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.
The Quantum Cascade Laser is becoming a key tool for plenty of applications, from the IR to the THz range. Progress in nearby areas, such as the development of ultra-low loss crystalline microresonators, optical frequency standards and optical fiber networks for time&frequency dissemination, are paving the way to unprecedented applications in many fields. For the most demanding applications, a thorough control of quantum cascade lasers (QCLs) emission must be achieved. In the last few years, QCLs unique spectral features have been unveiled, while multifrequency, comb-like QCLs have been demonstrated. Ultra-narrow frequency linewidths are necessary for metrological applications, ranging from cold molecules interaction and ultra-high sensitivity spectroscopy to infrared/THz metrology. In our group, we are combining crystalline microresonators, with a combined high quality factor in the infrared and ultra-broadband spectral coverage, with QCLs and other nonlinear highly coherent and frequency referenced sources. Frequency referencing to optical fiber-distributed optical primary standards offers astonishing stability values of 10-16 @1-sec timescales in laboratory environments but several hundred kilometres far away from the primary clocks. A review will be given of the present status of research in this field, with a view to perspectives and future applications.