Using the trion as a monitor we investigate the anisotropy of the single-hole state in epitaxial CdSe/ ZnSe quantum dots. Heavy-light hole mixing caused by a symmetry reduction below D 2d results in elliptical polarization of the optical transitions with a specific axis for each dot defined by strain and shape. In a transverse magnetic field, a quartet of strictly linearly polarized lines appears that reveals the off-diagonal coupling of both electron and hole states. Although induced by the field, the linear polarization is not related to the field orientation, but either along or perpendicular to the dot axis seen at zero field. We find an in-plane hole g factor as large as 0.3 with distinct anisotropic behavior.Semiconductor quantum dots (QDs) are often called "artificial atoms." However, crystal symmetry and specific band structure of the semiconductor make the energy eigenstates utterly different from those of the simple particle-in-a-box problem. 1,2 Detailed knowledge of the energy levels and the corresponding wave functions is of immense fundamental and practical interest. In the present work, the hole eigenstates of epitaxial Stranski-Krastanov (SK) QDs are examined in this respect. We consider the prototype case of a semiconductor with zinc-blende symmetry. Unlike colloidal nanocrystals, SK QDs exhibit a defined quantization axis ͑z͒ given by the growth direction. In the frequent case of [001] growth, the symmetry of the bulk semiconductor implies that x and y direction are equivalent ͑D 2d ͒. That equivalence is generally lifted in a QD due to shape and/or strain. The splitting of the optically allowed exciton into a line doublet caused by the electron-hole exchange interaction is a wellstudied consequence of this symmetry reduction. 3,4 The question of whether the in-plane anisotropy is also manifested on a single-particle level has not been addressed so far. As long as the confinement energy is much smaller than the band gap, the electron with its s-like Bloch function can be safely treated as an isotropic effective-mass particle with spin ±1 / 2. On the other hand, such an approximation fails even qualitatively for the p-type holes of total angular momentum ±3 / 2 connected with the fourfold-degenerate edge of the valence band. In what follows, we demonstrate that the single-hole states of a SK QD are indeed substantially affected and that the anisotropy translates into a specific coupling with a transverse magnetic field. Our experimental concept is based on the trion feature representing the fundamental optical excitation of charged QDs. 5,6 In case of a single negative resident charge, the trion consists of one hole and two electrons. As the total spin of the latter is zero in the singlet ground state, the trion represents a direct monitor of the angular momentum configuration of the hole. 7 The CdSe/ ZnSe QD samples used in this study are grown by molecular-beam epitaxy using a thermal activation procedure. 8,9 The height and diameter of the pure CdSe core are about 2 and 5 -10 nm, respectively,...

Single-photon interference is observed on the ultranarrow long-term stable exciton resonance of an individual semiconductor quantum dot. This interference is related to the fine-structure splitting and allows direct conclusions about the coherence properties of the exciton. When selectively addressing a particular dot by quasiresonant phonon-assisted excitation, despite a rapid orientation relaxation on a 1-ps time scale, coherence is partly maintained. No significant further decoherence occurs when the ground state is reached until the exciton recombines radiatively (approximately 300 ps).

The fundamental limits of inorganic semiconductors for light emitting applications, such as holographic displays, biomedical imaging and ultrafast data processing and communication, might be overcome by hybridization with their organic counterparts, which feature enhanced frequency response and colour range. Innovative hybrid inorganic/organic structures exploit efficient electrical injection and high excitation density of inorganic semiconductors and subsequent energy transfer to the organic semiconductor, provided that the radiative emission yield is high. An inherent obstacle to that end is the unfavourable energy level offset at hybrid inorganic/organic structures, which rather facilitates charge transfer that quenches light emission. Here, we introduce a technologically relevant method to optimize the hybrid structure's energy levels, here comprising ZnO and a tailored ladder-type oligophenylene. The ZnO work function is substantially lowered with an organometallic donor monolayer, aligning the frontier levels of the inorganic and organic semiconductors. This increases the hybrid structure's radiative emission yield sevenfold, validating the relevance of our approach.

We discuss density functional theory calculations of hybrid inorganic-organic systems that explicitly include the global effects of doping (i.e., position of the Fermi level) and the formation of a space-charge layer. For the example of tetrafluoro-tetracyanoquinodimethane on the ZnO(0001[over ¯]) surface we show that the adsorption energy and electron transfer depend strongly on the ZnO doping. The associated work function changes are large, for which the formation of space-charge layers is the main driving force. The prominent doping effects are expected to be quite general for charge-transfer interfaces in hybrid inorganic-organic systems and important for device design.

Electronic coupling between Wannier and Frenkel excitons in an inorganic/organic semiconductor hybrid structure is experimentally observed. Time-resolved photoluminescence and excitation spectroscopy directly demonstrate that electronic excitation energy can be transferred with an efficiency of up to 50% from an inorganic ZnO quantum well to an organic [2,2-p-phenylenebis-(5-phenyloxazol), alpha-sexithiophene] overlayer. The coupling is mediated via dipole-dipole-interaction analog to the Förster transfer in donor-acceptor systems.