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Within the framework of the T-matrix method, we present a modeling tool that predicts the optical response from the Nanosphere-on-Mirror (NSoM) construct. The nonclassical effects in metals are accounted for by the nonlocal hydrodynamic Drude model (NLHDM) or the surface response model (SRM). Two essential elements in the T-matrix method, i.e., the T-matrix of the sphere and the R matrix accounting for the effects of the mirror, have been fully upgraded to include longitudinal waves for the NLHDM and the augmented interface conditions for the SRM. The proposed tool is quantitatively validated both in the near and the far field by an in-house developed BEM solver for the NLHDM where the gap between the sphere and the mirror is as small as 1 nm. Two physical checks are performed, where the results from the classical local response model are compared with the ones from the NLHDM and the SRM. The observed shifts in resonances and reduced field enhancements in the gap region agree well with previous physical findings. The proposed tool may not only serve as a reference tool for other numerical methods, but also provides an ideal platform for investigating nonclassical optical processes in the NSoM, hence paving a semi-analytical way to understand the extreme optics at very small scales.

We investigate a fundamental electromagnetic theorem, namely the uniqueness theorem, in the context of nonlocal electromagnetics, as simulated by a popular semiclassical model, the Hydrodynamic Drude Model (HDM) and extensions thereof such as the Generalized Nonlocal Optical Response (GNOR). The derivations and proofs presented here give a theoretical foundation to the use of the Additional Boundary Conditions (ABCs), whose necessity is recognized and underlined in virtually all implementations and applications of HDM. Our proofs follow a mathematically relaxed style, borrowing from the literature of established electromagnetics textbooks that study the matter from an engineering perspective. Through this simpler route we deduce clear and intuitive material-response requirements for uniqueness to hold, while using a familiar parlance in a topic that is mostly studied through a physics perspective. Two numerical examples that examine the problem from either a semianalytical or a purely numerical viewpoint support our findings., Comment: This work has been submitted to the IEEE for possible publication

This paper presents a quasi-deterministic ray tracing (QD-RT) method for analyzing the propagation of electromagnetic waves in street canyons. The method uses a statistical bistatic distribution to model the Radar Cross Section (RCS) of various irregular objects such as cars and pedestrians, instead of relying on exact values as in a deterministic propagation model. The performance of the QD-RT method is evaluated by comparing its generated path loss distributions to those of the deterministic ray tracing (D-RT) model using the Two-sample Cramer-von Mises test. The results indicate that the QD-RT method generates the same path loss distributions as the D-RT model while offering lower complexity. This study suggests that the QD-RT method has the potential to be used for analyzing complicated scenarios such as street canyon scenarios in mmWave wireless communication systems.

In electromagnetic inverse scattering, the goal is to reconstruct object permittivity using scattered waves. While deep learning has shown promise as an alternative to iterative solvers, it is primarily used in supervised frameworks which are sensitive to distribution drift of the scattered fields, common in practice. Moreover, these methods typically provide a single estimate of the permittivity pattern, which may be inadequate or misleading due to noise and the ill-posedness of the problem. In this paper, we propose a data-driven framework for inverse scattering based on deep generative models. Our approach learns a low-dimensional manifold as a regularizer for recovering target permittivities. Unlike supervised methods that necessitate both scattered fields and target permittivities, our method only requires the target permittivities for training; it can then be used with any experimental setup. We also introduce a Bayesian framework for approximating the posterior distribution of the target permittivity, enabling multiple estimates and uncertainty quantification. Extensive experiments with synthetic and experimental data demonstrate that our framework outperforms traditional iterative solvers, particularly for strong scatterers, while achieving comparable reconstruction quality to state-of-the-art supervised learning methods like the U-Net., Comment: 13 pages, 11 figures

Plasmonic vortex lenses (PVLs), producing vortex modes, known as plasmonic vortices (PVs), in the process of plasmonic spin-orbit coupling, provide a promising platform for the realization of many optical vortex-based applications. Very recently, it has been reported that a single PVL can generate multiple PVs. This work exploits the representation theory of finite groups, reveals the symmetry origin of the generated PVs, and derives a new conservation relation based on symmetry principles. Specifically, the symmetry principles divide the near field of the PVL into regions, designate integers, which are the topological charges, to the regions, and, particularly, give an upper bound to the topological charge of the PV at the center of the PVL. Further application of the symmetry principles to the spin-orbit coupling process leads to a new conservation relation. Based on this relation, a two-step procedure is suggested to link the angular momentum of the incident field with the one of the generated PVs through the symmetries of the PVL. This theory is well demonstrated by numerical calculations. This work provides an alternative but essential symmetry perspective on the dynamics of spin-orbit coupling in PVLs, forms a strong complement for the physical investigations performed before, and therefore lays down a solid foundation for flexibly manipulating the PVs for emerging vortex-based nanophotonic applications., Comment: 23 pages and 3 figures

Electromagnetic (EM) skyrmions are an EM analogue of the skyrmions in condensed matter physics, which offer new degrees of freedom to structure light and manipulate light matter interactions and thus promise various groundbreaking applications in optics and photonics. Recently, there is a growing interest in composing EM skyrmions based on different field vectors of EM waves. Here, we realize an EM skyrmion, i.e., a spoof plasmonic skyrmion, using the electric field vectors of spoof localized surface plasmons (spoof LSPs) in a planar microwave resonator with rotational and mirroring symmetries. We construct the spoof plasmonic skyrmion, which holds a hedgehog-like configuration in its electric field vectors, by synthesizing a scalar vortex with a topological charge 0 in the out-of-plane component of the fields, and a polarization vortex with a topological charge 1 in the in-plane component of the fields. Besides an experimental demonstration of this skyrmion, we employ group representation theory and pinpoint the symmetry origin of the skyrmion. Such an investigation demonstrates the ubiquity of the existence of the skyrmion in any planar EM resonator holding rotational and mirroring symmetries, regardless the dimensions and the operating frequencies. The designed skyrmion not only promises novel microwave applications for sensing, processing, storing and transferring information, but also lays down a general guideline for devising skyrmions operating over a broad range in the EM spectra owing to the fact that the conducted symmetry investigation is independent of specific dimension or frequency.

Coherent interconversion of signals between optical and mechanical domains is enabled by optomechanical interactions. Extreme light-matter coupling produced by confining light to nanoscale mode volumes can then access single mid-infrared (MIR) photon sensitivity. Here we utilise the infrared absorption and Raman activity of molecular vibrations in plasmonic nanocavities to demonstrate frequency upconversion. We convert {\lambda}~10 {\mu}m incoming light to visible via surface-enhanced Raman scattering (SERS) in doubly-resonant antennas that enhance upconversion by >10^10. We show >200% amplification of the SERS antiStokes emission when a MIR pump is tuned to a molecular vibrational frequency, obtaining lowest detectable powers ~1 {\mu}W/{\mu}m^2 at room temperature. These results have potential for low-cost and large-scale infrared detectors and spectroscopic techniques, and bring single-molecule sensing into the infrared

In this paper, a route to ultra-broadband absorption is suggested and demonstrated by a feasible design. The high absorption regime (absorption above 90%) for the suggested structure ranges from visible to mid-infrared (MIR), i.e. for the wavelength from 478 to 3,278 nm that yields an ultra-wide bandwidth of 2,800 nm. The structure consists of a top-layer-patterned metal-insulator-metal (MIM) configuration, into the insulator layer of which, an ultra-thin 5 nm layer of Manganese (Mn) is embedded. The MIM configuration represents a Ti-Al2O3-Ti tri-layer. It is shown that, without the ultra-thin layer of Mn, the absorption bandwidth is reduced to 274 nm. Therefore, adding only a 5 nm layer of Mn leads to a more than tenfold increase in the width of the absorption band. It is explained in detail that the physical mechanism contributing to this ultra-broadband result is a combination of plasmonic and non-plasmonic resonance modes, along with the appropriate optical properties of Mn. This structure has the relative bandwidth (RBW) of 149%, while only one step of lithography is required for its fabrication, so it is relatively simple to fabricate. This makes it rather promising for practical applications., Comment: 17 pages, 12 figures

Massive MIMO is considered a key technology for 5G. Various studies analyze the impact of the number of antennas, relying on channel properties only and assuming uniform antenna gains in very large arrays. In this paper, we investigate the impact of mutual coupling and edge effects on the gain pattern variation in the array. Our analysis focuses on the comparison of patch antennas versus dipoles, representative for the antennas typically used in massive MIMO experiments today. Through simulations and measurements, we show that the finite patch array has a lower gain pattern variation compared with a dipole array. The impact of a large gain pattern variation on the massive MIMO system is that not all antennas contribute equally for all users, and the effective number of antennas seen for a single user is reduced. We show that the effect of this at system level is a decreased rate for all users for the zero-forcing MIMO detector, up to 20% for the patch array and 35% for the dipole array. The maximum ratio combining on the other hand, introduces user unfairness., Comment: 9 pages, IEEE Transactions on Antennas and Propagation

Though commonly used to calculate Q-factor and fractional bandwidth, the energy stored by radiating systems (antennas) is a subtle and challenging concept that has perplexed researchers for over half a century. Here, the obstacles in defining and calculating stored energy in general electromagnetic systems are presented from first principles as well as using demonstrative examples from electrostatics, circuits, and radiating systems. Along the way, the concept of unobservable energy is introduced to formalize such challenges. Existing methods of defining stored energy in radiating systems are then reviewed in a framework based on technical commonalities rather than chronological order. Equivalences between some methods under common assumptions are highlighted, along with the strengths, weaknesses, and unique applications of certain techniques. Numerical examples are provided to compare the relative margin between methods on several radiating structures., Comment: This update corrects the constant in (23), which was missing a factor of 1/k in the original version. Many thanks to Lars Jonsson for pointing out this error

This paper deals with the old yet unsolved problem of defining and evaluating the stored electromagnetic energy - a quantity essential for calculating the quality factor, which reflects the intrinsic bandwidth of the considered electromagnetic system. A novel paradigm is proposed to determine the stored energy in the time domain leading to the method, which exhibits positive semi-definiteness and coordinate independence, i.e. two key properties actually not met by the contemporary approaches. The proposed technique is compared with two up-to-date frequency domain methods that are extensively used in practice. All three concepts are discussed and compared on the basis of examples of varying complexity, starting with lumped RLC circuits and finishing with realistic radiators., Comment: 20 pages, 13 figures