Achromatic devices have wide application prospects in radar and imaging fields. However, chromatic aberration and limited bandwidth restrict their development. Moreover, broadband and highly efficient achromatic devices working in transmission mode are still difficult to realize. In this paper, broadband highly efficient achromatic transmission in the microwave region by a metasurface is achieved. First, the ideal dispersion conditions of achromatic meta-atoms are given. Then, a polarization selective grating metasurface and a split ring slot metasurface are designed using the transfer matrix method and equivalent circuit theory, respectively. The former is used to control phase characteristics while the latter enables controlling dispersion. Phase and dispersion can be controlled independently by cascading them and any phase curve can be designed as is desired. In order to verify the strategy, an achromatic deflector and an achromatic lens are designed and samples are fabricated. The experimental results show that the deflector can realize achromatic refraction from 9.3 to 12.3 GHz with average efficiency 77.5% and the lens can realize achromatic focusing from 9.8 to 12.2 GHz with average efficiency 78.9%, respectively. The experimental results are in good agreement with theory. The findings provide valuable strategy for achromatic devices design, which can be widely applied.

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Optical antennas enable efficient coupling between propagating light and bonding electromagnetic waves like surface plasmon polaritons (SPPs). Under the illumination of inhomogeneous optical fields, propagating SPPs mediated by multimode antennas could be spatially asymmetric and the asymmetry strongly depends on the position of the antennas relative to the illumination field. Here we develop such asymmetric excitation of SPPs through illuminating a pair of slot antennas with the (1,0) mode Hermite-Gaussian beam. The physical scenario of the interaction between the illumination optical field and the paired slot antennas are elaborated by full-wave electromagnetic simulations. We also carry out experiments to monitor the asymmetric SPPs propagation with a back-focal plane imaging technique. By retrieving the asymmetric intensity ratio of the SPP pattern in the back-focal plane image, lateral displacement of the antennas down to angstrom level is demonstrated.

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Carrying orbital angular momentum per photon, the optical vortex has elicited widespread interest. Here, we demonstrate that dual coaxial longitudinal polarization vortices can appear upon a nonparaxial propagation of a tightly focused Pancharatnam-Berry tailored Laguerre-Gaussian beam. Most importantly, it is capable of accessing arbitrary independent topological charges for both vortices, as well as predesigned tunable spacing distances between them.

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We introduce an optimal metagrating design for transverse-position metrology in presence of photon shot noise. The proposed working principle is closely related to the formation of a phase vortex in the diffraction orders in the parameter space. Using the topological robustness, we optimize the design and compress all the transverse-position information around a certain point into a small number of detected photons, saturating the shot-noise limit had all the photons used for probing the position been detected. The current scheme avoids the problem of detector saturation in the presence of high probe power while maintaining all the information detected, allowing one to make full use of the high power that is available. Besides, the direct link between the resonant property in the unit cell and the conditions to achieve the bound is given: one with the zeroth dipole resonance and the other one with the anapole condition of the first dipole. The connection between the metagrating design and the optimization using topological robustness along with the fundamental precision limit using classical light gives new insights in all of these fields.

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We propose a scheme to retrieve the size parameters of a nanoparticle on a glass substrate at a scale much smaller than the wavelength. This is achieved by illuminating the particle using two plane waves to create rich and nontrivial local polarization distributions, and observing the far-field scattering pattern into the substrate. By using this illumination to control the induced complex dipole moment, the exponential decay of power radiated into the supercritical region, as well as directional scattering due to spin-orbit coupling can be exploited to retrieve the particle's shape, size, and position directly from the far-field scattering with high sensitivity and without the need for a complicated and time-consuming optimization algorithm. Our method brings about a far-field superresolution nanometrology scheme based on the interaction of vectorial light with nanoparticles.

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It is generally believed that when a single metallic nanowire is sufficiently small, it scatters like a point electric dipole. We show theoretically when a metallic nanowire is placed inside specially designed beams, the magnetic dipole contribution along with the electric dipole resonance can lead to unidirectional scattering in the far field, fulfilling Kerker’s condition. Remarkably, this far-field unidirectional scattering encodes information that is highly dependent on the nanowire’s deflection at a scale much smaller than the wavelength. The special roles of small but essential magnetic response along with the plasmonic resonance are highlighted for this extreme sensitivity as compared with the dielectric counterpart. In addition, the same essential role of the magnetic dipole contribution is also presented for a very small metallic nanosphere.@en

Hyperbolic Meta-Materials (HMMs) are anisotropic materials with permittivity tensor that has both positive and negative eigenvalues. Here we report that by using a type II HMM as cladding material, a waveguide which only supports higher order modes can be achieved, while the lower order modes become leaky and are absorbed in the HMM cladding. This counter-intuitive property can lead to novel application in optical communication and photonic integrated circuit. The loss in our HMM-Insulator-HMM (HIH) waveguide is smaller than that of similar guided mode in a Metal-Insulator-Metal (MIM) waveguide.

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Identifying subwavelength objects and displacements is of crucial importance in optical nanometrology. We show in this Letter that nanoantennas with subwavelength structures can be excited precisely by incident beams with singularity. This accurate feeding beyond the diffraction limit can lead to dynamic control of the unidirectional scattering in the far field. The combination of the field discontinuity of the incoming singular beam with the rapid phase variation near the antenna leads to remarkable sensitivity of the far-field scattering to the displacement at a scale much smaller than the wavelength. This Letter introduces a far-field deep subwavelength position detection method based on the interaction of singular optics with nanoantennas.@en

Hyperbolic metamaterials (HMMs) are anisotropic materials with a permittivity tensor that has both positive and negative eigenvalues. Here we report that by using a type II HMM as a cladding material, a waveguide that only supports higher-order modes can be achieved, while the lower-order modes become leaky and are absorbed in the HMM cladding. This counter-intuitive property can lead to novel application in optical communications and photonic integrated circuits. The loss in our HMM insulator-HMM (HIH) waveguide is smaller than that of similar guided modes in a metal-insulator-metal (MIM) waveguide.

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The existence of non-radiating sources is of fundamental importance for inverse scattering problems and the design of invisible objects. However, the excitation of such radiationless sources is quite challenging. We present a method based on which the anapole mode of a high-index isotropic dielectric nanosphere can be excited but radiationless. We show that this radiationless anapole is attributed to the destructive interference of the Cartesian dipole and toroidal moment of the induced current by our proposed focused radially polarized beam illumination. Further, with a standing-wave illumination formed by two counter-propagating focused radially polarized beams under 4휋 configuration, the ideal radiationless anapole can be excited. This result illustrates a case where the reciprocity condition is not violated, and yet, a radiationless mode can be excited by external illumination.@en

We propose an approach to actively tune the scattering pat-tern of a Mie-type spherical antenna. The scheme is basedon separate control over the induced electric dipole and in-duced magnetic dipole using two coherent focused beams ofradial polarization and azimuthal polarization, respectively.By carefully tuning the amplitude and phase relation of thetwo beams, a broadband unidirectional scattering can beachieved, even at the antenna’s resonant wavelength wherethe antenna scatters efficiently. By moving the focus of onebeam, a drastic switch of the unidirectional scattering canbe observed. Such a scheme enables the design of ultra-compact optical switches and directional couplers basedon nanoantennas.@en