Experimental Demonstration of Controllable PT and Anti-PT Coupling in a Non-Hermitian Metamaterial.

Non-Hermiticity has recently emerged as a rapidly developing field due to its exotic characteristics related to open systems, where the dissipation plays a critical role. In the presence of balanced energy gain and loss with environment, the system exhibits parity-time (PT) symmetry, meanwhile as the conjugate counterpart, anti-PT symmetry can be achieved with dissipative coupling within the system. Here, we demonstrate the coherence of complex dissipative coupling can control the transition between PT and anti-PT symmetry in an electromagnetic metamaterial. Notably, the achievement of the anti-PT symmetric phase is independent of variations in dissipation. Furthermore, we observe phase transitions as the system crosses exceptional points in both anti-PT and PT symmetric metamaterial configurations, achieved by manipulating the frequency and dissipation of resonators. This work provides a promising metamaterial design for broader exploration of non-Hermitian physics and practical application with a controllable Hamiltonian.

A Thermally Controlled Multifunctional Metamaterial Absorber with Switchable Wideband Absorption and Transmission at THz Band.

This paper proposes a thermally controlled multifunctional metamaterial absorber with switchable wideband absorption and transmission at the THz band based on resistive film and vanadium dioxide (VO2). The function of the absorber can be adjusted by changing the phase transition characteristics of VO2. When VO2 is in a metallic state, the absorber can achieve wideband absorption with above 90% absorption from 3.31 THz to 10 THz and exhibits excellent absorption performance under a wide range of incident and polarization angles. When VO2 is in an insulating state, the metamaterial acts in transmission mode with a transmission coefficient of up to 61% at 5.15 THz. The transmission region is inside the absorption band, which is very important for practical applications. It has the advantages of having a simple structure, wideband absorption, and switchable absorption/transmission with potential application value in the fields of stealth of communication equipment and radar at the THz band.

A Photoexcited Switchable Dual-Function Metamaterial Absorber for Sensing and Wideband Absorption at THz Band.

Based on the tunable conductivity of silicon as a function of incident pump power, a photoexcited switchable dual-function metamaterial absorber for sensing and wideband absorption at the THz band is designed in this paper. The absorber has an absorption peak at 2.08 THz with the absorption up to 99.6% when the conductivity of silicon is 150 Sm-1, which can be used for sensing. The refractive index sensitivity of the absorption peak is up to 456 GHz/RIU. A wideband absorption is generated from 3.4 THz to 4.5 THz with the bandwidth of 1.1 THz as the conductivity σsi = 12,000 Sm-1. The generation mechanism of the sensing absorption peak and wideband absorption is explained by monitoring the surface current, electric, and magnetic field distribution at some absorption frequencies. It has the advantages of being simple and having a high sensitivity, and wideband absorption with wide application prospects on terahertz communication, electromagnetic stealth, and biochemical detection.

Fano-Resonant Hybrid Metamaterial for Enhanced Nonlinear Tunability and Hysteresis Behavior.

Artificial resonant metamaterial with subwavelength localized filed is promising for advanced nonlinear photonic applications. In this article, we demonstrate enhanced nonlinear frequency-agile response and hysteresis tunability in a Fano-resonant hybrid metamaterial. A ceramic cuboid is electromagnetically coupled with metal cut-wire structure to excite the high-Q Fano-resonant mode in the dielectric/metal hybrid metamaterial. It is found that the significant nonlinear response of the ceramic cuboid can be employed for realization of tunable metamaterials by exciting its magnetic mode, and the trapped mode with an asymmetric Fano-like resonance is beneficial to achieve notable nonlinear modulation on the scattering spectrum. The nonlinear tunability of both the ceramic structure and the ceramic/metal hybrid metamaterial is promising to extend the operation band of metamaterials, providing possibility in practical applications with enhanced light-matter interactions.

EIA metamaterials based on hybrid metal/dielectric structures with dark-mode-enhanced absorption.

Metamaterial analogue of electromagnetically induced absorption (EIA) has promising applications in spectroscopy and sensing. Here we propose an EIA metamaterial based on hybrid metal/dielectric structures, which are composed of a metallic wire and a dielectric block, and investigate the EIA-like effect by simulations, experiments, and the two-oscillator model. An EIA-like effect emerges in virtue of the near-field coupling between metallic wire and dielectric block, and the dielectric block exhibiting magnetic dipolar resonance makes a major contribution to the resonance absorption. The magnetic flux through the dielectric block engendered by the near filed of the metallic wire determines the coupling between dielectric block and metallic wire. With the variation of the separation between dielectric block and metallic wire, the EIA-like effect is preserved and does not convert into the EIT-like effect although the coupling and consequently the absorbance are altered. Based on the two-oscillator model, the absorption spectrum of the EIA metamaterial is quantitatively analyzed and the parameters of the oscillator system are retrieved.

Realization of a near-infrared active Fano-resonant asymmetric metasurface by precisely controlling the phase transition of Ge2Sb2Te5.

A metasurface is one of the most effectual platforms for the manipulation of complex optical fields. One of the current challenges in the field is to develop active or reconfigurable functionalities to extend its operation band which is limited by its intrinsic resonant nature. Here we demonstrate a kind of active Fano-resonant asymmetric metasurface in the near-infrared (NIR) region with heterostructures made of a layer of asymmetric split-ring resonators and a thin layer of phase-change material (PCM). In the asymmetric metasurface, significant tunability in the frequency, Q-factor and strength of the Fano resonance are all achieved by precisely controlling the phase transition of the contained PCM Ge2Sb2Te5 (GST), together with changing the geometric asymmetry of the split-ring resonators. Moreover, we provide a complete transition process of the optical properties for GST and an optimized modulation on the active Fano-resonant metasurface. Our approach to dynamically control a Fano-resonant metasurface paves the way to realizing various active photonic meta-devices involving PCM.

Controllable coherent perfect absorber made of liquid metal-based metasurface.

Coherent perfect absorber (CPA) is a novel strategy proposed and demonstrated for solving the challenge to attain efficient control of absorption by exploiting the inverse process of lasing. The operation condition of CPA results in narrow-band, which is the main limitation obstruct it from practical applications. Here, we demonstrate a CPA with tunable operation frequency employing the liquid metal made reconfigurable metasurface. The flow of liquid metal is restricted with a plastic pipe for realizing a controllable liquid metal cut-wire. The adjustable electric dipolar mode of the reconfigurable cur-wire metasurface ensures that the quasi-CPA point can be dynamically controlled; the measured CPA under proper phase modulation is in good agreement with the simulation results. The proposed CPA system involving liquid metal for dynamic control of operation frequency will have potential applications and may stimulate the exploitation of liquid based smart absorption control of optical waves.

Phase-Modulated Scattering Manipulation for Exterior Cloaking in Metal-Dielectric Hybrid Metamaterials.

Artificially structured metamaterials with metallic or dielectric inclusions are extensively studied for exotic light manipulations via controlling the local-resonant modes in the microstructures. The coupling between these resonant modes has drawn growing interest in recent years due to the advanced functional metamaterial making the microstructures more and more complex. Here, the suppression of magnetic resonance of a dielectric cuboid, an analogue to the scattering cancellation effect or radiation control system, realized with an exterior cloaking in a hybrid metamaterial system, is demonstrated. Furthermore, the significant modulation of the absorption of the dielectric resonator in the hybrid metamaterial is also demonstrated. The physical insight of the experimental results is well illuminated with a classical double-harmonic-oscillator model, from which it is revealed that the complex coupling, i.e., the phase of coupling coefficient, plays a crucial role in the overall response of the metal-dielectric hybrid system. The proposed design strategy is anticipated to form a more straightforward and efficient paradigm for practical applications based on radiation control via versatile mode couplings.

Thermally controllable Mie resonances in a water-based metamaterial.

Active control of metamaterial properties is of great significance for designing miniaturized and versatile devices in practical engineering applications. Taking advantage of the highly temperature-dependent permittivity of water, we demonstrate a water-based metamaterial comprising water cubes with thermally tunable Mie resonances. The dynamic tunability of the water-based metamaterial was investigated via numerical simulations and experiments. A water cube exhibits both magnetic and electric response in the frequency range of interest. The magnetic response is primarily magnetic dipole resonance, while the electric response is a superposition of electric dipole resonance and a smooth Fabry-Pérot background. Using temporal coupled-mode theory (TCMT), the role of direct scattering is evaluated and the Mie resonance modes are analyzed. As the temperature of water cube varies from 20 °C to 80 °C, the magnetic and electric resonance frequencies exhibit obvious blue shifts of 0.10 and 0.14 GHz, respectively.

Realization of switchable EIT metamaterial by exploiting fluidity of liquid metal.

Novel manipulation techniques for the propagation of electromagnetic waves based on metamaterials can only be performed in narrow operating bands, and this drawback is a major challenge for developing metamaterial-based practical applications. We demonstrate that the scattering of metamaterials can be switched and that their operating band can be tuned by introducing liquid metal in the design of functional metamaterials. The proposed liquid metal-based metamaterial is composed of a copper wire pair and a tiny pipe filled with a liquid metal, namely eutectic gallium-indium. The interference of the sharp magnetic resonance of the copper wire pair and the broad dipolar mode of the liquid metal rod lead to an electromagnetically induced transparency (EIT)-like spectrum. We experimentally demonstrate that this EIT-like behavior can be switched on or off by exploiting the fluidity of the liquid metal, which is useful for multi-frequency modulators. These findings will hopefully promote the development of fluid matter-based metamaterials for extending the operating band of novel electromagnetic functions.

Facile synthesis of hierarchical chrysanthemum-like copper cobaltate-copper oxide composites for enhanced microwave absorption performance.

Hierarchical chrysanthemum-like CuCo2O4-CuO composites were successfully synthesized by a facile one-pot hydrothermal method after calcination at 500 °C. Based on the results of X-ray diffraction (XRD), Raman spectra, thermogravimetric analysis (TGA) and transmission electron microscope (TEM), we found that not only the morphology (from 2 dimensional to 3 dimensional) but also the crystalline structure (Cu0 + Cu2O + CoOx → CuO + CuCo2O4) of the samples could be tuned by the calcination temperature. The existed interfaces of CuCo2O4-CuCo2O4, CuCo2O4-CuO, and CuO-CuO played a key role on the attenuation of electromagnetic waves. The effective absorption frequency bandwidth is up to 4.02 GHz with a matched thickness of 2.8 mm. The CuCo2O4-CuO/paraffin composites can even exhibit bigger effective frequency bandwidth (from 4.02 to 4.65 GHz) if we turn the incident angle of electromagnetic (EM) wave to a proper value (i.e., 45°). We believe that the hierarchical chrysanthemum-like CuCo2O4-CuO composites can be a good candidate for the high-performance Co-based spinel microwave absorbers.

Controlling optical polarization conversion with Ge2Sb2Te5-based phase-change dielectric metamaterials.

Recent progress in the metamaterial-based polarization manipulation of light highlights the promise of novel polarization-dependent optical components and systems. To overcome the limited frequency bandwidth of metamaterials resulting from their resonant nature, it is desirable to incorporate tunability into metamaterial-based polarization manipulations. Here, we propose a dielectric metamaterial for controlling linear polarization conversion using the phase-change characteristic of Ge2Sb2Te5 (GST), whose refractive index changes significantly when transforming from the amorphous phase to the crystalline phase under external stimuli. The polarization conversion phenomena are systematically studied using different arrangements of GST in this metamaterial. The performance of linear polarization conversion and the tunability are also analyzed and compared in three different designs. It is found that phase-change materials such as GST can be employed in dielectric materials for tunable and switchable linear polarization conversion in the telecom band. The conversion efficiency can be significantly modulated during the phase transition. Our results provide useful insights for incorporating phase-change materials with metamaterials for tunable polarization manipulation.

An electromagnetic modulator based on electrically controllable metamaterial analogue to electromagnetically induced transparency.

Electromagnetically induced transparency (EIT) is a promising technology for the enhancement of light-matter interactions, and recent demonstrations of the EIT analogue realized in artificial micro-structured medium have remarkably reduced the extreme requirement for experimental observation of EIT spectrum. In this paper, we propose to electrically control the EIT-like spectrum in a metamaterial as an electromagnetic modulator. A diode acting as a tunable resistor is loaded in the gap of paired wires to inductively tune the magnetic resonance, which induces remarkable modulation on the EIT-like spectrum through the metamaterial sample. The experimental measurements confirmed that the prediction of electromagnetic modulation in three narrow bands on the EIT-like spectrum, and a modulation contrast of up to 31 dB was achieved on the transmission through the metamaterial. Our results may facilitate the study on active/dynamical technology in translational metamaterials, which connect extraordinary manipulations on the flow of light in metamaterials, e.g., the exotic EIT, and practical applications in industry.

Electrically reconfigurable split ring resonator covered by nematic liquid crystal droplet.

In this paper, an electrically reconfigurable split ring resonator (SRR) covered by sessile droplet of nematic liquid crystal (LC) is demonstrated experimentally. The magnetic resonance of single SRR decreases gradually by 237 MHz as external bias voltage is applied, resulting from increasing fringing capacitance due to liquid crystal molecular reorientation along local electric field distribution. The transmission phase can be modulated by more than 100 degrees. Furthermore, frequency tuning range of SRR increases with the droplet height, because of the significant enhancement for SRR capacitance difference between LC states with/without bias voltage. This work will be of interest for the development of reconfigurable metasurface and related application.

Electrically tunable Fano-type resonance of an asymmetric metal wire pair.

We theoretically and experimentally investigate the electrically tunable Fano-type resonance of asymmetric metal wire pair loaded with varactor diodes. It is illustrated that Fano-type transmission spectrum with high quality factor Q appears as a result of interference between the dipole and quadrupole modes. The ohmic loss of series resistance in varactor diode makes major contribution to absorption. At the Fano-type resonance frequency, both the two metal wires exhibit the strongest electric resonance simultaneously, and the Fano-type resonance manifests a large group delay. As the bias voltage ranges from 0 V to 8 V, the Fano-type resonance frequency exhibits a prominent blueshift of 0.16 GHz and the transmission experiences a modulation with a modulation depth of 97%.

Tunable mid-infrared coherent perfect absorption in a graphene meta-surface.

Graphene has drawn considerable attention due to its intriguing properties in photonics and optoelectronics. However, its interaction with light is normally rather weak. Meta-surfaces, artificial structures with single planar function-layers, have demonstrated exotic performances in boosting light-matter interactions, e.g., for absorption enhancement. Graphene based high efficiency absorber is desirable for its potential applications in optical detections and signal modulations. Here we exploit graphene nanoribbons based meta-surface to realize coherent perfect absorption (CPA) in the mid-infrared regime. It was shown that quasi-CPA frequencies, at which CPA can be demonstrated with proper phase modulations, exist for the grapheme meta-surface with strong resonant behaviors. The CPA can be tuned substantially by merging the geometric design of the meta-surface and the electrical tunability of graphene. Furthermore, we found that the graphene nanoribbon meta-surface based CPA is realizable with experimentally achievable graphene sample.

90° polarization rotator with rotation angle independent of substrate permittivity and incident angles using a composite chiral metamaterial.

We propose a more efficient way to obtain much stronger polarization rotatory power by constructing a composite chiral metamaterial (CCMM) which is achieved via the combination of the cut-wire pairs (CWPs) and a purely chiral metamaterial (PCMM) composed of conjugated gammadion resonators. Owing to the strong coupling between the CWPs and PCMM, the polarization rotation in our CCMM is more gigantic than that of the PCMM. Furthermore, the CCMM proposed in this paper can function as a wide-angle 90° polarization rotator for different substrate permittivity without needing to adjust its geometric parameters. Due to the unique properties, the CCMM may greatly benefit potential applications including designing a tunable 90°-polarization rotator, microwave devices, telecommunication, and so on.