138 research outputs found
Cross polarization of nano-objects located on a flat substrate in the presence of a glass microparticle
No full textIn this work, we theoretically show that the deep subwavelength objects located on a dielectric substrate under a glass microcylinder sufficiently close to its bottom point are strongly polarized in the direction that is radial with respect to the microcylinder. This is even in the case when the structure is illuminated by the normally incident light. Though the incident electric field in the area of the objects is polarized almost tangentially to the cylinder surface, a significant cross polarization arises in the object due to its near-field coupling with the cylinder. In accordance with our previous works, the radial polarization is the key prerequisite of the super-resolution granted by a glass microsphere. Extending our results to the 3D case, we claim that the same cross-polarization effect should hold for a glass microsphere. In other words, the reported study shows that the parasitic spread image created by the tangential polarization of the objects should not mask the subwavelength image created by the radial polarization.Peer reviewe
Inverse propagation method for evaluation of super-resolution granted by dielectric microparticles
No full textPublisher Copyright: © 2022 Optica Publishing GroupIn this work we report a theoretical study of the lateral resolution granted by a simple glass microcylinder. In this 2D study, we had in mind the 3D analogue—a microsphere whose ability to form a deeply subwavelength and strongly magnified image of submicrometer objects has been known since 2011. Conventionally, the microscope in which such an image is observed is tuned to see the areas behind the microsphere. This corresponds to the location of the virtual source formed by the microsphere at a distance longer than the distance of the real source to the miscroscope. Recently, we theoretically found a new scenario of super-resolution, when the virtual source is formed in the wave beam transmitted through the microsphere. However, in this work we concentrated on the case when the super-resolution is achieved in the impractical imaging system, in which the microscope objective lens is replaced by a microlens located at a distance smaller than the Rayleigh range. The present paper theoretically answers an important question: Which scenario of far-field nanoimaging by a microsphere grants the finest spatial resolution at very large distances? We found that the novel scenario (corresponding to higher refractive indices) promises further enhancement of the resolution.Peer reviewe
Simovski, “Dynamic model of artificial reactive impedance surfaces
No full textAbstract|New arti cial reactive impedance surfaces have been recently suggested by Sievenpiper et al. for antenna and waveguide applications. In particular, high impedance values corresponding to a magnetic wall can be realized in dense arrays of conducting patches over a conducting plane. In this paper, a dynamic model for the electromagnetic properties of such structures is developed. The analytical model takes into account electromagnetic interactions between all patches in in nite arrays excited by normally incident plane waves, as well as higher-order Floquet modes between the array and the ground plane. The results are compared with the known experiments
Accuracy of homogenization models for finite high-impedance surfaces located in the proximity of a horizontal dipole
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Revisiting substrate-induced bianisotropy in metasurfaces
No full textRecently, it has been shown that a metasurface of plasmonic nanospheres deposited on a highly refractive substrate requires a bianisotropic magnetoelectric coupling for its effective description. The effect has been coined substrate-induced bianisotropy. It leads to an asymmetric reflectance similar to bianisotropic metasurfaces. In this work, through a circuit model, we show that such bianisotropy does not necessarily emerge for all substrated metasurfaces. Indeed, we show that the thickness of the metasurface plays a crucial role to encounter substrate-induced bianisotropy. Moreover, by taking advantage of substrate-induced bianisotropy, we present the necessary conditions for the circuit model parameters to compensate the asymmetric reflectance generated by an intrinsically bianisotropic metasurface. We finally express that, in substrated metasurfaces, the asymmetric reflectance and the bianisotropic response are two separate issues albeit with interdependencies.Peer reviewe
Dielectric microcylinder makes a nanocylindrical trap for atoms and ions
No full textFunding Information: Funding by Russian Foundation for Basic Research (Grant No. 18-02-00315) is acknowledged by V.K. Publisher Copyright: © 2021 American Physical Society. Copyright: Copyright 2021 Elsevier B.V., All rights reserved.In the diffraction of visible light by a dielectric microcylinder, packages of evanescent waves always arise. However, single plane-wave incidence corresponds to rather small impact of evanescent waves outside the cylinder. In this paper, we theoretically show that a pair of plane waves impinging a glass microcylinder under certain conditions may correspond to much higher impact of the evanescent waves. Namely, the interference of the evanescent waves with the propagating ones results in the suppression of the electromagnetic field in an area with very small cross section. This area is located in free space at a substantial distance from the rear side of the microcylinder and along its axis. It may serve a linear optical trap for cold atoms and ions.Peer reviewe
Unbounded spatial spectrum of propagating waves in a polaritonic wire medium
No full textIn this paper, we study a topological phase transition in a wire medium operating at infrared frequencies. This transition occurs in the reciprocal space between the indefinite (open-surface) regime of the metamaterial and its dielectric (closed-surface) regime. Due to the spatial dispersion inherent to a wire medium, a hybrid regime turns out to be possible at the transition frequency. Both such surfaces exist at the same frequency and touch one another. At this frequency, all values of the parallel wave vector correspond to propagating spatial harmonics. The implication of this regime is the overwhelming radiation enhancement. We numerically investigate the gain in radiated power for a subwavelength dipole source submerged into such medium. In contrast to previous works, this gain (called the Purcell factor) turns out to be higher for a parallel dipole than for a perpendicular one.Peer reviewe
Extending a Birdcage Coil for Magnetic Resonance Imaging of a Human Head with an Artificial Magnetic Shield
No full textIn magnetic resonance imaging, a birdcage coil is the most commonly used volumetric resonator creating a highly homogeneous radiofrequency magnetic field in a conductive sample. An artificial magnetic radiofrequency shield was recently shown to improve the magnetic field amplitude per unit power (transmit efficiency) of a preclinical birdcage coil by reducing the intrinsic losses in the coil and increasing power absorbed by the sample. In this paper, we propose a new application of an artificial shield in clinical MRI. Thanks to the proposed artificial shield a birdcage coil for human brain imaging operating at 300 MHz (Larmor frequency of protons at static fields of 7 T) can be expanded to increase free space. As a result, the coil becomes more comfortable for the patient and keeping comparable transmit efficiency. The same extended coil with a conventional copper shield would have at least 10% lower efficiency. The proposed artificial shield is implemented as an annular-ring cavity-backed slot in a copper cylinder that tightly surrounds the birdcage. To demonstrate the effect, radiofrequency magnetic field and specific absorption rate distributions were compared numerically and experimentally for the initial and extended coils with different shields.Peer reviewe
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