ISBN 9783868539394

978-3-86853-939-4, Reihe Physik

Robert Hafenbrak
Tuning the exciton fine structure of single (In,Ga)As/GaAs quantum dots to realize a triggered entangled photon source

183 Seiten, Dissertation Universität Stuttgart (2011), Softcover, A5

Zusammenfassung / Abstract

The radiative biexciton-exciton decay in a semiconductor quantum dot has the potential of being a source of triggered polarization-entangled photon pairs. In most cases the anisotropy-induced exciton fine structure splitting destroys this entanglement. In this thesis, measurements on self-assembled (In,Ga)As/GaAs quantum dot structures, providing both small inhomogeneous emission linewidth and near-zero fine structure splitting, are presented. A high-resolution detection technique based on a scanning Fabry-Perot interferometer is introduced. It allows to accurately determine the fine structure in the photoluminescence emission and therefore select appropriate quantum dots for quantum state tomography.

Two techniques to controllably tune the fine structure splitting are elaborated and discussed. Firstly, the application of uniaxial strain to the quantum dot structures is investigated in detail. Measurements on the emission energy tuning on different samples reveal controlled

near-linear tuning. We find red- and blue-shift on different quantum dots for the same strain in the experiment. Atomistic million-atom calculations imply, that direction and magnitude of this energy tuning depend on structural properties of the quantum dots. Applying uniaxial strain, we observe reversible and controlled linear tuning of the fine structure splitting. For alloyed (In,Ga)As/GaAs quantum dots, atomistic calculations predict an anticrossing of the exciton states, resulting in a minimum value of the fine structure splitting. The first experimental verification of this exciton state anticrossing is presented.

The second technique is local laser annealing. Here, microdisk structures are locally heated with a laser until the barrier material (GaAs) and the quantum dot material (InGaAs) intermix. We find permanent monotonic blue-shifts of the emission energy of single quantum

dots and observe a tuning of the fine structure splitting.

By measuring the polarization quantum state of the biexciton-exciton photon pair, we are able to verify the conditions of entangled or classically correlated photon pairs in full consistence with observed fine structure properties. Furthermore, we demonstrate reliable polarization-entanglement for elevated temperatures up to 30 K. The fidelity of the maximally entangled state decreases only little from 72 % at 4 K to 68 \% at 30 K. This is especially encouraging for future implementations in practical devices.