Andreas Tortschanoff

Non-dispersive infrared (NDIR) absorption spectroscopy is a widespread approach to gas sensing due to its selectivity and conceptual simplicity. One of the main challenges towards the development of fully integrated NDIR sensors is the design and fabrication of microstructures, typically waveguides, that can combine high sensitivity with the ease of integrability of other sensor elements (sources, filters, detectors). Here, we investigate theoretically and experimentally a class of coupled strip-array (CSA) waveguides realized on a SiO₂/Si₃N₄ platform with mass semiconductor fabrication processes. We demonstrate that this class of waveguides shows comparable sensitivity for a wide range of presented geometries, making it a very promising platform for satisfying multiple sensor and fabrication requirements without loss of performance.

In this work, we investigated the optimization of a plasmonic slot waveguide (PSWG) in the mid-IR region particularly for a representative wavelength of 4.26 µm, which is the absorption line of CO₂ and thus particularly relevant for applications. We analysed the mode features associated with metal-dielectric-metal (MDM), dielectric-metal-dielectric (DMD), and truncated metal film (TMF) structures with respect to the considered PSWG. Subsequently, the mode features of the PSWG were considered based on what we outlined for MDM, DMD, and TMF structures. Furthermore, as confinement factor and propagation length are two crucial parameters for absorption sensing applications, we optimized the PSWG based on a figure of merit (FOM) defined as the product of the aforementioned quantities. To characterize the propagation length, the imaginary part of the effective mode index of a guided mode was considered, leading to a dimensionless FOM. Finally, we investigated the PSWG also for other wavelengths and identified particularly attractive wavelengths and geometries maximizing the FOM.

We propose a design for coupling light in/out of a hybrid photonic crystal surface plasmon polariton (PhC-SPP) waveguide using the band folding concept and demonstrate its feasibility. The finite-difference time-domain (FDTD) technique and rigorously coupled-wave analysis (RCWA) was used to study the absorption spectrum and dispersion diagram. By superimposing a second periodicity into the waveguide region, vertical diffraction is introduced. The proposed grating coupler couples light incident from free space directly into the PCWG modes without using any extra taper structure.

Sensors based on the interaction between analytes and the evanescent field of a silicon nitride waveguide are emerging in the field of bio-medical and environmental applications. We designed and implemented the first single particle detector based on this sensor principle that consists of a silicon nitride waveguide with sub-micron dimensions. The detection capabilities of the prototype were demonstrated with polystyrene latex (PSL) spheres equal to or greater than Ø 200 nm. Single PSL spheres caused a decrease of the transmission through the waveguide from 0.2 up to 10%, depending on their diameter and position with respect to the waveguide. The experiments were supported by 3D finite element method (FEM) simulations of the particle-waveguide interaction. The simulated relative scattered power of a single sphere is in agreement with experimental results obtained from two different setups. The silicon nitride photonic chip was fabricated with a plasma-enhanced chemical vapor deposition (PECVD) process, which is compatible with established complementary metal-oxide-semiconductor (CMOS) processes for high-volume production. The demonstrator setup was realized with an external laser and photodetector, but with recent advances in light source and detector integration, our work leverages the realization of a fully integrated, low-cost photonic particle detector.

Optical measurement approaches have proven to provide intrinsic selectivity and the sensitivity, required for the development of integrated gas sensors. In an ongoing project, we work towards a Si-photonics non-dispersive infrared gas sensor and are investigating the possibility of the incorporation of IR-plasmonic materials, which could allow an increase in sensitivities and reduce the size of such sensors. Here we present the basic concept and discuss in some detail first results concerning fabrication and characterization of the plasmonic properties.

Plasmonic slot waveguides have attracted much attention due to the possibility of high light confinement, although they suffer from relatively high propagation loss originating from the presence of a metal. Although the tightly confined light in a small gap leads to a high confinement factor, which is crucial for sensing applications, the use of plasmonic guiding at the same time results in a low propagation length. Therefore, the consideration of a trade-off between the confinement factor and the propagation length is essential to optimize the waveguide geometries. Using silicon nitride as a platform as one of the most common material systems, we have investigated free-standing and asymmetric gold-based plasmonic slot waveguides designed for sensing applications. A new figure of merit (FOM) is introduced to optimize the waveguide geometries for a wavelength of 4.26 µm corresponding to the absorption peak of CO₂, aiming at the enhancement of the confinement factor and propagation length simultaneously. For the free-standing structure, the achieved FOM is 274.6 corresponding to approximately 42% and 868 µm for confinement factor and propagation length, respectively. The FOM for the asymmetric structure shows a value of 70.1 which corresponds to 36% and 264 µm for confinement factor and propagation length, respectively.

The field of mid-infrared (MIR) plasmonics has shown great potential applications in spectroscopic sensing, infrared light sources and detectors. MIR plasmonic materials that are compatible with common fabrication processes may enable cost-effective and reliable plasmonic device platforms. In this work, we examined aluminium metal (Al), gold-tin (AuSn) and titanium-tungsten (TiW) alloys regarding their usability for surface plasmon polariton (SPP) excitation in the MIR regime using a grating configuration. The angle dependence and the influence of varying depths of gratings were numerically and experimentally studied for the chosen materials. The structures were fabricated on eight-inch silicon (Si) substrates and characterized with a free-beam reflection measurement setup in the MIR regime. The fabricated gratings show narrow resonance dips, which are in good agreement with the simulations, revealing that Al, AuSn and TiW alloys are reliable plasmonic materials for MIR plasmonic devices.