A polarization-dependent and angle-insensitive absorber based on surface phonon polaritons (SPhPs) is proposed based on a sandwiched layer structure that consists of a patterned α - M o O 3 layer, a dielectric layer, and a gold layer. The absorption remains larger than 95% for TE incidence wave with incident angles from 0° to 30° in the wavelength range from 13.7 to 18.3 µm, which is a broadband absorber. At the same time, the absorption remains larger than 95% for TM incidence wave with incident angles from 0° to 60° in the wavelength range from 11.52 to 11.66 µm, which is a narrowband absorber. It is interesting that the absorber can also be used to fabricate the ideal filter of transverse electric (TE) wave in the wavelength range from 11.52 to 11.66 µm and an ideal filter of transverse magnetic (TM) wave from 15.03 to 18.37 µm. Moreover, one can combine an ideal filter of polarization and a perfect absorber into one single device. The proposed absorber shows promising performance, which has potential applications in sensing, imaging, and detection.

A tunable terahertz (THz) metasurface (MS) absorber based on the surface phonon polariton (SPhP) is proposed based on a layered structure that consists of a split-silver-ring array followed by a graphene layer, polar crystal layer, and silver layer. A dynamically tunable, polarization-independent, and angle-insensitive MS absorber is numerically investigated at THz frequencies. By changing the Fermi energy of graphene from 0 to 1.0 eV, the state of the absorber can switch between the OFF state (with an absorptivity above 90%) and ON state (with an absorptance below 3%). The switching intensity (SI) of the absorber remains greater than 80% for TE incidence wave with incident angles from 0° to 70° and for TM incidence wave incident angles from 0° to 40° in the frequency range from 2.74 to 3.51 THz. These results should be helpful in guiding the design of THz tunable devices such as optical switches, smart absorbers, and imaging.