Anisotropic epsilon-near-pole (ENP) resonance leads to hyperbolic photonic dispersion in homologous (Bi2)m(Bi2Se3)n topological quantum materials

Abstract

The hyperbolic iso-frequency surface (dispersion) of photons in materials that arise from extreme dielectric anisotropy is the latest frontier in nanophotonics with potential applications in subwavelength imaging, coherent thermal emission, photonic density of state engineering, negative refraction, thermal hyperconductivity, etc. Most hyperbolic materials utilize nanoscale periodic metal/dielectric multilayers (superlattices) or metallic nanowires embedded inside the dielectric matrix that require expensive growth techniques and possess significant fabrication challenges. Naturally occurring bulk materials that exhibit tunable hyperbolic photonic dispersion in the visible-to-near-IR spectral ranges will, therefore, be highly beneficial for practical applications. Due to the layered structure and extreme anisotropy, a homologous series of (Bi2)m(Bi2Se3)n could serve as a unique class of natural hyperbolic material with tunable properties derived from different stoichiometry. In this Letter, we demonstrate hyperbolic photonic dispersion in a single crystal of weak topological insulator BiSe (m = 1 and n = 2), where a Bi2 layer is inserted between Bi2Se3 (m = 0 and n = 1) quintuple layers in the visible (525–710 nm) and near-UV (210–265 nm) spectral range. The origin of hyperbolic dispersion in homologous (Bi2)m(Bi2Se3)n topological quantum materials arises from their anisotropic epsilon-near-pole resonance corresponding to the interband transitions that lead to different signs of its dielectric permittivity. The tunability of hyperbolic dispersion is further demonstrated by alloying Bi2Se3 with Mn that alters the interband transition positions and expands their hyperbolic spectral regime from 500–1045 to 500–1185 nm.