A new benzothiazolium-based crystal of HDB-T (C₂₅H₂₅NO₄S₂) was designed and synthesized. It achieves large SHG response and efficient terahertz-wave generation.

We report an experimental study on widely tunable terahertz (THz) wave difference frequency generation (DFG) with hydrogen-bonded crystals 2-(3-(4-hydroxystyryl)-5,5-dime-thylcyclohex-2-enylidene) malononitrile (OH1). The organic crystals were pumped by a ∼1.3 μm double-pass KTiOPO₄ optical parametric oscillator. A tuning range of 0.02–20 THz was achieved. OH1 crystals offer a long effective interaction length (also high output) for the generation below 3 THz, owing to the low absorption and favorable phase-matching. The highest energy of 507 nJ/pulse was generated at 1.92 THz with a 1.89-mm-thick crystal. Comprehensive explanations were provided, on the basis of theoretical calculations. Cascading phenomenon during the DFG process was demonstrated. The photon conversion efficiency could reach 2.9%.

Terahertz (THz) technology is an emerging field that is promising for future applications. Nonlinear optical (NLO) materials can effectively convert incident light into the THz frequency range using optics methods. Ionic-type organic π-conjugated NLO crystals containing electron donor-π-acceptor motifs have long attracted attention for their possibility to achieve large nonlinear optical coefficients. In this paper, an overview of the recent progress in ionic-type organic NLO crystals for highly efficient THz wave generation is presented. The substitution design strategies of cations and anions, for increasing optical nonlinearities and reducing absorptions in different structure series, are summarized. In addition, the progress in crystal growth and their THz output performance are also discussed.

Acentric indolium-based OHI-T (C₂₆H₂₇NO₄S) was obtained through rational design, and highly efficient terahertz-wave generation was realized using an OHI-T single crystal.