@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{3435, abstract = {{Semiconductor quantum dots are promising sources for polarization-entangled photons. As an alternative to the usual cascaded biexciton-exciton emission, direct two-photon emission from the biexciton can be used. With a high-quality optical resonator tuned to half the biexciton energy, a large proportion of the photons can be steered into the two-photon emission channel. In this case the degree of polarization entanglement is inherently insensitive to the exciton fine-structure splitting. In the present work we analyze the biexciton emission with particular emphasis on the influence of coupling of the quantum-dot cavity system to its environment. Especially for a high-quality cavity, the coupling to the surrounding semiconductormaterial can open up additional phonon-assisted decay channels. Our analysis demonstrates that with the cavity tuned to half the biexciton energy, the potentially detrimental influence of the phonons on the polarization entanglement is strongly suppressed—high degrees of entanglement can still be achieved. We further discuss spectral properties and statistics of the emitted twin photons.}}, author = {{Heinze, Dirk and Zrenner, Artur and Schumacher, Stefan}}, issn = {{1098-0121}}, journal = {{Physical Review B}}, number = {{24}}, title = {{{Polarization-entangled twin photons from two-photon quantum-dot emission}}}, doi = {{10.1103/PhysRevB.95.245306}}, 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{13909, author = {{Salewski, M. and Poltavtsev, S. V. and Yugova, I. A. and Karczewski, G. and Wiater, M. and Wojtowicz, T. and Yakovlev, D. R. and Akimov, I. A. and Meier, Torsten and Bayer, M.}}, issn = {{2160-3308}}, journal = {{Physical Review X}}, number = {{3}}, title = {{{High-Resolution Two-Dimensional Optical Spectroscopy of Electron Spins}}}, doi = {{10.1103/physrevx.7.031030}}, volume = {{7}}, 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{13289, author = {{Yulin, A. and Driben, R. and Meier, Torsten}}, issn = {{2469-9926}}, journal = {{Physical Review A}}, number = {{3}}, title = {{{Bloch oscillations and resonant radiation of light propagating in arrays of nonlinear fibers with high-order dispersion}}}, doi = {{10.1103/physreva.96.033827}}, volume = {{96}}, year = {{2017}}, } @inproceedings{3543, author = {{Hildebrandt, Andre and Alhaddad, Samer and Hammer, Manfred and Förstner, Jens}}, booktitle = {{Integrated Optics: Devices, Materials, and Technologies XX}}, editor = {{Broquin, Jean-Emmanuel and Nunzi Conti, Gualtiero}}, keywords = {{tet_topic_waveguide}}, publisher = {{SPIE}}, title = {{{Oblique incidence of semi-guided waves on step-like folds in planar dielectric slabs: Lossless vertical interconnects in 3D integrated photonic circuits}}}, doi = {{10.1117/12.2214460}}, year = {{2016}}, } @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{6539, abstract = {{Light is often characterized only by its classical properties, like intensity or coherence. When looking at its quantum properties, described by photon correlations, new information about the state of the matter generating the radiation can be revealed. In particular the difference between independent and entangled emitters, which is at the heart of quantum mechanics, can be made visible in the photon statistics of the emitted light. The well-studied phenomenon of superradiance occurs when quantum–mechanical correlations between the emitters are present. Notwithstanding, superradiance was previously demonstrated only in terms of classical light properties. Here, we provide the missing link between quantum correlations of the active material and photon correlations in the emitted radiation. We use the superradiance of quantum dots in a cavity-quantum electrodynamics laser to show a direct connection between superradiant pulse emission and distinctive changes in the photon correlation function. This directly demonstrates the importance of quantum–mechanical correlations and their transfer between carriers and photons in novel optoelectronic devices.}}, author = {{Jahnke, Frank and Gies, Christopher and Aßmann, Marc and Bayer, Manfred and Leymann, H. A. M. and Foerster, Alexander and Wiersig, Jan and Schneider, Christian and Kamp, Martin and Höfling, Sven}}, issn = {{2041-1723}}, journal = {{Nature Communications}}, number = {{1}}, publisher = {{Springer Nature America, Inc}}, title = {{{Giant photon bunching, superradiant pulse emission and excitation trapping in quantum-dot nanolasers}}}, doi = {{10.1038/ncomms11540}}, volume = {{7}}, 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{13920, abstract = {{We investigate the transient optical response in high-quality Cd0.88Zn0.12Te crystals in the regime of slow light propagation on the lower exciton-polariton branch. Femtosecond photoexcitation leads to very substantial transmission changes in a ∼10-meV broad spectral range within the transparency window of the unexcited semiconductor. These nonlinear optical signatures decay on picosecond time scales governed by carrier thermalization and recombination. The temporal and spectral dependence indicate the dynamical optical response as arising from excitation-induced dephasing and perturbed free induction decay. Model simulations for the optical response taking into account the actual exciton-polariton dispersion and excitation-induced dephasing of a nonlinearly driven two-level system support this interpretation.}}, author = {{Lohrenz, J. and Melzer, S. and Ruppert, C. and Akimov, I. A. and Mariette, H. and Reichelt, Matthias and Trautmann, Alexander and Meier, Torsten and Betz, M.}}, issn = {{2469-9950}}, journal = {{Physical Review B}}, number = {{7}}, title = {{{Ultrafast dynamical response of the lower exciton-polariton branch in CdZnTe}}}, doi = {{10.1103/physrevb.93.075201}}, volume = {{93}}, year = {{2016}}, } @article{1454, author = {{Grynko, Yevgen and Zentgraf, Thomas and Meier, Torsten and Förstner, Jens}}, issn = {{0946-2171}}, journal = {{Applied Physics B}}, keywords = {{tet_topic_meta, tet_topic_shg}}, number = {{9}}, pages = {{242}}, publisher = {{Springer Nature}}, title = {{{Simulations of high harmonic generation from plasmonic nanoparticles in the terahertz region}}}, doi = {{10.1007/s00340-016-6510-0}}, volume = {{122}}, year = {{2016}}, }