Highly sensitive photoacoustic NO2 measurement system based on an optimized ring-shaped resonant cell. - In: IEEE transactions on instrumentation and measurement, Bd. 72 (2023), 9504210, insges. 10 S.
This work proposes the feasibility of an ultraviolet (UV) photoacoustic (PA) gas sensing system based on an innovative resonant PA cell structure. The system design exploits the resonant characteristics of a ring-shaped cell whose design has been optimized through analytical and finite element method (FEM) analysis to meet the requirements of the target application. In particular, the device characteristics in terms of size, frequency response, limit of detection (LOD), and related metrological characteristics were taken into account. The overall designed sensing system is made up of low-cost and commercially available components and it represents a scalable solution for monitoring different gas species. The performance of the sensing system toward NO2 was experimentally assessed, showing a relative sensitivity of 0.38%/ppm NO2 and a LOD of 500 ppb with response and recovery times in the order of a few seconds. Moreover, the cross-sensitivity of the PA sensor toward CO, CH4, and NH3 was evaluated.
Diffractive Alvarez-Lohmann lenses for correcting aberrations of tunable membrane lenses. - In: Optical engineering, ISSN 1560-2303, Bd. 62 (2023), 3, S. 035103-1-035103-23
Focus-tunable lenses, e.g., liquid filled membrane lenses (MLs), have found increasingly widespread application in optical systems. If a large refractive power range is to be used, the correction of chromatic aberrations is particularly challenging: a group containing a single ML cannot be corrected over the whole refractive power range. In analogy to hybrid achromats for lenses with constant focal lengths, we present the combination of an ML and a diffractive Alvarez-Lohmann-lens (ALL) for the compensation of axial color over a large refractive power range. In contrast to the combination of multiple MLs, this does not increase the axial length of the system significantly. At the same time, the flexible adaption of the phase function of the diffractive ALL can reduce spherical aberration over the whole focal range. Design examples with ray-tracing and wave-optical simulations demonstrate the performance of the resulting hybrid tunable element. Experimental data from fabricated sample lenses provide a proof of principle.
Cochlear implant imaging in the mouse and guinea pig using light-sheet microscopy. - In: Hearing research, ISSN 1878-5891, Bd. 426 (2022), 108639, S. 1-5
Postmortem examination of the cochlea with a cochlear implant in the scala tympani presents several challenges. It is technologically difficult to section a cochlea with an implant due to the presence of its wires and metal components that are adjacent to the membranous and bony tissues of the cochlea. These metal components damage traditional steel blades of a microtome in celloidin, paraffin or frozen embedded tissues. However, plastic embedded implanted cochleas have been successfully sectioned using specialized methods (Irving et al., 2013). An alternative non-destructive method is to optically section a chemically cleared cochlea using light-sheet microscopy, which we will describe in this publication. However, since this method uses a light-sheet to section the cochlea the opaque and reflective metal components of the implant results in some artifacts in the 2D optical sections. The best image quality using light-sheet fluorescent microscopy is when the implant is removed prior to imaging.
Design method for zoom systems based on tunable lenses. - In: Optical engineering, ISSN 1560-2303, Bd. 61 (2022), 6, S. 065103-1-065103-30
It is well known that tunable lenses, with refractive power that can be varied, e.g., by changing the curvature of a membrane, can replace the motion of lens groups in zoom systems. Similar to classical zoom systems, the performance of these systems is heavily influenced by the fundamental first-order layout. Moreover, the first-order layout sets the most important requirements for the employed tunable lenses. In this contribution, we present a method for the analysis of a large number of possible first-order solutions for typical requirements and for the selection of the most promising layouts. The first-order solution space is mapped, allowing the layouts to be automatically filtered and plotted depending on pre-defined characteristics. Ray tracing of the marginal and chief rays combined with the traditional thin lens aberration theory provide efficient estimations of the expected installation space requirements and performance for each first-order layout. Using an example, we demonstrate good agreement between these estimations and the corresponding real lens layout, optimized by commercial raytracing software. The presented design method for zoom systems based on tunable lenses is compared with similar approaches for classical zoom lenses.
Lichtschichtfluoreszenzmikroskopische Untersuchung von Silikatmaterialien :
Light-sheet fluorescence microscopic probing of silicate materials. - In: Technisches Messen, ISSN 2196-7113, Bd. 89 (2022), 6, S. 447-454
Light-sheet fluorescence microscopy (LSFM) is a powerful method for 3D characterization of fluorescent samples. In this contribution we introduce the technique for the application in material analytics by demonstrating the 3D imaging of Ce 3+ -doped YAG (Y 3 Al 5 O 12 ) crystals isolated in a glass matrix. When excited with short wavelength laser radiation, the Ce 3+ doping enables fluorescence in the wavelength range between about 450 nm and 680 nm. Since the excitation wavelengths of Ce 3+ in the YAG and glass phases of the glass ceramic differ substantially, a suitable laser wavelength can be used to excite only the YAG phase. Thus, an imaging contrast to the surrounding glass matrix is generated. We exploit the crystal dendrites for monitoring the image contrast and improve it by a deconvolution operation of the images. This field of application of LSFM offers great potential, e. g. for fundamental understanding of the microstructuring processes in silicate glasses.
Manufacturing of nanostructures with high aspect ratios using soft UV-nanoimprint lithography with bi- and trilayer resist systems. - In: Micro and nano engineering, ISSN 2590-0072, Bd. 14 (2022), 100106, S. 1-7
In this contribution we introduce new multilayer (bilayer and trilayer) resist systems for the generation of nanostructures with high aspect ratios of up to 14:1 on 4-in. full wafer scale. The bilayer stack consists of a bottom resist layer (lift off polymer LOR1A) and an UV-curable top resist layer (UV-NIL resist mr-NIL210 200 nm). The top resist is structured by UV-nanoimprint lithography with a soft polydimethysiloxane (PDMS) stamp (soft UV-NIL). After removal of the residual layer a wet chemical development is performed to achieve an isotropic undercut underneath the nanostructures in the top layer. This undercut is mandatory in order to perform a reliable and precise lift-off. The bilayer system is applicable on both silicon and fused silica. For a higher variety and combination of different resists, a trilayer system is investigated. A layer stack with new materials for bottom and top layer is presented. An intermediate layer made of silicon oxide by low temperature ICP-PECVD is added between a tailor-made top resist (mr-NIL213FC 200 nm) and an organic transfer layer (UL1). The intermediate layer serves as hard mask in order to etch the bottom layer isotropically utilizing a plasma etch process and thus replacing the wet-chemical development step. Subsequently, a thin metal layer is deposited onto the structured resist stack by electron beam evaporation. After lift-off, a nanostructured metal mask remains on the substrate providing a high selectivity during the following plasma etch step. A cryogenic ICPRIE etch process creates high aspect ratio nanostructures within the substrate. An aspect ratio of 14:1 was achieved.
Perspectives of reactive ion etching of silicate glasses for optical microsystems. - In: Journal of optical microsystems, ISSN 2708-5260, Bd. 1 (2021), 4, S. 040901-1-040901-22
We provide a review of the latest research findings as well as the future potential of plasma-based etching technology for the fabrication of micro-optical components and systems. Reactive ion etching (RIE) in combination with lithographic patterning is a well-established technology in the field of micro- and nanofabrication. Nevertheless, practical implementation, especially for plasma-based patterning of complex optical materials such as alumino-silicate glasses or glass-ceramics, is still largely based on technological experience rather than established models. Such models require an in-depth understanding of the underlying chemical and physical processes within the plasma and at the glass-plasma/mask-plasma interfaces. We therefore present results that should pave the way for a better understanding of processes and thus for the extension of RIE processes toward innovative three-dimensional (3D) patterning as well as for the processing of chemically and structurally inhomogeneous silicate-based substrates. To this end, we present and discuss the results of a variety of microstructuring strategies for different application areas with a focus on micro-optics. We consider the requirements for refractive and diffractive micro-optical systems and highlight potentials for 3D dry chemical etching by selective tailoring of the material structure. The results thus provide first steps toward a knowledge-based approach to RIE processing of universal dielectric glass materials for optical microsystems, which also has a significant impact on other microscale applications.
Tailoring patterned visible-light scattering by silicon photonic crystals. - In: ACS applied materials & interfaces, ISSN 1944-8252, Bd. 13 (2021), 50, S. 60319-60326
Searching for the relationship between the nanostructure and optical properties has always been exciting the researchers in the field of optics (linear optics as well as non-linear optics), energy harvesting (anti-reflective Si solar cells, perovskite solar cells, ..., etc.), and industry (anti-reflection coating on car windows, sunglasses, etc.). In this work, we present an approach for nanostructuring the silicon substrate to silicon photonic crystals. By precisely controlling the etching time and etching path after using nanoimprint lithography, ordered arrays of inverted Si nanopyramids and Si nanopillars with good homogeneity, uniform surface roughness, high reproducibility of pattern transfer, and a controllable aspect ratio are prepared. Experimental investigation of the optical properties indicates that the reflections of these Si nanostructures are mainly determined by the aspect ratio as well as the period of nanostructures. Furthermore, we have experimentally observed visible-light scattering (V-LS) patterns on inverted Si nanopyramids and Si nanopillars, and their corresponding patterns can be precisely controlled by the patterned nanostructures. The V-LS pattern, background, and "ghost peaks" on the angle-resolved scattering results are caused by constructive interference, destructive interference, and the interference situation between both. This controllable nanopatterning on crystalline Si substrates with precisely tunable optical properties shows great potential for applications in many fields, for example, optics, electronics, and energy.
Development and implementation of a rotating nanoimprint lithography tool for orthogonal imprinting on edges of curved surfaces. - In: Nanomanufacturing and metrology, ISSN 2520-8128, Bd. 4 (2021), 3, S. 175-180
Uniform molding and demolding of structures on highly curved surfaces through conformal contact is a crucial yet often-overlooked aspect of nanoimprint lithography (NIL). This study describes the development of a NIL tool and its integration into a nanopositioning and nanomeasuring machine to achieve high-precision orthogonal molding and demolding for soft ultraviolet-assisted NIL (soft UV-NIL). The process was implemented primarily on the edges of highly curved plano-convex substrates to demonstrate structure uniformity on the edges. High-resolution nanostructures of sub-200-nm lateral dimension and microstructures in the range of tens of microns were imprinted. However, the nanostructures on the edges of the large, curved substrates were difficult to characterize precisely. Therefore, microstructures were used to measure the structure fidelity and were characterized using profilometry, white light interferometry, and confocal laser scanning microscopy. Regardless of the restricted imaging capabilities at high inclinations for high-resolution nanostructures, the scanning electron microscope (SEM) imaging of the structures on top of the lens substrate and at an inclination of 45˚ was performed. The micro and nanostructures were successfully imprinted on the edges of the plano-convex lens at angles of 45˚, 60˚,and 90˚ from the center of rotation of the rotating NIL tool. The method enables precise imprinting at high inclinations, thereby presenting a different approach to soft UV-NIL on curved surfaces.
Tip- and laser-based 3D nanofabrication in extended macroscopic working areas. - In: Nanomanufacturing and metrology, ISSN 2520-8128, Bd. 4 (2021), 3, S. 132-148
The field of optical lithography is subject to intense research and has gained enormous improvement. However, the effort necessary for creating structures at the size of 20 nm and below is considerable using conventional technologies. This effort and the resulting financial requirements can only be tackled by few global companies and thus a paradigm change for the semiconductor industry is conceivable: custom design and solutions for specific applications will dominate future development (Fritze in: Panning EM, Liddle JA (eds) Novel patterning technologies. International society for optics and photonics. SPIE, Bellingham, 2021. https://doi.org/10.1117/12.2593229). For this reason, new aspects arise for future lithography, which is why enormous effort has been directed to the development of alternative fabrication technologies. Yet, the technologies emerging from this process, which are promising for coping with the current resolution and accuracy challenges, are only demonstrated as a proof-of-concept on a lab scale of several square micrometers. Such scale is not adequate for the requirements of modern lithography; therefore, there is the need for new and alternative cross-scale solutions to further advance the possibilities of unconventional nanotechnologies. Similar challenges arise because of the technical progress in various other fields, realizing new and unique functionalities based on nanoscale effects, e.g., in nanophotonics, quantum computing, energy harvesting, and life sciences. Experimental platforms for basic research in the field of scale-spanning nanomeasuring and nanofabrication are necessary for these tasks, which are available at the Technische Universität Ilmenau in the form of nanopositioning and nanomeasuring (NPM) machines. With this equipment, the limits of technical structurability are explored for high-performance tip-based and laser-based processes for enabling real 3D nanofabrication with the highest precision in an adequate working range of several thousand cubic millimeters.