Krzysztof Dorywalski

Electronic and Optical Properties of Strontium Barium Niobate Single Crystals

Electronic structure of Sr x Ba1- x Nb2O6 crystals in the energy range involving the valence and conduction bands has been studied theoretically and experimentally using the DFT-based ab initio VASP code and the spectroscopic ellipsometry method with synchrotron radiation. The Sr0.61Ba0.39Nb2O6 sample of the congruently melting composition is characterized by the greatest difference of the experimental dielectric function ϵ(E) compared to the non congruently melting compositions of Sr x Ba1- x Nb2O6 (x = 0.40, 0.55, 0.65, 0.75). Electronic band structures of Sr0.60Ba0.40Nb2O6 and LiNbO3 crystals in the energy range of 6–8 eV and the corresponding dielectric functions ϵ(E) in the photon energy range of 3–10 eV have been found to be similar, what is caused by similar octahedral NbO6 complexes in the crystals.

Spectral Ellipsometry Study of SBN Single Crystals in Visible and Ultraviolet Region

The pseudo-dielectric function (E) = (E) + i(E) of strontium-barium niobate single crystals, Sr x Ba1-x Nb2O6 (SBN), was studied at room temperature for five nominal compounds indices x (x = 0.40, 0.55, 0.61, 0.65, 0.75) by spectroscopic ellipsometry using the synchrotron light ellipsometer in the photon energy range of 1.5– 10 eV. The spectra of (E) obtained appear to be similar to the analogous spectrum of LiNbO3 crystal, where the NbO6 octahedral groups are responsible for the features of (E). Differences in the dielectric function of SBN with various x have been found and analyzed. The nonlinear dependences of optical values on the compound index x have been found in SBN.

Electronic Properties of KDP and DKDP Crystals: Ab-Initio Calculations and Spectral Ellipsometry Experiment

Electronic band structure, density of states and complex dielectric function ϵ(E) of KDP (KH2PO4) crystal at Fdd2 and space groups of symmetry corresponding to the ferroelectric and paraelectric phases, have been calculated within the density functional theory using the VASP code. The experimental dielectric function ϵ(E) of KDP and DKDP (KD2PO4) crystals have been studied by the spectroscopic ellipsometry method in the photon energy range of 5–18 eV using synchrotron radiation. Temperature dependences of the dielectric function ϵ(T) and the intensity of reflected light IR (T) of KDP and DKDP crystals have been measured and discussed.

Ultraviolet vacuum ultraviolet optical functions for SrTiO3 and NdGaO3 crystals determined by spectroscopic ellipsometry

Complex dielectric functions e(E) = e1(E) + e2(E) were experimentally evaluated within the spectral range E = 2–25 eV and E = 2–20 eV for SrTiO3 and NdGaO3 single crystals, respectively, using synchrotron-based spectroscopic ellipsometry measurements. The ellipsometric spectra were evaluated within a framework of optical layer model taking into account sample surface roughness and anisotropy of NdGaO3. The parameters of Herzinger-Johs oscillator model were fitted to reproduce sufficiently all features of the optical spectra within the spectral range 2–10 eV. Only slight differences were revealed for spectra polarized along b and c crystallographic axes of the NdGaO3, which can confirm weak optical anisotropy.

Structural phase transitions in ferroelectric crystals and thin films studied by VUV spectroscopic ellipsometry with synchrotron radiation

Results of thermo-optical investigations for lead germanate (Pb5Ge3O11), potassium dihydrogen phosphate (KH2PO4) ferroelectric crystals, and lead titanate (PbTiO3) thin films expitaxially deposited on (001) SrTiO3 substrate are presented in this article. The measurements were performed using VUV spectroscopic ellipsometry with synchrotron radiation in the temperature range comprising structural phase transitions (PTs) in the materials under study. Distinct anomalies in the temperature dependencies of complex dielectric permittivity and the intensity of reflected synchrotron light are found in the vicinity of ferroelectric-to-paraelectric PT point and discussed. Different temperature points of the characteristic anomalies at the PT of the real and imaginary components of the pseudo-dielectric function are observed for Pb5Ge3O11 and KH2PO4. It is crucial that anomalies in the measured temperature dependences for PbTiO3 thin films on SrTiO3 substrates are observed at the temperature near 105 K, corresponding to the PT in the SrTiO3 single crystal.

Parametrized optical functions of strontium barium niobate crystals in the vacuum ultraviolet spectral range

Complex optical function e(E) = e1(E) + ie2(E) components parallel e∥(E) and perpendicular e⊥(E) to the optic axis are determined in the photon energy range E = 2–10 eV for SrxBa1–xNb2O6 single crystals with the composition parameters x = 0.40, 0.50, 0.61, 0.65, and 0.75. The spectra are obtained from the evaluation of the ellipsometric data using a relevant optical model which takes into account the optical anisotropy of the crystal and surface imperfection of the measured specimens. We report the parameters necessary to construct the optical function for each studied composition. Additionally, the energies of electronic inter-band transitions are obtained from the analysis in terms of standard analytical line shapes.