Showing papers on "Group velocity published in 2012"

Design for broadband on-chip isolator using stimulated Brillouin scattering in dispersion-engineered chalcogenide waveguides

Abstract: We propose a scheme for on-chip isolation in chalcogenide (As2S3) rib waveguides, in which Stimulated Brillouin Scattering is used to induce non-reciprocal mode conversion within a multi-moded waveguide. The design exploits the idea that a chalcogenide rib buried in a silica matrix acts as waveguide for both light and sound, and can also be designed to be multi-moded for both optical and acoustic waves. The enhanced opto-acoustic coupling allows significant isolation (> 20 dB) within a chip-scale (cm-long) device (< 10 cm). We also show that the bandwidth of this device can be dramatically increased by tuning the dispersion of the waveguide to match the group velocity between optical modes: we find that 20 dB isolation can be extended over a bandwidth of 25 nm.

Crustal structure of Australia from ambient seismic noise tomography

Abstract: [1] Surface wave tomography for Australian crustal structure has been carried out using group velocity measurements in the period range 1–32 s extracted from stacked correlations of ambient noise between station pairs. Both Rayleigh wave and Love wave group velocity maps are constructed for each period using the vertical and transverse component of the Green's function estimates from the ambient noise. The full suite of portable broadband deployments and permanent stations on the continent have been used with over 250 stations in all and up to 7500 paths. The permanent stations provide a useful link between the various shorter-term portable deployments. At each period the group velocity maps are constructed with a fully nonlinear tomographic inversion exploiting a subspace technique and the Fast Marching Method for wavefront tracking. For Rayleigh waves the continental coverage is good enough to allow the construction of a 3D shear wavespeed model in a two stage approach. Local group dispersion information is collated for a distribution of points across the continent and inverted for a 1D SV wavespeed profile using a Neighbourhood Algorithm method. The resulting set of 1D models are then interpolated to produce the final 3D wavespeed model. The group velocity maps show the strong influence of thick sediments at shorter periods, and distinct fast zones associated with cratonic regions. Below the sediments the 3D shear wavespeed model displays significant heterogeneity with only moderate correlation with surface tectonic features. For example, there is no evident expression of the Tasman Line marking the eastern edge of Precambrian outcrop. The large number of available inter-station paths extracted from the ambient noise analysis provide detailed shear wavespeed information for crustal structure across the Australian continent for the first time, including regions where there was no prior sampling because of difficult logistics.

Minimum thermal conductivity in superlattices: A first-principles formalism

Abstract: The thermal conductivity of silicon-germanium superlattices is computed from density-functional perturbation theory using relaxation times that include both anharmonic and interface roughness effects. A decrease in the group velocity of low-frequency phonons in addition to the interface-disorder-induced scattering of high-frequency phonons drives the superlattice thermal conductivity to below the alloy limit. At short periods, interplay between decrease in group velocity and increase in phonon lifetimes with increase in superlattice period leads to a minimum in the cross-plane thermal conductivity. Increasing the mass mismatch between the constituent materials in the superlattice further lowers the thermal conductivity below the alloy limit, pointing to avenues for higher efficiency thermoelectric materials.

Ultrafast plasmon propagation in nanowires characterized by far-field spectral interferometry.

Abstract: Spectral interferometry is employed to fully characterize amplitude and phase of propagating plasmons that are transmitted through silver nanowires in the form of ultrashort pulses. For nanowire diameters below 100 nm, the plasmon group velocity is found to decrease drastically in accordance with the theory of adiabatic focusing. Furthermore, the dependence of the plasmon group velocity on the local nanowire environment is demonstrated. Our findings are of relevance for the design and implementation of nanoplasmonic signal processing and in view of coherent control applications.

Sensing With Slow Light in Fiber Bragg Gratings

Abstract: On the edge of the bandgap in a fiber Bragg grating (FBG) narrow peaks of high transmission exist at frequencies where light interferes constructively in the forward direction. In the vicinity of these transmission peaks, light reflects back and forth numerous times across the periodic structure and experiences a large group delay. Since the sensitivity of a phase sensor to most external perturbations is proportional to the reciprocal of group velocity, in these slow-light regions the sensitivity of an FBG is expected to be significantly enhanced over traditional FBG sensors operated around the Bragg wavelength. In this paper, we describe means of producing and operating FBGs that support structural slow light with a group index that can be in principle as high as several thousand. We present simulations elucidating how to select the FBG parameters, in particular index modulation, length, and apodization, to generate such low group velocities, and quantify the very large improvement in strain and temperature sensitivities resulting from these new slow-light configurations. As a proof of concept, we report an FBG with a group index of 127, or a group velocity of ~2,360 km/s. This is by far the lowest group velocity reported to date in an FBG. Used as a strain sensor, this slow-light FBG is shown to be able to detect a strain as small as 880 fe/ √Hz , the lowest value reported for a passive FBG sensor.

Efficient frequency shifting of dispersive waves at solitons

Abstract: We demonstrate frequency redshifting and blueshifting of dispersive waves at group velocity horizons of solitons in fibers. The tunnelling probability of waves that cannot propagate through the fiber-optical solitons (horizons) is measured and described analytically. For shifts up to two times the soliton spectral width, the waves frequency shift with probability exceeding 90% rather than tunnelling through the soliton in our experiment. We also discuss key features of fiber optical Cherenkov radiation such as high efficiency and large bandwidth within this framework.

Robust optimization of adiabatic tapers for coupling to slow-light photonic-crystal waveguides.

Abstract: We investigate the design of taper structures for coupling to slow-light modes of various photonic-crystal waveguides while taking into account parameter uncertainties inherent in practical fabrication. Our short-length (11 periods) robust tapers designed for λ = 1.55μm and a slow-light group velocity of c/34 have a total loss of < 20dB even in the presence of nanometer-scale surface roughness, which outperform the corresponding non-robust designs by an order of magnitude. We discover a posteriori that the robust designs have smooth profiles that can be parameterized by a few-term (intrinsically smooth) sine series which helps the optimization to further boost the performance slightly. We ground these numerical results in an analytical foundation by deriving the scaling relationships between taper length, taper smoothness, and group velocity with the help of an exact equivalence with Fourier analysis.

Storage-recovery phenomenon in magnonic crystal.

Abstract: The phenomenon of coherent wave trapping and restoration is demonstrated experimentally in a magnonic crystal. Unlike the conventional scheme used in photonics, the trapping occurs not due to the deceleration of the incident wave when it enters the periodic structure but due to excitation of the quasinormal modes of the artificial crystal. This excitation occurs at the group velocity minima of the decelerated wave in narrow frequency regions near the edges of the band gaps of the crystal. The restoration of the traveling wave is implemented by means of phase-sensitive parametric amplification of the stored mode.

The effect of rotation on internal solitary waves

Abstract: In the weakly nonlinear long-wave regime, internal solitary waves are often modelled by the Korteweg– de Vries equation, which is well known to support an exact solitary wave solution. However, when the effect of background rotation is taken into account, an additional term is needed and the outcome is the Ostrovsky equation. Although the additional term would appear to be relatively mild, being a linear long-wave perturbation, it has the drastic effect of destroying the solitary wave solution. Instead an initial solitary-like disturbance decays into radiating oscillatory waves, with the eventual formation of a nonlinear envelope wave packet, whose carrier wavenumber is determined by an extremum in the group velocity. In this paper, we will use a combination of theoretical analyses, numerical simulations and laboratory experiments to describe this process.

On the performance of bender elements in triaxial tests

Abstract: This paper presents the results of a research programme in which the transfer functions of bender element/soil systems have been measured for various materials, set-ups and stress levels. The resulting database has allowed the characterisation of the inherent multi-vibrational nature of the system, and the interpretation challenges that this imposes. The excitation of multiple modes of vibration may introduce signal distortion and consequently differences between group velocity and phase velocity. This means that the common assumption of conventional interpretation techniques that group velocity is equal to shear-wave velocity is in error. Based on the features observed, system parameters have been introduced and a parametric analysis carried out in order to assess the performance of the system and to better understand the effects of multimodal vibration as an attempt to improve signal interpretation.

Mode conversion from quantized to propagating spin waves in a rhombic antidot lattice supporting spin wave nanochannels

Abstract: We report spin wave excitations in a nanopatterned antidot lattice fabricated from a 30-nm thick Ni80Fe20 film. The 250-nm-wide circular holes are arranged in a rhombic unit cell with a lattice constant of 400 nm. By Brillouin light scattering, we find that quantized spin wave modes transform to propagating ones and vice versa by changing the in-plane orientation of the applied magnetic field H by 30∘. Spin waves of either negative or positive group velocity are found. In the latter case, they propagate in narrow channels exhibiting a width of below 100 nm. We use the plane wave method to calculate the spin wave dispersions for the two relevant orientations of H. The theory allows us to explain the wave-vector-dependent characteristics of the prominent modes. Allowed minibands are formed for selected modes only for specific orientations of H and wave vector. The results are important for applications such as spin wave filters and interconnected waveguides in the emerging field of magnonics where the control of spin wave propagation on the nanoscale is key.

Experimental Validation of Frozen Modes Guided on Printed Coupled Transmission Lines

Abstract: Previous work has theoretically demonstrated that nonreciprocal slow-wave modes, namely, “frozen modes,” can be supported on a pair of coupled transmission lines printed on a magnetic substrate. Small antennas have also been designed by exploiting these modes. However, to date, we have yet to demonstrate and observe their existence experimentally. To this end, we construct two printed prototypes comprised of several unit-cells and employ the “T-matrix method” to determine the dispersion properties by measuring the S-parameters of these finite periodic prototypes. The printed unit-cell is designed to exhibit a unique stationary inflection point in the dispersion diagram corresponding to a frozen mode with almost zero group velocity. Through careful measurements and calculations, the frozen mode is observed to propagate at a significantly slower speed (286 times slower) than the speed of light. Importantly, this extraction method can be applied to any other periodic layout to obtain related dispersion properties.

Estimation of shear wave velocity in gelatin phantoms utilizing PhS-SSOCT

Abstract: We report a method for measuring shear wave velocity in soft materials using phase stabilized swept source optical coherence tomography (PhS-SSOCT). Wave velocity was measured in phantoms with various concentrations of gelatin and therefore different stiffness. Mechanical waves of small amplitudes (∼10 μm) were induced by applying local mechanical excitation at the surface of the phantom. Using the phase-resolved method for displacement measurement described here, the wave velocity was measured at various spatially distributed points on the surface of the tissue-mimicking gelatin-based phantom. The measurements confirmed an anticipated increase in the shear wave velocity with an increase in the gelatin concentrations. Therefore, by combining the velocity measurements with previously reported measurements of the wave amplitude, viscoelastic mechanical properties of the tissue such as cornea and lens could potentially be measured.

Characteristics of Seismic Noise: Fundamental and Higher Mode Energy Observed in the Northeast of the Netherlands

Abstract: We study seismic noise recorded in the northeast of the Netherlands by beamforming and by using empirical Green's functions obtained by seismic interfero- metry. From beamforming we found differences in noise directions in different fre- quency bands. The main source region for primary microseisms (0.05-0.08 Hz) is in the west-northwest direction, while the secondary microseisms (0.1-0.14 Hz) have a west-southwest back azimuth. Furthermore, we observed a fast (∼4 km=s) arrival corresponding to the Rayleigh wave overtone. This arrival is also in the secondary microseism band (between 0.15 and 0.2 Hz), but has a west-northwest back azimuth. Both arrivals in the secondary microseism band gain in strength during winter, as does the average wave height in the North Atlantic. We measured phase velocity dispersion curves from both beamforming and noise cross-correlations, as well as group velocity from the latter. These are then jointly inverted for an average 1D S-wave model. The results show how the combination of different methods leads to a more complete char- acterization of the propagation modes and an improved knowledge of the subsurface, especially as the group velocity measurements increase the upper frequency limit of analysis, providing valuable information of the shallowest subsurface.

Slip velocity of large neutrally buoyant particles in turbulent flows

Abstract: We discuss possible definitions for a stochastic slip velocity that describes the relative motion between large particles and a turbulent flow. This definition is necessary because the slip velocity used in the standard drag model fails when particle size falls within the inertial subrange of ambient turbulence. We propose two definitions, selected in part due to their simplicity: they do not require filtration of the fluid phase velocity field, nor do they require the construction of conditional averages on particle locations. A key benefit of this simplicity is that the stochastic slip velocity proposed here can be calculated equally well for laboratory, field and numerical experiments. The stochastic slip velocity allows the definition of a Reynolds number that should indicate whether large particles in turbulent flow behave (a) as passive tracers; (b) as a linear filter of the velocity field; or (c) as a nonlinear filter to the velocity field. We calculate the value of stochastic slip for ellipsoidal and spherical particles (the size of the Taylor microscale) measured in laboratory homogeneous isotropic turbulence. The resulting Reynolds number is significantly higher than 1 for both particle shapes, and velocity statistics show that particle motion is a complex nonlinear function of the fluid velocity. We further investigate the nonlinear relationship by comparing the probability distribution of fluctuating velocities for particle and fluid phases.

Mid-infrared photonic crystal waveguides in silicon.

Abstract: We demonstrate the design, fabrication and characterization of mid-infrared photonic crystal waveguides on a silicon-on-insulator platform, showing guided modes in the wavelength regime between 29 and 39 µm The characterization is performed with a proprietary intra-cavity Optical Parametric Oscillator in a free space optical setup and with a fibre coupled setup using a commercial Quantum Cascade Laser We discuss the use of an integrated Mach-Zehnder interferometer for dispersion measurements and report a measured group velocity of up to a value of ng = 12, and determine the propagation loss to be 20 dB/cm

Various photonic crystal bio-sensor configurations based on optical surface modes

Abstract: We design a new bio-sensor concept that incorporates photonic crystal (PC) surface modes to sense small refractive index changes. The initial attempt creates optical surface modes by first enlarging and then perforating the radii of rods residing along the end surface of the square-lattice PC. The strongly confined mode which decays both evanescently along transverse to propagation direction interacts with the substance while propagating along the PC-air interface. Due to index change of the ambient medium, the transmission spectrum experiences linear shift with a large dynamic range. The relocation of the surface defects enhances the sensitivity of bio-sensor from ∼8 to ∼93 nm/RIU. The second type of investigated PC structure is based on triangular-lattice PC and it provides a surface state bio-sensor with a sensitivity of 117 nm/RIU. In addition to these designs, we propose a final structure that incorporates air slot along one side of triangular-lattice PC. We succeeded to obtain a new sensitivity value of 396 nm/RIU. The investigation shows that even higher sensitivities can be achieved. The different RIU values are reminiscent of group velocity of the relevant modes which can be extracted from the dispersion analysis. Compact, sensitive and label-free optical sensors based on surface modes may become part of the important applications in opto-fluidic technology and lab-on-a-chip.

Constraints on the ICM velocity power spectrum from the X-ray lines width and shift

Abstract: Future X-ray observations of galaxy clusters by high spectral resolution missions will provide spatially resolved measurements of the energy and width for the brightest emission lines in the intracluster medium (ICM) spectrum. In this paper we discuss various ways of using these high-resolution data to constrain velocity power spectrum in galaxy clusters. We argue that variations of these quantities with the projected distance R in cool core clusters contain important information on the velocity field length scales (i.e. the size of energy-containing eddies) in the ICM. The effective length leff along the line of sight, which provides dominant contribution to the line flux, increases with R, allowing one to probe the amplitude of the velocity variations at different spatial scales. In particular, we show that the width of the line as a function of R is closely linked to the structure function of the 3D velocity field. Yet another easily obtainable proxy of the velocity field length scales is the ratio of the amplitude of the projected velocity field (line energy) variations to the dispersion of the velocity along the line of sight (line width). Finally the projected velocity field can be easily converted into 3D velocity field, especially for clusters like Coma with an extended flat core in the surface brightness. Under assumption of a homogeneous isotropic Gaussian 3D velocity field we derived simple expressions relating the power spectrum of the 3D velocity field (or structure function) and the observables. We illustrate the sensitivity of these proxies to changes in the characteristics of the power spectrum for a simple isothermal β-model of a cluster. The uncertainties in the observables, caused by the stochastic nature of the velocity field, are estimated by making multiple realizations of the random Gaussian velocity field and evaluating the scatter in observables. If large-scale motions are present in the ICM these uncertainties may dominate the statistical errors of line width and shift measurements.

Nonlinear complementary filters on the special linear group

Abstract: This article proposes a nonlinear complementary filter for the special linear Lie-group SL(3) that fuses low-frequency state measurements with partial velocity measurements and adaptive estimation of unmeasured slowly changing velocity components. The obtained results have direct application on the problem of filtering a sequence of image homographies acquired from low-quality video data. The considered application motivates us to derive results that provide adaptive estimation of the full group velocity or part of the group velocity that cannot be measured from sensors attached to the camera. We demonstrate the performance of the proposed filters on real world homography data.

Entanglement creation with negative index metamaterials

Abstract: We propose a scheme for creating a maximally entangled state comprising two field quanta. In our scheme, two weak light fields, which are initially prepared in either coherent or polarization states, interact with a composite medium near an interface between a dielectric and a negative index metamaterial. This interaction leads to a large Kerr nonlinearity, reduction of the group velocity of the light, and significant confinement of the light fields, while simultaneously avoiding amplitude losses of the incoming radiation. All these considerations make our scheme efficient.

Light trapping beyond the 4n2 limit in thin waveguides

Abstract: We describe a method for determining the maximum absorption enhancement in thin film waveguides based on optical dispersion relations. For thin film structures that support one, well-confined guided mode, we find that the absorption enhancement can surpass the traditional limit of 4n^2 when the propagation constant is large and/or the modal group velocity is small compared to the bulk value. We use this relationship as a guide to predicting structures that can exceed the 4n^2 light trapping limit, such as plasmonic and slot waveguides. Finally, we calculate the overall absorption for both single and multimode waveguides, and show examples of absorption enhancements in excess of 4n^2 for both cases.

Velocity and velocity gradient based properties of a turbulent plane mixing layer

Abstract: Experiments were carried out in a turbulent mixing layer designed to match, as closely as possible, the conditions of the temporally evolving direct numerical simulation of Rogers & Moser (Phys. Fluids, vol. 6, 1994, pp. 903–922). Two Reynolds numbers, based on the local momentum thickness in the self-similar region of the mixing layer, were investigated: and . Measurements were also made in the mixing layer in the pre-mixing transition region where . The three velocity components and their cross-stream gradients were measured with a small 12-sensor hot-wire probe that traversed the mixing layer. Taylor’s hypothesis was used to estimate the streamwise gradients of the velocity components so that reasonably good approximations of all the components of the velocity gradient tensor would be available. The signal from a single-sensor probe at a fixed position in the high-speed free stream flow provided a reference to the phases of the passage of large-scale, coherent, spanwise-oriented vortices past the 12-sensor probe. The velocity and velocity gradient data were analysed to determine turbulence statistical characteristics, including moments, probability density functions and one-dimensional spectra of the velocity and vorticity fields. Although the velocity statistics obtained from the experiment agree well with those from the direct numerical simulation of Rogers & Moser, there are significant differences in the vorticity statistics. The phase reference from the fixed single-sensor probe permitted phase averaging of the 12-sensor probe data so that the spanwise ‘roller’ vortices could be separated from the small-scale, more random turbulence, as had been previously demonstrated by Hussain & Zaman (J. Fluid Mech., vol. 159, 1985, pp. 85–104). In this manner, the data could be conditionally averaged to examine the spatial distributions, with respect to the roller vortices, of interesting and important characteristics of the turbulence, such as the turbulent kinetic energy production and dissipation rate, enstrophy and vorticity component covariances.

Dispersion and the dissipative characteristics of surface waves in the generalized S-transform domain

Abstract: Wavenumber, group velocity, phase velocity, and frequency-dependent attenuation characterize the propagation of surface waves in dispersive, attenuating media. We use a mathematical model based on the generalized S transform to simultaneously estimate these characteristic parameters for later use in joint inversion for near-surface shear wave velocity. We use a scaling factor in the generalized S transform to enable the application of the method in a highly dispersive medium. We introduce a cost function in the S-domain to estimate an optimum value for the scaling factor. We also use the cost function to generalize the application of the method for noisy data, especially data with a low signal-to-noise ratio at low frequencies. In that case, the estimated wavenumber is perturbed. As a solution, we estimate wavenumber perturbation by minimizing the cost function, using Simulated Annealing. We use synthetic and real data to show the efficiency of the method for the estimation of the propagation parameters o...

S-wave velocity structure of northeastern China from joint inversion of Rayleigh wave phase and group velocities

Abstract: SUMMARY We imaged the crust and uppermost mantle structure beneath northeastern (NE) China with fundamental mode Rayleigh waves recorded by 125 broad-band stations deployed in the region. Rayleigh wave phase and group velocities along more than 700 interstation paths were estimated using the wavelet transformation method and then these data were utilized to construct 2-D phase and group velocity maps in the period range of 15–60 s. Owing to the dense ray coverage and short ray path, our results provide better lateral resolution in the NE China region compared with previous phase and group velocity studies. The regularized Rayleigh wave phase and group velocity dispersions at each cell were jointly inverted to determine 1-D shear wave velocity structure using the linear inversion method and then assembled into a 3-D model. Our results show that obvious low velocities exist in the uppermost mantle beneath the Changbaishan volcanic region, which may be due to asthenospheric upwelling. The thin lithosphere with fast S-wave velocity in the lower crust of the Songliao Basin implies that the lithospheric mantle beneath NE China is partly removed.

Ion acceleration from thin foil and extended plasma targets by slow electromagnetic wave and related ion-ion beam instability

Abstract: When ions are accelerated by the radiation pressure of a laser pulse, their velocity cannot exceed the pulse group velocity which can be considerably smaller than the speed of light in vacuum. This is demonstrated in two cases corresponding to a thin foil target irradiated by high intensity laser light and to the hole boring produced in an extended plasma by the laser pulse. It is found that the beams of accelerated ions are unstable against Buneman-like and Weibel-like instabilities which results in the broadening of the ion energy spectrum.

Relativistic critical density increase and relaxation and high-power pulse propagation

Abstract: High‐power laser pulse propagation in an overdense plasma due to the relativistic critical density increase has been investigated in one dimension. In a first step the conditions for the existence of a relativistic critical density are delimited and supported by particle‐in‐cell simulations. Its accurate determination is made possible by the installation of a new numerical diagnostics. Guided by this we show that the critical density increase strongly depends on both laser polarization and plasma density profile. Further, we find a new relaxation time ranging from several to many laser cycles, which sets a limit for short laser pulse manipulation and tailoring. Paramountly, it is proved that in the power optics domain the pulse propagation velocity is inhibited by the relativistic energy density in the medium and by the efficient reflection, in contrast to the group velocity from standard dispersion optics.

A Rayleigh wave back‐projection method applied to the 2011 Tohoku earthquake

Abstract: [1] We study the rupture process of the 2011 Tohoku megathrust by analyzing 384 regional strong-motion records using a novel back-projection method for Rayleigh waves with periods between 13 and 100 s. The proposed approach is based on isolating the signal at the selected period with a continuous wavelet transform, and generating the stack using arrival times predicted from detailed fundamental mode Rayleigh wave group velocity maps. We verify the method by back-projecting synthetic time series representing a point source off the coast of Tohoku, which we generate with a 3D finite difference method and a mesh based on the Japan Integrated Velocity Structure Model. Application of the method to K-NET/KiK-net records of theMw9.1 Tohoku earthquake reveals several Rayleigh wave emitters, which we attribute to different stages of rupture. Stage 1 is characterized by slow rupture down-dip from the hypocenter. The onset of stage 2 is marked by energetic Rayleigh waves emitted from the region between the JMA hypocenter and the trench within 60 s after hypocentral time. During stage 3 the rupture propagates bilaterally towards the north and south at rupture velocities between 3 and 3.5 km · s−1, reaching Iwate-oki 65 s and Ibaraki-oki 105 s after nucleation. In contrast to short-period back-projections from teleseismic P-waves, which place radiation sources below the Honshu coastline, Rayleigh wave emitters identified from our long-period back-projection are located 50–100 km west of the trench. This result supports the interpretation of frequency-dependent seismic wave radiation as suggested in previous studies.

Characterization of mechanical properties of a hollow cylinder with zero group velocity Lamb modes

Abstract: Hollow cylinders used in the industry must be regularly inspected. Elastic guided waves, similar to Lamb modes in a plate, can propagate in the axial direction or around the circumference. They are sensitive to geometrical and mechanical parameters of the cylindrical shell. The objective of this paper is to show that zero group velocity (ZGV) Lamb modes can be used to bring out anisotropy and to measure elastic constants of the material. This study provides experimental and numerical investigations on a Zirconium alloy tube extensively used by the nuclear industry in reactor core components. A non-contact method, based on laser ultrasound techniques and ZGV Lamb modes, demonstrates that the difference observed between axial and circumferential guided waves cannot be explained by an isotropic model. Then, a transverse isotropic model is used for the Zircaloy tube. Four of the five elastic constants are directly extracted from ZGV resonance frequencies. The last one is deduced from the measured dispersion spectra. With this complete set of constants, a good agreement is obtained between theoretical and experimental dispersion curves for both axially and circumferentially propagating guided waves.

Slowing down terahertz waves with tunable group velocities in a broad frequency range by surface magneto plasmons

Abstract: This paper proposes one broadly tunable terahertz (THz) slow-light system in a semiconductor-insulator-semiconductor structure. Subject to an external magnetic field, the structure supports in total two surface magneto plasmons (SMPs) bands above and below the surface plasma frequency, respectively. Both the SMPs bands can be tuned by the external magnetic field. Numerical studies show that leveraging on the two tunable bands, the frequency and the group velocity of the slowed-down THz wave can be widely tuned from 0.3 THz to 10 THz and from 1 c to 10−6c, respectively, when the external magnetic field increases up to 6 Tesla. The proposed method based on the two SMPs bands can be widely used for many other plasmonic devices.