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We propose an alignment-free and polarization-selective bidirectional absorber composed of a one-dimensional bilayer Au grating array buried in a silicon nitride spacer. The absorptivity of the designed structure is more than 95% (77%) under normal forward (backward) TM-polarized light incidence, and is more than 80% (70%) within a forward (backward) incident angle up to 30°. The great bidirectional absorption performance is illustrated by the resonance coupling of the surface plasmon polaritons (SPPs) resonance, the propagating surface plasmon (PSP) resonance and the localized surface plasmon (LSP) resonance under TM-polarized wave illumination. Moreover, the excitation of the Fano-like resonance mode of the proposed metasurface can produce two significantly different peaks in the absorption spectrum under the oblique TM-polarized incidence, which is beneficial for the plasmon-sensing application. Therefore, the proposed bidirectional metasurface absorber can be a candidate in the application of optical camouflage, thermal radiation, solar cells and optical sensing.

With the rapid development of terahertz technology, tunable high-efficiency broadband functional devices have become a research trend. In this research, a dynamically tunable dual broadband terahertz absorber based on the metamaterial structure of vanadium dioxide (VO2) is proposed and analyzed. The metamaterial is composed of patterned VO2 on the top layer, gold on the bottom layer and silicon dioxide (SiO2) as the middle dielectric layer. Simulation results show that two bandwidths of 90% absorption reach as wide as 2.32 THz from 1.87 to 4.19 THz and 2.03 THz from 8.70 to 10.73 THz under normal incidence. By changing the conductivity of VO2, the absorptance dynamically tuned from 2% to 94%. Moreover, it is verified that absorptance is insensitive to the polarization angle. The physical origin of this absorber is revealed through interference theory and matching impedance theory. We further investigate the physical mechanism of dual broadband absorption through electric field analysis. This design has potential applications in imaging, modulation and stealth technology.

A number of past studies have focused on point and line defects in graphene epitaxially grown on SiC substrates. However, few studies have investigated closed-ring defects formed within grain boundary loops. The present study addresses this issue by applying low-temperature scanning tunneling microscopy/spectroscopy to investigate the atomic structures of closed-ring defects in graphene epitaxially grown on 4H-SiC, and to evaluate their effects on the electron state density. The results indicate that the orientations of the graphene lattice inside and outside of grain boundary loop structures are rotated uniformly by an angle of 30° relative to each other, suggesting that closed-ring defects are highly ordered and are mainly composed of clusters of pentagon-heptagon carbon rings and highly ordered pentagon-heptagon chains. In addition, the spectroscopy results reveal for the first time that the density of electron states inside a closed-ring defect is strongly localized and position-dependent. Moreover, these closed-ring defects can eliminate intervalley scattering while maintaining intravalley scattering. These findings are not only helpful for contributing to a deeper understanding of the effects of closed-ring defects in graphene, but also present a potentially useful valley-filtering mechanism for charge carries that can be applied to the practical development of all-electric valley-based devices.