Universitätspublikationen
Refine
Year of publication
- 2021 (27) (remove)
Document Type
- Doctoral Thesis (27) (remove)
Has Fulltext
- yes (27) (remove)
Is part of the Bibliography
- no (27)
Keywords
- 2D materials (1)
- ALICE upgrade (1)
- Astrophysics (1)
- Bohmian mechanics (1)
- CBM Experiment (1)
- Color superconductivity (1)
- Control System (1)
- EPICS (1)
- FEBID (1)
- Gravitational Waves (1)
Institute
- Physik (27) (remove)
Terahertz (THz) technology is an emerging field that considers the radiation between microwave and far-infrared regions where the electronic and photonic technologies merge. THz generation and THz sensing technologies should fill the gap between photonics and electronics which is defined as a region where THz generation power and THz sensing capabilities are at a low technology readiness level (TRL). As one of the options for THz detection technology, field-effect transistors with integrated antennae were suggested to be used as THz detectors in the 1990s by M. Dyakonov and M. Shur from where the development of field-effect transistor-based detector began. In this work, various FET technologies are presented, such as CMOS, AlGaN/GaN, and graphene-based material systems and their further sensitivity enhancement in order to reach the performance of well-developed Schottky diode-based THz sensing technology. Here presented FET-based detectors were explored in a wide frequency range from 0.1 THz up to 5 THz in narrowband and broadband configurations.
For proper implementation of THz detectors, the well-defined characterization is of high importance. Therefore, this work overviews the characterization methods, establishes various definitions of detector parameters, and summarizes the state-of-the-art THz detectors. The electrical, optical, and cryogenic characterization techniques are also presented here, as well as the best results obtained by the development of the characterization methods, namely graphene FET stabilization, low-power THz source characterization for detector calibration, and technology development for cryogenic detection.
Following the discussion about the detector characterization, a wide range of THz applications, which were tested during the last four years of Ph.D. and conducted under the ITN CELTA project from HORIZON2020 program, are presented in this work. The studies began with spectroscopy applications and imaging and later developed towards hyperspectral imaging and even passive imaging of human body THz radiation. As various options for THz applications, single-pixel detectors as well as multi-pixel arrays are also covered in this work.
The conducted research shows that FET-based detectors can be used for spectroscopy applications or be easily adapted for the relevant frequency range. State-of-the-art detectors considered in this work reach the resonant performance below 20 pW/√Hz at 0.3 THz and 0.5 THz, as well as 404 pW/√Hz cross-sectional NEP at 4.75 THz. The broadband detectors show NEP as low as 25 pW/√Hz at around 0.6 THz for the best AlGaN/GaN design and 25 pW/√Hz around 1 THz for the best CMOS design. As one of the most promising applications, metamaterial characterization was tested using the most sensitive devices. Furthermore, one of the single-pixel devices and a multi-pixel array were tested as an engineering solution for a radio astronomy system called GREAT in a stratosphere observatory named SOFIA. The exploration of the autocorrelation technique using FET-based devices shows the opportunity to employ such detectors for direct detection of THz pulses without an interferometric measurement setup.
This work also considers imaging applications, which include near-field and far-field visualization solutions. A considerable milestone for the theory of FET technology was achieved when scanning near-field microscopy led to the visualization of plasma (or carrier density) waves in a graphene FET channel. Whereas another important milestone for the THz technology was achieved when a 3D scan of a mobile phone was performed under the far-field imaging mode. Even though the imaging was done through the phone’s plastic cover, the image displayed high accuracy and good feature recognition of the smartphone, inching the FET-based detector technology ever so close to practical security applications. In parallel, the multi-pixel array testing was carried out on 6x7 pixel arrays that have been implemented in configurable-size aperture and imaging configurations. The configurable aperture size allowed the easier detector focusing procedure and a better fit for the beam size of the incident radiation. The imaging has been tested on various THz sources and compared to the TeraSense 16x16 pixel array. The experimental results show the big advantage of the developed multi-pixel array against the used commercial technology.
Furthermore, two ultra-low-power applications have been successfully tested. The application on hyper-frequency THz imaging tested in the specially developed dual frequency comb and our detector system for 300 GHz radiation with 9 spectral lines led to outstanding imaging results on various materials. The passive imaging of human body radiation was conducted using the most sensitive broadband CMOS detector with a log-spiral antenna working in the 0.1 – 1.5 THz range and reaching the optical NEP of 42 pW/√Hz. The NETD of this device reaches 2.1 K and overcomes the performance limit of passive room-temperature imaging of the human body radiation, which was less than 10 K above the room temperature. This experiment opened a completely new field that was explored before only by the multiplier chain-based or thermal detectors.
...
The Compressed Baryonic Matter (CBM) Experiment will investigate heavy ion collisions and reactions at interaction rates of 100 kHz in a targeted energy range of up to 11 AGeV for systems such as gold-gold or lead-lead. It will be one of the major scientific experiments of the Facility for Antiproton and Ion Research in Europe (FAIR) currently under construction at the site of the GSI Helmholtzzentrum für Schwerionenforschung (GSI) in Darmstadt, Germany. CBM is going to be a fixed target experiment consisting of a superconducting magnet, multiple detectors of various types, and high-performance computing for online event reconstruction and selection. The detector closest to the interaction point of the experiment will be the Micro Vertex Detector (MVD). Consisting of four planar stations equipped with custom CMOS pixel sensors, it will allow to reconstruct the primary vertex with high precision and will help to reconstruct secondary vertices and identify particles originating from conversion in the detector material.
Due to the high interaction rates foreseen for CBM, understanding and minimizing systematic errors due to the detectors’ operating conditions will become all the more important to obtain significant measurement results, as statistical errors in the measurements of many observables are diminishing due to the enormous amount of data available.
Furthermore, the MVD will be the first detector based on CMOS pixel sensors used in a large physics experiment, that will be operated in vacuum. As a result, many aspects of the mechanical and electrical integration of the detector require careful testing and validation.
This thesis addresses both those challenges specifically for the Micro Vertex Detector with the development of a control system for the operation and validation of the MVD prototype “PRESTO” in vacuum. The prototype was selected as device under test as the final MVD is not yet built.
The developed control system helps a) to operate the prototype safely and keep it at the desired working point and b) to record important time-series data of the state of the detector prototype. Those two aspects allow the control system (which might later serve as a ‘blueprint’ for the final detector) to minimize the mentioned systematic errors as much as possible and to contribute to the understanding of remaining systematic errors using correlations with the time-series data. The controlled operation of the prototype in vacuum allowed to validate the integration concepts from a wide range of mechanical and electrical aspects in an endurance test for more than a year with 24/7 operation.
The prototype for this study itself was named “PRESTO” (standing for ‘PREcursor of the Second sTatiOn of the CBM-MVD’). It represents one quadrant of an MVD detector plane, equipped with a total of 15 MIMOSA-26 sensors on the front and back side of a carrier plate. Within this thesis, major parts of the prototype itself were designed. Custom ultra-thin flat flexible cables for data and power were designed and validated. Furthermore, the CNC-machined Aluminium heatsink to mount and cool the prototype design was refined to increase thermal performance. A custom vacuum feedthrough for a total of 21 flat ribbon cables was designed and fabricated. The read-out chain for MIMOSIS-26 was extended to cover a total of 8 sensors with a single and newer TRB-3 FPGA board and was set-up with the prototype. Vacuum equipment including chambers, hoses, pumps, valves and gauges were integrated to form a large vacuum testing system. A cooling circuit for the prototype was assembled comprising an external chiller, hoses, vacuum feedthroughs, as well as temperature, flow and pressure sensors.
The control system was developed to serve the needs of the prototype, while taking the requirements of the final MVD already into account. The main design goals of the control system are:
• compatibility with the other detectors and the overall CBM experiment,
• access to real-time measurements of all necessary parameters (‘process values’),
• reliable, fail-safe operation of the detector,
• recording of all time-series data (‘archiving’),
• cost efficiency and acceptance within the physics community,
• good usability for the users (‘operators’),
• long-term maintainability.
The recorded time-series data of the process variables (i.e. sensor readings) allow a post-measurement analysis of variations in the detector performance. The longterm archiving of all relevant system parameters is therefore of outstanding importance, which is why the software intended for this purpose – called “archiver” – was given special attention in this thesis.
For this reason in particular, it is necessary to implement a comprehensive control system that allows the detector to be operated safely under these conditions and cooled effectively. Before the start of this doctoral thesis, vigilant and extensively trained operators were always necessary for this. The control system that has been developed makes it possible that, after basic training, the detector can also be operated by a less specialised shift supervisor during measurement campaigns.
...
Die vorliegende Dissertation behandelt das Thema der Wechselstromleitfähigkeit nano-granularer Metalle, welche mit Hilfe der fokussierten elektronenstrahlinduzierten Direktabscheidung (FEBID) hergestellt wurden, sowie der dielektrischen Relaxation in metall-organischen Gerüstverbindungen (MOFs). Sie war eingebettet in das interdisziplinäre Projekt „Dielectric and Ferroelectric Surface-Mounted Metal-Organic Frameworks (SURMOFs) as Sensor Devices“ im Rahmen des DPG-Schwerpunktsprogramms „Coordination Networks: Building Blocks for Functional Systems“ (SPP 1928, COORNETs). Dabei verfolgt sie ein Sensorkonzept zur selektiven Detektion von Analytgasen. Der zentrale Erfolg der Arbeit besteht dabei in neuen Erkenntnissen über die Wechselstromleitfähigkeit nano-granularer Pt(C)-FEBID-Deponate. Die hierbei gewonnen Erkenntnisse können in Zukunft einen weiteren Baustein in der theoretischen Beschreibung dieses grundlegend interessanten und für sensorische Anwendungen wichtigen Teilgebiets der Festkörperphysik darstellen.
High-energy astrophysics plays an increasingly important role in the understanding of our universe. On one hand, this is due to ground-breaking observations, like the gravitational-wave detections of the LIGO and Virgo network or the black-hole shadow observations of the EHT collaboration. On the other hand, the field of numerical relativity has reached a level of sophistication that allows for realistic simulations that include all four fundamental forces of nature. A prime example of how observations and theory complement each other can be seen in the studies following GW170817, the first detection of gravitational waves from a binary neutron-star merger. The same detection is also the chronological starting point of this Thesis. The plethora of information and constraints on nuclear physics derived from GW170817 in conjunction with theoretical computations will be presented in the first part of this Thesis. The second part goes beyond this detection and prepares for future observations when also the high-frequency postmerger signal will become detectable. Specifically, signatures of a quark-hadron phase transition are discussed and the specific case of a delayed phase transition is analyzed in detail. Finally, the third part of this Thesis focuses on the inclusion of radiative transport in numerical astrophysics. In the context of binary neutron-star mergers, radiation in the form of neutrinos is crucial for realistic long-term simulations. Two methods are introduced for treating radiation: the approximate state-of-the-art two-moment method (M1) and the recently developed radiative Lattice-Boltzmann method. The latter promises
to be more accurate than M1 at a comparable computational cost. Given that most methods for radiative transport or either inaccurate or unfeasible, the derivation of this new method represents a novel and possibly paradigm-changing contribution to an accurate inclusion of radiation in numerical astrophysics.
This thesis explores the phase diagrams of the Nambu--Jona-Lasinio (NJL) and quark-meson (QM) model in the mean-field approximation and beyond. The focus lies in the investigation of the interplay between inhomogeneous chiral condensates and two-flavor color superconductivity.
In the first part of this thesis, we study the NJL model with 2SC diquarks in the mean-field approximation and determine the dispersion relations for quasiparticle excitations for generic spatial modulations of the chiral condensate in the presence of a homogeneous 2SC-diquark condensate, provided that the dispersion relations in the absence of color superconductivity are known. We then compare two different Ansätze for the chiral order parameter, the chiral density wave (CDW) and the real-kink crystal (RKC). For both Ansätze we find for specific diquark couplings a so-called coexistence phase where both the inhomogeneous chiral condensate and the diquark condensate coexist. Increasing the diquark coupling disfavors the coexistence phase in favor of a pure diquark phase.
On the other hand, decreasing the diquark coupling favors the inhomogeneous phase over the coexistence phase.
In the second part of this thesis the functional renormalization group is employed to study the phase diagram of the quark-meson-diquark model. We observe that the region of the phase diagram found in previous studies, where the entropy density takes on unphysical negative values, vanishes when including diquark degrees of freedom. Furthermore, we perform a stability analysis of the homogeneous phase and compare the results with those of previous studies. We find that an increasing diquark coupling leads to a smaller region of instability as the 2SC phase extends to a smaller chemical potential. We also find a region where simultaneously an instability occurs and a non-vanishing diquark condensate forms, which is an indication of the existence of a coexistence phase in accordance with the results of the first part of this work.
Bohmian mechanics as formulated originally in 1952, has been useful in the implementation of numerical methods applied to quantum mechanics. The scientific community though has had ever since a critical thought about it. Therefore, there are still points to be clarified and rectified. The two main problems are basically: Bohmian mechanics gives a privilege role to the position representation. Secondly, the current interpretation of Bohmian trajectories has been recently proven wrong.
In this context, in Chapter 2, new complex Bohmian quantities are defined; so that they allow the capacity to formulate Bohmian mechanics in any arbitrary continuous representation, for instance, the momentum representation. This Chapter is fully based on two articles, regarding the proposed complex Bohmian formulation and its extension into momentum space.
Chapter 3 deals with a redefinition and reinterpretation of the Bohmian trajectories from the handling of the continuity equation, this is done without any need of additional postulates or interpretations. Also, it is proved that Bohmian mechanics is actually more than a projective aspect of the Wigner function.
As a third point, Chapter 4 presents a sytematic treatment of the hydrodynamic scheme of Bohmian mechanics. Then, a brief summary of the transport equations in Bohmian mechanics is done. Next, a unified hydrodynamic treatment is found for the Bohmian mechanics. This treatment is useful to sketch, a Bohmian treatment to efficiently find the steady value of the transmission integral.
In Chapter 5 conclusions of this thesis are drawn.
The realization of a fast and robust closed orbit feedback (COFB) system for the on-ramp orbit correction at SIS18 synchrotron of FAIR project is reported in this thesis. SIS18 has some peculiar behaviors including on-ramp optics variation, very short lengths of the ramps (200 ms to 1 s) and a cycle-to-cycle variation of beam parameters. The realized fast COFB system being robust against above mentioned features of SIS18 is a first of its kind and the course to its realization led to some novel contributions in the field of closed orbit correction. A new method relying on the discrete Fourier transform (DFT)-based decomposition of the orbit response matrix (ORM) has been introduced, exploiting the symmetry in the arrangement of beam position monitors (BPMs) and the corrector magnets in the synchrotrons. A nearest-circulant approximation has also been introduced for synchrotrons having slight deviation from the symmetry, making the method applicable to a vast majority of synchrotrons. Moreover, the performance and the stability analysis of COFB systems in the presence of ORM mismatch between the synchrotron and the feedback controller is presented. The COFB systems are divided into slow and fast regimes and a new stability criterion consistent with measurements, is introduced. The practicality of the criterion is verified experimentally at COSY Jülich and is used for the analysis of various sources of ORM mismatch at SIS18. The commissioning of the SIS18 COFB system is also reported in detail which relies on Libera Hadron as the main hardware resource for the controller implementation. The on-ramp orbit correction is demonstrated for the horizontal plane of SIS18, for the disturbance rejection up to 600 Hz.