Non-Hermitian Topology of a Subwavelength Resonator in Synthetic Space
Non-Hermitian singularities and the nontrivial topology of energy bands have been extensively explored in extended periodic systems, where phenomena such as exceptional points and bulk Fermi arcs are naturally associated with periodicity and momentum space. Here, we show that this non-Hermitian topology can be extended to the domain of subwavelength resonators using a synthetic space. By reconstructing the complex energy bands of a subwavelength ring resonator in the synthetic space of geometrical parameters, we experimentally observe a spectral triplet formed by two exceptional points (EPs) connected by a bulk Fermi arc and a topologically robust quasi-bound state in the continuum (q-BIC). This Fermi-arc q-BIC emerges exactly at the intersection of the bulk Fermi arc and the q-BIC line in synthetic space, establishing a direct topological link between two spectral singularities. Encircling the EPs in synthetic space visualizes the nontrivial topology of the complex energy-band structure and reveals the opposite half-integer topological charges of the EPs connected by the bulk Fermi arc. These results extend non-Hermitian topology to subwavelength structures and establish compact resonators as a versatile platform for applications in photonics, microwave physics, and acoustics.