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Dielectric spheres with high permittivity represent a Mie resonance-based metamaterial. Owing
to its high far-infrared permittivity and low dielectric losses, TiO2 is a suitable material for the
realization of magnetic metamaterials based on micro-resonators for the terahertz (THz) range.
In a previous work, we experimentally demonstrated the magnetic effective response of TiO 2
microspheres dispersed in air, forming nearly a single-layer sample enclosed between two sapphire
wafers [1]. Here we embedded the polycrystalline TiO2 microparticles into a polyethylene matrix,
which enabled us to prepare a rigid bulk metamaterial with a controllable concentration of micro-
resonators.
TiO2 microspheres with a diameter of a few tens of micrometers were prepared by a bottom up
approach. A liquid suspension of TiO2 nanoparticles was first spray-dried producing fragile TiO2
microspheres. These were subsequently sintered in a furnace at 1200° C for two hours, in order to
consolidate individually each sphere. The particles show polycrystalline rutile structure with a porosity
of 15%. The microspheres were finally sieved and sorted along their diameters in order to obtain a
narrow size distribution. They were mixed with polyethylene powder and a pressure of 14 MPa was
used to prepare rigid pellets with random spatial distribution of the TiO2 microspheres.
Using finite-difference time-domain simulations, we investigated how the filling fraction and
the ratio between the permittivities of the microspheres and the host matrix affect the position and the
strength of the magnetic response associated with the lowest Mie mode. We found that a range of
negative effective magnetic permeability can be achieved for sufficiently high filling factors and
contrasts between the permittivities of the resonators and the embedding medium.
Using time-domain THz spectroscopy we experimentally characterized the response of the
realized structures and confirmed the magnetic character of their response. The retrieved spectra of the
effective dielectric permittivity and magnetic permeability were analyzed within Mie theory and
Maxwell-Garnett effective medium model in a quasi-stationary regime. We found out that the TiO2
microparticles embedded in polyethylene to fabricate the rigid metamaterials were probably elliptical
[2].
To provide a better understanding of the electromagnetic behavior we will also show a near-
field THz response of both isotropic polycrystalline and anisotropic monocrystalline TiO2 microsphere
[3,4]. In the anisotropic case, the microparticles were sintered at 1400° C. The annealing process
melted polycrystalline particle clusters into single crystal TiO2 spheres. It resulted in a strong dielectric
anisotropy of the spheres since the ordinary and extraordinary permittivities of bulk rutile in the THz
range are 80 and 150, respectively. A splitting of the first Mie mode into two orthogonal magnetic
dipole modes was then detected.
The discussed examples show a high potential of TiO2 micro-resonators to realize magnetic THz
metamaterials, from cheap mechanically stable structures up to anisotropic resonators.
References
[1] H. Němec et al., App. Phys. Lett. 100, 061117 (2012)
[2] M. Šindler et al., Opt. Express 24, 18304 (2016)
[3] O. Mitrofanov et al., Opt. Express 22, 23034 (2014),
[4] I. Khromova et al., Laser Photon. Rev. 10, 681 (2016)
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