Refine
Year of publication
Document Type
- Doctoral Thesis (21)
- diplomthesis (1)
Has Fulltext
- yes (22)
Is part of the Bibliography
- no (22)
Keywords
- Laser (2)
- terahertz (2)
- 2D materials (1)
- Absolute Phase (1)
- Aluminiumarsenid (1)
- Antenne (1)
- Brechungsindex (1)
- Carrier Envelope Phase (1)
- E-beam induced deposition (1)
- EBID (1)
Institute
- Physik (22)
Terahertz (THz) radiation lies between the micro and far-infrared range in the electromagnetic spectrum. Compared with microwave and millimeter waves, it has a larger signal bandwidth and extremely narrow antenna beam. Thus, it is easier to achieve high-resolution for imaging and detection applications. The unique properties, such as penetration for majority non-polar materials, non-ionizing characteristic and the spectral fingerprint of materials, makes THz imaging an appealing artifice in the military, biomedical, astronomical communications, and other areas. However, THz radiation’s current low power level and detection sensitivity block THz imaging system from including fewer optical elements than the visible or infrared range. This leads to imaging resolution, contrast, and imaging field of view degenerate and makes the aberration more serious. THz imaging based on the space Fourier spectrum detection is developed in this thesis to achieve high-quality imaging. The main concept of Fourier imaging is by recording the field distribution in the Fourier plane (focal plane) of the imaging system; the information of the target is obtained. The numerical processing method is needed to extract the amplitude and phase information of the imaged target. With additional process, three-dimensional (3D) information can be obtained based on the phase information. The novel recording and reconstructing ways of the Fourier imaging system enables it to have a higher resolution, better contrast, and broader field of view than conventional imaging systems such as microscopy and plane to plane telescopic imaging system.
The work presented in this thesis consists of two imaging systems, one is working at 300 GHz based on the fundamental heterodyne detection of the THz radiation, the other is operated at 600 GHz by utilizing the sub harmonic heterodyne detection technique. The realization and test of the heterodyne detection are based on the THz antenna-coupled field-effect transistor (TeraFET) detector developed by Dr. Alvydas Lisauskas. Both systems use two synchronized electronic multiplier chains to radiate the THz waves. One radiation works as the local oscillator (LO), the other works as illumination with a slight frequency shift, the radiations are mixed on the detector scanning in the Fourier plane to record the complex Fourier spectrum of the imaged target. The LO has the same frequency range as the illuminating radiation for fundamental heterodyne detection but half the frequency range for the sub-harmonic heterodyne detection. The 2-mm resolution, 60-dB contrast, and 5.5-cm diameter imaging area at 300 GHz and the of 500-μm resolution, 40-dB contrast, and 3.5-cm diameter imaging area at 600 GHz are achieved (the 300-GHz illuminating radiation has the approximate power of 600 μW , the 600-GHz illuminating radiation has the approximate power of 60 μW ).
The thesis consists of 6 parts. After the introduction, the second chapter expands on the topic of Fourier optics from a theoretical point of view and the simulations of the Fourier imaging system. First, the theory of the electromagnetic field propagation in free space and through an optical system are investigated to elicit the Fourier transform function of the imaging system. The simulation is used for theoretical considerations and the implementation of a Fourier optic script that allows for numerical investigations on reconstruction. The preliminary imaging field of view and resolution are also demonstrated. The third chapter describes the Fourier imaging system at 300 GHz based on the fundamental heterodyne detection, including the experimental setup, the 2D, and 3D imaging results. The following fourth chapter reports the integration of the TeraFET detector with two substrate lenses (one is a Si lens on the back-side Si substrate, the other is a wax/PTFE lens on the front side containing the bonding wires) for sub-harmonic heterodyne detection at 600 GHz. The characteristic of the wax/PTFE lens at THz range is presented. After that, the compared imaging results between the detector with and without the wax/PTFE lens are shown. The fifth chapter extends the demonstration on the lateral and depth resolution of the Fourier imaging system in detail and uses the experimental results at 600 GHz to validate the analytical predictions. The comparison of the resolution between the Fourier imaging system and the conventional microscopy system proves that the Fourier imaging system has better imaging quality under the same system configuration. The last chapter in this thesis concludes on the findings of the THz Fourier imaging and gives an outlook for the enhancement of the Fourier imaging system at THz range.
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.
...
Classical light microscopy is one of the main tools for science to study small things. Microscopes and their technology and optics have been developed and improved over centuries, however their resolution is ultimately restricted physically by the diffraction of light based on its wave nature described by Maxwell’s equations. Hence, the nanoworld – often characterized by sub-100-nm structural sizes – is not accessible with classical far-field optics (apart from special x-ray laser concepts) since its lateral resolution scales with the wavelength.
It was not until the 20th century that various technologies emerged to circumvent the diffraction limit, including so-called near-field microscopy. Although conceptually based on Maxwell’s long known equations, it took a long time for the scientific community to recognize its powerful opportunities and the first embodiments of near-field microscopes were developed. One representative of them is the scattering-type Scanning Near-field Optical Microscope (s-SNOM). It is a Scanning Probe Microscope (SPM) that enables imaging and spectroscopy at visible light frequencies down to even radio waves with a sub-100-nm resolution regardless of the wavelength used. This work also reflects this wide spectral range as it contains applications from near-infrared light down to deep THz/GHz radiation.
This thesis is subdivided into two parts. First, new experimental capabilities for the s-SNOM are demonstrated and evaluated in a more technical manner. Second, among other things, these capabilities are used to study various transport phenomena in solids, as already indicated in the title.
On the technical side, preliminary studies on the suitability of the qPlus sensor – a novel scanning probe technology – for near-field microscopy are presented.
The scanning head incorporating the qPlus sensor–named TRIBUS – is originally intended and built for ultra-high vacuum, low temperature, and high resolution applications. These are desirable environments and properties for sensitive nearfield measurements as well. However, since its design was not planned for near-field measurements, several special technical and optical aspects have to be taken into account, among others the scanning tip design and a spring suspended measurement head.
In addition, in this thesis field-effect transistors are used as THz detectors in an s-SNOM for the first time. Although THz s-SNOM is already an emerging technology, it still suffers from the requirements of sophisticated and specialized infrastructure on both the detector and laser side. Field-effect transistors offer an alternative that is flexible, cost-efficient, room-temperature operating, and easy to handle. Here, their suitability for s-SNOM measurements, which in general require very sensitive and fast detectors, is evaluated.
In the scientific part of this thesis, electromagnetic surface waves on silver nanowires and the conductivity/charge carrier density in silicon are investigated. Both are completely different concepts of transport phenomena, but this already shows the general versatility of the s-SNOM as it can enter both fields. Silver nanowires are analysed by means of near-infrared radiation. Their plasmonic behaviour in this spectral region is studied complementing other simulations and studies in literature performed on them using for example far-field optics.
Furthermore, the surface wave imaging ability of the s-SNOM in the near-infrared regime is thoroughly investigated in this thesis. Mapping surface waves in the mid-infrared regime is widespread in the community, however for much smaller wavelengths there are several important aspects to be considered additionally, such as the smaller focal spot size.
After that, doped and photo-excited silicon substrates are investigated. As the characteristic frequencies of charge carriers in semiconductors – described by the plasma frequency and the Drude model – are within the THz range, the THz s-SNOM is very well suited to probe their behaviour and to reveal contrasts, which has already been shown qualitatively by numerous literature reports. Here, the photo-excitation enables to set and tune the charge carrier density continuously.
Furthermore, the analysis of all silicon samples focuses on a quantitative extraction of the charge carrier densities and doping levels ...
Die vorliegende Arbeit präsentiert Forschungsarbeiten basierend auf nanoskopischen Oberflächenmessungen an plasmonischen Metaoberflächen und zweidimensionalen Materialien, insbesondere dem halbleitenden Übergangsmetal-Dichalcogenid (TMDC) WS_2. Die Thesis ist in sieben Kapitel untergegliedert. Die Einleitung vermittelt einen Überblick über die treibenden Kräfte hinter der Forschung im Bereich der Nanophotonik an zweidimensionalen Materialsystemen. Die Untersuchung der Licht-Materie-Wechselwirkung an dünnen Materialgrenzflächen zieht sich als roter Faden durch die gesamte Arbeit.
Das zweite Kapitel beschreibt den experimentellen Aufbau, der für die Durchführung der nanoskopischen Messungen in dieser Arbeit implementiert wurde. Es werden theoretische Grundlagen, das Messprinzip und die Implementierung des optischen Rasternahfeldmikroskops (s-SNOM) skizziert. Außerdem wird ein Strom-Spannungs-Rasterkraftmikroskop (c-AFM) im Kontaktmodus genutzt, um elektrische Ströme auf mikroskopischen zweidimensionalen TMDC-Terrassen zu messen. In den darauffolgenden vier Kapiteln werden die Beiträge dieser Arbeit zur Untersuchung der Licht-Materie-Wechselwirkung auf der Nanoskala aus verschiedenen Perspektiven vorgestellt. Jedes Kapitel enthält eine kurze Einleitung, einen Theorieteil, Messdaten oder Simulationsergebnisse sowie eine Analyse; vervollständigt durch einen Schlussteil.
Die zentrale Arbeit an einer metallischen Metaoberfläche aus elliptischen Goldscheiben wird in Kapitel 3 vorgestellt. Der zugehörige Theorieteil führt in das Konzept von Oberflächen-Plasmon-Polaritonen (SPP) ein, das für den Forschungsbereich der Plasmonik im Allgemeinen wesentlich ist. Verschiedene Methoden zur Berechnung der Dispersionsrelation dieser Oberflächenmoden an ein- und mehrschichtigen Grenzflächen werden auf die untersuchte Metaoberflächenprobe angewendet. Das Modell sagt drei verschiedene Moden voraus, die sich an der Grenzfläche ausbreiten. Eine teil-gebundene ins Substrat abstrahlende Oberflächenmode sowie zwei vergrabene stark gebundene anisotrope Moden. Eine auf der Probe platzierte Nanokugel aus Silizium wird als radiale Anregungsquelle verwendet.
Der Vergleich mit s-SNOM-Nahfeldbildern zeigt, dass nur die schwach gebundene geführte Modenresonanz ausreichend angeregt wurde, um durch s-SNOM-Bildgebung nachgewiesen werden zu können. Die schwache Oberflächenbindung erklärt die scheinbar isotrope Ausbreitung auf der anisotropen Oberfläche. Die Beobachtung der verbleibenden stark eingegrenzten anisotropen vergrabenen Moden würde eine verbesserte tiefenempfindliche Auflösung des Systems erfordern, die im Prinzip für Schichtdicken von 20 nm möglich sein sollte. Darüber hinaus wirft die Beobachtung die Frage auf, ob die durch Impuls- und Modenvolumenanpassung der Nanokugel gegebene Anregungseffizienz einen ausreichenden Anregungsquerschnitt erzeugt, um nachweisbare vergrabene SPP-Moden zu erzeugen.
In Kapitel 4 wird die Idee der Visualisierung vergrabener elektrischer Felder mit s-SNOM fortgesetzt. Hier wird es auf die Untersuchung von WS_2 angewendet, einem zweidimensionalen TMDC-Material, welches Photolumineszenz zeigt. Durch die Strukturierung des Galliumphosphid-Substrats unter der hängenden Monolage, die von einer dünnen Schicht aus hBN getragen wird, wird die Photolumineszenzausbeute um den Faktor 10 erhöht. Dies wird durch den Entwurf einer lateralen DBR-Mikrokavität mit zusätzlich optimierter vertikaler Tiefe erreicht, die in das Substrat geätzt wurde.
Die hochauflösende Abbildung der elektrischen Feldverteilung im Resonator wird durch den Einsatz von s-SNOM ermöglicht, um die Verbesserung der Einkopplung durch diese beiden Ansätze zu bewerten. Es konnte festgestellt werden, dass die laterale Struktur überwiegend zur verstärkten Photolumineszenzausbeute beiträgt, während für die Einkopplung keine offensichtliche Verstärkung auf die vertikale Strukturoptimierung zurückgeführt werden konnte.
Das zweidimensionale Material WS_2 wird in Kapitel 5 erneut mit Hilfe von c-AFM untersucht. Unterschiedlich dicke Multilagen auf Graphen und Gold dienen als Tunnelbarrieren für vertikale Ströme zwischen Substrat und leitender c-AFM-Messpitze. Die Daten können mit einem Fowler-Nordheim-Modell mit Parametern für die Tunnelbreite und Schottky-Barrierenhöhen der beiden Grenzflächen erklärt werden. Die Messungen zeigen jedoch eine schwache Reproduzierbarkeit, was eine detailliertere Zusammenfassung der relevanten Fehlerquellen erfordert. In der Schlussfolgerung des Kapitels werden mehrere Schlüsselaspekte vorgeschlagen, die bei künftigen Messungen berücksichtigt werden sollten. Entscheidend ist, dass c-AFM sehr empfindlich auf die Adsorption von Wasserfilmen an der Probenoberfläche reagiert, worunter WS_2-Oberflächen unter Umgebungsbedingungen leiden...
With the discovery of light beyond human visibility, scientists strove to extend the range of observation to invisible parts of the light’s spectrum. Realising that light of all frequencies is part the same physical phenomenon, brought a leap in understanding about electromagnetic waves. With the development of more advanced technology, detectors with higher sensitivity for adjacent frequencies to the visible were built. From this, with each new observable wavelength, more insight into otherwise invisible processes and phenomenons were observed. Hand in hand with this went the enhancement of the output power of corresponding sources. This has lead to higher sensitivity setups throughout the spectrum, leading to observations which have given a deeper understanding in various fields of science. Nowadays, detectors and emitters in many different regions of the invisible electro magnetic spectrum have found their way in our every day life. Innovations in technology has lead to practical applications such as X-rays in medicine, motion sensors and remote controls using infrared light, distance sensors and data transmission using radar and radio devices. The frequency regions above infrared are optically generated and below radar can be produced using electric methods. There is no straight line that separates these frequencies. There rather is a whole intermediate region known as the terahertz (THz) regime. Due to the lack of sensitive detectors and efficient sources, the THz frequency region has not been exploited for application use on a widespread basis so far. It combines properties from the surrounding frequency ranges which make it an ideal spectrum for various applications. Consequently, THz radiation and THz imaging are active fields of research.
The work presented in this thesis consists of the development and testing of novel THz imaging concepts, which uses a THz antenna coupled field effect transistor (TeraFET) detector. Two detection principles are applied using two different optical setups. The first uses a pulsed optical parametric oscillator (OPO) THz source where the optical output power is detected. The source relies on a nonlinear effect of a lithium niobate crystal to generate tunable THz pulses from a Q-switched pump laser. The THz signal is detected and amplified by a double stage operational amplifier for monitoring the real time 20 ns pulses on an oscilloscope where a signal to noise ratio (SNR) of ⇠ 25 at a frequency range from 0.75 to 1.1 THz is reached. Imaging of the area of interest with a resolution of 1.2 mm is achieved through raster scanning of the THz pulses. Also spectroscopy with a frequency resolution of ⇠ 50 GHz is demonstrated using a para-aminobenzoic acid sample. The second setup utilises two synchronised electronic multiplier chain sources where their output is mixed on the detector. To form a heterodyne detection setup, the intermediate frequency is fed to a lock-in amplifier which then amplifies the so called beat signal from the TeraFET detector. One source is fixed relative to the detector even through scanning to ensure a stable signal. This detection method allows for amplitude and phase detection for every scanning position, making numerical light field propagation and object reconstruction possible. Numerical focussing is a key feature achieving a lateral resolution of the input transmittance of ⇡ 2 mm.
After the introduction, the second chapter describes the setup, measurement results and challenges which arise using a TeraFET together with the pulsed THz source “Firefly-THz”. In the description of the setup, special attention is given to the shielding of the detector and the electronics. General findings discuss first the overall performance and later spectroscopy and imaging as application examples. Another subsection continues with potential noise sources before the chapter is concluded. Chapter three expands on the topic of Fourier optics from a theoretical point of view. First, parts of the theory of the Fourier Transform (FT) are set out for the reader and how the Fast Fourier Transform (FFT) results from the Discrete Fourier Transform (DFT). This approach is used for theoretical considerations and the implementation of a Fourier optic script that allows for numerical investigations on electro magnetic field propagation through an optical system. The boundary conditions are chosen to be practical relevant to make predictions on measurements presented in chapter four. The following fourth chapter describes the realisation of a heterodyne THz detection setup. Before the measurement results are presented, the setup and its electric configuration are shown. The results come close to the analytical predictions so that the same algorithm which propagates the field from an object to the Fourier plane is used to propagate the measured field back to the object. The influence of phase noise on the measurement results are discussed before simulation and measurement is compared. The last chapter in this thesis concludes on the findings in the pulsed THz detection and the heterodyne THz Fourier imaging and gives an outlook for both configurations.
Terahertz (THz) physics are an emerging field of research dealing with electromagnetic radiation in the far-infrared to microwave region. The development of innovative technologies for the generation and detection of THz radiation has only in the recent past led to a tremendous rise of both fundamental research as well as investigation of possible fields of application for THz radiation. The most prominent reason has long been the scarce accessibility of the THz region of the electromagnetic spectrum - commonly loosely located between 0.1 and 30 THz - to broad research, and it was mostly limited to astronomy and high energy physics facilities. Over the recent years, numerous novel concepts on both the source and detector side have been proposed and successfully implemented to overcome this so-called THz gap. New technology has become available and paved the way for wide-spread experimental laboratory work and accompanying theoretical investigations. First application studies have emerged and in some cases even commercial development of the field of THz physics is on the rise. Despite these enormous progresses, a continuing demand for more efficient THz detectors still impels current technological research. Relatively low source powers are often a major limiting factor and the request for new detection concepts, their understanding and implementation, as well as the optimization on a device basis has been and still remains in place. One of these concepts is the use of field-effect transistors (FETs) high above their conventional cut-off frequencies as electronic THz detectors. The concept has been proposed in a number of theoretical publications by M. Dyakonov and M. Shur in the early 1990's, who pioneered to show that under certain boundary conditions, non-linear collective excitations of the charge carrier system of a two-dimensional electron gas (2DEG) by incident THz radiation can exhibit rectifying behaviour - a detection principle, which has become known as plasma wave or plasmonic mixing. Up until this day, the concept has been successfully implemented in many device realizations - most advanced in established silicon CMOS technology - and stands on the edge of becoming commercially available on a large scale. The main direction of the work presented in this thesis was the modeling and experimental characterization of antenna-coupled FETs for THz detection - termed TeraFETs in this and the author's previous works - which have been implemented in different material systems. The materials presented in this thesis are AlGaN/GaN HEMTs and graphene FETs. In a number of scientific collaborations, TeraFETs were designed based on a hydrodynamic transport model, fabricated in the respective materials, and characterized mainly in the lower THz frequency region from 0.2 to 1.2 THz. The theoretical description of the plasma wave mixing mechanism in TeraFETs, as initiated by Dyakonov and Shur, was based on a fluid-dynamic transport model for charge carriers in the transistor channel. The THz radiation induces propagating charge density oscillations (plasma waves) in the 2DEG, which via non-linear self-mixing cause rectification of the incident THz signals. Over the course of this work, it became evident in the on-going detector characterization experiments that this original theoretical model of the detection process widely applied in the respective literature does not suffice to describe some of the experimental findings in TeraFET detection signals. Thorough measurements showed signal contributions, which are identified in this work to be of thermoelectric origin arising from an inherent asymmetric local heating of charge carriers in the devices. Depending on the material, these contributions constituted a mere side effect to plasmonic detection (AlGaN/GaN) or even reached a comparable magnitude (graphene FETs). To include these effects in the detector model, the original reduced fluid-dynamic description was extended to a hydrodynamic transport model. The model yields at the current stage a reasonable qualitative agreement to the measured THz detection signals. This thesis presents the formulation of a hydrodynamic charge carrier transport model and its specific implementation in a circuit simulation tool. A second modeling aspect is that the transport equations cover only the intrinsic plasmonic detection process in the active gated part of the TeraFET's transistor channel. In order to model and simulate the behavior of real devices, extrinsic detector parts such as ungated channel regions, parasitic resistances and capacitances, integrated antenna impedance, and others must be considered. The implemented detector model allows to simulate THz detection in real devices with the above influences included. Besides presentation of the detector model, experimental THz characterization of the fabricated TeraFETs is presented in this work. Careful device design yielded record detection performance for detectors in both investigated materials. The respective results are shown and the experimental observations of the thermoelectric effect in TeraFETs are compared to modeling results. It is the goal of this work to provide a framework for further theoretical and experimental studies of the plasmonic and thermoelectric effect in TeraFETs, which could eventually lead to a new type of THz detectors particularly exploiting the thermoelectric effect to enhance the sensitivity of today's plasmonic TeraFETs.
This thesis deals with the simulation, optimization and realization of quasi-optical scanning systems for active THz cameras. Active THz cameras are sensitive in the THz regime of the electromagnetic spectrum and are suitable for the detection of metal objects such as weapons behind clothing or fabrics (maybe for security applications) or material investigation. An advantage of active THz-systems is the possibility to measure the phase of the THz-radiation and thus to reconstruct the surface topography of the objects under test. Due to the coherent illumination and the required system parameters (like image field size, working distance and lateral resolution) the optical systems (in the THz region often called quasi-optical systems) must be optimized. Specifically, the active illumination systems require highly optimized quasioptical systems to achieve a good image quality. Since currently no suitable multi-pixel detectors are available, the object has to be scanned in one or two dimensions in order to cover a full field of view. This further reinforces the occurring aberrations. The dissertation covers, alongside the underlying theory, the simulation, optimisation and realisation of three different active THz systems. The subdivision of the chapters is as follows: Chapter 1 deals with a motivation. Chapter 2 develops the underlying theory and it is demonstrated that the geometrical optics is an adequate and powerful description of the image field optimization. It also addresses the developed analytic on-axis and the off-axis image field optimization routine. Chapter 3, 4 and 5 are about the basis of various active THz cameras, each presented a major system aspect. Chapter 3 shows how active THz-cameras with very high system dynamics range can be realised. Within this chapter it could although be demonstrated how very high depth resolution can be achieved due to the coherent and active illumination and how high refresh rate can be implemented. Chapter 4 shows how absolute distance data of the objects under test can be obtained. Therefore it is possible to reconstruct the entire object topography up to a fraction of the wavelength. Chapter 5 shows how off-axis quasi-optical systems must be optimized. It is also shown how the illumination geometry of the active THz systems must be changed to allow for real-time frame rates. The developed widened multi-directional lighting approach also fixes the still existing problem of phase ambiguity of the single phase measurement. Within this chapter, the world’s first active real-time camera with very high frame rates around 10 Hz is presented. This could be only realized with the highly optimised quasioptical system and the multi-directional lighting approach. The paper concludes with a summary and an outlook for future work. Within the outlook some results regarding the simulation of synthetic aperture radar systems and metamaterials are shown.
Towards a THz Bloch laser
(2011)
The realisation of tunable THz laser sources working at room temperature would give
rise to further applications in this range of the electromagnetic spectrum. The THz
Bloch laser could therefore become the basis for a technological breakthrough. Beside
this practical relevance, the physics of the gain mechanism has been investigated
theoretically for a long time and the experimental implementation of a self-starting
laser still has not been achieved.
At the beginning of this thesis the basic principles of Bloch oscillations and the
related Bloch gain are described. The need of a superlattice structure to make Bloch
oscillations possible in a semiconductor material is discussed. In this context, the effect
of negative differential resistance and its influence on the field distribution due to Gunn
domains is explained. The latter lead to an inhomogeneous field which may suppress
the Bloch gain mechanism. The Krömer criterion is introduced and the concept of
field-pinning layers to improve the field homogeneity is deduced. Finally, the design of
the laser material is shown and different types of laser waveguides are compared.
In chapter 3 detailed recipes for the processing of samples are given. Different types of
contacts (ohmic and Schottky), the wafer bonding process required for double-metal
lasers and the application of different photoresists for different purposes are described.
An explanation of the formation of waveguides due to dry etching, wet etching
and ion implantation follows. Dry etching is an established technique in the field
of microstructure processing but the challenge of etching about 20 μm has led to
problems. The high etching depth also makes wet etching difficult but this method
could be improved due to a hard bake of the photoresist. The protection of critical
areas on the surface of the samples with photoresist during ion implantation was
increased by optimising the spin coating process. However, a full implantation of the
active layer between the waveguides was not achieved which was the reason for the
development of the hybrid technology. Here a prior wet etching of about 10 μm is
performed and the rest of the material is implanted.
The experimental setup is shown in chapter 4. An alternative method for the electrical
contacting with the help of a copper bar is introduced. This improves the current
distribution and the risk of an electrical breakdown during the measurements could
therefore be lowered. Devices for THz beam guidance and spectroscopic measurements
are shown and the method of biasing the samples with pulses below 100 ns and
determining the effective voltage applied to the sample is depicted. These short pulses
are required to prevent the samples heating up drastically due to high power.
Chapter 5 contains the current-voltage characterisation of several structures including
I-V-samples, Bloch laser samples and a quantum cascade laser. Different contacts
(ohmic and Schottky) and different techniques for the formation of the ridges have
been used in the processing of these samples (performed at the University of Frankfurt
in all cases) and their influence on the I-V-dependence is discussed. The properties of
the THz emission of the quantum cascade laser are in good agreement with published
results from lasers processed with the same material. Another important result of
this chapter is that the Bloch laser samples show unstable behaviour compared to the
quantum cascade structure even with short pulses (of about 10 ns) where the risk of an
electrical breakdown or the building of filaments is low. THz radiation emitted from
one of the Bloch laser samples could not be observed.
Two aspects that may have prevented the Bloch laser to emit are discussed in
chapter 6. The saturation of the gain for higher amplitudes of the THz wave is
investigated in single mode and multiple mode operation (the latter could occur due
to the Bloch gain being expected to be broadband). In both cases it is shown that
the saturation effect would limit the output power only to values clearly above the
detection limit. In the subsequent section the distribution of the electric field is
simulated with SILVACO software. Structures with transit layer lengths above the
Krömer criterion are compared with structures which include field-pinning layers. It is
shown that the latter are useful to avoid propagating Gunn domains as they build up
in similar structures without field-pinning layers. Nevertheless, the electric field inside
the superlattice regions is not stable. Beside spatial inhomogeneities also temporal
variations of the field magnitude are observed. The lack of a suitable field distribution
is expected to be the main reason for the samples not to work.
In der Doktorarbeit wurde ein Verfahren zur Ermittlung der Schwerpunkthöhe eines Fahrzeugs aus den Messwerten von Sensoren, die serienmäßig in vielen geländegängigen Fahrzeugen verbaut sind, entwickelt. Dieses Verfahren benötigt nur die Signale von Sensoren des elektronischen Stabilitätssystems (ESP) und eines Fahrwerks mit Luftfeder. Um die Höhe des Schwerpunkts zu bestimmen, wurde ein Modell entworfen, das die Drehbewegung des Fahrzeugs um seine Längsachse beschreibt. Eine der unbekannten Größen in diesem Modell ist das Produkt m_g\Deltah, wobei mit m_g die gefederte Masse des Fahrzeugs und mit Deltah der Abstand zwischen dem Schwerpunkt und der Wankachse des Fahrzeugs bezeichnet wird. Die Höhe des Schwerpunkts wird berechnet, indem zu diesem Abstand der als bekannt vorausgesetzte Abstand der Wankachse von der Straße addiert wird. Es wurden drei Varianten des Modells betrachtet. Die eine Modellvariante (stationäres Modell) beschreibt das Fahrzeugverhalten nur in solchen Fahrsituationen exakt, in denen die Wankgeschwindigkeit und die Wankbeschleunigung vernachlässigbar klein sind. In dieser Modellvariante wurden die Federkräfte mit einem detaillierten Modell der Luftfeder berechnet. Eine Eingangsgröße dieses Modells ist der Druck in den Gummibälgen der Luftfeder. Um diesen Druck zu ermitteln, wurde ein Algorithmus auf dem Steuergerät des Luftfedersystems implementiert. Um die Genauigkeit des Luftfedermodells zu testen und um die Abmessungen bestimmter Bauteile der Luftfeder zu ermitteln, wurden Messungen am Federungsprüfstand durchgeführt und eine Methode entwickelt, wie aus diesen Messungen die gesuchten Größen berechnet werden können. Bei den zwei übrigen Modellvarianten (dynamisches Modell) gelten die Einschränkung für die Fahrsituationen nicht. Die einzelnen Varianten des dynamischen Modells unterscheiden sich darin, dass das eine Mal die Feder- und Dämpferkonstanten als bekannt vorausgesetzt und das andere Mal aus den Sensorsignalen geschätzt werden. Passend zu jeder Modellvariante wurde ein Verfahren gewählt, mit dem Schätzwerte für das Produkt m_g\Deltah berechnet wurden. Des Weiteren wurde auch eine Methode entwickelt, mit der die Masse mg geschätzt wurde, ohne zuvor ein Wert für das Produkt m_g\Deltah zu ermitteln. Die Schätzwerte wurden unter Verwendung von Daten ermittelt, die bei einer Simulation und bei Messfahrten gewonnen worden sind. Das Ergebnis des Vergleiches der betrachteten Modellvarianten ist, dass die eine Variante des dynamischen Modells zum Teil falsche Werte für m_g\Deltah liefert, weil die Modellgleichungen ein nicht beobachtbares System bilden. Die andere Variante dieses Modells liefert nicht bei jeder Beladung exakte Werte, was vor allem daran liegt, dass in den Modellgleichungen dieses Modells ein konstanter Wert für die Federsteifigkeit angenommen wird. Bei Fahrzeugen mit Luftfeder ändert sich jedoch dieser Wert in Abhängigkeit von der Fahrzeugmasse. Die Werte von m_g\Deltah und mg können am genauesten mit dem stationären Modell ermittelt werden. Des Weiteren wurden Methoden entwickelt, die die Genauigkeit der durch den Schätzalgorithmus ermittelten Werte verbessern. So wurde zusätzlich zu dem Produkt m_g\Deltah und der Masse mg auch die Verteilung des Gewichtes auf die Vorder- und Hinterachse betrachtet. Es wurde ermittelt, welche Zusammenhänge zwischen dieser Verteilung und dem Produkt m_g\Deltah sowie zwischen dieser Verteilung und der Masse des Fahrzeugs bestehen. So konnte der Fehler in den Schätzwerten dieser Größen minimiert werden. Außerdem wurde auch der Zusammenhang zwischen dem Produkt m_g\Deltah und der Masse des Fahrzeugs ermittelt. Damit konnten die Schätzwerte dieser Größen genauer bestimmt werden. Aus den so gewonnenen Werten kann die Schwerpunkthöhe von einem Mercedes ML auf etwa 8cm genau berechnet werden. Diese Genauigkeit reicht aus, um das elektronische Stabilitätsprogramm auf die aktuelle Beladung des Fahrzeugs abzustimmen und damit einen Gewinn an Agilität für dieses Fahrzeug zu realisieren.