Fourier metasurface cloaking: unidirectional cloaking of electrically large cylinder under oblique incidence.

The existence of a non-electrically-small scatterer adjacent to the source can severely distort the radiation and lead to a poor electromagnetic compatibility. In this work, we use a conducting hollow cylinder to shield a cylindrical scatterer. The cylinder is shelled with a single dielectric layer enclosed by an electromagnetic metasurface. The relationship between the scattering field and the surface impedance is derived analytically. By optimizing the Fourier expansion coefficients of the surface impedance distribution along ϕ-dimension, the scattering cross-section can be effectively reduced. This unidirectional cloaking method is valid for both TM/TE and non-TM/TE incident field and is not limited to a plane-wave incident field. The accuracy and effectiveness of the method are verified by four cloaking scenarios in microwave regime. We demonstrate that with the surface impedance obtained by the proposed method, a metasurface is designed with physical subwavelength structures. We also show a cloaking scenario under a magnetic dipole radiation, which is closer to the case of a realistic antenna. This method can be further applied to cloaking tasks in terahertz and optical regimes.

Nearly Ideal Transparency with Artificially Designed Meta-Atoms.

The ideal electromagnetic transparency refers to the ability of an object to remain scatteringless to any incoming waves, resulting in vacuum invisibility. However, natural solid substances can hardly be transparent in free space as they are responsive to external polarizations. Completely eliminating the polarization effect of an obstacle under arbitrary field illumination is a long-standing scientific challenge. Here, it is shown that a subwavelength meta-atom can be nearly ideally transparent in the vacuum. The overall vacuum-like property of the meta-atom is achieved through judiciously designing its internal polarization and magnetization. Remarkably, any large-scale objects made by stacking the meta-atoms inherit the vacuum-like property and are scatteringless in free space. By both the simulations and experiments, the meta-atom's peculiar property is reasonably verified. The proposed meta-atoms are excellent candidates for a wide range of applications, such as perfect radar radomes, scatteringless walls, filtering devices, and self-stealth materials.

Ultra-wideband antireflection assisted by a continuously varying temporal medium.

We demonstrate that reflectionless propagation of electromagnetic waves between two different materials can be achieved by designing an intermediate temporal medium, which can work in an ultra-wide frequency band. Such a temporal medium is designed with consideration of a multi-stage variation of the material's permittivity in the time domain. The multi-stage temporal permittivity is formed by a cascaded quarter-wave temporal coating, which is an extension of the antireflection temporal coating by Pacheco-Peña et al. [Optica7, 323 (2020)10.1364/OPTICA.381175]. The strategy to render ultra-wideband antireflection temporal medium is discussed analytically and verified numerically. In-depth analysis shows that the multi-stage design of the temporal media implies a continuously temporal variation of the material's constitutive parameters, thus an ultra-wideband antireflection temporal medium is reasonably obtained. As an illustrative example for application, the proposed temporal medium is adopted to realize impedance matching between a dielectric slab and free space, which validates our new findings.

Observation of 2D omnidirectional invisibility from an electrically large cluster under TE polarization.

The concept of perfect invisibility in free space implies an object neither reflects nor refracts optical waves coming from arbitrary directions, regardless of its shape and size. An optimal solution to realize such a peculiar phenomenon is to tune the constitutive parameters of the object to be identical to air. In particular, to render zero extinction from an existing object by covering some additional structures, is of importance for practical implementations, which is challenging. Here, we demonstrate and propose that a thin metallic wire can be tuned to be air-like under TE polarization, with the aid of an external enclosure. This is achieved through a precise dispersion engineering with independently controllable electric and magnetic responses. Consequently, an electrically large cluster composed of multiple thin wires can be safely hidden in free space, without any macroscopic cloaking structure, which is verified by full-wave simulations and experiments. The measured results on an electrically large airplane-like sample show the excellent performance of 2D omnidirectional invisibility at the designed frequency. This proposed metamaterial would be helpful in enhancing the mechanical stability, electrical conduction, and heat dissipation of a device (or system) by extra wires (or pipes), without disturbing its electromagnetic characteristics.

Observation of reflectionless absorption due to spatial Kramers-Kronig profile.

As a fundamental phenomenon in electromagnetics and optics, material absorption has been extensively investigated for centuries. However, omnidirectional, reflectionless absorption in inhomogeneous media has yet to be observed. Previous research on transformation optics indicated that such absorption cannot easily be implemented without involving gain media. A recent theory on wave propagation, however, implies the feasibility to implement such absorption requiring no gain, provided that the permittivity profile of this medium can satisfy the spatial Kramers-Kronig relations. In this work, we implement such a profile over a broad frequency band based on a novel idea of space-frequency Lorentz dispersion. A wideband, omnidirectionally reflectionless absorption is then experimentally observed in the gigahertz range, and is in good agreement with theoretical analysis and full-wave simulations. The proposed method based on the space-frequency dispersion implies the practicability to construct gain-free omnidirectionally non-reflecting absorbers.Reflectionless absorption independent of the angle of incidence usually requires the introduction of gain media into the system. Here, Ye et al. implement a recent theoretical proposal to achieve this with a spatially varying permittivity, showing that this approach is experimentally feasible.

Microwave Imaging under Oblique Illumination.

Microwave imaging based on inverse scattering problem has been attracting many interests in the microwave society. Among some major technical challenges, the ill-posed, multi-dimensional inversion algorithm and the complicated measurement setup are critical ones that prevent it from practical applications. In this paper, we experimentally investigate the performance of the subspace-based optimization method (SOM) for two-dimensional objects when it was applied to a setup designed for oblique incidence. Analytical, simulation, and experimental results show that, for 2D objects, neglecting the cross-polarization scattering will not cause a notable loss of information. Our method can be potentially used in practical imaging applications for 2D-like objects, such as human limbs.

Invisible metallic mesh.

A solid material possessing identical electromagnetic properties as air has yet to be found in nature. Such a medium of arbitrary shape would neither reflect nor refract light at any angle of incidence in free space. Here, we introduce nonscattering corrugated metallic wires to construct such a medium. This was accomplished by aligning the dark-state frequencies in multiple scattering channels of a single wire. Analytical solutions, full-wave simulations, and microwave measurement results on 3D printed samples show omnidirectional invisibility in any configuration. This invisible metallic mesh can improve mechanical stability, electrical conduction, and heat dissipation of a system, without disturbing the electromagnetic design. Our approach is simple, robust, and scalable to higher frequencies.

TOPOLOGICAL MATTER. Experimental observation of Weyl points.

The massless solutions to the Dirac equation are described by the so-called Weyl Hamiltonian. The Weyl equation requires a particle to have linear dispersion in all three dimensions while being doubly degenerate at a single momentum point. These Weyl points are topological monopoles of quantized Berry flux exhibiting numerous unusual properties. We performed angle-resolved microwave transmission measurements through a double-gyroid photonic crystal with inversion-breaking where Weyl points have been theoretically predicted to occur. The excited bulk states show two linear dispersion bands touching at four isolated points in the three-dimensional Brillouin zone, indicating the observation of Weyl points. This work paves the way to a variety of photonic topological phenomena in three dimensions.

Observation of wave packet distortion during a negative-group-velocity transmission.

In Physics, causality is a fundamental postulation arising from the second law of thermodynamics. It states that, the cause of an event precedes its effect. In the context of Electromagnetics, the relativistic causality limits the upper bound of the velocity of information, which is carried by electromagnetic wave packets, to the speed of light in free space (c). In anomalously dispersive media (ADM), it has been shown that, wave packets appear to propagate with a superluminal or even negative group velocity. However, Sommerfeld and Brillouin pointed out that the "front" of such wave packets, known as the initial point of the Sommerfeld precursor, always travels at c. In this work, we investigate the negative-group-velocity transmission of half-sine wave packets. We experimentally observe the wave front and the distortion of modulated wave packets propagating with a negative group velocity in a passive artificial ADM in microwave regime. Different from previous literature on the propagation of superluminal Gaussian packets, strongly distorted sinusoidal packets with non-superluminal wave fronts were observed. This result agrees with Brillouin's assertion, i.e., the severe distortion of seemingly superluminal wave packets makes the definition of group velocity physically meaningless in the anomalously dispersive region.

Microwave gain medium with negative refractive index.

Artificial effective media are attractive because of the fantastic applications they may enable, such as super lensing and electromagnetic invisibility. However, the inevitable loss due to their strongly dispersive nature is one of the fundamental challenges preventing such applications from becoming a reality. In this study, we demonstrate an effective gain medium based on negative resistance, to overcompensate the loss of a conventional passive metamaterial, meanwhile keeping its original negative-index property. Energy conservation-based theory, full-wave simulation and experimental measurement show that a fabricated sample consisting of conventional sub-wavelength building blocks with embedded microwave tunnel diodes exhibits a band-limited Lorentzian dispersion simultaneously with a negative refractive index and a net gain. Our work provides experimental evidence to the assertion that a stable net gain in negative-index gain medium is achievable, proposing a potential solution for the critical challenge current metamateiral technology faces in practical applications.

Ultrawideband dispersion control of a metamaterial surface for perfectly-matched-layer-like absorption.

Narrow bandwidth is a fundamental issue plaguing practical applications of metamaterial absorbers. In this Letter, we show that by deliberately controlling the dispersion and dissipation of a metamaterial, an ultrawideband perfect metamaterial absorber with complex-valued constitutive parameters strictly satisfying the modified model of a perfectly matched layer, can be achieved. The nearly perfect power absorption, better than 99%, was experimentally observed in an unprecedented bandwidth of 39%, approaching the theoretical Rozanov limit. We expect a wide range of applications to emerge from this general concept.

Negative group velocity in the absence of absorption resonance.

Scientific community has well recognized that a Lorentzian medium exhibits anomalous dispersion behavior in its resonance absorption region. To satisfy the Krammers-Kronig relation, such an anomalous region has to be accompanied with significant loss, and thus, experimental observations of negative group velocity in this region generally require a gain-assisted approach. In this letter, we demonstrate that the negative group velocity can also be observed in the absence of absorption resonance. We show that the k-surface of a passive uniaxial Lorentzian medium undergoes a distortion near the plasma frequency. This process yields an anomalous dispersion bandwidth that is far away from the absorption resonance region, and enables the observation of negative group velocity at the plasma frequency band. Introducing anomalous dispersion in a well-controlled manner would greatly benefit the research of ultrafast photonics and find potential applications in optical delay lines, optical data storage and devices for quantum information processing.

Gyrotropic response in the absence of a bias field.

Electromagnetic materials lacking local time-reversal symmetry, such as gyrotropic materials, are of keen interest and importance both scientifically and technologically. Scientifically, topologically nontrivial phenomena, such as photonic chiral edge states, allow for reflection-free transport even in the presence of large disorder. Technologically, nonreciprocal photonic devices, such as optical isolators and circulators, play critical roles in optical communication and computing technologies because of their ability to eliminate cross-talk and feedback. Nevertheless, most known natural materials that lack local time-reversal symmetry require strong external fields and function only in a limited range of the electromagnetic spectrum. By taking advantage of metamaterials capable of translating the property of unidirectional active electronic circuits into effective dielectric response, we introduce a microwave gyrotropic metamaterial that does not require an external magnetic bias. Strong bulk Faraday-like effects, observed in both simulations and experiments, confirm nonreciprocity of the effective medium. This approach is scalable to many other wavelengths, and it also illustrates an opportunity to synthesize exotic electromagnetic materials.

Active microwave negative-index metamaterial transmission line with gain.

We studied the active metamaterial transmission line at microwave frequency. The active composite right-handed or left-handed transmission line was designed to incorporate a germanium tunnel diode with a negative differential resistance property as the gain device at the unit cell level. Measurements of the fabricated planar transmission line structures with one-, two-, and three-unit cells showed that the addition of the dc pumped tunnel diodes not only provided gain but also maintained the left handedness of the transmission line metamaterial. Simulation results agree well with experimental observation. This work demonstrated that negative index material can be obtained with a net gain when an external source is incorporated.

Inverse scattering problem in presence of a conducting cylinder.

This paper deals with the inverse scattering problem, in which a conducting cylinder is placed near samples that are to be reconstructed. Due to multiple scattering effect, the radius of the conducting cylinder and its distance to samples play an important role in inverse scattering problem. The paper investigates the role of the conducting cylinder under different arrangement of transmitting/receiving antennas. Numerical simulations show that with a proper arrangement of the cylinder and transmitting/receiving antennas, it is possible to achieve high-resolution reconstruction results with fewer antennas than when the conducting cylinder is absent.

Harmonic image reconstruction assisted by a nonlinear metmaterial surface.

We experimentally demonstrate a microwave far-field image reconstruction modality with the transverse resolution exceeding the diffraction limit by using a single layer of highly nonlinear metamaterial. The harmonic fields of the nonlinear metamaterial surface allow the far-field propagation of wave fronts with spatial frequencies several times higher than that of the fundamental field. Near-field images can thus be mathematically recovered from the far-field patterns of the harmonic fields.

Experimental characterization of the dispersive behavior in a uniaxial metamaterial around plasma frequency.

In this paper, the dispersive behavior around the plasma frequency in a magnetically uniaxial metamaterial is experimentally investigated. We show by theoretical analysis, parameter retrieval and experiment that when material loss is considered, while the plasma frequency is defined by the frequency where the real part of permeability approaches zero, ultra fast phase velocity actually appears at a slightly lower frequency, due to the change of the dispersion diagram. Both parameter retrieval and experimental data show that within a narrow frequency band to the left of the plasma frequency, the inherent loss keeps finite and is much less than that in the corresponding resonant region. In a real metamaterial sample, an ultra fast phase velocity of 24,440 times the speed of light in free space is measured, and negative phase propagation due to the only negative permeability is observed. The existence of such ultra fast phase velocity with finite loss perfectly explains how the highly directivity antennas based on near-zero refractive index metamaterial work, and can be further used in other applications such as in-phase wave divider and coherent wave sources.

Fractal plasmonic metamaterials for subwavelength imaging.

We show that a metallic plate with periodic fractal-shaped slits can be homogenized as a plasmonic metamaterial with plasmon frequency dictated by the fractal geometry. Owing to the all-dimensional subwavelength nature of the fractal pattern, our system supports both transverse-electric and transverse-magnetic surface plasmons. As a result, this structure can be employed to focus light sources with all-dimensional subwavelength resolution and enhanced field strengths. Microwave experiments reveal that the best achievable resolution is only lambda/15, and finite-difference-time-domain simulations demonstrate that similar effects can be realized at infrared frequencies with appropriate designs.

Active Microwave Metamaterials Incorporating Ideal Gain Devices.

Incorporation of active devices/media such as transistors for microwave and gain media for optics may be very attractive for enabling desired low loss and broadband metamaterials. Such metamaterials can even have gain which may very well lead to new and exciting physical phenomena. We investigate microwave composite right/left-handed transmission lines (CRLH-TL) incorporating ideal gain devices such as constant negative resistance. With realistic lumped element values, we have shown that the negative phase constant of this kind of transmission lines is maintained (i.e., left-handedness kept) while gain can be obtained (negative attenuation constant of transmission line) simultaneously. Possible implementation and challenging issues of the proposed active CRLH-TL are also discussed.

Rainbow-like radiation from an omni-directional source placed in a uniaxial metamaterial slab.

In this paper, the radiation of an omni-directional line source placed in a uniaxial metamaterial slab is experimentally presented. The anisotropic slab made of metallic symmetrical rings with dispersive permeability is investigated both theoretically and experimentally. For low value of the permeability, a directive radiation at the broadside of the slab can be obtained. Due to the excitation of the leaky wave mode supported by this structure, the emitted electromagnetic wave transmits at a greater angle from the normal of the slab as the value of permeability increases along with the frequency. Thus a rainbow-like radiation will be formed since waves of different frequencies will deflect into different directions.