New review paper on the impact of phonons on optical control of quantum dots
Semicond. Sci. Technol. 34, 063002 (2019)
The optical control of semiconductor quantum dots plays a crucial role in using quantum dots as photon sources; an important ingredient for applications in quantum information technology. In contrast to atoms, which are rather isolated from their environment, quantum dots are solid state systems and hence their electronic degrees of freedom interact strongly with the lattice vibrations, i.e. the phonons. In this paper, we review the latest developments on the theoretical description and experimental results regarding the influence of the electron-phonon interaction on the optical state preparation of a semiconductor quantum dot.
New publication on the theoretical description of optical vortices
Phys. Rev. B 99, 155306 (2019)
When a semiconductor nanostructure is excited by light, so-called excitons (electron-hole pairs) can form. The spatio-temporal dynamics of these excitons is strongly affects by the Coulomb interaction. When the strength of the excitation changes, the dynamics of the excitons is modified significantly. As an example, we have considered the exciton dynamics in a semiconductor quantum wire. We have developed and utilized a wave function approach. With this, we have studied the dynamics of a wave packet traveling along the wire under different excitation conditions.
This work results from a collaboration with Roberto Rosati, who is now at Chalmers, Gothenburg.
Read more in the publication ...
New publication on a new detection scheme for electron dynamics
J. Phys.: Condens. Matter 31 (2019) 28LT01
Electrons that move in a 2D semiconductor can scatter at obstacles resulting in a diffraction pattern due to their wave character. To detect such a pattern we propose to use the scattering of the electrons into a strongly confined potential. Because the scattering happens locally, the process is able to resolve the spatiotemporal dynamics of the electrons. Our method could be used as detection scheme for electron dynamics on ultrafast time and nanometric length scales.
This work results from a collaboration with Roberto Rosati, who is now at Chalmers, Gothenburg.
Read more in the publication ...
New publication on the theoretical description of optical vortices
Phys. Rev. A 99, 023845 (2019)
In addition to polarization, light can carry orbital angular moment. Such light has a helical wave front and, thus, a vortex forms. The theoretical description of these electro-magnetic waves can be derived from the corresponding potentials. By exploiting the gauge freedom, it is possible to obtain different beam profiles. This leads to the question, if the distinct descriptions are connected via a gauge transformation. We discuss, that there is no gauge transformation possible between different descriptions. We furthermore introduce a wider class of optical vortices connecting the well-established Bessel and Laguerre-Gauss beams and propose an experiment to measure different beams.
Congratulations to Dr. Richard Kerber to his PhD. Richard handed in his thesis „Numerical Study of the Interaction between Orbital Angular Momentum and Plasmonic Nanostructures“ last year and defended in January 2019. He will be awarded his PhD in the official ceremony on February 1st, 2019. During his PhD studies Richard has published three papers in high-ranked journals, performed research in London, gave tutorials, organized the PhD-student-seminar and much more. We wish Richard all the best for his future!
New publication on nanophotonic control of optical signals stemming from atomically thin semiconductors
Phys. Rev. B 98, 245307 (2018)
Atomically thin semiconductors like the transitions metal dichalcogenides interact strongly with light. Thereby not only linear signals like linear absorption emerge, but also nonlinear optical signals like higher harmonics are generated. To control the higher harmonic generation, one can use nanophotonic structures like cavities. To model and design such structures efficiently, we introduce a method which simultaneously solves Maxwell’s equations to describe the light field dynamics with the Bloch equations for the description on the carrier dynamics. With our method we calculate optimized structures for higher harmonic generation.
We look back on a great year 2018 with fascinating physics,
interesting publications and many successful theses.
We wish everybody a Merry Christmas
and a good start into the New Year!
New publication on dichroism induced by orbital angular momentum light
Communications Physics 1, 87 (2018)
When light interacts with matter, the optical response can be dependent on the polarization state of the light. This effect is known as dichroism. When the light beam additionally carries an orbital angular momentum, the optical response can also strongly depend on the orbital angular momentum and induce a dichroism. In the paper we define orbital angular momentum induced dichroism and discuss its appearance using the example of stacked golden nanorods.
In a thermal occupation of quantum states the higher energy states are less occupied than the lower energy states. The opposite case refers to inversion and is the foundation to create lasing. This gives rise to the question, if a thermal gradient is able to create an inversion. In our paper we present a quantum system, where by using two energy filters an inversion in the middle quantum system emerges. This inversion can then be used to drive a phonon laser.
The work is a collaboration with Pawel Karwat from Wroclaw University of Science and Technology, Poland, and Ortwin Hess from Imperial College London.
New publication in 'Physical Review B' on spatial control of carrier capture
Phys. Rev. B 98, 195411 (2018)
If a golf ball moves across the green and encounter the hole, it can fall into it. On the nanoscale electrons can undergo a similar process: When impinging on a potential with bound states, they may by captured inside the potential. The capture efficiency depends not only on the energy selection rules, but also on the spatial geometry of the problem. In the publication we show using the example of a quantum dot in a TMDC monolayer how the geometry can be varied to spatially control the capture
Semiconductor quantum dots in photonic structures: Influence of the electron-phonon interaction
Congratulations to Kevin Jürgens who finished his master theses. Kevin studied quantum dots which are embedded in a photonic cavity. He analysed both the dynamics in the quantum dot as well as the dynamics of the electric field via the Maxwell equations. In addition he considered the influence of the phonons. In his thesis Kevin showed that a complex dynamics emerges in such systems. Kevin will continue his studies in the group of Prof. Kuhn during his PhD studies. We wish him all the best for his future!
New publication in 'Physical Review B' on quantum effects in plasmonic spheres
Phys. Rev. B 98, 165411 (2018)
The confinement of carriers in a nanoparticle results in a quantization of the energies. On the other hand also classical effects like plasmon resonances are observed in such particles. In this publication we study a few-electron system and shed new light on the formation of the plasmon. In particular, we discuss the optically induced dynamics and the impact of the Coulomb interaction, which leads to a mixing of the states and strongly modifies the dynamics.
The calculations were performed by Alexandra Crai from Ortwin Hess’ group at Imperial College London.
New publication in 'New Journal of Physics' on the interaction of Archimedean Spirals with Twisted Light
New J. Phys. 20, 095005 (2018)
In addition to the polarization light can carry an orbital angular moment, which manifests itself is a helical wave front. At the beam axis such twisted light possesses a vortex and is thus also called vortex light. When the twisted light interacts with matter, unusual effects which are not visible for plane wave excitation can be observed. An Archimedean spiral also possesses some sort of helicity. Using a micro-meter sized spiral it is possible to create vortex light. For a nano-meter sized spiral, the excitation with twisted light results in untypical resonances with fascinating radiation patterns. The publication discusses this novel effects resulting from the unique geometry of the Archimedean spiral.
The work was initiated by a visit from Jamie Fitzgerald from Vincenzo Giannini's group at Imperial College London in our group and was done in collaboration with Sang Soon Oh from Cardiff.
Congratulation to Thomas Bracht, who successfully finished his bachelor’s thesis. In his thesis Thomas studied the optical excitation of bulk semiconductors. He showed that the excitation with frequency modulated pulses results in a robust population of the electronic states. A focus of his work was on the impact of the Coulomb interaction on the optical state preparation and the excitation of excitons. In the upcoming term, Thomas will start his master’s studies and we wish him all the best for this!
Simulation of spatially resolved carrier dynamics in two-dimensional semiconductors induced by optical excitation
The German Research Foundation (DFG) has approved our proposal on Simulation of spatially resolved carrier dynamics in two-dimensional semiconductors induced by optical excitation. In this project we plan to study the spatially resolved carrier dynamics in two-dimensional semiconductors acting on ultra-short time and length scales. Using a quantum mechanical description we want to study several interactions like the Coulomb- or the electron-phonon interaction. In addition we want to consider the resulting optical signals of the dynamics. We are looking forward to the project, which will run for the next 3 years.
Jan Olthaus receives his Master's degree
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.