# Organic Crystals for THz Photonics
## 5 Applications of Organic THz Sources
### General THz Spectroscopy and Imaging  
Most often employed THz systems for THz spectroscopy and imaging applications are the so-called THz-TDS (THz time-domain spectroscopy) systems, which are based on optical rectification and can simultaneously measure the amplitude and the phase of the THz electric field and therefore, for example, the absorption coefficient and the refractive index in the THz range. THz-TDS systems can be also used for non-destructive sample-thickness or coating-thickness measurements [198]. One of the additional features of THz-TDS systems is the possibility of terahertz pulsed imaging. Terahertz pulsed imaging can be performed in transmission or in reflection system geometry by raster-scanning of the sample at the focal plane of a THz-TDS system. In the time-domain, terahertz pulsed imaging can provide 2D amplitude images based on the maximum amplitude or the peak-to-peak amplitude of the transmitted (or reflected) THz electric field as well as 2D phase images by considering the arrival time of the waveform. THz spectroscopic images are obtained by the Fourier transform of time-domain signals at each sample position. Spectroscopic THz imaging and spatial distribution mapping provide visualization and localization of individual chemical compounds within heterogeneous samples, e.g., pharmaceutical pellets. Thus, the complete spectroscopic information about each pixel is obtained, which allows besides detection also identification as well as classification of materials. The chemical analysis is possible at frequencies higher than 500 GHz [199], where THz spectral responses express significant characteristics that can be assigned to inter and intramolecular vibrations of individual chemical substances. Additionally, data of spectroscopic THz imaging can be further enriched by the use of different chemometrics methods. Among them, the most used is principal component analysis, where a series of spectra are simultaneously compared through measuring the covariance [200]. The method is also known as chemical mapping and is often applied in the pharmaceutical industry [201; 202]. There have been already many examples of broadband THz spectroscopy measured with THz system based on organic NLO crystals. For example, optical properties in the THz range of organic NLO crystals themselves as those presented in Figure 8b and many other crystals have been measured by organic-based THz-TDS systems [53; 83; 91; 126; 129]. The refractive index and absorption coefficients have been also measured using THz systems based on difference-frequency generation in organic crystals [127; 169]. A system based on organic NLO crystal DSTMS as a THz emitter and a photoconductive antenna as a THz receiver operating in the 0.2-8 THz range [79] was used for measuring a lactose film and two metamaterials, U-shape split-ring resonators and rectangle hole arrays. The obtained results coincide with previous results measured by Fourier-transform infrared spectroscopy or inorganic-based THz systems. The refractive indices and the absorption of single crystals of aluminum nitride were measured in the 1-8 THz by THz-TDS based on DSTMS crystals [203]. The selected semiconductor has promising optical and optoelectronic properties that can be exploited for optical applications from the far infrared to deep ultraviolet spectral region. [PERSON] et al. [204] performed several exploratory experiments with high-resolution time-of-flight THz tomography using DAST crystal for THz waves generation. They measured the thickness of Teflon films as a function of actual thickness. THz tomography images of multilayered structures of semiconductor device sample were recorded where GaAs layer was approximately 2 \(\upmu\)m thick. This method should be valuable for car paint inspection. By using a metallic mesh, they tried to achieve imaging with high sensitivity in the THz region. They were able to detect 1 \(\upmu\)m thickness differences in ultrathin PET film which is about 300 times thinner than the wavelength in the THz frequency range [204].