Bulk Synthesis of Hyperbolic Metamaterials with Chemical Tunability

Herein, we report on a novel methodology and characterization of a finely tunable hyperbolic metamaterial (HMM) composed of self-assembled van der Waals crystals of h-BN and graphite. Bulk metamaterials were obtained by sintering well-stacked h-BN/graphite precipitate by spark plasma sintering. We established a facile and large-scale synthetic method of designing and realizing bulk metamaterials. The fabrication process consists of two steps: (1) self-assembly via controlled surface modification of the exfoliated nano building blocks and (2) densification under high pressure and current of nanohybrid powders. The final samples have a thickness of up to centimeters, readily further scalable. Intentionally controlled exfoliation and restacking enable us to obtain the h-BN/graphite HMM, in which building blocks layer thickness are varied systematically. The new hyperbolic metamaterials are composed of the alternating nanolayers of h-BN and graphite/graphene. They display a significant modulation in the position and intensity for both type-I and type-II hyperbolic resonances. Most importantly, their hyperbolic property can be tuned by controlling the thickness of building blocks and their relative concentration in bulk materials. Small amount of rhombohedral BN (r-BN) phase was also found to modulate the hyperbolic dispersions in the bulk phases, in which the r-BN polymorph is introduced by rhombohedral graphite (r-G) phases in graphite layers. This is the first example of bulk hyperbolic metamaterials, of which properties are tuned by controlling the chemical composition and crystal structure. Their permittivity obtained by Kramer-Kronig relation exhibits the capability in negative refraction of incident light and is delicately altered by introduction of graphite layer and r-BN phase. The resulting bulk material also exhibits unique reflectance spectra as a result of distinct interactions with unpolarized and polarized transverse magnetic and electronic beams. The outcome of our work can be a new platform for tailor-made synthesis of various bulk metamaterials without complicated nanofabrication techniques. It also gives an insight to a creation of new hyperbolic materials operating in a desirable range of frequencies.