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Fresnel zone

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


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TL;DR: In this article, it was shown that large-scale downwarping of the 670-km discontinuity near subduction zones derived from underside reflection data can be explained as the long-wavelength manifestation of a single, small-scale (wavelengths of several hundreds of kilometres) deflection of the discontinuity inside and near subducting slabs.
Abstract: SUMMARY Currnet models of upper-mantle discontinuity topography derived from traveltime variations of long-period underside reflections (PdP or SdS waves) may contain gross errors due to the use of geometrical optics in relating observed traveltimes to depth variations of the discontinuity near the reflection point. The use of geometrical optics is not valid if variations in the depth of the discontinuities exist on a lateral scale smaller than the size of the Fresnel zone in the data. Geometrical optics does not take into account the large size and the complex structure of the traveltime surface of underside reflections and may introduce spurious structure when used in inversions. Examples of synthetic long-period PdP waveforms for reflections near subducting lithosphere show that scattering from expected small-scale depth variations of the discontinuities inside slabs causes complex waveform variations. For a 15–20s PdP wave these small-scale depth variations (with a scale-length of 4) are smaller than the Fresnel zone (roughly 20°) but larger than the PdP wavelength at 420 km depth (about 150–200 km). The synthetic waveforms are processed to obtain PdP traveltimes, which are in turn converted to apparent discontinuity depth at the reflection point using geometrical optics. The apparent discontinuity-depth variations are not only of the same order of magnitude as those obtained from real data but also exhibit artefacts that closely resemble features observed in current models. It is shown that large-scale (wavelengths of the order of 1000 km) downwarping of the 670-km discontinuity near subduction zones derived from underside reflection data can be explained as the long-wavelength manifestation of a single, small-scale (wavelengths of several hundreds of kilometres) deflection of the discontinuity inside and near subducting slabs.

81 citations

Journal ArticleDOI
TL;DR: In this article, an in-plane Fresnel zone plate (FZP) for focusing surface plasmon polariton (SPP) fields has been designed, fabricated, and tested.
Abstract: An in-plane Fresnel zone plate (FZP) for focusing surface plasmon polariton (SPP) fields has been designed, fabricated, and tested. The fabricated device consists of 400nm tall by 5μm wide amorphous Si-based SPP FZP on an Al film integrated with a pair of two-dimensional nanohole arrays for excitation of the incident and detection of the diffracted SPP fields. Diffracted SPP fields from each Fresnel zone constructively interfere at the expected focal point to produce focusing with threefold intensity enhancement. Temporal and spatial characteristics of the focused SPP fields are studied with time-resolved spatial-heterodyne imaging technique. Good agreement with average power measurements is demonstrated. Diffractive focusing of SPP fields, based on Fourier plasmonics, represents an approach to SPP in-plane microscopy.

78 citations

Journal ArticleDOI
TL;DR: Fresnel zone plates are used as pupil apertures to extend the depth of field of an incoherent imaging system and the defocused optical transfer function is evaluated approximately.
Abstract: Fresnel zone plates are used as pupil apertures to extend the depth of field of an incoherent imaging system. The defocused optical transfer function is evaluated approximately, and the results are confirmed by experiments and by subjective evaluations of the images of a test chart. With a pupil having four zones, for example, the resolution limit at four depths of field out of focus is improved by a factor of 10.

78 citations

Journal ArticleDOI
TL;DR: In this paper, an analytical expression for the irradiance pattern, in both the far field and near field, associated with the field propagating from an aperture with a truncated-gaussian field distribution was obtained by evaluating the diffraction integral subject to the Fresnel approximation.
Abstract: In this paper we obtain an analytical expression for the irradiance pattern, in both the far field and near field, associated with the field propagating from an aperture with a truncated-gaussian field distribution. This is accomplished by evaluating the diffraction integral subject to the Fresnel approximation. Numerical results are presented, which indicate how the patterns vary as the width of the truncated-gaussian field distribution changes.

78 citations

Journal ArticleDOI
TL;DR: It is shown that for a beam with a small degree of ellipticity and even order of singularity, two isolated intensity zeroes appear in the Fresnel zone on a straight line at an angle of 45 deg or -45 deg, depending whether the beam's spin is right or left.
Abstract: An analytical expression for the diffraction of an elliptic Laguerre-Gaussian (LG) beam is derived and analyzed. We show that a beam with even singularity order has nonzero axial intensity for any degree of ellipticity and at any finite distance z from the initial plane, whereas at z=0 and z=∞ the axial intensity is zero. We show that for a beam with a small degree of ellipticity and even order of singularity, two isolated intensity zeroes appear in the Fresnel zone on a straight line at an angle of 45 deg or −45 deg, depending whether the beam's spin is right or left. The theoretical conclusions are confirmed by numerical simulation and physical experiments.

78 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202326
202249
202137
202052
201965
201878