# Organic Crystals for THz Photonics
## 4 Organic Crystalline THz Sources
### THz Sources Based on Difference-Frequency Generation  
By using the process of difference-frequency generation, narrowband and in most cases frequency-tunable THz wave generation in various NLO crystals has been demonstrated. While the accessible THz wavelength range is intrinsically limited by the absorption of the NLO material employed, it is difficult to generate waves above 5 THz efficiently in inorganic crystals due to strong optical phonon resonances. The THz sources based on difference-frequency generation in organic NLO crystals are characterized by a wide tunability range, often beyond 20 THz (see Figure 10), a narrow single frequency output with linewidths that may be below 5 GHz [134] and high THz pulse energies beyond 500 nJ [97]. The typical dynamic range of such systems is about 30 dB (see Figure 10), limited by incoherent THz detection techniques employed (see Section 3.3).  
The most often used dual-wavelength source is a nanosecond (in some cases picosecond) optical parametric oscillator that generates two closely spaced pump wavelengths that are possibly in the phase-matching range of a particular NLO crystal employed. In many of the reported investigations, DAST has been used as a THz-wave generation crystal [106; 134; 135; 136; 137; 138; 139; 140; 141; 142; 143; 144; 145; 146; 147; 148; 149; 150; 151], while more recently other organic NLO crystals such as BNA[103; 106], DSTMS [73] and OH1 [89; 91; 95; 97; 152] have been employed, with different benefits and differences in particular with respect to the achievable efficiencies, THz frequency ranges and optimal pump wavelengths of the dual-wavelength source.  
A slightly non-collinear difference-frequency generation geometry may be used to achieve exact phase-matching conditions and improve the efficiency in a desired THz range [144]. Specially cut crystals may be used to optimize the phase matching at other pump wavelengths [147; 148]. The possibility of quasi-phase matching has also been demonstrated in case the available dual-wavelength source is out of the best phase-matching conditions of a particular crystal [91]. Intrinsic gaps in the achievable spectrum are observed due to optical phonon and molecular vibrations (see Figure 8b), as shown in Figure 10a,b [106]. An interesting possibility to reduce these gaps is to combine the output generated by difference-frequency generation in different organic NLO crystals to obtain a more homogeneous THz spectrum, as demonstrated for DAST and BNA (see Figure 10c [106]).  
In general, it is relatively straightforward to produce narrowband THz waves by difference-frequency generation in organic NLO crystals, which may be most often phase matched without the need for a special crystal cut at certain pump-wavelength ranges [135; 136; 137; 138; 139; 140; 141; 142; 143; 144; 145; 146]. It is more challenging to develop an appropriate pump-beam pair with a desired separation in the THz frequency range. These systems are presently mostly based on high-power optical parametric oscillators and therefore relatively bulky and sensitive. More compact generation systems can be achieved by using optical rectification discussed in the following section.