The motivation of the work comes from the practical observation that the conventional approach to predicting the velocities of pulses or wave packets, that is, the simple group velocity calculation, breaks down when the guided waves are attenuative. The conventional approach is therefore not valid for guided waves in absorbing materials or for leaky waves. The paper presents a theoretical derivation of an expression to predict the energy velocity of guided waves in an isotropic plate, based on the integration of the Poynting energy vectors. When applied to modes with no attenuation, it is shown analytically from this expression that the energy velocity is always identical to the group velocity. On the other hand, when applied to attenuative modes, numerical integration of the expression to yield the true energy velocity shows that this can differ quite significantly from the group velocity. Experimental validation of the expression is achieved by measuring the velocity of wave packets in an absorbing plate, under such conditions when the energy velocity differs substantially from the group velocity. Excellent agreement is found between the predictions and the measurements. The paper also shows the Poynting vectors in the various model studies, and some interesting phenomena relating to their directions.

A new estimator for vector velocity estimation [medical ultrasonics]

Abstract: A new estimator for determining the two-dimensional velocity vector using a pulsed ultrasound field is derived. The estimator uses a transversely modulated ultrasound field for probing the moving medium under investigation. A modified autocorrelation approach is used in the velocity estimation. The new estimator automatically compensates for the axial velocity when determining the transverse velocity. The estimation is optimized by using a lag different from one in the estimation process, and noise artifacts are reduced by averaging RF samples. Further, compensation for the axial velocity can be introduced, and the velocity estimation is done at a fixed depth in tissue to reduce the influence of a spatial velocity spread. Examples for different velocity vectors and field conditions are shown using both simple and more complex field simulations. A relative accuracy of 10.1% is obtained for the transverse velocity estimates for a parabolic velocity profile for flow transverse to the ultrasound beam and a SNR of 20 dB using 20 pulse-echo lines. The overall bias in the estimates was -4.3%.

Group velocity and propagation losses measurement in a single-line photonic-crystal waveguide on InP membranes

Abstract: Single-line photonic-crystal waveguides are investigated. Photoluminescence experiments and three-dimensional calculation are performed and allow a clear identification of the guided modes. The propagation properties of the latter (group velocity, losses) are extracted from photoluminescence spectra obtained on closed waveguides which act as linear cavities.

Elastic waves in anisotropic laminates

Abstract: FUNDAMENTS OF WAVES IN ELASTIC SOLIDS Introduction Formulation of Longitudinal Wave in a Bar Free Wave Motion in Infinite Bars Free Wave Motion in a Finite Bar Forces Wave Motion in an Infinite Bar Forced Wave Motion in a Finite Bar Transient Waves in an Infinite Bar Remarks WAVES IN PLATES OF FUNCTIONALLY GRADED MATERIAL Introduction Element of Linear Property Variation Boundary and Continuity Conditions Transient Response Evaluation of Confluent Hypergeometric Function Examples Remarks FREE WAVE MOTION IN ANISOTROPIC LAMINATES Introduction Basic Equations Derivation of Dispersion Equation Strain Energy Distribution Examples Remarks FORCED WAVE MOTION IN COMPOSITE LAMINATES Introduction Basic Equations Boundary and Interface Conditions Displacement in the Wavenumber Domain A Technique for the Inverse Fourier Integration Response in Time Domain Poles and Complex Paths Examples Remarks CHARACTERISTICS OF WAVES IN COMPOSITE LAMINATES Introduction Dispersion Equation Group Velocities Phase Velocity Surface Phase Slowness Surface Phase Wave Surface Group Velocity Surface Group Slowness Surface Group Wave Surface Examples Remarks FREE WAVE MOTION IN ANISOTROPIC LAMINATED BARS: FINITE STRIP ELEMENT METHOD Introduction System Equation Examples Remarks FREE WAVE MOTION IN COMPOSITE LAMINATED BARS: SEMI-EXACT METHOD Introduction System Equation Examples of Harmonic Waves in Bars Edge Waves in Semi-Infinite Laminates Remarks TRANSIENT WAVES IN COMPOSITE LAMINATES Introduction HNM Formulation Equation in Wavenumber Domain Displacement in Wavenumber Domain Response in Space-Time Domain Response to Line Time-Step Load Response to Point Time-Step Load Techniques for Inverse Fourier Integral Response to Transient Load of Arbitrary Time Function Remarks WAVES IN FUNCTIONALLY GRADED PLATES Introduction Dynamic System Equation Dispersion Relation Group Velocity Response Analysis Two-Dimensional Problem Computational Procedure Dispersion Curves Transient Response to Line Time-Step Loads Remarks WAVES IN ANISOTROPIC FUNCTIONALLY GRADED PIEZOELECTRIC PLATES Introduction Basic Equations Approximated Governing Equations Equations in Transform Domain Characteristics of Waves in FBPM Plates Transient Response Analysis Interdigital Electrodes Excitation Displacement and Electrostatics Potential Response Computation Procedure Dispersion Curves Excitation of Time-Step Shear Force in y Direction Excitation of a Line Electrode Excitation of Interdigital Electrodes Remarks STRIP ELEMENT METHOD FOR STRESS WAVES IN ANISOTROPIC SOLIDS Introduction System Equation SEM for Static Problems (Flamant's Problem) SEM for Dynamic Problems Remarks WAVE SCATTERING BY CRACKS IN COMPOSITE LAMINATES Introduction Governing Differential Equations Particular Solution Application of the SEM to Cracked Laminates Solution in the Time Domain Examples of Scattered Wave Fields Characterization of Horizontal Cracks Characterization of Vertical Surface-Breaking Cracks Characterization of Middle Interior Vertical Cracks Characterization of Arbitrary Interior Vertical Cracks Remarks WAVES SCATTERING BY FLAWS IN COMPOSITE LAMINATES Introduction Applications of the SEM to Plates Containing Flaws Examples for Wave Scattering in Laminates SH Waves in Sandwich Plates Strip Element Equation for SH Waves Particular Solution Complementary Solution General Solution SH Waves Scattered by Flaws Remarks BENDING WAVES IN ANISOTROPIC LAMINATED PLATES Introduction Governing Equation Strip element Equation Assembly of Element Equations Static Problems for Orthotropic Laminated Plates Wave Motion in Anisotropic Laminated Plates CHARACTERISTICS OF WAVES IN COMPOSITE CYLINDERS Introduction Basic Equations Dispersion Relations Examples Remarks WAVE SCATTERING BY CRACKS IN COMPOSITE CYLINDERS Introduction Basic Equations Axisymmetric Strip Element Examples Remarks INVERSE IDENTIFICATION OF IMPACT LOADS USING ELASTIC WAVES Introduction Two-dimensional Line Load Two-dimensional Extended Load Three-dimensional Concentrated Load Examples Remarks INVERSE DETERMINATION OF MATERIAL CONSTANTS OF COMPOSITE LAMINATES Introduction Inverse Operation Uniform-Micro Genetic Algorithms Examples Remarks

Electromagnetic energy transport along arrays of closely spaced metal rods as an analogue to plasmonic devices

Abstract: The transport of electromagnetic energy along structures consisting of arrays of closely spaced metal rods (spacing = 0.2 cm) was investigated in the microwave regime at 8.0 GHz (lambda= 3.7 cm). The dispersion relation shows that information transport occurs at a group velocity of 0.6c. The electromagnetic energy is highly confined to the arrays (90% within a distance of 0.05lambda from the array). The propagation loss in a straight array is 3 dB/8 cm. Routing of energy around 90° corners is possible with a power loss of 3–4 dB. Analogies to plasmon wires consisting of arrays of nm-size metal clusters are discussed.

Lamb and SH waves generated and detected by air-coupled ultrasonic transducers in composite material plates

Abstract: Electrostatic, air-coupled, ultrasonic transducers are used to generate and detect guided waves in anisotropic solid plates. Waves considered in this study are Lamb-type and SH-type, guided modes. If the plane of propagation coincides with a plane of symmetry of the material, then Lamb modes only are launched and detected by the transducers. If the plane of propagation does not coincide with a plane of symmetry of the material, then Lamb modes are still generated and detected, but guided, SH-like modes are, too. The variation of phase velocity with frequency is measured for several modes propagating in different directions along a glass–epoxy composite plate. A numerical model that takes into account the anisotropy of composite materials is developed to predict the dispersion curves (phase velocity, group velocity or wave-number versus frequency) and the displacement fields of plate waves, the plane of propagation being either a plane of symmetry or not. The experimental phase velocities are in good agreement with the predicted dispersion curves, thus showing that the forward problem concerning the propagation of plate waves in anisotropic, homogeneous, composite material plates is properly solved. The dispersion curves associated with the predicted displacement fields show that guided modes in composite plates have different behaviors depending on their direction of propagation.

Comparison between theory and experiment of nonlinear propagation for a-few-cycle and ultrabroadband optical pulses in a fused-silica fiber

Abstract: Wave-propagation equations, including effectively the second derivative in time under the condition of a small difference between the group and phase velocities and the first derivative in position /spl xi/ in the group velocity coordinate, are derived based on the slowly evolving wave approximation. These can describe ultrabroadband optical pulse propagation with not only self-phase modulation (SPM), but also induced-phase modulation (IPM) in the monocycle regime in a fiber. It is shown that linear dispersion effects can be rigorously included in the numerical calculations. Calculations including SPM in a single-mode fused-silica fiber with the Raman effect are performed and compared with experimental results. Also, calculations including IPM in the fused-silica fiber are compared with experimental results. The effects of each term in the calculations on spectra are analyzed and it is shown that inclusion of the Raman effect and the dispersion of the effective core area is important for obtaining better agreement with experiments. It is shown that inclusion of more than third-order dispersion terms is necessary for calculations of monocycle pulse propagation.

Structure of the upper mantle under the Antarctic Plate from surface wave tomography

Abstract: We have analyzed surface wave data for 245 events occurring in Antarctica and surrounding oceans during years 1991-1999, We first apply dispersion analysis to the fundamental mode of Rayleigh waves for periods ranging from 20 to 150 sec. We then invert path-averaged dispersion curves to obtain 2D maps of group velocity for different periods, represented by splines on a grid with 250 km spacing, with considerably more detail than that achieved by global models. We finally use group velocity maps in a nonlinear inversion for the regional 3D v SV structure of the upper mantle. The resulting model shows thick continental roots under East Antarctica, reaching depths below 200 km; low velocities under oceanic ridges, mostly confined to the upper 150 km; and a slow area under the Ross Sea volcanic centres, extending all the way down to 200 km, beyond which our data loose resolving power.

Nonadiabatic approach to quantum optical information storage

Abstract: We show that there is no need for adiabatic passage in the storage and retrieval of information in the optically thick vapor of Lambda-type atoms. This information can be mapped into and retrieved out of long-lived atomic coherence with nearly perfect efficiency by strong writing and reading pulses with steep rising and falling edges. We elucidate similarities and differences between the ``adiabatic'' and ``instant'' light storage techniques, and conclude that for any switching time, an almost perfect information storage is possible if the group velocity of the signal pulse is much less than the speed of light in the vacuum c and the bandwidth of the signal pulse is much less then the width of the two-photon resonance. The maximum loss of the information appears in the case of instantaneous switching of the writing and reading fields compared with adiabatic switching, and is determined by the ratio of the initial group velocity of the signal pulse in the medium and speed of light in the vacuum c, which can be very small. Quantum restrictions to the storage efficiency are also discussed.

Group velocity of solitons

Abstract: It is shown that, in addition to the well-known phase accumulation of a traveling soliton, which may be interpreted as a change of phase velocity as a result of the Kerr nonlinearity, there is a change in the speed of travel of the envelope, the group velocity. This analysis is extended to dispersion-managed solitons, for which it is shown that the discrepancy between phase- and group-velocity changes is generally smaller.

Analysis of plate wave propagation in anisotropic laminates using a wavelet transform

Abstract: A new approach is presented for the analysis of transient waves propagating in anisotropic composite laminates. The wavelet transform (WT) using the Gabor wavelet is applied to the time-frequency analysis of dispersive flexural waves in these plates. It can be shown that the peaks of the magnitude of WT in a time-frequency domain is related to the arrival times of the group velocity. Experiments were performed using a lead break as the simulated acoustic emission source on the surface of unidirectional and quasi-isotropic laminates. A method was developed to obtain the group velocity of the flexural mode as a function of frequency. Theoretical predictions were made using the Mindlin plate theory, which includes the effects of shear deformation and rotatory inertia. Our predictions on the dispersion of the flexural mode showed good agreement with the experimental results.

Interfaces of Photonic Crystals for High Efficiency Light Transmission

Abstract: Input and output interfaces of photonic crystals are discussed for high efficiency transmission of light. The discussion focuses on the frequency range higher than the photonic bandgap, at which a small group velocity of light, the superprism effect, the strong structural birefringence and the negative refractive index effect are expected. The two-dimensional photonic crystal of triangular lattice airholes in a high-refractive-index dielectric medium is assumed as the fundamental model. The finite difference time domain simulation shows that two types of interfaces improve the transmission efficiency. One is composed of small airholes and the other is composed of projected airholes. In particular, the latter generates a wide transmission band of ~ 0.1 times the center frequency with the maximum efficiency of -0.01 dB against the normal or slightly inclined incidence of the TE-polarized wave.

Analysis of THz-wave surface-emitted difference-frequency generation in periodically poled lithium niobate waveguide

Abstract: It was shown that the periodically poled LiNbO3-waveguide with period of poling λ≈λ/ng (λ is the wavelength of emitted THz-wave, ng is a refractive index corresponding to optical group velocity) emits THz-wave difference-frequency generation (DFG) in the direction normal to the surface of the planar waveguide. The 5% distinction between the manufactured and required periods of gratings results only in a small deflection (∼6°) of the output THz-beam from the normal direction. The dependence of DFG efficiency on mode size is analyzed. The output THz power at λ=150 μm is estimated as 2 mW, taking into account imperfections in coupling incident beams with guided modes. It was shown that the efficiency of THz-wave DFG in surface-emitting geometry is more than for collinear geometry in bulk crystal, especially in the high-absorption wavelength region.

Superluminal to subluminal transition in the pulse propagation in a resonantly absorbing medium.

Abstract: We have examined the propagation of femtosecond laser pulses in an absorbing dye solution through a short to a long range of propagation distance. The transmitted pulses show strong spectral shift and a superluminal to subluminal transition in the propagation velocity keeping its initial shape almost intact. It is verified that the peak velocity is well described by a modified group velocity v(S) defined within the framework of the saddle-point method as well as by a recent prediction of the net group delay of surviving frequency.

Pc1 pearls revisited: Structured electromagnetic ion cyclotron waves on Polar satellite and on ground

Abstract: We study an event of structured electromagnetic ion cyclotron (EMIC) waves observed by the Polar satellite in good conjunction with the Finnish ground stations on April 25, 1997. Polar observed two EMIC wave bands around the plasmapause. He + band waves consisted of repetitive bursts which were observed on ground as a classical Pc1 pearl band. H + band waves occurred over a large latitude range of more than 5° invariant latitude and were observed on ground as a broad, diffuse Pc1 pearl band with several subbands. We found the same repetition period of ∼100 s for the Polar He + band bursts and ground Pc1 pearls, in conflict with the bouncing wave packet (BWP) model. Comparing the burst structure of He + band waves in Polar and on ground, we found a transit time of ∼45 s and an average group velocity of ∼500 km s -1 . Within the BWP model this velocity would lead to a pearl repetition period of more than 250 s, in dramatic contradiction with the observed repetition period. Moreover, the bursts of the two Polar bands were roughly simultaneous with no significant dispersion, contrary to the expectation of the BWP model. These results clearly reject the classical BWP model, i.e., that Pc1 pearls are generated by one wave packet bouncing from one hemisphere to another. Instead, we find that EMIC waves were accompanied by long-period ULF waves which had a period very close to the repetition period of the simultaneous EMIC bursts. Interestingly, plasma density showed simultaneous fluctuations with roughly the same period. As an alternative to the BWP model, we discuss models where the EMIC wave packet structure and ULF waves are connected. We note that the suggested relation of EMIC wave packets and ULF waves offers a new explanation to the well-known preference of Pc1 pearls for the plasmapause. We have also estimated the full Poynting flux of EMIC waves for the first time, using the three components of electric and magnetic fields. The magnitude of the total Poynting flux of the He + band waves was ∼20-25 μW m -2 and strongly directed downward away from the equator.

A surface wave dispersion study of the Middle East and North Africa for monitoring the comprehensive nuclear-test-ban treaty

Abstract: We present results from a large-scale study of surface-wave group velocity dispersion across the Middle East, North Africa, southern Eurasia and the Mediterranean Our database for the region is populated with seismic data from regional events recorded at permanent and portable broadband, three-component digital stations We have measured the group velocity using a multiple narrow-band filter on deconvolved displacement data Overall, we have examined more than 13,500 seismograms and made good quality dispersion measurements for 6817 Rayleigh- and 3806 Love-wave paths We use a conjugate gradient method to perform a group-velocity tomography Our current results include both Love- and Rayleigh-wave inversions across the region for periods from 10 to 60 seconds Our findings indicate that short-period structure is sensitive to slow velocities associated with large sedimentary features such as the Mediterranean Sea and Persian Gulf We find our long-period Rayleigh-wave inversion is sensitive to crustal thickness, such as fast velocities under the oceans and slow along the relatively thick Zagros Mts and Turkish-Iranian Plateau We also find slow upper mantle velocities along known rift systems Accurate group velocity maps can be used to construct phase-matched filters along any given path The filters can improve weak surface wave signals by compressing the dispersed signal The signals can then be used to calculate regionally determined M s measurements, which we hope can be used to extend the threshold of m b :M s discriminants down to lower magnitude levels Other applications include using the group velocities in the creation of a suitable background model for forming station calibration maps, and using the group velocities to model the velocity structure of the crust and upper mantle

Delay in light transmission through small apertures.

Abstract: We demonstrate a technique for measuring pulse propagation time delays with 0.5-fs resolution by use of a widely available 100-fs pulsed laser. Using this technique, we measured the time delay of a light pulse transiting through subwavelength apertures placed on a 0.3-mum metallic film. We measured a 7-fs total transit time, corresponding to an effective group velocity of c/7 . The experimental result yielded additional evidence that light interacts resonantly with oscillators formed by the surface modes near the small apertures.

Corrosion thickness gauging in plates using Lamb wave group velocity measurements

Abstract: The use of guided waves in ultrasonic inspection of plate and pipe structures is faster but more complicated than conventional bulk wave inspection. This is due to dispersion effects and multimode propagation. This paper describes an ultrasonic measurement method allowing us to inspect the corroded thickness of plates using Lamb waves. This method consists of measuring the group velocity of the S0 mode and then observing variations in velocity due to successive chemical attacks. From these data, it is possible to evaluate the progressively reduced thickness. Experimental and numerical results show that, depending on the chosen mode, the group velocity can be very sensitive to the reduction in thickness. Wavelet signal processing is suggested to extend this technique when several modes overlap.