Widely-tunable, monochromatic, and high-power terahertz sources and their applications

We have demonstrated that coherent terahertz (THz) waves can be efficiently generated and tuned in extremely-wide ranges based on phase-matched difference-frequency generation (DFG) in second-order nonlinear materials. Among all the nonlinear crystals available, we have chosen GaSe and ZnGeP2, which correspond to two of the best nonlinear materials for the efficient THz generation. This is due to the fact that these two materials have the lowest and next-to-lowest absorption coefficients in the THz domain. In addition, they possess large nonlinear coefficients. For a single GaSe crystal, continuously-tunable and coherent radiation in the extremely-wide ranges of 2.7-38.4 mum and 58.2-3540 mum has been achieved. Hence, a novel and promising THz source is finally available. This source has the additional advantages of high coherence (narrow linewidth) and simple alignment. The peak output power for the THz radiation reaches 209 W at the wavelength of 196 mum (1.53 THz), which corresponds to a power conversion efficiency of 0.055%. On the other hand, for a single ZnGeP2 Crystal we have efficiently generated a monochromatic THz wave continuously-tunable in the ranges of 83.1-1642 mum and 80.2-1416 mum. The highest output peak power achieved so far is 134 W. We have also investigated how to use THz waveguides and long coherence lengths for improving the conversion efficiencies. We have reviewed our results on some important applications using these unique sources.