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A hybrid metal?semiconductor nanosystem for the generation of THz radiation, based on the fabrication of GaAs quantum molecules-Ga metal nanoparticles complexes through a self assembly approach, is proposed. The role of the growth parameters, the substrate temperature, the Ga and As flux during the quantum dot molecule (QDM) fabrication and the metal nanoparticle alignment are discussed. The tuning of the relative positioning of QDMs and metal nanoparticles is obtained through the careful control of Ga droplet nucleation sites via Ga surface diffusion. The electronic structure of a typical QDM was evaluated on the base of the morphological characterizations performed by atomic force microscopy and cross sectional scanning electron microscopy, and the predicted results confirmed by micro-photoluminescence experiments, showing that the Ga metal nanoparticle-GaAs quantum molecule complexes are suitable for terahertz generation from intraband transition.

Several semiconductor quantum dot techniques have been investigated for the generation of entangled photon pairs. Among the other techniques, droplet epitaxy enables the control of the shape, size, density, and emission wavelength of the quantum emitters. However, the fraction of the entanglement-ready quantum dots that can be fabricated with this method is still limited to around 5%, and matching the energy of the entangled photons to atomic transitions (a promising route towards quantum networking) remains an outstanding challenge. Here, we overcome these obstacles by introducing a modified approach to droplet epitaxy on a high symmetry (111)A substrate, where the fundamental crystallization step is performed at a significantly higher temperature as compared to previous reports. Our method drastically improves the yield of entanglement-ready photon sources near the emission wavelength of interest, which can be as high as 95% due to the low values of fine structure splitting and radiative lifetime, together with the reduced exciton dephasing offered by the choice of GaAs/AlGaAs materials. The quantum dots are designed to emit in the operating spectral region of Rb-based slow-light media, providing a viable technology for quantum repeater stations., 14 pages, 3 figures

Entanglement plays a crucial role in many protocols for quantum cryptography and in various approaches for quantum computation. Semiconductor quantum dots (QDs) have been proposed as a source of polarization-entangled photons which can be integrated in an electrically driven solid state device [1]. The generation process relies on the biexciton-exciton radiative cascade. However, e-h exchange interaction often induces a fine structure splitting (FSS) between the bright exciton states and destroys quantum correlation. In the case of QDs grown on commonly used (100) substrates, this degeneracy lifting is caused by the C2v symmetry stemming from asymmetric interfaces, strain anisotropy, piezoelectric fields and shape elongation [2,3]. A viable alternative relies on growing QDs on the higher symmetry (111) substrate. Theoretical investigations have shown that QDs with C3v symmetry should exhibit zero FSS [4,5]. The most studied III-V materials do not grow in the Stranski-Krastanov mode on a (111) surface, however different techniques such as droplet epitaxy [6] or the use of patterned substrates [7] have demonstrated to be able to overcome this limitation. In our work we focus on GaAs/AlGaAs (111) QDs grown by droplet epitaxy. Polarization resolved single dot photoluminescence measurements on hexagonal QDs are presented. Charged and bi-excitonic complexes are consistently identified by means of power and polarization dependence analyses. A broad FSS energy distribution is observed, with an average value smaller than the one reported for QDs grown on (100) substrates using the same technique and emitting at similar wavelengths. The phase distribution of the polarization axis evidences the absence of systematic anisotropies. These results are in agreement with previous studies on similar samples [8]. Recent advances in fabrication have proven the ability to obtain atomically flat substrates and to gradually tune the shape from hexagonal to triangular by changing the growth parameters. This lays the groundwork for a systematic investigation of the impact of geometry on the excitonic fine structure, with the goal of finding the best conditions for vanishing FSS. [1] O. Benson, C. Santori, M. Pelton, Y. Yamamoto, in: Physical Review Letters 84, 2513 (2000). [2] G. Bester, S. Nair, A. Zunger, in: Physical Review B 67, 161306 (2003). [3] R. Seguin, A. Schliwa, S. Rodt, K. Poetschke, U. W. Pohl, D. Bimberg, in: Physical Review Letters 95, 257402 (2005). [4] R. Singh, G. Bester, in: Physical Review Letters 103, 063601 (2009). [5] A. Schliwa, M. Winkelnkemper, A. Lochmann, E. Stock, D. Bimberg, in: Physical Review B 80, 161307 (2009). [6] E. Stock, T. Warming, I. Ostapenko, S. Rodt, A. Schliwa, J. A. Toefflinger, A. Lochmann, A. Toropov, S. Moshchenko, D. Dmitriev, V. Haisler, D. Bimberg, in: Applied Physics Letters 96, 093112 (2010). [7] Y. Sugiyama, Y. Sakuma, S. Muto, N. Yokoyama, in: Applied Physics Letters 67, 256 (1995). [8] T. Mano, M. Abbarchi, T. Kuroda, B. McSkimming,, A. Ohtake, K. Mitsuishi, K. Sakoda, in: Applied Physics Express 3, 065203 (2010).

We present a fully self‐assembly technique, based on Droplet Epitaxy, to fabricate semiconductor surfaces functionalized with asymmetric planar quantum dot molecules –metal nanoparticles hybrid systems for efficient THz generation. The role of growth parameters on molecule fabrication and metal nanoparticle alignment for the matching of the grown system with the design requirements is discussed.