Infoscience

Thesis

Interaction between site-controlled quantum dot systems and photonic cavity structures

The goal of this thesis was to investigate light-matter interaction in nanophotonic devices based on site-controlled pyramidal quantum dots (QD) in photonic crystal (PhC) cavities. These QDs provide position and spectral control, which is hardly achievable by the widely exploited self-assembled QD-based systems, thus allowing almost ideal cloning of differently designed devices in thousands of copies on the same chip. Thus, we conducted statistical studies of the optical properties of a large variety of photonic structures without concerns about significant deviations from the targeted layout. In particular, we addressed the influence of the QD position with respect to the electrical field pattern of the cavity mode (CM) on the optical properties of the QD excitonic transitions. We integrated a single pyramidal QD in a linear PhC membrane cavity with three missing holes (L3 PhC cavity) at a set of well-defined positions, among which were points corresponding to the first and the second CM lobes as well as a CM node. Taking advantage of the high reproducibility of the fabricated devices, we aimed at providing statistical evidence of the impact of the positioning of a single dipole on the CM-induced Purcell enhancement. Interestingly, we observed a clear Fano-like resonance in the QD emission component co-polarized with a CM that vanished for devices with a QD at the CM node. Further developing pyramidal QD-based QD-PhC cavity integration technology, we successfully implemented the integration of up to 4 QDs with an L7 PhC cavity. For several such structures we identified the optical transitions of each QD by means of spatial scanning micro-photoluminescence, accompanied with correlations in spectral wandering traces. We demonstrated phonon-assisted weak coupling of 4 different QD excitons with the same CM. Using a combination of temperature- and water condensation- induced exciton-CM tuning allowed probing the coupling of the 4 QDs to different CMs, thereby probing the modal spatial profiles. In parallel, we explored spectral diffusion and spectral wandering processes of QD excitons. As a tool, we developed a correlation technique based on the observation of transitions between different excitonic energy levels induced by the quantum confined Stark effect (QCSE). This technique allowed us to study the nature of charged centers in the vicinity of the QD, leading to spectral jumps between discrete emission energies of the QD excitons. Relating the QD exciton energy to the amplitude of the electric field inducing the QCSE allowed observing unusual spectral response of the QD upon increasing the charge density in its vicinity. Additionally, it allowed probing the ratio between the dipole moments of different excitonic complexes. Scanning spectrally a CM with a single QD exciton tuned by the fluctuations of the built-in electric field, we observed emission intensity enhancement associated with the CM-induced Purcell effect. We also observed an irreversible QCSE-induced giant exciton spectral shift accompanied by the intensity intermittency. Finally, we evidenced a strong dependence of the observed spectral wandering and emission intermittency effects on the sample light exposure history, clearly exhibiting a photon-activated charge trapping in the QD vicinity.

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