Advanced waveguide technologies for enhancing the performance of infrared chemo/bio sensors

Abstract

Discovery and development of novel sensing concepts is an ongoing research topic across disciplines. With maturing manufacturing and microfabrication processes, deeper understanding of medical and biological processes as well as physical mechanisms, the development of appropriate analytical tools significantly expands the accessible range of information. Typically, sensors and sensing devices comprise a probe/transducer/sensing interface, a chemical, physico-chemical or biological interaction mechanism, and a detection scheme for probing increasingly complex measurement scenarios. While most probes rely on direct interaction of probe and specimen, more delicate sampling strategies enable interactions without direct mechanical contact, e.g. when operating in harsh environments, in remote sensing scenarios or if the species of interest is embedded, e.g. in biological tissue. While acoustic interrogation may be utilized to address such scenarios, low sensitivity, low spatial resolution and inadequate chemical information content are limiting factors. Instead, probing complex scenarios via electromagnetic waves provides unsurpassed sensitivity and selectivity. In principle, each wavelength regime within the electromagnetic spectrum, ranging from gamma spectroscopy for investigation of radio nuclei to the radio frequencies applied in nuclear magnetic resonance spectroscopy, are of interest for particular sensing demands. The present thesis is focused on the thermal spectral regime, which is commonly referred to as near-infrared and mid-infrared spectral window (NIR, 780 nm to 2.5 µm, and MIR, 2.5 µm to 20 µm, 4000 cm-1 to 500 cm-1 respectively) for the development of advanced spectroscopic sensing devices. Spectroscopy in the NIR and MIR spectral region has matured into a routine analytical strategy over the past decades as any organic and inorganic substances give rise to characteristic rotational, vibrational, and roto-vibrational transitions when illuminated with infrared radiation. While these transitions are considered well-pronounced, enabling the discrimination of molecular species in complex mixtures, NIR and MIR spectroscopy are conventionally subject to rather high detection limits given the associated modest molecular absorption cross-sections. Consequently, the present thesis is focused on identifying limitations in the state-of-the-art infrared spectroscopy and sensing with particular emphasis on the utility of advanced waveguide technologies, materials, and geometries toward enhanced signals in the optical chemo/bio sensing concepts. The developed waveguide technologies are specifically tailored to match the emission characteristics of the most advanced infrared laser light sources, i.e. quantum cascade lasers (QCLs). QCLs provide unsurpassed inherent scalability and tunability in size, output power, and emission wavelength which renders them ideal light sources for the development of compact IR chemo/bio sensing devices. The combination of QCLs with thin-film waveguide technologies enables the introduction of sophisticated photonic circuitry for advanced, highly miniaturized chemo/bio sensing concepts utilizing the waveguide as an active optical transducer element. Specifically, evanescent field sensing schemes derived from attenuated total reflection (ATR) spectroscopy have been explored, which are particularly useful for probing liquid phase samples with significant opaqueness in the MIR such as aqueous media. The developed concepts have been augmented by adapting appropriate surface and immobilization chemistries for tailoring semiconductor materials as well as nanocrystalline diamond towards a variety of chemo/bio sensing scenarios. Finally, these developments have been combined into first approaches, harnessing quantum states of light and evaluating their potential toward revolutionary chemo/bio sensing concepts beyond classical noise limits based on non-classical states of light.