Owing to its high penetrability with dielectric materials, terahertz time domain spectroscopy (THz-TDS) is a promising nondestructive measurement technology. The coating thickness deviation and defect of thermal barrier coatings (TBC) will affect its thermal insulation performance and lifetime. In this work, THz-TDS was applied to measure the coating thickness distribution of TBC. The refractive index was obtained by THz-TDS transmission mode. To avoid the normal incidence THz signal loss, the THz signal was reflected from the TBC with a 10° incident angle, which also made the measurement result insensitive to the unevenness and tilt of the TBC sample. In the experiment, the yttria-stabilized zirconia (YSZ) TBC was measured by THz-TDS to estimate the thickness distribution. To validate the thickness measurements, metallography was introduced to correlate the TBC thickness result. The measurement deviation was within 12.1 µm, i.e., 3.45% for the THz-TDS and metallography result. A piece of turbine blade was measured by THz-TDS and a eddy current test. The maximum deviation was 8.48 µm, i.e., 2.36% of these two methods. Unlike the eddy current test, the THz-TDS thickness result was not affected by the cooling holes. The effectiveness of the nondestructive thickness measurement of TBC for turbine blades by THz-TDS was verified.

All-silicon terahertz absorbers have attracted considerable interest. We present a design and numerical study of an all-silicon polarization-insensitive terahertz metamaterial absorber. The meta-atoms of the metamaterial absorber are square silicon rings which can be viewed as gratings. By properly optimizing the structure of the meta-atom, we achieve a broadband absorptivity that is above 90% ranging from 0.77 THz to 2.53 THz, with a relative bandwidth of 106.7%. Impedance matching reduces the reflection of the terahertz waves and the (0, ±1)-order diffraction induce the strong absorption. The absorption of this absorber is insensitive to the polarization of the terahertz wave and has a large incident angle tolerance of up to 60 degrees. The all-silicon metamaterial absorber proposed here provides an effective way to obtain broadband absorption in the terahertz regime. Metamaterial absorbers have outstanding applications in terahertz communication and imaging.

The discovery of topological phases in solid-state electronic materials opened a new chapter in condensed matter physics, which has not only deepened our understanding of how topology can enrich the classification of states of matter¹ but also offered unprecedented opportunities for device applications.² Inspired by these exciting developments, researchers in the photonics community have made great efforts to explore the possibilities of creating topological states for photons, leading to the emerging field of topological photonics.^{3 – 8} To date, a variety of photonic topological states have been proposed and demonstrated, such as photonic quantum Hall states,^{9 – 12} photonic quantum spin-Hall states,^{13 – 16} photonic quantum valley-Hall states,^{17 – 24} and high-order photonic topological corner states.^25,26 Due to the appealing features of photonic topological states, such as unidirectional backscattering-immune wave propagation and robustness against defects and imperfections, these states have found many interesting applications, such as robust delay lines,²⁷ reconfigurable waveguides,^28,29 topological quantum optics interfaces,³⁰ topological quantum light sources,³¹ topological lasers,^{32 – 34} and antichiral edge or surface states.^{35 – 38}

In this paper, a total of 4770 effective documents about metamaterial absorbers were retrieved from the Web of Science Core Collection database. We scientifically analyzed the co-occurrence network of co-citation analysis by author, country/region, institutional, document, keywords co-occurrence, and the timeline of the clusters in the field of metamaterial absorber. Landy N. I.’s, with his cooperator et al., first experiment demonstrated a perfect metamaterial absorber microwave to absorb all incidents of radiation. From then on, a single-band absorber, dual-band absorber, triple-band absorber, multi-band absorber and broad-band absorber have been proposed and investigated widely. By integrating graphene and vanadium dioxide to the metamaterial absorber, the frequency-agile functionality can be realized. Tunable absorption will be very important in the future, especially metamaterial absorbers based on all-silicon. This paper provides a new research method to study and evaluate the performance of metamaterial absorbers. It can also help new researchers in the field of metamaterial absorbers to achieve the development of research content and to understand the recent progress.

Terahertz vortex beams, carrying orbital angular momentum (OAM), are quite desirable for enhancing data transmission capability in telecommunication. However, it faces fundamental and technical challenges in a single metasurface to simultaneously generate orthogonal basis vortices with linear polarization ( x ‐ and y ‐polarity) and circular polarization (left‐ and right‐handed polarity) under the orthogonal polarized light incident. Here, we proposed a chip‐integrated all‐dielectric metasurface in the terahertz regime, to demonstrate the simultaneous generation of four‐channel orthogonal polarized vortex beams at various topological charges under the x ‐ and y ‐polarized light incident. The polarization multiplexed metasurface was designed only with a propagation phase strategy, consisting of polarization‐maintaining and polarization‐conversion meta‐atoms. Simultaneous control of polarization and topological charges in vortex beams was realized by properly arranging birefringent meta‐atom arrays to induce additional phases of x ‐ and y ‐polarization as customized, showing more degrees of freedom for carrying information. The experimental results are in good agreement with the simulations. Such a metasurface approach provides complete polarization bases for further synthesis of diverse polarization vortices required for huge‐capacity communication.