# Infrared Spectroscopy-Quo Vadis?
## 3 Infrared Spectroscopy: The Present
High-quality crystals as well as foreseeably reduced fabrication costs will certainly lead to a much wider implementation of diamond-based infrared optics given the advantage of being inert, non-toxic and will therefore be of increasing importance in food analysis and pharmaceutical analysis [56]. In addition, the fabrication of waveguides such as thin-film waveguides with envisioned applications in ICs requires the existence of optical insulators (i.e., optical buffers) for which such materials are suitable. Recently, smart combinations of, e.g., freestanding diamond waveguide strips [57; 58] and membranes as well as AgX, ZnSe [59] or GaAs/AlGaAs [45] -based thin-film waveguides have been developed as efficient ATR transducer elements for MIR sensing tasks. Given these trends, refined fabrication techniques are clearly resulting in the elevated quality of the fabricated structures with existing materials as well as the development of more sophisticated waveguide structures. Some, still more exotic, waveguide designs based on, e.g., slot guiding [60], plasmonic concepts [61], bimodal waveguides [62], on-chip Mach-Zehnder Interferometers [63; 64], micro-ring resonators [65] and photonic crystals (photonic bandgap structures) have recently been presented. These structures represent a well performing alternative to more traditional rib, strip and ridge designs of photonic chemo/bio sensors and will lead to highly sensitive implementations in chemo/bio sensors for various tasks. In combination with dedicated microfluidics, highly sensitive devices may be fabricated on a small footprint requiring only a few droplets of analyte and reaction chemicals [66]. Hollow or dielectrically filled waveguide technology with metallic waveguide walls is derived from microwave/terahertz (THz) and radar technology. Such waveguides work well for distinct reflections in dedicated MIR hollow waveguide structures. However, with reduced lateral dimensions towards single mode wave guiding, fast-alternating electric fields induce high losses in the waveguide walls, leading to limited lengths of such hollow waveguides [67; 68; 69]. Hence, dielectric waveguides (fibers) based on total internal reflection are more efficient as light guiding structures. In particular, waveguide fibers for localized light delivery are most commonly realized from solid materials instead of hollow core fibers. In this context, fibers can be utilized to route light in endoscopic scenarios, enabling IR spectroscopy to be used on inaccessible parts of a patient's body. Such tools have already been introduced for cancer detection and cartilage evaluation [70; 71; 72; 73]. With a foreseeable market entry of such tools, surgeons will be provided with a tool that facilitates diagnostic decision-making based on the molecular composition of the evaluated tissue. Apart from applications in medical scenarios, probes have been developed for the on-line monitoring of processes such as bio-fermenter status. IR spectroscopic monitoring facilitates the fast detection of reaction states as well as potential spoilage, allowing fast response times and elevated process control. The delivery of high intense laser light emitted from CO\({}_{2}\) lasers for application in industrial welding or chirurgical environments benefits from a high damage threshold and low insertion losses of hollow waveguides. Therefore, some hollow core  
Figure 6: Spectral transparency range of selected materials used for waveguiding applications in the infrared. fibers with inner walls that are tailored towards low attenuation at 10.6 \(\upmu\)m have emerged on the market. In addition to advancing capabilities in fabrication and material sciences that lead to advanced application scenarios, novel technological concepts and their adaption in IR spectroscopic techniques provide opportunities that have previously been inconceivable. Similarly, utilizing the photoacoustic effect in photoacoustic spectroscopy (PAS) has enabled the non-invasive analysis of glucose concentration in interstitial fluid [74].