Spinning thermal radiation is a unique phenomenon observed in condensed astronomical objects, including the Wolf-Rayet star EZ-CMa and the red degenerate star G99-47, due to the existence of strong magnetic fields. Here, by designing symmetry-broken metasurfaces, we demonstrate that spinning thermal radiation with a nonvanishing optical helicity can be realized even without applying a magnetic field. We design nonvanishing optical helicity by engineering a dispersionless band that emits omnidirectional spinning thermal radiation, where our design reaches 39% of the fundamental limit. Our results firmly suggest that metasurfaces can impart spin coherence in the incoherent radiation excited by thermal fluctuations. The symmetry-based design strategy also provides a general pathway for controlling thermal radiation in its temporal and spin coherence.

Properties of one-dimensional graphene photonic crystals with dual-layer defects are studied. Results show that two defect modes appear within the gaps, and the defect modes shift to the lower frequencies with the chemical potential increasing, the physical mechanism are also given based on the relative dielectric constant of the graphene. It is also found that the frequency, magnitude, and numbers of the defect modes can vary with the symmetrical changes of the dual-defect layers. For oblique incidence, the defect modes of the TE polarization follow a similar trend with the TM polarization, and all the defect modes shift to the higher frequencies and disappear while new defect modes appear at the larger incident angles. These properties of graphene photonic crystals with dual-layer defects have potential applications in tunable terahertz narrow multiband filters.

In this paper, a new type of broadband tunable metamaterial absorber is proposed. The structure consists of a single-layer ring graphene and a metal ground plane separated by a dielectric spacer. The graphene rings are connected each other to be easily tuned by the voltage. In addition, this absorber is polarization-independent at normal incidence due to the symmetric pattern of the unit cell. Results reveal the bandwidth of 90% absorption reaches to 2.25 THz for a single-ring structure. The bandwidth of 90% absorption increases to 3.2 THz by combining two similar rings with different sizes in one unit cell. The physical mechanism of the single-ring and dual-ring absorbers are given based on the impedance matching theory.

Two types of broadband and polarization independent graphene metamaterial absorbers are designed. One is a composite four-ring structure with azimuthally symmetry, the other is a nested multi-ring structure. The nested structure is more compact and has a broader band of absorption. The bandwidth with over 90% absorption is 7.1 THz for the composite four-ring structure. The bandwidth increases to 8.7 THz and 11.9 THz for the nested dual-ring and three-ring absorber, respectively. The wide bandwidth and polarization independent absorption owe to the combination of symmetric multi-resonators with different sizes in unit cell. Physical mechanisms of the broadband absorbers are given by the impedance matching theory as well as the surface current distributions. Instead of one single layer graphene as papers reported before, multilayers of graphene are coated on the dielectric surface to form the metal resonant-multilayer graphene-dielectric-metal ground structure. It is found that both the absorbing width and magnitude increase greatly as the graphene layers increase while a large dip appears when the number of graphene layers is large enough. Moreover, by investigating the absorptions under different graphene permittivities, we find that the reduced graphene permittivity is better to simplify the calculation.

In this work, we experimentally demonstrate a bifunctional terahertz (THz) metasurface that can achieve broadband linear-to-circular polarization conversion (LCPC) and linear polarization conversion (LPC) in two different bands, respectively. The measured 3-dB axial ratio relative bandwidth is 34.2% for the LCPC in the lower band from 0.34 THz to 0.48 THz, and the measured polarization conversion ratio is over 80% for the LPC in the higher band from 0.53 THz to 0.85 THz. The physical mechanism of the LCPC and LPC is explained by the surface current distribution and is further discussed by employing the multiple interference model. This bifunctional THz metasurface polarization converter has promising applications in THz systems, communications, spectroscopy, etc., and the design strategy can also be extended to microwave and millimeter bands.