Resonant magnetic response of TiO2 microspheres at terahertz frequencies


Metamaterials are artificial composite structures formed by common materials in a way to exhibit new interesting electromagnetic properties. The geometrical arrangement of the composite on the microscale is of prime importance; as a rule, they rely on regularly arranged resonators with sizes and mutual distances much smaller than the targeted wavelength of the radiation. A proper choice of materials and of their arrangement can induce an unusual electromagnetic behavior. In a given (narrow) spectral range, it is possible to conceive e.g. an "invisibility cloak" or a medium with a negative refractive index allowing one to overcome the diffraction limit in the optical imaging. This requires simultaneously negative dielectric permittivity and magnetic permeability; while a broad-band negative permittivity is typical for metals, it is difficult to achieve negative permeability, since the permeability of common materials is always positive.

Although crystals of TiO2 (rutile) do not exhibit magnetic properties, it is possible to use rutile in a suitable geometrical configuration to create a magnetic response. We proposed and studied a metamaterial composed of TiO2 microspheres with a magnetic resonance near 1 THz. Its preparation is based on spray-drying of a suspension of TiOnanoparticles which then self-assemble into microspheres. We also developed a new method of measuring unambiguously such a magnetic response. We have shown that a metamaterial of this kind can exhibit a negative effective permeability in the terahertz spectral range.

Left: Scanning electron microscope images of the fabricated TiO2 microspheres before the sorting procedure. Sorting procedure allows one to obtain a narrower distribution of microsphere diameters d. Right: effective magnetic response (real and imaginary part of the effective permeability) of samples with a 10% volume filling fraction of TiO2 microspheres and their sizes d = 45 ± 4 µm and 39 ± 3 µm. Symbols: experiment, lines: results of electromagnetic simulations.