We have developed a widely-tunable monochromatic THz source based on phase-matched difference-frequency generation (DFG) in a Gal? crystal. By using a 20-mm-long GaP crystal the output wavelength was tuned in the range of 71.1-2830 mu m whereas the highest peak power was 15.6 W. The advantage of using Gal? over birefringent non-linear-optical crystals is obvious: crystal rotation is no longer required for wavelength tuning. Instead, one just needs to tune the wavelength of one mixing beam within a bandwidth of as narrow as 15.3 nm. Most recently, we observed THz frequency upconversion by mixing a THz wave with an infrared laser beam in another GaP crystal. The lowest energy per pulse measured by us was 780 pJ whereas the wavelength range of the detectable THz waves was 113.6-128.3 mu m. This scheme allows us to measure the pulse energy density, wavelength, linewidth, and pulse width of a THz beam at noncryogenic temperatures. We have also discussed a few mechanisms for further enhancing the conversion efficiencies of the two parametric processes. (c) 2006 Elsevier Ltd. All rights reserved.