Armando Rastelli

Triggered polarization-entangled photon pairs from a single quantum dot up to 30 K

The radiative biexciton-exciton decay in a semiconductor quantum dot (QD) has the potential of being a source of triggered polarization-entangled photon pairs. However, in most cases the anisotropy-induced exciton fine structure splitting destroys this entanglement. Here, we present measurements on improved QD structures, providing both significantly reduced inhomogeneous emission linewidths and near-zero fine structure splittings. A high-resolution detection technique is introduced which allows us to accurately determine the fine structure in the photoluminescence emission and therefore select appropriate QDs for quantum state tomography. We were able to verify the conditions of entangled or classically correlated photon pairs in full consistence with observed fine structure properties. Furthermore, we demonstrate reliable polarization- entanglement for elevated temperatures up to 30 K. The fidelity of the maximally entangled state decreases only a little from 72% at 4 K to 68% at 30 K. This is especially encouraging for future implementations in practical devices.

SiOx∕Si radial superlattices and microtube optical ring resonators

Scanning and transmission electron microscopy reveals that SiOx∕Si layers can roll up into microtubes and radial superlattices on a Si substrate. These hybrid objects are thermally stable up to 850°C and emit light in the visible spectral range at room temperature. For tubes disengaged from the substrate surface, optically resonant emissions with mode spacings inversely proportional to the tube diameter are observed and agree excellently with those obtained from finite-difference time-domain simulations. The resonant modes recorded are strictly polarized along the tube axis.

Three-Dimensional Composition Profiles of Single Quantum Dots Determined by Scanning-Probe-Microscopy-Based Nanotomography

Scanning probe microscopy combined with selective wet chemical etching is employed to quantitatively determine the full three-dimensional (3D) composition profiles of single strained SiGe/Si(001) islands. The technique allows us to simultaneously obtain 3D profiles for both coherent and dislocated islands and to collect data with large statistics. Lateral and vertical composition gradients are observed, and their origin is discussed. X-ray scattering measurements performed on a large sample area are used to validate the results.

Enhancing the Optical Excitation Efficiency of a Single Self-Assembled Quantum Dot with a Plasmonic Nanoantenna

We demonstrate how the controlled positioning of a plasmonic nanoparticle modifies the photoluminescence of a single epitaxial GaAs quantum dot. The antenna particle leads to an increase of the luminescence intensity by about a factor of 8. Spectrally and temporally resolved photoluminescence measurements prove an increase of the quantum dot’s excitation rate.

Light emission and wave guiding of quantum dots in a tube

We present microphotoluminescence investigations of InAs quantum dots (QDs) integrated into self-rolling InGaAs∕GaAs strained layers. The emission signal from the QDs is redshifted due to strain relaxation and increased in intensity after the strained layers are released from the substrate and rolled up into tubes. We detect waveguided light at the tube ends, which originates from the QDs at the laser excitation spot. The possibility of integrating quantum emitters into the tube walls acting as waveguides represents a major step toward the realization of flexible high quality factor optical resonators based on rolled-up nano- and microtubes.

Universal shapes of self-organized semiconductor quantum dots: Striking similarities between InAs∕GaAs(001) and Ge∕Si(001)

The model systems for self-organized quantum dots formed from elemental and compound semiconductors, namely Ge grown on Si(001) and InAs on GaAs(001), are comparatively studied by scanning tunneling microscopy. It is shown that in both material combinations only two well-defined families of faceted and defect-free nanocrystals exist (and coexist). These three-dimensional islands, pyramids and domes, show common morphological characteristics, independently of the specific material system. A universal behavior is further demonstrated in the capping-passivation process that turns the nanocrystals in true quantum dots.

Self-assembled InAs quantum dots on patterned GaAs(001) substrates: Formation and shape evolution

We report on the formation of ordered and size homogeneous InAs quantum dot (QD) arrays on patterned GaAs(001) substrates. A material depletion region is observed around the patterned area while a long-range homogeneous distribution of QDs is found inside the patterned area. A sample with less InAs deposition shows a gradient in the material distribution over the patterned area. Based on these observations we propose a simple model to describe the QD formation in the patterned area. The QD shape evolution is also investigated and discussed.

Photoluminescence from seeded three-dimensional InAs∕GaAs quantum-dot crystals

We investigate the photoluminescence (PL) properties of three-dimensional InAs∕GaAs quantum-dot (QD) crystals grown on shallow modulated periodic hole arrays patterned on GaAs(001). We find that the PL spectra become narrower and more intense with increasing number of QD layers. A deconvoluted PL linewidth of 14.9 meV is obtained from a defect-free QD crystal consisting of 11 stacked QD layers. The PL spectra obtained for QD crystals containing QD vacancies show significantly broader spectra. The PL peak energy and linewidth of the QDs across the whole pattern (100×100μm2) remain constant within 1.278±0.001eV and 21.0±1.7meV, respectively. From power-dependent PL measurement, we can resolve up to seven excited-state PL peaks confirming the remarkable size homogeneity of our QD crystals. This experimental result can be reasonably fitted by a calculation based on random population theory and on a simple model for the QD confinement potential.

SiGe growth on patterned Si(001) substrates: Surface evolution and evidence of modified island coarsening

The morphological evolution of both pits and SiGe islands on patterned Si(001) substrates is investigated. With increasing Si buffer layer thickness the patterned holes transform into multifaceted pits before evolving into inverted truncated pyramids. SiGe island formation and evolution are studied by systematically varying the Ge coverage and pit spacing and quantitative data on the influence of the pattern periodicity on the SiGe island volume are presented. The presence of pits allows the fabrication of uniform island arrays with any of their equilibrium shapes.

Ordered GaAs quantum dot arrays on GaAs(001) : Single photon emission and fine structure splitting

Ordered GaAs∕AlGaAs quantum dots (QDs) are fabricated on patterned GaAs(001) substrates and their optical properties are investigated by microphotoluminescence (PL) spectroscopy. QDs exhibit sharp excitonic lines with typical single QD emission features. Photon-correlation spectroscopy shows single photon emission for the neutral exciton transition. Polarization-dependent PL measurements reveal a sharp exciton line and a fine structure exchange splitting of about 70μeV.