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Institute
- Physikalisches Institut (44) (remove)
Sonstige beteiligte Institutionen
- Wilhelm-Conrad-Röntgen-Forschungszentrum für komplexe Materialsysteme (2)
- Arizona State University, Tempe, Arizona, USA (1)
- Department of Cellular Therapies, University of Navarra, Pamplona, Spain (1)
- Department of X-ray Microscopy, University of Würzburg, Würzburg, Germany (1)
- Fraunhofer-Institute for Applied Optics and Precision Engineering IOF Jena, Germany (1)
- Friedrich Schiller University Jena, Germany (1)
- Max Planck School of Photonics Jena, Germany (1)
- National Institute for Materials Science, Tsukuba, Japan (1)
- Siemens Corporate Technology Munich (1)
- University of Oldenburg, Germany (1)
Laser spectroscopic gas sensing has been applied for decades for several applications
as atmospheric monitoring, industrial combustion gas analysis or fundamental research.
The availability of new laser sources in the mid-infrared opens the spectral fingerprint
range to the technology where multiple molecules possess their fundamental ro-vibrational
absorption features that allow very sensitive detection and accurate discrimination of
the species. The increasing maturity of quantum cascade lasers that cover this highly
interesting spectral range motivated this research to gain fundamental knowledge about
the spectra of hydrocarbon gases in pure composition and in complex mixtures as they
occur in the petro-chemical industry. The long-term target of developing accurate and fast
hydrocarbon gas analyzers, capable of real-time operation while enabling feedback-loops,
would lead to a paradigm change in this industry.
This thesis aims to contribute to a higher accuracy and more comprehensive understanding
of the sensing of hydrocarbon gas mixtures. This includes the acquisition of yet
unavailable high resolution and high accuracy reference spectra of the respective gases,
the investigation of their spectral behavior in mixtures due to collisional broadening of
their transitions and the verification of the feasibility to quantitatively discriminate the
spectra when several overlapping species are simultaneously measured in gas mixtures.
To achieve this knowledge a new laboratory environment was planned and built up to
allow for the supply of the individual gases and their arbitrary mixing. The main element
was the development of a broadly tunable external-cavity quantum cascade laser based
spectrometer to record the required spectra. This also included the development of a new
measurement method to obtain highly resolved and nearly gap-less spectral coverage as
well as a sophisticated signal post-processing that was crucial to achieve the high accuracy
of the measurements. The spectroscopic setup was used for a thorough investigation of
the spectra of the first seven alkanes as of their mixtures. Measurements were realized
that achieved a spectral resolution of 0.001 cm-1 in the range of 6-11 µm while ensuring an
accuracy of 0.001 cm-1 of the spectra and attaining a transmission sensitivity of 2.5 x 10-4
for long-time averaging of the acquired spectra.
These spectral measurements accomplish a quality that compares to state-of-the art
spectral databases and revealed so far undocumented details of several of the investigated
gases that have not been measured with this high resolution before at the chosen measurement
conditions. The results demonstrate the first laser spectroscopic discrimination of a
seven component gas mixture with absolute accuracies below 0.5 vol.% in the mid-infrared
provided that a sufficiently broad spectral range is covered in the measurements. Remaining
challenges for obtaining improved spectral models of the gases and limitations of the
measurement accuracy and technology are discussed.
In this work, a bridge was built between the so-far separate fields of spin defects and 2D systems: for the first time, an optically addressable spin defect (VB-) in a van der Waals material (hexagonal boron nitride) was identified and exploited. The results of this thesis are divided into three topics as follows:
1.) Identification of VB-:
In the scope of this chapter, the defect ,the negatively charged boron vacancy VB-, is identified and characterized. An initialization and readout of the spin state can be demonstrated optically at room temperature and its spin Hamiltonian contributions can be quantified.
2.) Coherent Control of VB-:
A coherent control is required for the defect to be utilized for quantum applications, which
Organic dyes offer unique properties for their application as room temperature single photon emitters. By means of photon‐correlation, the emission characteristics of macrocyclic para‐xylylene linked perylene bisimide (PBI) trimers and tetramers dispersed in polymethyl methacrylate matrices are analyzed. The optical data indicate that, despite of the strong emission enhancement of PBI trimers and tetramers according to their larger number of chromophores, the photon‐correlation statistics still obeys that of single photon emitters. Moreover, driving PBI trimers and tetramers at higher excitation powers, saturated emission behavior for monomers is found while macrocycle emission is still far‐off saturation but shows enhanced fluctuations. This observation is attributed to fast singlet–singlet annihilation, i.e., faster than the radiative lifetime of the excited S1 state, and the enlarged number of conformational arrangements of multichromophores in the polymeric host. Finally, embedding trimeric PBI macrocycles in active organic light‐emitting diode matrices, electrically driven bright fluorescence together with an indication for antibunching at room temperature can be detected. This, so far, has only been observed for phosphorescent emitters that feature much longer lifetimes of the excited states and, thus, smaller radiative recombination rates. The results are discussed in the context of possible effects on the g(2) behavior of molecular emitters.
Point-spread function engineering for single-molecule localization microscopy in brain slices
(2022)
Single-molecule localization microscopy (SMLM) is the method of choice to study biological specimens on a nanoscale level. Advantages of SMLM imply its superior specificity due to targeted molecular fluorescence labeling and its enhanced tissue preservation compared to electron microscopy, while reaching similar resolution. To reveal the molecular organization of protein structures in brain tissue, SMLM moves to the forefront: Instead of investigating brain slices with a thickness of a few µm, measurements of intact neuronal assemblies (up to 100 µm in each dimension) are required. As proteins are distributed in the whole brain volume and can move along synapses in all directions, this method is promising in revealing arrangements of neuronal protein markers. However, diffraction-limited imaging still required for the localization of the fluorophores is prevented by sample-induced distortion of emission pattern due to optical aberrations in tissue slices from non-superficial planes. In particular, the sample causes wavefront dephasing, which can be described as a summation of Zernike polynomials. To recover an optimal point spread function (PSF), active shaping can be performed by the use of adaptive optics. The aim of this thesis is to establish a setup using a deformable mirror and a wavefront sensor to actively shape the PSF to correct the wavefront phases in a super-resolution microscope setup. Therefore, fluorescence-labeled proteins expressed in different anatomical regions in brain tissue will be used as experiment specimen. Resolution independent imaging depth in slices reaching tens of micrometers is aimed.
Background
Fast and accurate T1ρ mapping in myocardium is still a major challenge, particularly in small animal models. The complex sequence design owing to electrocardiogram and respiratory gating leads to quantification errors in in vivo experiments, due to variations of the T\(_{1p}\) relaxation pathway. In this study, we present an improved quantification method for T\(_{1p}\) using a newly derived formalism of a T\(_{1p}\)\(^{*}\) relaxation pathway.
Methods
The new signal equation was derived by solving a recursion problem for spin-lock prepared fast gradient echo readouts. Based on Bloch simulations, we compared quantification errors using the common monoexponential model and our corrected model. The method was validated in phantom experiments and tested in vivo for myocardial T\(_{1p}\) mapping in mice. Here, the impact of the breath dependent spin recovery time T\(_{rec}\) on the quantification results was examined in detail.
Results
Simulations indicate that a correction is necessary, since systematically underestimated values are measured under in vivo conditions. In the phantom study, the mean quantification error could be reduced from − 7.4% to − 0.97%. In vivo, a correlation of uncorrected T\(_{1p}\) with the respiratory cycle was observed. Using the newly derived correction method, this correlation was significantly reduced from r = 0.708 (p < 0.001) to r = 0.204 and the standard deviation of left ventricular T\(_{1p}\) values in different animals was reduced by at least 39%.
Conclusion
The suggested quantification formalism enables fast and precise myocardial T\(_{1p}\) quantification for small animals during free breathing and can improve the comparability of study results. Our new technique offers a reasonable tool for assessing myocardial diseases, since pathologies that cause a change in heart or breathing rates do not lead to systematic misinterpretations. Besides, the derived signal equation can be used for sequence optimization or for subsequent correction of prior study results.
Quantum point contacts (QPCs) are one-dimensional constrictions in an otherwise extended two-dimensional electron or hole system. Since their first realization in GaAs based two-dimensional electron gases, QPCs have become basic building blocks of mesoscopic physics and are used in manifold experimental contexts. A so far unrealized goal however is the implementation of QPCs in the new material class of two-dimensional topological insulators, which host the emergence of the so-called quantum spin Hall (QSH) effect. The latter is characterized by the formation of conducting one-dimensional spin-polarized states at the device edges, while the bulk is insulating. Consequently, an implemented QPC technology can be utilized to bring the QSH edge channels in close spatial proximity, thus for example enabling the study of interaction effects between the edge states. The thesis at hand describes the technological realization as well as the subsequent experimental characterization and analysis of QPCs in a QSH system for the first time.
After an introduction is given in Chapter 1, the subsequent Chapter 2 starts with discussing the peculiar band structure of HgTe. The emergence of the QSH phase for HgTe quantum wells with an inverted band structure is explained. For the band inversion to occur, the quantum wells have to exhibit a well thickness d_QW above a critical value (d_QW > d_c = 6.3 nm). Subsequently, the concept of QPCs is explicated and the corresponding transport behaviour is analytically described. Following the discussion of relevant constraints when realizing a QPC technology in a QSH system, a newly developed lithography process utilizing a multi-step wet etching technique for fabricating QPC devices based on HgTe quantum wells is presented. Transport measurements of exemplary devices show the expected conductance quantization in steps of ΔG ≈ 2e^2/h within the conduction band for a topological as well as for a trivial (d_QW < d_c) QPC. For the topological case, the residual conductance within the bulk band gap saturates at G_QSH ≈ 2e^2/h due to presence of the QSH state, while it drops to G ≈ 0 for the trivial device. Moreover, bias voltage dependent measurements of the differential conductance of an inverted sample provide explicit proof of the unperturbed coexistence of topological and trivial transport modes.
In a next step, Chapter 3 describes the emergence of a QSH interferometer state in narrow QPC devices with a quantum well thickness of d_QW = 7 nm. Presented band structure calculations reveal that the spatial extension of the QSH edge states depends on the position of the Fermi energy within the bulk band gap. As a consequence, reservoir electrons with randomized spin couple to both edge channels with the same probability under certain conditions, thus causing the formation of a QSH ring. A straightforward model capturing and specifying the occurrence of such a QSH interferometer is provided as well as substantiated by two experimental plausibility checks. After relevant quantum phases are theoretically introduced, the discussion of the obtained data reveals the accumulation of an Aharonov-Bohm phase, of a dynamical Aharonov-Casher phase as well as of a spin-orbit Berry phase of π in appropriate QPC devices. These results are consistent with analytic model considerations.
The last part of this thesis, Chapter 4, covers the observation of an unexpected conductance pattern for QPC samples fabricated from quantum wells with d_QW = 10.5 nm. In these devices, an anomalous plateau at G ≈ e^2/h = 0.5 x G_QSH emerges in addition to the QSH phase entailed residual conductance of G_QSH ≈ 2e^2/h. This so-called 0.5 anomaly occurs only for a specific interval of QPC width values, while it starts to get lost for too large sample widths. Furthermore, presented temperature and bias voltage dependent measurements insinuate that the emergence of the 0.5 anomaly is related to a gapped topological state. Additional characterization of this peculiar transport regime is provided by the realization of a novel device concept, which integrates a QPC within a standard Hall bar geometry. The results of the experimental analysis of such a sample link the occurrence of the 0.5 anomaly to a backscattered QSH channel. Thus, following a single particle perspective argumentation, it is reasoned that only one edge channel is transmitted in the context of the 0.5 anomaly. Two theoretic models possibly explaining the emergence of the 0.5 anomaly -- based on electron-electron interactions -- are discussed.
To conclude, the implementation of a working QPC technology in a QSH system represents a paramount development in the context of researching two-dimensional topological insulators and enables a multitude of future experiments. QPC devices realized in a QSH system are for example envisaged to allow for the detection of Majorana fermions and parafermions. Furthermore, the reported formation of a QSH interferometer state in appropriate QPC devices is of high interest. The observed dynamical Aharonov-Casher phase in the QSH regime enables a controllable modulation of the topological conductance, thus providing the conceptual basis for a topological transistor. Moreover, due to the resilience of geometric phases against dephasing, the presence of a spin-orbit Berry phase of π represents a promising perspective with regard to possible quantum computation concepts. Besides that, the transmission of only one QSH edge channel due to the emergence of the 0.5 anomaly is equivalent to 100 % spin polarization, which is an essential ingredient for realizing spintronic applications. Hence, the thesis at hand covers the experimental detection of three effects of fundamental importance in the context of developing new generations of logic devices -- based on QPCs fabricated from topological HgTe quantum wells.
Realization and Spectroscopy of the Quantum Spin Hall Insulator Bismuthene on Silicon Carbide
(2022)
Topological matter is one of the most vibrant research fields of contemporary solid state physics since the theoretical prediction of the quantum spin Hall effect in graphene in 2005. Quantum spin Hall insulators possess a vanishing bulk conductivity but symmetry-protected, helical edge states that give rise to dissipationless charge transport.
The experimental verification of this exotic state of matter in 2007 lead to a boost of research activity in this field, inspired by possible ground-breaking future applications.
However, the use of the quantum spin Hall materials available to date is limited to cryogenic temperatures owing to their comparably small bulk band gaps.
In this thesis, we follow a novel approach to realize a quantum spin Hall material with a large energy gap and epitaxially grow bismuthene, i.e., Bi atoms adopting a honeycomb lattice, in a \((\sqrt{3}\times\sqrt{3})\) reconstruction on the semiconductor SiC(0001). In this way, we profit both from the honeycomb symmetry as well as the large spin-orbit coupling of Bi, which, in combination, give rise to a topologically non-trivial band gap on the order of one electronvolt.
An in-depth theoretical analysis demonstrates that the covalent bond between the Si and Bi atoms is not only stabilizing the Bi film but is pivotal to attain the quantum spin Hall phase.
The preparation of high-quality, unreconstructed SiC(0001) substrates sets the basis for the formation of bismuthene and requires an extensive procedure in ultra-pure dry H\(_2\) gas. Scanning tunneling microscopy measurements unveil the (\(1\times1\)) surface periodicity and smooth terrace planes, which are suitable for the growth of single Bi layers by means of molecular beam epitaxy. The chemical configuration of the resulting Bi film and its oxidation upon exposure to ambient atmosphere are inspected with X-ray photoelectron spectroscopy.
Angle-resolved photoelectron spectroscopy reveals the excellent agreement of probed and calculated band structure. In particular, it evidences a characteristic Rashba-splitting of the valence bands at the K point. Scanning tunneling spectroscopy probes signatures of this splitting, as well, and allows to determine the full band gap with a magnitude of \(E_\text{gap}\approx0.8\,\text{eV}\).
Constant-current images and local-density-of-state maps confirm the presence of a planar honeycomb lattice, which forms several domains due to different, yet equivalent, nucleation sites of the (\(\sqrt{3}\times\sqrt{3}\))-Bi reconstruction.
Differential conductivity measurements demonstrate that bismuthene edge states evolve at atomic steps of the SiC substrate. The probed, metallic local density of states is in agreement with the density of states expected from the edge state's energy dispersion found in density functional theory calculations - besides a pronounced dip at the Fermi level.
By means of temperature- and energy-dependent tunneling spectroscopy it is shown that the spectral properties of this suppressed density of states are successfully captured in the framework of the Tomonaga-Luttinger liquid theory and most likely originate from enhanced electronic correlations in the edge channel.
Resonant tunneling diode photodetectors appear to be promising architectures with a simple design for mid-infrared sensing operations at room temperature. We fabricated resonant tunneling devices with GaInAsSb absorbers that allow operation in the 2–4 μm range with significant electrical responsivity of 0.97 A/W at 2004 nm to optical readout. This paper characterizes the photosensor response contrasting different operational regimes and offering a comprehensive theoretical analysis of the main physical ingredients that rule the sensor functionalities and affect its performance. We demonstrate how the drift, accumulation, and escape efficiencies of photogenerated carriers influence the electrostatic modulation of the sensor's electrical response and how they allow controlling the device's sensing abilities.
Optical quantum information science and technologies require the capability to generate, control, and detect single or multiple quanta of light. The need to detect individual photons has motivated the development of a variety of novel and refined single-photon detectors (SPDs) with enhanced detector performance. Superconducting nanowire single-photon detectors (SNSPDs) and single-photon avalanche diodes (SPADs) are the top-performer in this field, but alternative promising and innovative devices are emerging. In this review article, we discuss the current state-of-the-art of one such alternative device capable of single-photon counting: the resonant tunneling diode (RTD) single-photon detector. Due to their peculiar photodetection mechanism and current-voltage characteristic with a region of negative differential conductance, RTD single-photon detectors provide, theoretically, several advantages over conventional SPDs, such as an inherently deadtime-free photon-number resolution at elevated temperatures, while offering low dark counts, a low timing jitter, and multiple photon detection modes. This review article brings together our previous studies and current experimental results. We focus on the current limitations of RTD-SPDs and provide detailed design and parameter variations to be potentially employed in next-generation RTD-SPD to improve the figure of merits of these alternative single-photon counting devices. The single-photon detection capability of RTDs without quantum dots is shown.
Die Fluoreszenzmikroskopie ist eine vielseitig einsetzbare Untersuchungsmethode für biologische Proben, bei der Biomoleküle selektiv mit Fluoreszenzfarbstoffen markiert werden, um sie dann mit sehr gutem Kontrast abzubilden. Dies ist auch mit mehreren verschiedenartigen Zielmolekülen gleichzeitig möglich, wobei üblicherweise verschiedene Farbstoffe eingesetzt werden, die über ihre Spektren unterschieden werden können.
Um die Anzahl gleichzeitig verwendbarer Färbungen zu maximieren, wird in dieser Arbeit zusätzlich zur spektralen Information auch das zeitliche Abklingverhalten der Fluoreszenzfarbstoffe mittels spektral aufgelöster Fluoreszenzlebensdauer-Mikroskopie (spectrally resolved fluorescence lifetime imaging microscopy, sFLIM) vermessen. Dazu wird die Probe in einem Konfokalmikroskop von drei abwechselnd gepulsten Lasern mit Wellenlängen von 485 nm, 532nm und 640nm angeregt. Die Detektion des Fluoreszenzlichtes erfolgt mit einer hohen spektralen Auflösung von 32 Kanälen und gleichzeitig mit sehr hoher zeitlicher Auflösung von einigen Picosekunden. Damit wird zu jedem detektierten Fluoreszenzphoton der Anregungslaser, der spektrale Kanal und die Ankunftszeit registriert. Diese detaillierte multidimensionale Information wird von einem Pattern-Matching-Algorithmus ausgewertet, der das Fluoreszenzsignal mit zuvor erstellten Referenzpattern der einzelnen Farbstoffe vergleicht. Der Algorithmus bestimmt so für jedes Pixel die Beiträge der einzelnen Farbstoffe.
Mit dieser Technik konnten pro Anregungslaser fünf verschiedene Färbungen gleichzeitig dargestellt werden, also theoretisch insgesamt 15 Färbungen. In der Praxis konnten mit allen drei Lasern zusammen insgesamt neun Färbungen abgebildet werden, wobei die Anzahl der Farben vor allem durch die anspruchsvolle Probenvorbereitung limitiert war. In anderen Versuchen konnte die sehr hohe Sensitivität des sFLIM-Systems genutzt werden, um verschiedene Zielmoleküle voneinander zu unterscheiden, obwohl sie alle mit demselben Farbstoff markiert waren. Dies war möglich, weil sich die Fluoreszenzeigenschaften eines Farbstoffmoleküls geringfügig in Abhängigkeit von seiner Umgebung ändern. Weiterhin konnte die sFLIM-Technik mit der hochauflösenden STED-Mikroskopie (STED: stimulated emission depletion) kombiniert werden, um so hochaufgelöste zweifarbige Bilder zu erzeugen, wobei nur ein einziger gemeinsamer STED-Laser benötigt wurde.
Die gleichzeitige Erfassung von mehreren photophysikalischen Messgrößen sowie deren Auswertung durch den Pattern-Matching-Algorithmus ermöglichten somit die Entwicklung von neuen Methoden der Fluoreszenzmikroskopie für Mehrfachfärbungen.