# Organic Crystals for THz Photonics
## 5 Applications of Organic THz Sources
### Pharmaceutical and Chemical Industry  
The potential of various THz systems in the pharmaceutical industry was explored by several studies. For pharmaceutical use, mainly molecular structure, polymorphic forms [205], cocrystals [202], hydrates [206], dehydration processes [207], tablet coating thickness [208], concentration of active pharmaceutical ingredient (API) [178], spatial density distribution [209; 210] and impurities [211] were analyzed. THz spectroscopic analysis based on organic NLO crystals was performed to distinguish between the absorption spectra of polymorphs I, II, and V of piroximam, particularly in the regions below approximately 2 THz and above approximately 3.8 THz [177]. The organic compounds like piroximam can come in different polymorphic forms that typically differ in their physicochemical properties, which also importantly affect their pharmaceutical performance. Since THz waves are sensitive to both intermolecular and intramolecular vibrations, THz-TDS can be used to investigate the crystalline state in different chemical species e.g., polymorphism. The same system based on DSTMS crystals was used to analyze hormone-based drug melatonin and its pharmaceutical product Circadin in the THz frequency range between 1.5 THz and 4.5 THz [178]. In the selected spectral range, melatonin showed characteristic spectral features at 3.21 THz and a weaker one at 4.20 THz allowing a quantitative analysis within the final product. Furthermore, spectroscopic THz imaging of different concentrations of Circadin and melatonin as an API within pellets was also performed, in order to verify the spatial recognition of different substances [178]. Another study with a DSTMS-based THz-TDS system was performed on paracetamol, where commercial pellets from two manufacturers were compared [212]. In both samples, the same characteristic spectral peaks were observed at 2.1 THz, 3.0 THz, 3.6 THz and 4.5 THz. The studies indicated that THz spectroscopy and imaging using organic THz sources could be a complementary tool to Raman and Fourier transform infrared spectroscopies, especially in applications of identification and quality control in the pharmaceutical industry. In different chemical industries ranging from construction materials, paints, glasses, ceramics, adhesives, composites, paper products, polymers and plastics to animal feeds, food ingredients, pharmaceuticals, and cosmetics, powders are main process entering material used as fillers, pigments or additives [213]. Its usefulness and effectiveness in manufacturing processes are defined by several physicochemical properties such as granulation, particle size distribution, hardness, morphology, specific surface area, polymorphism and chemical purity. One of the most abundant raw material calcium carbonate (CaCO\({}_{3}\)) was characterized by THz spectroscopic and imaging system operating with organic NLO crystals (see Figure 15) [174]. Several parameters such as CaCO\({}_{3}\) concentration in a polymer matrix, its granulation, and chemical treatment were analyzed based on the analysis of peak absorbance amplitude at around 3.2 THz, the appearance of additional spectral features such as shoulder and small peaks on the first derivative of absorbance spectra, and the frequency position and shift. In the last experiment, the calcite samples were treated by various chemicals. In that case, the frequency position of the characteristic spectral peak at 3.2 THz was shifted to higher frequencies although the powder granulation was the same. The less treated sample, i.e., natural calcite, had the most similar peak position as the pure calcite sample, whereas the samples with additives that modified the particle surface have the highest peak shift in comparison to the pure chemical. In addition, THz-TDS and amplitude THz pulsed imaging were also applied to distinguish CaCO\({}_{3}\) from other two calcium compounds, calcium oxide and calcium hydroxide, which appear during the carbonation/decarbonation production process.