Progress in terahertz sources based on difference-frequency generation [Invited]

We review our previous results on efficient, high-power, and compact terahertz (THz) sources based on parametric processes. By reversely stacking GaP plates, we have improved photon conversion efficiency from 25% to 40%, which corresponds to the maximum value. As the number of the plates is increased from 4 to 5, the output power is decreased due to back conversion. The corresponding peak power of the THz output reaches 4 kW. We have investigated THz generation by mixing two frequencies generated by a single Nd:YLF solid-state laser. The average output power reaches 1 μW. By introducing two Nd:YLF crystals, we significantly improved the output power to 4.5 μW. Such a configuration allows us to generate different output frequencies. We review our progress for making our THz source further compact by taking advantage of passively Q-switched laser pulses. We have demonstrated that an intracavity can be utilized to enhance the THz output powers by two orders of magnitude.