The Experts below are selected from a list of 276 Experts worldwide ranked by ideXlab platform
Akhlesh Lakhtakia - One of the best experts on this subject based on the ideXlab platform.
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Response to ‘On the orthogonality of the Phase Velocity and its feasibility for plane waves’
Optik, 2012Co-Authors: Tom G. Mackay, Akhlesh LakhtakiaAbstract:Abstract A recent theoretical study of plane waves with orthogonal Phase Velocity is premised on incorrect assumptions. The conclusion reached therein – namely, that “a plane wave with orthogonal Phase Velocity cannot possess linear momentum” and therefore “cannot propagate at all” – is incorrect in general.
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Optical refraction in silver: counterposition, negative Phase Velocity and orthogonal Phase Velocity
European Journal of Physics, 2011Co-Authors: Qaisar Abbas Naqvi, Tom G. Mackay, Akhlesh LakhtakiaAbstract:Complex behaviour associated with metamaterials can arise even in commonplace isotropic dielectric materials. We demonstrate how silver, for example, can support negative Phase Velocity and counterposition, but not negative refraction, at optical frequencies. The transition from positive to negative Phase Velocity is not accompanied by remarkable changes in the Abraham and Minkowski momentum densities. In particular, orthogonal Phase Velocity is associated with nonzero Abraham and Minkowski momentum densities.
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Photonic band gap materials comprising positive-Phase-Velocity and negative-Phase-Velocity layers in waveguides
Journal of Modern Optics, 2009Co-Authors: Alvaro Gomez, María Luz Martínez Ricci, Ricardo A. Depine, Akhlesh LakhtakiaAbstract:We have analyzed electromagnetic wave propagation in photonic bandgap (PBG) structures comprising alternating layers of isotropic dielectric-magnetic materials with positive Phase Velocity and negative Phase Velocity, implemented in different waveguides of uniform cross-section (parallel-plate, rectangular, circular, and coaxial) and perfectly conducting walls. The structures could be either ideal (i.e. of infinite extent along the waveguide axis) or real (i.e. terminated at both ends with homogeneously filled waveguide sections). The spectral locations of the band gaps do not directly depend on the cross-sectional shape and dimensions, but on the cut-off parameter instead, for ideal structures. The band gaps of an ideal structure are located in spectral regions where the reflectance of the corresponding real structure is large. The real structures show four types of band gaps, only one type of which is due to the periodically repetitive constitution of the PBG structure; the remaining three types are not...
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negative refraction negative Phase Velocity and counterposition in bianisotropic materials and metamaterials
Physical Review B, 2009Co-Authors: Tom G. Mackay, Akhlesh LakhtakiaAbstract:The planewave response of a linear passive material generally cannot be characterized by a single scalar refractive index, as directionality of energy flow and multiple wave vectors may need to be considered. This is especially significant for materials which support negative refraction, negative Phase Velocity, and counterposition. By means of a numerical example based on a commonly studied bianisotropic material, our theoretical investigation revealed that (i) negative (positive) refraction can arise even though the Phase Velocity is positive (negative), (ii) counterposition can arise in instances of positive and negative refraction, (iii) the Phase Velocity and time-averaged Poynting vectors can be mutually orthogonal, and (iv) whether or not negative refraction occurs can depend on the state of polarization and angle of incidence. A further numerical example revealed that negative Phase Velocity and positive refraction can coexist even in a simple isotropic dielectric material.
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Negative refraction, negative Phase Velocity, and counterposition
arXiv: Optics, 2009Co-Authors: Tom G. Mackay, Akhlesh LakhtakiaAbstract:The planewave response of a linear passive material generally cannot be characterized by a single scalar refractive index, as directionality of energy flow and multiple wavevectors may need to be considered. This is especially significant for materials which support negative refraction, negative Phase Velocity, and counterposition. By means of a numerical example based on a commonly studied bianisotropic material, our theoretical investigation revealed that: (i) negative (positive) refraction can arise even though the Phase Velocity is positive (negative); (ii) counterposition can arise in instances of positive and negative refraction; (iii) the Phase Velocity and time-averaged Poynting vectors can be mutually orthogonal; and (iv) whether or not negative refraction occurs can depend upon the state of polarization and angle of incidence. A further numerical example revealed that negative Phase Velocity and positive refraction can co-exist even in a simple isotropic dielectric material.
Tom G. Mackay - One of the best experts on this subject based on the ideXlab platform.
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Response to ‘On the orthogonality of the Phase Velocity and its feasibility for plane waves’
Optik, 2012Co-Authors: Tom G. Mackay, Akhlesh LakhtakiaAbstract:Abstract A recent theoretical study of plane waves with orthogonal Phase Velocity is premised on incorrect assumptions. The conclusion reached therein – namely, that “a plane wave with orthogonal Phase Velocity cannot possess linear momentum” and therefore “cannot propagate at all” – is incorrect in general.
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Optical refraction in silver: counterposition, negative Phase Velocity and orthogonal Phase Velocity
European Journal of Physics, 2011Co-Authors: Qaisar Abbas Naqvi, Tom G. Mackay, Akhlesh LakhtakiaAbstract:Complex behaviour associated with metamaterials can arise even in commonplace isotropic dielectric materials. We demonstrate how silver, for example, can support negative Phase Velocity and counterposition, but not negative refraction, at optical frequencies. The transition from positive to negative Phase Velocity is not accompanied by remarkable changes in the Abraham and Minkowski momentum densities. In particular, orthogonal Phase Velocity is associated with nonzero Abraham and Minkowski momentum densities.
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negative refraction negative Phase Velocity and counterposition in bianisotropic materials and metamaterials
Physical Review B, 2009Co-Authors: Tom G. Mackay, Akhlesh LakhtakiaAbstract:The planewave response of a linear passive material generally cannot be characterized by a single scalar refractive index, as directionality of energy flow and multiple wave vectors may need to be considered. This is especially significant for materials which support negative refraction, negative Phase Velocity, and counterposition. By means of a numerical example based on a commonly studied bianisotropic material, our theoretical investigation revealed that (i) negative (positive) refraction can arise even though the Phase Velocity is positive (negative), (ii) counterposition can arise in instances of positive and negative refraction, (iii) the Phase Velocity and time-averaged Poynting vectors can be mutually orthogonal, and (iv) whether or not negative refraction occurs can depend on the state of polarization and angle of incidence. A further numerical example revealed that negative Phase Velocity and positive refraction can coexist even in a simple isotropic dielectric material.
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Negative refraction, negative Phase Velocity, and counterposition
arXiv: Optics, 2009Co-Authors: Tom G. Mackay, Akhlesh LakhtakiaAbstract:The planewave response of a linear passive material generally cannot be characterized by a single scalar refractive index, as directionality of energy flow and multiple wavevectors may need to be considered. This is especially significant for materials which support negative refraction, negative Phase Velocity, and counterposition. By means of a numerical example based on a commonly studied bianisotropic material, our theoretical investigation revealed that: (i) negative (positive) refraction can arise even though the Phase Velocity is positive (negative); (ii) counterposition can arise in instances of positive and negative refraction; (iii) the Phase Velocity and time-averaged Poynting vectors can be mutually orthogonal; and (iv) whether or not negative refraction occurs can depend upon the state of polarization and angle of incidence. A further numerical example revealed that negative Phase Velocity and positive refraction can co-exist even in a simple isotropic dielectric material.
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Negative‐ and positive‐Phase‐Velocity propagation in an isotropic chiral medium moving at constant Velocity
Microwave and Optical Technology Letters, 2007Co-Authors: Tom G. Mackay, Akhlesh LakhtakiaAbstract:Analysis of electromagnetic planewave propagation in a medium which is a spatiotemporally homogeneous, temporally nonlocal, isotropic, chiral medium in a comoving frame of reference shows that the medium is both spatially and temporally nonlocal with respect to all non-co-moving inertial frames of reference. Using the Lorentz transformations of electric and magnetic fields, we show that plane waves which have positive Phase Velocity in the comoving frame of reference can have negative Phase Velocity in certain non-co-moving frames of reference. Similarly, plane waves which have negative Phase Velocity in the comoving frame can have positive Phase Velocity in certain non-co-moving frames. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2640–2643, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22810
Thomas Bodin - One of the best experts on this subject based on the ideXlab platform.
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resolution potential of surface wave Phase Velocity measurements at small arrays
Geophysical Journal International, 2008Co-Authors: Thomas Bodin, Valerie MaupinAbstract:SUMMARY The deployment of temporary arrays of broadband seismological stations over dedicated targets is common practice. Measurement of surface wave Phase Velocity across a small array and its depth-inversion gives us information about the structure below the array which is complementary to the information obtained from body-wave analysis. The question is however: what do we actually measure when the array is much smaller than the wave length, and how does the measured Phase Velocity relates to the real structure below the array? We quantify this relationship by performing a series of numerical simulations of surface wave propagation in 3-D structures and by measuring the apparent Phase Velocity across the array on the synthetics. A principal conclusion is that heterogeneities located outside the array can map in a complex way onto the Phase velocities measured by the array. In order to minimize this effect, it is necessary to have a large number of events and to average measurements from events well-distributed in backazimuth. A second observation is that the period of the wave has a remarkably small influence on the lateral resolution of the measurement, which is dominantly controlled by the size of the array. We analyse if the artefacts created by heterogeneities can be mistaken for azimuthal variations caused by anisotropy. We also show that if the amplitude of the surface waves can be measured precisely enough, Phase velocities can be corrected and the artefacts which occur due to reflections and diffractions in 3-D structures greatly reduced.
Ulf Leonhardt - One of the best experts on this subject based on the ideXlab platform.
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notes on waves with negative Phase Velocity
IEEE Journal of Selected Topics in Quantum Electronics, 2003Co-Authors: Ulf LeonhardtAbstract:Negative refraction is a wave phenomenon beyond geometrical optics - it depends on the way waves behave when their Phase Velocity reaches a zero. Various forms of linear wave processes in media can be concisely described in one wave equation that is inspired by the interpretation of a medium as an effective space-time geometry. Depending on the conformal factor of the effective metric, the waves may show positive or negative refraction. For electromagnetic waves in two-dimensional dielectrics the conformal factor corresponds to the impedance.
Sébastien Chevrot - One of the best experts on this subject based on the ideXlab platform.
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On the validity of the eikonal equation for surface-wave Phase-Velocity tomography
Geophysical Journal International, 2020Co-Authors: M Lehujeur, Sébastien ChevrotAbstract:S U M M A R Y The Phase Velocity of surface waves can be directly determined from the amplitude and Phase of the regional wavefield using the Helmholtz equation. However, the Helmholtz equation involves estimating the Laplacian of the amplitude field, a challenging operation to perform on noisy and sparsely sampled seismic data. For this reason, the amplitude information is often discarded. In that case, Phase-Velocity maps are reconstructed with the eikonal equation, which relates the local Phase slowness to the gradient of the Phase. Here, we derive analytical expression of the errors arising from neglecting the amplitude of the wavefield in eikonal tomography. In general, these errors are quite strong but they vary sinusoidally with the wave propagation direction. Consequently, if the azimuthal coverage is good, they will average out, and unbiased Phase-Velocity maps can be obtained with eikonal tomography. We numerically validate these results with a synthetic tomography experiment.
