Planar Metamaterials and Applications in Directive Antennas

The dispersion curves of surface waves have been widely used for the retrieval of subsurface structures. Because the surface-wave dispersion is not sensitive to Vp, classical dispersion inversion can only retrieve Vs structures. To retrieve Vp, the dispersion of guided-P waves, which belong to leaky waves, should be considered. Due to the difficulty in forward modeling, the quantitative analysis of the leaky-wave dispersion curves has been rarely reported. In this study, we first derive the sensitivity analysis method of the leaky-wave dispersion based on previously proposed the semi-analytical spectral element method. Then the quantitative sensitivity analysis of leaky-wave dispersion curves is carried out, which confirms the ability of guided-P wave dispersion to constrain the velocity structures. With the theoretical analysis, we propose a joint inversion method based on surface and guided-P wave dispersion curves, which can simultaneously retrieve Vp and Vs. To verify the effectiveness, the proposed joint inversion has been applied to different kinds of field data including ocean bottom seismometer data with active sources and the seismic data of Nevada in the United States in 2008. Both of the inversion tests show that the joint inversion of surface- and leaky-wave dispersion can effectively constrain the velocity structure of Vp and Vs at the same time, which helps to obtain more complete and accurate models than the traditional surface wave dispersion inversion.

The shallow subsurface can be extremely complex and challenging from a geological and geophysical point of view. Near-surface perturbations impact the imaging of the deeper events and therefore have to be corrected in the seismic reflection information. It is also known that solving the near surface is an important part of any depth imaging velocity modeling. Usually we use refracted waves for P-wave velocity model estimation. Refraction data provide the information about the deeper subsurface and gives directly Vp velocity. In areas with complex near surface estimation of Vp on refracted energy can be challenging: velocity inversions and hidden layers can produce ambiguity in the refraction solution. The near-surface model obtained through surface wave inversion gives geometric information about the near-surface layers, geology, velocity and attenuation distribution down to the investigation depth. Because the propagation properties of the surface waves are directly depend on elastic properties of the near surface, the dispersion of surface waves is the physical property used for its characterization. Presented example of the simultaneous joint inversion successfully shows that resulting compressional and shear depth velocity models provide significant improvement to support refraction statics and shallow velocity model building.

Dispersive surface waves in anisotropic linear electromagnetoelasticity

In this paper, the two-dimensional Fourier transform and wave decomposition technique are used to transform Maxwell’s equation in transversely isotropic (TI) formation into two independent sets of transmission line equations about transverse magnetic (TM) wave and transverse electric (TE) wave. According to both transmission line theory and superposition principle, an improved transmission line method is advanced to simplify the computational process of TM wave and TE wave only by introducing the transmission line Green’s functions of the current source. Thus, the new algorithm and the new expressions of current source electromagnetic (EM) dyadic Green’s functions in frequency-wavenumber domain are established for realizing the efficient simulation of marine controlled-source electromagnetic (MCSEM) responses in the horizontally layered TI formation. On this base, the basic solutions of the transmission line Green’s functions and EM boundary conditions are utilized to derive the recursive formula of the generalized reflections and amplitudes of the TE and TM waves, and the analytic solutions of the Green’s functions in each bed are obtained. Then, by using Fourier inverse transform and Bessel integral formula, the MCSEM responses in frequency-spatial domain are expressed in the Sommerfeld integral form. A combination of the cubic spline interpolation with the Lommel integral is used to efficiently compute the MCSEM responses. Finally, we validate the modeling algorithm and investigate the influences of changes in operation frequency and anisotropic coefficient on the MCSEM response in the TI formation.

In this paper, a modified laser induced grating technique (LIG) has been utilized to generate narrow band surface waves in an epoxy-bonded copper-aluminum layered structure. A high performance optical interferometer system was utilized to detect the laser-generated surface waves. The dispersion of surface wave in an epoxy-bonded copper-aluminum specimen was measured and compared with the theoretical solution. An inverse algorithm based on the simplex method was then introduced to determine the bonding thickness as well as the elastic properties of the bonding layer. The inversion results demonstrated that the thickness in the microm range or the elastic properties of the bonding layer could be successfully determined.

Surface waves are widely used for determining properties of the subsurface on different scales. In most applications, the dispersion of surface waves is analyzed to yield depthdependent velocity profiles. However, the heterogeneity of the shallow subsurface complicates such an analysis because of the scattering it causes. Here, we use an inverse scattering method to determine the location and properties of the heterogeneity. First, we illustrate this method, using a regular receiver array. A synthetic example shows that the scatterer is imaged at the correct location with good resolution. In practice, we may want to take advantage of existing non-uniform arrays, such as those deployed for global seismic studies. In our next example, we generate scattered data at irregularly spaced locations. By simultaneously inverting the scattered wave field excited by different sources, we observe that the image is well focused on the scatterer. Further work is on the way to better understand the influence of non-uniform (and sparse) sampling on the image quality and what are the sampling requirements.

In this study, we were interested in the dispersion of surface waves caused by the presence of a micrometric field of residual stress on the surface of an amorphous medium. We have shown that in relation to surface waves, a stressed structure like this is comparable to a layer on substrate type structure. The design and implementation of SAW-IDT MEMS sensors enabled quasi-monochromatic Rayleigh-type surface waves to be generated and the dispersion phenomenon to be studied over a wide range of frequencies for different superficial fields of residual stress. The thicknesses of the stressed cortical zones were estimated with good accuracy using an inverse method.

It is known that $\ensuremath{\epsilon}$-near-zero (ENZ) structures may provide exciting possibilities for light-matter interaction, including the phenomenon of supercoupling, i.e., channeling of wave energy through ultranarrow conduits connecting two waveguides. Here, we extend this concept to the surface waves using structures with effective ENZ properties, exploring how surface waves can tunnel through such structures with essentially no reflection, even in the presence of sharp bends. Such ENZ metastructures support tunneling of waves with longitudinal wave numbers near zero. The surface waves may include examples of transverse electric (TE) surface waves along the grounded dielectric slabs (e.g., in the microwave regime) and TE surface-plasmon polariton waves along the negative-permeability interfaces. The ENZ metastructures can transfer the surface wave energy from one side to another by an anomalous squeezing effect, which is effectively independent of the length and the shape of the ENZ metastructures. The interaction of electromagnetic waves with such systems offers exciting possibilities for the design of flat photonic devices and components.

Realizing and manipulating strong light–matter coupling in two-dimensional monolayer semiconductors is important in the research of cavity quantum electrodynamics and the development of photonic/plasmonic devices. Here, we investigate the coupling between the transverse-electric (TE) polarized wave supported by a Si3N4/Ag heterostructure and the two excitons in a WS2 monolayer (i.e., the A- and B-excitons) attached on such a heterostructure. It is found that the dissipation rate of the TE wave can be modified by simply varying the thickness of the Ag film. By appropriately designing the thickness of the Ag film, the strong coupling between the TE wave and the two excitons in the WS2 monolayer can be simultaneously realized, leading to hybridization of the two excitons. We numerically simulate the angle-resolved reflection spectra for the WS2/Si3N4/Ag heterostructure and reveal hybridization of the two excitons mediated by the TE wave. In experiments, we introduce Au nanorods as scatters for the TE wave, which enhance the in-plane electric field and achieve stronger photon–exciton coupling. We experimentally measured the angled-resolved scattering spectra for an Au nanorod placed on the WS2/Si3N4/Ag heterostructure and demonstrated the strong coupling between the TE wave and the two excitons, which results in the hybridization of the two excitons. Our findings open horizons for realizing and manipulating strong light–matter interaction in two-dimensional materials and pave the way for the construction of photonic/plasmonic devices by exploiting dielectric–metal heterostructure.

The scattering problem of acoustic plane waves from comb-like impedance gratings on a rigid surface has been investigated in this paper. A rigorous analytic approach for homogeneous plane-wave incidence is presented based on the periodicity of the grating structure, in which the problem was solved as a mixed boundary value problem and the scattered field was represented by the tangent velocity difference across a partition wall of the grating. A singular integral equation has been derived for the tangent velocity difference, which can directly be solved with the Gauss-Chebyshev procedure. The resulting solution consists of a series of Bloch-Floquet waves (plane bulk wave and surface wave modes) with explicit expressions for the expansion coefficients. When the grating period is much less than the incident wavelength (ka << 1), the grating structure is equivalent to a plane impedance surface and no surface waves can be excited with homogeneous plane-wave incidence. When the grating period is comparable to the incident wavelength, resonance phenomena are predicted under certain conditions and surface waves can form, even with homogeneous plane-wave incidence. The dispersion relation for surface waves has also been examined. The impedance effects of the grating on the reflection and diffraction waves as well as on the dispersion and formation of surface waves have been studied, with the acoustically hard grating being the special case of the general impedance grating.

Measurements of the Doppler spectra of airborne ultrasound backscattered by the rough dynamic surface of a shallow turbulent flow are presented in this paper. The interpretation of the observed acoustic signal behavior is provided by means of a Monte Carlo simulation based on the Kirchhoff approximation and on a linear random-phase model of the water surface elevation. Results suggest that the main scattering mechanism is from capillary waves with small amplitude. Waves that travel at the same velocity of the flow, as well as dispersive waves that travel at a range of velocities, are detected, studied, and used in the acoustic Doppler analysis. The dispersive surface waves are not observed when the flow velocity is slow compared to their characteristic velocity. Relatively wide peaks in the experimental spectra also suggest the existence of nonlinear modulations of the short capillary waves, or their propagation in a wide range of directions. The variability of the Doppler spectra with the conditions of the flow can affect the accuracy of the flow velocity estimations based on backscattering Doppler. A set of different methods to estimate this velocity accurately and remotely at different ranges of flow conditions is suggested.

Potential (electrostatic) surface waves (SWs) in a semi-bounded plasma with degenerate electrons are studied using the quasi-classical mean-field kinetic model. The SW spectrum and the collisionless damping rate are obtained numerically for a wide range of wavelengths. In the limit of long wavelengths, the SW frequency ω approaches the cold-plasma limit ω=ωp/2 with ωp being the plasma frequency, while at short wavelengths, the SW spectrum asymptotically approaches the spectrum of zero-sound mode propagating along the boundary. It is shown that the surface waves in this system remain weakly damped at all wavelengths (in contrast to strongly damped surface waves in Maxwellian electron plasmas), and the damping rate nonmonotonically depends on the wavelength, with the maximum (yet small) damping occurring for surface waves with wavelength of ≈5πλF, where λF is the Thomas-Fermi length. The applicability of the used approximations and of the obtained results is discussed in detail.

We studied the surface plasmon resonance properties of transverse electric (TE) wave in a μ-negative (MNG) material/dielectric/μ-negative (MNG) material waveguide with a finite length which works as a subwavelength cavity. The wavelength of the surface plasmon becomes shorter when decrease the thickness of the dielectric core and decrease the plasma frequency of MNG material. The resonance in this cavity can be understood as a Fabry-Perot resonance caused by the reflection of the TE guided mode at the entrance and the exit surfaces. The electromagnetic fields and power flow are concentrated around the dielectric core at the resonant frequency, the magnetic field is maximized at the dielectric core entrance and exit. When a subwavelength magnetic resonator is put at the core entrance and the resonance frequency is tuned to the plasmon cavity mode, Rabi splitting and Rabi oscillation can appear because of the strong coupling between this resonator and the cavity mode.

Dispersive surface waves are sensitive to the geometry of layered structures and the compressional and shear wave velocities within each layer. Therefore they can be used to estimate the structure and lithology of shallow soils. It is common practice to use the low frequency fundamental mode for inversion; thus, it is necessary to separate the single modes contained in the data interactively, which is time consuming and a source of error. The fast algorithm presented in this paper interprets all excited modes of a surface wave simultaneously and avoids the need to identify individual mode-numbers. The inversion is based on the group travel time of a Rayleigh wave, estimated from a single seismic trace by application of the Continuous Wavelet Transform (CWT). The resulting CWT distribution is fitted by a composition of wavelets determined from theoretical group-velocity dispersion relations of possible models, in connection with appropriate boundary conditions. A feasibility test on a synthetic dispersion dataset with dominant higher modes shows that the systematic inversion error associated with a fundamental-mode assumption does not arise, and both layer thicknesses and elastic properties can converge rapidly to the true values. However, two possible problems with the proposed method are the imperfect reconstruction (in this case especially at low frequencies), owing to the limited number of wavelets and theoretical modal dispersion curves used for approximation, and possible convergence to local minima, due to the increased nonlinearity of the misfit function.

In this presentation, we analytically derive the dispersion equation for surface waves traveling along reactive boundaries which are periodically modulated in time. In addition, we show numerical results for the dispersion curves and importantly uncover that time-varying boundaries generate band gaps that can be controlled by engineering the modulation spectrum. Furthermore, we also point out an interesting effect of field amplification related to the existence of such band gaps for surface waves. The effect of amplification does not require the synchronization of signal and pumping waves. This unique property is very promising to be applied in surface-wave communications from microwave to optical frequencies.