The research group focuses on the development of novel quantum architectures fabricated on silicon, GaAs, (flat and pre-patterned) and InP substrates using molecular beam epitaxy (MBE) and investigated their specific aspects of quantum optics. The first is considered to be as one of the key technologies combining the best of both materials leading to a highly versatile hybrid photonics platform which opens the way to large scale photonic integration; this could allow a direct combination of photonics and electronics on the same chip. The later could allow the implementation of efficient single-photon sources for long-distance quantum information.

The emphasis is on the fabrication (growth and processing) and studies the fundamental structural and quantum optical properties of the single quantum dots (QDs). Integration of III-V semiconductor light sources in silicon, fabrication and characterization of pillar and photonic crystal (PhC) microcavities with integrated single emitters and processing of nanostructured surfaces for optical devices.

The MBE grown QD structures have been characterized by different methods some of which are in collaboration with our project partners, atomic force microscopy (AFM), photoluminescence (PL), correlation measurements (Uni. Wrocław), Pump probe spectroscopy (TU Dortmund), photocurrent spectroscopy (Uni. Paderborn), transmission electron microscopy (TEM) measurements and atomistic tight-binding (TB) calculation (Uni. Bremen). By extensive optimization of the growth parameters, our group has successfully obtained almost round shaped and low density InP-based QDs. This leads to the realization of telecom single QDs with resolution limited linewidths and very low fine-structure splitting (see Fig. 2).

With further growth and etching optimization, we have successfully fabricated high quality L3 InP-based photonic crystal microcavities embedded with QDs by using electron beam lithography, inductively coupled plasma reactive ion etching and wet etching techniques. Furthermore, light emission from QDs could be increased by more than an order of magnitude by integrating the QDs on distributed Bragg reflector (DBR) and with 40 times enhancement by embedding the QDs in PhC microcavities. An SEM images of DBR and PhC structures are shown in Fig.1.

High quality telecom C-band single-photon emitter has been demonstrated, which is indicated in Fig. 3(a). We have also realized pin-diode structures with integrated QDs. High-resolution resonant photocurrent spectroscopy performed in collaboration with University Paderborn reveals Rabi oscillations as displayed in Fig. 3(b). Realization the Schottky-diode structures with delta-doping and embedded QDs. Low temperature pump-probe measurements in magnetic field have been performed in collaboration with TU Dortmund. The measured electron and hole g-factors is shown in Fig. 3(c).

A new research direction was recently stared on molecular quantum system will be developed in the framework of newly funded collaborative priority project in the frame of the state initiative for the development of scientific and economic excellence (LOEWE) entitled "Scalable Molecular Quantum Bits (SMolBits)". Seven CINSaT members (T. Baumert, M. Benyoucef, C. Koch, R. Pietschnig, J.P. Reithmaier, K. Singer, B. Witzigmann) from three disciplines (chemistry, physics and electrical engineering) are involved. The main objective is to realize ideal quantum systems that can be used as key building blocks for scalable quantum systems (see Fig. 4).

The SMolBits project consists of 7 project areas and the Nano Optics group is leading two of them. The first one is “B1- Spectroscopy of immobilized molecules”, which focuses on the optical investigation of immobilized molecules and study the photon statistics of the light emitted from single molecules. The second is “C-Integration in Photonic Chip”. The goal of this project area is to explore the potential of molecular-based quantum systems whose quantum transitions are essentially indistinguishable, and investigation of on-chip optically coupled spatially separated single molecules.