A thesis submitted February 12, 2015 for the degree of Doctor of Philosophy and defended April 25, 2015.
The PhD School of Science
Faculty of Science, Niels Bohr Institute, Quantum Photonics group, University of Copenhagen, Denmark
Electric and Magnetic Interaction between Quantum Dots and Light
The present thesis reports research on the optical properties of quantum dots by developing new theories and conducting optical measurements. We demonstrate experimentally singlephoton superradiance in interface-uctuation quantum dots by recording the temporal decay dynamics in conjunction with second-order correlation measurements and a theoretical model. We measure an oscillator strength of up to 960:8 and an average quantum eciency of (94:83:0)%. This enhanced light-matter coupling is known as the giant oscillator strength of quantum dots, which is shown to be equivalent to superradiance. We argue that there is ample room for improving the oscillator strength with prospects for approaching the ultra-strong-coupling regime of cavity quantum electrodynamics with optical photons. These outstanding gures of merit render interface-uctuation quantum dots excellent candidates for use in cavity quantum electrodynamics and quantum-information science.
We investigate exciton localization in droplet-epitaxy quantum dots by conducting spectral and time-resolved measurements. We nd small excitons despite the large physical size of dropletepitaxy quantum dots, which is attributed to material inter-diusion during the growth process. The small size of excitons leads to a small oscillator strength of about 10. These ndings are crosschecked by an analysis of the phonon-broadened spectra revealing a small exciton wavefunction. We conclude that engineering large excitons with giant oscillator strength remains a future challenge for the droplet-epitaxy technique.
A multipolar theory of spontaneous emission from quantum dots is developed to explain the recent observation that In(Ga)As quantum dots break the dipole theory. The analysis yields a large mesoscopic moment, which contains magnetic-dipole and electric-quadrupole contributions and may compete with the dipole moment in light-matter interactions. A theory for the quantum-dot wavefunctions is developed showing that the mesoscopic moment originates from distortions in the underlying crystal lattice. The resulting quantum-mechanical current density is curved leading to light-matter interaction of both electric and magnetic character. Our study demonstrates that In(Ga)As quantum dots lack parity symmetry and, as consequence, can be employed for locally probing the parity symmetry of complex photonic nanostructures. This opens the prospect for interfacing quantum dots with optical metamaterials for tailoring light-matter interaction at the single-electron and single-photon level.