Physikalisches Institut
Refine
Is part of the Bibliography
- yes (787)
Year of publication
Document Type
- Doctoral Thesis (409)
- Journal article (362)
- Conference Proceeding (4)
- Preprint (4)
- Master Thesis (3)
- Review (3)
- Habilitation (1)
- Other (1)
Keywords
- Molekularstrahlepitaxie (31)
- Topologischer Isolator (31)
- Quantenpunkt (30)
- Hadron-Hadron scattering (experiments) (29)
- Kernspintomografie (28)
- Parton Distributions (25)
- Photoelektronenspektroskopie (23)
- MRI (20)
- NMR-Tomographie (19)
- Organischer Halbleiter (19)
Institute
- Physikalisches Institut (787)
- Medizinische Klinik und Poliklinik I (15)
- Institut für Physikalische und Theoretische Chemie (9)
- Institut für diagnostische und interventionelle Neuroradiologie (ehem. Abteilung für Neuroradiologie) (8)
- Institut für diagnostische und interventionelle Radiologie (Institut für Röntgendiagnostik) (7)
- Deutsches Zentrum für Herzinsuffizienz (DZHI) (6)
- Institut für Molekulare Infektionsbiologie (5)
- Institut für Organische Chemie (5)
- Institut für Theoretische Physik und Astrophysik (5)
- Rudolf-Virchow-Zentrum (5)
Sonstige beteiligte Institutionen
- Wilhelm-Conrad-Röntgen-Forschungszentrum für komplexe Materialsysteme (4)
- Universitätsklinikum Würzburg (3)
- Röntgen Center for Complex Material Systems (RCCM), Am Hubland, 97074 W¨urzburg, Germany (2)
- ATLAS Collaboration (1)
- Arizona State University, Tempe, Arizona, USA (1)
- Bavarian Center for Applied Energy Research (ZAE Bayern), 97074 Würzburg, Germany (1)
- Bavarian Center for Applied Energy Research e.V. (ZAE Bayern) (1)
- Bayerisches Zentrum für Angewandte Energieforschung e.V. (1)
- Biomedizinische NMR Forschungs GmbH am Max-Planck-Institut fuer biophysikalische Chemie (1)
- CERN (1)
ResearcherID
- D-1250-2010 (1)
- N-7500-2014 (1)
Strong light matter coupling between excitons and microcavity photons, as described in the framework of cavity quantum electrodynamics, leads to the hybridization of light and matter excitations. The regime of collective strong coupling arises, when various excitations from different host media are strongly coupled to the same optical resonance. This leads to a well-controllable admixture of various matter components in three hybrid polariton modes. Here, we study a cavity device with four embedded GaAs quantum wells hosting excitons that are spectrally matched to the A-valley exciton resonance of a MoSe\(_{2}\) monolayer. The formation of hybrid polariton modes is evidenced in momentum resolved photoluminescence and reflectivity studies. We describe the energy and k-vector distribution of exciton-polaritons along the hybrid modes by a thermodynamic model, which yields a very good agreement with the experiment.
We report here that reconstruction on (100), (1lIlA, and (1l1lB CdTe surfaces is either C(2X2), (2X2), and (l X I) or (2X I), (l X I), and (l X I) when they are Cd or Te stabilized, respectively. There is a mixed region between Cd and Te stabilization in which the reflected high-energy electron-diffraction (RHEED) patterns contain characteristics of both Cd- and Te-stabilized surfaces. We have also found that the Cd-to-Te ratio of the x-ray photoelectron intensities of their 3d\(_{3/ 2}\) core levels is about 20% larger for a Cd-stabilized (1lIlA, (1lIlB, or (100) CdTe surface than for a Te-stabilized one. According to a simple model calculation, which was normalized by means of the photoelectron intensity ratio of a Cd-stabilized (lll)A and aTe-stabilized (1l1lB CdTe surface, the experimental data for CdTe surfaces can be explained by a linear dependence of the photoelectron-intensity ratio on the fraction of Cd in the uppermost monatomic layer. This surface composition can be correlated with the surface structure, i.e., the corresponding RHEED patterns. This correlation can in turn be employed to determine Te and Cd evaporation rates. The Te reevaporation rate is increasingly slower for the Te-stabilized (Ill) A, (l1l)B, and (100) surfaces, while the opposite is true for Cd from Cd-stabilized (Ill) A and (Ill)B surfaces. In addition, Te is much more easily evaporated from all the investigated surfaces than is Cd, if the substrate is kept at normal molecular-beam-epitaxy growth temperatures ranging from 2oo·C to 300 ·C.
The surface sublimation of Cd and Te atoms from the zinc blende (111)A CdTe surface has been investigated in detail by reflection high energy electron diffraction and x-ray photoelectron spectroscopy. These experiments verify that Te is much easier to evaporate than Cd. The experimental value for the Te activation energy from a Te stabilized (111)A CdTe surface is 1.41 ±0.1O eV, which is apparently inconsistent with recent theoretical results.
We have investigated oxygen on CdTe substrates by means of x-ray photoelectron spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED). A Te oxide layer that was at least 15 A thick was found on the surface of as-delivered CdTe substrates that were mechanically polished. This oxide is not easily evaporated at temperatures lower than 350°C. Furthermore, heating in air, which further oxidizes the CdTe layer, should be avoided. Etching with HCI acid (15% HCl) for at least 20 s and then rinsing with de-ionized water reduces the Te oxide layer on the surface down to 4% of a monoatomic layer. However, according to XPS measurements of the 0 Is peak, 20%-30% of a monoatomic layer of oxygen remains on the surface, which can be eliminated by heating at temperatures ranging between 300 and 340 cC. The RHEED patterns for a molecular beam epitaxially (MBE)-grown CdTe film on a (lOO) CdTe substrate with approximately one monoatomic layer of oxidized Te on the surface lose the characteristics of the normal RHEED pattems for a MBE-grown CdTe film on an oxygen-free CdTe substrate.
The influence of different CdZnTe substrate treatments prior to II-VI molecular beam epitaxial growth on surface stoichiometry, oxygen, and carbon contamination has been studied using x-ray photoelectron spectroscopy and reflection high energy electron diffraction. Heating the substrate at 300 °C can eliminate oxygen contamination, but cannot completely remove carbon from the surface. Heating at higher temperatures decreases the carbon contamination only slightly, while increasing the Zn-Cd ratio on the surface considerably. The magnitude of the latter effect is surprising and is crucial when one is using lattice matched CdZnTe (Zn 4%) substrates.
We report the results of a detailed investigation on the Te-stabilized (2 x 1) and the Cdstabilized c( 2 X 2) surfaces of ( 100) CdTe substrates. The investigation demonstrates for the first time that both laser illumination and, to a greater extent, high-energy electron irradiation increase the Te desorption and reduce the Cd desorption from ( 100) CdTe surfaces. Thus it is possible by choosing the proper growth temperature and photon or electron fluxes to change the surface reconstruction from the normally Te-stabilized to a Cd-stabilized phase.
We demonstrate single-photon emission with a low probability of multiphoton events of 5% in the C-band of telecommunication spectral range of standard silica fibers from molecular beam epitaxy grown (100)-GaAs-based structure with InAs quantum dots (QDs) on a metamorphic buffer layer. For this purpose, we propose and implement graded In content digitally alloyed InGaAs metamorphic buffer layer with maximal In content of 42% and GaAs/AlAs distributed Bragg reflector underneath to enhance the extraction efficiency of QD emission. The fundamental limit of the emission rate for the investigated structures is 0.5 GHz based on an emission lifetime of 1.95 ns determined from time-resolved photoluminescence. We prove the relevance of a proposed technology platform for the realization of non-classical light sources in the context of fiber-based quantum communication applications.
Super-resolution fluorescence imaging based on inglemolecule localization relies critically on the availability of efficient processing algorithms to distinguish, identify, and localize emissions of single fluorophores. In multiple current applications, such as threedimensional, time-resolved or cluster imaging, high densities of fluorophore emissions are common. Here, we provide an analytic tool to test the performance and quality of localization microscopy algorithms and demonstrate that common algorithms encounter difficulties for samples with high fluorophore density. We demonstrate that, for typical single-molecule localization microscopy methods such as dSTORM and the commonly used rapidSTORM scheme, computational precision limits the acceptable density of concurrently active fluorophores to 0.6 per square micrometer and that the number of successfully localized fluorophores per frame is limited to 0.2 per square micrometer.
The control of quantum mechanical processes, especially the selective manipulation of photochemical reactions by shaped fs laser pulses was successfully demonstrated in many experiments in the fields of physics, chemistry and biology. In this work, attention is directed to the control of two systems that mark a bridge to real synthetic chemistry. In a liquid phase environment the outcome of the photo-induced Wolff rearrangement of an industrially relevant diazonaphthoquinone compound, normally used in photoresists (e.g. Novolak) was optimized using shaped fs laser pulses. In the second series of experiments chemical reactions on a catalyst metal surface which comprise laser induced molecular bond formation channels were selectively manipulated for the first time. The control of liquid phase reactions necessitates adequate spectroscopic signals that are characteristic for the formed product species. Therefore, a pump-probe setup for transient absorption spectroscopy in the mid-infrared for the purpose of investigating ultrafast structural changes of molecules during photoreactions was constructed. This versatile setup enables to monitor structural changes of molecules in the liquid phase and to find appropriate feedback signals for the control of these processes. Prior to quantum control experiments, the photoinduced Wolff-rearrangement reaction of 2-diazo-1-naphthoquinone (DNQ) dissolved in water and methanol was thoroughly investigated. Steady state absorption measurements in the mid-infrared in combination with quantum chemical density functional theory (DFT) calculations revealed the characteristic vibrational bands of DNQ and of possible products. A mid-infrared transient absorption study was performed, to illuminate the structural dynamics of the ultrafast rearrangement reaction of DNQ. The experimental observations indicate, that the Wolff rearrangement reaction of DNQ proceeds within 300 fs. A model for the relaxation dynamics of the ketene photoproduct and DNQ after photoexcitation can be deduced that fits the measured data very well. The object of the quantum control experiments on DNQ was the improvement of the ketene yield. It was shown that the ketene formation after Wolff rearrangement of DNQ is very sensitive to the shape of the applied excitation laser pulses. The variation of single parameters, like the linear chirp as well as the pulse separation of colored double pulses lead to the conclusion that the well known intrapulse dumping mechanism is responsible for the impact of the frequency ordering within the excitation pulse on the photoproduct yield. Adaptive optimizations using a closed learning loop basically lead to the same result. Adaptive fs quantum control was also applied to surface reactions on a catalyst metal surface for the first time. Therefore, the laser-induced catalytic reactions of carbon monoxide (CO) and hydrogen (H2) on a Pd(100) single crystal surface were studied. This photochemical reaction initiated with fs laser pulses has not been observed before. Several product molecules could be synthesized, among them also species (e.g. CH^3+) for whose formation three particles are involved. The systematic variation of different parameters showed that the reactions are sensitive to the catalyst surface, the composition of the adsorbate and to the laser properties. A pump-probe study revealed that they occur on an ultrafast time scale. These catalytic surface reactions were then investigated and improved with phaseshaped fs laser pulses. By applying a feedback optimal control scheme, the reaction outcome could be successfully manipulated and the ratio of different reaction channels could be selectively controlled. Evidence has been found that the underlying control mechanism is nontrivial and sensitive to the specific conditions on the surface. The experiments shown here represent the first successful experiment on adaptive fs quantum control of a chemical reaction between adsorbate molecules on a surface. In contrast to previous quantum control experiments, reaction channels comprising the formation of new molecular bonds rather than the cleavage of already existing bonds are controlled. This work successfully showed that quantum control can be extended to systems closer to situations encountered in synthetic chemistry as was demonstrated in the two examples of the optimization of a complicated rearrangement reaction and the selective formation of chemical bonds with shaped fs laser pulses.