Optical Properties of Semiconductor Quantum Dots

This thesis presents different optical experiments performed on semiconductor quantum dots. These structures allow to confine a small number of electrons and holes to a tiny region of space, some nm across. The aim of this work was to study the basic properties of different types of quantum dots made of various materials and with different techniques. First we studied InAsP quantum dots in InP nanowires and demonstrated narrow optical transitions, with linewidths below 30 micro eV. It was also possible to produce electron-hole pairs in a given spin state and to show that, in the presence of a magnetic field, this state is preserved for a time comparable to the exciton lifetime. Measurements of the electron and hole g-factors in these dots are also presented. Other types of structures dealt in this thesis are GaAs quantum dots in AlGaAs and small InAs dots in GaAs. GaAs dots can be tuned to have optical transitions at the same energy as rubidium atoms. We studied InAs quantum rings and we observed energy oscillations that are compatible with the Aharonov-Bohm effect and that can be tuned by an electric field. The last chapter of this thesis deals with two-photon interference, a useful tool for different quantum information protocols. We demonstrated that a InAs quantum dot can emit pairs of indistiguishable photons with a delay of about 5~ns between them.Quantum NanoscienceApplied Science

Sharp emission from single InAs quantum dots grown on vicinal GaAs surfaces

We report on optical studies of single InAs quantum dots grown on vicinal GaAs(001) surfaces. To ensure low quantum dot density and appropriate size, we deposit InAs layers 1.4 or 1.5 ML thick, thinner than the critical thickness for Stranski–Krastanov quantum dot formation. These dots show sharp and bright photoluminescence. Lifetime measurements reveal an exciton lifetime of 500 ps. Polarization measurements show an exciton fine structure splitting of 15??eV and allow to identify the exciton and charged exciton transitions with linewidth as narrow as 23??eV.Kavli Institute of NanoscienceApplied Science

Tuning single GaAs quantum dots in resonance with a rubidium vapor

We study single GaAs quantum dots with optical transitions that can be brought into resonance with the widely used D2 transitions of rubidium atoms. We achieve resonance by Zeeman or Stark shifting the quantum dot levels. We discuss an energy stabilization scheme based on the absorption of quantum dot photoluminescence in a rubidium vapor. This offers a scalable means to counteract slow spectral diffusion in quantum dots.QN/Quantum NanoscienceApplied Science

Potential of semiconductor nanowires for single photon sources

The catalyst-assisted growth of semiconductor nanowires heterostructures offers a very flexible way to design and fabricate single photon emitters. The nanowires can be positioned by organizing the catalyst prior to growth. Single quantum dots can be formed in the core of single nanowires which can then be easily isolated and addressed to generate single photons. Diameter and height of the dots can be controlled and their emission wavelength can be tuned at the optical telecommunication wavelengths by the material composition. The final morphology of a wire can be shaped by the radial/axial growth ratio, offering the possibility to form single mode optical waveguides with a tapered end for efficient photon collection.Kavli Institute of NanoscienceApplied Science