@article{10023, abstract = {{We perform a comprehensive theoretical study of the structural and electronic properties of potassium niobate (KNbO3) in the cubic, tetragonal, orthorhombic, monoclinic, and rhombohedral phase, based on density-functional theory. The influence of different parametrizations of the exchange-correlation functional on the investigated properties is analyzed in detail, and the results are compared to available experimental data. We argue that the PBEsol and AM05 generalized gradient approximations as well as the RTPSS meta-generalized gradient approximation yield consistently accurate structural data for both the external and internal degrees of freedom and are overall superior to the local-density approximation or other conventional generalized gradient approximations for the structural characterization of KNbO3. Band-structure calculations using a HSE-type hybrid functional further indicate significant near degeneracies of band-edge states in all phases which are expected to be relevant for the optical response of the material.}}, author = {{Schmidt, Falko and Landmann, Marc and Rauls, Eva and Argiolas, Nicola and Sanna, Simone and Schmidt, Wolf Gero and Schindlmayr, Arno}}, issn = {{1687-8442}}, journal = {{Advances in Materials Science and Engineering}}, publisher = {{Hindawi}}, title = {{{Consistent atomic geometries and electronic structure of five phases of potassium niobate from density-functional theory}}}, doi = {{10.1155/2017/3981317}}, volume = {{2017}}, year = {{2017}}, } @article{10021, abstract = {{The optical properties of pristine and titanium-doped LiNbO3 are modeled from first principles. The dielectric functions are calculated within time-dependent density-functional theory, and a model long-range contribution is employed for the exchange-correlation kernel in order to account for the electron-hole binding. Our study focuses on the influence of substitutional titanium atoms on lithium sites. We show that an increasing titanium concentration enhances the values of the refractive indices and the reflectivity.}}, author = {{Friedrich, Michael and Schmidt, Wolf Gero and Schindlmayr, Arno and Sanna, Simone}}, issn = {{2475-9953}}, journal = {{Physical Review Materials}}, number = {{3}}, publisher = {{American Physical Society}}, title = {{{Optical properties of titanium-doped lithium niobate from time-dependent density-functional theory}}}, doi = {{10.1103/PhysRevMaterials.1.034401}}, volume = {{1}}, year = {{2017}}, } @article{13416, abstract = {{The optical properties of congruent lithium niobate are analyzed from first principles. The dielectric function of the material is calculated within time-dependent density-functional theory. The effects of isolated intrinsic defects and defect pairs, including the NbLi4+ antisite and the NbLi4+−NbNb4+ pair, commonly addressed as a bound polaron and bipolaron, respectively, are discussed in detail. In addition, we present further possible realizations of polaronic and bipolaronic systems. The absorption feature around 1.64 eV, ascribed to small bound polarons [O. F. Schirmer et al., J. Phys.: Condens. Matter 21, 123201 (2009)], is nicely reproduced within these models. Among the investigated defects, we find that the presence of bipolarons at bound interstitial-vacancy pairs NbV−VLi can best explain the experimentally observed broad absorption band at 2.5 eV. Our results provide a microscopic model for the observed optical spectra and suggest that, besides NbLi antisites and Nb and Li vacancies, Nb interstitials are also formed in congruent lithium-niobate samples.}}, author = {{Friedrich, Michael and Schmidt, Wolf Gero and Schindlmayr, Arno and Sanna, Simone}}, issn = {{2475-9953}}, journal = {{Physical Review Materials}}, number = {{5}}, publisher = {{American Physical Society}}, title = {{{Polaron optical absorption in congruent lithium niobate from time-dependent density-functional theory}}}, doi = {{10.1103/PhysRevMaterials.1.054406}}, volume = {{1}}, year = {{2017}}, } @article{13421, author = {{Landmann, M. and Rauls, E. and Schmidt, Wolf Gero}}, issn = {{2469-9950}}, journal = {{Physical Review B}}, number = {{15}}, title = {{{Understanding band alignments in semiconductor heterostructures: Composition dependence and type-I–type-II transition of natural band offsets in nonpolar zinc-blendeAlxGa1−xN/AlyGa1−yNcomposites}}}, doi = {{10.1103/physrevb.95.155310}}, volume = {{95}}, year = {{2017}}, } @article{13414, author = {{Riefer, A. and Schmidt, Wolf Gero}}, issn = {{2469-9950}}, journal = {{Physical Review B}}, number = {{23}}, title = {{{Solving the Bethe-Salpeter equation for the second-harmonic generation in Zn chalcogenides}}}, doi = {{10.1103/physrevb.96.235206}}, volume = {{96}}, year = {{2017}}, } @article{7481, abstract = {{The electronic band structures of hexagonal ZnO and cubic ZnS, ZnSe, and ZnTe compounds are determined within hybrid-density-functional theory and quasiparticle calculations. It is found that the band-edge energies calculated on the G0W0 (Zn chalcogenides) or GW (ZnO) level of theory agree well with experiment, while fully self-consistent QSGW calculations are required for the correct description of the Zn 3d bands. The quasiparticle band structures are used to calculate the linear response and second-harmonic-generation (SHG) spectra of the Zn–VI compounds. Excitonic effects in the optical absorption are accounted for within the Bethe–Salpeter approach. The calculated spectra are discussed in the context of previous experimental data and present SHG measurements for ZnO.}}, author = {{Riefer, Arthur and Weber, Nils and Mund, Johannes and Yakovlev, Dmitri R. and Bayer, Manfred and Schindlmayr, Arno and Meier, Cedrik and Schmidt, Wolf Gero}}, issn = {{1361-648X}}, journal = {{Journal of Physics: Condensed Matter}}, number = {{21}}, publisher = {{IOP Publishing}}, title = {{{Zn–VI quasiparticle gaps and optical spectra from many-body calculations}}}, doi = {{10.1088/1361-648x/aa6b2a}}, volume = {{29}}, year = {{2017}}, } @article{7480, author = {{Poltavtsev, S. V. and Kosarev, A. N. and Akimov, I. A. and Yakovlev, D. R. and Sadofev, S. and Puls, J. and Hoffmann, S. P. and Albert, M. and Meier, Cedrik and Meier, Torsten and Bayer, M.}}, issn = {{2469-9950}}, journal = {{Physical Review B}}, number = {{3}}, publisher = {{American Physical Society (APS)}}, title = {{{Time-resolved photon echoes from donor-bound excitons in ZnO epitaxial layers}}}, doi = {{10.1103/physrevb.96.035203}}, volume = {{96}}, year = {{2017}}, } @article{7484, author = {{Hoffmann, Sandro Phil and Albert, Maximilian and Meier, Cedrik}}, issn = {{0749-6036}}, journal = {{Superlattices and Microstructures}}, pages = {{397--408}}, publisher = {{Elsevier BV}}, title = {{{Fabrication of fully undercut ZnO-based photonic crystal membranes with 3D optical confinement}}}, doi = {{10.1016/j.spmi.2016.07.006}}, volume = {{97}}, year = {{2016}}, } @article{6533, abstract = {{We propose and implement a new concept for thermochromic plasmonic elements. It is based on vanadium dioxide (VO2) nanocrystals located in the near field of surface plasmon polaritons supported by an otherwise unstructured gold thin film. When the VO2 undergoes the metal-insulator phase transition, the coupling conditions for conversion of light into propagating surface plasmon polaritons change markedly. In particular, we realize thermochromic plasmonic grating couplers with substantial switching contrast as well as tunable plasmonic couplers in a Kretschmann configuration. The use of VO2 nanocrystals permits highly repetitive switching and room temperature operation. Simulations based on the actual dielectric function of our VO2 nanocrystals agree well with the experiment.}}, author = {{Jostmeier, Thorben and Mangold, Moritz and Zimmer, Johannes and Karl, Helmut and Krenner, Hubert J. and Ruppert, Claudia and Betz, Markus}}, issn = {{1094-4087}}, journal = {{Optics Express}}, number = {{15}}, publisher = {{The Optical Society}}, title = {{{Thermochromic modulation of surface plasmon polaritons in vanadium dioxide nanocomposites}}}, doi = {{10.1364/oe.24.017321}}, volume = {{24}}, year = {{2016}}, } @article{4239, abstract = {{Confocal Raman spectroscopy is applied to identify ferroelectric domain structure sensitive phonon modes in potassium titanyl phosphate. Therefore, polarization-dependent measurements in various scattering configurations have been performed to characterize the fundamental Raman spectra of the material. The obtained spectra are discussed qualitatively based on an internal mode assignment. In the main part of this work, we have characterized z-cut periodically poled potassium titanyl phosphate in terms of polarity- and structure-sensitive phonon modes. Here, we find vibrations whose intensities are linked to the ferroelectric domain walls. We interpret this in terms of changes in the polarizability originating from strain induced by domain boundaries and the inner field distribution. Hence, a direct and 3D visualization of ferroelectric domain structures becomes possible in potassium titanyl phosphate.}}, author = {{Rüsing, Michael and Eigner, Christof and Mackwitz, P. and Berth, Gerhard and Silberhorn, Christine and Zrenner, Artur}}, issn = {{0021-8979}}, journal = {{Journal of Applied Physics}}, number = {{4}}, publisher = {{AIP Publishing}}, title = {{{Identification of ferroelectric domain structure sensitive phonon modes in potassium titanyl phosphate: A fundamental study}}}, doi = {{10.1063/1.4940964}}, volume = {{119}}, year = {{2016}}, } @article{4237, abstract = {{We report the fabrication of periodically poled domain patterns in x-cut lithium niobate thin-film. Here, thin films on insulator have drawn particular attention due to their intrinsic waveguiding properties offering high mode confinement and smaller devices compared to in-diffused waveguides in bulk material. In contrast to z-cut thin film lithium niobate, the x-cut geometry does not require back electrodes for poling. Further, the x-cut geometry grants direct access to the largest nonlinear and electro-optical tensor element, which overall promises smaller devices. The domain inversion was realized via electric field poling utilizing deposited aluminum top electrodes on a stack of LN thin film/SiO2 layer/Bulk LN, which were patterned by optical lithography. The periodic domain inversion was verified by non-invasive confocal second harmonic microscopy. Our results show domain patterns in accordance to the electrode mask layout. The second harmonic signatures can be interpreted in terms of spatially, overlapping domain filaments which start their growth on the þz side.}}, author = {{Mackwitz, P. and Rüsing, Michael and Berth, Gerhard and Widhalm, A. and Müller, K. and Zrenner, Artur}}, issn = {{0003-6951}}, journal = {{Applied Physics Letters}}, number = {{15}}, publisher = {{AIP Publishing}}, title = {{{Periodic domain inversion in x-cut single-crystal lithium niobate thin film}}}, doi = {{10.1063/1.4946010}}, volume = {{108}}, year = {{2016}}, } @article{4240, abstract = {{Cubic gallium nitride (GaN) films are analyzed with highresolution X-ray diffraction (HRXRD) and Raman spectroscopy. Several cubic GaN layers were grown on 3C-SiC (001) substrate by radio-frequency plasma-assisted molecular beam epitaxy. The layer thickness of the cubic GaN was varied between 75 and 505 nm. The HRXRD analysis reveals a reduction of the full-width at half-maximum (FWHM) of omega scans for growing layer thicknesses, which is caused by a partial compensation of defects. The Raman characterization confirms well-formed c-GaN layers. A more detailed examination of the longitudinal optical mode hints at a correlation of the FWHM of the Raman mode with the dislocation density, which shows the possibility to determine dislocation densities by Ramanspectroscopy on a micrometer scale, which is not possible by HRXRD. Furthermore, this Raman analysis shows that normalized Raman spectra present an alternative way to determine layer thicknesses of thin GaN films.}}, author = {{Rüsing, Michael and Wecker, T. and Berth, Gerhard and As, Donat Josef and Zrenner, Artur}}, issn = {{0370-1972}}, journal = {{physica status solidi (b)}}, keywords = {{cubic gallium nitride, dislocation density, HRXRD, Raman spectroscopy}}, number = {{4}}, pages = {{778--782}}, publisher = {{Wiley}}, title = {{{Joint Raman spectroscopy and HRXRD investigation of cubic gallium nitride layers grown on 3C-SiC}}}, doi = {{10.1002/pssb.201552592}}, volume = {{253}}, year = {{2016}}, } @article{10026, abstract = {{Congruent lithium niobate and lithium tantalate mixed crystals have been grown over the complete compositional range with the Czochralski method. The structural and vibrational properties of the mixed crystals are studied extensively by x-ray diffraction measurements, Raman spectroscopy, and density functional theory. The measured lattice parameters and vibrational frequencies are in good agreement with our theoretical predictions. The observed dependence of the Raman frequencies on the crystal composition is discussed on the basis of the calculated phonon displacement patterns. The phononic contribution to the static dielectric tensor is calculated by means of the generalized Lyddane-Sachs-Teller relation. Due to the pronounced dependence of the optical response on the Ta concentration, lithium niobate tantalate mixed crystals represent a perfect model system to study the properties of uniaxial mixed ferroelectric materials for application in integrated optics.}}, author = {{Rüsing, Michael and Sanna, Simone and Neufeld, Sergej and Berth, Gerhard and Schmidt, Wolf Gero and Zrenner, Artur and Yu, H. and Wang, Y. and Zhang, H.}}, issn = {{2469-9950}}, journal = {{Physical Review B}}, title = {{{Vibrational properties of LiNb1−xTaxO3 mixed crystals}}}, doi = {{10.1103/physrevb.93.184305}}, year = {{2016}}, } @article{10024, abstract = {{The influence of electronic many-body interactions, spin-orbit coupling, and thermal lattice vibrations on the electronic structure of lithium niobate is calculated from first principles. Self-energy calculations in the GW approximation show that the inclusion of self-consistency in the Green function G and the screened Coulomb potential W opens the band gap far stronger than found in previous G0W0 calculations but slightly overestimates its actual value due to the neglect of excitonic effects in W. A realistic frozen-lattice band gap of about 5.9 eV is obtained by combining hybrid density functional theory with the QSGW0 scheme. The renormalization of the band gap due to electron-phonon coupling, derived here using molecular dynamics as well as density functional perturbation theory, reduces this value by about 0.5 eV at room temperature. Spin-orbit coupling does not noticeably modify the fundamental gap but gives rise to a Rashba-like spin texture in the conduction band.}}, author = {{Riefer, Arthur and Friedrich, Michael and Sanna, Simone and Gerstmann, Uwe and Schindlmayr, Arno and Schmidt, Wolf Gero}}, issn = {{2469-9969}}, journal = {{Physical Review B}}, number = {{7}}, publisher = {{American Physical Society}}, title = {{{LiNbO3 electronic structure: Many-body interactions, spin-orbit coupling, and thermal effects}}}, doi = {{10.1103/PhysRevB.93.075205}}, volume = {{93}}, year = {{2016}}, } @article{10025, abstract = {{The phonon dispersions of the ferro‐ and paraelectric phase of LiTaO3 are calculated within density‐functional perturbation theory. The longitudinal optical phonon modes are theoretically derived and compared with available experimental data. Our results confirm the recent phonon assignment proposed by Margueron et al. [J. Appl. Phys. 111, 104105 (2012)] on the basis of spectroscopical studies. A comparison with the phonon band structure of the related material LiNbO3 shows minor differences that can be traced to the atomic‐mass difference between Ta and Nb. The presence of phonons with imaginary frequencies for the paraelectric phase suggests that it does not correspond to a minimum energy structure, and is compatible with an order‐disorder type phase transition.}}, author = {{Friedrich, Michael and Schindlmayr, Arno and Schmidt, Wolf Gero and Sanna, Simone}}, issn = {{1521-3951}}, journal = {{Physica Status Solidi B}}, number = {{4}}, pages = {{683--689}}, publisher = {{Wiley-VCH}}, title = {{{LiTaO3 phonon dispersion and ferroelectric transition calculated from first principles}}}, doi = {{10.1002/pssb.201552576}}, volume = {{253}}, year = {{2016}}, } @article{6522, abstract = {{An electric field applied to a semiconductor reduces its crystal symmetry and modifies its electronic structure which is expected to result in changes of the linear and nonlinear response to optical excitation. In GaAs, we observe experimentally strong electric field effects on the optical second (SHG) and third (THG) harmonic generation. The SHG signal for the laser-light k vector parallel to the [001] crystal axis is symmetry forbidden in the electric-dipole approximation, but can be induced by an applied electric field in the vicinity of the 1s exciton energy. Surprisingly, the THG signal, which is allowed in this geometry, is considerably reduced by the electric field. We develop a theory which provides good agreement with the experimental data. In particular, it shows that the optical nonlinearities for the 1s exciton resonance are modified in an electric field by the Stark effect, which mixes the 1s and 2p exciton states of opposite parity. This mixing acts in opposite way on the SHG and THG processes, as it leads to the appearance of forbidden SHG in (001)-oriented GaAs and decreases the crystallographic THG.}}, author = {{Brunne, D. and Lafrentz, M. and Pavlov, V. V. and Pisarev, R. V. and Rodina, A. V. and Yakovlev, D. R. and Bayer, M.}}, issn = {{1098-0121}}, journal = {{Physical Review B}}, number = {{8}}, publisher = {{American Physical Society (APS)}}, title = {{{Electric field effect on optical harmonic generation at the exciton resonances in GaAs}}}, doi = {{10.1103/physrevb.92.085202}}, volume = {{92}}, year = {{2015}}, } @inproceedings{6529, author = {{Yakovlev, D. R. and Warkentin, W. and Brunne, D. and Mund, J. and Pavlov, V. V. and Rodina, A. V. and Pisarev, R. V. and Bayer, M.}}, booktitle = {{Nonlinear Optics and Applications IX}}, editor = {{Bertolotti, Mario and Haus, Joseph W. and Zheltikov, Alexei M.}}, location = {{Prague, Czech Rep}}, publisher = {{SPIE}}, title = {{{Novel mechanisms of optical harmonic generation on excitons in semiconductors}}}, doi = {{10.1117/12.2185309}}, year = {{2015}}, } @article{10027, author = {{Landmann, M. and Rauls, E. and Schmidt, Wolf Gero and Neumann, M. D. and Speiser, E. and Esser, N.}}, issn = {{1098-0121}}, journal = {{Physical Review B}}, title = {{{GaNm-plane: Atomic structure, surface bands, and optical response}}}, doi = {{10.1103/physrevb.91.035302}}, year = {{2015}}, } @article{10029, author = {{Braun, Christian and Sanna, Simone and Schmidt, Wolf Gero}}, issn = {{1932-7447}}, journal = {{The Journal of Physical Chemistry C}}, pages = {{9342--9346}}, title = {{{Liquid Crystal (8CB) Molecular Adsorption on Lithium Niobate Z-Cut Surfaces}}}, doi = {{10.1021/acs.jpcc.5b00894}}, year = {{2015}}, } @article{10031, author = {{Li, Yanlu and Schmidt, Wolf Gero and Sanna, Simone}}, issn = {{1098-0121}}, journal = {{Physical Review B}}, title = {{{Defect complexes in congruentLiNbO3and their optical signatures}}}, doi = {{10.1103/physrevb.91.174106}}, year = {{2015}}, }