Enhancement of low-frequency terahertz wave generation with a thick organic crystal DAST

The organic crystal 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) is an excellent material for terahertz (THz) photonics owing to its high second-order nonlinearity and favorable phase-matching properties. The inherent absorption centers substantially limit the generation at some frequencies, leaving several tuning gaps, especially in the low-frequency band below 2 THz. This study employs a 2.1-mm-thick DAST crystal as the difference-frequency THz emitter, and its unique output characteristics are investigated. Compared with the well-known results of thin DAST, the THz outputs at the previous dip frequencies are enhanced under sufficiently long interaction distances and high optical input. To our knowledge, an emission peak at 0.94 THz is observed for the first time. The output rapidly increases with increasing input optical energy, exceeding that predicted by the theoretical quadratic relationship. This work demonstrates the ability of DAST to generate low-frequency THz waves, which were previously considered to be outside the accessible range of DAST. This approach also provides the possibility for gap-free THz tuning.