# Organic Crystals for THz Photonics
## 4 Organic Crystalline THz Sources
### THz Sources Based on Optical Rectification  
The THz sources based on optical rectification in organic crystals are presently more widely employed compared to the sources based on difference-frequency generation. The THz electric field strength and its spectrum achieved by these sources depend on the characteristic properties of a particular THz generator, its nonlinearity and phase matching (see Section 3.4), as well as on the characteristics of the particular pump laser source through the source term \(I(\omega)\) in Equation (22). The shorter the pump pulses, the broader the THz spectrum that can be achieved by optical rectification. For example, using 17-fs pump pulses, it was possible to generate a spectrum extending from 0.1 to beyond 25 THz in DAST crystal by optical rectification [153], and using 5-fs short pulses, the spectrum extended even up to 200 THz [154]. THz-wave generation by optical rectification in organic crystals has been first demonstrated using DAST in 1992 [133], followed by investigations in many different groups, initially only in DAST (see, e.g., Refs. [155, 156, 157, 158, 159, 130  
set-up based on optical rectification is illustrated in Figure 7. The incoming pump laser pulse is split into a pump beam and a probe beam. The pump beam is used to generate a THz pulse in an organic NLO crystal and is then blocked by a filter transparent to THz radiation. For the optics to guide THz beams, parabolic or ellipsoidal mirrors are mostly used and different mirror combinations can be used to optimize the system for different THz photonics applications. The THz beam can be measured directly using incoherent THz detectors or coherently by overlapping THz pulses and probe optical pulses in a detector material, for which several different approaches are commonly used (see Section 3.3). The coherent detection, i.e., the measurement of the THz electric field as a function of time \(E_{\mathrm{THz}}(t)\), is most often achieved by varying the relative path length of pump with respect to probe pulses with a computer-controlled optical delay line. In some cases, e.g., when different materials are used for generation and detection with different optimal operation wavelength, probe pulses with a different wavelength can be used, which can be achieved e.g., by using a second-harmonic of the pump beam or a direct output of a Ti:Sapphire laser in case an additional optical parametric amplifier is used for the pump beam. For table-top THz sources, particularly for those aiming at high-power THz pulses that can be used for nonlinear THz photonics (see Section 5.1), conventional femtosecond Ti:Sapphire lasers, combined with high-power optical parametric amplifier systems, are most often employed. Conventional Ti:Sapphire lasers emit at the wavelength of around 800 nm and they can be directly used only for organic crystals that allow for phase matching at this wavelength, such as BNA [107; 108], COANP [109] and some other recently developed organic crystals, e.g., crystals based on the 2-(4-hydroxystyryl)-1-methylquinolinium (OHQ) chromophore [168] or the 4-(4-hydroxystyryl)-1-methyl-pyridinium (OHP) chromophore [167]. For highly NLO crystals such as DAST, DSTMS (see Figure 9), wavelengths of above 1200 nm are optimal, which is why an additional high-power optical parametric amplifier seeded by a Ti:Sapphire laser is used for these sources to convert the output to longer wavelengths. With such a Ti:Sapphire-based pump system, THz electric fields exceeding 150 MV/m [93] or even beyond 8 GV/m using special focusing optimization techniques have been demonstrated [94]. Figure 11a shows time domain traces achieved with such a system for DSTMS and OH1, where the pump wavelength has been chosen as the optimum for each of the crystals, 1500 nm and 1350 nm, respectively.