# Organic Crystals for THz Photonics
## 2 Second-Order Organic Nonlinear Optical (NLO) Crystals
### Optical and NLO Properties of Organic Crystals
#### 2.3.3 Non-Ionic Organic NLO Crystals  
Besides organic salt crystals, the non-ionic organic NLO crystals have also gained considerable attention during the last decade for integrated electro-optic applications and THz photonics. The configurationally locked polyene crystal OH1 [45] has become very attractive due to the electro-optic coefficients as high as for ionic DAST, combined with a blue-shifted absorption edge and different THz properties [85; 39]. OH1 exhibits a relatively high crystal symmetry (orthorhombic, see Figure 4c) compared to the majority of other highly nonlinear organic crystals (monoclinic or even triclinic), which simplifies the handling and characterization. OH1 has very good crystal processing possibilities from solution, including high optical quality bulk crystals [46; 68; 71] and large-area single crystalline thin films on substrates [86]. High-quality OH1 thin films can be also grown by the physical vapor deposition [87]. OH1 has been found particularly effective for THz-wave applications [88; 89; 90; 91; 92; 93; 94; 95; 96; 97; 98]. Another non-ionic NLO organic crystal used for THz-wave generation by several groups is BNA (N-benzyl-2-methyl-4-nitroaniline) [55]. BNA is based on the nitronaniline derivatives developed in the early days of organic NLO research [99]. Because of the yellow color of these crystals, due to the considerably blue-shifted absorption maximum (see Table 1 and Figure 5a), considerably lower NLO coefficients compared to the red-colored materials such as DAST are expected, based on the common nonlinearity-transparency trade-off (see Equation (11)). However, the measured NLO coefficients of BNA are surprisingly high, see Table 1, with the measured diagonal NLO coefficient of \(d_{333}=234\) pm/V at the fundamental wavelength of 1064 nm [56], which is however resonantly enhanced. From the measured dispersion of the refractive indices and using a two-level dispersion model according to Equation (10), one can estimate \(d_{333}\approx 92\) pm/V and \(d_{333}\approx 72\) pm/V for BNA at 1500-nm and 1900-nm fundamental wavelength, respectively. Considering the two-level dispersion model for both the NLO and the electro-optic coefficient, we can estimate the highest electro-optic coefficient of BNA is \(r_{333}=24\) pm/V at 1538 nm [100]. Although some experimental results suggest different values of the corresponding NLO and electro-optic coefficients [58; 59; 100], BNA crystal has been very successfully used for THz-wave generation applications, in particular in the pump wavelength regime at around 800 nm that is less optimal for high-nonlinearity red materials [101; 102; 103; 104; 105; 106; 107; 108]. Several research groups worldwide identified many other interesting groups of ionic and non-ionic crystals recently, but have not yet been extensively used for THz applications. Some more crystal types are mentioned in Section 4, where we discuss different organic THz sources. For recent reviews on the development of novel organic materials see [15; 30]. On the other hand, also some older organic NLO crystals with a lower nonlinearity may be interesting for THz photonics, which was not considered at the time of their development more than two decades ago. For a comprehensive review of these crystals, see, e.g., Ref. [11]. Only a few of these crystals, e.g., COANP (2-cyclooctylamino-5-nitropyridine) [109], DANPH (4-dimethylaminobenzaldehyde-4-nitrophenylhydrazzone), MNBA (4\({}^{\prime}\)-nitrobenzylidene-3-acetamino-4-methoxyaniline), MTTNPH (5-(methylthio)-thiophenecarboxaldehyde-4-nitrophenylhydrazzone), PNP (2-(\(N\)-proinol)-5-nitropyridine) have been investigated for their THz properties [110]. The natural morphology of the crystals is also very important for applications because the NLO response critically depends on the orientation of the interacting fields with respect to the crystal axes and therefore an appropriate sample orientation is essential.