Tetrafluorinated ionic organic terahertz crystals

Developing new organic crystals for efficient broad terahertz (THz) wave generation remains challenging, as multiple critical material properties must be achieved simultaneously within a single crystal. In this work, we introduce a new series of organic nonlinear optical salt crystals tailored for THz wave generators. The newly designed fluorinated cationic chromophore, 6-fluoro-2-(4-hydroxystyryl)-1-methylquinolinium (OH6FQ), has been incorporated with various fluorinated or non-fluorinated anions into six different ionic crystals. Notably, the tetrafluorinated crystals, composed of fluorinated OH6FQ cations and trifluorinated anions, meet essential requirements for efficient THz wave generation, including large macroscopic optical nonlinearity, optimal facet orientation in as-grown crystals with mm-size, plate-like morphology, and relatively low THz absorption across both the entire-molecular phonon and anion-stretching vibration regions. These tetrafluorinated crystals exhibit over 20 times higher optical-to-THz amplitude conversion efficiency compared to inorganic ZnTe, with a broad bandwidth extending up to 16 THz. To the best of our knowledge, this is the first report of tetrafluorinated ionic organic THz crystals, showing promising potential for diverse THz photonic and nonlinear optical applications.