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
29Si NMR data have been recorded for the apatite series La8+xSr2-x(SiO4)6O2+x/2 (0 < or = x < or = 1.0). For x = 0, a single NMR peak is observed at a chemical shift of approximately -77 ppm, while as the La : Sr ratio and hence interstitial oxygen content is increased, a second peak at a chemical shift of approximately -80 ppm is observed, which has been attributed to silicate groups neighbouring interstitial oxide ions. An increase in the intensity of this second peak is observed with increasing x, consistent with an increase in interstitial oxide ion content, and the data are used to estimate the level of interstitial oxide ions, and hence Frenkel-type disorder in these materials. The increase in second 29Si NMR peak intensity/interstitial oxide ion content is also shown to correlate with an increase in conductivity. The effect of interstitial oxygen content can also be studied by means of Raman spectroscopy, with a new mode at 360 cm(-1) appearing for samples with x > 0 in the symmetric bending mode energy region of the SiO4 group. The intensity of this mode increases with increasing oxygen content, yielding results comparable to those from the NMR studies, showing the complementarities of the two techniques.
ABSTRACT
Chromium-doped [Formula: see text] and [Formula: see text] have been studied by both EPR and ENDOR spectroscopy. [Formula: see text] ions enter the fluorite structure in distorted substitution cation sites. In both matrices the distortion observed is tetragonal. X- and Q-band EPR measurements at temperatures between 4 and 300 K allowed us to determine the ion symmetry and the following spin-Hamiltonian parameters: [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] for [Formula: see text]; and [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] for [Formula: see text]. For [Formula: see text], the weak superhyperfine interaction of [Formula: see text] with the surrounding [Formula: see text] ions has been studied by both EPR and ENDOR techniques for [Formula: see text]. No ENDOR signals were detected for [Formula: see text]. The results are tentatively explained in terms of a Jahn - Teller effect corresponding to [Formula: see text] coupling strongly stabilized by lattice stresses, although other possible origins for the distortion cannot be completely ruled out.
ABSTRACT
Overexpression of foreign proteins in Escherichia coli is usually tested by sodium dodecyl sulfate-polyacrylamide gel electrophoresis or Western blot analysis. However, when metalloproteins of flavoenzymes are expressed, the correct assembly of the polypeptide chain to the cofactor cannot be verified using these methods. We have used EPR spectroscopy and Mancini tests to monitor the expression of holoflavodoxin from the cyanobacteria Anabaena in E. coli. Flavodoxin from Anabaena sp PCC 7119 was cloned in the plasmid pTrc 99b after the trc promotor, which is activated in the presence of isopropyl-beta-D-thiogalactoside. Two hours after induction, most of the recombinant apoflavodoxin is already synthesized. However, only 65% of this protein is assembled to flavin mononucleotide (FMN). Harvesting of the cells to obtain all the flavodoxin in the holo form must be carried out 10 h after induction. Addition of FMN to the culture media increases the synthesis of holoflavodoxin by about 17%. The presence of flavin adenine dinucleotide or riboflavin in the culture inhibits the accumulation of semiquinone flavodoxin in the cells.
