Enhancement of an electromagnetic field by a terahertz metasurface based on near-field coupling and quasi-BIC.

The coupling effect between incident electromagnetic waves and a properly designed metasurface can excite surface plasmon resonance at the operating frequency, which can be used to achieve significant field enhancement. The bound state in the continuum (BICs), as a unique non-radiative eigenstate, is considered to have rich physical properties. In this work, what we believe to be a new approach to enhance high-order mode electromagnetic response by utilizing near-field coupling and quasi-BIC interaction has been proposed. Quasi-BICs can be excited to achieve significant optimization of high-order coupled resonant states by breaking the symmetry of one side of the dual-port resonant ring. The electric field enhancement factor (EFEF) has been boosted by about 4.09 times, the magnetic field enhancement factor (MFEF) has been boosted by about 5.19 times, and the quality factor has been boosted to 1050, an increase of 493.22%. At the same time, the increasing resonance strength of the mode coupling term ensures sufficient radiation energy for exciting electromagnetic response. The results show that the EFEF is increased by a factor of 205.3, the MFEF is increased by a factor of 110.4, and the modulation depth is from 0 to 99.89%. In addition, by embedding actively controllable materials in the metasurface. The dimension of dynamic tuning is increased, enabling ultrafast switching of high-order modes after photon coupling and quasi-BIC coupling, and the tunable range of the eigenstate can be successfully tuned from 0 to 98.9% at the highest level. Therefore, this research has great potential applications in terahertz devices that require precise control of frequency and signal strength.