Showing papers on "Reflection (physics) published in 2015"

A Reflection on Nonlinear Oscillations, Dynamical Systems, and Bifurcations of Vector Fields

Abstract: Contents: Introduction: Differential Equations and Dynamical Systems.- An Introduction to Chaos: Four Examples.- Local Bifurcations.- Averaging and Perturbation from a Geometric Viewpoint.- Hyperbolic Sets, Sympolic Dynamics, and Strange Attractors.- Global Bifurcations.- Local Codimension Two Bifurcations of Flows.- Appendix: Suggestions for Further Reading. Postscript Added at Second Printing. Glossary. References. Index.

Lessons learned from ocean acidification research

Abstract: Reflection on the rapidly growing field of ocean acidification research highlights priorities for future research on the changing ocean.

Anomalous Terahertz Reflection and Scattering by Flexible and Conformal Coding Metamaterials

Abstract: Arbitrary control of terahertz (THz) waves remains a significant challenge although it promises many important applications. Here, a method to tailor the reflection and scattering of THz waves in an anomalous manner by using 1-bit coding metamaterials is presented. Specific coding sequences result in various THz far-field reflection and scattering patterns, ranging from a single beam to two, three, and numerous beams, which depart obviously from the ordinary Snell's law of reflection. By optimizing the coding sequences, a wideband THz thin film metamaterial with extremely low specular reflection, due to the scattering of the incident wave into various directions, is demonstrated. As a result, the reflection from a flat and flexible metamaterial can be nearly uniformly distributed in the half space with small intensity at each specific direction, manifesting a diffuse reflection from a rough surface. Both simulation and experimental results show that a reflectivity less than −10 dB is achieved over a wide frequency range from 0.8 to 1.4 THz, and it is insensitive to the polarization of the incident wave. This work reveals new opportunities arising from coding metamaterials in effective manipulation of THz wave propagation and may offer widespread applications.

Functional metamirrors using bianisotropic elements.

Abstract: Conventional mirrors obey the simple reflection law that a plane wave is reflected as a plane wave, at the same angle. To engineer spatial distributions of fields reflected from a mirror, one can either shape the reflector or position some phase-correcting elements on top of a mirror surface. Here we show, both theoretically and experimentally, that full-power reflection with general control over the reflected wave phase is possible with a single-layer array of deeply subwavelength inclusions. These proposed artificial surfaces, metamirrors, provide various functions of shaped or nonuniform reflectors without utilizing any mirror. This can be achieved only if the forward and backward scattering of the inclusions in the array can be engineered independently, and we prove that it is possible using electrically and magnetically polarizable inclusions. The proposed subwavelength inclusions possess desired reflecting properties at the operational frequency band, while at other frequencies the array is practically transparent. The metamirror concept leads to a variety of applications over the entire electromagnetic spectrum, such as optically transparent focusing antennas for satellites, multifrequency reflector antennas for radio astronomy, low-profile conformal antennas for telecommunications, and nanoreflectarray antennas for integrated optics.

Metasurfaces for general transformations of electromagnetic fields.

Abstract: In this review paper I discuss electrically thin composite layers, designed to perform desired operations on applied electromagnetic fields. Starting from a historical overview and based on a general classification of metasurfaces, I give an overview of possible functionalities of the most general linear metasurfaces. The review is concluded with a short research outlook discussion.

Diffuse Reflectance Spectroscopy

Abstract: The appearance of a soil results from the interaction of its different constituents with incident light. Color and various other attributes of the appearance of soil are highly sensitive to the nature, proportion, particle size and morphology, and spatial association of its mineral and organic components. In fact, color has been used for more than 75 yr to obtain information about these soil properties with a goal of characterizing and distinguishing soil types. The demand for a standardized method to describe soil color was met by the adoption of the Munsell notation by the USA Soil Survey Program in 1949 and, about 10 yr later, by the International Society of Soil Science (Simonson, 1993). Since then, Munsell Soil Color Charts (Munsell Color, 1975) have been systematically used by pedologists. Visual estimation of soil color, however, is subject to substantial error due to various psychophysical and physical factors. For this reason, the use of colorimeters and spectrophotometers has gained widespread acceptance among soil scientists as a means to measure color accurately and precisely. Moreover, different types of spectrophotometers afford the elucidation of the spectrum of light reflected by a soil illuminated in various ways. Reflectance, which is the base quantity that characterizes the process of reflection, is defined as the ratio of the reflected radiant flux (or power) to the incident radiant flux (or power) (Wyszecki and Stiles, 1982). Generally, the reflectance of a soil at any wavelength λ can be considered to be the sum of two components: regular (or specular, or mirror) reflectance and diffuse (or volume or nondirectional) reflectance (defined in more detail below). Reflectance measurements in the field are usually made on relatively large areas (>10 cm2). Under these conditions, both specular and diffuse reflectance usually contribute to the total reflectance of a soil surface, the magnitude of which depends on particle size, structure, microrelief, and other properties that define the “surface state” (Escadafal, 1989). In contrast, laboratory measurements of soil reflectance are usually made on small areas (<10 cm2) of disturbed soil materials that are usually sieved or ground to a small size. In this case diffuse reflectance predominates, which depends mainly on soil composition. This chapter describes available laboratory methods for recording diffuse reflectance spectra for soil materials and ways to handle the information to identify and characterize soil minerals. Only the visible and narrow vicinal ultraviolet (UV) and infrared (IR) regions of the spectrum are considered here because reflectance in the IR region is the subject of another chapter (Johnston and Aochi, 1996)).

PT-symmetric acoustics

Abstract: The concept of acoustic parity-time (PT) symmetry is introduced and used for the study of extraordinary scattering behavior in acoustic PT-symmetric media consist of loss and gain units. The analytical study of acoustic PT-symmetric media shows that these media can be designed to achieve unidirectional transparency at specific frequencies named exceptional points (EPs). This unidirectional transparency occurs at the EPs is due to the asymmetrical arrangement of the periodic loss and gain units that results in different Bragg scatterings on the two sides of the PT-symmetric media. A close look at the phases of the reflections on both sides reveals a sudden jump of the reflection phase on one side at the EPs. This step-function like behavior causes an infinite delay time of the reflected wave on that side, and hence the media become reflectionless in that direction. Combining the concept of acoustic PT-symmetry with transformation acoustics, we design a two-dimensional acoustic cloak that is invisible in a prescribed direction. This kind of directional cloak is important especially for military use since a target object is hidden from the enemy in front can still be identified by friendly at the back. Other useful applications such as directional acoustic imaging, noise cancellation, architectural acoustics, acoustic amplification, etc., can also be developed.

Dynamic control of electromagnetic wave propagation with the equivalent principle inspired tunable metasurface

Abstract: Transmission and reflection are two fundamental properties of the electromagnetic wave propagation through obstacles. Full control of both the magnitude and phase of the transmission and reflection independently are important issue for free manipulation of electromagnetic wave propagation. Here we employed the equivalent principle, one fundamental theorem of electromagnetics, to analyze the required surface electric and magnetic impedances of a passive metasurface to produce either arbitrary transmission magnitude and phase or arbitrary reflection magnitude and phase. Based on the analysis, a tunable metasurface is proposed. It is shown that the transmission phase can be tuned by 360° with the unity transmissivity or the transmissivity can be tuned from 0 to 1 while the transmission phase is kept around 0°. The reflection magnitude and phase can also been tuned similarly with the proposed metasurface. The proposed design may have many potential applications, such as the dynamic EM beam forming and scanning.

Vector-based elastic reverse time migration

Abstract: Prestack elastic reverse time migration (RTM) of multicomponent seismic data requires separating PP and PS reflections before, or as part of, applying the image condition, and using image conditions that preserve the angle and amplitude information. Both of these requirements are best achieved when all operations are on vectors. We have created a new 2D migration context for isotropic, elastic RTM, which included decomposition of the elastic source and receiver wavefields into P- and S-wave vectors by decoupled elastodynamic extrapolation, which retained the same stress and particle velocity components as the input data. Then, the propagation directions of the incident and reflected P- and S-waves were calculated directly from the stress and particle velocity definitions of the P- and S-wave Poynting vectors. An excitation-amplitude image condition that scaled the receiver wavelet by the source vector magnitude produced angle-dependent images of PP and PS reflection coefficients with the correct p...

Broadband reflectionless metasheets: Frequency-selective transmission and perfect absorption

Abstract: Energy of propagating electromagnetic waves can be fully absorbed in a thin lossy layer, but only in a narrow frequency band, as follows from the causality principle. On the other hand, it appears that there are no fundamental limitations on broadband matching of thin absorbing layers. However, known thin absorbers produce significant reflections outside of the resonant absorption band. In this paper we explore possibilities to realize a thin absorbing layer which produces no reflected waves in a very wide frequency range, while the transmission coefficient has a narrow peak of full absorption. Here we show, both theoretically and experimentally, that a wide-band-matched thin resonant absorber, invisible in reflection, can be realized if one and the same resonant mode of the absorbing array unit cells is utilized to create both electric and magnetic responses. We test this concept using chiral particles in each unit cells, arranged in a periodic planar racemic array, utilizing chirality coupling in each unit cell but compensating the field coupling at the macroscopic level. We prove that the concept and the proposed realization approach also can be used to create non-reflecting layers for full control of transmitted fields. Our results can have a broad range of potential applications over the entire electromagnetic spectrum including, for example, perfect ultra-compact wave filters and selective multi-frequency sensors.

Studying atmosphere-dominated hot jupiter kepler phase curves: evidence that inhomogeneous atmospheric reflection is common

Abstract: We identify three Kepler transiting planets, Kepler-7b, Kepler-12b, and Kepler-41b, whose orbital phase-folded light curves are dominated by planetary atmospheric processes including thermal emission and reflected light, while the impact of non-atmospheric (i.e., gravitational) processes, including beaming (Doppler boosting) and tidal ellipsoidal distortion, is negligible. Therefore, those systems allow a direct view of their atmospheres without being hampered by the approximations used in the inclusion of both atmospheric and non-atmospheric processes when modeling the phase-curve shape. We present here the analysis of Kepler-12b and Kepler-41b atmosphere based on their Kepler phase curve, while the analysis of Kepler-7b was already presented elsewhere. The model we used efficiently computes reflection and thermal emission contributions to the phase curve, including inhomogeneous atmospheric reflection due to longitudinally varying cloud coverage. We confirm Kepler-12b and Kepler-41b show a westward phase shift between the brightest region on the planetary surface and the substellar point, similar to Kepler-7b. We find that reflective clouds located on the west side of the substellar point can explain the phase shift. The existence of inhomogeneous atmospheric reflection in all three of our targets, selected due to their atmosphere-dominated Kepler phase curve, suggests this phenomenon is common. Therefore, it is also likely to be present in planetary phase curves that do not allow a direct view of the planetary atmosphere as they contain additional orbital processes. We discuss the implications of a bright-spot shift on the analysis of phase curves where both atmospheric and gravitational processes appear, including the mass discrepancy seen in some cases between the companion's mass derived from the beaming and ellipsoidal photometric amplitudes. Finally, we discuss the potential detection of non-transiting but otherwise similar planets, whose mass is too small to show a gravitational photometric signal, but their atmosphere is reflective enough to show detectable phase modulations.

What is the Temporal Analog of Reflection and Refraction of Optical Beams

Abstract: It is shown numerically and analytically that when an optical pulse approaches a moving temporal boundary across which the refractive index changes, it undergoes a temporal equivalent of reflection and refraction of optical beams at a spatial boundary. The main difference is that the role of angles is played by changes in the frequency. The frequency dependence of the dispersion of the material in which the pulse is propagating plays a fundamental role in determining the frequency shifts experienced by the reflected and refracted pulses. Our analytic expressions for these frequency shifts allow us to find the condition under which an analog of total internal reflection may occur at the temporal boundary.

Terahertz wavefront control by tunable metasurface made of graphene ribbons

Abstract: We propose a tunable metasurface consisting of an array of graphene ribbons on a silver mirror with a SiO2 gap layer to control reflected wavefront at terahertz frequencies The graphene ribbons exhibit localized plasmon resonances depending on their Fermi levels to introduce abrupt phase shifts along the metasurface With interference of the Fabry-Perot resonances in the SiO2 layer, phase shift through the system is largely accumulated, covering the 0-to-2π range for full control of the wavefront Numerical simulations prove that wide-angle beam steering up to 53° with a high reflection efficiency of 60% is achieved at 5 THz within a switching time shorter than 06 ps

Facile synthesis of a CNT@Fe@SiO2 ternary composite with enhanced microwave absorption performance

Abstract: Magnetic/dielectric core–shell structures have been regarded as ideal high-performance electromagnetic absorption materials due to their novel multiple-loss mechanism. However, the poor impedance matching property of a dielectric shell may lead to the high reflection of electromagnetic waves from the interface of the shell. Thus, we ingeniously use the magnetic material as the shell while the dielectric material is used as the core. Such a change not only decreases the electromagnetic wave reflection, but also causes a strong interface polarization. Subsequently, the wave-transparent material SiO2 was further coated on the surface of the magnetic shell which not only protected it from oxidation but also increased the impedance matching performance. Based on the above design, in this study, we fabricated a CNT@Fe@SiO2 ternary core–shell structure composite using a simple two-step approach consisting of pyrolysis and decomposition processes. As compared with pure CNTs and CNT@Fe materials, the obtained CNT@Fe@SiO2 composite shows obviously enhanced electromagnetic absorption properties. In particular at a thin thickness of 1.5 mm, the optimal reflection loss value is as high as −14.2 dB which is better than most of the reported CNT based absorbers. The improved electromagnetic absorption properties can be attributed to the perfect impedance matching behavior and the multiple interface polarization effect.

Critical coupling to Tamm plasmons

Abstract: The conditions of critical coupling of light to Tamm plasmons are investigated with comprehensive numerical simulations, highlighting the parameters that maximize absorption of incident light in the metal layer. The asymmetric response in reflection and absorption with respect to the direction of incidence is discussed, the two cases yielding different optimal coupling conditions. These findings are relevant for the design of optimized Tamm structures, particularly in applications such as narrow-band thermal emitters, field-enhanced spectroscopy and refractive-index sensing.

Oblique Wave Trapping by Porous Structures Near a Wall

Abstract: The current study deals with the oblique wave trapping by bottom-standing and surface-piercing porous structures of finite width placed at a finite distance from a vertical rigid wall Using the Sollitt and Cross model for wave motion within the porous structure, the problems are analyzed based on the small-amplitude water wave theory in water of finite depth The solutions of the associated boundary value problems are obtained analytically using the eigenfunction expansion method and numerically using a multidomain boundary-element method In the boundary-element method, the boundary value problems are converted into integral equations over the physical boundaries The physical boundaries are discretized into a finite number of elements to obtain a system of linear algebraic equations Various aspects of structural configurations, in trapping surface gravity waves, are analyzed from the computed results on the reflection coefficients and the hydrodynamic forces Suitable arrangements of the rigid

Dynamically tunable electromagnetically induced reflection in terahertz complementary graphene metamaterials

Abstract: We presented a dynamically tunable electromagnetically induced reflection (EIR) based on the complementary graphene metamaterials composed of the wire-slot and split-ring resonators slot (SRRs-slot) array structures for the terahertz region. In this structure, the dark mode excited by the near field coupling between wire-slot and SRRs-slot structures, induces a reflection window. Moreover, the reflection window can be actively controlled by varying the lateral displacement between two slot-type resonant structures or Fermi energy of graphene without reoptimizing and re-fabricating structure. In addition, the large positive group delay obtained within the reflection peak can be also tuned over a broad terahertz region by changing the Fermi energy of graphene. Therefore, the work opens up the possibility for the development of compact elements such as modulators, tunable sensor, switches and slow light devices.

Graphene-Based Ultrathin Flat Lenses

Abstract: Flat lenses when compared to curved surface lenses have the advantages of being aberration free, and they offer a compact design necessary for a myriad of electro-optical applications. In this paper we present flat and ultrathin lenses based on graphene, the world’s thinnest known material. Monolayers and multilayers of graphene were fabricated into Fresnel zones to produce Fresnel zone plates, which utilize the reflection and transmission properties of graphene for their operation. The working of the lenses and their performance in the visible and terahertz regimes were analyzed computationally. Experimental measurements were also performed to characterize the lens in the visible regime, and a good agreement was obtained with the simulations. This work demonstrates the principle of atom-thick graphene-based lenses, with perspectives for ultracompact integration.

The NuSTAR extragalactic survey: First direct measurements of the ≳10 keV X-ray luminosity function for active galactic nuclei at z > 0.1

Abstract: We present the first direct measurements of the rest-frame 10–40 keV X-ray luminosity function (XLF) of active galactic nuclei (AGNs) based on a sample of 94 sources at 0.1 < z < 3, selected at 8–24 keV energies from sources in the Nuclear Spectroscopic Telescope Array (NuSTAR) extragalactic survey program. Our results are consistent with the strong evolution of the AGN population seen in prior, lower-energy studies of the XLF. However, different models of the intrinsic distribution of absorption, which are used to correct for selection biases, give significantly different predictions for the total number of sources in our sample, leading to small, systematic differences in our binned estimates of the XLF. Adopting a model with a lower intrinsic fraction of Compton-thick sources and a larger population of sources with column densities N_H ~ 10^(23-24) cm^(−2) or a model with stronger Compton reflection component (with a relative normalization of R ~ 2 at all luminosities) can bring extrapolations of the XLF from 2–10 keV into agreement with our NuSTAR sample. Ultimately, X-ray spectral analysis of the NuSTAR sources is required to break this degeneracy between the distribution of absorbing column densities and the strength of the Compton reflection component and thus refine our measurements of the XLF. Furthermore, the models that successfully describe the high-redshift population seen by NuSTAR tend to over-predict previous, high-energy measurements of the local XLF, indicating that there is evolution of the AGN population that is not fully captured by the current models.

Extremely Thin Dielectric Metasurface for Carpet Cloaking

Abstract: We demonstrate a novel and simple approach to cloaking a scatterer on a ground plane. We use an extremely thin dielectric metasurface ({\lambda}/12) to reshape the wavefronts distorted by a scatterer in order to mimic the reflection pattern of a flat ground plane. To achieve such carpet cloaking, the reflection angle has to be equal to the incident angle everywhere on the scatterer. We use a graded metasurface and calculate the required phase gradient to achieve cloaking. Our metasurface locally provides additional phase to the wavefronts to compensate for the phase difference amongst light paths induced by the geometrical distortion. We design our metasurface in the microwave range using highly sub-wavelength dielectric resonators. We verify our design by full-wave time-domain simulations using micro-structured resonators and show that results match theory very well. This approach can be applied to hide any scatterer on a ground plane not only at microwave frequencies, but also at higher frequencies up to the near infrared.

Reflections of Electromagnetic Waves Obliquely Incident on a Multilayer Stealth Structure With Plasma and Radar Absorbing Material

Abstract: To overcome some drawbacks of the plasma stealth technology in real-life application, a practical multilayer stealth structure composed of enclosed plasma slab and radar absorbing material (RAM) is presented in this paper. Based on a technique referred to as the transmission line analogy method, reflection coefficients of the perpendicularly polarized wave, the parallel polarized wave, and the circularly polarized wave obliquely incident upon this multilayer structure are determined, respectively. The effects of the incident angle, kinds of RAMs, and parameters of the plasma slab including electron density, collision frequency, and thickness on the stealthy effectiveness of this composite stealth structure have been studied systematically. The numerical results indicate that by a proper design, the power of reflected wave is significantly reduced over a wide frequency bandwidth, which provides some useful references to the plasma stealth technology applied to aircrafts, ships, and missiles.

Application of laser ultrasonic technique for non-contact detection of structural surface-breaking cracks

Abstract: Based on the finite element method (FEM), the surface-breaking cracks have been investigated by using the laser-generated Rayleigh wave. The features of laser-generated Rayleigh wave interaction with cracks are analyzed in time and frequency domain. The simulation results show that the surface acoustic wave induced by the pulsed laser is sensitive to the surface-breaking cracks. As the crack depth increases, the transmission coefficients almost linearly decrease and the reflection coefficients show a dip. The corresponding experimental results have verified the feasibility of numerical calculation and reached a good agreement with simulation results. The research findings would provide a potential application for testing surface-breaking cracks of aircraft parts.

On the Numerical Simulation of Metasurfaces With Impedance Boundary Condition Integral Equations

Abstract: Metasurfaces are thin metamaterial layers characterized by unusual dispersion properties of surface/guided wave and/or reflection properties of otherwise incident plane waves. At the scales intervening in their design, metasurfaces can be described through a surface impedance boundary condition. The impedance, possibly tensorial, is often “modulated,” i.e., it can vary from place to place on the surface (by design). We investigate on different integral equation formulations of the problem, with special attention to the stability properties of the resulting system matrix.

When are Surface Plasmon Polaritons Excited in the Kretschmann-Raether Configuration?

Abstract: It is widely believed that the reflection minimum in a Kretschmann-Raether experiment results from direct coupling into surface plasmon polariton modes. Our experimental results provide a surprising discrepancy between the leakage radiation patterns of surface plasmon polaritons (SPPs) launched on a layered gold/germanium film compared to the K-R minimum, clearly challenging this belief. We provide definitive evidence that the reflectance dip in K-R experiments does not correlate with excitation of an SPP mode, but rather corresponds to a particular type of perfectly absorbing (PA) mode. Results from rigorous electrodynamics simulations show that the PA mode can only exist under external driving, whereas the SPP can exist in regions free from direct interaction with the driving field. These simulations show that it is possible to indirectly excite propagating SPPs guided by the reflectance minimum in a K-R experiment, but demonstrate the efficiency can be lower by more than a factor of 3. We find that optimal coupling into the SPP can be guided by the square magnitude of the Fresnel transmission amplitude.

A New Direct Retrieval Method of Refractive Index for the Metamaterial

Abstract: With the advent of metamaterial the approaches of automated extraction of effective parameters for a metamaterial have recently attracted considerable attention among researchers. Evaluation of refractive index has received huge importance especially for the left-handed metamaterial. This communication presents a new simplified equation for the direct retrieval of refractive index from the transmission and reflection co-efficient. Refractive index is calculated from the new equation for two recently published metamaterials. The resultant curve for the proposed method shows good conformity with the classical NRW-method and shows a result closer to that achieved using the TR-method. In addition, a comparative study of the three methods is presented.

Gate-controlled mid-infrared light bending with aperiodic graphene nanoribbons array.

Abstract: Graphene plasmonic nanostructures enable subwavelength confinement of electromagnetic energy from the mid-infrared down to the terahertz frequencies. By exploiting the spectrally varying light scattering phase at the vicinity of the resonant frequency of the plasmonic nanostructure, it is possible to control the angle of reflection of an incoming light beam. We demonstrate, through full-wave electromagnetic simulations based on Maxwell equations, the electrical control of the angle of reflection of a mid-infrared light beam by using an aperiodic array of graphene nanoribbons, whose widths are engineered to produce a spatially varying reflection phase profile that allows for the construction of a far-field collimated beam towards a predefined direction.

Localized photonic jets from flat, three-dimensional dielectric cuboids in the reflection mode.

Abstract: A photonic jet (a terajet at terahertz frequencies) commonly denotes a specific, spatially localized region in the near field on the front side of a dielectric particle with a diameter comparable with the wavelength illuminated by a plane wave on its back side (i.e., the jet emerges from the shadow surface of a dielectric particle). In this Letter, the formation of a photonic jet is demonstrated using the recently proposed three-dimensional (3D) dielectric cuboids working in the "reflection" mode when the specific, spatially localized region is localized in the direction of the incident wavefront. The results of the simulations based on the Finite Integration Technique are discussed. All dimensions are given in wavelength units so that all results can be scaled to any frequency of interest, including optical frequencies, thus simplifying the fabrication process compared with spherical dielectrics. The results presented here may be of interest for novel applications, including microscopy techniques and sensors.