# Infrared Spectroscopy-Quo Vadis?
## 3 Infrared Spectroscopy: The Present
Aviation, space and high acceleration applications demand exceedingly lightweight yet robust substrate materials [40; 41]. Here, silicon carbide (SiC) has recently emerged as a stiff, durable and lightweight substrate material for metallic mirrors that are reflective in a wide spectral window, especially covering the MIR. Most recently, SiC Mirrors were used as the primary mirrors of the EUKLID telescope by the European Space Agency's Cosmic Vision program. Within this project, a visible imager and a near-infrared spectrometer and photometer together with a Korsch Telescope were built [42]. As an alternative, dielectric mirrors can provide enhanced reflection performance at a narrower spectral range. However, such Bragg reflectors and photonic bandgap structures can be fabricated via gas phase deposition processes (i.e., chemical vapour deposition, CVD; physical vapour deposition PVD; molecular beam epitaxy, MBE; atomic layer deposition, ALD, etc.). Here, alternating layers of specific atomic composition can be deposited to specifically tune the reflected wavelength. The very same principle has been introduced to prepare highly reflective coatings for MIR optics. Dichroic mirrors represent an implementation of the combination of dielectric mirrors and highly reflective coatings. With such mirrors, narrow-band radiation of different wavelengths can be combined into a single multi-colour beam. Such mirrors represent the standard way for beam combinations where diffractive elements (gratings) or tilted combinations cannot be deployed. Very recently, substrate integrated hollow waveguides (iHWGs) have been developed to achieve such a combination without the need to pass a dispersive element, i.e., the substrate material, of such a dielectric mirror [43]. With efforts towards the reduction of moving parts and the ideally monolithic integration linear variable filter (LVF) elements have also been introduced for IR spectroscopic demands. Such filters, in combination with detector arrays fabricated, for example., from lead zirconate titanate (PZT), provide promising wavelength selective elements for robust spectroscopic sensors in the IR. It is worthy of note that such alternating structures have also been introduced as light emitting and light detecting structures [44]. In general, dielectric materials are the material of choice in the MIR. Similar to UV/Vis and NIR optics, distinct dielectric materials are suitable for the fabrication of lenses, windows, beam-splitters, fibers and waveguides. Germanium (Ge), diamond, zinc selenide (ZnSe), zinc sulphide (ZnS), germanium based chalcogenide glasses (such as AMTR-1 and AMTR-3), other germanium based chalcogenide glasses (e.g., Schott IRG, Ge-Sb-Se, Ge-Ga-Sb-S) [45; 46], thallium bromine chloride or iodide (KRS-5, KRS-6), silver halides (AgX, AgCl\({}_{\mathrm{x}}\)Br\({}_{\mathrm{1-x}}\)) [47; 48; 49; 50], potassium bromide (KBr), sodium chloride (NaCl) and gallium arsenide (GaAs) [51; 52] are only some examples of materials that have been implemented as routinely deployed dielectrics in MIR spectroscopy of late. Many other chalcogenide-based materials are the topics of today's research on spectroscopy as shown in the articles by [PERSON] et al. and [PERSON] et al. [53; 54]. These materials are transparent in the MIR (the respective transparency regions are given in Figure 6) and provide sufficient robustness, inertness, hardness, and pH-stability. Diamond infrared optics have gained much interest in the military, such as for defence applications for night vision or guidance systems in the form of infrared domes or windows due to their excellent durability. The formerly used ZnS and ZnSe optics were cheaper to produce but much more prone to material damage, such as abrasion and darkening. As fabrication capabilities of diamonds have made this material increasingly available for civil stakeholders, diamond has become the quasi-gold standard for vacuum windows and ATR transducer materials in the MIR [55].