# Organic Crystals for THz Photonics
## 4 Organic Crystalline THz Sources
### THz Sources Based on Optical Rectification
The spectral shape (see Figure 11b) such as the gaps in the spectrum for OH1, are due to the phonon and vibrational modes in this crystal, which corresponds well to the measured THz properties shown in Figure 8b and the theoretical calculations of phonon modes in OH1 [169]. Figure 11: Intense THz-wave generation using a high-power Ti:Sapphire/optical parametric amplifier system (65 fs pulse duration, 100 Hz repetition rate) in DSTMS (0.44-mm thick, 1500 nm pump wavelength and 3.8 mJ pump energy, blue curves) and in OH1 (0.48-mm thick, 1350 nm pump wavelength and 3.5 mJ pump energy, red curves without and black dotted curves with a 3-THz low-pass filter): (**a**) THz time-domain signals detected by air-breakdown-coherent-detection and (**b**) the corresponding amplitude spectra. The green curves in (**a**) represent the estimated generated electric field \(E_{\mathrm{THz}}(t)\) considering the mirror losses, with a maximum of 8.3 GV/m for DSTMS and 6.2 GV/m for OH1 [94]. Reprinted from Ref. [94] with permission from Springer Nature. One interesting alternative to Ti-Sapphire-based pump systems is a high-energy Cr:Forsterite laser emitting at 1250 nm, which is still in the optimal pump range for the state-of-the-art organic NLO crystals (see Figure 9). With Cr:Forsterite femtosecond pump, THz electric fields higher than 1 GV/m in organic crystals DAST, DSTMS and OH1 have been generated [74; 170; 171]. With the variety of organic NLO crystals that are being developed, the possibility for phase matching should be also soon available for many other available high-power pump fs laser systems. For example, an ytterbium laser operating at 1030 nm [84] has been successfully used in combination with HMQ-TMS crystal, for which the best pump wavelength range is blue-shifted compared to DAST, DSTMS and OH1 (see Figure 9), also yielding high THz electric fields with a peak of about 20 MV/m [84]. In addition to high-power systems, relatively compact and ultra-broadband THz systems based on optical rectification in organic NLO crystals can be realized based on compact and relatively low-cost femtosecond laser sources operating at around 1560 nm, which is about the best phase-matching range for the state-of-the-art organic crystals such as DAST and DSTMS. Since the available pump pulse energies of these fs lasers are relatively small (in the nJ-range, which is six orders of magnitude smaller in comparison with the values up to several mJ for the high-power systems, see, e.g., the values in the caption of Figure 11), these systems do not provide an intense THz power, but they are still very interesting in combination with a sensitive detection technique, providing a high dynamic range needed e.g., for THz time-domain spectroscopy applications. Two of such examples based on a DSTMS organic crystal as THz generator are presented in Figures 12 and 13. In the first example, a highly sensitive photoconductive antenna optimized at 1560 nm wavelength has been used and in the second example, another DSTMS crystal has been used for the THz-induced lensing detection. Using organic crystals as both THz generation and detection materials enables achieving an ultra-broad THz spectral bandwidth beyond 15 THz, profiting from the phase-matching on both generation and detection part. One drawback of broadband systems based on optical rectification in organic NLO crystals is the modulation of the resulting THz spectra (see, e.g., the spectrum in Figure 11b for OH1 and in Figure 13b for DSTMS). This is due to the intrinsic phonon and vibrational modes of the generating materials, resulting in THz absorption peaks as shown in Figure 8b, leading to the absorption of the generated THz waves and varying phase-matching figures.