Showing papers on "Group velocity published in 2007"
TL;DR: In this article, a femtosecond optical pump pulse from a 10Hz Ti:sapphire laser to match the phonon-polariton phase velocity to the group velocity of the pump pulses in a lithium niobate crystal was achieved.
Abstract: Generation of near single-cycle pulses centered at 0.5THz frequency with up to 10μJ energy, 100μW average power, and 5.0MW peak power was demonstrated by tilting the intensity front of a femtosecond optical pump pulse from a 10Hz Ti:sapphire laser to match the phonon-polariton phase velocity to the group velocity of the pump pulses in a lithium niobate crystal. Terahertz pulse intensity as high as 10MW∕cm2 was achieved. The photon conversion efficiency was 45% and the calculated peak electric field strength at the focus of an off-axis parabolic mirror was 250kV∕cm.
582 citations
TL;DR: This physical picture encompasses a variety of different approaches to pulse storage ranging from adiabatic reduction of the photon group velocity and pulse-propagation control via off-resonant Raman fields to photon-echo-based techniques and derives an optimal control strategy for storage and retrieval of a photon wave packet of any given shape.
Abstract: We present a universal physical picture for describing storage and retrieval of photon wave packets in a Lambda-type atomic medium. This physical picture encompasses a variety of different approaches to pulse storage ranging from adiabatic reduction of the photon group velocity and pulse-propagation control via off-resonant Raman fields to photon-echo-based techniques. Furthermore, we derive an optimal control strategy for storage and retrieval of a photon wave packet of any given shape. All these approaches, when optimized, yield identical maximum efficiencies, which only depend on the optical depth of the medium.
323 citations
TL;DR: In this article, the dispersive and anisotropic behavior of Lamb waves in a two different types of symmetric laminates is studied theoretically and experimentally, with emphasis on group velocity and characteristic wave curves.
280 citations
TL;DR: In this paper, the authors present the first New Zealand-wide study of surface wave dispersion, using ambient noise observed at 42 broad-band stations in the national seismic network (GeoNet) and the Global Seismic Network (GSN).
Abstract: SUMMARY
We present the first New Zealand-wide study of surface wave dispersion, using ambient noise observed at 42 broad-band stations in the national seismic network (GeoNet) and the Global Seismic Network (GSN). Year-long vertical-component time-series recorded between 2005 April 1 and 2006 March 31 have been correlated with one another to yield estimated fundamental mode Rayleigh wave Green's functions. We filter these Green's functions to compute Rayleigh wave group dispersion curves at periods of 5–50 s, using a phase-matched filter, frequency–time analysis technique. The uncertainties of the measurements are estimated based on the temporal variation of the dispersion curves revealed by 12 overlapping 3-month stacks. After selecting the highest quality dispersion curve measurements, we compute group velocity maps from 7 to 25 s period. These maps, and 1-D shear wave velocity models at four selected locations, exhibit clear correlations with major geological structures, including the Taranaki and Canterbury Basins, the Hikurangi accretionary prism, and previously reported basement terrane boundaries.
251 citations
01 Jan 2007
TL;DR: In this article, the authors present the first New Zealand-wide study of surface wave dispersion, using ambient noise observed at 42 broad-band stations in the national seismic network (GeoNet) and the Global Seismic Network (GSN).
Abstract: SUMMARY We present the first New Zealand-wide study of surface wave dispersion, using ambient noise observed at 42 broad-band stations in the national seismic network (GeoNet) and the Global Seismic Network (GSN). Year-long vertical-component time-series recorded between 2005 April 1 and 2006 March 31 have been correlated with one another to yield estimated fundamental mode Rayleigh wave Green’s functions. We filter these Green’s functions to compute Rayleigh wave group dispersion curves at periods of 5‐50 s, using a phase-matched filter, frequency‐time analysis technique. The uncertainties of the measurements are estimated based on the temporal variation of the dispersion curves revealed by 12 overlapping 3-month stacks. After selecting the highest quality dispersion curve measurements, we compute group velocity maps from 7 to 25 s period. These maps, and 1-D shear wave velocity models at four selected locations, exhibit clear correlations with major geological structures, including the Taranaki and Canterbury Basins, the Hikurangi accretionary prism, and previously reported basement terrane boundaries.
245 citations
TL;DR: In this article, the dispersion characteristics of Rayleigh waves using frequency-time analysis were measured for the EarthScope USArray Transportable Array (TA) between October 2004 and January 2007.
Abstract: We have applied ambient noise surface wave tomography to data that have emerged continuously from the EarthScope USArray Transportable Array (TA) between October 2004 and January 2007 Estimated Green's functions result by cross-correlating noise records between every station-pair in the network The 340 stations yield a total of more than 55,000 interstation paths Within the 5- to 50-s period band, we measure the dispersion characteristics of Rayleigh waves using frequency-time analysis High-resolution group velocity maps at 8-, 16-, 24-, 30-, and 40-s periods are presented for the western United States The footprint of the TA encloses a region with a resolution of about the average interstation spacing (∼70 km) Velocity anomalies in the group velocity maps correlate well with the dominant geological features of the western United States Coherent velocity anomalies are associated with the Sierra Nevada, Peninsular, and Cascade Ranges, Great Valley, Salton Trough, and Columbia basins, the Columbia River flood basalts, the Snake River Plain and Yellowstone, and mantle wedge features associated with the subducting Juan de Fuca plate
222 citations
TL;DR: The LVLD characteristics of waveguides with slightly small innermost holes were experimentally confirmed by using modulation phase shift measurement and transmission of ultrashort optical pulses to apply to buffering and nonlinearity enhancement of optical signals.
Abstract: Photonic crystal slab line defect waveguides with slightly small innermost holes are theoretically expected to show light transmission with low-group-velocity and low-dispersion (LVLD) characteristics owing to a linear and almost flat photonic band In this study, the LVLD characteristics of such waveguides were experimentally confirmed by using modulation phase shift measurement and transmission of ultrashort optical pulses These results will be applicable to buffering and nonlinearity enhancement of optical signals
184 citations
TL;DR: In this article, a tomographic S wave velocity model for the upper mantle beneath South America is presented, which combines Rayleigh wave group velocity dispersion curves and 1537 regional wave trains with paths principally passing through the South American continent.
Abstract: [1] A new tomographic S wave velocity model for the upper mantle beneath South America is presented. We developed and applied a new method of simultaneously inverting regional S and Rayleigh waveforms and fundamental mode Rayleigh wave group velocities, to better constrain upper mantle S velocity structure and Moho depth. We used � 5700 Rayleigh wave group velocity dispersion curves and 1537 regional wave trains with paths principally passing through the South American continent. The joint inversion of this data set provided a new three-dimensional (3-D) upper mantle S velocity model and a Moho depth model for South America, which fits both the group velocity and regional waveform data sets well. New features of the final three-dimensional (3-D) S velocity and Moho depth model correlate well with known geotectonic units on a regional scale. The Moho depth ranges from 30 km in the central Chaco basin to 42 km beneath the Amazonian craton and 45–70 km beneath the orogenic Andean belt. The imaged S velocity indicates an average lithosphere thickness of around 160 km for the Amazonian craton. High velocities are imaged beneath the Amazon and part of the Parana´ and Parnao´ba basins down to about 150 km. Low to very low velocities are imaged ! ! !
170 citations
TL;DR: In this article, a series of ternary glass systems namely, Na2O, B2O3, and RO (R ¼ Ba or Mg) doped with TiO2 are synthesized.
156 citations
TL;DR: The optical analysis shows that the random fabrication errors of the structures, made on a standard e-beam lithography system, are below 1 nm root mean square, which indicates two types of loss.
Abstract: We examine the effects of disorder on propagation loss as a function of group velocity for W1 photonic crystal (PhC) waveguides. Disorder is deliberately and controllably introduced into the photonic crystal by pseudo-randomly displacing the holes of the photonic lattice. This allows us to clearly distinguish two types of loss. Away from the band-edge and for moderately slow light (group velocity c/20-c/30) loss scales sub-linearly with group velocity, whereas near the band-edge, reflection loss increases dramatically due to the random and local shift of the band-edge. The optical analysis also shows that the random fabrication errors of our structures, made on a standard e-beam lithography system, are below 1 nm root mean square.
151 citations
TL;DR: A finite element technique for modelling the dispersion characteristics of guided waves in a waveguide of arbitrary cross section subjected to axial load is presented and the results from the FE model are compared to results obtained from a simple Euler-Bernoulli beam model.
TL;DR: Numerical examples indicate that appropriate gradient distributing of the material properties make Love waves to propagate along the surface of the piezoelectric layer, or a bigger electromechanical coupling factor can be obtained, which is in favor of acquiring a better performance in surface acoustic wave (SAW) devices.
TL;DR: Leemans et al. as discussed by the authors presented detailed results on the generation of GeV-class electron beams using an intense femtosecond laser beam and a 3.3cm long preformed discharge-based plasma channel.
Abstract: Laser wakefield accelerators can produce electric fields of order 10–100GV∕m, suitable for acceleration of electrons to relativistic energies. The wakefields are excited by a relativistically intense laser pulse propagating through a plasma and have a phase velocity determined by the group velocity of the light pulse. Two important effects that can limit the acceleration distance and hence the net energy gain obtained by an electron are diffraction of the drive laser pulse and particle-wake dephasing. Diffraction of a focused ultrashort laser pulse can be overcome by using preformed plasma channels. The dephasing limit can be increased by operating at a lower plasma density, since this results in an increase in the laser group velocity. Here we present detailed results on the generation of GeV-class electron beams using an intense femtosecond laser beam and a 3.3cm long preformed discharge-based plasma channel [W. P. Leemans et al., Nature Physics 2, 696 (2006)]. The use of a discharge-based waveguide per...
TL;DR: In this article, it was shown that the one-dimensional potential well V(x)=−(ℏ2ν(ν+1)∕2ma2)sech2(x∕a) does not reflect waves of any energy when ν is a positive integer.
Abstract: The one-dimensional potential well V(x)=−(ℏ2ν(ν+1)∕2ma2)sech2(x∕a) does not reflect waves of any energy when ν is a positive integer. We show that in this reflectionless case the solutions of Schrodinger’s equation can be expressed in terms of elementary functions. Wave packets can be constructed from these energy eigenstates, and the propagation of such wave packets through the potential region can be studied analytically. We find that the group velocity of a particular packet can substantially exceed the group velocity of a free-space Gaussian packet. The bound states of the potential can also be expressed in terms of elementary functions when ν is an integer. The special properties of the integer ν potentials are associated with critical binding.
TL;DR: In this article, a neural network is used to find 1-dimensional marginal probability density functions (pdfs) of global crustal parameters, which can quantify the extent to which a parameter is constrained by the data.
Abstract: [1] We use neural networks to find 1-dimensional marginal probability density functions (pdfs) of global crustal parameters. The information content of the full posterior and prior pdfs can quantify the extent to which a parameter is constrained by the data. We inverted fundamental mode Love and Rayleigh wave phase and group velocity maps for pdfs of crustal thickness and independently of vertically averaged crustal shear wave velocity. Using surface wave data with periods T > 35 s for phase velocities and T > 18 s for group velocities, Moho depth and vertically averaged shear wave velocity of continental crust are well constrained, but vertically averaged shear wave velocity of oceanic crust is not resolvable. The latter is a priori constrained by CRUST2.0. We show that the resulting model allows to compute global crustal corrections for surface wave tomography for periods T > 50 s for phase velocities and T > 60 s for group velocities.
TL;DR: In this paper, the authors exploited the resonance that occurs at the minimum frequency thickness product of the first order symmetric (S1) and of the second order antisymmetric (A2) Lamb modes.
Abstract: An original method for material characterization with acoustic waves is presented. The measurement of the longitudinal and shear wave velocities in thin isotropic plates or shells is performed locally on the same face without any mechanical contact. We exploit the resonance that occurs at the minimum frequency thickness product of the first order symmetric (S1) and of the second order antisymmetric (A2) Lamb modes. At these frequencies the group velocity vanishes, whereas the phase velocity remains finite. Then, the energy, which cannot propagate in the structure, is localized in a zone of diameter half the wavelength. The vibrations are excited in the thermoelastic regime by a laser pulse and detected at the same point by an optical interferometer. For these two Lamb modes we have computed the variations of the frequency thickness product versus Poisson’s ratio. The resonance frequency ratio, which is independent of the plate or shell thickness, provides an absolute and local measurement of Poisson’s rat...
TL;DR: Using a new general approach to limits in optical structures that counts orthogonal waves generated by scattering, an upper limit to the number of bits of delay possible in one-dimensional slow light structures that are based on linear optical response to the signal field is derived.
Abstract: Using a new general approach to limits in optical structures that counts orthogonal waves generated by scattering, we derive an upper limit to the number of bits of delay possible in one-dimensional slow light structures that are based on linear optical response to the signal field. The limit is essentially the product of the length of the structure in wavelengths and the largest relative change in dielectric constant anywhere in the structure at any frequency of interest. It holds for refractive index, absorption, or gain variations with arbitrary spectral or spatial form. It is otherwise completely independent of the details of the structure's design, and does not rely on concepts of group velocity or group delay.
TL;DR: Photonic crystal tapers have been designed for coupling of light from ridge waveguides into low group velocity photonic crystal channel guides, and the coupling efficiency is increased from 3 % to 97 % for frequencies in the band-edge region, corresponding to a group index close to 100.
Abstract: Photonic crystal tapers have been designed for coupling of light from ridge waveguides into low group velocity photonic crystal channel guides. The coupling efficiency is increased from 3 % (case of butt-coupling) to 97 % for frequencies in the band-edge region, corresponding to a group index close to 100, as predicted using 2D finite-difference time-domain simulations.
TL;DR: In this paper, the results from Brillouin light scattering experiments on guided spin waves propagating along metallic magnetic stripes are presented and analyzed, and it is shown that each standing spin-wave resonance across the stripe width is associated with a particular guided mode.
Abstract: Results from Brillouin light scattering experiments on guided spin waves propagating along metallic magnetic stripes are presented and analyzed. The spin waves propagate along the stripe axis and form mode families due to geometrical confinement in the stripe geometry. In consequence, the allowed wave vectors are quantized in the transverse directions by stripe width and height. We show that each standing spin-wave resonance across the stripe width is associated with a particular guided mode. In the case of stripes magnetized along their length, the group velocity of the guided modes is negative and all modes have a volume character. When the stripes are magnetized along their width, the modes are characterized by a positive group velocity, and the spectrum consists of a series of volume and localized modes.
TL;DR: It is shown that certain choices of mixed interpolation schemes may lead to significant phase and group velocity errors and spurious solutions in the calculation of fast waves.
Abstract: The numerical approximation of shallow-water models is a delicate problem. For most of the discretization schemes, the coupling between the momentum and the continuity equations usually leads to anomalous dispersion in the representation of fast waves. A dispersion relation analysis is employed here to ascertain the presence and determine the form of spurious modes as well as the dispersive nature of the finite-element Galerkin mixed formulation of the two-dimensional linearized shallow-water equations. Nine popular finite-element pairs are considered using a variety of mixed interpolation schemes. For each pair the frequency or dispersion relation is obtained and analyzed, and the dispersion properties are compared analytically and graphically with the continuous case. It is shown that certain choices of mixed interpolation schemes may lead to significant phase and group velocity errors and spurious solutions in the calculation of fast waves. The $P^{NC}_{1} - P^{}_{1}$ and RT0 pairs are identified as a promising compromise, provided the grid resolution is high relative to the Rossby radius of deformation for the RT0 element. The numerical solutions of two test problems to simulate fast waves are in good agreement with the analytical results.
TL;DR: In this paper, a novel periodic structure, made of an arbitrary number of stacked subwavelength hole arrays, exhibiting simultaneously electromagnetic band gap, extraordinary transmission and a longitudinal left-handed propagation is presented.
Abstract: A novel periodic structure, made of an arbitrary number of stacked subwavelength hole arrays, exhibiting simultaneously electromagnetic band gap, extraordinary transmission and a longitudinal left handed propagation is presented in this paper. If the longitudinal period of the stacked structure is chosen adequately, it is possible under normal incidence to mold the electromagnetic wave properties inside the structure from right-handed to left-handed wave propagation passing through a zero-group velocity band. The transmission response of the fabricated prototype has been measured with a millimeter wave quasioptical vector network analyzer in the range between 40 GHz and 110 GHz confirming the possibility to tune the left- or right-handed characteristics of the propagating waves. These results can give rise to interesting applications such as novel lenses and other quasioptical structures.
TL;DR: This Letter demonstrates Mie resonances mediated transport of light in randomly arranged, monodisperse dielectric spheres packed at high filling fractions and finds that the energy transport velocity, which is lower than the group velocity, also displays a resonant behavior.
Abstract: In this Letter we demonstrate Mie resonances mediated transport of light in randomly arranged, monodisperse dielectric spheres packed at high filling fractions. By means of both static and dynamic optical experiments we show resonant behavior in the key transport parameters and, in particular, we find that the energy transport velocity, which is lower than the group velocity, also displays a resonant behavior.
TL;DR: In this article, a generalised wave approach based on reflection, transmission and propagation of waves is used for the analysis of non-uniform waveguides, where positive and negative-going wave motions are separated so that the problem is always wellposed.
TL;DR: In this article, a phase-locking mechanism was proposed for pulsed second-harmonic generation under conditions of phase and group velocity mismatch, and generally low conversion efficiencies and pump intensities.
Abstract: The present investigation is concerned with the study of pulsed second-harmonic generation under conditions of phase and group velocity mismatch, and generally low conversion efficiencies and pump intensities. In positive-index, nonmetallic materials, we generally find qualitative agreement with previous reports regarding the presence of a double-peaked second harmonic signal, which comprises a pulse that walks off and propagates at the nominal group velocity one expects at the second-harmonic frequency, and a second pulse that is ``captured'' and propagates under the pump pulse. We find that the origin of the double-peaked structure resides in a phase-locking mechanism that characterizes not only second-harmonic generation, but also ${\ensuremath{\chi}}^{(3)}$ processes and third-harmonic generation. The phase-locking mechanism that we describe occurs for arbitrarily small pump intensities, and so it is not a soliton effect, which usually relies on a threshold mechanism, although multicolor solitons display similar phase locking characteristics. Thus, in second harmonic generation a phase-matched component is always generated, even under conditions of material phase mismatch: This component is anomalous, because the material does not allow energy exchange between the pump and the second-harmonic beam. On the other hand, if the material is phase matched, phase locking and phase matching are indistinguishable, and the conversion process becomes efficient. We also report a similar phase-locking phenomenon in negative index materials. A spectral analysis of the pump and the generated signals reveals that the phase-locking phenomenon causes the forward moving, phase-locked second-harmonic pulse to experience the same negative index as the pump pulse, even though the index of refraction at the second-harmonic frequency is positive. Our analysis further shows that the reflected second-harmonic pulse generated at the interface and the forward-moving, phase-locked pulse appear to be part of the same pulse initially generated at the surface, part of which is immediately back-reflected, while the rest becomes trapped and dragged along by the pump pulse. These pulses thus constitute twin pulses generated at the interface, having the same negative wave vector, but propagating in opposite directions. Almost any break of the longitudinal symmetry, even an exceedingly small ${\ensuremath{\chi}}^{(2)}$ discontinuity, releases the trapped pulse which then propagates in the backward direction. These dynamics are indicative of very rich and intricate interactions that characterize ultrashort pulse propagation phenomena.
TL;DR: In this article, a perfect electric conductor surface with one-dimensional periodic rectangle holes is proposed as a surface-plasmon-like waveguide, where designed surface plasmon modes with very low group velocity are confined in a subwavelength region.
Abstract: A perfect electric conductor surface with one-dimensional periodic rectangle holes is proposed as a surface-plasmon-like waveguide, where designed surface plasmon modes with very low group velocity are confined in a subwavelength region. It is shown that the half maximum of electric field intensity of the mode can be confined in a 0.20λ×0.10λ subwavelength region on the transversal plane and the group velocity approaches to zeros, where λ is the wavelength in vacuum.
TL;DR: In this paper, angle-and frequency-resolved optical extinction measurements were used to determine the dispersion relation of plasmon modes on Ag and Au nanoparticle chains with pitches down to 75 nm.
Abstract: We present angle- and frequency-resolved optical extinction measurements to determine the dispersion relation of plasmon modes on Ag and Au nanoparticle chains with pitches down to 75 nm. The large splitting between transverse and longitudinal modes and the band curvature are inconsistent with reported electrostatic near-field models and confirm that far-field retarded interactions are important, even for λ∕5-sized structures. The data imply that lower propagation losses, larger signal bandwidth, and larger maximum group velocity than expected can be achieved for wave vectors below the light line. We conclude that for the design of optical nanocircuits coherent far-field couplings across the entire circuit need to be considered, even at subwavelength feature sizes
TL;DR: A model of the effects of disorder on slow light in structures such as coupled photonic crystal cavities, coupled microresonators, and similar coupled-resonator optical waveguides is derived, obtaining a relationship between the root-mean-square variation in the coupling coefficients and how slow the light is at the band edge.
Abstract: Slow-wave optical structures such as coupled photonic crystal cavities, coupled microresonators, and similar coupled-resonator optical waveguides are being proposed for slowing light because of the nature of their dispersion relationship. Since the group velocity becomes small, slow light and enhanced light-matter interaction may be observed at the edges of the waveguiding band. We derive a model of the effects of disorder on slow light in such structures, obtaining a relationship between the root-mean-square variation in the coupling coefficients and how slow the light is at the band edge.
TL;DR: Optical transmission spectroscopy on metal films with slit-groove pairs is conducted to determine the group velocity of surface plasmons on flat gold and silver surfaces.
Abstract: Optical transmission spectroscopy on metal films with slit-groove pairs is conducted. Spectra of the light transmitted through the slit exhibit Fabry-Perot-type interference fringes due to surface plasmons propagating between the slit and the groove. The spectral dependence of the period of interference fringes is used to determine the group velocity of surface plasmons on flat gold and silver surfaces.
TL;DR: In this article, a reflective-mirror-based 4f-compressor is proposed to compensate the angular and group velocity dispersion of the ultrashort Laguerre-Gaussian (LG) pulses.
Abstract: Coherent optical vortices are generated from ultrashort 6.4 fs pulses. Our results demonstrate angular dispersion compensation of ultrashort 6.4 fs Laguerre-Gaussian (LG) pulses as well as what is believed to be the first direct autocorrelation measurement of 80 fs LG amplified pulses. A reflective-mirror-based 4f-compressor is proposed to compensate the angular and group velocity dispersion of the ultrashort LG pulses.
TL;DR: In this article, the authors review the studies of gyrotron-type microwave devices whose electrodynamic system has the form of an oversized metal waveguide with a helically corrugated internal surface, such a corrugation changes radically the waveguide dispersion ensuring an almost constant group velocity of the eigenmode for a small (close to zero) longitudinal wave number in a wide frequency band.
Abstract: We review the studies of gyrotron-type microwave devices whose electrodynamic system has the form of an oversized metal waveguide with a helically corrugated internal surface. For certain parameters, such a corrugation changes radically the waveguide dispersion ensuring an almost constant group velocity of the eigenmode for a small (close to zero) longitudinal wave number in a wide frequency band. The use of “helical” waveguides along with electron optical systems which form near-axis electron beams makes it possible to create high-efficiency amplifiers based on gyro-traveling-wave tubes (gyro-TWTs) with a wide instantaneous frequency band of amplification and gyro-backward-wave oscillators (gyro-BWOs) with continuous wideband tuning of the oscillation frequency. The studied devices are superior to the well-studied microwave sources of this type (gyroklystrons and gyrotrons) in frequency band, by more than an order of magnitude, and are not inferior to them in efficiency even for a wide spread of electron velocities.