Zoran V. Popović

Spectroscopic ellipsometry and polarimetry for materials and systems analysis at the nanometer scale: state-of-the-art, potential, and perspectives

This paper discusses the fundamentals, applications, potential, limitations, and future perspectives of polarized light reflection techniques for the characterization of materials and related systems and devices at the nanoscale. These techniques include spectroscopic ellipsometry, polarimetry, and reflectance anisotropy. We give an overview of the various ellipsometry strategies for the measurement and analysis of nanometric films, metal nanoparticles and nanowires, semiconductor nanocrystals, and submicron periodic structures. We show that ellipsometry is capable of more than the determination of thickness and optical properties, and it can be exploited to gain information about process control, geometry factors, anisotropy, defects, and quantum confinement effects of nanostructures.

Radiation and scattering from imperfect cylindrical electromagnetic cloaks

The design of electromagnetic invisibility cloaks is based on singular mappings prescribing zero or infinite values for material parameters on the inner surface of the cloak. Since this is only approximately feasible, an asymptotic analysis is necessary for a sound description of cloaks. We adopt a simple and effective approach for analyzing electromagnetic cloaks - instead of the originally proposed singular mapping, nonsingular mappings asymptotically approaching the ideal one are considered. Scattering and radiation from this type of imperfect cylindrical cloaks is solved analytically and the results are confirmed by full-wave finite element simulations. Our analysis sheds more light on the influence of this kind of imperfection on the cloaking performance and further explores the physics of cloaking devices.

Ce1−xY (Nd)xO2−δ nanopowders: potential materials for intermediate temperature solid oxide fuel cells

Nanopowdered solid solution Ce 1-x Y(Nd) x O 2-δ samples (0.1 ≤ x ≤ 0.25) were made by self-propagating room temperature (SPRT) synthesis. The first-order Raman spectra of Ce 1-x Y(Nd) x O 2-δ samples measured at room temperature exhibit three broad features: the main Raman active F 2g mode at about 450 cm -1 and two broad features at about 550 (545) and 600 cm -1 . The mode at ∼600 cm -1 was assigned to the intrinsic oxygen vacancies due to the nonstoichiometry of ceria nanopowders. The mode at about 550 (545) cm -1 was attributed to the oxygen vacancies introduced into the ceria lattice whenever Ce 4+ ions are replaced with trivalent cations (Y 3+ , Nd 3+ ). The intensity of this mode increases with doping in both series of samples, indicating a change of O 2- vacancy concentration. The mode frequency shifts in opposite direction in Y- and Nd-doped samples with doping level, suggesting that different types of defect space can occur in Y- and Nd-doped ceria nanopowders.

Raman scattering study of the anharmonic effects in CeO2−y nanocrystals

We have studied the temperature dependence of the F2g Raman mode phonon frequency and broadening in CeO2−y nanocrystals. The phonon softening and phonon linewidth are calculated using a model which takes into account the three-and four-phonon anharmonic processes. A detailed comparison of the experimental data with theoretical calculations revealed the predominance of four-phonon anharmonic processes in the temperature dependence of the phonon energy and broadening of the nanocrystals. On the other hand, three-phonon processes dominate the temperature behavior of phonons in polycrystalline samples. The anti-Stokes/Stokes peak intensity ratio was also investigated and found to be smaller for nanosized CeO2 powders than in the bulk counterpart.

Raman and infrared studies of La1- ySryMn1- xMxO3 (M=Cr, Co, Cu, Zn, Sc or Ga) : Oxygen disorder and local vibrational modes

We present results of our study of polarized Raman scatteringnand infrared reflectivity of rhombohedral ceramicnLa1-ySryMn1-xMxO3 manganites in the temperature range betweenn77 and 320 K. In our samples, a part of the Mn atoms isnsubstituted by M = Cr, Co, Cu, Zn, Sc, or Ga with x in thenrange 0-0.1. The hole concentration was kept at the optimalnvalue of about 32% by tuning the Sr content y. We havenmonitored distortions of the oxygen sublattice by the presencenof broad bands in the Raman spectra, the increase of dcnresistivity extracted from the infrared reflectivity, and thenchange of the critical temperature of the ferromagneticntransition. Our results support the idea that these propertiesnare mainly determined by the radius of the substituent ion, itsnelectronic and magnetic structure playing only a minor role.nFurthermore, the Raman spectra exhibit an additional Ag-likenhigh frequency mode attributed to the local breathing vibrationnof oxygens surrounding the substituent ion.

Lattice vibrations in spin-Peierls compound NaV2O5

Abstract We present the room temperature far-infrared reflectivity and Raman scattering spectra of NaV2O5 single crystals. The frequencies of infrared active modes are obtained by an oscillator fitting procedure of reflectivity data. The assignment of the vibrational modes is given according to a lattice dynamical calculation based on the valence shell model. Besides phonon modes we observed broad features centered at about 280, 550 and 3500xa0cm−1 in infrared spectra for E∥a polarization and at about 640xa0cm−1 in Raman spectra for (aa) polarization. These structures are explained as crystal-field-splitting-induced d–d electronic transitions of V4+ ions.

Raman scattering from magnetic excitations in the spin-ladder compoundsCaV2O5andMgV2O5

We present the Raman-scattering spectra of CaV2O5 and MgV2O5. The magnetic contribution in the Raman spectra of CaV2O5 is found in the form of a strong asymmetric line, centered at 2Delta, with a tail on the high-energy side. Our analysis of its spectral shape shows that the magnetic ordering in CaV2O5 can be described using a S=1/2 two-leg ladder Heisenberg antiferromagnetic model with Jparallel/Jperp = 0.1, and a small interladder exchange. The spin gap and exchange constant are estimated to be Delta = 400 cm-1 (570 K) and Jperp = 640 K. No magnon bound states are found. In contrast to CaV2O5 the existence of the spin-gap is not confirmed in MgV2O5, since we found no feature in the spectra which could be associated with the onset of the two-magnon continuum. Instead, we observe two-magnon excitation at 340 cm-1, presumably related to the top of the two-magnon brunch.

Raman scattering spectra of indium-doped PbTe

The non-polarized Raman scattering spectra of indium-doped PbTe single crystals were measured in the temperature range between 10 and 300 K. Well resolved peaks at about 68, 126 and 143 cm-1 were observed for all temperatures. An additional mode appears at about 115 cm-1 for temperatures below 100 K. The intensity of this mode increases sharply when the temperature is lowered below 25 K, the temperature where a persistent photoconductivity effect in PbTe(In) appears. This mode is assigned as a local In impurity mode and represents a population of metastable states due to the transfer of electrons from two-electron to one-electron metastable impurity states.

Far infrared spectra of Hg1−xMnxSe

Abstract The far-infrared reflectivity spectra of Hg 1− x Mn x Se ( x = 2%, 6%, 8%, 9%, 14% and 16%) were measured in the 50 to 650 cm -1 range at room temperature. These spectra were fitted using the plasmon- phonon interaction model. Plasmon and phonon characteristic parameters were determined. It is shown that Hg 1− x Mn x Se phonons exhibit two-mode behaviour.

Evidence for Electron-phonon Interaction in Fe1−xMxSb2 (M=Co and Cr; 0 x 0.5) Single Crystals

We have measured polarized Raman scattering spectra of the Fe{sub 1-x}Co{sub x}Sb{sub 2} and Fe{sub 1-x}Cr{sub x}Sb{sub 2} (0 {le} x {le} 0.5) single crystals in the temperature range between 15 and 300 K. The highest energy B{sub 1g} symmetry mode shows significant line asymmetry due to phonon-mode coupling-width electronic background. The coupling constant achieves the highest value at about 40 K and after that it remains temperature independent. Origin of additional mode broadening is pure anharmonic. Below 40 K the coupling is drastically reduced, in agreement with transport properties measurements. Alloying of FeSb{sub 2} with Co and Cr produces the B{sub 1g} mode narrowing, i.e., weakening of the electron-phonon interaction. In the case of A{sub g} symmetry modes we have found a significant mode mixing.