The design and synthesis of three organic nonlinear optical crystals is presented for terahertz (THz) generation that incorporates the optimal characteristics of known organic nonlinear optical crystals 4‐(4‐(dimethylamino)styryl)‐1‐methylpyridinium 4‐methylbenzenesulfonate (DAST) and 2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium 2,4,6‐trimethylbenzenesulfonate (HMQ‐TMS). The three crystals feature the 4‐((4‐(dimethylamino)phenyl)ethynyl)‐1‐methylpyridin‐1‐ium (4DEP) cation and three different anion combinations. Gas phase ab initio calculations of the cation show that 4DEP has a larger hyperpolarizability than cations from both DAST and HMQ‐TMS. To obtain the molecular packing needed for large nonlinear optical susceptibility and efficient THz generation, three anions are tested that result in noncentrosymmetric crystals: 4‐methylbenzenesulfonate, 3‐nitrobenzenesulfonate, and napthalene‐2‐sulfonate. After synthesis and crystallization, the crystal structure is characterized via X‐ray diffraction (XRD) analysis, and the relative THz generation efficiency for ideal crystals is predicted. 4DEP‐N2S shows a roughly 30% larger relative second‐order nonlinear optical susceptibility compared to HMQ‐TMS and DAST and, therefore, shows promise as a new advanced THz generation crystal.
One of the most effective ways of generating terahertz (THz) radiation involves the conversion of short-pulsed IR or visible laser light into THz pulses at significantly lower frequencies. This conversion can be accomplished using organic crystals with nonlinear optical crystal (NLO) properties for IR to THz conversion through optical rectification. Due to the high refractive indices of organic crystals, pump laser light as well as generated THz radiation is lost from reflections at crystal surfaces. Here we report a structure composed of a layered series of materials with intermediate refractive indices designed to reduce reflective losses and improve the THz generation from organic crystals. This structure increases the transmission coefficients for both infrared pump input and THz output. We combine simple theoretical calculations with experimental data to show that a structure composed of materials with intermediate refractive indices can be used to increase generated THz intensity by nearly 50%.
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