As a typical lightweight material, carbon materials have great application prospects in fabricating microwave absorbers. In this work, the Fe3C@Fe/C composites with porous microstructure are obtained by a simple and environmentally friendly method with wasted cornstalks as raw materials. The reflection loss of absorbing composites reaches the minimum value which is less than −50 dB when the thickness is 1.13 mm, while widest effective absorbing bandwidth reaches 5.1 GHz when thickness is 1.50 mm. The stalk-made nanocomposites’ excellent absorbing ability is mainly credit to excellent impedance matching and attenuation characteristics of composites, which is further credit to a synergistic influence of the dielectric loss from the multiple interfaces in porous microstructure and magnetic loss from iron nanoparticles. A low-cost method to obtain microwave absorption materials and realize high-value utilization of agricultural waste to reduce pollution caused by burning cornstalks is put forward.

Lightweight Fe3C@Fe/C nanocomposites derived from wasted cornstalks with high-efficiency microwave absorption and ultrathin thickness

Manipulating the properties of the isofrequency contours (IFCs) of materials provides a powerful means of controlling the interaction between light and matter. Hyperbolic metamaterials (HMMs), an important class of artificial anisotropic materials with hyperbolic IFCs, have been intensively investigated. Because of their open dispersion curves, HMMs support propagating high-k modes and possess an enhanced photonic density of states. As a result, HMMs can be utilized to realize hyperlenses breaking the diffraction limit, metacavity lasers with subwavelength scale, high-sensitivity sensors, long-range energy transfer, and so on. Aimed at those who are about to enter this burgeoning and rapidly developing research field, this tutorial article not only introduces the basic physical properties of HMMs but also discusses dispersion manipulation in HMMs and HMM-based structures such as hypercrystals. Both theoretical methods and experimental platforms are detailed. Finally, some potential applications associated with hyperbolic dispersion are introduced.

Hyperbolic metamaterials: From dispersion manipulation to applications

Manipulating the property of iso-frequency contour (IFC) will provide a powerful control for the interaction between light and matter. Importantly, hyperbolic metamaterials (HMMs), a class of artificial anisotropic materials with hyperbolic IFC have been intensively investigated. Because of the open dispersion curves, HMMs support propagating high-k modes and possess enhanced photonic density of states. As a result, HMMs can be utilized to realize hyper-lens breaking the diffraction limit, meta-cavity laser with subwavelength scale, high sensitivity sensor, long-range energy transfer and so on. In order to make it easier for people who are about to enter this burgeoning and rapidly developing research field, this tutorial article not only introduces the basic physical properties of HMMs, but also discusses the dispersion manipulation of HMMs and HMM-based structures such as hypercrystals. The theoretical methods and experimental platforms are given in this tutorial. Finally, some potential applications associated with hyperbolic dispersion are also introduced

Hyperbolic Metamaterials: From Dispersion Manipulation to Application

Enhancing the light-matter interaction in two-dimensional (2D) materials with high $Q$ resonances in photonic structures has boosted the development of optical and photonic devices. Herein we intend to build a bridge between the radiation engineering and the bound states in the continuum (BIC), and present a general method to control light absorption at critical coupling through the quasi-BIC resonance. In a single-mode, two-port system composed of graphene, coupled with silicon nanodisk metasurfaces, the maximum absorption of 0.5 can be achieved when the radiation rate of the magnetic dipole resonance equals to the dissipate loss rate of graphene. Furthermore, the absorption bandwidth can be adjusted more than two orders of magnitude, from 0.9 nm to 94 nm, by simultaneously changing the asymmetric parameter of metasurfaces, the Fermi level, and the layer number of graphene. This work reveals the essential role of BIC in radiation engineering and provides promising strategies in controlling light absorption of 2D materials for the next-generation optical and photonic devices, e.g., light emitters, detectors, modulators, and sensors.

Controlling light absorption of graphene at critical coupling through magnetic dipole quasi-bound states in the continuum resonance

I review the four mechanisms of bound states in the continuum (BICs) in application to microwave and acoustic cavities open to directional waveguides. The most simple are the symmetry protected BICs which are localized inside the cavity because of the orthogonality of the eigenmodes to the propagating modes of waveguides. However, the most general and interesting is the Friedrich-Wintgen mechanism when the BICs are result of full destructive interference of outgoing resonant modes. The third type of the BICs, the Fabry-Perot BICs, occur in a double resonator system when each resonator can serve as an ideal mirror. At last, the accidental BICs can be realized in the open cavities with no symmetry like the open Sinai billiard in which the eigenmode of the resonator can become orthogonal to the continuum of the waveguide accidentally by a smooth deformation of the eigenmode. We also review the one-dimensional systems in which the BICs occur owing to full destructive interference of two waves separated by spin or polarization or by paths in the Aharonov-Bohm rings. We widely use the method of effective non-Hermitian Hamiltonian equivalent to the coupled mode theory which detects bound states in the continuum (BICs) by finding zero widths resonances.

Interference traps waves in open system: Bound states in the continuum

In optics, the two main mechanisms for enhancing the Goos-H\"anchen (GH) shift of a reflected light beam have a common shortcoming: The maximum shift is located exactly at the reflectance dip, which makes the reflected beam hard to detect. Here the authors tune the excitation of guided modes in a compound grating-waveguide structure, to realize quasibound states in the continuum (quasi-BICs) with ultrahigh $Q$-factors. Assisted by these quasi-BICs, the GH shift at the reflectance peak can be greatly enhanced. This giant GH shift with high reflectance can be used for $e.g.$ ultrasensitive sensors, wavelength-division (de)multiplexers, optical switches, and polarization beam splitters.

Giant Enhancement of the Goos-Hänchen Shift Assisted by Quasibound States in the Continuum

The band gaps of photonic crystals (PCs) play an important role in light manipulation for many applications, such as reflectors, filters, and lasers. In traditional, one-dimensional all-dielectric PCs, as the angle of incidence increases, the gaps are blueshifted in wavelength for both transverse-electric (TE) and transverse-magnetic (TM) polarizations. However, this work predicts and demonstrates that redshifted gaps can be realized in one-dimensional PCs composed of alternating hyperbolic metamaterials and dielectrics for TM polarization, while for TE polarization the gaps remain blueshifted. This property facilitates the design of polarization selectors working in a wide angle range.

Redshift gaps in one-dimensional photonic crystals containing hyperbolic metamaterials

Recently, broadband optical Tamm states (OTSs) in heterostructures composed of highly lossy metal layers and all-dielectric one-dimensional (1D) photonic crystals (PhCs) have been utilized to realize broadband absorption. However, as the incident angle increases, the broadband OTSs in such heterostructures shift towards shorter wavelengths along the PBGs in all-dielectric 1D PhCs, which strongly limits the bandwidths of wide-angle absorption. In this paper, we realize a broadband omnidirectional OTS in a heterostructure composed of a Cr layer and a 1D PhC containing layered hyperbolic metamaterials with an angle-insensitive photonic band gap. Assisted by the broadband omnidirectional OTS, broadband wide-angle absorption can be achieved. High absorptance (A > 0.85) can be remained when the wavelength ranges from 1612 nm to 2335 nm and the incident angle ranges from 0° to 70°. The bandwidth of wide-angle absorption (0°-70°) reaches 723 nm. The designed absorber is a lithography-free 1D structure, which can be easily fabricated under the current magnetron sputtering or electron-beam vacuum deposition technique. This broadband, wide-angle, and lithography-free absorber would possess potential applications in the design of photodetectors, solar thermophotovoltaic devices, gas analyzers, and cloaking devices.

Broadband wide-angle multilayer absorber based on a broadband omnidirectional optical Tamm state.

Ultra-large omnidirectional photonic band gaps in one-dimensional ternary photonic crystals composed of plasma, dielectric and hyperbolic metamaterial

We theoretically and experimentally investigate the wide-angle perfect absorptance in a photonic heterostructure composed of a metal film and a truncated photonic crystal (PC) with layered hyperbolic metamaterials (HMMs) in the near ultraviolet and visible regions. The wide-angle perfect optical absorption depends on the dispersionless Tamm plasmon polarition (TPP) under TM polarization, which originates from reflection phase compensation condition between the metal and the truncated PC with HMMs. Our experimental results show nearly perfect absorptance over 0.91 in an angle range of 0-45 degree, which facilitates the design of perfect optical absorbers working in a wide angle range.

Feng Wu

Papers

Giant Enhancement of the Goos-Hänchen Shift Assisted by Quasibound States in the Continuum

Redshift gaps in one-dimensional photonic crystals containing hyperbolic metamaterials

Broadband wide-angle multilayer absorber based on a broadband omnidirectional optical Tamm state.

Ultra-large omnidirectional photonic band gaps in one-dimensional ternary photonic crystals composed of plasma, dielectric and hyperbolic metamaterial

Perfect optical absorbers in a wide range of incidence by photonic heterostructures containing layered hyperbolic metamaterials