@article{SteglichMaiVillringeretal.2020, author = {Steglich, Patrick and Mai, Christian and Villringer, Claus and Mai, Andreas}, title = {Direct observation and simultaneous use of linear and quadratic electro-optical effects}, series = {Journal of Physics D: Applied Physics}, volume = {53}, journal = {Journal of Physics D: Applied Physics}, number = {12}, issn = {1361-6463}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-13032}, year = {2020}, abstract = {We report on the direct observation and simultaneous use of the linear and quadratic electro-optical effect and propose a method by which higher-order susceptibilities of electro-optical materials can be determined. The evaluation is based on the separation of the second- and third-order susceptibilities and the experimental technique uses a slot waveguide ring resonator fabricated in integrated photonic circuit technology, which is embedded by a guest-host polymer system consisting of the azobenzene dye Disperse Red 1 in a poly(methyl methacrylate) matrix as an active electro-optical material. The contribution of both effects on the electro-optical response under the influence of static and time-varying electrical fields is investigated. We show that the quadratic electro-optical effect has a significant influence on the overall electro-optical response even with acentric molecular orientated molecules. Our findings have important implications for developing electro-optical devices based on polymer-filled slot waveguides and give rise to advanced photonic circuits.}, language = {en} } @article{LuxEdlingLuccietal.2019, author = {Lux, Helge and Edling, Matthias and Lucci, Massimiliano and Kitzmann, Julia and Villringer, Claus and Siemroth, Peter and De Matteis, Fabio and Schrader, Sigurd}, title = {The Role of Substrate Temperature and Magnetic Filtering for DLC by Cathodic Arc Evaporation}, series = {Coatings}, volume = {9}, journal = {Coatings}, number = {5}, issn = {2079-6412}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-12295}, pages = {345}, year = {2019}, abstract = {Diamond-like carbon (DLC) films were deposited using two different types of high current arc evaporation. The first process used a magnetic particle filter to remove droplets from the plasma. For the second process, the samples were put into a metallic cage which was placed directly above the plasma source. For both processes, we varied the substrate temperature from 21 to 350 °C in order to investigate the temperature effect. The samples were characterized using SEM, AFM, XPS, Raman Spectroscopy, Ellipsometry, Photometry, and Nano Indentation in order to compare both methods of deposition and provide a careful characterization of such DLC films. We found that the sp3 content and the hardness can be precisely adjusted by changing the substrate temperature. Furthermore, in the case of unfiltered deposition, the optical constants can be shifted in the direction of higher absorbance in order to produce black and hard carbon coatings.}, language = {en} } @article{SteglichVillringerDietzeletal.2019, author = {Steglich, Patrick and Villringer, Claus and Dietzel, Birgit and Mai, Christian and Schrader, Sigurd and Casalboni, Mauro and Mai, Andreas}, title = {On-Chip Dispersion Measurement of the Quadratic Electro-Optic Effect in Nonlinear Optical Polymers Using a Photonic Integrated Circuit Technology}, series = {IEEE Photonics Journal}, volume = {11}, journal = {IEEE Photonics Journal}, number = {3}, issn = {1943-0655}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-12307}, year = {2019}, abstract = {A novel method to determine the dispersion of the quadratic electro-optic effect in nonlinear optical materials by using a silicon-on-insulator microring resonator is presented. The microring consists of a silicon slot waveguide enabling large dc electric field strength at low applied voltages. The dispersion of third-order hyperpolarizability of a linear conjugated dye is approximated by using a two-level model for the off-resonant spectral region. As an example, the dispersion of the resonance wavelength of the resonator filled with a dye doped polymer was measured in dependence of the applied dc voltage. The polymer was poly (methylmethacrylate) doped with 5 wt\% disperse red 1 (DR1), and the measurements have been carried out at the telecommunication wavelength band around 1550 nm (optical C-band). The described measurements represent a new technique to determine the dispersion of the third-order susceptibility and molecular hyperpolarizability of the material filled into the slot of the ring-resonator.}, language = {en} } @article{BondarenkoVillringerSteglich2019, author = {Bondarenko, Siegfried and Villringer, Claus and Steglich, Patrick}, title = {Comparative Study of Nano-Slot Silicon Waveguides Covered by Dye Doped and Undoped Polymer Cladding}, series = {Applied Sciences}, volume = {9}, journal = {Applied Sciences}, number = {1}, publisher = {MDPI}, issn = {2076-3417}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-10716}, year = {2019}, abstract = {Nonlinear optical dyes doped in optical polymer matrices are widely used for electro-optical devices. Linear optical properties change with dye concentration, which leads to a change in modal properties, especially in nano-structured integrated waveguides such as silicon slot-waveguides. Here, we investigate the influence of a nonlinear optical dye on the performance of a silicon-organic hybrid slot-waveguide. A simulation study of the modal and optical confinement properties is carried out and dependence of the structural parameters of the slot-waveguide and the organic cladding material is taken into account. As cladding material, a guest-host polymer system is employed comprising the nonlinear optical dye Disperse Red 1 (DR1) doped in a poly[methyl methacrylate] (PMMA) matrix. The refractive indices of doped and undoped PMMA were deduced from ellipsometric data. We present a guideline for an optimized slot-waveguide design for the fabrication in silicon-on-insulator technology giving rise to scalable, high-performance integrated electro-optical modulators.}, language = {en} } @inproceedings{SteglichVillringerDuemeckeetal.2015, author = {Steglich, Patrick and Villringer, Claus and D{\"u}mecke, Silvio and Padilla-Michel, Yazmin and Casalboni, Mauro and Schrader, Sigurd}, title = {Silicon-On-Insulator Slot-Waveguide Design Trade-Offs}, series = {Proceedings of the 3rd International Conference on Photonics, Optics and Laser Technology : vol. 2}, booktitle = {Proceedings of the 3rd International Conference on Photonics, Optics and Laser Technology : vol. 2}, publisher = {SciTePress}, address = {Set{\´u}bal}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-20104}, pages = {47 -- 52}, year = {2015}, abstract = {Silicon-on-insulator slot-waveguide structures are designed and analysed numerically. We present our theoretical investigation of field confinement factors and effective nonlinear areas for different waveguide structures in order to find optimized geometrical dimensions. It is shown that a slot-waveguide with a height of 220 nm, a slot width of 180 nm and a silicon rail width of 180 nm provides a five times higher field confinement in the cladding region compared to conventional strip-waveguides which explains the high sensitivity of slotwaveguide based label-free bio-sensors.}, language = {en} } @article{VillringerSteglichPulweretal.2021, author = {Villringer, Claus and Steglich, Patrick and Pulwer, Silvio and Schrader, Sigurd and Laufer, Jan}, title = {Electro-optical properties of doped polymers with high transparency in the visible wavelength range}, series = {Optical Materials Express}, volume = {11}, journal = {Optical Materials Express}, number = {11}, publisher = {Optica Publishing Group}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-15480}, pages = {3801 -- 3811}, year = {2021}, abstract = {The electro-optical (EO) properties of poly(methyl methacrylate) and the photopolymer poly(vinyl cinnamate) doped with varying concentrations of the EO chromophore 2-Methyl-4-nitroaniline were measured. The EO polymers were embedded in Fabry-P{\´e}rot etalons for the simultaneous determination of the Pockels and Kerr coefficients from measurements of the fringe shift induced by an external electric field. It was found that the host polymer has a significant impact on the EO performance and that the undoped host polymers exhibit a significant Pockels effect. Moreover, the Kerr effect provides a substantial contribution of 27\% to the total change of the refractive index at relatively high electric field strengths of E = 91.2 MV m-1.}, language = {en} } @article{KirchnerVillringerLaufer2023, author = {Kirchner, Thomas and Villringer, Claus and Laufer, Jan}, title = {Evaluation of ultrasound sensors for transcranial photoacoustic sensing and imaging}, series = {Photoacoustics}, volume = {33}, journal = {Photoacoustics}, publisher = {Elsevier}, issn = {2213-5979}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-17993}, year = {2023}, abstract = {Photoacoustic imaging through skull bone causes strong attenuation and distortion of the acoustic wavefront, which diminishes image contrast and resolution. As a result, transcranial photoacoustic measurements in humans have been challenging to demonstrate. In this study, we investigated the acoustic transmission through the human skull to design an ultrasound sensor suitable for transcranial PA imaging and sensing. We measured the frequency dependent losses of human cranial bones ex vivo, compared the performance of a range of piezoelectric and optical ultrasound sensors, and imaged skull phantoms using a PA tomograph based on a planar Fabry-Perot sensor. All transcranial photoacoustic measurements show the typical effects of frequency and thickness dependent attenuation and aberration associated with acoustic propagation through bone. The performance of plano-concave optical resonator ultrasound sensors was found to be highly suitable for transcranial photoacoustic measurements.}, language = {en} } @article{SopinskyyGrytsenkoVillringeretal.2023, author = {Sopinskyy, Mykola and Grytsenko, Kostyantyn and Villringer, Claus and Kolomzarov, Yurii and Schrader, Sigurd}, title = {Determination of scattering and Urbach absorption contributions to the light extinction in PTFE films by using graphical representation technique and numerical solution of the inverse problem}, series = {Semiconductor Physics, Quantum Electronics \& Optoelectronics}, volume = {26}, journal = {Semiconductor Physics, Quantum Electronics \& Optoelectronics}, number = {3}, publisher = {National Academy of Sciences of Ukraine. Institute for Semiconductor Physics}, address = {Kyiv}, issn = {1605-6582}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-18035}, pages = {303 -- 314}, year = {2023}, abstract = {Ellipsometrically obtained spectral dependences of ordinary αxy and extra-ordinary αz extinction/attenuation coefficients within the spectral range λ = 300…980 nm of uniaxially anisotropic polytetrafluoroethylene (PTFE) films were analyzed. We considered the capabilities and specific features of the graphical representation technique for determining the contribution of Rayleigh scattering and Urbach absorption to light attenuation in the spectral range beyond fundamental absorption. It has been shown that the graphical approach enables to estimate these contributions qualitatively, semi-quantitatively or quantitatively, depending on the situation. The conclusions made using the analysis of graphical representation are confirmed by numerical solution of the inverse problem via simulation of the αxy (λ), αz (λ) experimental dependences within the framework of a best-fit procedure. Being based on both of these approaches, we have ascertained that, in the as-prepared PTFE films, the so-called anomalous light scattering (ALS) with the spectral dependence of scattering coefficient αs ≈ as λ-p (p > 4) takes place. Transformation of scattering from ALS to the Rayleigh one with p ≈ 4 due to annealing is accompanied by an increase of Urbach (subband) absorption. Both of these factors cause narrowing the dynamic range of extinction coefficient values. Both scattering and absorption coefficients are higher for the component of light polarized along the normal to the substrate as compared to the component polarized in parallel to it. The relationship between observed behavior of the scattering and absorption coefficients and the film structure has been discussed.}, language = {en} } @article{GrytsenkoKolomzarovLytvynetal.2023, author = {Grytsenko, Kostyantyn and Kolomzarov, Yurii and Lytvyn, Peter and Kondratenko, Olga and Sopinskyy, Mykola and Lebedyeva, Iryna and Niemczyk, Agata and Baranowska, Jolanta and Moszyński, Dariusz and Villringer, Claus and Schrader, Sigurd}, title = {Optical and Mechanical Properties of Thin PTFE Films, Deposited from a Gas Phase}, series = {Macromolecular Materials and Engineering}, volume = {308}, journal = {Macromolecular Materials and Engineering}, number = {6}, publisher = {Wiley}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-17587}, year = {2023}, abstract = {Thin polytetrafluoroethylene (PTFE) films are produced by deposition from a gas phase by two methods: electron-enhanced vacuum deposition (EVD) and EVD + low-temperature plasma (LTP). Structure, morphology, and composition of the films are studied by IR spectroscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. They are close to the structure of bulk PTFE. The roughness of the films' surface is changed with gas pressure and LTP power variations. Films are transparent from UV to near-infrared regions. Refractive and extinction indices and their anisotropy are measured by spectral ellipsometry. They are tuned by variations of deposition conditions. Hardness and Young modulus of the films are increased if EVD + low power LTP is used for film deposition. Use of EVD + LTP also increases thermal stability of the films. Contact angle of the films corresponds to the bulk PTFE. The PTFE molecules oriented are preferentially in perpendicular direction to the substrate surface.}, language = {en} } @inproceedings{SteglichMaiStolareketal.2016, author = {Steglich, Patrick and Mai, Christian and Stolarek, David and Lischke, Stefan and Kupijai, Sebastian and Villringer, Claus and Pulwer, Silvio and Heinrich, Friedhelm and Bauer, Joachim and Meister, Stefan and Knoll, Dieter and Casalboni, Mauro and Schrader, Sigurd}, title = {Partially slotted silicon ring resonator covered with electro-optical polymer}, series = {Proceedings of SPIE}, booktitle = {Proceedings of SPIE}, issn = {1996-756X}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-13771}, year = {2016}, abstract = {In this work, we present for the first time a partially slotted silicon ring resonator (PSRR) covered with an electro-optical polymer (Poly[(methyl methacrylate)-co-(Disperse Red 1 acrylate)]). The PSRR takes advantage of both a highly efficient vertical slot waveguide based phase shifter and a low loss strip waveguide in a single ring. The device is realized on 200 mm silicon-on-insulator wafers using 248 nm DUV lithography and covered with the electro-optic polymer in a post process. This silicon-organic hybrid ring resonator has a small footprint, high optical quality factor, and high DC device tunability. A quality factor of up to 105 and a DC device tunability of about 700 pm/V is experimentally demonstrated in the wavelength range of 1540 nm to 1590 nm. Further, we compare our results with state-of-the-art silicon-organic hybrid devices by determining the poling efficiency. It is demonstrated that the active PSRR is a promising candidate for efficient optical switches and tunable filters.}, language = {en} } @inproceedings{FursenkoLukosiusLupinaetal.2017, author = {Fursenko, Oksana and Lukosius, Mindaugas and Lupina, G. and Bauer, Joachim and Villringer, Claus and Mai, Andreas}, title = {Development of graphene process control by industrial optical spectroscopy setup}, publisher = {SPIE}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-15319}, pages = {250 -- 259}, year = {2017}, abstract = {The successful integration of graphene into microelectronic devices depends strongly on the availability of fast and nondestructive characterization methods of graphene grown by CVD on large diameter production wafers [1-3] which are in the interest of the semiconductor industry. Here, a high-throughput optical metrology method for measuring the thickness and uniformity of large-area graphene sheets is demonstrated. The method is based on the combination of spectroscopic ellipsometry and normal incidence reflectometry in UV-Vis wavelength range (200-800 nm) with small light spots (~ 30 μm2) realized in wafer optical metrology tool. In the first step graphene layers were transferred on a SiO2/Si substrate in order to determine the optical constants of graphene by the combination of multi-angle ellipsometry and reflectometry. Then these data were used for the development of a process control recipe of CVD graphene on 200 mm Ge(100)/Si(100) wafers. The graphene layer quality was additionally monitored by Raman spectroscopy. Atomic force microscopy measurements were performed for micro topography evaluation. In consequence, a robust recipe for unambiguous thickness monitoring of all components of a multilayer film stack, including graphene, surface residuals or interface layer underneath graphene and surface roughness is developed. Optical monitoring of graphene thickness uniformity over a wafer has shown an excellent long term stability (s=0.004 nm) regardless of the growth of interfacial GeO2 and surface roughness. The sensitivity of the optical identification of graphene during microelectronic processing was evaluated. This optical metrology technique with combined data collection exhibit a fast and highly precise method allowing one an unambiguous detection of graphene after transferring as well as after the CVD deposition process on a Ge(100)/Si(100) wafer. This approach is well suited for industrial applications due to its repeatability and flexibility.}, language = {en} } @inproceedings{BuchmannZhangScharfenorthetal.2016, author = {Buchmann, Jens and Zhang, Edward and Scharfenorth, Chris and Spannekrebs, Bastian and Villringer, Claus and Laufer, Jan}, title = {Evaluation of Fabry-Perot polymer film sensors made using hard dielectric mirror deposition}, publisher = {Society of Photo-Optical Instrumentation Engineers (SPIE)}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-15289}, pages = {920 -- 928}, year = {2016}, abstract = {Fabry-Perot (FP) polymer film sensors offer high acoustic sensitivity, small element sizes, broadband frequency response and optical transmission to enable high resolution, backward mode photoacoustic (PA) imaging. Typical approaches to sensor fabrication involve the deposition of stacks of alternating dielectric materials to form interferometer mirrors, which are separated by a polymer spacer. If hygroscopic soft dielectric materials are used, a protective polymer layer is typically required. In this study, methods for the deposition of water-resistant, hard dielectric materials onto polymers were explored to improve the robustness and performance of the sensors. This involved the optimisation of the fabrication process, the optical and acoustic characterisation of the sensors, and a comparison of the frequency response with the output of an acoustic forward model. The mirrors, which were separated by a 20 μm Parylene spacer, consisted of eight double layers of Ta2O5 and SiO2 deposited onto polymer substrates using temperature-optimised electron vapour deposition. The free spectral range of the interferometer was 32 nm, its finesse FR = 91, and its visibility V = 0.72. The noise-equivalent pressure was 0.3 kPa (20 MHz bandwidth). The measured frequency response was found to be more resonant at 25 MHz compared to sensors with soft dielectric mirrors, which was also in good agreement with the output of a forward model of the sensor. The sensors were used in a PA scanner to acquire 3-D images in tissue phantoms.}, language = {en} } @inproceedings{PulwerSteglichVillringeretal.2016, author = {Pulwer, Silvio and Steglich, Patrick and Villringer, Claus and Bauer, Joachim and Burger, Martin and Franz, M. and Grieshober, K. and Wirth, F. and Blondeau, J. and Rautenberg, J. and Mouti, S. and Schrader, Sigurd}, title = {Triangulation-based 3D surveying borescope}, publisher = {SPIE}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-15307}, pages = {51 -- 56}, year = {2016}, abstract = {In this work, a measurement concept based on triangulation was developed for borescopic 3D-surveying of surface defects. The integration of such measurement system into a borescope environment requires excellent space utilization. The triangulation angle, the projected pattern, the numerical apertures of the optical system, and the viewing angle were calculated using partial coherence imaging and geometric optical raytracing methods. Additionally, optical aberrations and defocus were considered by the integration of Zernike polynomial coefficients. The measurement system is able to measure objects with a size of 50 μm in all dimensions with an accuracy of ± 5 μm. To manage the issue of a low depth of field while using an optical high resolution system, a wavelength dependent aperture was integrated. Thereby, we are able to control depth of field and resolution of the optical system and can use the borescope in measurement mode with high resolution and low depth of field or in inspection mode with low resolution and higher depth of field. First measurements of a demonstrator system are in good agreement with our simulations.}, language = {en} } @article{FursenkoLukosiusBaueretal.2019, author = {Fursenko, Oksana and Lukosius, Mindaugas and Bauer, Joachim and Villringer, Claus and Lux, Helge and B{\"a}rwolf, Florian and Lisker, Marco and Mai, Andreas}, title = {Diagnostic of graphene on Ge(100)/Si(100) in a 200 mm wafer Si technology environment by spectroscopic ellipsometry/reflectometry}, series = {Journal of Vacuum Science \& Technology B}, volume = {37}, journal = {Journal of Vacuum Science \& Technology B}, number = {6}, publisher = {American Vacuum Society (AVS)}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-16083}, year = {2019}, abstract = {Comprehensive diagnostics is a prerequisite for the application of graphene in semiconductor technologies. Here, the authors present long-term investigations of graphene on 200-mm Ge(100)/Si(100) wafers under clean room environmental conditions. Diagnostic of graphene was performed by a fast and nondestructive metrology method based on the combination of spectroscopic ellipsometry and reflectometry (SE/R), realized within a wafer optical metrology tool. A robust procedure for unambiguous thickness monitoring of a multilayer film stack, including graphene, interface layer GeOx underneath graphene, and surface roughness is developed and applied for process control. The authors found a relationship between the quality of graphene and the growth of GeOx beneath graphene. Enhanced oxidation of Ge beneath graphene was registered as a long-term process. SE/R measurements were validated and complemented using atomic force microscopy, scanning electron microscopy, Raman spectroscopy, and secondary ion mass spectrometry. This comparative study shows a high potential for optical metrology of graphene deposited on Ge/Si structures, due to its great sensitivity, repeatability, and flexibility, realized in a nondestructive way.}, language = {en} } @inproceedings{PulwerJahnsVillringeretal.2020, author = {Pulwer, Silvio and Jahns, Daniel and Villringer, Claus and Schrader, Sigurd}, title = {Dynamic pattern generation by singlemode fibers for endoscopic 3D measurement systems}, publisher = {SPIE}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-15364}, year = {2020}, abstract = {This work presents experimental and simulation results of a 3D measurement concept based on fiber-optic interference pattern generation. A demonstrator system which is able to adjust different optical and mechanical parameters is described. The results of parameter variation studies are discussed along with possible applications in the field of micro-optical 3D measurement systems like endoscopes. The experiments were prepared with different structures with various scattering properties. To implement that approach into a micro-optical 3D measurement system, the combination of distance variation between the fibers and a controlled phase shift introduced by a retarder can be used to design the system versatile.}, language = {en} } @inproceedings{GilaniVillringerZhangetal.2018, author = {Gilani, Taravat Saeb and Villringer, Claus and Zhang, Edward and Gundlach, H. and Buchmann, Jens and Schrader, Sigurd and Laufer, Jan}, title = {Parallelised photoacoustic signal acquisition using a Fabry-Perot sensor and a camera-based interrogation scheme}, publisher = {SPIE}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-15335}, pages = {217 -- 222}, year = {2018}, abstract = {Tomographic photoacoustic (PA) images acquired using a Fabry-Perot (FP) based scanner offer high resolution and image fidelity but can result in long acquisition times due to the need for raster scanning. To reduce the acquisition times, a parallelised camera-based PA signal detection scheme is developed. The scheme is based on using a sCMOScamera and FPI sensors with high homogeneity of optical thickness. PA signals were acquired using the camera-based setup and the signal to noise ratio (SNR) was measured. A comparison of the SNR of PA signal detected using 1) a photodiode in a conventional raster scanning detection scheme and 2) a sCMOS camera in parallelised detection scheme is made. The results show that the parallelised interrogation scheme has the potential to provide high speed PA imaging.}, language = {en} } @inproceedings{KirchnerVillringerGutkeetal.2023, author = {Kirchner, Thomas and Villringer, Claus and Gutke, Marko and Laufer, Jan}, title = {Plano-concave optical sensor for transcranial photoacoustic measurements}, series = {Opto-Acoustic Methods and Applications in Biophotonics VI}, booktitle = {Opto-Acoustic Methods and Applications in Biophotonics VI}, publisher = {Society of Photo-Optical Instrumentation Engineers (SPIE)}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-17840}, year = {2023}, abstract = {Biomedical photoacoustics is usually used to image absorption-based contrast in soft tissues up to depths of several centimeters and with sub-millimeter resolution. By contrast, measuring Photoacoustic (PA) signals through hard bone tissue shows severe signal degradation due to aberration and high attenuation of high frequency acoustic signal components. This is particularly noticeable when measuring through thicker, human, skull bone. Which is the main reason why transcranial PA imaging in humans has so far proved challenging to implement. To tackle this challenge, we developed an optical resonator sensor based on a previous planar-concave design. This sensor was found to be highly suitable for measuring the low-pressure amplitude and low acoustic frequency signals that are transmitted through human cranial bone. A plano-concave optical resonator sensor was fabricated to provide high sensitivity in the acoustic frequency range of DC to around 2 MHz, a low noise equivalent pressure and a small active element size enabling it to significantly outperform conventional piezoelectric transducers when measuring PA waves transmitted through ex vivo human cranial bones.}, language = {en} } @inproceedings{SieversVillringerLebeketal.2023, author = {Sievers, Jan and Villringer, Claus and Lebek, Werner and Gilani, Taravat Saeb and Laufer, Jan}, title = {Photoacoustic tomography using a Fabry-Perot sensor with homogeneous optical thickness and wide-field camera-based detection}, series = {Opto-Acoustic Methods and Applications in Biophotonics VI}, booktitle = {Opto-Acoustic Methods and Applications in Biophotonics VI}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-17850}, year = {2023}, abstract = {Fabry-Perot (FP) sensors are typically read out using a raster scan to acquire tomographic Photoacoustic (PA) images. To speed up the recording time, wide-field illumination of the sensor in combination with a camera as detector can be used. In this study, an sCMOS camera and wavelengths around 517 nm are used to interrogate a FP sensor with a homogeneous optical thickness over a 4 cm2 aperture. The recorded time series show PA signals are acquired over the entire area of the interrogation beam. The performance of the system, such as the noise equivalent pressure, is evaluated.}, language = {en} } @inproceedings{LebekHeyrothSyrowatkaetal.2023, author = {Lebek, Werner and Heyroth, Frank and Syrowatka, Frank and Villringer, Claus and Goerlitz, Sylvia and Laufer, Jan}, title = {Evaluation of fabrication methods for Fabry-Perot polymer film ultrasound sensors}, series = {Opto-Acoustic Methods and Applications in Biophotonics VI}, booktitle = {Opto-Acoustic Methods and Applications in Biophotonics VI}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-17864}, year = {2023}, abstract = {Fabry-P{\`e}rot (FP) interferometer sensors enable highly sensitive backward mode acoustic detection in Photoacoustic (PA) imaging. They are transparent to the excitation wavelength, can be placed directly next to the PA source, and offer a broadband frequency response and high acoustic sensitivity. PA tomography using parallelized detection requires high spatial uniformity of the optical and acoustic properties, which can be hampered by contaminations during fabrication that lead to the formation of inhomogeneities and artefacts. The quality and homogeneity of the dielectric and polymer layers have a direct effect on the maximum optical phase sensitivity, and hence acoustic sensitivity. In this study, cross-sectional images of FP sensors were obtained using focused ion beam milling and ultramicrotomy followed by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) to evaluate different fabrication methods.}, language = {en} } @inproceedings{VillringerGilaniZhangetal.2019, author = {Villringer, Claus and Gilani, Taravat Saeb and Zhang, Edward and Pulwer, Silvio and Steglich, Patrick and Schrader, Sigurd and Laufer, Jan}, title = {Development of tuneable Fabry-P{\´e}rot sensors for parallelised photoacoustic signal acquisition}, series = {Proc. SPIE 10878, Photons Plus Ultrasound: Imaging and Sensing 2019, 108780M (8 March 2019)}, booktitle = {Proc. SPIE 10878, Photons Plus Ultrasound: Imaging and Sensing 2019, 108780M (8 March 2019)}, publisher = {Society of Photo-Optical Instrumentation Engineers (SPIE)}, issn = {1996-756X}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-13769}, year = {2019}, abstract = {Fabry-P{\´e}rot (FP) sensors have enabled high resolution 3D photoacoustic (PA) imaging in backward mode. However, raster-scanning of the interrogation laser beam across the sensor can result in slow 3D image acquisition. To overcome this limitation, parallelized PA signal acquisition can be used for which FP sensors with uniform optical thickness are required. In this work, the optical thickness is tuned a) irreversibly through the use of a photopolymer host matrix and b) actively using embedded electro-optic (EO) chromophores. Polymer spacers (5 μm) were deposited using spin coating and sandwiched between two dielectric mirrors and transparent ITO electrodes. The employed polymer guest-host system consists of an EO chromophore (2-methyl-4-nitroaniline) and poly(vinyl cinnamate). EO tuneability was induced using contact poling and a tuneability of 68 pm was demonstrated. The optical thickness was homogenised by raster scanning a UV beam whilst varying the exposure time across a 4 mm2 detection aperture.}, language = {en} } @inproceedings{BauerHeinrichFursenkoetal.2017, author = {Bauer, Joachim and Heinrich, Friedhelm and Fursenko, Oksana and Marschmeyer, Steffen and Bluemich, Adrian and Pulwer, Silvio and Steglich, Patrick and Villringer, Claus and Mai, Andreas and Schrader, Sigurd}, title = {Very high aspect ratio through silicon via reflectometry}, series = {Proceedings of SPIE}, booktitle = {Proceedings of SPIE}, issn = {1996-756X}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-13758}, year = {2017}, abstract = {Through Silicon Via (TSV) technology is a key feature of new 3D integration of circuits by creation of interconnections using vias, which go through the silicon wafer. Typically, the highly-selective Bosch Si etch process, characterized by a high etch rate and high aspect ratio and forming of scallops on the sidewalls is used. As presented in this paper, we have developed an experimental setup and a respective evaluation algorithm for the control and monitoring of very high aspect ratio TSV profiles by spectroscopic reflectometry. For this purpose square via arrays with lateral dimension from 3 to 10 μm were fabricated by a Bosch etch process and analyzed by our setup. By exploiting interference and diffraction effects of waves reflected from the top and bottom surfaces as well as from the side walls of the TSV patterns, the measurements provided etch depths, CD values and scallop periods. The results were compared with data obtained by a commercial wafer metrology tool. Aspect ratios of up to 35:1 were safely evaluable by our setup.}, language = {en} } @article{KlingspornKirnerVillringeretal.2016, author = {Klingsporn, M. and Kirner, S. and Villringer, Claus and Abou-Ras, D. and Costina, Ioan and Lehmann, M. and Stannowski, Bernd}, title = {Resolving the nanostructure of plasma-enhanced chemical vapor deposited nanocrystalline SiOx layers for application in solar cells}, series = {Journal of Applied Physics}, volume = {119}, journal = {Journal of Applied Physics}, number = {22}, issn = {1089-7550}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-9727}, year = {2016}, abstract = {Nanocrystalline silicon suboxides (nc-SiOx) have attracted attention during the past years for the use in thin-film silicon solar cells. We investigated the relationships between the nanostructure as well as the chemical, electrical, and optical properties of phosphorous, doped, nc-SiO0.8:H fabricated by plasma-enhanced chemical vapor deposition. The nanostructure was varied through the sample series by changing the deposition pressure from 533 to 1067 Pa. The samples were then characterized by X-ray photoelectron spectroscopy, spectroscopic ellipsometry, Raman spectroscopy, aberration-corrected high-resolution transmission electron microscopy, selected-area electron diffraction, and a specialized plasmon imaging method. We found that the material changed with increasing pressure from predominantly amorphous silicon monoxide to silicon dioxide containing nanocrystalline silicon. The nanostructure changed from amorphous silicon filaments to nanocrystalline silicon filaments, which were found to cause anisotropic electron transport.}, language = {en} } @article{KsianzouVillringerGrytsenkoetal.2025, author = {Ksianzou, Viachaslau and Villringer, Claus and Grytsenko, Kostyantyn and Pekur, Demyd and Lytvyn, Peter and Sopinskyy, Mykola and Lebedyeva, Iryna and Niemczyk, Agata and Baranowska, Jolanta}, title = {Structural and Optical Anomalies in Thin Films Grown in a Magnetic Field by Electron-Assisted Vacuum Deposition of PTFE}, series = {Macromolecular Materials and Engineering}, volume = {310}, journal = {Macromolecular Materials and Engineering}, number = {3}, publisher = {Wiley}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:526-opus4-19712}, year = {2025}, abstract = {Polytetrafluoroethylene (PTFE) films are deposited in parallel and perpendicular magnetic fields (MF) by electron-enhanced vacuum deposition (EVD) and EVD + low-temperature plasma (LTP) methods. The structure, morphology, and nanomechanical properties of the films are studied by infrared spectroscopy (IRS), atomic force microscopy (AFM), and spectroscopic ellipsometry. The structure of the thicker films is closer to that of bulk PTFE than that of thin films. The films' crystallinity and surface roughness are higher than those deposited without MF. The birefringence of the refractive index (n) of the films deposited in the MF is inverse to the anisotropy of the n of the films deposited without MF. The hardness of the films is close to that of bulk PTFE.}, language = {en} }