Helena Hilmer

Homogeneous core/shell ZnO/ZnMgO quantum well heterostructures on vertical ZnO nanowires.

Low-area density ZnO nanowire arrays, growing perpendicularly to the substrate, are synthesized with high-pressure pulsed laser deposition. The introduction of a ZnO buffer layer enables us to fabricate individual nanowires several micrometres apart (area density<0.1 nanowire microm(-2)), suppressing any shadowing effect by neighbouring nanowires during subsequent growth. These low density ZnO nanowires, whose c-axis is perpendicular to the substrate surface, are then used as templates to grow ZnO/ZnMgO core-shell nanowire heterostructures with conventional low-pressure pulsed laser deposition. Cathodoluminescence spectroscopy as well as transmission electron microscopy show that a sharp interface forms between the ZnO core and the ZnMgO shell. Based on these findings, we have grown a series of radial ZnO/ZnMgO quantum wells with different thicknesses that exhibit quantum confinement effects, with thicker quantum wells emitting at lower energies. Spatially resolved cathodoluminescence confirms the homogeneity of the quantum well structure along the full nanowire length of about 3 microm.

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.

Tuning the lateral density of ZnO nanowire arrays and its application as physical templates for radial nanowire heterostructures

The lateral density of ZnO nanowire arrays grown with pulsed laser deposition (PLD) can be tuned from 1 to 10−2 μm−2 by introducing a ZnO nucleation layer and optimizing the distance between the substrate and the ablated target. High-density (∼10 μm−2) nanowire arrays can be grown on sapphire substrates with or without gold catalysts. However, if a ZnO wetting layer was adopted, the density of ZnO nanowires could be controlled with high reproducibility. The decreasing growth density is attributed to a competition between the two-dimensional film epitaxy and one-dimensional nanowire growth. The dependence of nanowire density on the substrate–target distance mainly arises from the expansion dynamics of the plasma plume and the chamber geometry. Using low-density nanowires as templates, a general PLD route was developed to grow radial nanowire heterostructures. Here we demonstrate MgZnO/ZnO/MgZnO nanowire quantum wells and ZnO/ZnO:P core–shell nanowire p–n junctions.

Oxide Thin Film Heterostructures on Large Area, with Flexible Doping, Low Dislocation Density, and Abrupt Interfaces: Grown by Pulsed Laser Deposition

Advanced Pulsed Laser Deposition (PLD) processes allow the growth of oxide thin film heterostructures on large area substrates up to 4-inch diameter, with flexible and controlled doping, low dislocation density, and abrupt interfaces. These PLD processes are discussed and their capabilities demonstrated using selected results of structural, electrical, and optical characterization of superconducting (YBa 2Cu 3O 7−δ), semiconducting (ZnO-based), and ferroelectric (BaTiO 3-based) and dielectric (wide-gap oxide) thin films and multilayers. Regarding the homogeneity on large area of structure and electrical properties, flexibility of doping, and state-of-the-art electronic and optical performance, the comparably simple PLD processes are now advantageous or at least fully competitive to Metal Organic Chemical Vapor Deposition or Molecular Beam Epitaxy. In particular, the high flexibility connected with high film quality makes PLD a more and more widespread growth technique in oxide research.

Strong exciton-photon coupling in ZnO based resonators

The authors report on the fabrication of high quality all-oxide Bragg reflectors (BRs) and ZnO based resonators. The resonator consists of a bulk half-wavelength ZnO microcavity embedded between two BRs, each made of 10.5 layer pairs of yttria stabilized zirconia and Al2O3. Scanning transmission electron microscopy and atomic force microscopy, yield smooth interfaces and low surface roughness for the BR as well as the resonator. For the BR with 10.5 layer pairs the authors obtain reflectivities up to 99.2% within the Bragg stop band. The exciton-polariton dispersion was determined by both, polarization- and angle-resolved photoluminescence (PL) and reflectivity (R) measurements. The detuning between the uncoupled exciton mode and photon mode was changed by shifting the exciton mode energy in the temperature range of 10–290 K. Thereby we observed that a strong exciton-photon coupling regime up to room temperature is present in our resonators with maximum values of the Rabi splitting of about 68 meV (PL, T=...

Exciton-polaritons in a ZnO-based microcavity: polarization dependence and nonlinear occupation

We report on the occupation of the lower exciton?polariton branch in a ZnO-based microcavity as a function of the detuning between the exciton and the uncoupled cavity-photon mode and on the optical excitation density. We emphasize the difference in the dispersion and occupation of the lower polariton branch as a function of the linear polarization of the emitted light. For the negative detuning regime, we found an energy splitting between the transverse electric (TE)- and transverse magnetic (TM)-polarized states at in-plane wave vectors between 0.4?107?m?1 and 1.2?107?m?1, which is caused by the polarization dependence of the dispersion of the uncoupled cavity-photon mode. The maximum energy splitting of about 6?meV was observed for a detuning of about ?=?70?meV. From the integrated photoluminescence peak, we deduce the occupation of the lower polariton branch as well as the scattering rates of exciton?polaritons into the lower polariton branch. We found that the energy splitting causes an enhanced scattering of exciton?polaritons into the lower polariton branch for the TM-polarized light compared with that of the TE-polarized light. By varying the excitation density, we observe a superlinear growth of the lower polariton branch occupation for negative and intermediate detuning regimes. For an accumulation of exciton?polaritons in the ground state at low temperatures (T=10?K), we found an intermediate detuning regime (?20?meV

Gold nanostructure matrices by diffraction mask-projection laser ablation: extension to previously inaccessible substrates

Gold nanodot matrices made by diffraction mask-projection laser ablation (DiMPLA) are presented. The nanodots are well ordered and can be synthesized on so far not accessible substrates by virtue of intermediate thin AlO(x) layers deposited by pulsed-laser deposition (PLD). Investigations were made on the influence of layer thickness, roughness and type of substrate on the nanodots and their fabrication. It is shown that all of these parameters are crucial for the generation of nanodots on thin AlO(x) layers. The roughness of the layer and the substrate material determine whether the layer cracks upon laser patterning. The layer thickness, on the other hand, influences the size of gold nanodots on top. Extinction spectra show that the particle size is the dominant contribution that shifts the plasmon resonance peak.

PLD Growth of High Reflective All‐Oxide Bragg Reflectors for ZnO Resonators

We report on high‐reflective all‐oxide Bragg reflectors for future applications in ZnO based resonators. Our YSZ‐Al2O3 Bragg reflectors were grown by pulsed laser deposition and show reflectivity values of up to 99.99% with a layer pair number of 15.5. A very smooth surface roughness of Ra = 0.8 nm was observed for these Bragg reflectors. By using these Bragg reflectors, we were able to reproducibly grow well tuned microcavity resonators with smooth interfaces, low surface roughness, top and bottom Bragg Reflectors with almost identical properties, and quality factors of about 500.

Exciton‐polaritons in ZnO microcavity resonators

We report on the observation of exciton‐polaritons in all‐oxide resonator structures with a quasi‐bulk ZnO film as active medium at temperatures between 10 K and room temperature. The observed energy splitting of the exciton‐polariton branches of about 76 meV in maximum reflects the resonator to be in the strong coupling regime.

Occupation behaviour of the lower exciton‐polariton branch in ZnO‐based microresonators

We present the occupation behaviour of the lower exciton‐polariton branch in a ZnO‐based all‐oxide microresonator for different detunings and excitation powers. We found that there is an intermediate detuning range which should be preferred in order to reach an enhancement of the ground state occupation with increasing excitation power density. This is not possible for large negative and large positive detunings caused by the strong bottleneck effect and a large thermal escape rate, respectively.