Congratulation to Jan Olthaus, who successfully finished his master’s studies. In his thesis he simulated nanoscale wave guides with a focus on how to include a single emitter in the wave guide. Wave guides are promising candidates to be used in applications in quantum information technology. Jan analysed what the optimal position of the emitter is to ensure an efficient coupling to the modes of the wave guide. For this, Jan optimized several structures. During his master’s thesis he worked closely with members of the AG Schuck, who are fabrication such structures. Jan will now continue his work within his PhD studies.
New publication in 'Physical Review B' about the influence of Coulomb interaction on quantum dot spectra
Phys. Rev. B 97, 075308 (2018)
In a semiconductor quantum dot excitons (electron-hole pairs) can be optically excited. When an additional carrier is present, a charged exciton (or trion) is formed. Besides the well-studied ground state exciton, excited excitons become more and more important. For the excited excitons, the Coulomb interaction mixes different states, such that complex spectra emerge. In this publication, we studied how the Coulomb interaction with regard to the charge state and geometry of the quantum dots, influences the spectra. Thereby, we focussed on the excited exciton transitions.
5th International Workshop on the Optical Properties of Nanostructures in Münster
Last week we had the 5th International Workshop on the Optical Properties of Nanostructures (OPON 2018) in Münster. About 100 participants from different countries attended interesting talks and had vivid discussions. The latest results were also exchanged during the poster session. The scientific programme was complemented by a conference dinner. We thank everybody for making this a successful conference and we are looking forward to the next OPON
New publication in 'Physical Review B' about ultrafast dynamics in quantum dots
When a semiconductor quantum dot is excited into a higher state, an ultrafast relaxation into a lower states takes place. This process depends crucially on the details of the participating states as well as the available relaxation channels. We studied this dependence in this work theoretically and experimentally. After the relaxation, a single photon can be created. One can use our set-up to design a single photon amplifier.
Simulating Four Wave Mixing Spectroscopy of a Semiconductor Quantum Dot Doped with a Single Manganese Ion
Magnus Molitor finished his master’s thesis last year. In his thesis, Magnus has simulated four wave mixing signals of quantum dots doped with a single magnetic ion. Magnus calculated and analysed the complex spectra, which emerge due to the exchange interaction. In addition he studied the influence of phonons on the four wave mixing spectra. With finishing his thesis, Magnus has completed his master’s studied and he plans to stay in solid state theory for his PhD studies. Congratulations to Magnus and we wish him all the best.
New publication in 'Physical Review B' about electron-phonon interaction in quantum dots
Phonons influence the optical control of semiconductor quantum dots significantly. The strength of the interaction is among other things determined by the geometry of the dot and there can be a difference between, e.g., lens-shaped quantum dots (A) and spherical ones (B). In the publication we show that the influence of the phonons on the electronic properties can always by described by a spherical quantum dot (C) by an appropriate choice of parameters. In contrast, for the phonon properties the geometry is decisive
New publication in 'Physical Review B' discusses the sensitivity of four wave mixing signals with respect to the pulse area
Four wave mixing spectroscopy it a sophisticated method to explore quantum systems with a focus on couplings and coherences. We have been studying the four wave mixing signals of a single quantum dots (see also: https://doi.org/10.1364/OPTICA.3.000377). In this publication we now show that the four wave mixing signals depend sensibly on the properties of the exciting laser pulses. We conclude that is important to know precisely the strength of the exciting pulses, because the shape of the signals may differ significantly for different pulse strengths.
New publication in 'Acta Physica Polonica A' about the optical signals of carrier capture processes
When electrons in a semiconductor impinge on a localized potential, carrier can be captured into the potential mediated by the interaction with phonons. We were able to show that such a capture process happens locally, i.e., only carriers in the vicinity of the potential can be captured (read more about this here). In this publication we now show that the local nature of the capture process is also reflected in optical signals by looking at time resolved pump probe signals.
During the past month Andreas Völker worked on his Bachelor thesis in our group. Now he has handed in his theses about Theoretical analysis of bound states in 2D materials and presented his work in a talk. In his thesis he showed by numerically solving the Schrödinger equation, that in potential wells, which might appear in monolayers, there are numerous bound states. He further analysed the corresponding optical transitions. Congratulations to Andreas and we wish him all the best for his Master studies!
Jamie Fitzgerald from Imperial College London is visiting us
For the next two weeks Jamie Fitzgerald is staying in our group. Jamie is a PhD student working at Imperial College London. For his visit he successfully applied for funding within the EU COST action Nanoscale Quantum Optics. During his stay in Münster Jamie wants to study the excitation of a quantum dot using light carrying orbital angular momentum.
New publication in 'New Journal of Physics' shows how phonons control the laser emission of quantum dot ensemble
Semiconductor quantum dots can be used as active laser medium. Recently we have shown (News archive: 28.03.2017) that the laser output can be drastically enhanced by the interaction with phonons. Here, we now systematically study the phonon influence on the laser output accounting for different pulse forms and ensemble shapes. We thereby discriminate between two effects: the shaking and the adiabatic shift. Our work paves the way for a tailored laser control using phonons.
New publication in 'Physical Review B (Rapid)' about entering the reappearance regime
Optically excited carriers in semiconductor quantum dots interact with phonons. This interaction can hinder the optical state preparation. For large pulse intensities the phonon coupling becomes less efficient again and one enters the so called the reappearance regime. In collaboration with our experimental colleagues from Basel and samples from Bochum, we were able to demonstrate that it is possible to enter the reappearance regime.
New publication in 'Physical Review B' about the phonon assisted preparation of dark excitons
Dark excitons in quantum dots can typically not be directly excited by a laser pulse. However, if a tilted magnetic field is applied, it becomes possible to optically excite the dark exciton. We show that the dark exciton preparation is not hindered by phonons. Furthermore we demonstrate that phonons widen the parameter range where an optical excitation of the dark exciton can take place.
Richard Kerber is going for a research stay to London
Starting in May Richard Kerber will do research at the Department of Physics at Imperial College London within his PhD studies. To finance his stay he successfully applied for a fellowship of the German Academic Exchange Service (DAAD). In London, Richard will continue his research on the interaction of complex light fields with nano structures with a new focus on metamaterials. We wish Richard all the best for his stay in London!
New publication in 'Physical Review B (Rapid)' about the optical generation of the biexciton using chirped pulses
Biexcitons in quantum dots can be used as source of entangled photons. For this, the biexciton should be created efficiently and with a large fidelity. We here show that by using an excitation with chirped laser pulses the biexciton can be created in a robust and resonant way, which is advantageous over previously used methods.
This work was done together with the group of Richard Warburton from Basel, Switzerland, and sampled from the group of Andreas Wieck from Bochum, Germany.
New publication in 'Physical Review B' about a new method to describe quantum mechanics
The carrier capture from a quantum wire in a quantum dot happens on ultrafast time and length scales and involves different dimensionalities. We have developed a Lindblad formalism to describe the dynamical capture process quantum mechanically. The computationally light formalisms give insights into the inhomogeneous dynamics at the nanoscale.
New publication in 'Physical Review Letters' about the phonon controlled lasing
To make a laser one need a gain medium, which emits light into a resonator or cavity mode. Consider an ensemble of quantum dots with a certain energy distribution, most of them are out of resonance. Using a coherent phonon pulse, one can modify the quantum dot energies on a picosecond time scale. By this, the laser intensity can be enhanced or quenched. The intensity is further influenced by dynamical effects.
New publication in 'ACS Photonics' about the interaction of twisted light with a nanostructure
A light pulse can be characterized by its polarization, denoting its spin angular momentum. In addition it can carry an orbital angular moment, which modifies the interaction with matter significantly. When a light beam carrying orbital angular momentum hits a plasmonic nanostructure, also modes which are usually dark can be excited. This makes it possible to read out information about the orbital angular momentum of the incident light.
This work was done together with the group of Ortwin Hess from Imperial College London, UK.
New publication in 'Physical Review B' about the influence of phonons on the optical signals of a quantum dot
When a quantum dot is excited by a continuous light field, the coupled quantum dot-light states are the new eigenstates of the system. The absorption spectra then consists of up to three lines, forming the so called Mollow triplet. By simulating a pump-probe set-up we analyse the dynamics of the quantum dot-light system. Further, we study the influence of phonons on the optical signals reflecting the relaxation into the ground state in the coupled system.