Fresnel zone

About: Fresnel zone is a research topic. Over the lifetime, 2337 publications have been published within this topic receiving 37650 citations.

Physical and numerical model study of diffraction effects on seismic profiles over simple structures

Abstract: SUMMARY 2-D scale models and synthetic seismograms have been used to study diffraction artifacts and interpretation pitfalls on seismic profiles over two classes of simple-shaped structures: a vertical fault model and a rectangular mound model. The fault throw was varied from one eighth of a wavelength (λ/8) to two wavelengths (2λ). For throws greater than λ/2, the structure is resolved in the x-t domain but diffractions which are present convey misleading structure. Amplitude and wavelet anomalies are observed for small fault displacement (λ/2) which serve to detect but not to define the exact fault position and extent. Migration improves but does not necessarily overcome the problem. Mound structures having heights of λ/8 to 2λ, and length of one half to twice the Fresnel zone radius, were investigated. Lateral resolution is improved as the length of the structure increases, but only for mounds which are vertically resolved (height greater than λ/2). For low relief mounds, resolution deteriorates with increasing mound length. The resulting edge diffractions, which interfere with the reflection events, serve to detect and identify the target. Amplitude anomalies are observed across the structures, but it is not possible (without calibration) to interpret the edges of the model. Even for thick mounds the diffraction patterns complicate the seismic picture and can lead to false interpretation. The migration aperture needs to be about 10 times the Fresnel zone radius to produce satisfactory results. However, for vertically unresolved structures, migration may not significantly help. Complicating factors with post-stack migration are lack of knowledge of the true velocity function and inadequacy of the CMP zero-offset equivalence assumption.

Embossing Reactive Mesogens: A Facile Approach to Polarization-Independent Liquid Crystal Devices

Abstract: Reactive mesogens (RMs) have found their way into different branches of science and application. They can be processed as standard liquid crystals and can then be polymerised to stabilise their shape and anisotropic properties. Their birefringence is used in optical compensation films while the complex director field is exploited for actuators. However, creating complex shapes while maintaining good alignment is a challenge in these applications. In the present work, the embossing of reactive mesogens is introduced. Embossing is a fast and large-scale method, which allows to form a large variety of structured reactive mesogen devices. To create a polarisation-independent lens, two birefringent Fresnel zone plates (FZPs) are embossed. A lens is formed by assembling the FZPs in a twisted nematic cell and filling the cell with a nematic crystal index matched to the RM. The device can be switched from a non-focusing to a focusing state by applying a small voltage. After characterising the efficiency and beam properties, the method is used to fabricate a switchable multi-level Fresnel zone plate with optical efficiencies beyond 50%. Finally, manufactured polarisation-independent gratings and microlens arrays are presented using the method to illustrate the wide range of applicability.

Fabrication of high-resolution zone plates with wideband extreme-ultraviolet holography

Abstract: We report an achromatic holographic method to fabricate high-resolution x-ray optics using coherent extreme-ultraviolet radiation from an undulator source. The interference pattern between two spherical beams, which are created using Fresnel zone plates, is recorded to produce a higher-resolution zone plate. Analytical and simulation results showing the formation of the zone plate pattern was confirmed experimentally with the production and testing of a lens with 60-nm outermost zone width. The combination of extreme-ultraviolet light, which exposes photoresists with practically no proximity effect, and holography, which guarantees the accurate placement of zones, addresses the main difficulties faced in the improvement of the resolution of x-ray lenses. Holography with extreme-ultraviolet light has the potential to produce lenses with sub-10-nm resolution.

Vectorial Diffraction Analysis of Near-Field Focusing of Perfect Black Fresnel Zone Plates Under Various Polarization States

Abstract: Near-field focusing properties of perfect black Fresnel zone plates are analyzed by solving Maxwell equations using the vectorial angular spectrum representation and the equivalent Rayleigh diffraction formulae of the first kind given by the Luneburg vector diffraction theory. Linear and circular polarizations are compared. As a result, an exact analytical expression for axial intensity is obtained showing that, in the near field, the shift of focal length is inherent in the diffraction process and independent of incident field polarization. The numerical intensity distribution of the electromagnetic field is also obtained, which allows us to demonstrate that when vectorial analysis of the system is performed, subwavelength focusing cannot be achieved for very short focal distances. This result is valid for linearly and circularly polarized incident fields. Finally, it is shown that symmetry of the focalized field is strongly dependent on the polarization of the incident field and that the vectorial diffraction theory can be a good alternative to finite-difference time-domain methods.