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The open-ended coaxial probe (OECP) technique is one of the most commonly used methods for the characterization of homogeneous media properties, especially in the biomedical sciences. However, when considering inhomogeneous media, the effect of the heterogeneity on the probe terminal admittance is unclear, making the measured admittance hard to interpret and relate to the medium properties. In this paper we present an analytical model for the contribution of an isotropic scatterer embedded in an otherwise homogeneous medium to the probe admittance. We utilize rigorous scattering theory and various approximations to obtain simplified, closed-form expressions. Using the obtained results we present a method to accurately extract the scatterer properties from a measurement of the admittance. In addition, we define the sensing depth, and show how it can be mapped as a function of the expected scatterer properties. Full-wave simulations are used to verify the analytical model, and the proposed method paves a path for further generalization to additional scenarios of open-coaxial probe sensing of an inhomogeneous medium., Comment: 7 pages, 6 figures

Nanoscale magnetic resonance imaging (nanoMRI) aims at obtaining structure at the single molecule level. Most of the techniques for effecting a nanoMRI gradient use small permanent magnets. Here, we present a switchable magnetic field gradient on a tip, which is designed to provide a local and controllable magnetic field with a high gradient on the nanometer scale. We incorporate the gradient field with a nanoscale magnetic resonance sensor, a single nitrogen-vacancy (NV) center in diamond, to provide high-resolution magnetic resonance imaging. The device is a metal microwire deposited along a quartz tip, with the current flowing along the tip inducing a magnetic field around its apex. This field can be manipulated throughout a measurement by controlling the current along the wire. We achieved gradients as high as 1 $\mathrm{\mu}\text{T/nm}$ at fields weaker than 200 $\mathrm{\mu}\text{T}$. Such a gradient can facilitate electron spin mapping with 1 nm resolution using single NV sensors, allowing for nanoscale imaging of electrons. The ability to switch the current on and off and to position the device with high precision overcomes limitations such as limited emitter contrast and the flexibility in sample preparation. Moreover, we show that proximity of the metallic tip to the sensor modifies the Rabi power in a spatially dependent manner, providing regions with enhanced ($\times$3.5) and decreased Rabi power. This spatial gradient, induced by the tip, offers the opportunity for selective pulses on nearby spin species where the same microwave power will result in different spin manipulation characteristics.

We analytically and numerically study the nonreciprocal surface waves guided by a magnetized metasurface tube. When applying the magnetic bias perpendicularly to the cylinder axis, the conductivity profile cross-section is nonuniform, which enables us to obtain pronounced nonreciprocal modes with different field distributions for propagation in opposite directions due to the coupling between different values of orbital angular momentum. We show that this property can be leveraged for directional power delivery when changing the source location, and we study the isolation ratio as a function of different parameters. The simple, continuous dependence of the isolation on the direction of magnetization allows simple yet robust control over the wave propagation properties., Comment: 10 pages, 5 figures

The advent of ultrafast science with pulsed electron beams raised the need in controlling the temporal features of the electron pulses. One promising suggestion is the nano-selective quantum optics with multi-electrons, which scales quadratically with the number of electrons within the coherence time of the quantum system. Terahertz (THz) radiation from optical nonlinear crystals is an attractive methodology to generate the rapidly varying electric fields necessary for electron compression, with an advantage of an inherent temporal locking to laser-triggered electrons, such as in untrafast electron microscopes. Longer (picosecond-) pulses require sub-THz field for their compression, however, the generation of such low frequencies require pumping with energetic optical pulses and their focusability is fundamentally limited by their mm-wavelength. This work proposes electron-pulse compression with sub-THz fields directly in the vicinity of their dipolar origin, thereby avoiding mediation through radiation. We analyze the merits of nearfields for compression of slow electrons particularly in challenging regimes for THz radiation, such as small numerical apertures, micro-joule-level optical pump pulses, and low frequencies. This sheme can be implemented within the tight constraints of electron microscopes and reach fiels of a few kV/cm below 0.1 THz at high repetition rates. Our paradigm offers a realistic approach for controlling electron pulses spatially and temporally in many experiments, opening the path of flexible multi-electron manipulation for analytic and quantum sciences., Comment: 22 pages, 3 figures

Electromagnetic wave interaction with time-varying systems has gained a lot of research interest in recent years. The temporal modulation gives unprecedented control over the response, allowing us to go beyond the state-of-the-art in passive systems. In this work, we use time variation to derive a model for a deep-subwavelength direction-of-arrival (DoA) sensing apparatus with enhanced performance and sensitivity. We formulate the problem, derive an analytical model, and discuss the various physical mechanisms responsible for the enhancement. We show that time modulation enables a new degree of control that can be used to optimize the response for various incident frequencies, allowing for wideband operation. Additionally, we show that incorporating the currents from higher generated harmonics into the sensing scheme allows us to extract more accurate information about the impinging wave., Comment: 19 pages, 6 figures

Twisted two-dimensional bi-layers offer exquisite control on the electronic bandstructure through the interlayer rotation and coupling, enabling magic-angle flat-band superconductivity and moir\'e excitons. Here, we demonstrate how analogous principles, combined with large anisotropy, enable extreme control and manipulation of the photonic dispersion of phonon polaritons (PhPs) in van der Waals (vdW) bi-layers. We experimentally observe tunable topological transitions from open (hyperbolic) to closed (elliptic) dispersion contours in twisted bi-layered {\alpha}-MoO3 at photonic magic angles, induced by polariton hybridization and robustly controlled by a topological quantity. At these transitions the bilayer dispersion flattens, exhibiting low-loss tunable polariton canalization and diffractionless propagation with resolution below {\lambda}0/40. Our findings extend twistronics and moir\'e physics to nanophotonics and polaritonics, with great potential for nano-imaging, nanoscale light propagation, energy transfer and quantum applications., Comment: Nature, in press

Recent advances in twistronics of low-dimensional materials, such as bilayer graphene and transition-metal dichalcogenides, have enabled a plethora of unusual phenomena associated with moir\'e physics. However, several of these effects require demanding manipulation of superlattices at the atomic scale, such as the careful control of rotation angle between two closely spaced atomic lattices. Here, we study moir\'e hyperbolic plasmons in pairs of hyperbolic metasurfaces (HMTSs), unveiling analogous phenomena at the mesoscopic scale. HMTSs are known to support confined surface waves collimated towards specific directions determined by the metasurface dispersion. By rotating two evanescently coupled HMTSs with respect to one another, we unveil rich dispersion engineering, topological transitions at magic angles, broadband field canalization, and plasmon spin-Hall phenomena. These findings open remarkable opportunities to advance metasurface optics, enriching it with moir\'e physics and twistronic concepts., Comment: 17 pages, 4 figures

Using the formulation of electrodynamics in rotating media, we put into explicit quantitative form the effect of rotation on interference and diffraction patterns as observed in the rotating medium's rest-frame. As a paradigm experiment we focus the interference generated by a linear array of sources in a homogeneous medium. The interference is distorted due to rotation; the maxima now follow curved trajectories. Unlike the classical Sagnac effect in which the rotation induced phase is independent of the refraction index $n$, here the maxima bending increases when $n$ decreases, suggesting that $\epsilon$-near-zero metamaterials can enhance optical gyroscopes and rotation-induced non-reciprocal devices. This result is counter intuitive as one may expect that a wave that travels faster would bend less. The apparent contradiction is clarified via the Minkowski momentum picture for a quasi-particle model of the interference that introduces the action of a Coriolis force, and by the Abraham picture of the wave-only momentum. our results may also shed light on the Abraham-Minkowski controversy as examined in non-inertial electrodynamics.

Moving metasurfaces support guided waves exhibiting unusual optical properties, including strong anisotropy, nonreciprocity, and hyperbolic dispersion. However, for these phenomena to be noticeable, high speeds are typically required, challenging their practical implementation. Here, we show a viable route towards the realization of one-way hyperbolic propagation in metasurfaces placed synthetically in motion through traveling-wave space-time modulation. In addition to non-reciprocal hyperbolic propagation, the proposed time-modulation scheme induces additional exotic opportunities for nanophotonic systems, such as efficient nonreciprocal frequency conversion and mode transfer., Comment: 31 pages, 9 figures, Includes supporting material following the article. arXiv admin note: text overlap with arXiv:1809.01638

Highly confined surface waves present unique opportunities to enhance light interactions with localized emitters or molecules. Hyperbolic dispersion in metasurfaces allows us to tailor and manipulate surface waves, enhancing the local density of states over broad bandwidths. So far, propagation on this platform was mainly studied in planar geometries, which facilitates the analysis but somehow limits the realm of possibilities. Here we show that "wrapping" hyperbolic metasurfaces into tubes may greatly enrich the wave propagation dynamics along their axis. This system shows strong interaction with fields and sources carrying optical angular momentum, pronounced field asymmetries, and opens pathways to valley-specific excitation and routing. In addition, we demonstrate that various parameter regimes enable strong spin and helicity to momentum locking., Comment: 26 pages, 5 figures

Hyperbolic propagation offers exciting opportunities in nanophotonics, from sub-diffraction imaging to enhanced local density of states. This transport regime is typically induced by strong modulation of conductivity, i.e., with alternating metallic and dielectric material properties. Here, we analyze a moving impedance surface, showing that suitably tailored homogeneous metasurfaces can support one-way hyperbolic propagation when in motion, adding non-reciprocity to hyperbolic propagation phenomena, and without suffering from nonlocal effects stemming from discretization or finite granularity of the surface., Comment: 16 pages, 3 figures

1D lattice summations of the 3D Green's function are needed in many applications such as photonic crystals, antenna arrays, and so on. Such summations are usually divided into two cases, depending on the location of the observer: Out of the summation axis, or on the summation axis. Here, as a service for the community, we present and summarize the summation formulas for both cases. On the summation axis, we use polylogarithmic functions to express the summation, and Away from the summation axis we use Poisson summation (equivalent to the expansion of the field to cylindrical harmonics)

Almost periodic particle chains exhibit peculiar propagation properties that are not observed in perfectly periodic ones. Furthermore, since they inherently support non-negligible long-range interactions and radiation through the surrounding free-space, nearest-neighbor approximations cannot be invoked. Hence the governing operator is fundamentally different than that used in traditional analysis of almost periodic structures, e.g. Harper's model and Almost-Mathieu difference equations. We present a mathematical framework for the analysis of almost periodic particle chains, and study their electrodynamic properties. We show that they support guided modes that exhibit a complex interaction mechanism with the light-cone. These modes possess a two-dimensional fractal-like structure in the frequency-wavenumber space, such that a modal phase-velocity cannot be uniquely defined. However, a well defined \emph{group velocity} is revealed due to the fractal's inner-structure., Comment: 10 pages, 9 figures

Confluent with the single dimension of time, breach of time-reversal symmetry is usually perceived as a one-dimensional concept. In its ultimate realization--the one-way guiding device--it allows optical propagation in one direction, say $+z$, and forbids it in the opposite direction, $-z$. Hence, in studies of time-reversal asymmetry the mapping $t\mapsto -t$ is naturally associated with $z\mapsto -z$. However, strongly non-reciprocal or one-way nano-scale threads can be used \emph{to weave meta-surfaces} thus adding dimensions to this concept. In this new family of surfaces the aforementioned association \emph{cannot be made}. An example of appropriate threads are the planar one-way particle chains based on the two-type rotation principle. The resulting surfaces--the meta-weaves--posses generalized non-reciprocity such as "sector-way" propagation, and offer new possibilities for controlling light in thin surfaces. We study several meta-weave designs and their asymmetries in the wave-vector space.

When a linear chain of plasmonic nano-particles is exposed to a transverse DC magnetic field, the chain modes are elliptically polarized, in a single plane parallel to the chain axis; hence, a novel longitudinal plasmon-rotation is created. If, in addition, the chain geometry possesses longitudinal rotation, e.g. by using ellipsoidal particles that rotate in the same plane as the plasmon rotation, strong non-reciprocity is created. The structure possesses a new kind of chirality--the longitudinal chirality--and supports one-way guiding. Since all particles rotate in the same plane, the geometry is planar and can be fabricated by printing leaf-like patches on a single plane. Furthermore, the magnetic field is significantly weaker than in previously reported one-way guiding structures. These properties are examined for ideal (lossless) and for lossy chains., Comment: to appear in PRB