Modulation Of Surface Waves Research Articles

Full Complex-Amplitude Modulation of Surface Waves Based on Spin-Decoupled Metasurface.

This work proposes a method for surface wave (SW) coupling along with flexible complex amplitude modulation of its wavefront. The linearly polarized incident plane wave is coupled into the surface mode with complex wavefront by exploiting the spin-decouple nature of a reflective chiral meta-atom. As verification, two kinds of metasurface couplers are designed. The first kind contains two examples for SW airy beam generation with and without deflection under linearly polarized illumination, respectively. The second kind is a bi-functional device capable of SW focusing under left-handed circularly polarized illumination, and propagating wave deflection under right-handed circularly polarized illumination, respectively, to verify the fundamental spin-decoupled character. Simulated and experimental results are in good agreement. We believe that this method provides a flexible approach for complex SW applications in integrated optics, optical sensing, and other related fields.

Complex-amplitude modulation of surface waves based on a metasurface coupler.

Simultaneous and independent modulation of the amplitude and phase of surface waves (SWs) is critical in photonics and plasmonics. Here, we propose a method for flexible complex-amplitude modulation of SWs based on a metasurface coupler. Benefiting from the full range complex-amplitude modulation ability of the meta-atoms over the transmitted field, the coupler can convert the incident wave into a driven surface wave (DSW) with an arbitrary combination of amplitude and initial phase. By placing a dielectric waveguide that supports guided SWs below the coupler, the DSWs can resonantly couple to SWs while preserving complex-amplitude modulation. The proposed scheme provides a practical way for freely tailoring the phase and amplitude profiles of SWs wavefronts. As verification, meta-devices for normal and deflected SW Airy beam generation and SW dual focusing are designed and characterized in the microwave regime. Our findings may stimulate various advanced surface optical meta-devices.

A Model Sensitivity Study of Ocean Surface Wave Modulation Induced by Internal Waves

AbstractNonlinear internal waves are an upper ocean phenomenon that drives horizontal surface current gradients, which in turn modulate ocean surface waves. Under certain conditions, this wave‐current interaction creates ocean surface roughness heterogeneity, in the form of alternating rough/smooth bands. In this study, we investigate the sensitivity of the modulation effect to internal wave properties and develop sea states using simulations of individual internal wave solitons. We utilize a phased‐resolved two‐layer fluid model to capture the evolution of surface waves deterministically. We conduct extensive simulations with a wide range of parameters, including fluid layer density ratio, internal wave amplitude, and parametric wind speed. We use the ratio of the mean surface slope between the rough and smooth bands, which are identified in the simulated surface wave field, to systematically investigate their response to the internal wave forcing across all our simulation cases. Our results show that, among the internal wave parameters, the upper‐lower layer density ratio causes the strongest surface heterogeneity. Spectral analysis of the surface wave elevation and slope variance reveals that the wavenumbers above the peak are most impacted. We demonstrate that accounting for the internal wave‐induced modulation requires including wave steepness statistics, for example, when modeling air‐sea exchange using a surface roughness, z0, parameter. Currently, these statistics are not included in typically coupled modeling schemes, and these systems cannot account for the impact of internal waves, even if the solitary wave phenomena are resolved.

Group resonant interactions between surface and internal gravity waves in a two-layer system

Nonlinear interactions between surface and internal gravity waves in a two-layer system are studied using explicit second-order nonlinear evolution equations in Hamiltonian form. Motivated by the detailed experiment of Lewis, Lake & Ko (J. Fluid Mech., vol. 63, 1974, pp. 773–800), our focus is on surface wave modulation by the group resonance mechanism that corresponds to near-resonant triad interactions between a long internal wave and short surface waves. Our numerical solutions show good agreement with laboratory measurements of the local wave amplitude and slope, and confirm that the surface modulation becomes significant when the group velocity of the surface waves matches the phase speed of the internal wave, as the linear modulation theory predicts. It is shown, however, that, after the envelope amplitude is increased sufficiently, the surface and internal waves start to exchange energy through near-resonant triad interactions, which is found to be crucial to accurately describe the long-term surface wave modulation by an internal wave. The reduced amplitude equations are also adopted to validate this observation. For oceanic applications, numerical solutions are presented for a density ratio close to one and it is found that significant energy exchanges occur through primary and successive resonant triad interactions.

Reflectionless magnetic-to-optical differential modulator based on gyroelectric material

In this work, we present a theoretical scheme to create a reflectionless magnetic-to-optical differential modulator constructed with two mirror symmetrical star structures of three interfaces formed of two gyroelectric materials and a bulk of metal. Taking advantage of two star structures, we receive the modulated signal through coherent superposition of two one-way surface waves. The modulator we have designed can realize the modulation of surface wave in response to magnetic signal. Reflected waves affecting the surrounding devices are prohibited. In addition, the modulator is mini-sized with simple construction and can detect the magnetic signal in an integrated optical system.

Strain-induced modulation of near-field radiative transfer.

In this theoretical study, we present a near-field thermal modulator that exhibits change in radiative heat transfer when subjected to mechanical stress/strain. The device has two terminals at different temperatures separated by vacuum: one fixed and one stretchable. The stretchable side contains one-dimensional grating. When subjected to mechanical strain, the effective optical properties of the stretchable side are affected upon deformation of the grating. This results in modulation of surface waves across the interfaces influencing near-field radiative heat transfer. We show that for a separation of 100 nm, it is possible to achieve 25% change in radiative heat transfer for a strain of 10%.

Refraction and phase modulation of surface electromagnetic waves propagating at the boundary of a one-dimensional photonic crystal

Refraction, reflection, and phase modulation of surface electromagnetic waves propagating along the interface of a one-dimensional dielectric photonic crystal and homogeneous medium are studied theoretically and numerically. Surface waves are refracted at dielectric steps positioned directly on the surface of their propagation (on the surface of the photonic crystal). Phase modulation is implemented due to varying the geometric parameters of the steps. Refraction and phase modulation are studied at an arbitrary angle of incidence. It has been found that phase modulation and refraction of surface waves at certain parameters are well described by an analytical model based on Fresnel formulas for a plane electromagnetic wave. At the same time, refraction and phase modulation of a surface wave can significantly differ from the behavior of plane wave and have some features of interest. The obtained results can be applied in developing elements of “two-dimensional” optics for surface electromagnetic waves.

Owen Martin Phillips

Owen Martin Phillips

Exposure of internal waves on the sea surface

We theoretically analyse the impact of subsurface currents induced by internal waves on nonlinear Stokes surface waves. We present analytical and numerical solutions of the modulation equations under conditions that are close to group velocity resonance. Our results show that smoothing of the downcurrent surface waves is accompanied by a relatively high-frequency modulation, while the profile of the opposing current is reproduced by the surface wave's envelope. We confirm the possibility of generating an internal wave forerunner that is a modulated surface wave packet. Long surface waves can create such a wave modulation forerunner ahead of the internal wave, while other relatively short surface waves comprise the trace of the internal wave itself. Modulation of surface waves by a periodic internal wavetrain may exhibit a characteristic period that is less than the internal wave period. This period can be non-uniform while the wave crosses the current zone. Our results confirm that surface wave excitation by means of internal waves, as observed at their group resonance frequencies, is efficient only in the context of opposing currents.

Application of “bull's eye” corrugated grooves integrated with artificially soft surfaces structure in the patch antenna to improve radiation performance

AbstractIn this article, a Bull's eye corrugated structure integrated with artificially soft surfaces structure is proposed as the ground plane in the patch antenna. This structure is based on the modulation of surface wave by the bull's eye corrugated structure and the suppression of surplus surface wave at grounded edge by artificially soft surfaces structure. A numerical investigation for the patch antenna surrounded with combined corrugated grooves ground is presented using finite difference time domain method. The gain of the patch antenna is significantly improved by 12.3 dB, and half power beamwidth in the E and H‐plane are dramatically suppressed. The backward radiation is also reduced by 10 dB, which greatly enhances the Front to Backward (F/B) radiation ratio. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 1676–1679, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24443

Grooves-Assisted Surface Wave Modulation in Two-Slot Array for Mutual Coupling Reduction and Gain Enhancement

In this letter, a method of reducing mutual coupling between the rectangular waveguide end-slot array and improving its gain is presented. In this antenna, the novel surface corrugated structure is symmetrically loaded at both sides of each slot element. Different from the suppression of surface wave in the choke grooves, the proposed corrugated structure would modulate surface wave propagation, resulting in the variation of surface energy strength distribution. The effectiveness of the proposed surface grooves is verified by numerical simulations and experimental testing. It is found that the mutual coupling level between two slot elements is observably decreased to be less than -40 dB after employment of surface grooves, and the gain of two slots array is also greatly enhanced by 4 dB. Furthermore, the influences of groove parameters on the mutual coupling level and radiation performance are discussed, respectively, and physical mechanism of this corrugated structure is investigated and compared to that of choke grooves as well.

The Rectangular Waveguide Board Wall Slot Array Antenna Integrated with One Dimensional Subwavelength Periodic Corrugated Grooves and Artificially Soft Surface Structure

In this letter, we propose a high gain rectangular waveguide board wall slot array antenna, which is integrated with one dimensional subwavelength periodic corrugated grooves and artificially soft surface structure. The corresponding far field radiation characteristics are investigated. The simulation results show that the gain of planar 2 x 8 slot array antenna with the introduction of corrugated grooves structure is increased to 26.1 dB, and half power beamwidth in the E plane is considerably reduced. Compared with the subwavelength periodic grooves slits array antenna proposed by Huang et al.[Appl. Phys. Lett. 91, 143512 (2007)], the side and back lobe level of this antenna are also significantly reduced by 6 dB and 10 dB, respectively. The physical mechanism for radiation-pattern improvement has been well explained by the modulation of surface wave, the reradiation of surface energy, and suppression of surplus surface wave at the grounded edge.

Internal and surface wave effects on inverted echo sounder measurements

Inverted echo-sounders operating at 10 kHz, long used to observe mesoscale synoptic variability from the sea-floor through changes in acoustic travel time and pressure, were recently deployed in the South China Sea to explore their potential for detecting nonlinear internal waves. The observations, based on a 6-s transmission rate, reveal both the internal tide and high-frequency nonlinear internal wave packets and are consistent with remotely sensed images and nearby temperature time series. The results motivate exploration of a refinement in instrument operation in which the raw signal is digitized and stored at the full transducer bandwidth, rather than the present scheme of narrow band filtering and threshold detection. If the raw signal is available, it will be related to the surface wave field, leading to the prospect of ocean surface environmental measurements from the sea-floor. An intriguing possibility is that internal wave modulation of the surface wave field, so evident in remotely sensed images, including the consequence of surface wave breaking in convergence zones, might be detectable in this way from the ocean floor, thus linking internal wave observations directly with surface wave modulation. Model calculations of surface scatter are used to gain insight on this approach.

On the cascade mechanism of short surface wave modulation

Abstract. Modulation of short surface ripples by long surface or internal waves by a cascade mechanism is considered. At the first stage, the orbital velocity of the long wave (LW) adiabatically modulates an intermediate length nonlinear gravity wave (GW), which generates a bound (parasitic) capillary wave (CW) near its crest in a wide spatial frequency band. Due to strong dependence of the CW amplitude on that of the GW, the resulting ripple modulation by LW can be strong. Adiabatic modulation at the first stage is calculated for an arbitrarily strong LW current. The CWs are calculated based on the Lonquet-Higgins theory, in the framework of a steady periodic solution, which proves to be sufficient for the cases considered. Theoretical results are compared with data from laboratory experiments. A discussion of related sea clutter data is given in the conclusion.

Role of surface films in ERS SAR signatures of internal waves on the shelf: 1. Short‐period internal waves

Analysis of synthetic aperture radar (SAR) images of internal waves (IWs) on the Iberian shelf and classification of the IW surface signatures is presented. There are three types of IW signatures in the form of bright/dark, dark, and bright bands that correspond to positive/negative, negative, or positive variations of radar backscatter, respectively. Strong positive sign IW signatures occur at very low wind velocities (less than 2m/s). At winds higher than 2 m/s both positive/negative and negative signatures are observed for range‐propagating IWs. For azimuth‐propagating IWs, negative signatures prevail. Two groups of experiments carried out in the frame of the Multidisciplinary Oceanographic Research in the Eastern North Atlantic (MORENA) project on the Iberian shelf in August 1994 are described: (1) IW measurements simultaneous with ERS‐1 SAR overpasses and (2) IW measurements, visual observations of the surface, and sampling of films from the sea surface. IW manifestations at low to moderate winds took the form of slicks locating over IW troughs; at the near‐threshold wind velocity they were of the form of intensified decimeter‐scale waves (antislicks) located over IW crests. Measurements of wave damping due to films collected from the slick and nonslick areas showed the concentration of surfactants and retrieved film elasticity to be higher in the slicks than in the nonslick areas. A theoretical model of the surface wave modulation by IWs is developed to include surface wave straining by the IW current and surface wave damping due to surfactant films. Pressure‐area curves for real marine films are used in the model. The relation between the film and the straining effects on the surface wave spectrum in the IW field is shown to depend on film parameters, surface wave‐length, and wind velocity. The model predicts strong damping of centimeter‐scale waves over IW troughs mainly due to the film effect and intensification and depression of decimeter‐scale waves due to both straining and film effects. Dependencies of the model spectrum variations as a function of wind velocity for range and azimuth propagating IWs are obtained, the negative sign contrast being shown to predominate for azimuth propagating IWs. Model variations of the spectrum of decimeter‐scale waves are shown to be significantly sensitive to surfactants (film pressure), the negative sign contrasts due to films being superimposed on the straining effects, and these are eventually capable of suppressing the positive sign contrasts. It is concluded that the occurrence of the different classes of IW signatures in the SAR images can be explained by the action of surface‐active films.

Parametric and modulation instabilities of magnetostatic surface spin waves in ferromagnetic films

First-order parametric instability, modulation instability and second-order instability processes for magnetostatic surface waves propagating in a pure yttrium iron garnet film are investigated in accordance with the relationship between the magnetostatic wave frequency and the internal magnetic field. The transitions from the second-order parametric instability, via the modulation instability, to the first-order parametric instability (when the magnetic field is 720 Oe and the carrier frequency varies from 3.87 to 4.39 GHz), and the transitions from the first-order parametric instability to the second-order parametric instability via the modulation instability (when the carrier frequency is 3.5 GHz and the magnetic field varies from 427 to 647 Oe) are experimentally observed and explained. Dispersion characteristics of the magnetostatic surface waves are modified by introduction of a metallic ground plane in order that the Lighthill criterion, as one of the prerequisites for onset of the modulation instability, is fulfilled in a limited frequency range. Characteristics of magnetostatic surface wave modulation instability are studied by solving the nonlinear Schrödinger equation. Dependencies of the modulation frequency induced by the modulation instability on the carrier frequency and the magnetic field are found to be consistent with experimental data.

High‐resolution measurements made across a tidal intrusion front

High spatial resolution current and hydrographic measurements have been made across a tidal intrusion front in the James River estuary, Virginia, during the first few hours of flood. The current measurements were made with a surface‐towed acoustic Doppler current profiler which was used in both profile and side‐scan configurations. The profile data show a 30–50 cm s−1 inflow of water from the Chesapeake Bay, a time‐evolving across‐front density gradient, and downward velocity beneath the front of about 15 cm s−1. The side‐scan observations directly measure the across‐front change in current and provide a view of the three‐dimensional character of the frontal interface. The near‐surface horizontal strain rate, a critical parameter for understanding the modulation of surface waves across the front, is estimated to be about 0.04 s−1. Using Phillips' [1984] theory as a guide, fronts with such large strain rates as these should be discernible in imagery from the ERS 1 C band synthetic aperture radar for wind speeds of ≤4 m s−1.

The Coupling of Surface and Internal Gravity Waves: Revisited

A new investigation is made of internal wave generation by surface waves. Previous theories are put into a unified form that includes a model of surface wave damping. Calculations using the complete theory, which do not seem to have been made previously, indicate that for wind speeds between 7 and 20 m s−1 the internal wave field loses about 10−4 W m−2 to the surface wave field. This would lead to a decay time of about 10 days for the high frequency portion of the internal wave field if an energy source were not available to maintain it. Possible sources for this energy are discussed. In contrast to this result for wind waves, a strong highly collimated ocean swell can lead to rapid growth of high frequency internal waves. The effects of nonlinear surface wave modulation and wave blocking are also discussed.

Short surface waves with growth and dissipation

The modulation of surface waves by larger‐scale flows is central to many forms of remote imaging of the sea surface yet is an incompletely understood process. An excellent example is the modulation of short waves by long ones (e.g., by swell). Observed modulations are of the order of 10 times the long‐wave steepness, decrease with both the long‐wave frequency and windspeed, and have stable phase, with maximum short‐wave amplitudes just forward of the long‐wave crests. Assuming a balance between propagation, growth, and drift‐enhanced dissipation, the short‐wave modulations arise owing to (1) direct straining by the long‐wave orbital motion, (2) straining of the wind drift layer (modulating the dissipation rate), (3) variations of the apparent gravity in the short‐wave frame, and (4) induced variations of the applied wind stress (affecting both the short‐wave growth and the drift‐enhanced dissipation). As modeled here, only the last (stress variation) can reproduce the observed wind speed and frequency dependence. In addition, the drift layer and the short waves are closely matched, suggesting direct coupling. The implied fractional modulation of stress is of the order of 20 times the long‐wave steepness.

On the interaction of internal waves and surface gravity waves

The perturbation of pre-existing surface gravity waves caused by the presence of an internal wave was studied both experimentally and analytically. An extensive series of experiments was performed, and quantitative results were obtained for the one-dimensional monochromatic interaction of internal waves and surface gravity waves. Internal wave-induced surface slope, amplitude and wavenumber modulations were measured for a wide range of interaction conditions. A complementary theoretical analysis, based on the conservation approach of Whitham (1962) and Longuet-Higgins & Stewart (1960,1961), was performed and a closed form solution obtained for the one-dimensional wave interaction. Both the theory and the experiment demonstrate that the effect increases with interaction distance. The maximum interaction effect is found to occur when the phase speed of the internal wave and the group velocity of the surface wave are matched. The phase of the internal wave at which maximum surface-wave modulation occurs is found to be a sensitive and continuous function of the relative wave speeds. The experimental data are in good agreement with the present theoretical analysis.