arXiv subjects

Marius Grundmann

Publications and source records attributed to Marius Grundmann.

35 records · Page 2Linked to original sources

arXiv2016

Exceptional points in anisotropic planar microcavities

Planar microcavities allow the control and manipulation of spin-polarization, manifested in phenomena like the optical spin Hall effect due to the intrinsic polarization mode splitting. Here, we study a transparent microcavity with broken rotational symmetry, realized by aligning the optical axis of a uniaxial cavity material in the cavity plane. We demonstrate that the in-plane optical anisotropy gives rise to exceptional points in the dispersion relation, which occur pair-wise, are circularly polarized, and are cores of polarization vortices. These exceptional points are a result of the non-Hermitian character of the system, and are in close relationship to singular optical axes in absorptive biaxial systems.

physics.optics↗

arXiv2016

Fundamental absorption edges in heteroepitaxial YBiO$_3$ thin films

The dielectric function of heteroepitaxial YBiO$_3$ grown on $a$-Al$_2$O$_3$ single crystals via pulsed laser deposition is determined in the spectral range from 0.03 eV to 4.5 eV by simultaneous modeling of spectroscopic ellipsometry and optical transmission data of YBiO$_3$ films of different thickness. The (111)-oriented YBiO$_3$ films are nominally unstrained and crystallize in a defective fluorite-type structure with $Fm\bar{3}m$ space group. From the calculated absorption spectrum, a direct electronic bandgap energy of 3.6(1) eV and the signature of an indirect electronic transition around 0.5 eV are obtained. These values provide necessary experimental feedback to previous conflicting electronic band structure calculations predicting either a topologically trivial or non-trivial insulating ground state in YBiO$_3$.

cond-mat.mtrl-sci↗

arXiv2016

Raman tensor elements of $β\text{-Ga}_2\text{O}_3$

The Raman spectrum and particularly the Raman scattering intensities of monoclinic $β\text{-Ga}_2\text{O}_3$ are investigated by experiment and theory. The low symmetry of $β\text{-Ga}_2\text{O}_3$ results in a complex dependence of the Raman intensity for the individual phonon modes on the scattering geometry which is additionally affected by birefringence. We measured the Raman spectra in dependence on the polarization direction for backscattering on three crystallographic planes of $β\text{-Ga}_2\text{O}_3$ and modeled these dependencies using a modified Raman tensor formalism which takes birefringence into account. The spectral position of all 15 Raman-active phonon modes and the Raman tensor elements of 13 modes were determined and are compared to results from ab-initio calculations.

cond-mat.mtrl-sci↗

arXiv2016

Coexistence of strong and weak coupling in ZnO nanowire cavities

We present a high quality two-dimensional cavity structure based on ZnO nanowires coated with concentrical Bragg reflectors. The spatial mode distribution leads to the simultaneous appearance of the weak and strong coupling regime even at room temperature. Photoluminescence measurements agree with FDTD simulations. Furthermore the ZnO core nanowires allow for the observation of middle polariton branches between the A- and B-exciton ground state resonances. Further, lasing emission up to room temperature is detected in excitation dependent photoluminescence measurements.

cond-mat.mes-hall↗

arXiv2016

Dipole Analysis of the Dielectric Function of Colour Dispersive Materials: Application to Monoclinic Ga$_2$O$_3$

We apply a generalized model for the determination and analysis of the dielectric function of optically anisotropic materials with colour dispersion to phonon modes and show that it can also be generalized to excitonic polarizabilities and electronic band-band transitions. We take into account that the tensor components of the dielectric function within the cartesian coordinate system are not independent from each other but are rather projections of the polarization of dipoles oscillating along directions defined by the, non-cartesian, crystal symmetry and polarizability. The dielectric function is then composed of a series of oscillators pointing in different directions. The application of this model is exemplarily demonstrated for monoclinic ($β$-phase) Ga$_2$O$_3$ bulk single crystals. Using this model, we are able to relate electronic transitions observed in the dielectric function to atomic bond directions and orbitals in the real space crystal structure. For thin films revealing rotational domains we show that the optical biaxiality is reduced to uniaxial optical response.

cond-mat.mtrl-sci↗

arXiv2016

Carrier density driven lasing dynamics in ZnO nanowires

We report on the temporal lasing dynamics of high quality ZnO nanowires using time-resolved micro-photoluminescence technique. The temperature dependence of the lasing characteristics and of the corresponding decay constants demonstrate the formation of an electron-hole plasma to be the underlying gain mechanism in the considered temperature range from 10 K to 300 K. We found that the temperature dependent emission onset-time ($t_{\text{on}}$) strongly depends on the excitation power and becomes smallest in the lasing regime, with values below 5 ps. Furthermore, the observed red shift of the dominating lasing modes in time is qualitatively discussed in terms of the carrier density induced change of the refractive index dispersion after the excitation laser pulse. This theory is supported by extending an existing model for the calculation of the carrier density dependent complex refractive index for different temperatures. This model coincides with the experimental observations and reliably describes the evolution of the refractive index after the excitation laser pulse.

physics.optics↗

arXiv2016

The Singular Optical Axes in Biaxial Crystals and Analysis of Their Spectral Dispersion Effects in $β$-Ga$_2$O$_3$

We classify and distinguish optically biaxial materials, which can have triclinic, monoclinic or orthorhombic crystal symmetry, by the degeneracy of the indices of refraction of their four singular optical axes (Windungsachsen) in the absorption regime. We provide explicit analytical solutions for angular orientations of the singular optical axes in monoclinic crystals and orthorhombic crystals. As a model material we analyze monoclinic gallia ($β$-Ga$_2$O$_3$) and discuss in detail the dispersion (i.e. the spectral variation of the angular position) of its singular optical axes. For a certain energy range ($E \approx 7.23$-$7.33$ eV) we find quasi-uniaxial symmetry. At two energies ($E \approx 8.14$ eV and $E \approx 8.37$ eV) we find triaxial spectral points for which one regular optical axis and two singular optical axes exist. Concurrently a Stokes analysis of the spectral dependence of the electrical field eigenvectors is made and discussed for various crystal orientations. For a singular optical axis $|S_3|=1$; for the two degenerate singular axes at the triaxial point the Stokes vector is undefined. For a certain energy ($E=6.59$ eV), the $\langle 010 \rangle$-orientation is close to a singular optical axis, $|S_3|=0.977$. The analysis provided here is prototypical for the treatment of the optical properties of optically biaxial functional materials in the absorption and gain regimes.

cond-mat.mtrl-sci↗

arXiv2015

Cavity Polariton Condensate in a Disordered Environment

We report on the influence of disorder on an exciton-polariton condensate in a ZnO based bulk planar microcavity and compare experimental results with a theoretical model for a non-equilibrium condensate. Experimentally, we detect intensity fluctuations within the far-field emission pattern even at high condensate densities which indicates a significant impact of disorder. We show that these effects rely on the driven dissipative nature of the condensate and argue that they can be accounted for by spatial phase inhomogeneities induced by disorder, which occur even for increasing condensate densities realized in the regime of high excitation power. Thus, non-equilibrium effects strongly suppress the stabilization of the condensate against disorder, contrarily to what is expected for equilibrium condensates in the high density limit. Numerical simulations based on our theoretical model reproduce the experimental data.

cond-mat.mes-hall↗

arXiv2015

Comparative Study of Optical and Magneto-Optical Properties of Normal, Disordered and Inverse Spinel Type Oxides

Co$_3$O$_4$, ZnFe$_2$O$_4$, CoFe$_2$O$_4$, ZnCo$_2$O$_4$, and Fe$_3$O$_4$ thin films were fabricated by pulsed laser deposition at high and low temperatures resulting in crystalline single-phase normal, inverse, as well as disordered spinel oxide thin films with smooth surface morphology. The dielectric function, determined by spectroscopic ellipsometry in a wide spectral range from 0.5 eV to 8.5 eV, is compared with the magneto-optical response of the dielectric tensor, investigated by magneto-optical Kerr effect (MOKE) spectroscopy in the spectral range from 1.7 eV to 5.5 eV with an applied magnetic field of 1.7 T. Crystal field, inter-valence and inter-sublattice charge transfer transitions, and transitions from O$_{2p}$ to metal cation 3d or 4s bands are identified in both the principal diagonal elements and the magneto-optically active off-diagonal elements of the dielectric tensor. Depending on the degree of cation disorder, resulting in local symmetry distortion, the magneto-optical response is found to be strongest for high crystal quality inverse spinels and for disordered normal spinel structure, contrary to the first principle studies of CoFe$_2$O$_4$ and ZnFe$_2$O$_4$. The results presented provide a basis for deeper understanding of light-matter interaction in this material system that is of vital importance for device-related phenomena and engineering.

cond-mat.mtrl-sci↗

arXiv2015

Dielectric tensor of monoclinic Ga$_2$O$_3$ single crystals in the spectral range $0.5 - 8.5\,$eV

The dielectric tensor of $β$-Ga$_2$O$_3$ was determined by generalized spectroscopic ellipsometry in a wide spectral range from $0.5\,\mathrm{eV}$ to $8.5\,\mathrm{eV}$ as well as by calculation including quasiparticle bands and excitonic effects. The dielectric tensors obtained by both methods are in excellent agreement with each other and the observed transitions in the dielectric function are assigned to the corresponding valence bands. It is shown that the off-diagonal element of the dielectric tensor reaches values up to $|\varepsilon_{xz} | \approx 0.30 $ and cannot be neglected. Even in the transparent spectral range where it is quite small ($|\varepsilon_{xz} | < 0.02 $) it causes a rotation of the dielectric axes around the symmetry axis of up to $20^\circ$.

cond-mat.mtrl-sci↗

arXiv2015

Antiferromagnetic phase transition in the temperature-dependent NIR-VUV dielectric function of hexagonal YMnO$_3$

Hexagonal YMnO$_3$ is well known for the co-occurrence of ferroelectricity and antiferromagnetism at low temperatures. Using temperature-dependent spectroscopic ellipsometry at an $a$-plane oriented single crystal, we show how the dielectric function is affected by the magnetic order transition at the Néel temperature. We focus especially on the pronounced charge transfer transitions around (1.6-1.7)eV which are strongly connected to Mn 3$d$ electrons. If described with a Bose-Einstein model, the temperature dependency of their energy and broadening is characterized by effective phonon energies not larger than 8meV. We argue that this is a hint for the occurrence of a soft phonon mode related to the antiferromagnetic phase transition. This is observed in both tensor components of the dielectric function, parallel and perpendicular to the crystallographic $c$-axis. Furthermore, a suitable parametrization for the uniaxial dielectric function is presented for the NIR-VUV spectral range. The broad transitions at energies higher than a critical point-like bandgap do not show a clear temperature dependence. We also observe some weak discrete absorption features around the strong charge transfer transitions with energies matching well to low-temperature photoluminescence signals.

cond-mat.mtrl-sci↗

arXiv2015

Electronic excitations and structure of Li2IrO3 thin films grown on ZrO$_{2}$:Y (001) substrates

Thin films are a prerequisite for application of the emergent exotic ground states in iridates that result from the interplay of strong spin-orbit coupling and electronic correlations. We report on pulsed laser deposition of Li$_{2}$IrO$_{3}$ films on ZrO$_{2}$:Y(001) single crystalline substrates. X-ray diffraction confirms preferential (001) and (10-1) out-of-plane crystalline orientations with well defined in-plane orientation. Resistivity between 35 and 300 K is dominated by a three-dimensional variable range hopping mechanism. The dielectric function is determined by means of spectroscopic ellipsometry and, complemented by Fourier transform infrared transmission spectroscopy, reveals a small optical gap of $\approx$ 300 meV, a splitting of the $5d$-$t_{2g}$ manifold, and several in-gap excitations attributed to phonons and possibly magnons.

cond-mat.str-el↗

arXiv2015

Room-temperature condensation in whispering gallery microresonators assisted by longitudinal optical phonons

We report condensation of hexagonal whispering gallery modes (WGM) at room temperature in ZnO microwires that embody nearly perfect polygonal whispering gallery microresonators. The condensate regime is achieved in the UV spectral range only at energies below the first longitudinal optical (LO) phonon replica of the free ZnO A-exciton transition and at non-zero wave vectors. We demonstrate that the multimodality of the WGM system and the high population of free excitons and phonons with various momenta strongly enhance the probability of an interaction of quasiparticles of the cavity exciton-photon system with LO phonons. We further examine the far-field mode pattern of lasing WGM and demonstrate their spatial coherence.

cond-mat.mes-hall↗

arXiv2013

Mott Variable Range Hopping and Weak Antilocalization Effect in Heteroepitaxial Na2IrO3 Thin Films

Iridate thin films are a prerequisite for any application utilizing their cooperative effects resulting from the interplay of stron spin-orbit coupling and electronic correlations. Here, heteroepitaxial Na2IrO3 thin films with excellent (001) out-of-plane crystalline orientation and well defined in-plane epitaxial relationship are presented on various oxide substrates. Resistivity is dominated by a three-dimensional variable range hopping mechanism in a large temperature range between 300 K and 40 K. Optical experiments show the onset of a small optical gap of about 200 meV and a splitting of the Ir 5d-t2g manifold. Positive magnetoresistance below 3 T and 25 K shows signatures of a weak antilocalization effect. This effect can be associated with surface states in a topological insulator and hence supports proposals for a topological insulator phase present in Na2IrO3.

cond-mat.str-el↗

arXiv2011

Microscopic Mechanism of Specific Peptide Adhesion to Semiconductor Substrates

The design of hybrid peptide-solid interfaces for nanotechnological applications such as biomolecular nanoarrays requires a deep understanding of the basic mechanisms of peptide binding and assembly at solid substrates. Here we show by means of experimental and computational analyses that the adsorption properties of mutated synthetic peptides at semiconductors exhibit a clear sequence-dependent adhesion specificity. Our simulations of a novel hybrid peptide-substrate model reveal the correspondence between proline mutation and binding affinity to a clean silicon substrate. After synthesizing theoretically suggested amino-acid sequences with different binding behavior, we confirm the relevance of the selective mutations upon adhesion in our subsequent atomic force microscopy experiments.

cond-mat.mes-hall↗

arXiv2008

Room temperature ferromagnetism in carbon-implanted ZnO

Unexpected ferromagnetism has been observed in carbon doped ZnO films grown by pulsed laser deposition [Phys. Rev. Lett. 99, 127201 (2007)]. In this letter, we introduce carbon into ZnO films by ion implantation. Room temperature ferromagnetism has been observed. Our analysis demonstrates that (1) C-doped ferromagnetic ZnO can be achieved by an alternative method, i.e. ion implantation, and (2) the chemical involvement of carbon in the ferromagnetism is indirectly proven.

cond-mat.mtrl-sci↗

arXiv2007

Specific Adhesion of Peptides on Semiconductor Surfaces in Experiment and Simulation

We report on self-assembly, clustering, and conformational phases of peptides on inorganic semiconductor surfaces. The peptide-covered surface fraction can differ by a factor of 25, depending mainly on surface and peptide polarity. Low adhesion induces large and soft clusters, which also have high contact angles to the surface. Direct surface adhesion of a peptide molecule competes with forming molecular aggregates which offer an overall reduced surface contact. Simulating a simple hybrid model yields a pseudophase diagram with a rich, temperature and solvent-quality dependent variety of subphases which are specific to the hydrophobicity and polarity of the considered substrates.

cond-mat.soft↗