# Infrared Spectroscopy-Quo Vadis?
## 3 Infrared Spectroscopy: The Present  
Currently, basic instrumentation for NIR and MIR spectroscopic devices has been continuously refined and improved. Again, developments in several technological fields have led to significant improvements in IR spectroscopic instruments (see Figure 5). Again, from a technological point of view the introduction of silicon-based detectors and inexpensive thermopile detectors (i.e., low cost down to few euro-cents), as well as high-end sensitive room-temperature-operated MCT detectors has enabled rapid detection schemes with high signal to noise levels. Commonly, MCT detectors had to be cooled with liquid nitrogen, requiring the respective infrastructure, or at least thermoelectric cooling (TEC) via Peltier elements, which results in some bulky cooling components as well as the need for heat dissipation. In addition, various concepts that have been developed for UV/Vis and NIR spectroscopy are increasingly transitioned into MIR spectroscopic tools. NIR spectroscopy has benefited from development pressure at telecom wavelengths. Global data communication greatly benefits from glass fibers that provide ultra-low losses down to few dB/km at adequate manufacturing efforts. As a consequence, the cost and performance of optical components such as sources, detectors, lenses and mirrors have been optimized for wavelengths around 1300 nm. Furthermore, silicon-based source and detector concepts greatly benefit from advances in microfabrication technologies derived from integrated microelectronic circuits. Yet, these co-parallel developments, interconnections within microelectronics, and UV/Vis and NIR spectroscopy benefitting from communications technology are dictated by the utilized material platforms, which is by far and large silicon. Hence, first smartphone-based handheld spectroscopic devices have emerged on the consumer marked, e.g., for the detection of nutrition values as well as potential food spoilage based on NIR techniques. Traditionally, IR spectroscopy has relied on broadband thermal infrared emitters that emit black body radiation via resistive heating across a certain wavelength band. The respective emission is dictated by the material (e.g., tungsten (W) or SiC, etc.) and the temperature of operation. With the increasing availability of light emitting diodes (LEDs) that are capable of emitting IR wavelengths, small footprint IR sources with a reduced power consumption have recently become available and will become increasingly common within the next decades [19; 20]. Recent progress in microfabrication has led to MEMS based sources that provide a broad selection of emission wavelength/frequencies as well  
Figure 5: Selected application scenarios where infrared spectroscopy is routinely applied, and which provide growing market shares for infrared spectroscopic tools. Selected images were used from pixabay (pixabay,com; accessed on 18 October 2020). as high spectral energy densities [21]. In this context, non-coherent IR light sources such as NIR-LEDs, MEMS-based thermal infrared emitters, as well as coherent IR light sources such as inter-band cascade lasers (ICLs) [22], and quantum cascade lasers (QCL) [23; 24], have since become high-performance alternatives replacing traditional lead salt laser diodes or CO\({}_{2}\) lasers. ICLs are characterized by a particular low energy consumption, which enables applications in remote or wearable scenarios, e.g., for spectroscopic sensing of biomarkers in breath or volatile organic compounds in environmental monitoring [25; 26]. QCLs, on the other hand, provide high emission power at the cost of increased power consumption. First, applications of QCLs for remote gas sensing or chemo/bio sensing in transmission mode, as well as spectroscopic sensing with ATR accessories have been shown to and will become more ubiquitous within the next few years.