Modulating and tuning relative permittivity of dielectric composites at metamaterial unit cell level for microwave applications

Dielectric materials with desirable relative permittivity (ε_r) may not be readily available in nature and are usually difficult to be tailored precisely. We hereby predict and experimentally achieve the flexible tuning of ε_r for dielectric materials at the unit cell level in microwave range, as inspired by the recent developments in metamaterials. A series of composite dielectric stock sheets with ε_r in the range of 3–7 and low loss tangent (tanδ < 0.01) were successfully fabricated by dispersing different volume fractions of TiO₂ nanoparticles into polypropylene polymer matrix. To further enable the flexible tuning of ε_r, sub-wavelength holes with different diameters were drilled on the composite dielectric sheets based on 5 × 5 × 5 mm³ metamaterial unit cells. The resultant effective ε_r of the metamaterials conformed well to the simulation results by CST STUDIO SUITE^® software. By utilizing the drilled-hole PP/TiO₂ composite metamaterials, a potential scheme to implement a dielectric gradient index lens with discretized shells of varying ε_r is presented. This drilling-hole approach to modulate and tune the effective ε_r at the metamaterial unit cell level can be generalized to produce a variety of desirable permittivity values with great flexibility. The applications of these materials can be easily extended to other microwave or optical devices including cloaks, lenses, and beam shifters.