Polarization-Orthogonal Nondegenerate Plasmonic Higher-Order Topological States.

Photonic topological states, providing light-manipulation approaches in robust manners, have attracted intense attention. Connecting photonic topological states with far-field degrees of freedom (d.o.f.) has given rise to fruitful phenomena. Recently emerged higher-order topological insulators (HOTIs), hosting boundary states two or more dimensions lower than those of bulk, offer new paradigms to localize or transport light topologically in extended dimensionalities. However, photonic HOTIs have not been related to d.o.f. of radiation fields yet. Here, we report the observation of polarization-orthogonal second-order topological corner states at different frequencies on a designer-plasmonic kagome metasurface in the far field. Such phenomenon stands on two mechanisms, i.e., projecting the far-field polarizations to the intrinsic parity d.o.f. of lattice modes and the parity splitting of the plasmonic corner states in spectra. We theoretically and numerically show that the parity splitting originates from the underlying interorbital coupling. Both near-field and far-field experiments verify the polarization-orthogonal nondegenerate second-order topological corner states. These results promise applications in robust optical single photon emitters and multiplexed photonic devices.

Low loss sensitivity of the anapole mode in localized defective nanoparticles.

The excitation of a nonradiating anapole in a high-index dielectric nanosphere is an effective pathway for enhancing light absorption. Here, we investigate the effect of localized lossy defects on the nanoparticle based on Mie scattering and multipole expansion theories and find its low sensitivity to absorption loss. The scattering intensity can be switched by tailoring the defect distribution of the nanosphere. For a high-index nanosphere with homogeneous loss distributions, the scattering abilities of all resonant modes reduce rapidly. By introducing loss in the strong field regions of the nanosphere, we achieve independent tuning of other resonant modes without breaking the anapole mode. As the loss increases, the electromagnetic scattering coefficients of the anapole and other resonant modes show opposite trends, along with strongly suppressed corresponding multipole scattering. While regions with strong electric fields are more susceptible to loss, the anapole's inability to emit or absorb light as a dark mode makes it hard to change. Our findings provide new opportunities for the design of multi-wavelength scattering regulation nanophotonic devices via local loss manipulation on dielectric nanoparticles.

Broadband nonreciprocal spoof plasmonic phase shifter based on transverse Faraday effects.

Spoof surface plasmon polaritons (SSPPs) have aroused widespread concern due to their strong ability in field confinement at low frequencies. For miniaturized integrated circuits, there is a pressing need for nonreciprocal spoof plasmonic platforms that provide diode functionalities. In this letter, we report the realization of nonreciprocal phase shifting in SSPPs using the transverse Faraday effect. A plasmonic coupled line is constructed by flipped stacking two corrugated metallic strips, in order to enhance the mode coupling between evanescent waves that carry opposite transverse spin angular momenta. With a transverse magnetized ferrite cladding, the SSPP mode is split into two circularly-polarized ones that show different propagation constants over a broad band. A nonreciprocal phase shifter compatible to standard microstrips is designed to validate the breaking of time-reversal symmetry in SSPPs. Microwave measurement demonstrates a differential phase shift up to 46.2°/cm from 12 GHz to 15 GHz. Owing to the advantages of strong field confinement and contactless ferrite integration, the proposed method enables an alternative pathway for nonreciprocal spoof interconnects.

Generation of Airy beams in Smith-Purcell radiation.

The metasurface has recently emerged as a powerful platform to engineer wave packets of free electron radiation at the mesoscale. Here, we propose that Airy beams can be generated when moving electrons interact with bianisotropic metasurfaces. By changing the intrinsic coupling strength, full amplitude coverage and 0-to-π phase switching of Smith-Purcell radiation can be realized from the meta-atoms. This unusual property shifts the wave front of the assembled Airy beam toward a parabolic trajectory. Experimental implementation displays that evanescent fields bounded at slotted waveguides can be coupled into Airy beams via Smith-Purcell radiation from a designed bianisotropic metasurface. Our method and design strategy offer an alternative route toward free-electron lasers with diffraction-free, self-accelerating, and self-healing beam properties.

Reconfigurable meta-radiator based on flexible mechanically controlled current distribution in three-dimensional space.

In this paper, we provide an experimental proof-of-concept of this dynamic three-dimensional (3D) current manipulation through a 3D-printed reconfigurable meta-radiator with periodically slotted current elements. By utilizing the working frequency and the mechanical configuration comprehensively, the radiation pattern can be switched among 12 states. Inspired by maximum likelihood method in digital communications, a robustness-analysis method is proposed to evaluate the potential error ratio between ideal cases and practice. Our work provides a previously unidentified model for next-generation information distribution and terahertz-infrared wireless communications.

Giant nonreciprocal transmission in low-biased gyrotropic metasurfaces.

Strong magneto-optical effect with low external magnetic field is of great importance to achieve high-performance isolators in modern optics. Here, we experimentally demonstrate a significant enhancement of the magneto-optical effect and nonreciprocal chiral transmission in low-biased gyrotropic media. A designer magneto-optical metasurface consists of a gyrotropy-near-zero slab doped with magnetic resonant inclusions. The immersed magnetic dopants enable efficient nonreciprocal light-matter interactions at the subwavelength scale, providing a giant macroscopic nonreciprocity and strong robustness against the bias disturbance. Microwave measurements reveal that the metasurface can act as a chiral isolator for circular polarization, with extremely weak intrinsic gyromagnetic activity. We also demonstrate its capability of signal isolation for circularly polarized antennas. Our findings provide an experimental verification of nonreciprocal photonic doping with low static magnetic fields.

Enhanced reflective dichroism from periodic graphene ribbons via total internal reflection.

A rigorous homogenization theory is developed to characterize the effective conductivity tensor of periodic graphene ribbons. This way, the obtained conductivity simplifies the study of the exotic scattering properties of periodic graphene ribbons. As a typical example, we find that the performance of reflective dichroism from the designed graphene ribbons can be enhanced (up to a maximum linear dichroism of 0.98) when the total internal reflection happens. Moreover, by rotating its optical axis, the functionality of the periodic graphene ribbon can change from an absorber for linearly polarized waves to another absorber for circularly polarized waves (maximum circular dichroism of 0.93). The revealed indispensable property of graphene ribbons in controlling the reflective dichroism indicates their promising wide applications including energy harvesting and optical sensing.

Optically transparent metamirror with broadband chiral absorption in the microwave region.

Chiral metamirror is one of the recently developed metadevices which can reflect designated circularly polarized waves, mimicking the exoskeleton of iridescent green beetles. Here, an optically transparent metamirror that can absorb microwave chiral photons in a broadband spectrum is demonstrated. A coupled mode theory is adopted to reveal the underlying physics for the improved bandwidth performance. Excellent agreements have been observed between numerical and experimental results, indicating a bandwidth for chiral absorption as high as 2.37 GHz. The optical transparence of the resistive patterns and substrate make the designed metamirrors suitable as microwave coatings in front of optical devices, which may find potential applications in cascaded optical systems working for both microwave and optical signals.

Enhancing the magneto-optical effects in low-biased gyromagnetic media via photonic doping.

Enhancing nonreciprocal light-matter interaction at subwavelength scales has attracted enormous attention due to high demand for compact optical isolators. Here, we propose a significant enhancement of the magneto-optical effect in low-biased gyromagnetic media via photonic doping. Magnetic particles immersed in a gyrotropy-near-zero medium act as dopants that largely modify the macroscopic gyromagnetic effects as well as the gyroelectric ones. Around the resonance frequency, the gyromagnetic activity is largely increased and even exceeds unity, thus providing a photonic band in which the wavenumber of one circularly polarized wave becomes purely imaginary. The sign of gyromagnetic activity flips at two chiral modes, and an equivalent switching of the external bias is revealed. A proof-of-concept low-biased planar isolator is designed with a thickness of only 1/28 wavelength and a degree of isolation achieving as high as 0.94. This methodology is robust against disturbance of the biased magnetic field and can be flexibly extended to other frequencies, thus offering a promising platform to achieve giant optical isolation with infinitesimally intrinsic magneto-optical effects and reduced sizes.

Dispersion engineering of hyperbolic plasmons in bilayer 2D materials.

Recent progress on anisotropic 2D materials brings new technologies for directional guidance of hyperbolic plasmons. Here, we investigate the plasmonic modes in twisted bilayer 2D materials (e.g., black phosphorous). Calculated dispersion curves show that two hyperbolas split as the twisted angle increases. The topological transition from closed ellipses to open hyperbolas is achieved by varying the frequency, indicating switching between highly directional and omnidirectional plasmons. These findings will provide potential applications of anisotropic 2D materials in the design of tunable field effect transistors and waveguides.

Manipulating surface plasmon polaritons with infinitely anisotropic metamaterials.

Guiding surface states through disorders recently has attracted attention of scientists from diverse backgrounds. In this work, we report a robust method to guide surface plasmon polaritons (SPPs) through arbitrary distorted metal surfaces (a kind of disorder), including slopes, bumps, and sharp corners. Almost total transmissions over a broad frequency range can be achieved by use of infinitely anisotropic metamaterials (IAMs). The SPPs are coupled into and out of the bulk modes in the IAMs, where the bulk modes are routed by altering the principle axis of the IAMs. Due to unique non-diffraction property of the IAMs, all processes are of high efficiency, which are explained from both microscopic and macroscopic perspectives. Several functional SPP devices, including adapter, cloak, and sharp bending waveguide, are presented in the simulations. Two proof-of-concept SPP devices are experimentally demonstrated, where the SPPs are mimicked by the designer SPPs at microwave frequency.

Manipulating Smith-Purcell Emission with Babinet Metasurfaces.

Swift electrons moving closely parallel to a periodic grating produce far-field radiation of light, which is known as the Smith-Purcell effect. In this letter, we demonstrate that designer Babinet metasurfaces composed of C-aperture resonators offer a powerful control over the polarization state of the Smith-Purcell emission, which can hardly be achieved via traditional gratings. By coupling the intrinsically nonradiative energy bound at the source current sheet to the out-of-plane electric dipole and in-plane magnetic dipole of the C-aperture resonator, we are able to excite cross-polarized light thanks to the bianisotropic nature of the metasurface. The polarization direction of the emitted light is aligned with the orientation of the C-aperture resonator. Furthermore, the efficiency of the Smith-Purcell emission from Babinet metasurfaces is significantly increased by 84%, in comparison with the case of conventional gratings. These findings not only open up a new way to manipulate the electron-beam-induced emission in the near-field region but also promise compact, tunable, and efficient light sources and particle detectors.

Optical chiral metamaterials: a review of the fundamentals, fabrication methods and applications.

Optical chiral metamaterials have recently attracted considerable attention because they offer new and exciting opportunities for fundamental research and practical applications. Through pragmatic designs, the chiroptical response of chiral metamaterials can be several orders of magnitude higher than that of natural chiral materials. Meanwhile, the local chiral fields can be enhanced by plasmonic resonances to drive a wide range of physical and chemical processes in both linear and nonlinear regimes. In this review, we will discuss the fundamental principles of chiral metamaterials, various optical chiral metamaterials realized by different nanofabrication approaches, and the applications and future prospects of this emerging field.

Free-space carpet cloak using transformation optics and graphene.

Free-space carpet cloak designed with transformation optics requires materials exhibiting simultaneously anisotropic properties and plasma-like behaviors, but materials that simultaneously meet these requirements are rarely found in nature. The recently discovered graphene has shown unique anisotropic plasma-like behavior benefitting from its natural two-dimensional structure and in-plane ultrahigh electron mobility, and therefore, can be a good candidate for the free-space carpet cloak design. In this Letter, we theoretically propose a novel free-space carpet cloak by using periodically stacking layered graphene for the first time. Simulation results show an object under the graphene-based carpet cloak becomes invisible in the THz frequencies. By exploiting the large tunability of graphene's conductivity, we also demonstrate the working frequency of the designed cloak is continuously tunable in a wide spectrum.

Kirigami-Triggered Spoof Plasmonic Interconnects for Radiofrequency Elastronics.

The flexible and conformal interconnects for electronic systems as a potential signal transmission device have great prospects in body-worn or wearable applications. High-efficiency wave propagation and conformal structure deformation around human body at radio communication are still confronted with huge challenges due to the lack of methods to control the wave propagation and achieve the deformable structure simultaneously. Here, inspired by the kirigami technology, a new paradigm to construct spoof plasmonic interconnects (SPIs) that support radiofrequency (RF) surface plasmonic transmission is proposed, together with high elasticity, strong robustness, and multifunction performance. Leveraging the strong field-confinement characteristic of spoof surface plasmons polaritons, the Type-I SPI opens its high-efficiency transmission band after stretching from a simply connected metallic surface. Meanwhile, the broadband transmission of the kirigami-based SPI exhibits strong robustness and excellent stability undergoing complex deformations, i.e., bending, twisting, and stretching. In addition, the prepared Type-II SPI consisting of 2 different subunit cells can achieve band-stop transmission characteristics, with its center frequency dynamically tunable by stretching the buckled structure. Experimental measurements verify the on-off switching performance in kirigami interconnects triggered by stretching. Overcoming the mechanical limitation of rigid structure with kirigami technology, the designer SPIs exhibit high stretchability through out-of-plane structure deformation. Such kirigami-based interconnects can improve the elastic functionality of wearable RF electronics and offer high compatibility to large body motion in future body network systems.

Topologically reconfigurable magnetic polaritons.

Hyperbolic polaritons in extremely anisotropic materials have attracted intensive attention due to their exotic optical features. Recent advances in optical materials reveal unprecedented dispersion engineering of polaritons, resulting in twistronics for photons, canalized phonon polaritons, shear polaritons, and tunable topological polaritons. However, the on-demand reconfigurability of polaritons, especially with magnetic anisotropic dispersions, is restricted by weak natural magnetic anisotropy and hence remains largely unexplored. Here, we show how origami fused with artificial magnetism unveils a versatile pathway to topologically reconfigure magnetic polaritons. We experimentally demonstrate that the three-dimensional origami deformation allows to reconfigure hyperbolic or elliptic topology of polariton dispersion and modulate group velocity. With group velocity transitioning from positive to negative directions, we further report reconfigurable origami polariton circuitry in which the polariton propagation and phase distribution can be tailored. Our findings provide alternative perspectives on on-chip polaritonics, with potential applications in energy transfer, sensing, and information transport.

Type-I hyperbolic metasurfaces for highly-squeezed designer polaritons with negative group velocity.

Hyperbolic polaritons in van der Waals materials and metamaterial heterostructures provide unprecedented control over light-matter interaction at extreme nanoscales. Here we propose a concept of type-I hyperbolic metasurface supporting highly-squeezed magnetic designer polaritons, which act as magnetic analogs of hyperbolic polaritons in the hexagonal boron nitride (h-BN) in the first Reststrahlen band. Compared with the natural h-BN, the size and spacing of the metasurface unit cell can be readily engineered, allowing for manipulating designer polaritons in frequency and space with greater flexibility. Microwave experimental measurements display a cone-like dispersion in momentum space, exhibiting an effective refractive index up to 60 and a group velocity down to c/400. Tailoring the metasurface, we demonstrate an ultra-compact integrated designer polariton circuit including high-transmission 90° sharp bending waveguides and waveguide splitters. The present metasurface could serve as a platform for polaritonics, and find applications in waveguiding, terahertz sensing, subdiffraction focusing/imaging, low-threshold terahertz Cherenkov radiation, and wireless energy transfer.

Origami Metawall: Mechanically Controlled Absorption and Deflection of Light.

Metamaterials/metasurfaces, which have subwavelength resonating unit cells (i.e., meta-atoms), can enable unprecedented control over the flow of light. Despite their significant progress, achieving dynamical control of both energy and momentum of light remains a challenge. Here, a mechanically tunable metawall capable of either absorbing light energy or modulating light momentum, by incorporating the magnetic meta-atoms into a 3D printed origami grating, is theoretically designed and experimentally realized. Through mechanical stretching or compressing of the Miura-ori pattern, the function of metawall can transit from an absorber, a mirror, to a negative reflector. Particularly, the continuously geometric deformation of the Miura-ori lattice is a promising approach to compensate the angular dispersion in gradient metasurfaces. Considering the prominent mechanical properties and strong deformation abilities of origami structures, the findings may open an alternative avenue toward lightweight and deployable metadevices with diversified and continuously alterable electromagnetic properties.

Spiral Field Generation in Smith-Purcell Radiation by Helical Metagratings.

Moving electrons interacting with media can give rise to electromagnetic radiations and has been emerged as a promising platform for particle detection, spectroscopies, and free-electron lasers. In this letter, we investigate the Smith-Purcell radiation from helical metagratings, chiral structures similar to deoxyribonucleic acid (DNA), in order to understand the interplay between electrons, photons, and object chirality. Spiral field patterns can be generated while introducing a gradient azimuthal phase distribution to the induced electric dipole array at the cylindrical interface. Experimental measurements show efficient control over angular momentum of the radiated field at microwave regime, utilizing a phased electromagnetic dipole array to mimic moving charged particles. The angular momentum of the radiated wave is determined solely by the handedness of the helical structure, and it thus serves as a potential candidate for the detection of chiral objects. Our findings not only pave a way for design of orbital angular momentum free-electron lasers but also provide a platform to study the interplay between swift electrons with chiral objects.

Direct current remote cloak for arbitrary objects.

Hiding an arbitrary object with a cloak at a distance from an object is of great significance in scientific research, but remains unrealized as a practical device. In this paper, we propose the first experimental realization of a remote cloaking device that makes any object located at a certain distance invisible at direct current (DC) frequency. A negative resistor network with active elements is used to achieve the remote function of the DC cloak. Based on this network, the cloak can remotely generate a hidden region without distorting the currents far from the cloaked region. The experimental results show that any object in the hidden region is invisible to a DC detector. Our cloak does not require any knowledge of the hidden object. The experimental demonstration shows the superiority of this remote cloaking device, which may find potential applications in medical or geologic research.