A systematic variation of the exciton fine-structure splitting with quantum dot size in single InAs/GaAs quantum dots grown by metal-organic chemical vapor deposition is observed. The splitting increases from -80 to as much as 520 µeV with quantum dot size. A change of sign is reported for small quantum dots. Model calculations within the framework of eight-band k·p theory and the configuration interaction method were performed. Different sources for the fine-structure splitting are discussed, and piezoelectricity is pinpointed as the only effect reproducing the observed trend.The exchange interaction of electron-hole pairs (excitons) in semiconductor quantum dots (QDs) has been subject of a lively debate in recent years [1,2,3,4,5,6,7,8]. In such strongly confined systems it is supposed to be enhanced with respect to the bulk case due to the close proximity of electrons and holes. However, the influence of the exact geometry of the confining potential on the exchange interaction still needs to be clarified. A detailed understanding of the resulting exciton fine structure in quantum dots is of fundamental interest and of largest importance for potential applications of QDs in single-photon emitters and entangled two-photon sources for quantum cryptography [9].The total angular momentum M of heavy-hole excitons (X) in QDs is composed of the electron spin (s = ± 1 2 ) and the heavy hole angular momentum (j = ± 3 2 ), consequently producing four degenerate exciton states frequently denoted as dark (M = ±2) and bright (M = ±1) states indicating whether they couple to the photon field or not. Independent of the given confinement symmetry electron-hole exchange interaction causes a dark-bright splitting. Furthermore it mixes the dark states lifting their degeneracy and forming a dark doublet (|2 ±|−2 ). Likewise, additional lowering of the confinement symmetry to C 2v or lower mixes the bright states producing a nondegenerate bright doublet (|1 ± | − 1 ).While emission lines involving pure states are circularly polarized, the mixed states usually produce lines showing linear polarization along the [110] and [110] crystal directions, respectively (Fig. 1). The two bright states are thus directly observable as linearly polarized transitions in luminescence experiments. The energetic difference between these lines is called exciton fine-structure splitting (FSS).The biexciton (XX) ground state is not split by the exchange interaction, since the net spin of the involved electrons and holes is 0. However, the XX to X decay involves two allowed transitions with the final states being the bright states of the X. Therefore, the FSS is reproduced (yet inverted) in the XX to X decay (Fig. 1).Recently reported experimental values of the FSS in * Email: seguin@sol.physik.tu-berlin.de

We probe the indistinguishability of photons emitted by a semiconductor quantum dot (QD) via time- and temperature-dependent two-photon interference (TPI) experiments. An increase in temporal separation between consecutive photon emission events reveals a decrease in TPI visibility on a nanosecond time scale, theoretically described by a non-Markovian noise process in agreement with fluctuating charge traps in the QD's vicinity. Phonon-induced pure dephasing results in a decrease in TPI visibility from (96±4)% at 10 K to a vanishing visibility at 40 K. In contrast to Michelson-type measurements, our experiments provide direct access to the time-dependent coherence of a quantum emitter on a nanosecond time scale.

The success of advanced quantum communication relies crucially on non-classical light sources emitting single indistinguishable photons at high flux rates and purity. We report on deterministically fabricated microlenses with single quantum dots inside which fulfil these requirements in a flexible and robust quantum device approach. In our concept we combine cathodoluminescence spectroscopy with advanced in situ three-dimensional electron-beam lithography at cryogenic temperatures to pattern monolithic microlenses precisely aligned to pre-selected single quantum dots above a distributed Bragg reflector. We demonstrate that the resulting deterministic quantum-dot microlenses enhance the photon-extraction efficiency to (23±3)%. Furthermore we prove that such microlenses assure close to pure emission of triggered single photons with a high degree of photon indistinguishability up to (80±7)% at saturation. As a unique feature, both single-photon purity and photon indistinguishability are preserved at high excitation power and pulsed excitation, even above saturation of the quantum emitter.

ZnO single crystals, epilayers, and nanostructures often exhibit a variety of narrow emission lines in the spectral range between 3.33 and 3.35 eV which are commonly attributed to deeply bound excitons (Y lines). In this work, we present a comprehensive study of the properties of the deeply bound excitons with particular focus on the Y 0 transition at 3.333 eV. The electronic and optical properties of these centers are compared to those of the shallow impurity related exciton binding centers (I lines). In contrast to the shallow donors in ZnO, the deeply bound exciton complexes exhibit a large discrepancy between the thermal activation energy and localization energy of the excitons and cannot be described by an effective mass approach. The different properties between the shallow and deeply bound excitons are also reflected by an exceptionally small coupling of the deep centers to the lattice phonons and a small splitting between their two electron satellite transitions. Based on a multitude of different experimental results including magnetophotoluminescence, magnetoabsorption, excitation spectroscopy (PLE), time resolved photoluminescence (TRPL), and uniaxial pressure measurements, a qualitative defect model is developed which explains all Y lines as radiative recombinations of excitons bound to extended structural defect complexes. These defect complexes introduce additional donor states in ZnO. Furthermore, the spatially localized character of the defect centers is visualized in contrast to the homogeneous distribution of shallow impurity centers by monochromatic cathodoluminescence imaging. A possible relation between the defect bound excitons and the green luminescence band in ZnO is discussed. The optical properties of the defect transitions are compared to similar luminescence lines related to defect and dislocation bound excitons in other II-VI and III-V semiconductors.

The quantum confined Stark effect is observed for quantum dots (QD's) exposed to randomly fluctuating electric fields in epitaxial structures. These fields, attributed to charges localized at defects in the vicinity of the QD's, lead to a jitter in the emission energies of individual QD's. This jitter has typical frequencies of below about 1 Hz and is characteristic for each QD thus providing a unique means to unambiguously identify the emission spectra of single QD's. Up to eight lines are identified for individual QD's and attributed to excitonic, biexcitonic, and LO-phonon-assisted transitions. The intensity of the LO-phonon replica is surprisingly large corresponding to Huang-Rhys factors of about one