The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202107900.A data mining approach to discover and develop new organic nonlinear optical crystals that produce intense pulses of terahertz radiation is demonstrated. The Cambridge Structural Database is mined for noncentrosymmetric materials and these structural data are used in tandem with density functional theory calculations to predict new materials that efficiently generate terahertz radiation. This enables us to (in a relatively short time) discover, synthesize, and grow large, high-quality crystals of four promising materials and characterize them for intense terahertz generation. In a direct comparison to the current state-of-the-art organic terahertz generation crystals, these new materials excel. The discovery and characterization of these novel terahertz generators validate the approach of combining data mining with density functional theory calculations to predict properties of high-performance organic materials, potentially for a host of exciting applications.

We characterize the THz generation of N-benzyl-2-methyl-4-nitroaniline (BNA), with crystals ranging in thickness from 123-700 μm. We compare excitation using 1250-1500-nm and 800-nm wavelengths. Pumping BNA with both 800-nm wavelengths and longer near-infrared wavelengths results in a broad spectrum, producing out to 6 THz using a 100-fs pump, provided the BNA sample is thin enough. ~200 μm or thinner crystals are required to produce a broad spectrum with an 800-nm pump, whereas ~300 μm crystals are optimal for broadband THz generation using the longer wavelengths. We report the favorable THz generation and optical characteristics of our BNA crystals that make them attractive for broadband, highfield THz generation, and we also find significant differences to BNA results reported in other works.

The organic terahertz (THz) generation crystal BNA has recently gained traction as a valuable source to produce broadband THz pulses. Even when pumped with 800-nm light, thin BNA crystals can produce relatively high electric fields with frequency components out to 5 THz. However, the THz output when pumped with 800-nm light is limited by the damage threshold of the organic crystal. Here we report that the damage threshold of BNA can be significantly improved by physically bonding BNA to a high-thermal conductivity sapphire window. When pumped with 800-nm light from an amplified Ti:sapphire laser system, our bonded BNA (BNA-sapphire) generates 2.5× higher electric field strengths compared to bare BNA crystals. We characterize the average damage threshold for bare BNA and BNA-sapphire, measure peak-to-peak electric field strengths and THz waveforms, and determine the nonlinear transmission in BNA. Pumping BNA-sapphire with 800-nm light results in peak-to-peak electric fields exceeding 1 MV/cm, with strong broadband frequency components from 0.5-5 THz. Our BNA-sapphire THz source is a promising alternative to tilted pulse front LiNbO3 THz sources, which will enable many research groups without optical parametric amplifiers to perform high-field, broadband THz spectroscopy.

We report the full characterization of a new organic nonlinear optical (NLO) crystal for intense THz generation: PNPA ((E)-4-((4-nitrobenzylidene)amino)-N-phenylaniline). We discuss crystal growth and structural characteristics. We present the wavelength dependence of THz generation, the thickness dependence of the THz spectrum for PNPA crystals and measure the efficiency. PNPA enables intense THz generation that surpasses NLO crystals DAST and OH-1, which have been the standard in organic high-field THz generators for several years. With our experimental conditions, PNPA can generate a peak-topeak field strength of 2.9 MV/cm compared to 2.2 and 1.5 MV/cm from DAST and OH-1, respectively. This corresponds to a THz generation efficiency exceeding 4%.

Despite being identified 4 decades ago as a potentially powerful organic material for intense terahertz (THz) generation, 2-amino-5-nitrotoluene (MNA) has not been extensively used as a THz source because of challenges associated with synthesizing large single crystals of the material. We report a consistent two-step process for growing large single crystals of MNA that are suitable for high intensity terahertz (THz) generation via optical rectification of IR light. Our process includes initial sublimation growth of thin sheets or needles of MNA, followed by solution phase slow evaporation growth using the sublimated crystals as seeds. To demonstrate the usefulness of MNA as a nonlinear optical crystal, we characterize the THz generation properties of MNA and compare these results to state-of-the-art organic THz generators such as OH-1, 4-N,N-dimethylamino-4′-N′-methylstilbazolium 4-methylbenzenesulfonate (DAST), and N-benzyl-2-methyl-4-nitroaniline (BNA). We further determine the dependence of THz intensity on crystal thickness and pump wavelength, determine the THz efficiency at different pump powers, and report the THz refractive index and absorption coefficient. These results demonstrate that high-quality MNA crystals provide a useful source for high intensity THz generation.