Fresnel zone

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

Fresnel zones for broadband data

Abstract: For monochromatic waves, the term “Fresnel zone” is well‐defined even though different authors use different terminology. Most authors use the definition originating from optics. There, the first Fresnel zone is defined as the area of a circular hole in a screen between a light source and an observation point that produces maximum light intensity in the observation point (Figure 1). If the radius of the hole is enlarged, minima and maxima in light intensity alternate. The first maximum is reached if the raypath difference between the direct ray and the ray traveling via the edge of the hole equals half a wavelength. The extension of the definition to energy reflected from a circular disk is straightforward (if we restrict ourselves to ray theory and neglect the angle dependency of the reflection coefficient) and is illustrated in Figure 2 (see also Sheriff, 1991).

Coherent x-ray wavefront reconstruction of a partially illuminated Fresnel zone plate

Abstract: A detailed characterization of the coherent x-ray wavefront produced by a partially illuminated Fresnel zone plate is presented. We show, by numerical and experimental approaches, how the beam size and the focal depth are strongly influenced by the illumination conditions, while the phase of the focal spot remains constant. These results confirm that the partial illumination can be used for coherent diffraction experiments. Finally, we demonstrate the possibility of reconstructing the complex-valued illumination function by simple measurement of the far field intensity in the specific case of partial illumination.

Calculation of fractional orbital angular momentum of superpositions of optical vortices by intensity moments

Abstract: Two simple and high-efficiency techniques for measuring the orbital angular momentum (OAM) of paraxial laser beams are proposed and studied numerically and experimentally. One technique relies on measuring the intensity in the Fresnel zone, followed by calculating the intensity that is numerically averaged over angle at discrete radii and deriving squared modules of the light field expansion coefficients via solving a linear set of equations. With the other technique, two intensity distributions are measured in the Fourier plane of a pair of cylindrical lenses positioned perpendicularly, before calculating the first-order moments of the measured intensities. The experimental error grows almost linearly from ~1% for small fractional OAM (up to 4) to ~10% for large fractional OAM (up to 34).

Modulation of a super-Gaussian optical needle with high-NA Fresnel zone plate.

Abstract: A high NA Fresnel zone plate (FZP) is studied using vectorial angular spectrum theory for realizing the sharpest possible super-Gaussian optical needle with purely longitudinal polarization illuminated by a radially polarized vector beam. Strong dispersion of the FZP results in a light field resembling a super-Gaussian optical needle by selecting an optimal FZP structural wavelength relative to the illumination wavelength and inserting a narrow comb window function into the center-shaded FZP. A 25 μm long longitudinally polarized flattop optical needle with a transverse beam width of about 0.366λ is focused at a distance of 222.5 μm away from a binary amplitude 3.46 mm diameter FZP for a 532.4 nm wavelength in free space.

Near-field behavior of zone-plate-like plasmonic nanostructures

Abstract: The near-field behavior of a new plasmonic structure, the plasmonic micro-zone-plate (PMZP), is presented. The PMZP can realize superfocusing at a working distance on the micrometer scale and a resolving power beyond the diffraction limit. Compared with conventional Fresnel zone plates (CFZPs), its unique characteristics of a significantly elongated depth of focus (DOF) and focal length will make autofocusing easier for the relevant optical systems. These characteristics imply that it is possible to realize a free feedback control system for autofocusing systems in which probe scanning is performed with a constant working distance from the probe to the sample surface, provided that the flatness variation of the sample substrate is within the DOF. Moreover, unlike the CFZPs, there is no series of focal points appearing for beam propagation in the near-field region with a propagation distance ranging from λ to 8λ or even longer. In addition, transmission properties in the near-field region are investigated by means of a computational simulation based on a finite-difference time-domain numerical algorithm. Peak transmission wavelength shifts were observed while the metal film thickness was changed. Focusing characteristics were analyzed for different numerical apertures of the PMZPs.