Intrinsic negative permittivity at far infrared region based on crystal lattice vibration

A new strategy to realize negative permittivity at infrared region without metamaterials was demonstrated based on intrinsic polar lattice vibration of crystals. The dependence of the dielectric properties on composition of Ca_1-xSr_xF₂ solid solution was measured. The region of the negative permittivity mainly relied on the effective parameters of the cation and the anion consisted of the material. It is promising in construction of isotropic electromagnetic cloak or negative index material by nature existed materials.