Launching of Visible-Range Hyperbolic Polaritons by Gold Nanoantennas in a natural van der Waals crystal

Abstract

Anisotropic van der Waals materials provide a powerful platform for nanoscale optoelectronics, enabling strong light$-$matter interaction and deep electromagnetic field confinement mediated by polaritons, hybrid light$-$matter excitations with unique dispersion properties. While polaritonic phenomena in van der Waals heterostructures have been extensively explored in the mid-infrared frequency range, their behaviour at the visible frequencies remains largely unexplored, in part due to the lack of knowledge on natural materials supporting anisotropic and highly confined visible-range polaritons. In this context, MoOCl$_2$, an anisotropic van der Waals metal, is particularly interesting, since it supports hyperbolic plasmon polaritons (PPs) that enable directional propagation and subwavelength light compression. Here, we investigate the strategy for launching anisotropic PPs in MoOCl$_2$ in the visible frequency range using gold rod nanoantennas. The nanoantennas, placed on top of the MoOCl$_2$ crystal, excite in-plane anisotropic PP modes, effectively overcoming the momentum mismatch between waves in free-space and nanoscale PPs. We demonstrate a strong electromagnetic field confinement, angle-dependent absorption, and controlled anisotropic PP launching enabled by gold nanoantennas, highlighting the potential of MoOCl$_2$ as a compact platform for nanoscale waveguiding and optical signal processing. By providing a practical antenna-based strategy for exciting visible-range PPs, this work addresses the lack of compact elements for optical signal manipulation and opens new opportunities for optoelectronic devices based on van der Waals polaritonics.