Photonic bound states in the continuum (BIC) promise a versatile platform for construction of optical resonators with highly confined electromagnetic fields and ultralong radiative lifetime. Merging multiple BIC can enable suppression of out-of-plane radiative losses and further boost the quality (Q) factor of the resonance state. However, current studies on merging BIC (M-BIC) are primarily restricted to the elementary BIC with topological charge $\pm 1$, which hinders further improvement of the performance of optical resonators. Here, an all-silicon terahertz metasurface (THz-MS) is investigated that supports the symmetry-protected BIC (SP-BIC) with topological charge $-2$ and accidental BIC (A-BIC) with topological charge $\pm 1$ simultaneously. Empowered by the topological nature of BIC, we merge twelve A-BIC with a higher-order SP-BIC at the $\mathrm{\Gamma }$ point. By tailoring in-plane mirror symmetry of the THz-MS, we achieve M-BIC at almost arbitrary position in the momentum space. Unlike original isolated BIC, M-BIC manifests the dramatic enhancement of the Q factors of nearby resonances and are robust against radiation losses introduced by fabrication imperfections. Our work presents a paradigm for realizing higher-order at-$\mathrm{\Gamma }$ M-BIC and momentum-steerable M-BIC that can substantially enhance light-matter interaction and further improve the performance of terahertz optoelectronic devices.

We have designed, fabricated, and experimentally verified highly sensitive all-dielectric metasurface THz absorbers (AMTAs) device in the terahertz (THz) frequency range. By designing different structures of the resonant, different types of absorber which have perfect absorption can be realised. The proposed AMTA with simplified designs is demonstrated by single-layer all-silicon array. Furthermore, according to the simulated results of absorption frequency changes under different refractive index demonstrate that the proposed AMTAs can act as refractive index sensors in the biochemical application, such as detection of liquid paraffin, cancer cells and ethanol. Also, experiments of detecting different concentrations of 2, 4-Dichlorophenoxyacetic acid (2, 4-D) have demonstrated that the proposed absorbers have great potential to improve the performance in biological and chemical sensing. These results show that the AMTAs can used as sensors, which can detect the presence of trace pesticides and residues of some chemicals in adsorption analyses, among other practical applications.

Perfect metasurface absorbers play a significant role in imaging, detecting, and manipulating terahertz radiation. We utilize all-dielectric gratings to demonstrate tunable multi-band absorption in the terahertz region. Simulation reveals quad-band and tri-band absorption from 0.2 to 2.5 THz for different grating depths. Coupled-mode theory can explain the absorption phenomenon. The absorption amplitude can be precisely controlled by changing the pump beam fluence. Furthermore, the resonant frequency is sensitive to the medium’s refractive index, suggesting the absorber may be of great potential in the sensor detection field. The experimental results exhibit a high detectivity of pesticides.

The extraordinary emergence of all-dielectric resonant meta-photonics underpinned by high refractive index and low optical loss materials promises a standout platform for unprecedented manipulation and subwavelength control of light. Recent significant breakthroughs in meta-photonics have to do with the exploration of the non-radiative eigenmodes that lie inside the light cone, called the bound states in the continuum (BICs), which have demonstrated considerable potential to achieve high quality (Q) factors but require ultra-small structure asymmetry, complicating fabrication and hindering precise control of Q factors. Here, we propose a scheme to excite quasi-BICs by breaking symmetry in the permittivity of the comprising meta-atoms where small permittivity perturbation renders the same effect in the conventional geometrically asymmetric design. Empowered by the insignificant permittivity perturbation introduced from chemical doping and carrier injection, arbitrarily small permittivity asymmetry, that is, infinitely high-Q factor, can be precisely and dynamically tailored. As an example of application, we numerically show the THz range, exceptionally high conversion efficiency for both third-harmonic and fifth-harmonic generation from the permittivity-broken metasurface (MS) benefiting from the extreme field confinement at high-Q quasi-BICs resonance. Our results present a feasible and efficient strategy to mitigate the shortcomings of previous photonic platform and shall shed light on the advances of nonlinear all-dielectric THz-MS.

As a high‐degree‐of‐freedom approach to manipulate the electromagnetic wave, metasurfaces are widely used in high‐capacity information technology. Extensive investigations have explored multiplexing techniques using polarization, incident angle, wavelength, and infinite‐dimensional multiplexing through Orbital Angular Momentum (OAM). However, due to the limited spatial resolution and array size of the metasurface, the number of multiplexing channels that can be actually realized is limited. Therefore, research on the combination of OAM multiplexing and polarization degrees of freedom is of great significance. Here, we propose and experimentally demonstrate a metasurface holography multiplexing scheme based on multiple polarization channels combined with OAM. Taking advantage of the orthogonal independence of spin angular momentum and orbital angular momentum, multiple OAM multiplexing holograms are constructed in multiple different spin‐polarization channels. Utilizing the well‐established compatibility between OAM multiplexing and polarization multiplexing, we successfully integrated two multiplane holograms and 15 OAM multiplexing holograms on a single metasurface. Subsequently, we introduced an optical nested encryption framework designed for parallel communication. This work facilitates high‐capacity and high‐security holography by employing multiplexing metasurfaces, thereby providing innovative design concepts for optical communication, information encryption, and related domains.