Rüdiger Schmidt-Grund

Whispering gallery mode lasing in zinc oxide microwires

Lasing on whispering gallery modes was excited by optical pumping in single zinc oxide microwires fabricated by a simple carbothermal evaporation process. The experimentally observed laser modes agree precisely with the predicted energetic positions obtained from a plane wave model. Systematic diameter dependent measurements have been carried out for diameters of the microwires between 3 and 12μm. The investigated microlasers are found to have a lasing threshold of about 170kW∕cm2 at 10K.

A Practical, Self‐Catalytic, Atomic Layer Deposition of Silicon Dioxide

The outstanding chemical, electrical, and optical properties of silicon dioxide have made it ubiquitous in science and technology. The ability to create SiO2 nanostructures of well-defined geometry would broaden its range of applications even further, in particular in the chemical, electrokinetic, and biomedical realms. Atomic layer deposition (ALD) is especially suited to nanostructuring, since its kinetics are controlled by surface chemistry rather than mass transport from the gas phase. However, reports on the ALD of silica are few and far between in the open literature to date. All published reactions suffer from some weakness: a corrosive by-product or catalyst, poor reproducibility, or impurities in the deposited film. Herein, we describe a practical ALD process for SiO2 that overcomes such limitations. Based on the NH3-catalyzed hydrolysis of tetraethoxysilane (Si(OEt)4), chemical intuition dictates that a triethoxysilane bearing an aminoalkyl moiety be readily hydrolyzed without any extraneous catalyst. The basic functionality will labilize the strong Si!O bonds, a phenomenon that can be called “self-catalysis” to emphasize the fact that one chemical species is both substrate and catalyst. Subsequent oxidative cleavage of the tethered moiety should afford a silanol, amenable to further reaction with aminoalkyltriethoxysilane molecules. Accordingly, a three-step reaction sequence based on 3-aminopropyltriethoxysilane, water, and ozone (O3) can be envisioned for the ALD of SiO2 (Scheme 1).

Strain distribution in bent ZnO microwires

ZnO microwires grown by carbothermal reduction were mechanically bent and the uniaxial stress state was studied with spatially resolved low-temperature photoluminescence. Inhomogeneous (tensile and compressive) stress (up to ±1 GPa) is visualized by the redshift and blueshift of the wire luminescence. Experimentally determined tensile and compressive strain along the c-axis (wire axis) of up to 1.5 %, symmetrically distributed with respect to the central axis (neutral fiber), is achieved, resulting in maximum energetic shifts of ±30 meV. Within these experiments, we are able to precisely determine the direct relation between energetic shift of the free A-exciton energy and strain to (−2.04±0.02) eV.

Magnetoresistance and anomalous Hall effect in magnetic ZnO films

Magnetotransport measurements were performed on n-type conducting Co-doped ZnO and Mn-doped ZnO films prepared by pulsed laser deposition on a-plane sapphire substrates, and positive magnetoresistance (MR) was observed at low temperature. The positive MR decreases drastically with the free electron concentration n exceeding 1019cm−3 and reveals almost the same dependency on n for Co-doped ZnO and Mn-doped ZnO. This hints towards a similar s-d exchange constant in both types of magnetic ZnO films. For Co-doped ZnO, the saturated anomalous Hall resistivity increases with decreasing electron concentration. No anomalous Hall effect was observed in Mn-doped ZnO. Within a free electron approximation the positive MR may be related with the spin polarization of conducting electrons due to s-d exchange interactions. The modeled spin splitting of the conduction band is smaller than 10meV.

Observation of strong exciton–photon coupling at temperatures up to 410 K

We report on the observation of strong exciton–photon coupling in a ZnO-based microresonator consisting of a half medium wavelength ZnO cavity embedded between two dielectric Bragg reflectors made of 10.5 layer pairs of yttria stabilized zirconia and Al2O3. The microresonator was investigated by photoluminescence and reflectivity measurements in a wide temperature range between 10 and 550 K. With both techniques a lower polariton branch (LPB) was observable. As expected no signal from an upper polariton branch could be detected caused by the strong absorption of ZnO in this spectral range. The dispersion behaviour of the LPB (in both energy and broadening) is well described by a model that takes into account the coupling between one exciton mode and one cavity-photon mode. From this analysis we can conclude that the microresonator is in the strong coupling regime up to 410 K. Maximum values of the coupling strength at 10 K of 51 meV, respectively 55 meV, could be derived from the photoluminescence and from the reflectivity. These results demonstrate the high potential of ZnO microresonators for the realization of a Bose–Einstein condensation at room temperature and above.

Lattice parameters and Raman-active phonon modes of β-(AlxGa1−x)2O3

We present X-ray diffraction and Raman spectroscopy investigations of a (100)-oriented (AlxGa1–x)2O3 thin film on MgO (100) and bulk-like ceramics in dependence on their composition. The thin film grown by pulsed laser deposition has a continuous lateral composition spread allowing to determine precisely the dependence of the phonon mode properties and lattice parameters on the chemical composition. For x < 0.4, we observe the single-phase β-modification. Its lattice parameters and phonon energies depend linearly on the composition. We determined the slopes of these dependencies for the individual lattice parameters and for nine Raman lines, respectively. While the lattice parameters of the ceramics follow Vegards rule, deviations are observed for the thin film. This deviation has only a small effect on the phonon energies, which show a reasonably good agreement between thin film and ceramics.

One- and two-dimensional cavity modes in ZnO microwires

We demonstrate that one-dimensional (1D) Fabry–Perot modes (FPM) and 2D whispering gallery modes (WGM) occur separately or simultaneously within a single, tapered ZnO microwire. Their observation is strongly correlated with the size and shape of the microwire cross-section. FPM can preferentially be observed in thick wires with an elongated wire cross-section without hexagonal symmetry. The formation of WGM otherwise requires hexagonal symmetry—they can be observed in thin wires having a regular hexagonal cross-section. All optical eigenmodes were unambiguously identified by their in- and out-of- (cross-sectional) plane photonic mode dispersion.

Raman tensor elements of β-Ga2O3.

The Raman spectrum and particularly the Raman scattering intensities of monoclinic

Magnetoresistance effects in Zn0.90Co0.10O films

\beta\text{-Ga}_2\text{O}_3

Dielectric tensor of monoclinic Ga2O3 single crystals in the spectral range 0.5–8.5 eV

are investigated by experiment and theory. The low symmetry of