Geometrical Shadow Region Research Articles

Trajectory-Based Interpretation of Laser Light Diffraction by a Sharp Edge

In the diffraction pattern produced by a half-plane sharp edge when it obstructs the passage of a laser beam, two characteristic regions are noticeable. There is a central region, where it can be noticed the diffraction of laser light in the region of geometric shadow, while intensity oscillations are observed in the non-obstructed area. On both sides of the edge, there are also very long light traces along the normal to the edge of the obstacle. The theoretical explanation to this phenomenon is based on the Fresnel-Kirchhoff diffraction theory applied to the Gaussian beam propagation behind the obstacle. Here we have supplemented this explanation by considering electromagnetic flow lines, which provide a more complete interpretation of the phenomenon in terms of electric and magnetic fields and flux lines, and that can be related, at the same time, with average photon paths.

Study of internal energy flows in dipole vortex beams by knife edge test

The rotation of Poynting vector in dipole vortex beam (DVB) during propagation has been experimentally detected in a knife edge test. The dipole vortex beam is generated, when a collimated laser beam is incident on a phase mask, displayed on spatial light modulator (SLM) in reflection mode. The knife edge test reveals dipole configurations through strikingly distinct intensity intrusions in the geometrical shadow region.

A Novel 270$^{\circ}$ Bent-Axial-Type Electron Gun and Positioning of Its Electron Beam Spot on the Target

In this paper, we propose and evaluate the performance of a practical 270° bent-axial-type electron gun, in which focusing is bidirectional through an axial magnetic field. Such an electron gun has two advantages: (1) a high-voltage cathode assembly placed in a geometrical shadow region of the emerging vapor and (2) a compact electron beam (e-beam) spot with circular symmetry that results in high power density. It is also shown that the position of the e-beam spot on the target is controllable along any direction without compromising the compactness and circular symmetry of the e-beam spot just like a straight-axial-type electron gun, but not typically possible for conventional 270° bent-transverse-type electron guns.

Young's Diagnostics of Phase Singularities into Polychromatic Light Fields

Two original approaches for the diagnostics of phase singularities (such as optical vortices, screw dislocations of a wave front) based on the Young-Rubinowicz mOdel of diffraction phenomena are represented. Both techniques are implemented without using a separate reference wave, as in common interference techniques. That is why they are very convenient for analysis of spatially coherent polychromatic fields. The first technique is based on the use of an opaque strip as the diffraction device. Bending Young's interference fringes produced by the edge diffraction waves from two rims of the strip within the geometrical shadow region reflect helicity of a wave front, so that the direction and magnitude of bending correspond directly to the sign and the modulus of topological charge of the optical vortex, respectively. This technique is practicable for diagnostics of isolated polychromatic vortices, such as rainbow Laguerre-Gaussian mode, where the condition of mutual spectral purity is satisfied. Another technique is based on the use of knife-edge diffraction. The edge of an opaque screen serves as the source of a reference wave, which interferes with the tested field within the directly illuminated region. One observes typical interference forklets near the geometrical shadow boundary, which detect optical vortices. This technique is more applicable to diagnostics of phase singularities in polychromatic speckle fields due to the condition of mutual spectral purity is satisfied automatically.

Lamb Wave Interaction with Impact-induced Damage in Aircraft Composite: Use of the A0 Mode Excited by Air-coupled Transducer

A novel technique has been experimented involving the use of an air-coupled transducer to excite Lamb mode in composite material, a pinducer to pick-up local response and a turn-table set-up to reveal impact damage-induced anisotropy. So, a fundamental antisymmetric A0 Lamb mode has been excited under unusual conditions. Its efficiency for detecting impact-induced damage in a multilayered thin quasi-isotropic carbon–epoxy composite laminate has been demonstrated and the knowledge on the A0 Lamb mode interaction with the damage has been improved using an original mapping. Some material characteristics have been obtained. Intrinsic material attenuation of this guided mode at a frequency of 370 kHz and a frequency-thickness product of 740 kHz mm has been assessed and in-plane energy distribution of the A0 mode has been mapped, revealing a relatively directive acoustic field. However, the composite material exhibits slightly anisotropic acoustic behaviour due to a lack of fibres in certain directions. Interactions between the A0 mode and multidelamination of the critical area were also investigated. In-plane mappings of the ultrasonic field have revealed two main phenomena: attenuation and scattering due to damage. Splitting of the ultrasonic wave was observed, creating a region of geometric shadow where attenuation occurs. It is shown on the mappings that the energy lost in this direction of propagation appears in other directions. An increase in the received energy was detected in certain directions, showing that the damage can be considered as a soft scattering centre.

Molecular beam epitaxy growth over shadow edges: a new method for in situ growth of nanostructures

We propose a new method to obtain well defined nanostructures in situ during MBE growth. The method is based on the MBE growth over UHV compatible shadow edges. These edges define a geometrical shadow region which can be modified by tilting the sample with respect to the direction of the molecular beams, so that layers with different lateral extension can be combined to form a buried junction of two quantum wells in the form a wire with lateral confinement. We show for ZnSe quantum wells in ZnMgSSe that the formation of facets at the shadow edge favours the formation of geometrically well defined structures, and quantum well junctions as narrow as 150 nm have been realized. The wires emit pronounced blue cathodoluminescence at room temperature, proving that even for simple structures the confinement of carriers in the resulting wire is on the order of kT at room temperature.

Aharonov-Bohm scattering on two antiparallel flux lines of the same magnitude-without return flux

The problem of Aharonov-Bohm (AB) scattering on two parallel flux lines of arbitrary magnitude is solved exactly; the expression for the scattering cross-section in the region of geometric shadow is derived. It is shown that in the particular case of two antiparallel lines of the same magnetic flux, though the return flux does not exist, the AB scattering still exists.

Axisymmetric monostatic and bistatic resonance excitation mechanisms of large aspect ratio targets: Impulse excitations and bipolar couplings

An experimental/theoretical study of the scattering from large aspect ratio, finite elastic cylinders is presented. The study focuses on the excitation mechanisms of the axisymmetric resonances. It is found that significant coupling of the incident field to these resonances occurs in the region of geometric shadow at low kD/2 (D is the cylinder diameter). The implications of this coupling for the relative excitation strengths of the resonances and for the associated bistatic beam pattern are examined. It is argued on physical grounds that the results must apply to all large aspect ratio targets (both solids and shells).

Diffraction by Discs for a Point Source in the Near Field

For a point source of light within the near field of circular, elliptical or tilted circular discs, diffraction into the geometrical shadow region is studied theoretically. For observation in the near field on the shadow side of the disc, the geometry of the caustic formed is shown to be closely approximated by the evolute of the diffracting edge, a result which holds strictly for plane wave incidence. For observation in the far field, the quite different patterns observed by Coulson and Becknell are explained using two approaches. First, a suitable approximation to the phase term in the Kirchhoff integral leads to a largely analytical treatment of the circular disc, and for the field on the axis of the elliptical disc. Second, the field away from the axis for the elliptical disc is explained by applying stationary phase methods to the Rubinowitz edge integral, in a modified form which is uniformly valid across the shadow boundary. Finally, the behaviour of wavefront dislocations formed in the diffracted ...

Quantum theory of neutral-atom scattering at long range from solid cylinders

Qualitative arguments suggest that diffraction effects might be important for the interpretation of the experiments of Shih et al. We derive a tractable quantum theory to describe the scattering of neutral atoms at long range by van der Waals forces from solid cylinders. The theory is based on an angular-momentum expansion and evaluates the scattered intensity close to the cylinder in the geometrical shadow region. The formal quantum theory indicates that one is far from the conventional classical limit, in the sense that WKB and stationary-phase approximations are not valid. Yet an essentially exact evaluation appears to agree with the classical prediction, at least over the range of deflections that have been experimentally studied. This surprising classical-quantum correspondence is clarified by a Regge-pole analysis. Finally we also set limits on the extent of specular reflection from the cylinder surfaces. The discrepancy between theory and experiment remains unresolved.

Analysis and reduction of scattering from the feed of a cheese antenna

The far field of a cheese antenna can be described in terms of three components: a) one due to the unobstructed aperture field, b) one due to that portion of the primary feed energy not intercepted by the reflector, and c) a scattered component due to the fact that the feed acts as an obstacle in the path of energy emerging from the reflector. This scattered component is usually calculated by considering a perturbation in the aperture field, in the form of an out-of-phase component sufficient to produce zero field in the geometrical shadow region behind the feed. A more exact analysis of a reflector excited by a longitudinally-slotted circular cylinder shows this engineering approach to be satisfactory, provided the out-of-phase component is assumed to exist over an area of width 1.5 times the projected width of the feed. An empirical investigation shows a similar result for a slotted rectangular waveguide feed and for a horn feed. An antenna is described in which the scattered field and the backlobe of the primary feed are made to partially cancel. An additional control over these field components is provided by a series of vanes or waveguides located either side of the feed.