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This cumulative thesis focuses on the development of thin-film waveguide technologies and their application for chem/bio sensors in food and feed quality monitoring scenarios. The thesis is based on seven peer-reviewed journal articles, which were published in collaboration with several research groups and industrial partners. The research presented in this thesis covers individual development steps towards mid-infrared (MIR) lab-on-chip systems based on thin-film waveguides and quantum cascade laser technology. Firstly, an extensive study on the modal behavior of GaAs/AlGaAs thin-film waveguides was performed. A simulation protocol was established for maximizing the sensitivity of such waveguide systems with main emphasis on their combination with broadly tunable quantum cascade lasers (QCL). Based on these design studies, optimized frequency-matched (i.e., matched to the respective analyte MIR signatures) waveguide structures and geometries may be derived for fabricating highly sensitive transducers applied in MIR chem/bio sensors and lab-on-chip systems. Consequently, the first functional monolithically integrated mid-infrared Mach-Zehnder interferometer was demonstrated based on the GaAs/AlGaAs semiconductor waveguide system. In a second step a modular waveguide assembly was designed and fabricated providing an easy-to-handle device despite micrometer-sized waveguide dimensions. This device was pivotal towards establishing advanced QCL-based analytical devices facilitating handling and alignment of thin-film waveguides, which to date was a limiting factor for the application of such miniaturized MIR sensing systems in real-world scenarios. Finally, the combination of various QCL light sources with the developed on-chip waveguide assemblies comprising frequency-matched thin-film waveguides paved the way towards miniaturized MIR sensors and more elaborate lab-on-chip systems. The potential of waveguide-and laser-based MIR analyzers as a viable alternative to commonly used Fourier transform infrared spectroscopy (FTIR) was demonstrated for several analytical scenarios. Last but not least, a new approach for spectroscopic mycotoxin analysis at EU regulatory limits utilizing the developed MIR sensor was developed, prototyped, and tested (MYCOSPEC). Thereby, an extraction and analysis procedure was established for a variety of agricultural commodities including wheat, maize and peanuts, which enabled rapid and potentially on-site classification of mycotoxin contaminated and uncontaminated samples at the respective limits demanded by EU regulations, thereby providing an innovative technology for food and feed importers and manufacturers. In summary, the developed MIR sensing technologies may readily evolve into MIR lab-on-chip systems facilitating rapidly responding and on-site applicable analytical platforms for screening and monitoring in a wide range of analytical application scenarios.

The performance and versatility of GaAs/AlGaAs thin-film waveguide technology in combination with quantum cascade lasers for mid-infrared spectroscopy in comparison to conventional FTIR spectroscopy is presented. Infrared radiation is provided by a quantum cascade laser (QCL) spectrometer comprising four tunable QCLs providing a wavelength range of 5–11 μm (1925−885 cm–1) within a single collimated beam. Epitaxially grown GaAs slab waveguides serve as optical transducer for tailored evanescent field absorption analysis. A modular waveguide mounting accessory specifically designed for on-chip thin-film GaAs waveguides is presented serving as a flexible analytical platform in lieu of conventional attenuated total reflection (ATR) crystals uniquely facilitating macroscopic handling and alignment of such microscopic waveguide structures in real-world application scenarios., acceptedVersion

This Feature highlights recent advances on mid-infrared thin-film waveguide technology and on-chip photonics facilitating next-generation label-free chem/bio sensor and assay platforms. Complemented by more recent advancements toward on-chip semiconductor waveguides, it is anticipated that label-free integrated mid-infrared sensing schemes will readily complement existing chem/bio sensor technologies in applications ranging from process monitoring and environmental analysis to biomedical diagnostics and point-of-care devices, acceptedVersion

Diamond has excellent optical properties including broadband transmissivity, low self-absorption, and a high refractive index, which have prompted its use for optical sensing applications. Thin- film diamond strip waveguides (DSWGs) combined with tunable quantum cascade lasers (tQCLs) providing an emission wave- length range of 5.78–6.35 mm (1735–1570 cm"1) have been used to obtain mid-infrared (MIR) spectra of proteins, thereby enabling the analysis of their secondary structure via the amide I band. Three different proteins were analyzed, namely bovine serum albumin (BSA), myoglobin, and g-globulin. The secondary structure of BSA and myoglobin has a major contribution of a-helices, whereas g-globulins are rich in b-sheet structures, which is reflected in the amide I band. A comparison of the spectra obtained via the combination of the tQCL and DSWG with spectra obtained using conventional Fourier transform infrared (FTIR) spectroscopy and a commercial diamond attenuated total reflection (ATR) element has been performed. It is shown that the main features evident in FTIR-ATR spectra are also obtained using tQCL-DSWG sensors., acceptedVersion

The rapid identification of mycotoxins such as deoxynivalenol and aflatoxin B1 in agricultural commodities is an ongoing concern for food importers and processors. While sophisticated chromatography-based methods are well established for regulatory testing by food safety authorities, few techniques exist to provide a rapid assessment for traders. This study advances the development of a mid-infrared spectroscopic method, recording spectra with little sample preparation. Spectral data were classified using a bootstrap-aggregated (bagged) decision tree method, evaluating the protein and carbohydrate absorption regions of the spectrum. The method was able to classify 79% of 110 maize samples at the European Union regulatory limit for deoxynivalenol of 1 750 μg kg –1 and, for the first time, 77% of 92 peanut samples at 8 μg kg –1 of aflatoxin B1. A subset model revealed a dependency on variety and type of fungal infection. The employed CRC and SBL maize varieties could be pooled in the model with a reduction of classification accuracy from 90% to 79%. Samples infected with Fusarium verticillioides were removed, leaving samples infected with F. graminearum and F. culmorum in the dataset improving classification accuracy from 73% to 79%. A 500 μg kg –1 classification threshold for deoxynivalenol in maize performed even better with 85% accuracy. This is assumed to be due to a larger number of samples around the threshold increasing representativity. Comparison with established principal component analysis classification, which consistently showed overlapping clusters, confirmed the superior performance of bagged decision tree classification., acceptedVersion

Diamond has excellent optical properties including broadband transmissivity, low self- absorption and a high refractive index, which have prompted its use in for optical sensing applications. Thin-film diamond strip waveguides (DSWGs) combined with tunable quantum cascade lasers (tQCLs) providing an emission wavelength range of 5.78-6.35 μm (1735-1570 cm-1) have been used to obtain mid-infrared (MIR) spectra of proteins, thereby enabling the analysis of their secondary structure via the amide I band. Three different proteins were analyzed, namely bovine serum albumin (BSA), myoglobin, and γ-globulin. The secondary structure of BSA and myoglobin has a major contribution of -helices, whereas γ-globulins are rich in β-sheet structures, which is reflected in the amide I band. A comparison of the spectra obtained via the combination of the tQCL and DSWG with spectra obtained using conventional Fourier transform infrared (FTIR) spectroscopy and a commercial diamond attenuated total reflection (ATR) element has been performed. It is shown that the main features evident in FTIR-ATR spectra are also obtained using tQCL-DSWG sensors., submittedVersion

Mid-infrared (MIR) sensors based on attenuated total reflection (ATR) spectroscopy provide robust, rapid and sensitive platforms for the detection of low levels of organic molecules and pollutants. Nowadays, MIR (3–15 μm) spectroscopy has evolved into a versatile sensing technique providing inherent molecular selectivity for the detection of organic and inorganic molecules. The excitation of vibrational and rotational transitions enables the qualitative and quantitative analysis of molecular constituents in solid, liquid, and vapor phases, which facilitates the application of MIR chem/bio sensors for on-site environmental analysis in scenarios such as trace pollutant monitoring or spill detection. This report presents the first integration of thin-film gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) into a miniaturized liquid flow cell designed for continuous trace analysis of chlorinated hydrocarbons (CHCs) in water coupled to a broadly tunable quantum cascade laser (QCL), which facilitates in-field deployment of QCL-based sensing devices ensuring water quality and water safety., publishedVersion