Dielectric function of layered Ga4Se3+δTe1-δ and emergent all van der Waals optical elements

Van der Waals (vdW) crystals provide an expanding platform for designing compact and reconfigurable functional optical devices mostly owing to their wide range of refractive indices, strong optical anisotropy and heterostructure compatibility. Among them, group III monochalcogenides form a prominent family of layered semiconductors exhibiting diverse crystalline phases and showing great potential for nanoscale optoelectronic applications. Despite this promise, a quantitative knowledge of their dielectric functions is yet unrevealed for many representatives of vdW family, not to mention their mixed composition ternary compounds. Here, we address this gap by presenting the complex anisotropic dielectric function of layered Ga ₄ Se _3+δ Te _1−δ in visible (Vis) to near-infrared (NIR) spectral region. It exhibits a response typical for uniaxial crystals enabling simultaneous access to the high refractive indices and low optical losses within a broad spectral region. We also suggest that the optical dispersion of vdW Ga ₄ Se _3+δ Te _1−δ can be adjusted by varying its ternary composition implying a tunability within a window defined by the dispersion limits of the parent binary compounds. Furthermore, we display an approach towards designing an ultrathin all-vdW optical elements exemplified by unpolarized plate-type beam splitters built on a few multilayer heterostructures of Ga ₄ Se _3+δ Te _1−δ / h BN.