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(2021): Towards a 3.5D Camera. Multispectral Time-of-Flight Range Imaging Provides Datasets Comprising 3 Spatial and 0.5 Spectral Dimensions. Poster session. FTAL Conference. Scuola universitaria professionale della Svizzera italiana SUPSI. Dipartimenteo tecnologie innovative. Lugano, 28. Oktober, 2021
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(2020): Multispektrales TOF (Einblicke in die Forschung). Online verfügbar unter https://www.fhgr.ch/fileadmin/publikationen/forschungsbericht/fhgr-Einblicke_in_die_Forschung_2020.pdf, zuletzt geprüft am 09.04.2021
Abstract: Im Projekt «Multispektrales TOF» wurde eine Kamera entwickelt, mit der sowohl drei räum-liche Dimensionen aufgelöst als auch eine begrenzte Anzahl spektraler Datenpunkte auf-genommen werden können. Da die spektrale Achse nur als halbe Dimension gewertet wird, ergibt sich die Bezeichnung des Systems als 3.5D-Kamera. Die Umsetzung konnte mittels zweier unterschiedlicher experimenteller Ansätze erfolgreich demonstriert werden.
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(2020) : Multispectral Time-of-Flight Range Imaging. Towards a 3.5 D Camera: Imaging and Applied Optics: Proceedings: Imaging and Applied Optics Congress: Online, 22. - 26. Juni: Washington: Optica
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(2019): 3D-Time-of-Flight-Kameras für Unterwasserapplikationen. In: Wissensplatz, S. 16-17. Online verfügbar unter https://www.fhgr.ch/fhgr/medien-und-oeffentlichkeit/publikationen/wissensplatz/februar-2019/, zuletzt geprüft am 14.02.2019
Abstract: Durch die Verwendung von Leuchtdioden im sichtbaren Bereich konnten 3D-Time-of-Flight-Kameras für den Einsatz in Unterwasserumgebungen optimiert werden. Dies ermöglicht es, Bewegungsabläufe in Pool-Umgebungen mit Gegenstandsweiten im Meterbereich zeitlich und in drei räumlichen Dimensionen aufzulösen.
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(2018): Time-of-Flight-Kameras für Unterwasserapplikationen (Einblicke in die Forschung). Online verfügbar unter https://www.fhgr.ch/fileadmin/publikationen/forschungsbericht/fhgr-Einblicke_in_die_Forschung_2018.pdf, zuletzt geprüft am 09.04.2021
Abstract: Durch die Verwendung von Leuchtdioden im sichtbaren Bereich wurden 3D-Time-of-Flight-Kameras für den Einsatz in Unterwasserumgebungen optimiert. Dies ermöglicht es, Bewegungsabläufe mit Gegenstandsweiten im Meterbereich zeitlich und in drei räumlichen Dimensionen aufzulösen.
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(2018): Optimizing time-of-flight cameras for underwater applications. Graubünden forscht. Academia Raetica. Davos, 19. September, 2018
Abstract: In recent years, time-of-flight (ToF) cameras have emerged as a reliable and cost-effective way to provide high frame-rate range imaging. Today, this technology is used to address a wealth of applications including object detection, collision prevention, or gesture recognition. Here, we describe the implementation of the technology for underwater applications. The challenge arising from this adaptation is that the near-IR regime, in which current ToF sensors typically operate, is far from ideal for this application due to the strong absorption of liquid water. We show that by operating existing ToF imaging chips in combination with LED light sources at visible wavelengths, distance sensing with ranges on the meter level can be realized. We find that the attainable performance depends on a variety of parameters, such as the wavelength dependent absorption of water, the emitted optical power and response times of the LEDs, or the spectral sensitivity of the TOF chip. An in-depth analysis of the interplay between the different parameters is given and the performance of underwater TOF imaging using different visible illumination wavelengths is analyzed.
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(2018) : Time-of-flight range imaging for underwater applications In: Soskind, Yakov G.: Photonic Instrumentation Engineering V: Proceedings: SPIE Photonics West: San Francisco. SPIE The international society for optics and photonics (Proceedings of SPIE). Online verfügbar unter https://doi.org/10.1117/12.2283476, zuletzt geprüft am 21.02.2020
Abstract: Precise and low-cost range imaging in underwater settings with object distances on the meter level is demonstrated. This is addressed through silicon-based time-of-flight (TOF) cameras operated with light emitting diodes (LEDs) at visible, rather than near-IR wavelengths. We find that the attainable performance depends on a variety of parameters, such as the wavelength dependent absorption of water, the emitted optical power and response times of the LEDs, or the spectral sensitivity of the TOF chip. An in-depth analysis of the interplay between the different parameters is given and the performance of underwater TOF imaging using different visible illumination wavelengths is analyzed.
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(2017) : Spatiotemporal imaging of THz wave propagation: Polaritonics versus near-field imaging In: IEEE: Proceedings of the 16th Workshop on Information Optics: Workshop on Information Optics (WIO): Interlaken. EPFL, Nanophotonics and Metrology Laboratory NAM, S. 1-3. Online verfügbar unter https://doi.org/10.1109/WIO.2017.8038178, zuletzt geprüft am 21.02.2020
DOI: https://doi.org/10.1109/WIO.2017.8038178 Abstract: Two recently developed methods that allow visualizing the interaction of THz radiation with arbitrary structures, namely polaritonics and near-field imaging, are reviewed and compared. THz polaritonics, which relies on complex amplified laser systems, allows single-shot image acquisition in a plane perpendicular to the structures. THz near-field imaging, on the other hand, requires only femtosecond oscillators but is a slower raster scanning technique providing images parallel to the structure plane.
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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. Online verfügbar unter https://doi.org/10.1364/OE.19.002537, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1007/s10762-010-9648-6, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1364/OE.19.007262, zuletzt geprüft am 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): High aspect ratio plasmonic nanostructures for sensing applications. In: ACS nano 5, S. 6374-6382. DOI: 10.1021/nn201529x
DOI: https://doi.org/10.1021/nn201529x Abstract: We present an experimental and theoretical study of plasmonic modes in high aspect ratio nanostructures in the visible wavelength region and demonstrate their high performance for sensing applications. Ordered and well-defined plasmonic structures with various cross-sectional profiles and heights are obtained using a top-down fabrication process. We show that, compared to cylindrical nanorods, structures with split-ring resonator-like cross sections have great potential for powerful sensing due to a pronounced polarization dependence, strong field enhancement, structural tunability, and improved mechanical stability. The plasmonic structures under study exhibit high sensitivities, up to nearly 600 nm/RIU, and figures of merit above 20.
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(2011): Concurrent field enhancement and high transmission of THz radiation in nanoslit arrays. In: Applied Physics Letters 99. DOI: 10.1063/1.3617476
DOI: https://doi.org/10.1063/1.3617476 Abstract: We experimentally and numerically investigate the transmission of THz radiation through uniform nanoslit arrays. These structures are capable of inducing plasmon-mediated field enhancement while concurrently providing high field transmission. Combined with intense THz radiation, estimated field strengths as high as 26 MV/cm are obtained in the near-field regime which will facilitate nonlinear THz experiments.
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(2010): Slit waveguide based terahertz near-field microscopy: Prospects and limitations. In: Journal of Applied Physics 107. DOI: 10.1063/1.3294623
DOI: https://doi.org/10.1063/1.3294623 Abstract: We experimentally and numerically investigate the transmission of single cycle terahertz pulses through subwavelength slit apertures featuring zero cutoff frequency and very low attenuation. Employing a polaritonic approach we demonstrate that wave forms transmitted through slit samples with slit widths as small as λ/1000 can be visualized and analyze the applicability of this approach to terahertz near-field microscopy. Finite element simulations are used to quantitatively investigate resolution limitations due to imperfect experimental configurations. Our results show that resolutions on the scale of the slit width are possible; however, they demand an accurate control of the distance between the imaging aperture and the sample. This is because the presence of small gaps leads to around-the-bend waveguiding effects resulting in a significant reduction of the attainable resolution.
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(2010): Imaging of THz waves in 2D photonic crystal structures embedded in a slab waveguide. In: New Journal of Physics 12. Online verfügbar unter doi.org/10.1088/1367-2630/12/1/013014, zuletzt geprüft am 18.06.2021
Abstract: We present space- and time-resolved simulations and measurements of single-cycle terahertz (THz) waves propagating through two-dimensional (2D) photonic crystal structures embedded in a slab waveguide. Specifically, we use a plane wave expansion technique to calculate the band structure and a time-dependent finite-element method to simulate the temporal evolution of the THz waves. Experimentally, we measure the space–time evolution of the THz waves through a coherent time-resolved imaging method. Three different structures are laser machined in LiNbO3 crystal slabs and analyzing the transmitted as well as the reflected THz waveforms allows determination of the bandgaps. Comparing the results with the calculated band diagrams and the time-dependent simulations shows that the experiments are consistent with 3D simulations, which include the slab waveguide geometry, the birefringence of the material, and a careful analysis of the excited modes within the band diagrams.
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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. Online verfügbar unter https://doi.org/10.1364/OE.17.003826, zuletzt geprüft am 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. Online verfügbar unter https://doi.org/10.1364/OE.17.022108, zuletzt geprüft am 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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: Laser-Based Oxygen Monitoring in Waste Incineration Facilities: Benefits and Challenges: Advanced Photonics 2014: Proceedings: Nonlinear Photonics: Barcelona, 27. - 31. Juli: 2015
Abstract: Oxygen measurements using tunable diode laser absorption spectroscopy were performed at a full scale waste incineration facility. A comparative analysis to a reference method allowed identifying benefits and challenges inherent to the laser method.
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: Sensing the Dielectric Properties of Metals at THz Frequencies Using Inverse Symmetry Broken Metamaterial: Advanced Photonics 2015: Proceedings: Optical Sensors: Boston, 27. Juni - 1. Juli: 2015
Abstract: Positive and inverse, asymmetric, double split-ring resonator structures are analyzed experimentally and numerically. The structures exhibit very sharp dark resonances which allow probing the dielectric properties of metals at THz frequencies.
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: Multiparameter sensing of paper sheets using terahertz time-domain spectroscopy. Caliper, fiber orientation, moisture, and the role of spatial inhomogeneity: 2016 IEEE SENSORS: Orlando, 10. Oktober - 3. November: 2016, S. 1-3
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: High precision material characterization method using THz spectroscopy: 39th International Conference on Infrared, Millimeter, and Terahertz waves (IRMMW-THz): Tucson, 14. - 19. September: 2014
Abstract: We propose a novel THz material analysis approach which consists of a combination of a stratified dispersive model, an appropriate measurement configuration and a time-domain based fitting procedure. The measurement configuration is modeled as a stratified system where for each layer the light-matter interaction is realistically described. The method is illustrated for the industrial quality control of (coated) paper sheets and is shown to give highly accurate results in humid air and irrespective of the position of the reference mirror in reflection.
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: Paper sheet parameter determination using terahertz spectroscopy: 41st International Conference on Infrared, Millimeter, and Terahertz waves (IRMMW-THz): Kopenhagen, 25. - 30. September: 2016
Abstract: Using THz spectroscopy, we show how to uniquely determine parameters for industrial quality control of paper sheets. We carry out measurements within the industrial setting: no averaging, fast acquisition in ambient conditions, and quasi-real-time analysis. Paper parameters are extracted using a combination of statistical evaluation and physical interpretation of the light-matter interaction. We show that THz spectroscopy offers additional value as compared to currently used industrial sensors.