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Abstract Yellow organic crystals, like BNA, MNA, and NMBA, can be used to generate terahertz (THz) pulses of light through optical rectification of infrared ultrafast laser pulses. When producing THz with these organic crystals, one needs to consider that 1) their damage thresholds are low due to having low melting points and 2) Fresnel reflection losses due to multiple interfaces reduce the efficiency of the generated THz output. In this work, new heterogeneous multi‐layer “sandwich” structures are developed with these yellow organic crystals by 1) fusing them to sapphire plates to permit the crystal to withstand higher laser fluences and 2) using an index‐matching fluid (liquid crystal MBBA) to decrease Fresnel reflection losses and improve the THz output. It is shown that the sapphire plates increase the damage threshold of these yellow organic crystals by a factor of two or more, thus allowing the crystals to generate higher THz electric fields. Furthermore, it is shown that the THz light output efficiency increases by assembling the yellow crystals into multi‐layered sandwich structures. For some yellow organic crystals, the sandwich structures increase the THz intensity by more than a factor of two. 

Abstract Large crystal growth and characterization of the optical and terahertz (THz) properties of organic nonlinear optical (NLO) crystal NMBA (4‐Nitro‐4′‐methylbenzylidene aniline) is presented. The reported method of crystal growth consistently produces high‐quality single crystals. The THz generation efficiency and optical properties of NMBA to other THz generation crystals including BNA and MNA are compared. The low THz absorptions that make NMBA an ideal source for THz time‐domain spectroscopy are highlighted. 

N-Benzyl-2-methyl-4-nitroanaline (BNA) is currently the industry standard for terahertz generation at 800 nm using organic materials. We developed four derivatives of BNA by replacing a hydrogen atom on the benzyl substituent with either a fluorine (F-BNA), a methoxy group (MeO-BNA), a cyano group (CN-BNA), or a methyl group (Me-BNA). X-ray diffraction measurements confirmed that all derivatives were crystallized in the same space group as BNA (Pna21). The THz generation capability of F-BNA is higher than that of BNA, with the maximum Fourier amplitude increasing by an average of 14% when pumped with wavelengths of 800, 1250, and 1550 nm. The damage threshold of F-BNA was also found to be higher than that of BNA, increasing by a factor of 1.14. The phase matching of F-BNA and BNA was found to be comparable. The increased THz output is, therefore, likely due to the higher induced nonlinear polarization, molecular hyperpolarizability, and closer molecular packing of F-BNA compared to BNA. 

Second-harmonic generation (SHG) is a common technique with many applications. Common inorganic single-crystalline materials used to produce SHG light are effective using short IR/visible wavelengths but generally do not perform well at longer, technologically relevant IR wavelengths such as 1300, 1550, and 2000 nm. Efficient SHG materials possess many of the same key material properties as terahertz (THz) generators, and certain single-crystalline organic THz generation materials have been reported to perform at longer IR wavelengths. Consequently, this work focuses on characterizing three efficient organic THz generators for SHG, namely, DAST (trans-4-[4-(dimethylamino)-N-methylstilbazolium] p-tosylate), DSTMS (4-N,N-dimethylamino-4’-N’-methylstilbazolium 2,4,6-trimethylbenzenesulfonate), and the recently discovered generator PNPA ((E)-4-((4-nitrobenzylidene)amino)-N-phenylaniline). All three of these crystals outperform the beta-barium borate (BBO), an inorganic material commonly used for SHG, using IR pump wavelengths (1200–2000 nm). 

We have improved the THz output and damage threshold of yellow organic THz generation crystals by fusing them to sapphire and using liquid crystals as index matching fluid to reduce reflective losses.