A dual broadband terahertz bifunction absorber that can be actively tuned is proposed. The optical properties of the absorber were simulated and numerically calculated using the finite-difference time-domain (FDTD) method. The results show that when the conductivity of vanadium dioxide is less than ${\sigma }_0=8.5\times {10}^3$ S/m, the absorptance can be continuously adjusted between 2% and 100%. At vanadium dioxide conductivity greater than ${\sigma }_0=8.5\times {10}^3$ S/m, the absorption bandwidth of the absorber can be switched from 3.4 THz and 3.06 THz to 2.83 THz and none, respectively, and the absorptance remains above 90%. This achieves perfect modulation of the absorptance and absorption bandwidth. The physical mechanism of dual-broadband absorptions and perfect absorption is elucidated by impedance matching theory and electric field distribution. In addition, it also has the advantage of being polarization insensitive and maintaining stable absorption at wide angles of oblique incidence. The absorber may have applications in emerging fields such as modulators, stealth and light-guided optical switches.

A kind of plasmonic structure consisted of an equilateral triangle-shaped cavity (ETSC) and a metal-insulator-metal (MIM) waveguide is proposed to realize triple Fano resonances. Numerically simulated by the finite difference time domain (FDTD) method, Fano resonances inside the structure are also explained by the coupled mode theory (CMT) and standing wave theory. For further research, inverting ETSC could dramatically increase quality factor to enhance resonance wavelength selectivity. After that, a bar is introduced into the ETSC and the inverted ETSC to increase resonance wavelengths through changing the structural parameters of the bar. In addition, working as a highly efficient narrowband filter, this structure owes a good sensitivity (S = 923 nm/RIU) and a pretty high-quality factor (Q = 322) along with a figure of merit (FOM = 710). Additionally, a narrowband peak with 1.25 nm Full-Width-Half-Maximum (FWHM) can be obtained. This structure will be used in highly integrated optical circuits in future.

In this paper, a side-coupled triangle cavity in a plasmonic waveguide structure is proposed and numerically analyzed by the finite-difference time-domain (FDTD) method and coupled mode theory (CMT). Triple plasmonically induced transparency (PIT) was achieved when an extra triangle was added into the structure, and the transmission characteristics were investigated. This novel structure has a maximal sensitivity of 933 nm/RIU when used as a sensor and a contrast ratio of 4 dB. Moreover, the tunability of PIT can be realized by filling the nematic liquid crystal (NLC) E7 into the triangles. The refractive index of E7 changes with the applied electric field. Given that E7 is also sensitive to temperature, this structure can be used as a temperature sensor with a sensitivity of 0.29 nm/°C. It is believed that this tunable structure with PIT may have potential applications in highly integrated optical circuits.

In this paper, we propose a dual-channel mid-infrared toroidal metasurface that consists of split equilateral triangular rings. The electromagnetic responses are analyzed by the finite-difference-time-domain (FDTD) method and temporal coupled-mode theory (TCMT). The results show that one channel of the metasurface is insensitive to the polarization angle of the incident light and temperature, while the other channel is sensitive. The reflectance and resonance wavelength can be manipulated by the polarization angle and temperature independently. Based on such a mechanism, we propose metasurfaces for two-bit programmable imaging and thermal imaging. The metasurfaces are believed to have potential applications in information processing and thermal radiation manipulation.