ABSTRACT
We present and analyze the unique phenomena of enhanced THz transmission through a subwavelength LiF dielectric rod lattice embedded in an epsilon-near-zero KCl host. Our experimental results in combination with theoretical calculations show that subwavelength waveguiding of terahertz radiation is achieved within an alkali-halide eutectic metamaterial as result of the coupling of Mie-resonance modes arising in the dielectric lattice.
ABSTRACT
In this paper we discuss the fabrication and the electromagnetic (EM) characterization of anisotropic eutectic metamaterials, consisting of cylindrical polaritonic LiF rods embedded in either KCl or NaCl polaritonic host. The fabrication was performed using the eutectics directional solidification self-organization approach. For the EM characterization the specular reflectance at far infrared, between 3 THz and 11 THz, was measured and also calculated by numerically solving Maxwell equations, obtaining good agreement between experimental and calculated spectra. Applying an effective medium approach to describe the response of our samples, we predicted a range of frequencies in which most of our systems behave as homogeneous anisotropic media with a hyperbolic dispersion relation, opening thus possibilities for using them in negative refractive index and imaging applications at THz range.
Subject(s)
Metals/chemistry , Models, Chemical , Models, Molecular , Computer Simulation , Light , Scattering, Radiation , Terahertz RadiationABSTRACT
Pb2MnW(1-x)Re(x)O6 samples have been synthesized and their structure determined by powder x-ray diffraction. These samples undergo a first order structural phase transition between 413 and 445 K depending on the composition. Above this temperature, the samples are cubic. Below the transition temperature, solid solutions are found for x ≤ 0.2 and x ≥ 0.5. The W-rich samples adopt an orthorhombic cell whereas the Re-rich compounds are monoclinic. In the intermediate region, 0.2 < x < 0.5, both phases coexist. X-ray absorption spectra did not reveal significant changes in the local structure for Pb, Mn or Re atoms across the structural phase transition. All the atoms exhibit distorted environments in the whole series. In the case of Pb and W(Re) atoms, the local distortion remains in the high temperature phase. Samples with x ≤ 0.2 also show a sharp discontinuity in the dielectric permittivity at the phase transition temperature indicating the presence of a concomitant electrical ordering in the bulk grains. Such an anomaly in the dielectric constant is not observed for the x ≥ 0.5 samples, compatible with the lack of dipole ordering for this composition range. The different electrical behaviours also explain the differences in the entropy content for the two types of transition.
ABSTRACT
The Raman spectrum of NiAl(2)O(4) inverse spinel has been studied in quenched polycrystalline pellets produced by solid-state reaction and in single crystals grown by the floating zone method. The lattice parameters and inversion degrees were determined by x-ray diffraction. Polarization measurements in single crystals allow mode symmetry assignment. Then, a correlation is established between the bands observed in polycrystalline samples and those of single crystals. Both kinds of sample present more bands than the five expected (A(1g)+E(g)+3T(2g)) in a cubic Fd3m spinel. This multiplicity is attributed to the almost fully inverted cation distribution in NiAl(2)O(4), with inversion parameter x≈0.9. The multiplicity of the high-frequency A(1g) band, in particular, is attributed to the different possible configurations of Ni(2+) and Al(3+) cations occupying the three octahedral sites close to a given oxygen ion. A strong downshift of the E(g) mode frequency, as compared to the normal spinel MgAl(2)O(4), is attributed to the longer bonding distance between oxygen and octahedral cations in inverse II-III spinels. Due to the small range of variation of x upon thermal treatment in NiAl(2)O(4), no significant differences were found between the spectra of samples quenched at different temperatures, from 800 to 1200 °C.