The impedance matching to free space in metamaterial perfect absorbers has been believed to involve and rely on magnetic resonant response, with direct evidence provided by the anti-parallel surface ...currents in the metal structures. Here I present a different theoretical interpretation based on interference, which shows that the two layers of metal structures in metamaterial absorbers are linked only by multiple reflections with negligible near-field interactions or magnetic resonances. This is further supported by the out-of-phase surface currents derived at the interfaces of resonator array and ground plane through multiple reflections and superpositions. The theory developed here explains all features observed in narrowband metamaterial absorbers and therefore provides a profound understanding of the underlying physics.
Polarization is one of the basic properties of electromagnetic waves conveying valuable information in signal transmission and sensitive measurements. Conventional methods for advanced polarization ...control impose demanding requirements on material properties and attain only limited performance. We demonstrated ultrathin, broadband, and highly efficient metamaterial-based terahertz polarization converters that are capable of rotating a linear polarization state into its orthogonal one. On the basis of these results, we created metamaterial structures capable of realizing near-perfect anomalous refraction. Our work opens new opportunities for creating high-performance photonic devices and enables emergent metamaterial functionalities for applications in the technologically difficult terahertz-frequency regime.
Harnessing light for modern photonic applications often involves the control and manipulation of light polarization and phase. Traditional methods require a combination of multiple discrete optical ...components, each of which contributes to a specific functionality. Here, plasmonic metasurfaces are proposed that accomplish the simultaneous manipulation of polarization and phase of the transmitted light. Arbitrary spatial field distribution of the optical phase and polarization direction can be obtained. The multifunctional metasurfaces are validated by demonstrating a broadband near‐perfect anomalous refraction with controllable linear polarization through introducing a constant phase gradient along the interface. Furthermore, the power of the proposed metasurfaces is demonstrated by generating a radially polarized beam. The new degrees of freedom of metasurfaces facilitate arbitrary manipulation of light and will profoundly affect a wide range of photonic applications.
A radially polarized beam is generated based on proposed plasmonic metasurfaces that allow simultaneous manipulation of the polarization and phase of the transmitted light. Arbitrary spatial field distribution of the optical phase and polarization direction are obtained by accordingly designed plasmonic metasurfaces. The multifunctional metasurfaces are also validated by demonstrating a broadband near‐perfect anomalous refraction with controllable linear polarization.
Emerging photonic functionalities are mostly governed by the fundamental principle of Lorentz reciprocity. Lifting the constraints imposed by this principle could circumvent deleterious effects that ...limit the performance of photonic systems. Most efforts to date have been limited to waveguide platforms. Here, we propose and experimentally demonstrate a spatio-temporally modulated metasurface capable of complete violation of Lorentz reciprocity by reflecting an incident beam into far-field radiation in forward scattering, but into near-field surface waves in reverse scattering. These observations are shown both in nonreciprocal beam steering and nonreciprocal focusing. We also demonstrate nonreciprocal behavior of propagative-only waves in the frequency- and momentum-domains, and simultaneously in both. We develop a generalized Bloch-Floquet theory which offers physical insights into Lorentz nonreciprocity for arbitrary spatial phase gradients, and its predictions are in excellent agreement with experiments. Our work opens exciting opportunities in applications where free-space nonreciprocal wave propagation is desired.
Due to the scarcity of circular polarization light sources, linear-to-circular polarization conversion is required to generate circularly polarized light for a variety of applications. Despite ...significant past efforts, broadband linear-to-circular polarization conversion remains elusive particularly in the terahertz and midinfrared frequency ranges. Here we propose a novel mechanism based on coupled mode theory, and experimentally demonstrate at terahertz frequencies that highly efficient (power conversion efficiency approaching unity) and ultrabroadband (fractional bandwidth up to 80%) linear-to-circular polarization conversion can be accomplished by the judicious design of birefringent metasurfaces. The underlying mechanism operates in the frequency range between well separated resonances, and relies upon the phase response of these resonances away from the resonant frequencies, as well as the balance of the resonant and nonresonant channels. This mechanism is applicable for any operating frequencies from microwave to visible. The present Letter potentially opens a wide range of opportunities in wireless communications, spectroscopy, and emergent quantum materials research where circularly polarized light is desired.
Complex multiphase nanocomposite designs present enormous opportunities for developing next-generation integrated photonic and electronic devices. Here, a unique three-phase nanostructure combining a ...ferroelectric BaTiO
, a wide-bandgap semiconductor of ZnO, and a plasmonic metal of Au toward multifunctionalities is demonstrated. By a novel two-step templated growth, a highly ordered Au-BaTiO
-ZnO nanocomposite in a unique "nanoman"-like form, i.e., self-assembled ZnO nanopillars and Au nanopillars in a BaTiO
matrix, is realized, and is very different from the random three-phase ones with randomly arranged Au nanoparticles and ZnO nanopillars in the BaTiO
matrix. The ordered three-phase "nanoman"-like structure provides unique functionalities such as obvious hyperbolic dispersion in the visible and near-infrared regime enabled by the highly anisotropic nanostructures compared to other random structures. Such a self-assembled and ordered three-phase nanocomposite is obtained through a combination of vapor-liquid-solid (VLS) and two-phase epitaxy growth mechanisms. The study opens up new possibilities in the design, growth, and application of multiphase structures and provides a new approach to engineer the ordering of complex nanocomposite systems with unprecedented control over electron-light-matter interactions at the nanoscale.
Terahertz (THz) radiation attracted great interest in the fields of material characterization, nondestructive security screening, clinical diagnostics, and identification of chemicals and molecules. ...Label-free THz sensing of trace amount of targets including biomolecules is promising because of their rich spectral fingerprint in this electromagnetic region; however, improving the sensitivity remains to be a challenge, partially due to the limitations of THz sources and detectors. The resonantly enhanced electromagnetic fields in metamaterials and metasurfaces offer a potentially viable solution, although highly complicated decoration process is still needed for biosensing on the surface of metamaterials. Here we demonstrate a simple biosensing platform by integrating a monolayer graphene on a THz metamaterial absorber cavity, where the introduction of sensing targets results in a large change of the metamaterial resonant absorption (or reflection) because of their strong interaction with graphene. We experimentally show its ultrahigh sensitivity through detecting trace amount of chlorpyrifos methyl down to 0.2 ng. Using simple decoration steps and utilizing DNA to capture thrombin, we further show the feasibility of this platform serving as a sensitive biosensor.
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Metasurfaces with actively tunable features are highly demanded for advanced applications in electronic and electromagnetic systems. However, realizing independent dual-tunability remains challenging ...and requires more efforts. In this paper, we present an active metasurface where the magnitude and frequency of the resonant absorption can be continuously and independently tuned through application of voltage biases. Such a dual-tunability is accomplished at microwave frequencies by combining a varactor-loaded high-impedance surface and a graphene-based sandwich structure. By electrically controlling the Fermi energy of graphene and the capacitance of varactor diodes, we experimentally demonstrate the independent shifting of the working frequency from 3.41 to 4.55 GHz and tuning of the reflection amplitude between −3 and −30 dB, which is in excellent agreement with full-wave numerical simulations. We further employed an equivalent lumped circuit model to elucidate the mechanism of the dual-tunability resulting from the graphene-based sandwich structure and the active high-impedance surface. We speculate that such a dual-tunability scheme can be potentially extended to terahertz and optical regimes by employing different active/dynamical tuning methods and materials integration, thereby enabling a variety of practical applications.
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