  Two-dimensional metals, such as graphene, have undergone extensive
exploration, with graphene exhibiting a metallic response limited to the
infrared spectral range. Overcoming the challenge of extending the electron
mobility in two-dimensional metals to achieve plasmonic behaviors in the
visible range necessitates innovative synthesis procedures. In this study, we
showcase the successful realization of the phase of \c{hi}_3 borophene on
diverse substrates using aluminum-based chemical vapor deposition. Leveraging
first-principle density-functional theory alongside advanced deep-subwavelength
cathodoluminescence spectroscopy, we reveal the extreme anisotropic response of
this material in the visible range, transitioning from hyperbolic polaritonic
to an elliptic wavefront. Our calculations substantiate the experimental
findings, positioning borophene as an unprecedented candidate for the in-plane
hyperbolic response in the visible range. These results open avenues for
pioneering optoelectronic applications in the visible spectrum, particularly
through the incorporation of borophene into hybrid metallic-semiconducting
heterostructures.
