Nonspecular Reflection Research Articles

Total internal reflection at a boundary between vacuum and an optical medium with quasi-zero refractive index

It is shown that, on a surface of optical transparency of a medium with quasi-zero refractive index, surface optical waves traveling along the surface can be excited for different angles of incidence of external radiation. Expressions for the amplitudes of nonspecular reflection and transmission of light waves at an inhomogeneous boundary between two media are derived. Using these expressions, the boundary reflectance and transmittance for different angles of incidence and refraction can be calculated.

Resonant excitation of intense acoustic waves in crystals

The resonant excitation of an intense elastic wave through nonspecular reflection of a special pump wave in a crystal is described. The choice of the plane and angle of incidence is dictated by the requirement that the excited reflected wave be close to the bulk eigenmode with its energy flow along a free boundary. The resonance parameters have been found for a medium with an arbitrary anisotropy. General relations are concretized for monoclinic, rhombic, and hexagonal systems. A criterion is formulated for an optimal selection of crystals in which the resonant reflection is close to the conversion one, when almost all of the energy from the incident beam of the pump wave falls into the near-surface narrow high-intensity reflected beam. Estimates and illustrations are given for such crystals as an example. The intensity of the reflected beam increases with its narrowing, but its diffraction divergence also increases with this narrowing. Nevertheless, the intensity of the beam can be increased by a factor of 5–10 at sufficiently high frequencies while keeping its divergence at an acceptable level. Amplification by two orders of magnitude can be achieved by compressing the beam in two dimensions through its double reflection.

Metamaterial-Coated Plates for Emulating Non-Specular Reflection and Antenna Reflectors

The optical transformation is generally used to deform free space in order to obtain novel effects on waves. Such effects can be realized by replacing the deformed region by a “metamaterial” characterized by \mbi e and \mbi μ tensors that are determined by the transformation. A new type of transformation is proposed which not only deforms a free space region, but transforms a non-trivially shaped conducting surface that bounds this region as well. The deformations are designed to map the original region into a planar slab of metamaterial, and to map the conducting surface into a plate which bounds this slab. Since the fields outside the deformed region-which include the fields scattered by the original conducting surface-are unaffected by the transformation, they can, in effect, be produced by scattering from the planar metamaterial-coated plate created from the transformation. This substitution of a non-trivially shaped conducting surface by a planar metamaterial-coated plate is applied to (a) a face of a wedge to produce reflection from the planar metamaterial-coated plate that is not in the specular direction, and (b) a parabolic reflector antenna to design a planar system which produces the same radiation pattern as the reflector. Both devices are numerically validated.

Non-specular reflections in a macroscopic system with wave-particle duality: Spiral waves in bounded media

Spiral waves in excitable media possess both wave-like and particle-like properties. When resonantly forced (forced at the spiral rotation frequency) spiral cores travel along straight trajectories, but may reflect from medium boundaries. Here, numerical simulations are used to study reflections from two types of boundaries. The first is a no-flux boundary which waves cannot cross, while the second is a step change in the medium excitability which waves do cross. Both small-core and large-core spirals are investigated. The predominant feature in all cases is that the reflected angle varies very little with incident angle for large ranges of incident angles. Comparisons are made to the theory of Biktashev and Holden. Large-core spirals exhibit other phenomena such as binding to boundaries. The dynamics of multiple reflections is briefly considered.

Leaky-surface-wave generated acoustic beam displacement upon reflection in lithium tantalate crystals

The non-specular reflection of a leaky surface acoustic wave radiated beam from the surface of piezoelectric lithium tantalate crystal has been investigated using an acoustoelectric and acousto-optic technique. The Schoch-type beam displacement upon reflection strongly depends on whether the surface is free or metalized. This effect is attributed to the excitation (on the reflecting surface) of the leaky surface wave, the acoustoelectric attenuation of which strongly depends on the surface conductivity.

Investigation of Non-specular Reflection of Rayleigh Waves for Evaluation of the Material Properties of Surface Area

Investigation of Non-specular Reflection of Rayleigh Waves for Evaluation of the Material Properties of Surface Area

Non-specular reflection of self-collimated beams in photonic-crystal-made prism and waveguide system

We demonstrate a dual-beam-reflection phenomenon for a Gaussian beam illuminating at a Kretschmann configuration composed of a photonic-crystal-made prism and a dielectric waveguide. One reflection beam has a positive shift and the other has a negative shift. The finite-difference time-domain (FDTD) shows that the specific phenomenon takes place only when the corresponding quasi-guided mode supported in the Kretschmann configuration is excited. Field profile of the quasi-guided mode demonstrates a strong localized stationary field in the dielectric waveguide. We found that the maximum positive lateral shift (LS) is 14.27a (where a is the lattice constant), corresponding to 3.07 times of the incident wavelength, which is 0.7135 times of the beam waist and much larger than that in some previous reports.

Neutron magnetic resonance and non-specular reflection from a magnetic film placed in an oscillating magnetic field

Experimental results on neutron reflection from a magnetic film placed in an oscillating magnetic field are reported. We found evidence for neutron spin resonance in the film and observed a spatial beam-splitting. The beam-splitting finds its origin in the exchange of an energy quantum ℏω between the oscillating field and the neutron.

NON-SPECULAR REFLECTION OF ELECTROMAGNETIC WAVES FROM A TWO-DIMENSIONALLY PERIODIC GRATING OF SHORT-CIRCUITED COAXIAL SECTOR WAVEGUIDES

The effect of non-specular reflection of electromagnetic waves from a grating made of short-circuited coaxial sector waveguides is investigated. It is shown that the electric field energy of the incident wave can be transformed into the wave energy reflected in a non-specular direction. The Stokes parameters are analyzed in dependence on the length of the short-circuited coaxial sector waveguides. The possibility of converting incident linearly polarized waves into reflected circularly polarized waves is considered for the regimes of non-specular reflection of electromagnetic waves.

Cavity soliton billiards

The motion of a self-propelled cavity soliton in a laser where the pump profile acts as a square billiard is investigated. In the long-term dynamics, only closed trajectories are possible, exhibiting nonspecular reflections with striking similarities to walking droplets in a vibrated liquid bath. Open orbits can be achieved either by introducing scattering defects in the pump profile or in the presence of more than two solitons, due to their interaction. Such dynamical properties can be exploited for applications such as a compact soliton-force microscope.

Quick measurement of crystal truncation rod profiles in simultaneous multi-wavelength dispersive mode

To conduct time-resolved measurements in the wide momentum transfer (q = 4π sinθ/λ, θ: the glancing angle of the x-ray beam, λ: x-ray wavelength) range of interest, we developed a method that can simultaneously measure the whole profile of x-ray diffraction and crystal truncation rod scattering of interest with no need of rotation of the specimen, detector, and monochromator crystal during the measurement. With a curved crystal polychromator (Si 111 diffraction), a horizontally convergent x-ray beam having a one-to-one correlation between wavelength (energy: 16.24–23.0 keV) and direction is produced. The convergent x-ray beam components of different wavelengths are incident on the specimen in a geometry where θ is the same for all the x-ray components and are diffracted within corresponding vertical scattering planes by a specimen ([GaAs(12ML)/AlAs(8 ML)]50 on GaAs(001) substrate) placed at the focal point. Although θ is the same for all the directions, q continuously varies because λ changes as a function of direction. The normalized horizontal intensity distribution across the beam, as measured using a two-dimensional pixel array detector downstream of the specimen, represents the reflectivity curve profile both near to and far from the Bragg point. As for the crystal truncation rod scattering around the 002 reflection, the diffraction profile from the Bragg peak down to reflectivity of 1.0 × 10−9 was measured with a sufficient data collection time (1000–2000 s). With data collection times of 100, 10, 1.0, and 0.1 s, profiles down to a reflectivity of ∼6 × 10−9, ∼2 × 10−8, ∼8 × 10−8, and ∼8 × 10−7 were measured, respectively. To demonstrate the time-resolving capability of the system, reflectivity curves were measured with time resolutions of 1.0 s while rotating the specimen. We have also measured the diffraction profile around the 113 reflection in the non-specular reflection geometry.

Planar Reflector Panels with Convex Edges

When designing rooms for speech or music, acoustical designers often use reflector panels to direct additional sound energy to listeners. Reflector panels introduce some coloration into the reflected signal due to edge diffraction. Furthermore, planar reflector panels reflect sound primarily into a single specular reflected direction. This project investigates the reflected sound field from planar acoustical reflectors with rounded reflector edges away from the source side, creating a convex edge. The authors use 2D boundary element method (BEM) sound field simulations to compare the performance of the convex-edged reflectors against flat reflectors. Results show that the convex edge increases nonspecular reflections by 3 dB with each doubling of curvature radius. Because the simulations were run in two dimensions, results generalize only to long reflectors mounted above long sources, such as an orchestra or choir.

Coherent reconstruction of flaw images taking the effect of nonspecular reflection of ultrasonic pulses from the boundaries of a test object into account

The influence of the effect of nonspecular reflection of transverse ultrasonic waves from the boundaries of a test object on the quality of coherent images of flaws is considered. The existence of an effect of nonspecular reflection in a pulsed operating mode and the accuracy of its description within the framework of complex reflection coefficients is experimentally verified. It is shown that the effect does not influence the delay times of propagating pulses and only the amplitude of echo pulses changes. Such distortions slightly impair the quality of flaw images that are reconstructed by coherent methods. Nevertheless, the methods for eliminating the influence of this effect are considered for two methods, viz., FT-SAFT and SAFT. Images of flaws that were reconstructed by the FT-SAFT method without taking the nonspecular reflection effect and with its compensation into account are presented.

Small-angle neutron spin-flip scattering in ferromagnetic films

This paper reports on the results of reflectometric measurements of anisotropic (Co67Fe31V2) and almost isotropic (Fe) films prepared by magnetron sputtering. Nonspecular reflections and the corresponding peaks of the intensities of refracted neutrons have been observed for the alloy samples in magnetic fields H ≤ 7 Oe applied in the film plane along the easy magnetization axis. For iron films, angular splitting of the reflected neutron beam becomes observable only at H > 100 Oe and increases with an increase in the magnetic field. A general scheme has been proposed for this small-angle scattering with allowance made for different variants of changes in the Zeeman energy of neutrons. This scheme has allowed us to identify the magnetic structures of Co-Fe films. The magnetization of 0.15-μm-thick films with uniaxial and unidirectional textures leads to the formation of unidirectional textures characterized by different intensity distributions, for which the qualitative differences are retained with an increase in the magnetic field from 7 to 800 Oe. It has been revealed that, for 2.5-μm-thick films with the initial unidirectional texture, the oppositely magnetized states are nonequivalent.

Annealing behaviour of structural and magnetic properties of evaporated Co thin films

Cobalt thin films of 50 nm nominal thickness were e-beam evaporated on silicon substrates covered with thermal oxide. Two series of independent and cumulative vacuum annealings up to 600 °C and 650 °C, respectively, were performed. The x-ray diffraction, specular and non-specular x-ray reflectivity and longitudinal magneto-optical Kerr effect measurements were applied to probe the annealing behaviour of the film structure and magnetic properties. A gradual transition from the hexagonal close-packed (hcp) to the face-centred cubic (fcc) structure was observed. Evolution of the in-plane magnetic anisotropy is dominated by residual stresses which relax during the structural transformation. The coercivity follows the stress behaviour in the hcp phase up to 300 °C and increases abruptly above 400 °C due to improving the magneto-crystalline anisotropy in the growing fcc crystallites and enhanced surface/interface roughness.

Casimir energy and geometry: beyond the proximity force approximation

We review the relation between the Casimir effect and geometry, emphasizing deviations from the commonly used proximity force approximation (PFA). We use, to this aim, the scattering formalism which is nowadays the best tool available for accurate and reliable theory–experiment comparisons. We first recall the main lines of this formalism when the mirrors can be considered to obey specular reflection. We then discuss the more general case where non-planar mirrors give rise to non-specular reflection with wavevectors and field polarizations mixed. The general formalism has already been fruitfully used for evaluating the effect of roughness on the Casimir force as well as the lateral Casimir force or Casimir torque appearing between corrugated surfaces. In this paper, we focus our attention to the case of the lateral force which should make possible in the future an experimental demonstration of the nontrivial (i.e. beyond PFA) interplay of the geometry and Casimir effect.

Lateral Casimir–Polder force with corrugated surfaces

We derive the lateral Casimir–Polder force on a ground-state atom on top of a corrugated surface, up to first order in the corrugation amplitude. Our calculation is based on the scattering approach, which takes into account nonspecular reflections and polarization mixing for electromagnetic quantum fluctuations impinging on real materials. We compare our first order exact result with two commonly used approximation methods. We show that the proximity force approximation (large corrugation wavelengths) overestimates the lateral force, while the pairwise summation approach underestimates it due to the non-additivity of dispersion forces. We argue that a frequency shift measurement for the dipolar lateral oscillations of cold atoms could provide a striking demonstration of non-trivial geometrical effects on the quantum vacuum.

Casimir torque between corrugated metallic plates

We consider two parallel corrugated plates and show that a Casimir torque arises when the corrugation directions are not aligned. We follow the scattering approach and calculate the Casimir energy up to second order in the corrugation amplitudes, taking into account nonspecular reflections, polarization mixing and the finite conductivity of the metals. We compare our results with the proximity force approximation, which overestimates the torque by a factor 2 when taking the conditions that optimize the effect. We argue that the Casimir torque could be measured for separation distances as large as 1 µm.

Anomalous momentum states, non-specular reflections, and negative refraction of phase-locked, second-harmonic pulses

We simulate and discuss novel spatio-temporal propagation effects that relate specifically to pulsed, phase-mismatched second-harmonic generation in a material characterized by simultaneously negative permittivity and permeability, having finite length. Using a generic Lorentz model for the dielectric permittivity and magnetic permeability, the fundamental and second-harmonic frequencies are tuned so that the corresponding indices of refraction are negative for the pump and positive for the second-harmonic signal. A phase-locking mechanism causes part of the second-harmonic signal generated at the entry surface to become trapped and dragged along by the pump and to refract negatively, even though the index of refraction at the second-harmonic frequency is positive. These circumstances culminate in the creation of an anomalous state consisting of a forward-moving second-harmonic wave packet that has negative wave vector and momentum density, which in turn leads to non-specular reflections at intervening material interfaces. The forward-generated second-harmonic signal trapped under the pump pulse propagates forward, but has all the attributes of a reflected pulse, similar to its twin counterpart generated at the surface and freely propagating backward away from the interface. This describes a new state of negative refraction, associated with nonlinear frequency conversion and parametric processes, whereby a beam generated at the interface can refract negatively even though the corresponding index of refraction at that wavelength is positive.

Real-space neutron scattering methods

Using precession devices with tilted interfaces with respect to the polarised neutron beam enables measurement of structures by scattering. The scattering techniques can be small-angle neutron scattering, neutron diffraction and both specular and non-specular neutron reflectivity. We describe in this article all these techniques in a real-space concept. In all the techniques this concept describes in which direction structural information is obtained. In some cases the concept shows directly how to interpret the measured data in real-space.