Mirrorless open cavities enabled by boundary incompatibility between perfect electric conductor and perfect magnetic conductor parallel-plate waveguides.

The most basic approach to confining electromagnetic waves is to place mirrors along their path, yet achieving confinement without conventional mirrors remains a major challenge. Here, we demonstrate theoretically and numerically an open-geometry cavity that enables electromagnetic field confinement without physical mirrors. This mirrorless confinement arises from a mode gap induced by incompatible boundary conditions at the interface between perfect electric conductor (PEC) and perfect magnetic conductor (PMC) parallel-plate waveguides. By employing a metallic-groove artificial magnetic conductor to emulate PMC behavior, we realize open PEC-PMC cavities that exhibit strong field localization and an enhanced magnetic-dipole Purcell factor. Waveguide-coupling simulations reveal a sharp transmission resonance with a quality factor of [Formula: see text], even in the presence of realistic Ohmic and dielectric losses. These mirrorless open cavities remove the physical boundaries imposed by conventional mirrors, allowing atoms, molecules, quantum dots, and biological nanoparticles to freely access and strongly couple to confined modes, thereby providing a versatile platform for exploring electromagnetic-wave phenomena in open geometries.