Ultranarrow polaritonic cavities formed by one-dimensional junctions of two-dimensional in-plane heterostructures.

We propose two-dimensional (2D) in-plane heterostructures, composed of a 2D crystal adjoining a perfect electric conductor (PEC) plane, that enable ultranarrow polaritonic resonant cavities. Specifically, we theoretically investigate the interaction of 2D surface polaritons (2DSPs) with the junction between the 2D crystal and a PEC plane. We reveal that when 2DSPs are strongly confined, the reflected 2DSPs experience a phase shift of 3π/4, which exhibits π/2 deviation from the so-called edge reflection value. This non-trivial phase shift is shown to play a crucial role in enabling resonant cavities whose size can be far smaller than the wavelength of the 2DSPs. Furthermore, we demonstrate that the spatial dimensionality of our heterostructure allows a direct mapping to metasurface-based heterostructures, where the 2D crystal is replaced by a metasurface supporting spoof surface polaritons (SSPs). This correspondence extends the feasibility of our concept to SSP-based resonators and broadens the accessible frequency range into the terahertz and microwave regimes. Our work provides not only deeper insight into low-dimensional polariton optics but also a design strategy for ultracompact polaritonic metaresonators.