Self-organized In͑Ga͒As/GaAs quantum dots ͑QDs͒ grown on ͑111͒ substrate are proposed as ideal sources for the generation of entangled photon pairs. Due to the threefold rotational symmetry of the ͑111͒ surface, QDs with C 3v symmetry or higher are expected to develop during growth. In contrast to QDs on ͑001͒-oriented substrates, the symmetry of the confinement potential of ͑111͒ QDs is not lowered by piezoelectric effects. As a result the excitonic bright splitting vanishes and the biexciton→ exciton→ 0 recombination cascade can be used for the generation of entangled photons. We evaluate the spectroscopic separability of excitonic and biexcitonic emissions as a function of QD size, shape, and composition using the configuration-interaction model in conjunction with eight-band k · p theory. The piezoelectric field in ͑111͒ QDs predominantly aligns along the growth direction and gives rise to vertical charge separation. First-and second-order piezoelectric fields are oriented in opposite directions. The In/Ga ratio inside the QD determines the leading contribution and can be employed to balance both terms in order to achieve a field-free situation with maximal electron-hole overlap. The biexciton binding energy depends on the net piezoelectric potential drop across the QD vertical extension and becomes maximal if the first-and second-order fields outweigh each other within the QD interior.

Coupling of acoustic and optical phonons to excitons in single InGaAs/GaAs quantum dots is investigated in detail experimentally and theoretically as a function of temperature. For the theoretical description of the luminescence spectrum, including acoustic and optical phonon scattering, we used the exactly solvable independent boson model. Surprisingly, only GaAs bulk-type longitudinal-optical (LO) phonons are detected in experiment. A quantitatively correct theoretical description of the optical-phonon replica is obtained by including a limited lifetime of the phonons and the dispersion of the LO phonon energy. Similarly, a numerically correct description of the acoustic phonon wings is again based on GaAs bulk material parameters for the phonon dispersion and deformation coupling. In addition, the line shape of the calculated spectra agrees with experiment only when realistic wave functions (e.g., based on eight-band k•p theory) are used for the electron-phonon coupling matrix elements. Gaussian wave functions describing the ground state of a harmonic oscillator fail to describe high-energy tails. Thus, fundamental insights of importance for the correct prediction of properties of nonclassical light sources, based on semiconductor nanostructures, are obtained.

In this letter, we demonstrate that self-organized InGaAs quantum dots ͑QDs͒ grown on GaAs ͑111͒ substrate using droplet epitaxy have great potential for the generation of entangled photon pairs. The QDs show spectrally sharp luminescence lines and low spatial density. A second order correlation value of g ͑2͒ ͑0͒ Ͻ 0.3 proves single-photon emission. By comparing the power dependence of the luminescence from a number of QDs we identify a typical luminescence fingerprint. In polarization dependent microphotoluminescence studies a fine-structure splitting ranging Յ40 eV down to the determination limit of our setup ͑10 eV͒ was observed.

A combined analysis of microphotoluminescence ͑PL͒ and microphotoluminescence excitation ͑PLE͒ spectra of the same single quantum dot ͑QD͒ enables an unambiguous identification of four sharp resonances in the excitation spectrum detected on the positive trion transition ͑h 0 → e 0 h 0 h 1 ͒ and reveals the complete fine structure of the hot trion. Transitions into states normally forbidden by ͑spin͒ selection rules for optical transitions between pure spin states are observed. The splittings of all triplet states are found to be large ͑up to 3 meV͒, asymmetric, and QD size and shape dependent. The experimental data are in excellent agreement with theoretical calculations in the framework of eight-band k · p theory and the configuration-interaction method. To account for the physical effects which lead to the observed fine-structure splitting, parts of the complex model are successively omitted. This approach identifies the anisotropic hole-hole exchange interaction as well as correlation effects dominating the observed fine-structure splitting of the hot trion.

A novel concept for on-chip quantum optics using an internal electrically pumped microlaser is presented. The microlaser resonantly excites a quantum dot microcavity system operating in the weak coupling regime of cavity quantum electrodynamics. This work presents the first on-chip application of quantum dot microlasers, and also opens up new avenues for the integration of individual microcavity structures into larger photonic networks.

980 nm vertical-cavity surface-emitting lasers based on submonolayer growth of quantum dots show clearly open eyes and operate error free with bit error rates better than 10 −12 at 25 and 85 • C for 20 Gb/s without current adjustment. The peak differential efficiency only reduces from 0.71 to 0.61 W/A between 25 and 85 • C; the maximum output power at 25 • C is above 10 mW.

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