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Plane-wave scattering by a three-dimensional chiralobject immersed in a simple Lorentz-nonreciprocalmedium with magnetoelectric gyrotropy

Hamad Alkhoori

DOI: 10.1364/JOSAA.419010 Received 06 Jan 2021; Accepted 24 Feb 2021; Posted 24 Feb 2021  View: PDF

Abstract: Scattering by a three-dimensional object composed of a chiral medium (the interior medium) and immersed in a simple Lorentz-nonreciprocal medium with magnetoelectric gyrotropy (the exterior medium)was treated using the extended boundary condition method (EBCM). The exterior medium is quantifiedby εre, µre, and Γ, whereas the interior medium by εri, µri, and β. When irradiated by a plane wave, the differential scattering efficiency does not depend on the polarization state of the incident plane wave if theexterior medium is impedance-matched with the interior medium, regardless of the shape of the object,Γ, and β. Zero backscattering is possible if, in addition to impedance matching condition, the object isrotationally symmetric about the propagation direction, and Γ is parallel to the propagation direction. Numerical results confirm these remarks for scattering by spheroids. On fixing εri, µri, εre, and µre, the effectsof Γ and β on the total scattering efficiency were examined for a sphere. The total scattering efficiencydoes not depend on the polarization state of the incident plane wave, even when the exterior medium isnot impedance-matched with the interior medium, and despite the presence of Γ and β. The total scattering efficiency when Γ is coparallel to the propagation direction can be either equal to, larger than, orsmaller than when Γ is antiparallel or perpendicular to the propagation direction, depending on β and theelectrical size of the sphere. It is found that parallel propagation of the incident plane wave with respectto Γ has a stronger influence than perpendicular propagation, regardless of β and the electrical size of thesphere. The effect of increasing/decreasing the magnitude of Γ can be envisioned only when its directionis parallel to the propagation direction.

In-Vacuum Measurements of Optical Scatter VersusAnnealing Temperature for Amorphous Ta2O5 andTiO2:Ta2O5 Thin Films

Elenna Capote, Amy Gleckl, Jazlyn Guerrero, Michael Rezac, Robert Wright, and Joshua Smith

DOI: 10.1364/JOSAA.415665 Received 01 Dec 2020; Accepted 23 Feb 2021; Posted 24 Feb 2021  View: PDF

Abstract: Optical coatings formed from amorphous oxide thin films have many applications in precision measurements. The Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) and AdvancedVirgo use coatings of SiO2 (silica) and TiO2:Ta2O5 (titania doped tantala) and post-deposition annealing to 500◦C to achieve low thermal noise and low optical absorption. Optical scattering by these coatings is a key limit to the detectors’ sensitivity. This paper describes optical scattering measurementsfor single-layer ion-beam-sputtered thin films on fused silica substrates: two samples of Ta2O5 and twoof TiO2:Ta2O5. Using an imaging scatterometer at a fixed scattering angle of 12.8◦, in-situ changes inthe optical scatter of each sample were assessed during post-deposition annealing to 500◦C in vacuum.The scatter of three of the four coated optics was observed to decrease during the annealing process, by25-30% for tantala and up to 74% for titania-doped tantala, while scatter from the fourth sample held constant. Angle-resolved scatter measurements performed before and after vacuum annealing suggests someimprovement in three of four samples. These results demonstrate that post-deposition high-temperatureannealing of single-layer tantala and titania-doped tantala thin films in vacuum does not lead to an increase in scatter, and may actually improve their scatter.

The refined EUV mask model

Igor Makhotkin, MEIYI WU, Victor Soltwisch, Frank Scholze, and Vicky Philipsen

DOI: 10.1364/JOSAA.416235 Received 04 Dec 2020; Accepted 09 Feb 2021; Posted 09 Feb 2021  View: PDF

Abstract: A refined model of an extreme ultraviolet (EUV) mask stack consisting of the Mo/Si multilayer coated by a Ru protective layer and a TaBN/TaBO absorber layer was developed to facilitate accurate simulations of EUV mask performance for high-NA EUV photo-lithography (EUVL) imaging. The model is derived by combined analysis of the measured EUV and X-ray reflectivity of a state-of-the-art mask blank. These two sets of measurements were analyzed using a combined free-form analysis procedure that delivers high-resolution X-ray and EUV optical constant depth profiles based on self-adapted sets of sublayers as thin as 0.25nm providing a more accurate description of the reflectivity than obtained from only EUV reflectivity. “Free-form analysis” means that the shape of the layer-interfaces in the model is determined experimentally and is not given a priori by the structure model. To reduce the numerical effort for EUV imaging simulations a low-resolution model of the multilayer and absorber stack with sublayer thicknesses larger than 2nm, that fits to only the EUV reflectance, was derived from the high-resolution model. Rigorous high-NA EUVL simulations were done to compare the performance of the new model to our previous work [1].

Mid-Fusion of Road Scene Polarization Images onPre-trained RGB Neural Networks

Khalid Omer and MEREDITH KUPINSKI

DOI: 10.1364/JOSAA.413604 Received 29 Oct 2020; Accepted 09 Feb 2021; Posted 10 Feb 2021  View: PDF

Abstract: This work presents a mid-fusion pipeline that can increase the detection performance of a convolutionalneural network (RetinaNet) by including polarimetric images even though the network is trained on alarge-scale database containing RGB and monochromatic images (Microsoft COCO). Here, the Average Precision (AP) for each object class quantifies performance.The goal of this work is to evaluate the usefulnessof polarimetry for object detection and recognition ofroad scenes and determine the conditions for AP increase. Shadows, reflections, albedo, and other objectfeatures that reduce RGB image contrast also decreaseAP. This work demonstrates specific cases for which APincreases using linear Stokes and polarimetric flux images. Images are fused during the neural network evaluation pipeline and referred to as mid-fusion. Here, theAP of polarimetric mid-fusion is greater than RGB APin 54 out of 80 detection instances. The recall for carsand buses were similar for RGB and polarimetry butincreases from 36% to 38% when using polarimetry fordetecting people. Videos of linear Stokes images forfour different scenes are collected at three times of theday and two driving directions. Despite this limiteddata set and use of a pre-trained network, this workdemonstrates selective enhancement to object detectionthrough mid-fusion of polarimetry to neural networkstrained on RGB images.

Dense stereo matching in fish-eye images using modified hemispherical ASW algorithm

Yakun Zhang, haibin li, Wenming Zhang, and Cunjun Xiao

DOI: 10.1364/JOSAA.413120 Received 21 Oct 2020; Accepted 06 Feb 2021; Posted 08 Feb 2021  View: PDF

Abstract: In this paper, we concentrate on dense estimation of disparities between fish-eye images without corrections. Because of the distortions, fish-eye images cannot be processed directly utilizing the classical ASW (adaptive support weight) method for perspective images. In order to addresses this problem, we propose a modified hemispherical ASW method in a hemispherical framework. First, 3D epipolar curves are calculated directly on a hemispherical model to deal with the problem that 2D epipolar curves cannot cover the whole image disc. Then a modified ASW method with hemispherical support window and hemispherical geodesic distance is presented. Moreover, 3DEDT (3-dimensional epipolar distance transform) is proposed and fused into the matching cost to copy with the textureless region problem. The benefit of this approach is demonstrated by realizing the dense stereo matching for fish-eye images using a public fish-eye data set, for which both objectively evaluated as well as visually convincing results are provided.

Two-point calibration method for zoom camera with approximate focal-invariant radial distortion model

Pei An, Jie Ma, Tao Ma, Bin Fang, Kun Yu, Xiaomao Liu, and Jun Zhang

DOI: 10.1364/JOSAA.414504 Received 11 Nov 2020; Accepted 06 Feb 2021; Posted 08 Feb 2021  View: PDF

Abstract: Zoom camera can change its focal length and pose to track moving objects with adjustable resolution. To extract precise geometric information of the tracked objects, an accurate calibration method for the zoom camera is in requirement. However, high-precision camera calibration methods usually require a number of control points that are not guaranteed in some practical situations. Most of zoom cameras suffer radial distortion. Traditional method can recover undistorted image with known intrinsic parameters. But it fails to work for the zoom camera with unknown focal length. Motivated by these problems, we propose a novel two-point calibration method (TPCM). In this scheme, we first propose an approximate focal-invariant radial distortion (AFRD) model. With AFRD model, RGB image can be undistorted with unknown focal length. After that, TPCM is presented to estimate focal length and rotation matrix with only two control points of one image. Synthetic and real data experiments demonstrate that AFRD model is efficient and TPCM has superior performance than the state-of-the-art methods.

Radon-to-Helmholtz mappings and nonlineardiffraction tomography

Gregory Samelsohn

DOI: 10.1364/JOSAA.412553 Received 15 Oct 2020; Accepted 05 Feb 2021; Posted 08 Feb 2021  View: PDF

Abstract: This paper addresses a number of approximate, analytically invertible, solutions ofthe scalar Helmholtz equation. Primary attention is devoted to the Glauber approximation(GA) derived for the far-field pattern. It is shown that the GA has the form of a nonlinearRadon-to-Helmholtz (RtH) mapping, which transforms a sinogram of the scattering potentialinto an approximate solution of the Helmholtz equation. A proposal of how to construct aposition space counterpart of the GA is formulated. Also, it is established that a paraxialversion of the Glauber model coincides, up to an inessential constant factor, with amomentum-space representation of the Mazar-Felsen propagator, which describes forwardscattered waves. For weakly scattering objects, these solutions are reduced to theconventional Born/Rytov approximations, which may, however, preserve the parametrizationand sampling formats of the original nonlinear models. Since all RtH mappings areanalytically invertible, they can be applied to the (nonlinear) diffraction tomography ofpenetrable objects. In particular, the Glauber model, which has been largely ignored for years,is shown to provide efficient inversion of synthetic data. The resulting tomograms clearlyoutperform the Born inversions, even for moderately scattering potentials.

Reflectance and transmittance of terahertz waves fromgraphene embedded into metamaterial structures

Irfan Toqeer, Muhammad Yaqoob, Abdul Ghaffar, Majeed Alkanhal, Y Khan, and Yosef Aladadi

DOI: 10.1364/JOSAA.412649 Received 14 Oct 2020; Accepted 04 Feb 2021; Posted 05 Feb 2021  View: PDF

Abstract: n this work, the theoretical study of the interaction of terahertz (THz) waves withgraphene embedded into two different semi-infinite metamaterials is carried out. To model thegraphene, the effective surface conductivity approach based upon the Kubo formalism is used.Two types of metamaterials i.e., double-positive DPS and double-negative DNG metamaterialsare studied in the THz regime. The numerical modeling of metamaterials is done in theframework of causality principle-based Kramers-Kronig relations. The reflectance andtransmittance from the graphene-embedded metamaterial structures are studied for fourdifferent configurations i.e., DPS-Graphene-DPS, DPS-Graphene-DNG, DNG-Graphene-DPS,and DNG-Graphene-DNG. The influence of the chemical potential and scattering rate on thereflectance and transmittance for each configuration is analyzed. It is concluded that the DPSGraphene-DPS and DNG-Graphene-DNG configurations behave as anti-reflectors for the THzwaves, while the DPS-Graphene-DNG and DNG-Graphene-DPS configurations are suitablefor THz reflector applications. Moreover, a parametric study reveals that the relativepermittivity of the partnering metamaterial can be used as an additional degree of freedom tocontrol the reflectance and transmittance of THz waves. It is concluded that the transmissiveand reflective characteristics of THz waves can be controlled effectively with the appropriatechoice of graphene parameters as well as the configuration of metamaterial structures. Theconvergence of the analytical and numerical results is found with the published results underspecial conditions. The present work may have potential applications in the design of THzwave controllers, THz reflectors, THz absorbers, and THz anti-reflectors.

Iterative Wavefront Reconstruction for StrongTurbulence Using Shack-Hartmann Wavefront SensorMeasurements

Jae Kim, Bautista Fernandez, and Brij Agrawal

DOI: 10.1364/JOSAA.413934 Received 04 Nov 2020; Accepted 03 Feb 2021; Posted 04 Feb 2021  View: PDF

Abstract: An iterative wavefront reconstruction method using Shack-Hartmann Wavefront Sensor (SHWFS) measurements ispresented in this paper. A new cost function for the wavefront reconstruction problem is derived and the solutionis obtained iteratively using the gradient descent method. The proposed method aims to effectively handle thescintillated SHWFS measurements and to provide simpler and accurate ways to achieve branch-point-tolerantwavefront reconstruction suitable for adaptive optics compensation of strong turbulence. Simulated iterativewavefront reconstruction results show the effectiveness of the proposed method. A laboratory optical testbed isalso presented to show the experimental implementation of the proposed method.

Complexity guided Fourier phase retrieval fromnoisy data

MANSI BUTOLA, Sunaina Rajora, and Kedar Khare

DOI: 10.1364/JOSAA.414276 Received 05 Nov 2020; Accepted 18 Jan 2021; Posted 21 Jan 2021  View: PDF

Abstract: Reconstruction of a stable and good quality solution from noisy single-shot Fourierintensity data is a challenging problem for phase retrieval algorithms. We examine the behaviorof solution provided by hybrid input-output (HIO) algorithm for noisy data from the perspectiveof complexity guidance methodology that was introduced by us in an earlier article [JOSA A36, 202 (2019)]. In our studies, Poisson noise with mean photon counts per pixel in the Fourierintensity data ranges over four orders of magnitude. We find that for noisy Fourier intensitydata, the complexity of the solution outside the support keeps increasing as the HIO iterationsprogress. Based on this observation, a strategy for controlling the solution complexity withinand outside the support during the HIO iterations is proposed and tested for Fourier intensitydata corrupted with Poisson noise. In particular, we actively track and control the growth ofcomplexity of the solution outside the support region with iterations. This in turn provides uswith guidance regarding the level to which complexity of the solution within the support regionneeds to be adjusted, such that, the total solution complexity is equal to that estimated from theraw Fourier intensity data. We observe that the performance of this strategy is noise robust inthe sense that with increasing noise, the quality of the phase solution degrades gradually. Forhigher noise levels the solution loses textural details while retaining the main object features.Our numerical experiments show that the proposed strategy can uniformly handle pure phaseobjects, mixed amplitude-phase objects and the case of dc blocked Fourier intensity data. Theresults may find a number of applications where single-shot Fourier phase retrieval is critical tothe success of the corresponding applications.

LEARNING COMPLEX TEXTURE DISCRIMINATION

Ted Maddess, Dominique Coy, Jessica Herrington, Corinne Carle, Faran Sabeti, and marconi barbosa

DOI: 10.1364/JOSAA.413065 Received 20 Oct 2020; Accepted 10 Jan 2021; Posted 11 Jan 2021  View: PDF

Abstract: Higher-order spatial correlations contribute strongly to visual structure and salience, and are common in the natural environment. One method for studying such structure has been through the use of highly controlled texture patterns whose obvious structure is defined entirely by third and higher order correlations. Here we examine for the first time the effects that longer term training has upon discrimination of 17 such texture types. Training took place in 14 sessions over 42 days. Discrimination performance increased at different rates for different textures. The time required to complete a visit reduced by 25.4% (p=0.0004). Factor analysis was applied to data from the learning and experienced phases of the experiment. That indicated that the gain in speed was accompanied by an increase in the number of mechanisms contributing to discrimination. Learning was not affected by sleep quality but was affected by extreme tiredness (p<0.01). The improved discrimination and speed was retained for 2.5 months. Overall the effects were consistent with perceptual learning. The observed learning is likely related to adaptation of innate mechanisms that underlie our ability to identify non-redundant, visually salient structure in natural images. It may involve V2 and appears to involve increased strength, speed and expanse of connections within our internal representation of this complex perceptual space.

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