Hyperbolic Bismuth–Dielectric Structure for Terahertz Photonics

Hyperbolic medium is a special class of strongly anisotropic materials described by diagonal permittivity tensor with the principal components being of the opposite signs, which results in a hyperbolic shape of isofrequency contours. These media support propagating electromagnetic waves with extremely large wave vectors exhibiting unique optical properties and applications such as negative refraction, subwavelength imaging, radiative heat transfer manipulation, enhancing spontaneous emission rate (Purcell factor), biosensing, and nanoscale light confinement. Hyperbolic metamaterials have been experimentally realized for optical, infrared, and microwave frequency ranges. For the terahertz (THz) frequency range, graphene‐based and bismuth‐based media are only theoretically predicted to have a hyperbolic dispersion relation. Herein, the experimental evidence of such dispersion in bismuth–dielectric materials at THz frequencies is shown. THz waveforms transmitted through ultrathin bismuth film/dielectric substrate structures are measured and the negative time delay caused by transition between the elliptic and hyperbolic dispersion at bismuth thickness increase is revealed. In the hyperbolic regime, the switching between effective near‐zero and negative refractive index regime is demonstrated, which depends on the bismuth film thickness and dielectric substrate optical properties. The outcomes demonstrate the possibility for realizing easy planar hyperbolic media for THz photonics, sensing, imaging, and communication systems. Bismuth–dielectric bilayer structure can operate as a hyperbolic metamaterial in terahertz (THz) frequency range. Using THz time‐domain spectroscopy, the negative phase shift and refractive index below 1 are clearly observed. These structures are promising for realization of THz hyperbolic media using natural materials. Their extreme anisotropy and hyperbolic dispersion give a new route to design innovative THz photonics devices.