Nanophotonic Cavity Modes from Subwavelength Polaritons of Polar Two-Dimensional Crystals

Cavity modes have fundamental importance in nanophotonics as in the two-dimensional (2D) crystals' optical response supporting subwavelength polaritons. Typically, high momenta polaritons in 2D crystals create cavity modes dependent on boundary conditions dictated by the material geometrical forms, substrate electrical permittivity and the optical probes used to interrogate the phenomenon. Here, we use a synchrotron infrared nanospectroscopy to probe polaritonic cavities in hexagonal boron nitride 2D crystals lying on metallic and dielectric substrates. From theoretical approach in quantitative agreement with experimental real-space imaging of polariton waves, we derive the dispersion relation and the reflection/transition coefficients governing the polaritonic cavities.