Overview
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Publications
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(2022): Rettungs-Assistenzsystem für die alpine Rettung (Einblicke in die Forschung). Available online at https://www.fhgr.ch/fileadmin/publikationen/forschungsbericht/fhgr-Einblicke_in_die_Forschung_2022.pdf, last checked on 19.05.2022
Abstract: Das Projekt zielt darauf ab, innovative Ansätze aus der Robotik, Sensorik, Regelungstechnik, Datenübertragung und Bilderkennung zu evaluieren, um die alpine Luftrettung technologisch voranzutreiben. Zu diesem Zweck soll ein Prototyp für ein Longline-Rettungs-Assistenzsystem entwickelt werden.
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(2021): A Transfer Function Measurement Setup With an Electro-Optic Sensor for MR Safety Assessment in Cascaded Media. In: IEEE Transactions on Electromagnetic Compatibility 63, S. 662-672. DOI: 10.1109/TEMC.2020.3040756
Abstract: This article introduces a transfer function (TF) measurement setup for cascaded media (CM) and demonstrates that the full immersion of an implant in a lossy medium might result in an underestimation of the RF-induced heating compared to partial immersion. A CMTF measurement setup was constructed using medium-specific local electric field sources and electro-optical sensor (EOS). TFs of bare and insulated wire samples were measured in the CM air/saline. Finite-difference time-domain (FDTD) simulations were implemented using a reciprocal approach with a monopole excitation source as well as a piecewise plane wave excitation method. As a theoretical guideline for understanding the CM behavior of the devices, an analytical model based on the buried wire models was derived. CMTFs agree with good precision between FDTD simulations, analytical modeling, and the EOS measurements. Measured TFs were calibrated and validated similar to ISO/TS 10974:Ed.2. Calculated tip SAR values under uniform incident field show an increase up to a factor of 5.7 for partially immersed wires compared to the full immersion. On average, FDTD-based TFs overestimate SAR by 25% compared to measurement-based TFs. As safety assessments for fully immersed implant leads or devices might not represent the worst-case scenario, single-medium assessments might underestimate the RF-induced heating.
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(2021): Analysis of the RF Excitation of Endovascular Stents in Small Gap and Overlap Scenarios Using an Electro-Optical E-field Sensor. In: IEEE Transactions on Biomedical Engineering 68, S. 783-792. DOI: 10.1109/TBME.2020.3009869
Abstract: OBJECTIVE To assess the effect of the electro-magnetic coupling of endovascular stents on their RF heating potential in MRI. METHODS A custom-built electro-optic E-field probe is used to perform measurements of the scattered E-field at a distance of 2 mm to stent samples with submillimeter resolution. Various combinations of stent lengths are measured at 124 MHz (3T MRI Larmor frequency) with varying gap and overlap between the stents, with and without stent coating, and with distilled water and saline solution as surrounding media. The results are compared to theoretically derived E-field distributions. RESULTS At an overlap of 10 mm the E-field pattern of two stents collapses to a single dipole indicating excellent coupling between the stents. E-field intensities substantially increase/decrease up to 5-fold/2.5-fold if the total length of the two combined stents is closer/further away from the resonance length of the single stents. Stent coating and conductivity of the surrounding medium strongly influence the E-field patterns of overlapping stents. Measured and calculated E-field patterns are in good agreement. CONCLUSION Electro-optic E-field measurements are a valuable tool for RF safety assessments in both single as well as coupled stents. SIGNIFICANCE RF induced heating of single stents during MRI has been extensively studied. However, in clinical practice often two or more stents are implanted in close proximity which can substantially change the pattern of the scattered electric fields and the localization and intensity of hot spots. In this study a detailed assessment of the coupling of stents during RF excitation is given.
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(2019) : Effiziente Entwicklung von Prototypen . Blog (Studentenfutter) . Available online at https://www.suedostschweiz.ch/blogs/studentenfutter/2019-06-28/effiziente-entwicklung-von-prototypen , last checked on 19.02.2021
Abstract: Prototypen versetzen einen Betrachtenden für einen kurzen Moment in die Zukunft und liefern so wichtige Beiträge bei der Entwicklung neuer Produkte. Auch fürs Management dienen sie als anschauliche und verständliche Entscheidungsgrundlage.
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(2019) : Effiziente Entwicklung von Prototypen . Blog (FHGR Blog) . Available online at https://blog.fhgr.ch/blog/effiziente-entwicklung-von-prototypen/ , last checked on 22.02.2021
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(2015): Terahertz imaging modalities of ancient Egyptian mummified objects and of a naturally mummified rat. In: The anatomical record 298, S. 1135-1143. DOI: 10.1002/ar.23143
Abstract: During the last few years, terahertz (THz) imaging has been used to investigate artwork and historic artifacts. The application of THz imaging to mummy investigations is very attractive since it provides spectroscopic information over a broad frequency range and its radiation has proven to be harmless to human cells. However, compared with the current standard imaging methods in mummy imaging-X-ray and computed tomography (CT)--it remains a novel, emerging technique whose potential still needs to be fully evaluated. Here, ancient Egyptian mummified objects as well as a naturally mummified rat have been investigated by two different THz imaging systems: a broadband THz time domain imaging system and an electronic THz scanner. The obtained THz images are compared with conventional CT, X-ray, and magnetic resonance images. While the broadband THz time domain setup permits analyses of smaller samples, the electronic THz scanner allows the recording of data of thicker and larger samples at the expense of a limited spectral bandwidth. Terahertz imaging shows clear potential for mummy investigations, although currently CT imaging offers much higher spatial resolution. Furthermore, as commercial mobile THz scanners become available, THz imaging could be applied directly in museums or at excavation sites.
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(2015): An optical setup for electric field measurements in MRI with high spatial resolution. In: Physics in medicine and biology 60, S. 4355-4370. DOI: 10.1088/0031-9155/60/11/4355
Abstract: Electric field measurements in the magnetic resonance (MR) imaging environment are important to assess potentially dangerous radio-frequency (RF) heating in the vicinity of metallic structures such as coils, implants or catheters. So far, E-field measurements have been performed with dipole antennas that lag of limited spatial resolution and which are difficult to use in the magnet bore as they interfere with the RF transmit field of the MRI system. In this work an electro-optic sensor is presented that utilizes the Pockels effect to measure the E-field in a clinical MR system with high spatial resolution. This sensor consists of dielectric materials only and thus, it only minimally influences the measured E-field distribution. A 10 m long flexible optical fiber connects the small sensor head to a remote processing unit where the optical signal is transformed into an electrical output signal. Spatially resolved qualitative E-field measurements were performed in a 1.5 T clinical MR system in the vicinity of metallic samples and an active tracking catheter with a resolution of up to 1 mm. The near-field pattern of a resonant U-shaped metallic sample was clearly identified and compared with numerical simulations. A more complex field behavior was found for the tracking catheter where strong E-field enhancements were observed at the distal tip and at its proximal part outside the phantom solution. Due to its sub-mm spatial resolution the optical sensor approach provides detailed insight into the complex and difficult to access field distributions close to implants and metallic structures and has turned out to be promising tool for MRI field and safety inspections.
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(2012) : Adjusting the functionality of terahertz split-ring resonators through geometry In: Betz, Markus; Elezzabi, Abdulhakem Y.; Song, Jin-Joo; Tsen, Kong-Thon (Hg.): Ultrafast Phenomena and Nanophotonics XVI: SPIE OPTO: San Francisco, California, USA, 21.-26. Januar: SPIE. Digital Library (SPIE Proceedings)
DOI: https://doi.org/10.1117/12.908107 Abstract: We examine planar double split-ring resonators (SRRs) consisting of two concentric rings with either opposite, similar, or asymmetric gap orientation. Depending on the geometry we observe resonance hybridization, metamaterial induced transparency, or the excitation of dark resonances. These properties can be used for SRR based sensing applications, to realize strongly dispersive behavior, or for determining the optical properties of metals. We further find that THz SRRs featuring very narrow gaps on the micro- or nanoscale can provide in-gap enhancement factors of several 10,000, a property particularly useful for the realization of nonlinear THz experiments.
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(2011): Terahertz near-field microscopy of complementary planar metamaterials: Babinet's principle. In: Optics express 19, S. 2537-2545. Available online at https://doi.org/10.1364/OE.19.002537, last checked on 18.06.2021
Abstract: Using terahertz near-field imaging we experimentally investigate the resonant electromagnetic field distributions behind a split-ring resonator and its complementary structure with sub-wavelength spatial resolution. For the out-of-plane components we experimentally verify complementarity of electric and magnetic fields as predicted by Babinet's principle. This duality of near-fields can be used to indirectly map resonant magnetic fields close to metallic microstructures by measuring the electric fields close to their complementary analogues which is particularly useful since magnetic near-fields are still extremely difficult to access in the THz regime. We find excellent agreement between the results from theory, simulation and two different experimental near-field techniques.
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(2011): Spatiotemporal Visualization of THz Near-Fields in Metamaterial Arrays. In: Journal of Infrared, Millimeter, and Terahertz Waves 32, S. 570-579. Available online at https://doi.org/10.1007/s10762-010-9648-6, last checked on 18.06.2021
Abstract: We present an experimental approach to record the spatiotemporal electric field distribution of coherent broadband THz pulses propagating through planar metamaterial arrays. The electric field can be measured with sub-wavelength precision within a volume that is several wavelengths in size, thus, having the potential to map the near-field to far-field transition of the resonant structures constituting the metamaterial. To demonstrate the potential we present measurements of THz pulses propagating through a planar array of double split-ring resonators and their inverse analogues.
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(2011): Near-field investigation of induced transparency in similarly oriented double split-ring resonators. In: Optics Letters 36, S. 1683-1685. DOI: 10.1364/OL.36.001683
DOI: https://doi.org/10.1364/OL.36.001683 Abstract: We present near-field measurements of an induced transparency behavior using a double split-ring resonator geometry. Mapping the out-of-plane electric field component directly reveals that the induced transparency is linked to an asymmetric mode profile with the subunits oscillating in antiphase. The measurements are compared to complementary numerical simulations, and excellent agreement is found.
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(2011): Second harmonic generation based on strong field enhancement in nanostructured THz materials. In: Optics express 19, S. 7262-7273. Available online at https://doi.org/10.1364/OE.19.007262, last checked on 18.06.2021
Abstract: The THz response of slit structures and split-ring resonators (SRRs) featuring extremely small gaps on the micro- or nanoscale is investigated numerically. Both structures exhibit strong field enhancement in the gap region due to light-induced current flows and capacitive charging across the gap. Whereas nanoslits allow for broadband enhancement the resonant behavior of the SRRs leads to narrowband amplification and results in significantly higher field enhancement factors reaching several 10,000. This property is particularly beneficial for the realization of nonlinear THz experiments which is exemplarily demonstrated by a second harmonic generation process in a nonlinear substrate material. Positioning nanostructures on top of the substrate is found to result in a significant increase of the generation efficiency for the frequency doubled component.
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(2011): Spatiotemporal Visualization of THz Near-Fields in Metamaterial Arrays. In: Journal of Infrared, Millimeter, and Terahertz Waves 32, S. 570-579. DOI: 10.1007/s10762-010-9648-6
Abstract: We present an experimental approach to record the spatiotemporal electric field distribution of coherent broadband THz pulses propagating through planar metamaterial arrays. The electric field can be measured with sub-wavelength precision within a volume that is several wavelengths in size, thus, having the potential to map the near-field to far-field transition of the resonant structures constituting the metamaterial. To demonstrate the potential we present measurements of THz pulses propagating through a planar array of double split-ring resonators and their inverse analogues.
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(2011): Near-field signature of electromagnetic coupling in metamaterial arrays: a terahertz microscopy study. In: Optics express 19, S. 17283-17292. Available online at https://doi.org/10.1364/OE.19.017283
Abstract: Using terahertz near-field imaging we experimentally investigate the interaction between split-ring resonators (SRRs) in metamaterial arrays. Depending on the inter-SRR spacing two regimes can be distinguished for which strong coupling between SRRs occurs. For dense arrays SRRs couple via their electric and magnetic near-fields. In this case distinct deformations of the SRRs' characteristic near-field patterns are observed as a signature of their strong interaction. For larger separations with a periodicity matching the resonance wavelength, the SRRs become diffractively coupled via their radiated fields. In this regime hybridization between plasmonic and lattice modes can be clearly identified in the experimentally obtained near-field maps.
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(2011): Electromagnetic Wave Propagation Close to Microstructures Studied by Time and Phase-Resolved THz Near-Field Imaging. In: Journal of Infrared, Millimeter, and Terahertz Waves 32, S. 1020-1030. DOI: 10.1007/s10762-011-9772-y
Abstract: We demonstrate microscopic mapping of electromagnetic waves close to metal microstructures with sub-ps temporal and sub-wavelength spatial resolution by pulsed THz near-field imaging. The inherent phase-sensitivity of this technique allows mapping wavefronts of propagating modes and the measured amplitude distributions provide information on field concentration and localization close to the structures. Using this approach we investigate wave propagation through a sub-wavelength aperture, as well as the formation of traveling and standing surface waves along a metal microwire.
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(2010): Terahertz near-field microscopy with subwavelength spatial resolution based on photoconductive antennas. In: Applied Optics 49. DOI: 10.1364/AO.49.0000E1
Abstract: Imaging and sensing applications based on pulsed terahertz radiation have opened new possibilities for scientific and industrial applications. Many exploit the unique features of the terahertz (THz) spectral region, where common packaging materials are transparent and many chemical compounds show characteristic absorptions. Because of their diffraction limit, THz far-field imaging techniques lack microscopic resolution and, if subwavelength features have to be resolved, near-field techniques are required. Here, we present a THz near-field microscopy approach based on photoconductive antennas as the THz emitter and as a near-field probe. Our system allows us to measure amplitude, phase, and polarization of the electric fields in the vicinity of a sample with a spatial resolution on the micrometer scale (approximately lambda/20). Using a dielectric (plant leaf) and a metallic structure (microwire) as examples, we demonstrate the capabilities of our approach.
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(2010): Spectral collapse in ensembles of metamolecules. In: Physical review letters 104. DOI: 10.1103/PhysRevLett.104.223901
Abstract: We report on the first direct experimental demonstration of a collective phenomenon in metamaterials: spectral line collapse with an increasing number of unit cell resonators (metamolecules). This effect, which is crucial for achieving a lasing spaser, a coherent source of optical radiation fuelled by coherent plasmonic oscillations in metamaterials, is linked to the suppression of radiation losses in periodic arrays. We experimentally demonstrate spectral line collapse at microwave, terahertz and optical frequencies.
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(2010): Terahertz imaging of ancient mummies and bone. In: American Journal of Physical Anthropology 142, S. 497-500. DOI: 10.1002/ajpa.21292
Abstract: Ancient mummified soft-tissues are a unique source to study the evolution of disease. Diagnostic imaging of such historic tissues is of foremost interest in paleoanthropology or paleopathology, with conventional x-ray and computed tomography (CT) being the gold-standard. Longer wavelength radiation in the far-infrared or Terahertz region allows diagnostic close-to-surface tissue differentiation of bone morphology while being harmless to human cells. The aim of this study is to show the feasibility and the morpho-diagnostic impact of THz imaging of historic remains. Images of an artificially embalmed ancient Egyptian human mummy hand, an artificially embalmed ancient Egyptian mummified fish and a macerated human lumbar vertebra were obtained by THz-pulse imaging and compared with conventional X-ray and CT images. Although conventional x-ray imaging provides higher spatial resolution, we found that THz-imaging is well-suited for the investigation of ancient mummified soft tissue and embalming-related substances / wrappings. In particular, bone and cartilaginous structures can be well differentiated from surrounding soft-tissues and bandage-wrappings by THz imaging. Furthermore, THz-pulse imaging also measures the time-delay of the pulsed signal when passing through the sample, which provides supplementary information on the optical density of the sample that is not obtained by X-ray and CT. Terahertz radiation provides a completely non-invasive diagnostic imaging modality for historic dry specimens. We anticipate this modality also to be used for detection of hidden objects in historic samples such as funerary amulets still in situ in wrapped mummies, as well as potentially for the identification of spectral signatures from chemical substances, e.g., in embalming essences.
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(2010): Chemical sensing and imaging with pulsed terahertz radiation. In: Analytical and bioanalytical chemistry 397, S. 1009-1017. DOI: 10.1007/s00216-010-3672-1
Abstract: Over the past decade, terahertz spectroscopy has evolved into a versatile tool for chemically selective sensing and imaging applications. In particular, the potential to coherently generate and detect short, and hence, broadband terahertz pulses led to the development of efficient and compact spectrometers for this interesting part of the electromagnetic spectrum, where common packaging materials are transparent and many chemical compounds show characteristic absorptions. Although early proof-of-principle demonstrations have shown the great potential of terahertz spectroscopy for sensing and imaging, the technology still often lacks the required sensitivity and suffers from its intrinsically poor spatial resolution. In this review we discuss the current potential of terahertz pulse spectroscopy and highlight recent technological advances geared towards both enhancing spectral sensitivity and increasing spatial resolution.
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(2009): Terahertz near-field imaging of electric and magnetic resonances of a planar metamaterial. In: Optics express 17, S. 3826-3834. Available online at https://doi.org/10.1364/OE.17.003826, last checked on 18.06.2021
Abstract: Experimental investigations of the microscopic electric and in particular the magnetic near-fields in metamaterials remain highly challenging and current studies rely mostly on numerical simulations. Here we report a terahertz near-field imaging approach which provides spatially resolved measurements of the amplitude, phase and polarization of the electric field from which we extract the microscopic magnetic near-field signatures in a planar metamaterial constructed of split-ring resonators (SRRs). In addition to studying the fundamental resonances of an individual double SRR unit we further investigate the interaction with neighboring elements.
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(2009): Lattice modes mediate radiative coupling in metamaterial arrays. In: Optics express 17, S. 22108-22113. Available online at https://doi.org/10.1364/OE.17.022108, last checked on 18.06.2021
Abstract: We show that a resonant response with very high quality factors can be achieved in periodic metamaterials by radiatively coupling their structural elements. The coupling is mediated by lattice modes and can be efficiently controlled by tuning the lattice periodicity. Using a recently developed terahertz (THz) near-field imaging technique and conventional far-field spectroscopy together with numerical simulations we pinpoint the underlying mechanisms. In the strong coupling regimes we identify avoided crossings between the plasmonic eigenmodes and the diffractive lattice modes.
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(2008): Beam-Profiling and Wavefront-Sensing of THz Pulses at the Focus of a Substrate-Lens. In: IEEE Journal of Selected Topics in Quantum Electronics 14, S. 476-481. DOI: 10.1109/JSTQE.2007.910709
Abstract: We report combined wavefront detection and beam profiling of single-cycle terahertz (THz) pulses. In our system, the electric field is recorded highly resolved in two spatial and one temporal dimension before and after propagation through an optical component. Using this approach, we examine the imaging properties of a hyperhemispherical silicon lens as it is commonly used in THz dipole antennas. We observe an asymmetric spatiotemporal field dynamic in the focus, which can be attributed to distortion of the incident wavefront in combination with the image properties of the lens. Diffraction on the lens aperture influences the spectral beam profile at the focus. The frequency dependence of the Airy pattern indicates a rapidly degrading Strehl ratio with increasing frequency.
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(2008): Terahertz near-field imaging of metallic subwavelength holes and hole arrays. In: Applied Physics Letters 92. DOI: 10.1063/1.2936303
Abstract: Metallic microstructures are investigated by time-resolved terahertz near-field imaging. By our approach, we can directly follow field diffraction from subwavelength structures as well as coupling to the surface. Near-field images of the spectral amplitude and phase of the electric field show the formation, propagation, and attenuation of surface waves and allow us to distinguish between propagating and stationary modes. Our results show that the field enhancement in an individual hole, together with the formation of standing waves on the metal surface between the holes, are key mechanisms for the extraordinary transmission phenomenon through periodic hole arrays.