# Organic Crystals for THz Photonics
## 2 Second-Order Organic Nonlinear Optical (NLO) Crystals
### Optical and NLO Properties of Organic Crystals
#### 2.3.1 Ionic Organic Crystal DAST
]** & **(Point Group)** & **(nm)** & **(10\({}^{-30}\) esu)** & **(c)** & **(10\({}^{-30}\) esu)** & **(pm/V)** \\ \hline DAST & \(Cc\) & 475 & 194 & 20 & \(\beta^{\text{eff}}_{111}=161\) & \(d_{111}(1318\,\text{nm})=1010\) \\
[22; 23; 41; 42; 43; 50; 51] & \((m)\) & & & & \(\beta^{\text{eff}}_{221}=21\) & \(d_{111}(1542\,\text{nm})=290\) \\  & & & & & & \(\beta^{\text{eff}}_{331}=-3.2\) & \(d_{111}(1907\,\text{nm})=210\) \\  & & & & & & \(\beta^{\text{eff}}_{113}=0.0\) & \(d_{122}(1907\,\text{nm})=32\) \\  & & & & & & \(\beta^{\text{eff}}_{223}=1.0\) & \(d_{212}(1907\,\text{nm})=25\) \\  & & & & & & \(\beta^{\text{eff}}_{333}=-0.3\) & \(r_{111}(720\,\text{nm})=92\) \\  & & & & & & & \(r_{111}(1313\,\text{nm})=53\) \\  & & & & & & & \(r_{111}(1535\,\text{nm})=47\) \\  & & & & & & & \(r_{221}(1535\,\text{nm})=21\) \\ \hline DSTMS & \(Cc\) & 475 & – & 23 & – & \(d_{111}(1907\,\text{nm})=214\) \\
[32; 44; 50; 52] & \((m)\) & & & & & \(d_{122}(1907\,\text{nm})=31\) \\  & & & & & & & \(d_{212}(1907\,\text{nm})=35\) \\ \hline OH1 & \(Pna2_{1}\) & 426 & 93 & 28 & \(\beta^{\text{eff}}_{333}=63\) & \(d_{333}(1907\,\text{nm})=120\) \\
[39; 45; 46] & \((mm2)\) & & & & \(\beta^{\text{eff}}_{223}=7\) & \(r_{333}(633\,\text{nm})=109\) \\  & & & & & & \(\beta^{\text{eff}}_{113}=9\) & \(r_{333}(1319\,\text{nm})=52\) \\  & & & & & & & \(r_{123}(1319\,\text{nm})=30\) \\  & & & & & & & \(r_{113}(1319\,\text{nm})=6.8\) \\ \hline HMQ-TMS & \(Pn\) & 439 & 185 & 1 & \(\beta^{\text{eff}}_{333}=185\) & \(r_{333}(633\,\text{nm})\gtrsim 50\) \\
[47; 53; 54] & \((m)\) & & & & & \(\beta^{\text{eff}}_{223}=0.7\) & \\  & & & & & & \(\beta^{\text{eff}}_{111}=-0.8\) & \\  & & & & & & \(\beta^{\text{eff}}_{211}=0.7\) & \\  & & & & & & \(\beta^{\text{eff}}_{221}=0.3\) & \\  & & & & &The structure of DAST is shown in Figure 4a. The chromophores are packed with their main charge transfer axis oriented at about \(\theta_{kz}=20^{\circ}\) with respect to the polar axis \(k=a\), resulting in a high diagonal acentric order parameter of \(\cos^{3}\theta_{kz}=0.83\) (see also Figure 1f), which is close to optimal for the electro-optic and THz photonics applications. Due to the high diagonal acentric order parameter, DAST crystals are strongly anisotropic with a refractive index difference \(n_{1}-n_{2}>0.5\) in the visible and infrared wavelength range [22; 23; 48]. The dispersion of the refractive index is essential for phase-matched THz-wave generation and scales with the absorption band (electronic resonances) of materials according to the general Kramers-Kronig relation. DAST crystals are well suited in particular for applications at around telecommunications wavelengths, with a material absorption that is smaller than 0.1 mm\({}^{-1}\) at 1300 nm and 1550 nm (see Figure 5b). As discussed later, this range is also best suited as pump wavelength range for THz-wave generation and detection with DAST. DAST has very high NLO coefficients and electro-optic coefficients, as listed in Table 1. The highest low-frequency electro-optic tensor components \(r_{ijk}\) measured in DAST are shown in Figure 6a. The measured dispersion has been modeled by the two-level model according to Equation (9) and is represented by solid or dashed curves in Figure 6a [23]; the deviation at shorter wavelengths is due to the limits of the simple two-level model close to the absorption edge as discussed below Equation (10). The particular orientation of the NLO stilbazolium chromophores shown in Figure 4a defines the main diagonal and non-diagonal components of the macroscopic NLO susceptibility.