In this paper, we design a metasurface terahertz perfect absorber with multi-frequency selectivity and good incident angle compatibility using a double-squared open ring structure. Simulations reveal five selective absorption peaks located at 0−1.2 THz with absorption 94.50% at 0.366 THz, 99.99% at 0.507 THz, 95.65% at 0.836 THz, 98.80% at 0.996 THz, and 86.70% at 1.101 THz, caused by two resonant absorptions within the fundamental unit (fundamental mode of resonance absorption, FRA) and its adjacent unit (supermodel of resonance absorption, SRA) in the structure, respectively, when the electric field of the electromagnetic wave is incident perpendicular to the opening. The strong frequency selectivity at 0.836 THz with a Q-factor of 167.20 and 0.996 THz with a Q-factor of 166.00 is due to the common effect of the FRA and SRA. Then, the effect of polarized electromagnetic wave modes (TE and TM modes) at different angles of incidence (θ) and the size of the open rings on the device performance is analyzed. We find that for the TM mode, the absorption of the resonance peak changes only slightly at θ = 0−80°, which explains this phenomenon. The frequency shift of the absorption peaks caused by the size change of the open rings is described reasonably by an equivalent RLC resonant circuit. Next, by adjusting two-dimensional materials and photosensitive semiconductor materials embedded in the unit structure, the designed metasurface absorber has excellent tunable modulation. The absorption modulation depth (MD) reaches ≈100% using the conductivity of photosensitive semiconductor silicon (σ SI-ps ), indicating excellent control of the absorption spectrum. Our results can greatly promote the absorption of terahertz waves, absorption spectrum tunability, and frequency selectivity of devices, which are useful in the applications such as resonators, biodetection, beam-controlled antennas, hyperspectral thermal imaging systems, and sensors.

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Metasurfaces solve the lack of materials in the terahertz (THz) band and control precisely the amplitude, phase, polarization, and transmission characteristics of THz waves, providing an effective way to realize THz functional devices. This article focuses on the design of THz metasurface modulators with a unit structure consisting of metal square rings, including resonance frequency, phase, and amplitude modulators. By embedding photosensitive semiconductor silicon (Si) in the unit structure, the unit structure is built from meta‐atom to molecularization model under the optical pumping condition, and the resonance frequencies are switched between high and low frequencies. The resonance frequency switchable characteristic is demonstrated using the equivalent LC oscillation circuit model, and the theoretical calculation results agree well with the simulations. Through theoretical calculations, the modulators achieve ultrafast switching times of less than 0.141 ps by the optical pumps, which have significant advantages in ultrafast THz modulators. By continuing to change the embedded position of the silicon in the unit structure, not only is a wide range of THz phase modulation achieved, but also multilevel modulation of the phase is realized. It is found that there is a strong relationship between the modulation depth and phase variation of THz waves, and a reasonable analysis is given. Further the amplitude modulator with a larger modulation depth (MD) is developed, and when the conductivity of photosensitive semiconductor silicon (σSi) reaches 2.5 × 106 S m−1, return loss (RL) is ≈0 dB, and the maximum MD reaches ≈100%; in order to gain insight into the nature of modulation, the modulation mechanism of THz waves under optical pumping conditions is analyzed. In addition, graphene‐based THz metasurface amplitude modulators are designed. When the depth of amplitude modulation is achieved by bias voltage modulation of the Fermi energy level of graphene, the maximum modulation amplitude is 23.42 dB, with a minimal modulation accuracy of 0.05 THz eV−1. In the article, the designed modulators have extremely excellent modulation performance. It has great potential applications in silicon‐based THz photonic devices, ultrahigh frequency electronic devices, high sensitivity sensors, and high‐precision imaging.