A dual-band terahertz metamaterial narrowband absorber is investigated based on a single simple windmill-shaped structure. The proposed metamaterial absorber achieves near-perfect absorption at 0.371 THz and 0.464 THz. The full width at half-maximum is 0.76% and 0.31% relative to absorption frequency. The multireflection interference theory is used for analyzing the absorption mechanism at low absorption frequency. The theoretical predictions of the decoupled model have excellent agreement with simulation results. By investigating the absorber’s distribution of electric field and surface current density at high absorption frequency, the absorber’s near-perfect absorption at the high absorption frequency originating from the magnetic resonance formed between the top metal structure and the bottom metal plane is explained. Besides, the absorber proposed is independent of the polarization angle. It is significant to various applications such as narrowband thermal radiation, photoelectric detection, biological sensing, and other fields.

The phase evolution of terahertz (THz) radiation from single-color femotsecond laser-induced air plasma controlled by a DC-bias is investigated experimentally. When the DC-bias is moved from the end to the beginning of the laser plasma filament, the produced THz waveform is advanced temporally, and its carrier-envelope phase is changed. Our phase spectrum analysis suggests that the slope and the intercept of the phase spectrum, respectively, determine the temporal shift and the carrier-envelope phase of the THz waveform. Therefore, the observed THz waveform evolution is mainly due to the THz propagation effect in plasma filament and the Gouy phase shift associated with the detection scheme. This Letter also illustrates explicitly the temporal order of THz radiation from different parts of a filament.

This paper proposes a terahertz absorber based on a U-shaped metal patch that exhibits both narrowband and wideband absorption characteristics. The absorber comprises a metal–medium–metal structure, with a U-shaped metal patch as the top layer, a lossy medium as the middle layer, and a fully covered metal base plate at the bottom. There are two narrowband absorption peaks at 1.368 THz and 2.648 THz and a broadband absorption band from approximately 4 to 5 THz. The formation of resonance frequencies was elucidated by analyzing the current and electric field distributions at the resonance frequency. The resonant mechanism of the U-shaped patterned structure was clarified using equivalent circuit models to evaluate the resonant frequencies of different absorption peaks. Subsequently, the absorber’s characteristics were examined under varying structural parameters. Dual-function terahertz absorbers are applicable in terahertz communications, imaging, and various other fields.