2D Borophene: In‐Plane Hyperbolic Polaritons in the Visible Spectral Range

Abstract 2D metals, such as graphene, are extensively explored, with graphene exhibiting a metallic response limited to the infrared spectral range. Extending the electron mobility in 2D metals to achieve plasmonic behaviors in the visible range requires innovative synthesis procedures. In this study, the χ 3 phase of borophene is successfully realized using chemical vapor deposition. Leveraging first‐principle density‐functional theory alongside advanced deep‐subwavelength cathodoluminescence spectroscopy, the extreme anisotropic response of this material in the visible range is revealed, transitioning from hyperbolic polaritonic to an elliptic wavefront. The calculations substantiate the experimental findings, positioning borophene as a candidate for the in‐plane hyperbolic response in the visible range. These results open an avenue for optoelectronic applications in the visible spectrum, particularly through the incorporation of borophene into hybrid metallic‐semiconducting heterostructures.