Lower index hybrid infrared waveguide enhanced by hyperbolic phonon polariton in hexagonal boron nitride

Abstract

In recent years, the field of nanophotonics has been used to miniaturize optical interconnects with long propagation lengths. In the ultraviolet to near-infrared regime, noble metals support light coupling to free electron oscillations, or plasmons, resulting in propagating surface waves called surface plasmon polaritons (SPPs). From mid to long infrared light applications, SPP-like effects in polar dielectrics with phonons can be achieved. Hexagonal boron nitride (hBN) is a two-dimensional van der Waals material in which a slab of hBN can support hyperbolic phonon polaritons (HPhPs). Hybrid waveguides result from the coupling between polariton modes and a high-index dielectric waveguide. Coupled-mode theory is used to describe the resulting hybrid modes and predicts both upper and lower hybrid modes from mode splitting or repulsion. However, only upper hybrid branches have been studied extensively in hybrid polariton waveguides. The lower mode is shown to exist and characterized in terms of mode area and propagation length. The lower hybrid mode formed by the coupling between HPhPs in hBN to a high-index waveguide is compared with the upper hybrid mode. It will be shown via a figure of merit that the lower hybrid mode is advantageous compared to the upper hybrid mode when the cylinder diameters correspond to the maximum coupling strength.