Excitation of Longitudinal Bound States in a Weyl Metamaterial cavity

Bulk plasmons, which are longitudinal electromagnetic modes (LMs) with electric fields oriented along the propagation direction, play a key role in the formation of Weyl points and hyperbolic dispersions in metamaterials. However, LMs in their pure form are challenging to study due to the difficulty in coupling them with free space radiation, and their sensitivity to material loss. Here a Fabry–Pérot resonance of LMs inside a Weyl metamaterial cavity is realized by introducing a tunable defect into the cavity to control the coupling between the LMs and the transverse-polarized states. A quasi-bound state in the continuum-like effect with FP resonance of LMs is successfully observed in microwave experiments with the help of the synthesized complex frequency approach. Experimental demonstration of longitudinal electromagnetic resonances can open up a wealth of potential applications in areas such as lasing, optical sensing, pass band filtering, and waveguiding.