Boosting transparent electromagnetic interference shielding by multi-cavity resonances.

We propose a multi-cavity resonant architecture that is established by employing two opposing ultrathin silver-based films to form a Fabry-Pérot (F-P) cavity and inserting one or two metallic mesh layers in between. Compared with the single F-P cavity, the multi-cavity architecture with one metallic mesh layer experimentally exhibits a ∼37% improvement in the average shielding effectiveness and maintains a transmittance over 80% at 550 nm. A more significant improvement of ∼108% in shielding effectiveness (SE) can be achieved by inserting two metallic mesh layers. The proposed multi-cavity architecture provides a strategy for removal of the hindrance to transparent electromagnetic interference shielding.

Switchable multifunctional terahertz metasurfaces employing vanadium dioxide.

In this paper, we design a type of switchable metasurfaces by employing vanadium dioxide (VO2), which possess tunable and diversified functionalities in the terahertz (THz) frequencies. The properly designed homogeneous metasurface can be dynamically tuned from a broadband absorber to a reflecting surface due to the insulator-to-metal transition of VO2. When VO2 is in its insulating state, the metasurface can efficiently absorb the normally incident THz wave in the frequency range of 0.535-1.3 THz with the average absorption of ~97.2%. Once the VO2 is heated up and switched to its fully metallic state, the designed metasurface exhibits broadband and efficient reflection (>80%) in the frequency range from 0.5 to 1.3 THz. Capitalizing on such meta-atom design, we further extend the functionalities by introducing phase-gradients when VO2 is in its fully metallic state and consequently achieve polarization-insensitive beam-steering and polarization-splitting, while maintaining broadband absorption when VO2 is in insulating state.

Transparent transmission-selective radar-infrared bi-stealth structure.

We report a multifunctional metamaterial composite structure that not only provides the broadband radar and thermal infrared bi-stealth function but also possesses an in-band microwave transmission window and high optical transparency. It is composed of four metasurface layers made of indium tin oxide (ITO) films with different surface resistances, which are specifically designed to sequentially control the infrared emission, microwave absorption and transmission. The fabricated sample exhibits a low reflectivity less than 10% in 1.5-9 GHz and a transmission peak of 50% around 3.8 GHz up to the incident angle of 30 degrees. In the infrared atmosphere window, a low thermal emissivity of about 0.52 is achieved. Meanwhile, it keeps good optical transparency by the use of the ITO films. The optically transparent, low-infrared-emissivity, radar-reflectionless and frequency-selective-transmission properties will enable the promising application of communication-compatible multispectral stealth technology.

Giant power enhancement for quasi-omnidirectional light radiation via ε-near-zero materials.

We theoretically demonstrate a giant power enhancement effect for a line current source in a ε-near-zero (ENZ) two-dimensional (2D) shell with proper physical dimensions. Compared with the traditional high-ε dielectric approach, the ENZ scheme has the prominent advantage that the radiation performance is less sensitive to the outer radius of the shell, which is critically important for real applications where micro-nano fabrications are often involved. The enhancing performance is independent on the position of the source inside the ENZ shell and could be substantially strengthened by incorporating more sources, while the quasi-omnidirectional radiation pattern could be managed to have negligible variance, as evidenced by a particular example with an inner radius of the shell equal to 0.156λ0. Compared with the single source case, two identical sources with a phase difference less than 134° will raise the total radiation power more than 4 folds and the maxima will be about 30 when they are in phase. The field analysis shows that this quasi-isotropic radiation enhancement is mainly contributed by the amplification of the isotropic zeroth order mode radiation while the higher orders with anisotropic emission patterns are effectively suppressed by the specifically designed ENZ shell. In the end, a practicable device employing 4H-silicon carbide (4H-SiC) naturally available with ENZ properties in the mid-infrared regime is numerically proposed, which could provide more than 10 times of radiation enhancement through optimizing the permittivity of the inner dielectric cylinder. These results may find very important applications in the design of novel devices for mid-infrared photon sources or detectors.

Ultrathin and lightweight microwave absorbers made of mu-near-zero metamaterials.

We present a theory of perfect absorption in a bilayer model composed of a mu-near-zero (MNZ) metamaterial (MM) absorbing layer on a metallic substrate. Our analytical solutions reveal that a MM layer with a large purely imaginary permeability and a moderate permittivity backed by a metallic plane has a zero reflection at normal incidence when the thickness is ultrathin. The impedance-mismatched metamaterial absorber (MA) can be 77.3% thinner than conventional impedance-matched MAs with the same material loss in order to get the same absorption. A microwave absorber using double-layered spiral MMs with a thickness of only about one percent of the operating wavelength is designed and realized. An absorption efficiency above 93% at 1.74 GHz is demonstrated experimentally at illumination angles up to 60 degrees. Our absorber is 98% lighter than traditional microwave absorbers made of natural materials working at the same frequencies.