Plasmonic responses in Janus bAsP with elliptic-to-hyperbolic transition: an ab-initio study

Abstract

Plasmonic responses in materials with actively tunable elliptic-to-hyperbolic transition are rare in nature. Based on ab-initio calculations, we have theoretically predicted that Janus black arsenic phosphorus (bAsP) supports both elliptic and hyperbolic in-plane surface plasmon polaritons in the infrared after being doped with electrons. In the elliptic regime, anisotropic plasmonic responses have been observed, which can be explained by the anisotropic dispersions at the bottom of the conduction bands. In the hyperbolic regime, the total permittivity along the armchair/zigzag edge is negative/positive, which is the result of positive interband permittivities and largely different Drude plasma frequencies along two directions making the total permittivities change signs at different photon energies. In this material, changing the topology (elliptic or hyperbolic) of the plasmonic responses via doping is possible. Then, strains along the zigzag and armchair directions have been applied to modify the band structures as well as the plasmonic responses. Since plasmonic responses are mostly related to the bands near the Fermi energy, a relatively small strain along the zigzag direction can make bAsP become an indirect-bandgap material and change the Drude plasma frequencies under proper doping. With both strain and doping present in this material, we have even found a special case of hyperbolicity where the total permittivity in the zigzag/armchair direction is negative/positive, which is opposite to the normal case. In the end, we have extended our investigations to bAsP-graphene heterostructures. Since bAsP is a Janus material, such direct contact can change the Fermi energy through charge transfer making this heterostructure support strong plasmons without extra doping. Our investigations propose bAsP as a promising Janus material platform for plasmonic applications.