# Organic Crystals for THz Photonics
###### Abstract  
Organic crystals with second-order optical nonlinearity feature very high and ultra-fast optical nonlinearities and are therefore attractive for various photonics applications. During the last decade, they have been found particularly attractive for terahertz (THz) photonics. This is mainly due to the very intense and ultra-broadband THz-wave generation possible with these crystals. We review recent progress and challenges in the development of organic crystalline materials for THz-wave generation and detection applications. We discuss their structure, intrinsic properties, and advantages compared to inorganic alternatives. The characteristic properties of the most widely employed organic crystals at present, such as DAST, DSTMS, OH1, HMQ-TMS, and BNA are analyzed and compared. We summarize the most important principles for THz-wave generation and detection, as well as organic THz-system configurations based on either difference-frequency generation or optical rectification. In addition, we give state-of-the-art examples of very intense and ultra-broadband THz systems that rely on organic crystals. Finally, we present some recent breakthrough demonstrations in nonlinear THz photonics enabled by very intense organic crystalline THz sources, as well as examples of THz spectroscopy and THz imaging using organic crystals as THz sources for various scientific and technological applications.  
organic nonlinear optics; organic crystals; terahertz waves; electro-optics; difference-frequency generation; optical rectification; THz spectroscopy; THz imaging; DAST; DSTMS; OH1; HMQ-TMS; BNA 2019 9 882 019 9 882 019 9 882 019 9 882 019 9 882 019 9 882 019 9 882 019 9 882 019 9 82 020 9 9 82 0219 9 82 022 03 9 9 82 04 9 82 05 06 07 08 08 09 08 09 08 09 08 0  
susceptibility and should possibly satisfy the phase-matching condition between the fundamental optical pump waves and the generated THz waves. For this purpose, mainly three different material systems are considered, semiconducting NLO crystals such as zinc telluride (ZnTe) or gallium phosphide (GaP), the conventional inorganic NLO crystals such as lithium niobate (LiNbO\({}_{3}\)) and organic NLO materials.  
The purpose of this review is to discuss the progress in the development of THz sources and applications based on organic NLO crystals. Section 2 presents the origins of NLO response in organic materials, the intrinsic differences between organic and inorganic NLO materials, and the relations between the relevant microscopic and macroscopic properties of organic NLO crystals. We present and compare the linear and the nonlinear optical properties of some of the presently most widely used organic crystals for THz photonics, including DAST, DSTMS, OH1, HMQ-TMS, and BNA. Section 3 presents theoretical fundamentals of THz-wave generation via difference-frequency generation and via optical rectification. We summarize THz incoherent and coherent detection techniques that are most widely employed in combination with organic THz sources. The important phase-matching requirements for efficient THz-wave generation and detection based on NLO principles are also discussed. We compare THz refractive indices and THz absorption properties of most widely used organic crystals in relation to their phase-matching characteristics. Section 4 presents some of the most successful examples of organic-based THz sources in terms of power or bandwidth, both the narrowband tunable organic THz sources based on difference-frequency generation and broadband THz sources based on optical rectification. Section 5 summarizes various photonics application demonstrations based on organic crystalline THz sources. These are for example important fundamental demonstrations in the exciting new field of nonlinear THz photonics, as well as various non-destructive THz spectroscopy and THz imaging approaches, interesting for a variety of future scientific and industrial applications.