# Organic Crystals for THz Photonics
## 3 THz-Wave Generation and Detection via NLO Processes
### THz-Wave Detection  
The detection of the generated THz waves is often as challenging as THz generation. There are several possibilities used for THz-wave detection, which also depend on the type of the THz source. We mention here the approaches that are most often used for organic crystalline THz sources. For difference-frequency generation systems that produce narrowband THz pulses, most often incoherent detectors such as Golay cells, bolometers and pyroelectric detectors are used. These detectors are commercially available and they can be directly used to determine the intensity of the generated THz radiation. For systems based on optical rectification, one can additionally use coherent detection methods, which are able to detect the amplitude and the phase of the oscillating THz field, i.e., the exact time dependence of the generated electric field \(E_{\rm THz}(t)\). The coherent detection methods can give a lot of additional information and are therefore very attractive for applications. We briefly describe the most common methods that are used for coherent detection of THz fieldsproduced by optical rectification in organic NLO crystals. Some of the often employed coherent detection methods for the optical rectification systems are illustrated in Figure 7.  
Photoconductive antennas measure the transient photoconductivity induced by the probe optical field. When the generated THz electric field overlaps with the probe optical beam on a photoconductive antenna, a current is induced across the electrodes of the antenna and its magnitude and sign are proportional to the magnitude and the sign of the THz field. By delaying the probe pulse with respect to the THz pulse, a time-domain trace of the THz electric field \(E_{\mathrm{THz}}(t)\) is obtained. Most commonly, the response of the photoconductive antennas is limited to a few THz only, but, using special antenna designs, broadband detection has also been demonstrated [116]. When fs optical pulses with relatively high pulse energies (in the order of 1 m)) are available, the detection is even possible in air by the so-called air-breakdown-coherent-detection (ABCD) [117; 118]. Most commonly used technique for detecting the THz waves based on NLO crystals is the electro-optic sampling. This method employs electro-optic crystals with zero birefringence such as ZnTe or GaP [119]. When the THz field illuminates the detection electro-optic crystal, it induces a birefringence through the conventional electro-optic effect according to Equation (3). The induced birefringence changes the polarization state of the probe beam, which is usually detected using a combination of a quarter-wave plate, Wollaston prism, and two balanced photodiodes [119] (see Figure 7). For highly birefringent materials such as organic NLO crystals, conventional electro-optic sampling detection cannot be used. Several alternative techniques have been proposed for birefringent materials [120; 121; 80; 122; 123]. A relatively simple approach is the so-called THz-induced lensing technique, which also relies on the electro-optic effect [121]. In this method, the refractive index changes induced by a focused THz electric field through the electro-optic effect are exploited. The spatially varying (focused) THz field induces a spatially varying refractive index change  
\[\Delta n(x,y,t)=-\frac{1}{2}n^{3}rE_{\mathrm{THz}}(x,y,t). \tag{23}\]  
Figure 7: Schematics of a possible experimental geometry for THz-wave generation based on optical rectification with an organic electro-optic crystal as THz generator, indicating some of the possible coherent detection approaches: electro-optic sampling (EOS) with inorganic electro-optic crystals such as ZnTe or GaP, THz-induced lensing with organic electro-optic crystals, or detection with a photoconductive antenna. The pump and the probe fs beams can be of the same or of a different wavelength, but they have to be synchronized.