Cerebral microbleeds (CMBs) are small haemorrhages nearby blood vessels. They have been recognized as important diagnostic biomarkers for many cerebrovascular diseases and cognitive dysfunctions. In ...current clinical routine, CMBs are manually labelled by radiologists but this procedure is laborious, time-consuming, and error prone. In this paper, we propose a novel automatic method to detect CMBs from magnetic resonance (MR) images by exploiting the 3D convolutional neural network (CNN). Compared with previous methods that employed either low-level hand-crafted descriptors or 2D CNNs, our method can take full advantage of spatial contextual information in MR volumes to extract more representative high-level features for CMBs, and hence achieve a much better detection accuracy. To further improve the detection performance while reducing the computational cost, we propose a cascaded framework under 3D CNNs for the task of CMB detection. We first exploit a 3D fully convolutional network (FCN) strategy to retrieve the candidates with high probabilities of being CMBs, and then apply a well-trained 3D CNN discrimination model to distinguish CMBs from hard mimics. Compared with traditional sliding window strategy, the proposed 3D FCN strategy can remove massive redundant computations and dramatically speed up the detection process. We constructed a large dataset with 320 volumetric MR scans and performed extensive experiments to validate the proposed method, which achieved a high sensitivity of 93.16% with an average number of 2.74 false positives per subject, outperforming previous methods using low-level descriptors or 2D CNNs by a significant margin. The proposed method, in principle, can be adapted to other biomarker detection tasks from volumetric medical data.
When hitting interfaces between two different media, light beams may undergo small shifts. Such beam shifts cannot be described by the geometrical optics based on Snell's law and their underlying ...physics has attracted much attention. Conventional beam shifts like Goos-Hänchen shifts and Imbert-Fedorov shifts not only require obliquely incident beams but also are mostly very small compared to the wavelength and waist size of the beams. Here we propose a method to realize large and controllable polarization-dependent lateral shifts for normally incident beams with photonic crystal slabs. As a proof of the concept, we engineer the momentum-space geometric phase distribution of a normally incident beam by controlling its interaction with a photonic crystal slab whose momentum-space polarization structure is designed on purpose. The engineered geometric phase distribution is designed to result in a large shift of the beam. We fabricate the designed photonic crystal slab and directly observe the beam shift, which is ~5 times the wavelength and approaches the waist radius. Based on periodic structures and only requiring simple manipulation of symmetry, our proposed method is an important step towards practical applications of beam shifting effects.
We propose to use transformation optics to generate a general illusion such that an arbitrary object appears to be like some other object of our choice. This is achieved by using a remote device that ...can transform the scattered light outside a virtual boundary into that of the object chosen for the illusion, irrespective of the profile and direction of the incident light. This type of illusion device also enables people to see through walls. Our work extends the concept of cloaking as a special form of illusion to the wider realm of illusion optics.
•A selective emitter for thermophotovoltaic systems is theoretically and experimentally demonstrated.•The wavelength-selective emission is realized by multipole resonances excited in dielectric ...nanodisks.•The coupling between multipole resonances and the tungsten layer induces magnetic dipolar resonances.•The anapole-induced emission peak can couple with the lattice resonance and achieve higher emissivity.•The proposed selective emitter is successfully fabricated and experimentally verified.
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Thermophotovoltaic systems can harvest electric energy from heat sources with a potential efficiency exceeding the Shockley–Queisser limit due to the selective emission of an elaborate thermal emitter. In this work, a two-dimensional nanodisks/thin-film metamaterial is proposed as a wavelength-selective emitter, which can coordinate well with the photovoltaic cell in a thermophotovoltaic system. Compared to conventional emitters based on surface plasmon polaritons, the emittance peaks of the proposed emitter are realized by the excitations of both electric dipole and anapole modes in silicon nanodisks, which can be easily tailored due to the non-dispersive optical constants of dielectric materials. Meanwhile, the effect of polarization and polar angle on the emittance spectra is also investigated, suggesting that the proposed emitter has high emittance and efficiency not only in the normal direction but also at large oblique angles. Electromagnetic field and current density distributions reveal that the coupling between multipole resonances and the bottom tungsten layer can induce a magnetic dipolar resonance. Therefore, the wavelengths of both emittance peaks are sensitive to the period paralleled to the incident electric field. Besides, the anapole-induced mode can couple with the lattice resonance, resulting in higher emittance. Moreover, the proposed emitter is successfully fabricated, and the measured spectra agree well with the theoretical results. The fundamental understanding and insights obtained here will facilitate the active design and application of novel multipole-based emitters in enhancing energy conversion.
Propagation behaviors of electromagnetic waves are governed by the equifrequency surface of the medium. Up to now, ordinary materials, including the medium exist in nature and the man-made ...metamaterials, always have an equifrequency surface (ellipsoid or hyperboloid) centered at zero k-point. Here we propose a new type of metamaterial possessing multiple index ellipsoids centered at arbitrary nonzero k-points. Their locations in momentum space are determined by the connectivity of a set of interpenetrating metallic scaffolds, whereas the group velocities of the modes are determined by the geometrical details. Such system is a new class of metamaterial whose properties arise from global connectivity and hence can have broadband functionality in applications such as negative refraction, orientation-dependent coupling effect, and cavity without walls, and they are fundamentally different from ordinary resonant metamaterials that are inherently bandwidth limited. We perform microwave experiments to confirm our findings.
Abstract
Singularities of non-Hermitian systems typified by exceptional points (EPs) are critical for understanding non-Hermitian topological phases and trigger many intriguing phenomena. However, it ...remains unexplored what happens when EPs meet one another. Here, in a typical four-level model with both touching and crossing intersections of EP hypersurfaces, we report the interconversion mechanisms between EPs of different orders. By examining both the eigenvalues and eigenvectors, we show analytically that all EPs of higher orders are formed at the touching intersections of two different types of EP hypersurfaces of lower orders. Contrarily, the crossing intersection of EP structures lowers the order of EPs. The mechanisms of the increase and decrease in defectiveness discovered here are expected to hold for EPs of any order in various non-Hermitian systems, providing a comprehensive understanding of EPs and inspiration toward advanced applications such as biosensing and information processing.
Abstract
Carbon dioxide (CO
2
) has been linked to many deleterious health effects, and it has also been used as a proxy for building occupancy measurements. These applications have created a need ...for low-cost and low-power CO
2
sensors that can be seamlessly incorporated into existing buildings. We report a resonant mass sensor coated with a solution-processable polymer blend of poly(ethylene oxide) (PEO) and poly(ethyleneimine) (PEI) for the detection of CO
2
across multiple use conditions. Controlling the polymer blend composition and nanostructure enabled better transport of the analyte gas into the sensing layer, which allowed for significantly enhanced CO
2
sensing relative to the state of the art. Moreover, the hydrophilic nature of PEO resulted in water uptake, which provided for higher sensing sensitivity at elevated humidity conditions. Therefore, this key integration of materials and resonant sensor platform could be a potential solution in the future for CO
2
monitoring in smart infrastructure.
Two-dimensional layers of molybdenum disulfide, MoS2, have been recognized as promising materials for nanoelectronics due to their exceptional electronic and optical properties. Here we develop a new ...ReaxFF reactive potential that can accurately describe the thermodynamic and structural properties of MoS2 sheets, guided by extensive density functional theory simulations. This potential is then applied to the formation energies of five different types of vacancies, various vacancy migration barriers, and the transition barrier between the semiconducting 2H and metallic 1T phases. The energetics of ripplocations, a recently observed defect in van der Waals layers, is examined, and the interplay between these defects and sulfur vacancies is studied. As strain engineering of MoS2 sheets is an effective way to manipulate the sheets’ electronic and optical properties, the new ReaxFF description can provide valuable insights into morphological changes that occur under various loading conditions and defect distributions, thus allowing one to tailor the electronic properties of these 2D crystals.
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