Research topics

Thanks to modern technology it is nowadays possible to fabricate structures on the size of a few nanometers. In other words, thousands of these nanostructure fit into the diameter of single hair! For the dynamics of the particles in such structures a picosecond, which is a millionth of a millionth of a second, already feels like ages. In contrast to what we experience in everyday life, on these extreme scale new effects come into play, because everything is governed by the rules of quantum mechanics. These effects can be explored using laser pulses, which can also be used to control the dynamics in these system. To understand the new phenomena, a theoretical description and numerical modelling is required, which is the basis of our work.

We consider on the one hand nanostructures made of semiconductors, mostly semiconductor quantum dots, and on the other hand metallic structures, which act as antennas for visible light. Our simulations allow for predicting the dynamics and developing control schemes, which is a basic ingredient for applications in the field of quantum information technology.

Currently we work on:

  • Carrier dynamics in ultra-thin semiconductors

    Nowadays it is possible to fabricate ultrathin semiconductors consisting only of one or a few monolayers of atoms. Examples are graphene, which has a vanishing band gap, and transition metal dichalcogenides (TMDCs) having a finite band gap. Those materials are effectively two-dimensional. Using deformation one can create a potential dip in these sheets, which acts like a potential well for the charge carriers. Carriers can be captured inside the potential through interaction with, e.g., phonons.

    We study the dynamics of charge carriers traveling inside these new semiconductor materials. To account for the ultrashort time and length scales, a quantum mechanical description is required. We take special care of the locality of the scattering processes and analyse the spatio-temporal dynamics of the carriers considering different interaction mechanisms.

    Current projects (in collaboration with AG Kuhn):

    • Spatio-temporal carrier dynamics in TMDCs (Frank Lengers, Roberto Rosati)
  • Phonon influence on optically excited semiconductor quantum dots

    Semiconductor quantum dots exhibit a discrete energy spectrum which can be tailored by the quantum dot geometry. Therefore, quantum dots are well suited to study solid state quantum optics effects, where we consider the optical control of the quantum dot states using laser light. In contrast to atoms, quantum dots are always subject to the coupling to phonons, which can result in unwanted effects, but also in fascinating, new phenomena.

    The phonon effects on the optical control of quantum dots are of interest to us. Using different analytical and numerical methods we analyse the influence of phonons on the electronic states of the quantum dot and the resulting optical signals. On the other hand, we also study the properties of the phonons which are generated during the optical excitation of the quantum dot. We work closely together with the AG Kuhn on the topics of carrier dynamics in optically semiconductor quantum dots and generation and dynamics of phonons in semiconductor nanostructures.

    Current projects (in collaboration with AG Kuhn):

    • Modelling of optical signals of quantum dots (Magnus Molitor, Daniel Wigger: in collaboration with Jacek Kasprzak and his group from CNRS Grenoble)
    • Phonon influence on optical controlled quantum dots (Sebastian Lüker: in collaboration with Timo Kaldewey and Richard Warburton from the University of Basel)
    • Phonon controlled lasing of a quantum dot ensemble (Daniel Wigger: in collaboration with Thomas Czerniuk and Manfred Bayer from the TU Dortmund)
    • Optical excitation of dark excitons in quantum dots (Sebastian Lüker)
    • Structure of higher excited states in quantum dots (Matthias Holtkemper)

  • Interaction of nanostructures with complex light fields

    A beam of light is often described by a plane wave, which has spatially homogeneous wavefronts. In contrast, complex light fields have spatially inhomogeneous wavefronts resulting in a complex phase relation. One example is the so called twisted light, which has a helical wave front and a phase singularity implying a zero of intensity at the beam axis. A twisted light beam carries an orbital angular momentum, which could be used in communication technology to encode information. When such a light beam interacts with matter, different processes can be excited compared to the excitation with a plane wave. One of our projects aims at the mathematical formulation of the interaction of twisted light with matter. Other projects deal with the numerical simulation of the interaction of twisted light with metallic nanostructures exhibiting plasmonic resonances.

    Current projects:

    • Interaction of complex light fields with plasmonic nano-antennas (Richard Kerber, in cooperation with Jamie Fitzgerald, Sang Soon Oh and Ortwin Hess from Imperial College London, UK).

    • Theoretical formulation of the interaction of twisted light with matter (in cooperation with Guillermo Quinteiro from the University of Buenos Aires, Argentina).

  • Controlling light using nanostructures

    Using nanostructured materials the properties of light can be controlled and manipulated. One example are photonic crystals, where a periodic structure of the material leads to a photonic band gap, in which light cannot propagate through the system. By putting a defect in a photonic crystal, a localized mode can be confined in the defect area, i.e., a cavity is formed. One important question is how the localized mode inside the cavity can be excited by a quantum emitter.

    A second example is the control of light using metallic nanostructures showing plasmonic resonances. Using, e.g., a pair of plasmonic nanostructures, light can be extremely enhanced in-between them. This results in a strong light-matter coupling between the field and a quantum system which is placed in the field maximum.

    To study these fascinating nanostructures, we mostly perform numerical simulations, often using already existing software packages. Our focus is to understand the light-matter interaction at the nanoscale.

    Current projects:

    • Coupling of single emitters into photonic crystal cavities (Jan Olthaus, in collaboration with the AG Schuck of the WWU Münster).
    • Light enhancement by plasmonic nanostructures (Richard Kerber).