# Organic Crystals for THz Photonics
## 2 Second-Order Organic Nonlinear Optical (NLO) Crystals
### Optical and NLO Properties of Organic Crystals
#### 2.3.2 Alternative Ionic Organic NLO Crystals  
Theoretical evaluations additionally show that the upper limits of NLO susceptibilities in organic crystals have not been reached yet [11; 31]. This is one of the reasons that the research and development of novel NLO organic crystals is continuing. Another reason for the reinforced development during the last couple of years is a requirement for different optical and THz properties that would allow applications in different wavelength regimes, as discussed in details in Section 3.4. Several salt-type organic crystals, i.e., with NLO cation and a counter-anion that promotes non-centrosymmetric  
Figure 5: **(a)** refractive index and **(b)** absorption coefficient as a function of wavelength for some of the state-of-the-art organic crystals for light polarized along the polar axis of the particular crystal: DAST [22; 23], DSTMS [52], OH1 [39], HMQ-TMS [53] and BNA [56].  
Figure 6: Dispersion of the largest electro-optic coefficients in **(a)** DAST [22; 23] and **(b)** OH1 [39].  
packing, have been identified with similar or even superior NLO properties than DAST [32; 49; 50; 60; 61; 62; 63; 64; 65]. For photonics applications, not only high nonlinear and appropriate linear optical properties are required. Many of the novel crystals may be very challenging to grow into suitable samples for applications or even simple characterization. Therefore, it is also important to develop organic NLO crystals with easier crystal growth procedures [44; 66] or better crystal processing possibilities. For example, a possibility for cleaving as-grown crystals to produce samples with a desired thickness and optical-quality surfaces is very attractive because the expensive and time-consuming cutting and polishing may not be necessary anymore [47; 67]. In addition, various crystal engineering approaches are being developed that can lead to crystals with a desired final morphology [68; 69; 70; 71]. Two of the most successful organic NLO salt crystals developed during the last decade, considering their properties and growth/processing possibilities as well as reports on applications, are DSTMS and HMQ-TMS, which are discussed in the following subsections.  
DSTMS [44] is structurally very similar to DAST: it has the same NLO cation chromophore as DAST, but the counter-anion is slightly modified (from 4-methylbenzenesulfonate to 2,4,6-trimethylbenzenesulfonate). The additional methyl groups on the anion of DSTMS do not considerably affect both the crystal structure and the nonlinear optical properties (see Figure 4 and Table 1). However, DSTMS has an important advantage for the crystal growth because the solubility in methanol is twice as large as in DAST [44]. In addition, the strongest THz phonon absorption peak at about 1 THz is in DSTMS only about half as that of DAST [72]. DSTMS has been employed by several research groups for THz photonics applications [72; 73; 74; 75; 76; 77; 78; 79; 80; 81].  
Another ionic crystal HMQ-TMS has a quinolinium-based ionic-cation core structure. The quinolinium derivatives have been recently identified as very attractive for NLO applications, resulting in non-centrosymmetric crystals with a relatively high probability [15]. HMQ-TMS has been first reported in 2013 [47] and has been already employed in different THz photonics schemes [82; 83; 54; 71; 84]. The quinolinium chromophores in HMQ-TMS are oriented almost perfectly parallel (see Figure 4d) and therefore the diagonal NLO response is maximized. Although the NLO properties of HMQ-TMS may be comparable or slightly lower compared to DAST (see Table 1[15; 54]), a big advantage of this crystal is a possibility to obtain high optical-quality samples by a simple cleaving method [47]. HMQ-TMS has a blue-shifted absorption edge (see Figure 5) and different optical and THz properties compared to DAST and DSTMS, so it may be preferred for specific applications.