Biaxial hyperbolic metamaterials using anisotropic few-layer black phosphorus.

Most of hyperbolic metamaterials (HMMs) investigated to date are based on isotropic materials resulting in uniaxial HMMs in which dielectric permittivities perpendicular to the propagation direction are the same. Using an anisotropic material constituent to form a HMM is a promising research direction providing opportunities to control the dielectric permittivity in all three directions independently. Herein, we propose and theoretically demonstrate novel biaxial HMMs composed of multilayer stacks of few-layer black phosphorus (BP) and Au thin films. Black phosphorus is an anisotropic material exhibiting crystal axis-dependent dielectric permittivity due to its puckered crystal structure. The proposed HMM provides previously unattained hyperbolic dispersion relations in which the dielectric permittivity in Z-direction of the structure shows opposite sign from that in X- and Y-directions in the most wavelengths from 400~900nm. Furthermore, we calculated the Purcell factor of the proposed biaxial HMMs using full-field electromagnetic simulations.