DOI: 10.1002/adom.201901548 energy with 100% efficiency can be very useful in energy harvesting and invisibility-related applications.[2–4] Moreover, it is even more fascinating if one can combine those two functionalities into one single device, which can behave either as an ideal filter (as shown in Figure 1a) or a perfect absorber (as shown in Figure 1b), controlled dynamically by certain external knobs. Such tunable devices can be potentially useful in many application scenarios (e.g., smart radar radomes and solar cells), but are extremely challenging to realize since the two EM-wave properties (i.e., scattering and absorption) of an ultrathin slab are strongly coupled with each other. In fact, early theoretical studies have demonstrated that the largest absorption enabled by an ultrathin screen is 50% (as shown in Figure 1c,d),[5] not mentioning further difficulties in making the device actively tunable. Recent developments on metasurface (particularly on tunable metasurfaces) provide possible solutions to make such optical devices. Metasurfaces,[6–12] ultrathin metamaterials composed by planar meta-atoms with tailored EM properties, have exhibited strong abilities to control EM waves,[6,7,13–16] with perfect transmission[17] and absorption[18] of EM waves both realized. With active elements integrated, one can further construct tunable metasurfaces with dynamically controlled functionalities[19–29] including tunable absorbers and filters. However, most tunable metadevices realized so far are based on reflection geometry[20–23,26–29] which are relatively easy to design, since only two channels exist in such systems for EM waves to radiate or dissipate (i.e., the reflection port and absorption).[30] In contrast, high-performance tunable metadevices in transmission modes are rarely seen, not mentioning those exhibiting dynamically switched dual functionalities. The intrinsic physics is that a transmissive metasurface has three channels (i.e., transmission and reflection ports, and absorption) to transport/dissipate energy, which must be carefully balanced to yield the desired functionalities (i.e., perfect transmission or perfect absorption). However, the competitions between these three channels are very complicated, which make a transmission-absorption switchable metadevice very difficult to realize, especially on an ultrathin platform. In this paper, we design and experimentally realize such a metadevice with ultrathin thickness. In Section 2, we start from examining the EM properties a trilayer metasurface with PIN diodes loaded and controlled by the same voltages, and show that such a device cannot exhibit freely controlled transmission and absorption properties. We next employ the coupled-mode A thin screen exhibiting dynamically switchable transmission/absorption functionalities is highly desired in practice. However, a trilayer transmissive metasurface with active elements controlled in a uniform manner cannot exhibit independently controlled transmission and absorption properties due to intriguing interplays between the scattering and absorbing properties in systems exhibiting inversion symmetries. This motivates to employ the coupled-mode theory to establish a generic phase diagram for such transmissive metasurfaces with active elements loaded in different layers tuned independently and to guide researchers design tunable metadevices with completely decoupled transmission/absorption responses. Based on such a phase diagram, a microwave metasurface is designed/fabricated with PIN diodes incorporated, and it is experimentally demonstrated that its functionality can switch from perfect transparency to perfect absorption, controlled by external voltages applied across the diodes. In addition to finding immediate applications in practice (e.g., smart radomes), the results of this study also provide a new type of tunable meta-atom for building metasurfaces with flexible wave-front control abilities.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adom.201901548

A Tunable Metasurface with Switchable Functionalities: From Perfect Transparency to Perfect Absorption

In this study, we used a batch-foaming method to prepare closed-cell poly(vinylidene fluoride) (PVDF) foams with tailored microcellular structures. This is a simple, cost-effective, and environmentally-friendly method, and it offers a wide range of tunable microcellular structures. The cell size and porosity volume of PVDF foams can be effectively monitored at their saturation temperatures. The PVDF foam prepared at 169.5 °C possesses an ultra-low dielectric constant of k ∼ 1.1 and a thermal conductivity of 0.027 W m−1 K−1. We used numerous theoretical models to predict the effective dielectric permittivity of the microcellular PVDF foams, and found that the Bruggeman model was very close to the measured results. The ultra-low thermal conductivities of the PVDF foams were the result of a large number of air pockets in the microcellular structure. Based on these results, we concluded that this PVDF foam would be a promising low-k dielectric and heat insulating material.

Poly(vinylidene fluoride) foams: a promising low-k dielectric and heat-insulating material

Cavity Magnonics is an emerging field that studies the strong coupling between cavity photons and collective spin excitations such as magnons. This rapidly developing field connects some of the most exciting branches of modern physics, such as quantum information and quantum optics, with one of the oldest science on the earth, the magnetism. The past few years have seen a steady stream of exciting experiments that demonstrate novel magnon-based transducers and memories. Most of such cavity magnonic devices rely on coherent coupling that stems from direct dipole-dipole interaction. Recently, a distinct dissipative magnon-photon coupling was discovered. In contrast to coherent coupling that leads to level repulsion between hybridized modes, dissipative coupling results in level attraction. It opens an avenue for engineering and harnessing losses in hybrid systems. This article gives a brief review of this new frontier. Experimental observations of level attraction are reviewed. Different microscopic mechanisms are compared. Based on such experimental and theoretical reviews, we present an outlook for developing open cavity systems by engineering and harnessing dissipative couplings.

Dissipative couplings in cavity magnonics

We investigate electromagnetically induced transparency (EIT)-like effect in a metal-dielectric-metal (MDM) waveguide coupled to a single multimode stub resonator. Adjusting the geometrical parameters of the stub resonator, we can realize single or double plasmon-induced transparency (PIT) windows in the plasmonic structure. Moreover, the consistency between analytical results and finite difference time domain (FDTD) simulations reveals that the PIT results from the destructive interference between resonance modes in the stub resonator. Compared with previous EIT-like scheme based on MDM waveguide, the plasmonic system takes the advantages of easy fabrication and compactness. The results may open up avenues for the control of light in highly integrated optical circuits.

Plasmon-induced transparency in a single multimode stub resonator.

Materials with anisotropic material parameters can be utilized to fabricate many fascinating devices, such as hyperlenses, metasolids, and one-way waveguides. In this study, we analyze the effects of geometric anisotropy on a two-dimensional metamaterial composed of a rectangular array of elliptic cylinders and derive an effective medium theory for such a metamaterial. We find that it is possible to obtain a closed-form analytical solution for the anisotropic effective medium parameters, provided the aspect ratio of the lattice and the eccentricity of the elliptic cylinder satisfy certain conditions. The derived effective medium theory not only recovers the well-known Maxwell-Garnett results in the quasi-static regime, but is also valid beyond the long-wavelength limit, where the wavelength in the host medium is comparable to the size of the lattice so that previous anisotropic effective medium theories fail. Such an advance greatly broadens the applicable realm of the effective medium theory and introduces many possibilities in the design of structures with desired anisotropic material characteristics. A real sample of a recently theoretically proposed anisotropic medium, with a near-zero index to control the flux, is achieved using the derived effective medium theory, and control of the electromagnetic waves in the sample is clearly demonstrated.

/pdf/effective-medium-theory-for-anisotropic-metamaterials-42dc1fq9m7.pdf

Effective medium theory for anisotropic metamaterials

A hybrid coupling model containing both near- and far-field couplings is developed for radiating two-resonator structures. We demonstrate that the near- and far-field couplings make distinguished contributions to electromagnetic responses. Compared to the classical electromagnetically induced transparency configurations, the presence of far-field coupling provides more flexibility in tuning lineshapes. Planar metamaterials composed of metal wires are designed based on this model, and various electromagnetic responses are experimentally observed.

Manipulating electromagnetic responses of metal wires at the deep subwavelength scale via both near- and far-field couplings

Design of guided Bloch surface wave resonance bio-sensors with high sensitivity

Optical Bloch oscillation and resonant Zener tunneling in one-dimensional quasi-period structures containing single negative materials

For usual lossy and lossless dielectric spherical particles, it can never realize zero forward scattering, since their Mie expansion coefficients cannot be totally offset at forward direction. By introducing suitable gain in dielectric spherical particles, we find that the electric and magnetic dipolar responses can achieve complete destructive interference at forward direction under certain incident wavelength. When the radius of particle is much smaller than the incident wavelength, the quadrupole and other higher order Mie coefficients are negligible, thus zero forward scattering can be observed and simultaneously backscattering will get enhanced. However, if the quadrupole Mie coefficients are of the same order of magnitude with dipolar terms, coherent effect between them may result in the scattering minimum and maximum shift away from forward and backward directions, respectively. These results may provide new insight into the manipulation of light scattering direction at nanoscale.

Anomalous forward scattering of dielectric gain nanoparticles.

Experimental study of quasi-one-dimensional comb-like photonic crystals containing left-handed material

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