A study of radiative lifetimes, spectral wandering and radiative coupling of individual InAs quantum dots

S.C.M. Grijseels

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In this work we investigate light matter interaction at the nanoscale. We focus on zero-dimensional objects, dubbed quantum dots, which are compositional defined nanometer-sized semiconductor structures in which charge carriers are spatially confined in all three dimensions. Using a tunable laser source, we can selectively address a few quantum dots in order to collect and analyze their photoluminescence spectrum. The aim is to probe the radiative recombination dynamics of the electron–hole pair created inside the quantum dot, which constitutes the exciton, and study the interaction with its environment. The charge fluctuations in the close neighborhood of the quantum dot are studied at low temperatures, when the influence of lattice vibrations is minimized. Besides the radiative broadening, the optical transition linewidth reflects the amount of changes in electric field caused by the charging and de-charging of local defects. This is a dynamical process, which is referred to as spectral diffusion or spectral wandering. On the other hand, the quantum dot static electrical environment is studied when an ultrafast pulsed laser source in combination with an ultra sensitive detector are employed. From these measurements, we can determine the radiative recombination time of the exciton. This recombination time reflects the coherence time of the electron and hole inside the quantum dot, which is important for quantum optoelectronic operation processes.

Another important aspect of the interaction between the exciton inside the quantum dot and its environment is the influence of lattice vibrations caused by thermally activated oscillations of the surrounding atoms. When the temperature is raised their role will become more apparent and the dynamics can be studied. We found that the linewidth broadening of self-assembled InAs quantum dots inside an AlxGa(1−x)As matrix at low temperature is determined solely by the spectral wandering rather than the radiative broadening process. We infer that the spectral wandering is caused by local charge defects that are created at the InAs wetting layer to AlxGa(1−x)As matrix interface. Furthermore, the radiative lifetime of the exciton is dispersed at a given photon energy as a result of the in-plane electric field that is created by the local charge traps.

In this work, we also explore the blooming field of nanoplasmonics, which studies the electromagnetic field interaction with a reservoir of free electrons inside a metal at the nanoscale. We place metal nanoparticles in close proximity to the quantum dot where they act as optical resonators. The exciton wave function can couple radiatively to an external cavity to enhance the oscillator strength of the electron-hole transition. The idea behind the experiments is to sufficiently increase the internal quantum efficiency of the quantum dot and metal nanoparticle system, thereby establishing device operation up to room temperature. Lithography techniques were employed to create the structures necessary to perform the measurements. We used a shallow etching technique to create freestanding nanomesas that contain the quantum dot layer. Subsequently, colloidal gold metal nanoparticles were deposited on the substrate surface where they tend to stick to the mesas by means of adhesive forces. Although the radiative enhancement is moderate at low temperatures, we were able to establish radiative coupling between the quantum dots and nanoparticles.
Originele taal-2Engels
KwalificatieDoctor in de Filosofie
Toekennende instantie
  • Applied Physics
  • Koenraad, Paul M., Promotor
  • Bacher, G., Promotor, Externe Persoon
  • Silov, Andrei Y., Co-Promotor
Datum van toekenning28 jun 2016
Plaats van publicatieEindhoven
Gedrukte ISBN's978-90-386-4098-3
StatusGepubliceerd - 28 jun 2016

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