New Electro‐Optic Salt Crystals for Efficient Terahertz Wave Generation by Direct Pumping at Ti:Sapphire Wavelength

Broadband terahertz (THz) wave generators based on widely used ultrafast solid‐state lasers operating near 800 nm are very attractive for diverse linear and nonlinear THz applications. Here new electro‐optic molecular salt crystals are developed for efficient THz wave generation directly pumpable by a conventional femtosecond (fs) Ti:sapphire amplifier. Electron‐rich naphthalene‐sulfonate anions (naphthalene‐2‐sulfonate and naphthalene‐1‐sulfonate) are introduced in new electro‐optic quinolinium crystals, instead of previously used benzene‐sulfonate anions with smaller size. In new electro‐optic crystals, highly nonlinear quinolinium‐based molecular cations are optimally aligned for maximizing the diagonal electro‐optic response. The quinolinium‐based crystals prepared using a simple cleaving method exhibit excellent crystal characteristics suitable for efficient THz wave generation, good optical quality with two parallel surfaces having high flatness, and desired crystal thickness. Pumped with 100 fs pulses from a Ti:sapphire amplifier at 800 nm, the quinolinium‐based crystals enable efficient THz wave generation. Compared to the standard inorganic 1.0 mm thick ZnTe crystal, 4.7 times higher field amplitude and a broader bandwidth with the upper cutoff frequency of 6.0 THz are obtained. Therefore, introducing naphthalene‐sulfonate anions in molecular salt electro‐optic crystals is a promising way for designing electro‐optic crystals for THz wave generation by pumping at the technologically important Ti:sapphire wavelength.