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Semiconductor for breakfast - Episode 3
The popular online series lecture series "Semiconductors for breakfast" will return. From 28.02.22 till 11.03.22 the semiconductor division presents each day at breakfast time a talk about an exciting topic in semiconductor physics with speakers from all around the world. The talks shall be accessible to a general audience and be addressed to students as well. The participation is possible without registration via the online links at the website.
New publication proposes to use Manganese doped quantum dots as photon buffer
In quantum technologies there is the demand to store photons. In this paper we propose to use quantum dots doped with a single Manganese ion for this purpose. The photon can be transformed into a spin excitation of the Manganese, which is rather long-lived. After a buffer time up to several nanoseconds, the photon can again be emitted. With our protocol we hope to stimulate a new usage of quantum dots for quantum technologies.
The work was performed in collaboration with AG Axt from Bayreuth.
doi.org/10.1002/qute.202100131
The work was performed in collaboration with AG Axt from Bayreuth.
doi.org/10.1002/qute.202100131
New publication introduces the SUPER scheme
To use quantum dots as single photon source, they need to be prepared in an excited state. In this paper we propose a new scheme for this, namely the „Swing UP of Quantum EmitteR population“ (SUPER) scheme. The paper explains, how this scheme works and why it is super. It was also selected as “Editor’s suggestion”.
Read a detailed description in our press release.The work was performed in collaboration with AG Axt from Bayreuth and AG Heindel from Berlin.
doi.org/10.1103/PRXQuantum.2.040354
New publication on the spectra of TMDC polaritons
When an atomically thin semiconductor is placed into an optical cavity with high quality, new quasiparticles called polaritons emerge. The semiconductor material we consider is a monolayer of a transition metal dichalcogenide (TMDC). Polaritons typically exhibit two lines in their spectra. In the paper we discuss the influence of the lattice vibration, i.e., the phonons, on the optical spectra. We show that the phonons have a different impact on the two polariton lines. Our results help to better understand polariton spectra, which is important to develop applications with polariton systems.
doi.org/10.1103/PhysRevB.104.L241301
doi.org/10.1103/PhysRevB.104.L241301
New publication introduces the electronic Poincaré sphere
Two-dimensional materials can host potential, into which carriers can be captured. One example such potentials are formed by so called “nano bubbles”. Via the capture process the dynamics of the captured carriers inside the nanobubble can be controlled. With this, linear and circular movements can be created. The different movements can be interpreted as points on an electronic Poincaré sphere and, thus, as two-level system. It is possible to initialize and switch the states. This opens new possibilities to use electronic degrees of freedom in quantum information technology.
The paper was done in collaboration with Roberto Rosati from the Ultrafast Quantum Dynamics Group from Marburg and the Semiconductor Theory Group from Bremen and the AG Kuhn.
doi.org/10.1021/acs.nanolett.1c02864
doi.org/10.1021/acs.nanolett.1c02864
New publication on the light field dynamics induced by emitters in a cavity
If an optical cavity contains several quantum emitters, all of them couple to the same light mode. In quantum optics, this is described within the Tavis-Cummings Model. In this paper, we compare the quantum optical model with a semiclassical one. While the spectra and the dynamics of the expectation values can for certain parameters be described using the semi-classical description, the photon states show a clearly non-classical behaviour. Our results give a good insight into the border between classical and quantum descriptions.
The paper is the result from a collaboration with AG Kuhn.
doi.org/10.1103/PhysRevB.104.205308
New publication on driving the radiative Auger process in quantum dots
In charged quantum dots radiative Auger processes can occur due to the Coulomb interaction. In such a process during the recombination of an electron-hole pair the remaining electron is promoted to a higher state and the emitted photon has a reduced energy. In this paper we show that using two laser pulses it is possible to actively drive this process. This opens up new possibilities of optically controlling quantum dots.
The paper is a joint publication with the Nano Photonics Group from Basel, the Lehrstuhl für Angewandte Festkörperphysik from Bochum and Pawel Machnikowski from Wroclaw.
see also press release RUB (24.11.2021)Congratulations to Marc Hunkemöller
Congratulations to Marc Hunkemöller for finishing his bachelor thesis “Machine Learning for the simulation of photonic crystal wave guides using the finite difference time domain method”. Photonic crystal wave guides play a crucial role in photonic quantum technologies. Hence, it is important to optimize the structures such that losses are minimal. Marc has shown in his work, that it is possible to use neuronal networks for the optimization. With this, he designed photonic wave guides with a high mirror strength or a high-quality cavity. His results will help in designing further structures.
DPG poster price for Thomas Bracht
Congratulations to Thomas Bracht. He won the 1. poster price of the semiconductor physics division in his poster session at the virtual meeting of the German Physical Society (DPG). The price contains a certificate and a cash price.
PosterCongratulations to Jan Kaspari
Congratulations to Jan Kaspari for finishing his master thesis “Theoretical description of higher excited quantum dot state in an external magnetic field”. In his work Jan starts by analysing the effect of a magnetic field on a hole spin, which due to the splitting into heavy and light holes is different that a usual 3/2 spin. The findings can be directly transferred to the behaviour of the exciton. Subsequently, Jan studies higher excited states, which can be strongly mixed, and shows the influence of the magnetic field on these states. Jan’s results give an interesting insight into the interplay of the discrete quantum dot states and a magnetic field.
New publication on the validity of the quantum regression theorem
The quantum regression theorem (QRT) from quantum optics can be used to describe optical signals of an open quantum system. It is an approximate method, which can be used to estimate the influence of phonons on the properties of photons that are emitted by a quantum dot. In the paper we compare the results obtained within the QRT with ones from a numerically exact path integral formalism. We demonstrate that the QRT can overestimate the influence of the phonons, which is very important for quantum communication, where photons should have almost ideal properties. The work was performed in collaboration with AG Axt from Bayreuth.
doi.org/10.1103/PhysRevLett.127.100402
doi.org/10.1103/PhysRevLett.127.100402
New publication on the usage of the Stark effect for dark exciton preparation
When a few-level system is driven by a strong laser pulse, the energy eigenstates change. This is called Stark effect. In the publication we show that the Stark effect can be used to deterministically address dark excitons in a quantum dot being in a magnetic field. Phonons do not significantly influence the preparation protocols. We also discuss the properties of the laser pulses that should be used for the Stark effect and whether these pulses can be realised in the lab. Dark excitons can play a decisive role to create entangled photons from quantum dots.
Congratulations to Frank Lengers
Congratulations to Dr. Frank Lengers to his PhD! This year Frank has handed in his thesis „Spatially resolved quantum kinetics of optically excited carriers in low-dimensional semiconductors“ and successfully defended and published it. He will be awarded his PhD in the official ceremony on July 23rd. During his PhD Frank has written and contributed to 10 publications. His talks at conferences and seminars were always very well received and he won two poster prices at NOEKS 2020. Frank was also active in teaching. Everybody was happy to meet and discuss with Frank at the coffee machine, which was always stocked up and clean thanks to Frank’s care!
We wish Frank all the best for his future!Congratulations to Miriam Neumann
Congratulations to Miriam Neumann for finishing her master thesis. In her work, Miriam shows how to prepare dark excitons in a quantum dot in a magnetic field using the optical Stark effect. She derives and analysis the conditions to successfully prepare the dark exciton. Miriam further discusses that the phonons do not influence the preparation in most of the cases. Dark excitons in quantum dots can be useful for photon storage or to enhance the generation of time-bin entangled photons.
New publication on optical signals of a driven quantum dot
The occupation of the excited state in a quantum dot oscillates as function of time. These Rabi oscillations are damped due the electron-phonon interaction. To detect the dynamics, we propose to use time-resolved optical measurements. We calculate analytical equations for the optical signals. In this paper, we discuss the influence of the detuning and the phonons on the dynamics, shedding new light on the electron-phonon interaction in quantum dots.
doi.org/10.1002/andp.202100086
doi.org/10.1002/andp.202100086
New publication on Cat-state in quantum dot-cavity systems
Named after the famous thought experiment, a Schrödinger cat state is a superposition of macroscopically distinguishable states. Nowadays such states can be realized using photons in cavities. In this paper, we discuss, whether semiconductor quantum dots embedded in a microcavity can be used to generate photonic cat states. We analyse the different loss channels of the system and show, that for certain parameters one can obtain a cat state in a state-of-the-art system. The work resulted from a collaboration with the AG Axt from Bayreuth.
doi.org/10.1103/PhysRevResearch.3.023088
doi.org/10.1103/PhysRevResearch.3.023088
New publication how to switch between different types of entanglement
Entangled photons are essential for quantum communication. They typically occur in the so-called Bell states, which are maximally entangled states. There are two types of Bell states, which are called Phi- and Psi-Bell states. In this paper we show, how to switch between these two types of entangled photons generated by a driven quantum emitter in a cavity. The switching is achieved by changing the driving strength of the external laser field.
The paper is part of the Special Issue „Non-Classical Light Emitters and Single-Photon Detectors”. The work resulted from a collaboration with the AG Axt from Bayreuth.
doi.org/10.1063/5.0045377
doi.org/10.1063/5.0045377
Doris Reiter is 'Physikerin der Woche'
The project „Physikerin der Woche“ of the German Physical Society (DPG) highlights every week women in physics in Germany or German women in physics abroad. This week (March 22-28) Doris is “Physikerin der Woche” as presented on the DPG-Website and the social media accounts Twitter and Instagram of the “Physikerinnen”.
'Semiconductors for breakfast' is back
March is usually the time of the spring meeting of the German Physical Society (DPG). This year we only have online events. Starting from March 15 the semiconductor division presents the online lecture series “Semiconductors for breakfast”. Each day at breakfast time we host a talk about an exciting topic in semiconductor physics. The talks shall be accessible to a general audience and be addressed to students as well. The participation is possible without registration via the online links at the website.
New publication on long-lived coherence in charged quantum dots
If an electron-hole pair is excited in a quantum dot, a trion forms. Analogous to excitons, trions can either be in their ground state or in any excited state. We generate a superposition of excited trion states and show that this superposition survives a fast relaxation and is surprisingly long-lived. We can detect the superposition in optical pump-probe experiments which were performed in the AG Leitenstorfer from Konstanz. The experiments agree excellently with our theoretical calculations. Thereby, the superposition is visible as dynamical feature and can be controlled via the excitation conditions.
doi.org/10.1103/PhysRevLett.126.067402
doi.org/10.1103/PhysRevLett.126.067402
New publication on the preparation of dark excitons in quantum dots
In quantum dots several excitons (electron-hole pairs) can be optically excited, while other excitons do not couple to the light field. These dark excitons have a long lifetime, which makes their preparation interesting. In this paper we propose a method to prepare the dark ground-state exciton. Our method relies on the excitation with a longitudinal light field and valence band mixing of higher excited states. We show that our method should enable an efficient dark state preparation in many quantum dots.
This work was done together with Guillermo Quinteiro from Argentina.
doi.org/10.1103/PhysRevResearch.3.013024
Congratulations to Thomas Bracht
Congratulations to Thomas Bracht for completing his master studies. In his master’s thesis he worked on “Machine learning approach to quantum control in optically driven few-level systems”. Using an artificial neutral network he was able to find new protocols to prepare quantum states in quantum dots. He explored the limits of using machine learning and showed that sometimes a very unreasonable outcome is found, but that machine learning can also be very helpful to study quantum systems. This month Thomas starts his PhD studies and we wish him all the best!
2020
New publication on the generation of entangled states of different type
Entangled photon states are fascinating quantum mechanical objects with applications in quantum information technology. Using a few-level system like in a quantum dot or an atom, it is possible to generate entangled photons. If such a system is actively driven, different types of entangled states can be created. We analyse which conditions lead to the different entanglement types and identify special points of no entanglement.
This work was done together with the AG Axt from Bayreuth.
doi.org/10.1002/qute.202000108
New publication on the excitation of quantum dots with spatially structured light
Semiconductor quantum dots have a discrete energy structure. When excited with light, the symmetries of the wave function and the light impose selection similar to the situation in atoms. For spatially structured light different transitions compared to plane wave excitation can be excited. Using a realistic model of a quantum dot, we discuss the absorption spectra for different light beams. Furthermore, we show that using spatially structured light one can reconstruct the wave function of the exciton.
The publication is a joint work with Guillermo Quinteiro from Argentina.
doi.org/10.1103/PhysRevB.102.165315
New publication on quantum emitters in photonic crystals
Nanophotonic is a promising approach to quantum technology. Together with the AG Schuck we studied in a joint theoretical and experimental effort photonic crystals with embedded nanodiamonds hosting NV centers. The photons which are emitted from the NV center were coupled into the tantalum pentoxide wave guide, showing a high Purcell factor. Furthermore, we demonstrate that we could address two circuits simultaneously and optically detect magnetic resonances.
Read more in the official news.
doi.org/10.1021/acs.nanolett.0c03262
New publication on exciton capture in TMDCs
Transition metal dichalcogenides (TMDCs) when thinned down to a monolayer are direct band gap semiconductors, in which carriers can move in the plane. TMDC monolayers can also host localized potentials. In this publication we have analyzed how to describe the capture of excitons (electon-hole pairs) from the 2D plane into the 0D potential quantum mechanically. Focussing on the interaction with phonons, we show that the excitons are captured mainly due to scattering with acoustic phonons and that the polarons formation is of high importance in these systems.
doi.org/10.1103/PhysRevResearch.2.043160
doi.org/10.1103/PhysRevResearch.2.043160
Congratulations to Marius Klaßen
Congratulations to Marius Klaßen for finishing his bachelor thesis. In his work Marius considered the pump-probe signals of quantum dots. For a constantly driven quantum dot under the influence of phonon he was able to derive analytical equations for the probe polarization and calculate the corresponding spectra. He showed that significant differences between the detuned and resonant case occur. Now Marius wants to start his master studies and we wish him all the best!
The Quantum Dinner
In October, the online seminar series “The Quantum Dinner” takes place. Every evening at 5:15 p.m., we present a lecture on current topics in the field of quantum physics and quantum technologies. Everybody who is interested is welcome to attend. All talks will be steamed live. Host is the DPG working group AGyouLeaP (young Leaders in Physics).
New publication on the preparation of photonic Fock states
There are several proposals to create photons in a Fock state using atoms or superconductors. In this manuscript we show that photonic Fock states can also be realized in a quantum dot—cavity system. Using a sequence of pi-pulses and longer Stark pulses, we propose two protocols for the Fock state preparation: PIC (protocol with interrupted coupling) and PUC (protocol with uninterrupted coupling). Even under realistic conditions, i.e., taking into account losses and coupling to phonons, a high fidelity Fock state preparation is possible.
This work has been performed in collaboration with AG Axt from Bayreuth.
doi.org/10.1103/PhysRevResearch.2.033489
Congratulation to Andreas Völker
Congratulations to Andreas Völker for finishing his master thesis. In his work Andreas studied semiconductor hetero structures like quantum wires. For his analysis, he combined the FDTD method for the description of the light field dynamics with the equations of motion for the carrier dynamics in the semiconductor. In this way he showed how the dynamics of a localized carrier distribution are visible in the optical signals. This method can be easily extended to also describe semiconductors in photonic nanostructures.
New publication on semiclassical modelling of the coupled quantum-dot cavity system
Quantum dots embedded in photonic structures often interact only with a single mode of the electric field. The dynamics and accordingly the spectrum depends sensitively on the strength of this mode. Interestingly, there is a sharp transition between a polariton-like regime and a regime displaying Rabi oscillations. In this paper, we use a numerical simulation based on FDTD to obtain this transition. We furthermore develop an analytical model using coupled, non-linear equations to explain the transition using a Newton-type potential.
10.1103/PhysRevB.101.235311
10.1103/PhysRevB.101.235311
New publication on the theoretical description of absorption spectra in TMDCs
Monolayers of transition metal dichalcogenides (TMDCs) are two-dimensional semiconductors with a direct band gap and pronounced excitonic absorption lines. In TMDC monolayers the electron-phonon interaction is exceptionally strong, which becomes visible in the shape of the absorption spectra. In this publication we discuss different theoretical method to describe the spectra. We show that the comparatively simple time-convolutionless master equation is surprisingly well suited to model the absorption lines.
doi.org/10.1103/PhysRevB.101.155304
doi.org/10.1103/PhysRevB.101.155304
Semiconductors for Breakfast
Together with the DPG and the semiconductor division, we are organising the lecture series “Semiconductors for breakfast”. Every day we present an interesting talk about semiconductor physics until the start of the lectures. Everybody who is interested is welcome to attend. All talks will be steamed live.
Happy New Year!
We wish everybody a great and successful year 2020! The year starts with a number of conferences: The OPON workshop in Warsaw, the Winterschool in Mautendorf and the DPG spring meeting in Dresden. We are looking forward to great physics and interesting discussions in 2020!
2019
New publication on optimization of photonic structures for quantum technology
Photonic structures will play a crucial role in future chip designs for quantum technology, because photonic crystal waveguides and cavities can guide or confine light on the nanoscale. Such cavities can be used to in-couple light from single emitters into the chips. In this paper, we analyse photonic crystal nanobeam waveguides on substrate for the coupling to NV centers in nanodiamonds. We have modelled and compared three different design approaches. The structures were then fabricated and characterized by Philip Schrinner in the group of Carsten Schuck. We showed that the mode-matching design is best in terms of quality factor and mode volume.
Read more in the publication or the research news in OPTICS & PHOTONICS News.
New review paper on electron-phonon interaction in quantum dots
The electron-phonon interaction in quantum dots is different to the one in bulk semiconductors due to the nano structuring. In this review article the consequences of this special electron-phonon interaction for optically driven semicondcutor quantum dots are discussed. A special emphasis is given for the impact of the phonons on the photons emitted from the quantum dot. The review also gives an overview over the recent literature in this field. The paper was written in collaboration with Martin Axt from University of Bayreuth.
doi.org/10.1080/23746149.2019.1655478
doi.org/10.1080/23746149.2019.1655478
Jan Olthaus is out for a research stay in Cardiff
This month Jan Olthaus has left his desk and moved to Cardiff, Wales, for a 4- research stay in the group of Dr. Sang Soon Oh at Cardiff University, Wales. During his stay he will work on chirality and topology in nanophotonic structures. To finance his stay Jan successfully applied for a fellowship of the German Academic Exchange Service (DAAD). We wish Jan all the best for his stay in Cardiff and are looking forward for his return in January!
New review paper on the impact of phonons on optical control of quantum dots
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
24.08.2019
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.
New publication on a new detection scheme for electron dynamics
26.04.2019
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.
Neue Veröffentlichung über die Beschreibung von optischen Vortizes
25.02.2019
Licht kann neben der Polarisation auch einen Drehimpuls besitzen. Solches Licht hat dann eine helikale Wellenfront ähnlich wie bei einem Wirbel. Die theoretische Beschreibung dieser elektro-magnetischen Wellen kann aus den entsprechenden Potentialen hergeleitet werden. Dabei kann die Eichfreiheit genutzt werden, um verschiedene Strahlprofile zu beschreiben. Daher stellt sich die Frage, ob die verschiedenen Strahlprofile über eine Eichtransformation verknüpft sind. Wir diskutieren, dass dies nicht der Fall ist, und führen eine Klasse von optischen Vortizes ein, die die bekannten Strahlprofile wie Besselstrahlen oder Laguerre-Gauss-Strahlen verknüpft. Wir schlagen des Weiteren ein Experiment zur Messung der verschiedenen Strahlen vor.
Die Arbeit ist in Kollaboration mit Guillermo Quinteiro und Christian Schmiegelow aus Argentinien entstanden.
doi.org/10.1103/PhysRevA.99.023845
Congratulations to Dr. Richard Kerber
31.01.2019
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!
2018
New publication on nanophotonic control of optical signals stemming from atomically thin semiconductors
31.12.2018
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.
This work is a collaboration with Stefano Guazzotti and Ortwin Hess from Imperial College London as well as Andreas Pusch from UNSW Sydney.
doi.org/10.1103/PhysRevB.98.245307
Happy holidays
20.12.2018
We look back on a great year 2018 with fascinating physics,
interesting publications and many successful theses.
interesting publications and many successful theses.
We wish everybody a Merry Christmas
and a good start into the New Year!
and a good start into the New Year!
Neue Veröffentlichung über Dichroismus induziert durch Licht mit Drehmoment
05.12.2018
Wenn Licht mit Materie wechselwirkt, kann die Effizienz der Wechselwirkung stark von der Polarisation des Licht abhängen. Diese Abhängigkeit ist bekannt als Dichroismus. Betrachtet man nun Licht mit Drehmoment kann es auch hier eine starke Abhängigkeit der optischen Antwort auf den Drehmoment geben, also einen Drehmoment-induzierten Dichrosimus. In der Veröffentlichungen definieren wir den Drehmoment-induzierten Dichroismus und diskutieren sein Auftreten am Beispiel von gestapelten Nanostäbchen.
Die Arbeit ist in Kollaboration mit Jamie Fitzgerald und Ortwin Hess vom Imperial College London mit Sang Soon Oh von der Univervität Cardiff entstanden.
doi.org/10.1038/s42005-018-0088-2
New publication on Thermal Phonon Lasing
03.12.2018
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.
doi.org/10.1103/PhysRevA.98.053855
New publication on spatial control of carrier capture
15.11.2018
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.
doi.org/10.1103/PhysRevB.98.195411
Kevin Jürgens finishes Master Thesis
31.10.2018
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 on quantum effects in plasmonic spheres
10.10.2018
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.
doi.org/10.1103/PhysRevB.98.165411
New publication in 'New Journal of Physics' on the interaction of Archimedean Spirals with Twisted Light
02.10.2018
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.
doi.org/10.1088/1367-2630/aae105/meta
Start eines DFG geförderten Projekts
21.08.2018
Die Deutsche Forschungsgemeinschaft (DFG) hat unseren Antrag über die Simulation der optisch induzierten und räumlich aufgelösten Ladungsträgerdynamik in zweidimensionalen Halbleitern genehmigt. In dem Projekt planen wir die räumlich aufgelöste Ladungsträgerdynamik in zweidimensionalen Halbleitern auf ultrakurzen Zeit- und Längenskalen zu untersuchen. Mit Hilfe einer quantenmechanischen Beschreibung wollen wir verschiedene Wechselwirkungen wie die Coulomb- oder die Elektron-Phonon-Wechselwirkung betrachten und die zugehörigen optischen Signale beschreiben. Wir freuen uns sehr auf das Projekt, welches für die nächsten 3 Jahre läuft.
Jan Olthaus receives his Master's degree
10.04.2018
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.
Neue Veröffentlichung über den Einfluss der Coulomb Wechselwirkung auf Quantenpunkten-Spektren
01.03.2018
In einem Quantenpunkt können Exzitonen (Elektron-Loch-Paare) optisch angeregt werden. Ist noch ein zusätzliches Elektron (oder Loch) vorhanden, spricht man von einem geladenen Exziton oder Trion. Neben dem viel beachteten Grundzustands-Exziton spielen mittlerweile auch angeregte Exzitonen eine wichtige Rolle. Bei dieses Exzitonen mischen Coulomb-Effekte verschiedene Zustände, so dass komplexe Spektren entstehen. In dieser Publikation haben wir untersucht, wie die Coulomb-Wechselwirkung in Abhängigkeit der Ladung und Geometrie des Quantenpunkts die Spektren, insbesondere für angeregte Exzitonen, beeinflusst.
doi.org/10.1103/PhysRevB.97.075308
5th International Workshop on the Optical Properties of Nanostructures in Münster
20.02.2018
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 about ultrafast dynamics in quantum dots
16.01.2018
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.
The work was a joint project with the AG Leitenstorfer [en] from the University of Konstanz.
The work was a joint project with the AG Leitenstorfer [en] from the University of Konstanz.
dx.doi.org/10.1103/PhysRevB.97.045302
Magnus Molitor obtains his Master's degree
09.01.2018
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.
2017
New publication 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.
doi.org/10.1103/PhysRevB.96.245306
doi.org/10.1103/PhysRevB.96.245306
New publication 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.
This work is a collaboration with the experimental group of Jacek Kasprzak from Grenoble.
doi.org/10.1103/PhysRevB.96.165311
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.
doi.org/10.12693/APhysPolA.132.372
doi.org/10.12693/APhysPolA.132.372
Andreas Völker finishes his Bachelor thesis
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.
This work was done together with the group of Manfred Bayer from TU Dortmund, Germany.
doi.org/10.1088/1367-2630/aa78bf
New publication 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.
doi.org/10.1103/PhysRevB.95.241306
New publication 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.
doi.org/10.1103/PhysRevB.95.195305
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 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.
doi.org/10.1103/PhysRevB.95.161302
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.
doi.org/10.1103/PhysRevB.95.161302
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.
dx.doi.org/10.1103/PhysRevB.95.165302
New publication 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.
The work was performed together with the group of Manfred Bayer from the TU Dortmund, Germany, and samples from the University of Würzburg, Germany.
doi.org/10.1103/PhysRevLett.118.133901
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.
doi.org/10.1021/acsphotonics.6b00980
This work was done together with the group of Ortwin Hess from Imperial College London, UK.
doi.org/10.1021/acsphotonics.6b00980
New publication 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.
doi.org/10.1103/PhysRevB.95.125308