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In this paper, a multifunctional terahertz (THz) absorber based on Dirac semimetal and vanadium dioxide ( V O 2 ) is proposed. By modulating the temperature of V O 2 , the absorber can be switched between the narrow band and wide band. When V O 2 is in the metallic state, the absorber has a broadband absorption effect with a bandwidth of approximately 4 THz. It has the advantages of insensitivity to polarization and wide-angle absorption. When V O 2 is in the insulating state, the absorber has two absorption peaks with absorptivity exceeding 90% and sensitivities of 297.7 and 402 GHz/RIU, and thus can be used as a highly sensitive sensor for cell detection. When the Fermi level of the Dirac semimetal is changed, the absorption characteristics can be modulated. The absorber has broad application prospects in multifunctional modulated devices.

Electromagnetic metasurface with many special electromagnetic properties can be utilized to manipulate electromagnetic wave propagation and reflection. If the metasurfaces were designed for coding, random, phase discontinuities, perfect absorber and so on，they can manipulate the scattering or the reflection of electromagnetic wave and achieve the reduction of the radar cross section(RCS). This paper mainly presents the recent progress concerning the reduction of RCS using non - directional scattering or the absorption characteristic in microwave and terahertz wave. The analysis results show that coding electromagnetic metasurface can disperse the reflection into a variety of direction by designing the specific coding sequences for different elements. The coding metasurface which are composed of different digital elements, and the reflection phase difference of these digital elements is constant in a wide frequency range, and higher bit coding metasurface can flexible manipulate electromagnetic wave. The random electromagnetic metasurface can achieve broadband phase shifter by adjusting the size parameters of array element, and can diffuse characteristic by scattering into random wave of the reflection peaks for metal target. Phase discontinuities metasurface can achieve anomalous or diffuse of wave because the phase distribution is not uniform at the surface. The absorber metasurfaces which are designed reasonably with the physical dimensions of the devices can reduce reflection by absorbing electromagnetic wave energy. So, the electromagnetic metasurfaces have a large potential application for radar stealth, broadband communications, imaging and so on. Finally, we discussed the future development of RCS reduction by using the electromagnetic metasurface. In order to satisfy the needs of practical application, the research of metasurface will continue development in broadband, flexible, large angle and other aspects.

The aim of this study was to investigate the bandgap characteristics of three-dimensional photonic crystals of SIC, GaP, InP, GaAs, InAs, InSb, HgTe, and HgSe materials with typical structures under the following different conditions: diamond, face-centered cubic (fcc), and wood. Based on the plane wave expansion method, the influence of factors such as the dielectric constant, density on the upper and lower boundary, and width on the bandgap characteristics of three-dimensional photonic crystals was studied, in addition to the influence of factors such as the filling ratio and the radius of the ball on the bandgap variation in three-dimensional photonic crystals. The results show that the bandgap width of diamond and fcc structures can be improved by increasing the dielectric constant and density. However, the bandgap width of wood structures decreases, as they have wider photonic bandgaps when the filling rate is 0.325 and the ball radius is highly symmetrical. These results provide the theoretical basis for the design of three-dimensional photonic crystal devices.

The plasmon properties of the concentric cavity consisting of a semi-disk inside a thin ring are theoretically investigated by the finite difference time domain and plasmon hybridization methods. According to the simulated absorption and scattering spectra, electric field distributions and charge diagrams, multiplar Fano resonances are successfully generated, mainly due to the coupling between the dark multipolar modes and the bright dipolar mode originating from the hybridization between the primitive modes of the semi-disk and the ring. Moreover, the intensities and spectral positions of the Fano resonances can be manipulated by modifying the geometric parameters of the cavity. The cavity for potential use as a multi-wavelength biochemical sensor is evaluated with the refractive index sensitivity and figure of merit.

In the present paper, the recent progress in terahertz metamaterials-based sensing is reviewed with the principle of metamaterial biosensor,metamaterial substrate, and structure design, respectively. The paper introduces the principle in detail, analyzes the sensitivity of the biosensor with the material and the thickness of the substrate and the structure of metamaterial. The analysis shows that we can enhance the sensitivity and resolution of biosensor by designing specific metamaterial structure, using low dielectric constant and low loss thin substrate, especially many materials have a specific response in the terahertz frequency. So, there is a large potential application for label-free sensing by using the terahertz metamaterials. This paper also presents the future development of THz metamaterial sensors.

Versatile devices, especially tunable ones, for terahertz imaging, sensing and high-speed communication, are in high demand. Liquid crystal based components are perfect candidates in the optical range; however, they encounter significant challenges in the terahertz band, particularly the lack of highly transparent electrodes and the drawbacks induced by a thick cell. Here, a strategy to overcome all these challenges is proposed: Few-layer porous graphene is employed as an electrode with a transmittance of more than 98%. A subwavelength metal wire grid is utilized as an integrated high-efficiency electrode and polarizer. The homogeneous alignment of a high-birefringence liquid crystal is implemented on both frail electrodes via a non-contact photo-alignment technique. A tunable terahertz waveplate is thus obtained. Its polarization evolution is directly demonstrated. Furthermore, quarter-wave plates that are electrically controllable over the entire testing range are achieved by stacking two cells. The proposed solution may pave a simple and bright road toward the development of various liquid crystal terahertz apparatuses. Light: Science & Applications (2015) 4, e253; doi:10.1038/lsa.2015.26; published online 27 February 2015