We provide an overview on THz frequency metrology, starting from the nowadays available continuous wave THz sources, discussing their main features such as tunability, spectral purity and possibility of frequency referencing to the primary frequency standard. A comparison on the achieved results in high precision molecular spectroscopy is given and discussed, and finally a special emphasis is placed on the future developments of this upcoming field. In fact, particular attention will be given to new generation metrological-grade THz sources, such as a novel 3-octaves-spanning roomtemperature continuous-wave source based on difference frequency generation, and the latest developments regarding quantum cascade laser frequency combs based on four-wave-mixing nonlinear processes.
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