# Organic Crystals for THz Photonics
## 5 Applications of Organic THz Sources
### THz Nonlinear Photonics  
Recent progress in generation of intense THz pulses opened a range of new possibilities to study nonlinear physical phenomena. Interaction between mater and electric field produces various linear and nonlinear effects that can modify material properties. When a high external field is applied, the induced polarization inside the medium is a sum of the linear and nonlinear components (similarly as in the optical range described by Equation (2)), giving rise to many nonlinear effects [180; 181], such as  
Figure 13: (**a**) THz time-domain signal and (**b**) the corresponding power spectrum achieved in dry-air environment using a compact terahertz setup based on a DSTMS crystal as THz generator and another DSTMS crystal as THz detector (each crystal was about 0.6 mm thick) and a pump laser with pulse length of 40 fs, 190 mW average power, 100 MHz repetition rate and 1560 nm central wavelength [173]. coherent THz manipulation of quantum states, high-order harmonic generation, nonlinear optical processes, and nonlinear transport phenomena in solids [182]. Various THz generation techniques are employed to achieve intense THz fields needed for nonlinear effects and most of them are based on optical rectification where either LiNbO\({}_{3}\)[182; 183; 184; 185; 186] or organic NLO crystals are used. Here we summarize some of the reported examples of nonlinear THz research based on organic crystalline THz sources. DSTMS THz emitter crystal was used for generating THz pulses with a high peak electric field of about 300 kV/cm for a system enabling the THz Kerr effect spectroscopy and two-dimensional terahertz-terahertz-Raman spectroscopy [77; 187; 188], demonstrating a 2D experimental technique for measuring the nonlinear response of liquids in the THz field. The terahertz-terahertz-Raman pulse sequences that are sensitive to intermolecular modes (e.g., molecular orientational alignment) were employed to initiate intramolecular vibrational modes of liquids (see Figure 14). Vibrational coherence arising from intramolecular modes were observed in liquids such as diiodomethane at 3.66 THz and in simple halogenated liquid carbon tetrachloride at 6.50 THz [187]. These experiments open the door to THz spectroscopy that can study the dynamics of condensed-phase molecular systems such as the ability of liquids to solvate different compounds, the dynamics of biological macromolecules such as polypeptides, protein folding and DNA internal conversion as well as for amorphous solids and hydrogen-bonded liquids. Instantaneous Kerr nonlinearity and the retarded alignment of air molecules CO\({}_{2}\), N\({}_{2}\), and O\({}_{2}\) when triggered by intense THz pulses have been demonstrated by [PERSON] et al [189], for which intense single-cycle THz pulses generated in a DSTMS crystal have been employed [94]. The same group demonstrated the visualization of broadband THz radiation in the 5-13 THz frequency range, based on nonlinear THz absorption mechanism in the standard silicon charge-coupled (CCD) sensor [190]. Intense THz pulses have been shown to induce heat-induced demagnetization in the ferromagnetic nickel [191] and to change dielectric properties and simultaneous electronic and magnetic response in a cobalt thin film [192]. [PERSON] et al. investigated n-doped silicon with high-field THz transmission experiment where they observed huge transmittance enhancement of approximately 90 times under high THz field strengths generated by a DSTMS crystal [161]. The observable high increase in the transmission of n-doped silicon in the THz range by almost two orders can lead to the development of novel THz absorbers and THz field controlled electronic devices [161]. An ultrafast THz-induced optical modulator for pulse switching in a thin diamond window has also been demonstrated [193]. Figure 14: Measured THz Kerr-effect signals, generated by a terahertz–terahertz–Raman pulse sequence, of various liquids (black curves) [187]. Vibrational coherences are visible as oscillations in the measured signal.