# Infrared Spectroscopy-Quo Vadis?
## 3 Infrared Spectroscopy: The Present
Rather limited spectral coverage of tuneable diode lasers (TDL) absorption spectroscopy (TDLAS) [27] scenarios has recently been brought to a new level, as, e.g., broadly tuning QCLs and QCL-arrays have become available that are capable of covering broad wavelength ranges of up to 1200 cm\({}^{-1}\)[28]. Those lasers have gained interest for the development of exceedingly sensitive spectroscopic applications such as cavity ring down spectroscopy (CRDS) [26; 29; 30], providing well defined, short pulses combined with accurate wavelength selection at a compact form factor. Additionally, the 3D-printing of gas cells and even very complex optical structures such as multi-pass gas cells further increase the accessibility of complex TDLAS experiments [31; 32; 33]. Thus, trace gas detection via IR techniques continue to be an emerging topic, in the decades to come, as those lasers ideally fit the needs for sensing narrow rotational bands in complex matrices. Recent trends in laser resonator techniques allow for the envisioning of even broader spectral coverage with complex resonator structures such as tuneable external cavities [34] or multiple integrated DFB resonator concepts. An exciting strategy for such advanced structures are ring-or dual-ring-QCLs, whereby the quantum cascade structure is arranged in a circular way that facilitates high integration levels. The combination of a QCL source and QC detector in close vicinity are also interesting, e.g., for gas sensing scenarios and will provide an interesting alternative to, for example, existing non-dispersive infrared sensors [35; 36]. Increased levels of control on the quality of quantum layers now enables the fabrication of devices with tailored electronic band structures at wafer scale production levels. Hence, low-cost MIR emitting IC-LEDs made, for example, from indium arsenide/ antimonide phosphide (InAsSbP), are within reach. Furthermore, in the context of FTIR spectroscopy, diode lasers have recently been applied as stable and long-lived wavelength reference replacing He-Ne laser tubes, thus facilitating an additional reduction of device footprints. Advancing from NIR wavelengths to longer wavelengths further into the mid-infrared spectral region results in demanding technological barriers, mainly concerning the development of suitable substrate materials or material mixes. Silicon (Si) and silicon dioxide (i.e., SiO\({}_{2}\), silica, silica glass, quartz) are suitable materials for fabricating windows or lenses in the UV/Vis and NIR range, but become increasingly opaque in the MIR. On the other hand, micro-bolometers may be fabricated from silicon, which facilitates the straightforward integration of such bolometers into integrated circuits (ICs) for MIR light detection. Bulk manufacturing capabilities that have been developed for Si and SiO\({}_{2}\) processing led to the availability of high-quality substrates that can be milled via computerized numerical control (CNC) machines. Such substrates are well suited for depositing such things as highly reflective metallic mirrors. Here, most metallic materials are useful for the fabrication of broadband reflective mirrors due to their high reflectivity in the MIR. It is worthy of note that, gold (Au), silver (Ag), aluminum (Al), copper (Cu) and alloys thereof (i.e., brass and bronze) are appropriate materials for the fabrication of highly reflective mirrors in the MIR. Here, the selection of the specific material is dependent on the required performance as resistivity towards, for example, corrosive media and the acceptable costs. For application in analytical spectroscopy in the MIR, substrate integrated hollow waveguides have been developed that provide a rugged and highly integrated material platform for analysing gaseous environmental and (bio)medical samples [37; 38; 39]. Another parameter that can be crucial for the material decisions may be weight, especially in application scenarios where the device payload is critical (e.g., drone-mounted IR sensing devices).