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We present the design of a volumetric metamaterial (MTM) structure and its interaction with a relativistic electron beam. This novel structure has promising applications in particle beam diagnostics, acceleration, and microwave generation. The volumetric MTM has a cubic unit cell allowing structures of arbitrary size to be configured as an array of identical cells. This structure allows the exploration of the properties of a metamaterial structure without having to consider substrates or other supporting elements. The dispersion characteristics of the unit cell are obtained using eigenmode simulations in the hfss code and also using an effective medium theory with spatial dispersion. Good agreement is obtained between these two approaches. The lowest-order mode of the MTM structure is found to have a negative group velocity in all directions of propagation. The frequency spectrum of the radiation from a relativistic electron beam passing through the MTM structure is calculated analytically and also calculated with the cst code, with very good agreement. The radiation pattern from the relativistic electron beam is found to be backward Cherenkov radiation, which is a promising tool for particle diagnostics. Calculations are also presented for the application of a MTM-based wakefield accelerator as a possible all-metal replacement for the conventional dielectric wakefield structure. The proposed structure may also be useful for MTM-based vacuum electron devices for microwave generation and amplification., United States. Dept. of Energy. Office of High Energy Physics (Award DE-SC0010075), United States. Air Force Office of Scientific Research. Multidisciplinary University Research Initiative (Grant FA 550-12-1-0489)

11 págs.; 10 figs.; PACS number(s): 42.65.Tg, 05.45.Yv, 47.54.−r, 47.20.Ky, © 2015 American Physical Society. We provide a systematic study of the rogue-wave dynamics in phase-matched weakly dispersive nonlinear media, which arise either from the parametric mixing or from the stimulated backscattering process, both of which are modeled by the three-wave resonant interaction equations. The explicit rogue-wave solutions up to the second order are presented. These solutions reveal the markedly different vector rogue-wave behaviors of the parametric and the scattering processes. In the degenerate situation where two interacting optical fields share the same group velocity, the sum of the intensities of this pair of optical wave components is spatiotemporally invariant. The close link between the existence of rogue-wave solutions and the presence of baseband modulation instability is confirmed. Finally, numerical simulations confirm that the generation of vector rogue waves is robust in the presence of spontaneous modulation instability activated by quantum noise. ©2015 American Physical Society, This work was supported by the National Natural Science Foundation of China (Grants No. 11174050 and No. 11474051) and by the Italian Ministry of University and Research (MIUR, Project No. 2012BFNWZ2). Ph.G. was supported by the Agence Nationale de la Recherche (Projects No. ANR-2010-BLANC-0417-01 and No. ANR-2012-BS04- 0011). The work of J.M.S.C. was supported by MINECO under Contract No. TEC2012-37958-C02-02, by C.A.M. under Contract No. S2013/MIT-2790, and by the Volkswagen Foundation. M.C. acknowledges the support from the Agence Nationale de la Recherche (Project No. ANR-14-ACHN-0014 NoAWE). F.B. and S.W. are also with Istituto Nazionale di Ottica del Consiglio Nazionale delle Ricerche.

We discuss the nature of leaky modes with large phase velocity and an effective refractive index and group velocity near zero in planar arrays of subwavelength dielectric or metallic spheres far from their resonances. Large field enhancements with $Q$ factors exceeding $10{}^{10}$ are theoretically predicted for these modes. Those theoretical findings may have important applications in optical switching and memories, quantum information processing, optical storage, and enhancement of optical nonlinearities. Preliminary experimental results at microwave frequencies qualitatively confirm these predictions.

In this paper, we present theoretical and simulation-based analyses of a novel, normal-conducting, multiple-cell, traveling wave accelerating structure. Instead of the conventional circular apertures, we utilize asymmetric complementary split-ring resonators to couple pillbox cavities and bring the phase velocity below that of the speed of light in vacuo. We show that this architecture exhibits a low, negative, group velocity and that the 0 through $\ensuremath{\pi}$ modes decrease in order of frequency---in contrast to conventional electrically coupled structures in which the 0 mode has the lowest frequency and the $\ensuremath{\pi}$ mode the highest. We illustrate the efficacy of the proposed design via electromagnetic and particle simulation results for a four-cell structure operating around 1.9 GHz. Results are given for operation in the $\ensuremath{\pi}$, $2\ensuremath{\pi}/3$, and $\ensuremath{\pi}/3$ modes. Our design achieves accelerating gradients of around $3.3\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ and a cavity voltage of 0.594 MV for an applied rf power of 82 kW ($\ensuremath{\pi}$ mode). The accelerating gradients achieved are up to 3.3 times that of a conventional circular aperture-coupled design with the same phase velocity, rf excitation power, operating frequency, mode type, and number of cells.

With the use of a face-centered cubic model of lattice dynamics we calculate the group velocity of acoustic phonons in the growth direction of periodic superlattices. Comparing with the case of bulk solids, this component of the phonon group velocity is reduced due to the flattening of the dispersion curves associated with Brillouin-zone folding. The results are used to estimate semiquantitatively the effects on the lattice thermal conductivity in Si/Ge and GaAs/AlAs superlattices. For a Si/Ge superlattice an order of magnitude reduction is predicted in the ratio of superlattice thermal conductivity to phonon relaxation time [consistent with the results of P. Hyldgaard and G. D. Mahan, Phys. Rev. B 56, 10 754 (1997)]. For a GaAs/AlAs superlattice the corresponding reduction is rather small, i.e., a factor of 2--3. These effects are larger for the superlattices with larger unit period, contrary to the recent measurements of thermal conductivity in superlattices.

We report on the dispersion relations of surface plasmon resonance (SPR) modes in spatially extended self-organized two-dimensional arrays of closely spaced gold nanoparticles (NPs). The arrays are supported on uniaxially rippled LiF substrates that trigger the organization of the metallic gold NPs in sequences of closely spaced chains. We measured the dispersion of the longitudinal and transverse SPR modes of these chains and successfully reproduced the experimental data by means of a modified effective-medium model. The calculations show that the interchain electromagnetic coupling increases by almost one order of magnitude the SP group velocity in well-defined regions of the Brillouin zone with respect to isolated chains.

The dynamics of optical switching in semiconductor microcavities in the strong coupling regime is studied by using time- and spatially resolved spectroscopy. The switching is triggered by polarized short pulses which create spin bullets of high polariton density. The spin packets travel with speeds of the order of 106 m/s due to the ballistic propagation and drift of exciton polaritons from high to low density areas. The speed is controlled by the angle of incidence of the excitation beams, which changes the polariton group velocity, We thank R. Houdré for the microcavity sample. This work was supported by the IFRAF, the Agence Nationale pour la Recherche, the Spanish MEC (MAT2008-01555 and QOIT-CSD2006-00019), CAM (S-2009/ESP-1503) and FP7 ITN’s ‘‘Clermont4’’ (235114), and Spin-optronics (237252). We acknowledge the Institut Universitaire de France

In lossless periodic media, the concept of group velocity is fundamental to the study of propagation dynamics. When spatially averaged, the group velocity is numerically equivalent to energy velocity, defined as the ratio of energy flux to energy density of modal fields. However, in lossy media, energy velocity diverges from group velocity. Here, we define a modal field velocity which remains equal to the complex modal group velocity in homogeneous and periodic media. The definition extends to the more general situation of modal fields that exhibit spatial or temporal decay due to lossy elements or Bragg reflection effects. Our simple expression relies on a generalization of the concepts of energy flux and density. Numerical examples, such as a two-dimensional square array of silver rods in vacuum, are provided to confirm the result. Examples demonstrate how the dispersion relation of the periodic structure, the properties of its modes, and their group velocities change markedly in lossy media.

We present experimental observations of evanescent waves in a Smith-Purcell free-electron laser (FEL). These waves, predicted by both theory and simulations, have wavelengths longer than the Smith-Purcell radiation, group velocity antiparallel to the electron beam, and for sufficiently high current, provide feedback to bunch the electron beam. This feedback is the basis of oscillator operation of the Smith-Purcell FEL. The wavelengths observed agree with theoretical predictions, and strong radiation from the upstream end of the grating confirms the negative group velocity. Radiation observed at the second harmonic may indicate electron bunching by the evanescent wave.

A scheme of generating a uniform ellipsoidal laser pulse for high-brightness photoinjectors is discussed. The scheme is based on the chromatic aberration of a dispersive lens. Fourier optics simulation reveals the interplay of group velocity delay and dispersion in the scheme, as well as diffractions. Particle tracking simulation shows that the beam generated by such a laser pulse approaches the performance of that by an ideal ellipsoidal laser pulse and represents a significant improvement from the traditionally proposed cylindrical beam geometry. The scheme is tested in an 800-nm, optical proof-of-principle experiment at lower peak power with excellent agreement between the measurement and simulation.

We propose a technique to obtain slow and fast light propagations based on coherent population oscillations forced by a modulated pump. This mechanism produces an enhancement of 1 order of magnitude of the delay or advancement of light signals. The relative phase between the pumps to the signal fields is used as a knob for changing light propagation from ultraslow group velocities to negative group velocities. The experimental realization of the phenomenon was carried out in an erbium-doped fiber amplifier at room temperature.

We study the propagation of electromagnetic waves in one-dimensional quasiperiodic photonic band gap structures made of serial loop structures separated by segments. Different quasiperiodic structures such as Fibonacci, Thue-Morse, Rudin-Shapiro, and double period are investigated with special focus on the Fibonacci structure. Depending on the lengths of the two arms constituting the loops, one can distinguish two particular cases. (i) There are symmetric loop structures, which are shown to be equivalent to impedance-modulated mediums. In this case, it is found that besides the existence of extended and forbidden modes, some narrow frequency bands appear as defect modes in the transmission spectrum inside the gaps. These modes are shown to be localized within only one of the two types of blocks constituting the structure. An analysis of the transmission phase time enables us to derive the group velocity as well as the density of states in these structures. In particular, the stop bands (localized modes) may give rise to unusual (strong normal) dispersion in the gaps, yielding fast (slow) group velocities above (below) the velocity of light. (ii) There are also asymmetric loop structures, where the loops play the role of resonators that may introduce transmission zeros and hence additional gaps unnoticed in the case of simple impedance-modulated mediums. A comparison of the transmission amplitude and phase time of Fibonacci systems with those of other quasiperiodic systems is also outlined. In particular, it was shown that these structures present similar behaviors in the transmission spectra inside the regions of extended modes, whereas they present different localized modes inside the gaps. Experiments and numerical calculations are in very good agreement., This work was supported by “Le Fond Européen de Développement Régional FEDER, INTERREG III France-Wallonie-Flandre PREMIO” and “Le Conseil Régional Nord-Pas de Calais.” The work of V.R.V. has been partially supported by the Ministerio de Educación y Ciencia Spain through Grant No. MAT2003-04278.

It has previously been shown that the electron beam in a Smith-Purcell free-electron laser interacts with a synchronous evanescent wave. At high electron energy, the group velocity of this wave is positive and the device operates on a convective instability, in the manner of a traveling-wave tube. For operation as an oscillator, the gain must exceed the losses in the external feedback system. At low electron energy, the group velocity of the synchronous evanescent wave is negative and the device operates on an absolute instability, like a backward-wave oscillator, and no external feedback is required. For oscillation to occur, the current must exceed the so-called start current. At an intermediate energy, called the Bragg condition, the group velocity ${v}_{g}$ of the evanescent wave vanishes and both the gain and the attenuation due to resistive losses in the grating diverge. It is shown that near the Bragg condition the gain depends on ${v}_{g}^{\ensuremath{-}1/3}$, while the attenuation depends on ${v}_{g}^{\ensuremath{-}1}$. Since the attenuation increases faster than the gain near the Bragg condition, the Smith-Purcell free-electron laser cannot operate at the point of maximum gain. The effects of resistive losses become increasingly important as Smith-Purcell free-electron lasers move to shorter wavelengths.

We study the dispersion and absorption spectra of a weak probe in a $\ensuremath{\Lambda}$-type three-level atomic system with closely ground sublevels driven by a strong field and damped by a broadband squeezed vacuum. We analyze the interplay between the spontaneous generated coherence and the squeezed field on the susceptibility of the atomic system. We find that by varying the intensity of the squeezed field the group velocity of a weak pulse can change from subluminal to superluminal. In addition we exploit the fact that the properties of the atomic medium can be dramatically modified by controlling the relative phase between the driving field and the squeezed field, allowing us to manipulate the group velocity at which light propagates. The physical origin of this phenomenon corresponds to a transfer of the atomic coherence from electromagnetically induced transparency to electromagnetically induced absorption. Besides, this phenomenon is achieved under nearly transparency conditions and with negligible distortion of the propagation pulse.

A formula is derived for the small-signal gain of a Smith-Purcell free-electron laser. The theory describes the electron beam as a moving plasma dielectric, and assumes that the electron beam interacts with an evanescent mode traveling along the surface of a periodic waveguide with a rectangular profile. The phase velocity of the evanescent wave is synchronous with the electron velocity, but the group velocity is actually negative. The electron beam amplifies the evanescent wave, which does not itself radiate. According to this picture, the radiation observed emanating from the grating is Smith-Purcell radiation enhanced by the bunching of the electrons due to the interaction with the evanescent mode. There will also be radiation from the part of the evanescent mode that is outcoupled from the ends of the grating. This radiation appears at a lower frequency than the Smith-Purcell radiation. The new results explain both the gain and the radiation observed in the experiments of Urata and Walsh, and the cube-root current dependence of the gain inferred by Bakhtyari, Walsh, and Brownell.

Finite-difference time-domain simulations show direct evidence of optical pulse propagation below the diffraction limit of light along linear arrays of spherical noble metal nanoparticles with group velocities up to 0.06c. The calculated dispersion relation and group velocities correlate remarkably well with predictions from a simple point-dipole model. A change in particle shape to spheroidal particles shows up to a threefold increase in group velocity. Pulses with transverse polarization are shown to propagate with negative phase velocities antiparallel to the energy flow.

Elastic waves of short wavelength propagating through the upper layer of the Earth appear to move faster at large separations of source and receiver than at short separations. Existing perturbation theories predict a linear increase of the velocity shift with increasing separation and cannot describe the saturation of the velocity shift at large separations that is seen in computer simulations. We point out that this nonperturbative problem can be solved using a model developed originally for the study of directed polymers. The saturation velocity is found to scale with the four-thirds power of the root-mean-square amplitude of the velocity fluctuations, in good agreement with the computer simulations.

The losses in a photonic crystal waveguide were measured with a near-field microscope in the group velocity range of c/7 down to c/200. Our measurements show that the losses scale proportional to v{g};{-2} for group velocities above c/30. Below c/30, the losses are no longer described by the same power-law dependence on v{g} and the modal pattern becomes irregular, indicative of multiple scattering. The findings indicate the existence of two regimes of slow-light losses: one where a perturbative approach describes propagation with fabrication disorder and one where it breaks down.

We present the experimental results of the measurement of fragment velocity in an impact disruption. Cylindrical projectiles impact on a side (edge) of thin glass plates, and the dispersed fragments were observed using a high-speed camera. The fragment velocity did not depend on the mass but rather on the initial position of the fragment; the velocity component parallel to the projectile direction increased with the distance from the impacted side, while the component perpendicular to the projectile direction increased with the distance from the central axis parallel to the projectile direction. It appears that there are two mechanisms for fragment ejection: one is "spallation," where the fragment velocities depend on the particle velocity induced by shock waves, and the other is "elastic ejection," where the velocities are controlled by the strain energy stored in targets and are at most a few tens of meters per second. We performed a one-dimensional numerical simulation of elastic ejection with a discrete element method and obtained the velocity distribution as a function of the initial position. The numerical results are qualitatively consistent with the experimental ones.

application/pdf, Optical dephasing of excitonic polaritons in CuCl is directly measured for the first time in the picosecond time domain by means of time-resolved, nondegenerate four-wave mixing. Dephasing of the ω1 polariton pulse is probed by an interrogation ω2 polariton pulse whose group velocity is faster than that of the ω1 polariton pulse. It is found that the dephasing damping constant Γ / 2 of excitonic polaritons is of order of 0.01 meV and increases as hω1 approaches the exciton resonance energy from below.